WorldWideScience

Sample records for atlantic meridional overturning

  1. The Atlantic Meridional Overturning Circulation's Response to Variable Buoyancy Forcing

    Science.gov (United States)

    Butler, Edward; Oliver, Kevin; Hirschi, Joël

    2014-05-01

    The Meridional Overturning Circulation (MOC) is a large-scale global circulation of water (and heat) throughout the world's ocean. It is an integral part of the climate system, responsible for significant anomalous warming of the North Atlantic region. Much of our current understanding of the MOC is based on equilibrium theories. However, the MOC is not a steady circulation and exhibits variability across a broad range of timescales. We examine the transient response of global ocean overturning, with particular emphasis on the Atlantic MOC (AMOC), to periodic variations in the North Atlantic meridional density gradient on decadal, centennial, and millennial timescales within the Nucleus for European Modelling of the Ocean (NEMO) model framework. We use the ORCA2 global ocean configuration of NEMO (with realistic topography and a horizontal resolution of 2°) and impose periodic variations in air temperature over the North Atlantic. In response, we see large oscillations in the strength of the AMOC which peak in magnitude at 128-year timescales. A scaling relationship of the form Ψ ~ ΔρH2 (in which Δρ is a measure of meridional density gradient and H is the depth scale of maximal overturning) is found to hold for the AMOC in these transient simulations with strongest correlations observed at centennial timescales. We explore the validity of this scaling relationship across a broad range of spatial and temporal scales and discuss its validity in a global context.

  2. A multimodel comparison of centennial Atlantic meridional overturning circulation variability

    Energy Technology Data Exchange (ETDEWEB)

    Menary, Matthew B.; Vellinga, Michael; Palmer, Matthew D. [Met Office Hadley Centre, Exeter, Devon (United Kingdom); Park, Wonsun; Latif, Mojib [IFM-GEOMAR, Leibniz-Institut fuer Meereswissenschaften, Kiel (Germany); Lohmann, Katja; Jungclaus, Johann H. [Max Planck Inst Meteorol, Hamburg (Germany)

    2012-06-15

    A mechanism contributing to centennial variability of the Atlantic Meridional Overturning Circulation (AMOC) is tested with multi-millennial control simulations of several coupled general circulation models (CGCMs). These are a substantially extended integration of the 3rd Hadley Centre Coupled Climate Model (HadCM3), the Kiel Climate Model (KCM), and the Max Plank Institute Earth System Model (MPI-ESM). Significant AMOC variability on time scales of around 100 years is simulated in these models. The centennial mechanism links changes in the strength of the AMOC with oceanic salinities and surface temperatures, and atmospheric phenomena such as the Intertropical Convergence Zone (ITCZ). 2 of the 3 models reproduce all aspects of the mechanism, with the third (MPI-ESM) reproducing most of them. A comparison with a high resolution paleo-proxy for Sea Surface Temperatures (SSTs) north of Iceland over the last 4,000 years, also linked to the ITCZ, suggests that elements of this mechanism may also be detectable in the real world. (orig.)

  3. Surface changes in the North Atlantic meridional overturning circulation during the last millennium

    OpenAIRE

    Wanamaker, Alan D.; Butler, Paul G.; James D Scourse; Heinemeier, Jan; Eiríksson, Jón; Knudsen, Karen Luise; Richardson, Christopher A.

    2012-01-01

    Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local rad...

  4. Surface changes in the North Atlantic meridional overturning circulation during the last millennium.

    Science.gov (United States)

    Wanamaker, Alan D; Butler, Paul G; Scourse, James D; Heinemeier, Jan; Eiríksson, Jón; Knudsen, Karen Luise; Richardson, Christopher A

    2012-01-01

    Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation dynamics likely amplified the relatively warm conditions during the Medieval Climate Anomaly and the relatively cool conditions during the Little Ice Age within the North Atlantic sector. PMID:22692542

  5. Atlantic Meridional Overturning Circulation response to idealized external forcing

    Energy Technology Data Exchange (ETDEWEB)

    Park, W.; Latif, M. [Leibniz-Institut fuer Meereswissenschaften an der Universitaet Kiel, Kiel (Germany)

    2012-10-15

    The response of the Atlantic Meridional Overturning Circulation (AMOC) to idealized external (solar) forcing is studied in terms of the internal (unforced) AMOC modes with the Kiel Climate Model (KCM), a coupled atmosphere-ocean-sea ice general circulation model. The statistical investigation of KCM's internal AMOC variability obtained from a multi-millennial control run yields three distinct modes: a multi-decadal mode with a period of about 60 years, a quasi-centennial mode with a period of about 100 years and a multi-centennial mode with a period of about 300-400 years. Most variance is explained by the multi-centennial mode, and the least by the quasi-centennial mode. The solar constant varies sinusoidally with two different periods (100 and 60 years) in forced runs with KCM. The AMOC response to the external forcing is rather complex and nonlinear. It involves strong changes in the frequency structure of the variability. While the control run depicts multi-timescale behavior, the AMOC variability in the experiment with 100 year forcing period is channeled into a relatively narrow band centered near the forcing period. It is the quasi-centennial AMOC mode with a period of just under 100 years which is excited, although it is heavily damped in the control run. Thus, the quasi-centennial mode retains its period which does not correspond exactly to the forcing period. Surprisingly, the quasi-centennial mode is also most strongly excited when the forcing period is set to 60 years, the period of the multi-decadal mode which is rather prominent in the control run. It is largely the spatial structure of the forcing rather than its period that determines which of the three internal AMOC modes is excited. The results suggest that we need to understand the full modal structure of the internal AMOC variability in order to understand the circulation's response to external forcing. This could be a challenge for climate models: we cannot necessarily expect that the

  6. Surface changes in the North Atlantic meridional overturning circulation during the last millennium

    DEFF Research Database (Denmark)

    Wanamaker, Jr., Alan D.; Butler, Paul G.; Scourse, James D.;

    2012-01-01

    Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance...... of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning...... the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation...

  7. Role of the Southern Ocean in setting the Atlantic stratification and meridional overturning circulation

    OpenAIRE

    Kamenkovich, Igor; Radko, Timour

    2011-01-01

    This study examines the importance of the Southern Ocean (SO) stratification in determining the upper cell of the Atlantic meridional overturning circulation (MOC) and stratification. Main results are based on a suite of idealized numerical simulations of the Atlantic with the prescribed density structure at the Atlantic southern boundary, intended to explore the importance of various factors. The results demonstrate that the density distribution at the SO-Atlantic boundary is the...

  8. Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH

    NARCIS (Netherlands)

    Wouters, B.; Drijfhout, D.; Hazeleger, W.

    2012-01-01

    The Atlantic meridional overturning circulation (AMOC) in a 600 years pre-industrial run of the newly developed EC-EARTH model features marked interdecadal variability with a dominant time-scale of 50–60 years. An oscillation of approximately 2 Sverdrup (1 Sv = 106 m3 s-1) is identified, which manif

  9. Frequency-dependent effects of the Atlantic meridional overturning on the tropical Pacific Ocean

    NARCIS (Netherlands)

    Raa, L.A. te; Oldenborgh, G.J. van; Dijkstra, H.A.; Philip, S.Y.

    2009-01-01

    Using the ECHAM5/MPI-OM model, we study the relation between the variations in the Atlantic meridional overturning circulation (AMOC) and both the Pacific sea surface temperature (SST) and the El Niño-Southern Oscillation (ENSO) amplitude. In a 17-member 20C3M/SRES-A1b ensemble for 1950–2100 the Pac

  10. Multi-spatial variability modes of the Atlantic Meridional Overturning Circulation

    Institute of Scientific and Technical Information of China (English)

    ZHOU Tianjun

    2003-01-01

    The multi-spatial variability modes of the Atlantic Meridional Overturning Circulation (MOC) are identified in the natural coupled simulation of two climate models, the MOC either oscillates at decadal scales with strong cross- equatorial flow or fluctuates locally at interannual scales with weaker cross-equatorial flow. Former studies mainly emphasize the paleo-environmental and paleo-climatic impacts of the meridional overturning states transition; this analysis indicates the existence of the multi-spatial variability modes of the MOC at interannual to decadal scales. Further analysis indicates that the conventionally used MOC index, which is defined as the maximum zonal mean meridional stream-function of the North Atlantic, cannot properly describe the multi-spatial variability characteristics of the MOC.

  11. Response of the South Atlantic circulation to an abrupt collapse of the Atlantic meridional overturning circulation

    Energy Technology Data Exchange (ETDEWEB)

    Laurian, Audine [University of Hawai' i at Manoa, International Pacific Research Center, School of Ocean and Earth Science and Technology, Honolulu, HI (United States); Drijfhout, Sybren S. [Royal Netherlands Meteorological Institute, De Bilt (Netherlands)

    2011-08-15

    The South Atlantic response to a collapse of the North Atlantic meridional overturning circulation (AMOC) is investigated in the ECHAM5/MPI-OM climate model. A reduced Agulhas leakage (about 3.1 Sv; 1 Sv = 10{sup 6} m{sup 3} s{sup -1}) is found to be associated with a weaker Southern Hemisphere (SH) supergyre and Indonesian throughflow. These changes are due to reduced wind stress curl over the SH supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced than Agulhas leakage in relation with an AMOC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic circulation due to global warming, featuring a reduced AMOC, are qualitatively similar to the response to an AMOC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an AMOC collapse. (orig.)

  12. An electrical analogy relating the Atlantic multidecadal oscillation to the Atlantic meridional overturning circulation.

    Directory of Open Access Journals (Sweden)

    Bruce E Kurtz

    Full Text Available The Atlantic meridional overturning circulation (AMOC is the northward flow of surface water to subpolar latitudes where deepwater is formed, balanced by southward abyssal flow and upwelling in the vicinity of the Southern Ocean. It is generally accepted that AMOC flow oscillates with a period of 60-80 years, creating a regular variation in North Atlantic sea surface temperature known as the Atlantic multidecadal oscillation (AMO. This article attempts to answer two questions: how is the AMOC driven and why does it oscillate? Using methods commonly employed by chemical engineers for analyzing processes involving flowing liquids, apparently not previously applied to trying to understand the AMOC, an equation is developed for AMOC flow as a function of the meridional density gradient or the corresponding temperature gradient. The equation is based on the similarity between the AMOC and an industrial thermosyphon loop cooler, which circulates a heat transfer liquid without using a mechanical pump. Extending this equation with an analogy between the flow of heat and electricity explains why the AMOC flow oscillates and what determines its period. Calculated values for AMOC flow and AMO oscillation period are in good agreement with measured values.

  13. Changes of the surface cell of the Atlantic and Indian Meridional overturning circulations

    OpenAIRE

    Pérez Hernández, María Dolores

    2014-01-01

    [EN]This research thesis focuses on the Meridional Overturning Circulations (MOC) of both the Atlantic and Indian Oceans (AMOC and IMOC). The MOC plays a very important role in the ocean and the climate systems. The MOC connects the flow of the different world oceans, distributing heat and other properties through the different latitudes. The AMOC at 26.5o N transports 1.33 ± 0.14PW, this is about half of the total heat carried together by the climate system form the Equator towards hi...

  14. Meridional coherence and divergence of the Atlantic Meridional Overturning Circulation (AMOC)

    Science.gov (United States)

    Kanzow, T.; Lankhorst, M.; Frajka-Williams, E.; Send, U.; Cunningham, S. A.; Bingham, R.; Karstensen, J.; Hughes, C. W.

    2012-04-01

    The AMOC is responsible for the meridional transport of heat on climate relevant timescales. Numerical model studies suggest, that the AMOC varies on broad range of periods: Wind forcing dominates local and intraseasonal variability; Buoyancy forcing is important at longer periods and sets - at least the mean strength - of a meridionally coherent AMOC. We shall present contemporaneous observations targeting AMOC fluctuations for the period 2000 to 2010. Three continuous data sets from 16°N, 26°N and 40°N are available. Continuous mooring-based density observations of the deep southward transport of North Atlantic Deep Water in the western basin are available since the year 2000 at 16°N in the framework of the MOVE experiment (operated by SIO, San Diego, USA). From 2006 the western basin transports are extended to the eastern basin using data from the TENATSO mooring near Cape Verde. This enables us to compute NADW transports integrated across almost the entire zonal width of the Atlantic at 16°N. From 2004 the RAPID / MOCHA (NOC, UK / RSMAS & AOML, USA) a purposefully designed monitoring array has been monitoring the strength of the AMOC (both the northward transport of warm water and cold NADW return flow) at 26°N To put the decade-long observations at 16°N and 26°N into a long-term perspective, we construct a 146 year-long index of changes of the AMOC near 40°N based on tide gauge records along the US east coast (which goes back in time with interruptions to the year 1856). We present evidence for the occurrence of coherent changes of the AMOC between 16°N and 26°N, suggesting that AMOC anomalies with meridional scales larger than 1000 km are observed. We find evidence for periods of pronounced divergence in NADW transports between 16°N and 26°N. Isolating different transport contributions of the AMOC signal at both latitudes we trace the origin of coherent and divergent events. Extreme low AMOC values (observation period 2004 to 2010) were observed

  15. Deep water provenance and dynamics of the (de)glacial Atlantic meridional overturning circulation

    Science.gov (United States)

    Lippold, Jörg; Gutjahr, Marcus; Blaser, Patrick; Christner, Emanuel; de Carvalho Ferreira, Maria Luiza; Mulitza, Stefan; Christl, Marcus; Wombacher, Frank; Böhm, Evelyn; Antz, Benny; Cartapanis, Olivier; Vogel, Hendrik; Jaccard, Samuel L.

    2016-07-01

    Reconstructing past modes of ocean circulation is an essential task in paleoclimatology and paleoceanography. To this end, we combine two sedimentary proxies, Nd isotopes (εNd) and the 231Pa/230Th ratio, both of which are not directly involved in the global carbon cycle, but allow the reconstruction of water mass provenance and provide information about the past strength of overturning circulation, respectively. In this study, combined 231Pa/230Th and εNd down-core profiles from six Atlantic Ocean sediment cores are presented. The data set is complemented by the two available combined data sets from the literature. From this we derive a comprehensive picture of spatial and temporal patterns and the dynamic changes of the Atlantic Meridional Overturning Circulation over the past ∼25 ka. Our results provide evidence for a consistent pattern of glacial/stadial advances of Southern Sourced Water along with a northward circulation mode for all cores in the deeper (>3000 m) Atlantic. Results from shallower core sites support an active overturning cell of shoaled Northern Sourced Water during the LGM and the subsequent deglaciation. Furthermore, we report evidence for a short-lived period of intensified AMOC in the early Holocene.

  16. A method to permanently collapse the Atlantic Meridional Overturning Circulation in climate models

    CERN Document Server

    Cimatoribus, Andrea A; Drijfhout, Sybren S; Dijkstra, Henk A

    2011-01-01

    The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the Atlantic. Biases in the freshwater budget strongly affect the stability of the AMOC. The impact of these biases is investigated in a coupled ocean-atmosphere model by adding local anomaly patterns to the freshwater fluxes at the surface. These anomalies impact the freshwater and salt transport by the different components of the ocean circulation, in particular the basin-scale salt-advection feedback, completely changing the response of the AMOC to arbitrary perturbations. It is found that an appropriate dipole anomaly pattern at the southern border of the Atlantic Ocean can collapse the AMOC entirely even without a further hosing. Based on these results, a method is proposed for permanently collapsing the AMOC in climate models and to systematically test the existence of multiple equilibria in these models.

  17. Glacial climate sensitivity to different states of the Atlantic Meridional overturning circulation : results from the IPSL model

    OpenAIRE

    Kageyama, M.; J. Mignot; D. Swingedouw; Marzin, C.; Alkama, R.; O. Marti

    2009-01-01

    Numerous records from the North Atlantic and the surrounding continents have shown rapid and large amplitude climate variability during the last glacial period. This variability has often been associated to changes in the Atlantic Meridional Overturning Circulation (AMOC). Rapid climate change on the same time scales has also been reconstructed for sites far away from the North Atlantic, such as the tropical Atlantic, the East Pacific and Asia. The mechanisms explaining these climatic respons...

  18. Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review

    Science.gov (United States)

    Buckley, Martha W.; Marshall, John

    2016-03-01

    This is a review about the Atlantic Meridional Overturning Circulation (AMOC), its mean structure, temporal variability, controlling mechanisms, and role in the coupled climate system. The AMOC plays a central role in climate through its heat and freshwater transports. Northward ocean heat transport achieved by the AMOC is responsible for the relative warmth of the Northern Hemisphere compared to the Southern Hemisphere and is thought to play a role in setting the mean position of the Intertropical Convergence Zone north of the equator. The AMOC is a key means by which heat anomalies are sequestered into the ocean's interior and thus modulates the trajectory of climate change. Fluctuations in the AMOC have been linked to low-frequency variability of Atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses. On intra-annual timescales, variability in AMOC is large and primarily reflects the response to local wind forcing; meridional coherence of anomalies is limited to that of the wind field. On interannual to decadal timescales, AMOC changes are primarily geostrophic and related to buoyancy anomalies on the western boundary. A pacemaker region for decadal AMOC changes is located in a western "transition zone" along the boundary between the subtropical and subpolar gyres. Decadal AMOC anomalies are communicated meridionally from this region. AMOC observations, as well as the expanded ocean observational network provided by the Argo array and satellite altimetry, are inspiring efforts to develop decadal predictability systems using coupled atmosphere-ocean models initialized by ocean data.

  19. Impact of the Indonesian Throughflow on the Atlantic Meridional Overturning Circulation

    Science.gov (United States)

    Le Bars, Dewi; Dijkstra, Henk

    2014-05-01

    Understanding the mechanisms controlling the strength and variability of the Atlantic Meridional Overturning Circulation (AMOC) is one of the main topics of climate science and in particular physical oceanography. Current simple representations of the global ocean overturning separates the surface return flow to the Atlantic basin into a cold water path through the Drake Passage and a warm water path through the Indonesian Throughflow and Agulhas leakage. The relative importance of these two paths has been investigated in non-eddying ocean models. In these models the Agulhas retroflection cannot be modelled properly, which leads to an important overestimation of the Agulhas leakage. Furthermore, it seems that the in these models the relation between the meridional density gradient and the overturning strength is greatly simplified and changes significantly when eddies are resolved (Den Toom et al. 2013). As a result, the impact of the Pacific-Indian Oceans exchange through the Indonesian Throughflow on the AMOC is still unknown. To investigate this question we run a state-of-the-art ocean model, the Parallel Ocean Program (POP), globally, at eddy resolving resolution (0.1º). Using climatological forcing from the CORE dataset we perform two simulations of 110 years, a control experiment with realistic coastlines and one in which the Indonesian Passages are closed. Results show that, for a closed Indonesian Throughflow, the Indian Ocean cools down but its salinity increases. The Agulhas leakage reduces also by 3Sv (Le Bars et al. 2013) and the net effect on the south Atlantic is a cooling down and decrease salinity. The anomalies propagate slowly northward and a significant decrease of the AMOC is found at 26ºN after 50 years. This decrease AMOC also leads to reduced northward heat flux in the Atlantic. These processes are investigated with a detailed analysis of the heat and freshwater balances in the Atlantic-Arctic region and in the region south of 34ºS where

  20. Punctuated Shutdown of Atlantic Meridional Overturning Circulation during Greenland Stadial 1

    Science.gov (United States)

    Hogg, Alan; Southon, John; Turney, Chris; Palmer, Jonathan; Bronk Ramsey, Christopher; Fenwick, Pavla; Boswijk, Gretel; Friedrich, Michael; Helle, Gerhard; Hughen, Konrad; Jones, Richard; Kromer, Bernd; Noronha, Alexandra; Reynard, Linda; Staff, Richard; Wacker, Lukas

    2016-05-01

    The Greenland Stadial 1 (GS-1; ~12.9 to 11.65 kyr cal BP) was a period of North Atlantic cooling, thought to have been initiated by North America fresh water runoff that caused a sustained reduction of North Atlantic Meridional Overturning Circulation (AMOC), resulting in an antiphase temperature response between the hemispheres (the ‘bipolar seesaw’). Here we exploit sub-fossil New Zealand kauri trees to report the first securely dated, decadally-resolved atmospheric radiocarbon (14C) record spanning GS-1. By precisely aligning Southern and Northern Hemisphere tree-ring 14C records with marine 14C sequences we document two relatively short periods of AMOC collapse during the stadial, at ~12,920-12,640 cal BP and 12,050-11,900 cal BP. In addition, our data show that the interhemispheric atmospheric 14C offset was close to zero prior to GS-1, before reaching ‘near-modern’ values at ~12,660 cal BP, consistent with synchronous recovery of overturning in both hemispheres and increased Southern Ocean ventilation. Hence, sustained North Atlantic cooling across GS-1 was not driven by a prolonged AMOC reduction but probably due to an equatorward migration of the Polar Front, reducing the advection of southwesterly air masses to high latitudes. Our findings suggest opposing hemispheric temperature trends were driven by atmospheric teleconnections, rather than AMOC changes.

  1. Reduced interdecadal variability of Atlantic Meridional Overturning Circulation under global warming.

    Science.gov (United States)

    Cheng, Jun; Liu, Zhengyu; Zhang, Shaoqing; Liu, Wei; Dong, Lina; Liu, Peng; Li, Hongli

    2016-03-22

    Interdecadal variability of the Atlantic Meridional Overturning Circulation (AMOC-IV) plays an important role in climate variation and has significant societal impacts. Past climate reconstruction indicates that AMOC-IV has likely undergone significant changes. Despite some previous studies, responses of AMOC-IV to global warming remain unclear, in particular regarding its amplitude and time scale. In this study, we analyze the responses of AMOC-IV under various scenarios of future global warming in multiple models and find that AMOC-IV becomes weaker and shorter with enhanced global warming. From the present climate condition to the strongest future warming scenario, on average, the major period of AMOC-IV is shortened from ∼50 y to ∼20 y, and the amplitude is reduced by ∼60%. These reductions in period and amplitude of AMOC-IV are suggested to be associated with increased oceanic stratification under global warming and, in turn, the speedup of oceanic baroclinic Rossby waves.

  2. Linkages between atmospheric blocking, sea ice export through Fram Strait and the Atlantic Meridional Overturning Circulation.

    Science.gov (United States)

    Ionita, M; Scholz, P; Lohmann, G; Dima, M; Prange, M

    2016-01-01

    As a key persistent component of the atmospheric dynamics, the North Atlantic blocking activity has been linked to extreme climatic phenomena in the European sector. It has also been linked to Atlantic multidecadal ocean variability, but its potential links to rapid oceanic changes have not been investigated. Using a global ocean-sea ice model forced with atmospheric reanalysis data, here it is shown that the 1962-1966 period of enhanced blocking activity over Greenland resulted in anomalous sea ice accumulation in the Arctic and ended with a sea ice flush from the Arctic into the North Atlantic Ocean through Fram Strait. This event induced a significant decrease of Labrador Sea water surface salinity and an abrupt weakening of the Atlantic Meridional Overturning Circulation (AMOC) during the 1970s. These results have implications for the prediction of rapid AMOC changes and indicate that an important part of the atmosphere-ocean dynamics at mid- and high latitudes requires a proper representation of the Fram Strait sea ice transport and of the synoptic scale variability such as atmospheric blocking, which is a challenge for current coupled climate models. PMID:27619955

  3. Linkages between atmospheric blocking, sea ice export through Fram Strait and the Atlantic Meridional Overturning Circulation

    Science.gov (United States)

    Ionita, M.; Scholz, P.; Lohmann, G.; Dima, M.; Prange, M.

    2016-09-01

    As a key persistent component of the atmospheric dynamics, the North Atlantic blocking activity has been linked to extreme climatic phenomena in the European sector. It has also been linked to Atlantic multidecadal ocean variability, but its potential links to rapid oceanic changes have not been investigated. Using a global ocean-sea ice model forced with atmospheric reanalysis data, here it is shown that the 1962-1966 period of enhanced blocking activity over Greenland resulted in anomalous sea ice accumulation in the Arctic and ended with a sea ice flush from the Arctic into the North Atlantic Ocean through Fram Strait. This event induced a significant decrease of Labrador Sea water surface salinity and an abrupt weakening of the Atlantic Meridional Overturning Circulation (AMOC) during the 1970s. These results have implications for the prediction of rapid AMOC changes and indicate that an important part of the atmosphere-ocean dynamics at mid- and high latitudes requires a proper representation of the Fram Strait sea ice transport and of the synoptic scale variability such as atmospheric blocking, which is a challenge for current coupled climate models.

  4. Variations of the Atlantic meridional overturning circulation in control and transient simulations of the last millennium

    Directory of Open Access Journals (Sweden)

    D. Hofer

    2011-02-01

    Full Text Available The variability of the Atlantic meridional overturing circulation (AMOC strength is investigated in control experiments and in transient simulations of up to the last millennium using the low-resolution Community Climate System Model version 3. In the transient simulations the AMOC exhibits enhanced low-frequency variability that is mainly caused by infrequent transitions between two semi-stable circulation states which amount to a 10 percent change of the maximum overturning. One transition is also found in a control experiment, but the time-varying external forcing significantly increases the probability of the occurrence of such events though not having a direct, linear impact on the AMOC. The transition from a high to a low AMOC state starts with a reduction of the convection in the Labrador and Irminger Seas and goes along with a changed barotropic circulation of both gyres in the North Atlantic and a gradual strengthening of the convection in the Greenland-Iceland-Norwegian (GIN Seas. In contrast, the transition from a weak to a strong overturning is induced by decreased mixing in the GIN Seas. As a consequence of the transition, regional sea surface temperature (SST anomalies are found in the midlatitude North Atlantic and in the convection regions with an amplitude of up to 3 K. The atmospheric response to the SST forcing associated with the transition indicates a significant impact on the Scandinavian surface air temperature (SAT in the order of 1 K. Thus, the changes of the ocean circulation make a major contribution to the Scandinavian SAT variability in the last millennium.

  5. Observed decline of the Atlantic Meridional Overturning Circulation 2004 to 2012

    Directory of Open Access Journals (Sweden)

    D. A. Smeed

    2013-09-01

    Full Text Available The Atlantic Meridional Overturning Circulation (AMOC has been observed continuously at 26° N since April 2004. The AMOC and its component parts are monitored by combining a transatlantic array of moored instruments with submarine-cable based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April~2008 to March 2012 the AMOC was an average of 2.7 Sv weaker than in the first four years of observation (95% confidence that the reduction is 0.3 Sv or more. Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the subtropical gyre above the thermocline. The increased southward flow of warm waters is balanced by a decrease in the southward flow of Lower North Atlantic Deep Water below 3000 m. The transport of Lower North Atlantic Deep Water slowed by 7% per year (95% confidence that the rate of slowing is greater than 2.5% per year.

  6. On the relation between Meridional Overturning Circulation and sea-level gradients in the Atlantic

    Directory of Open Access Journals (Sweden)

    H. Kienert

    2012-04-01

    Full Text Available On the basis of model simulations, we examine what information on changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC can be extracted from associated changes in sea surface height (SSH, specifically from a broad Atlantic north-south gradient as has been suggested previously in the literature. Since a relation between AMOC and SSH changes can only be used as an AMOC diagnostic if it is valid independently of the specific forcing, we consider three different forcing types: increase of CO2 concentration, freshwater fluxes to the northern convection sites and the modification of Southern Ocean winds. We concentrate on a timescale of 100 yr. We find approximately linear and numerically similar relations between a sea-level difference within the Atlantic and the AMOC for freshwater as well as wind forcing. However, the relation is more complex in response to atmospheric CO2 increase, which precludes this sea-level difference as an AMOC diagnostic under climate change. Finally, we show qualitatively to what extent changes in SSH and AMOC strength that are caused by simultaneous application of different forcings correspond to the sum of the changes due to the individual forcings, a potential prerequisite for more complex SSH-based AMOC diagnostics.

  7. Wind effect on the Atlantic meridional overturning circulation via sea ice and vertical diffusion

    Science.gov (United States)

    Yang, Haijun; Wang, Kun; Dai, Haijin; Wang, Yuxing; Li, Qing

    2016-06-01

    Effects of wind and fresh water on the Atlantic meridional overturning circulation (AMOC) are investigated using a fully coupled climate model. The AMOC can change significantly when perturbed by either wind stress or freshwater flux in the North Atlantic. This study focuses on wind stress effect. Our model results show that the wind forcing is crucial in maintaining the AMOC. Reducing wind forcing over the ocean can cause immediately weakening of the vertical salinity diffusion and convection in the mid-high latitudes Atlantic, resulting in an enhancement of vertical salinity stratification that restrains the deep water formation there, triggering a slowdown of the thermohaline circulation. As the thermohaline circulation weakens, the sea ice expands southward and melts, providing the upper ocean with fresh water that weakens the thermohaline circulation further. The wind perturbation experiments suggest a positive feedback between sea-ice and thermohaline circulation strength, which can eventually result in a complete shutdown of the AMOC. This study also suggests that sea-ice variability may be also important to the natural AMOC variability on decadal and longer timescales.

  8. Expert judgements on the response of the Atlantic meridional overturning circulation to climate change

    International Nuclear Information System (INIS)

    We present results from detailed interviews with 12 leading climate scientists about the possible effects of global climate change on the Atlantic Meridional Overturning Circulation (AMOC). The elicitation sought to examine the range of opinions within the climatic research community about the physical processes that determine the current strength of the AMOC, its future evolution in a changing climate and the consequences of potential AMOC changes. Experts assign different relative importance to physical processes which determine the present-day strength of the AMOC as well as to forcing factors which determine its future evolution under climate change. Many processes and factors deemed important are assessed as poorly known and insufficiently represented in state-of-the-art climate models. All experts anticipate a weakening of the AMOC under scenarios of increase of greenhouse gas concentrations. Two experts expect a permanent collapse of the AMOC as the most likely response under a 4xCO2 scenario. Assuming a global mean temperature increase in the year 2100 of 4 K, eight experts assess the probability of triggering an AMOC collapse as significantly different from zero, three of them as larger than 40%. Elicited consequences of AMOC reduction include strong changes in temperature, precipitation distribution and sea level in the North Atlantic area. It is expected that an appropriately designed research program, with emphasis on long-term observations and coupled climate modeling, would contribute to substantially reduce uncertainty about the future evolution of the AMOC

  9. Detectability of changes to the Atlantic meridional overturning circulation in the Hadley Centre Climate Models

    International Nuclear Information System (INIS)

    The Atlantic meridional overturning circulation (MOC) is responsible for a climatically significant northward heat transport that is expected to decrease in response to anthropogenic global warming. Here, simulations from an ensemble of UK Met Office Hadley Centre Climate Models (HadGEM1, HadGEM2 and a 22 member perturbed physics ensemble of HadCM3-like models) are used to evaluate detection times for different MOC observing strategies. Six different detection statistics are compared, including direct observations of the MOC at two latitudes (26 N and 50 N) and several multivariate detection variables based on an optimal fingerprint of MOC change previously identified using HadCM3 (Vellinga and Wood in Geophys Res Lett 31(14):L14203, 2004). Using these models, and assuming perfectly observed conditions, we find no evidence to suggest that detection times would be significantly reduced by measuring the MOC at 50 N instead of (or in addition to) measurements at 26 N. Our results suggest that complementary observations of hydrographic properties in the North Atlantic may help reduce MOC detection times, but the benefits are not universal across models, nor as large as previously suggested. In addition, detection times calculated using optimal fingerprint methods are sensitive to the model-dependent estimates of covariances describing internal climate variability. This last result presents a strong case for deriving fingerprints of MOC change using dynamical/physical arguments, rather than statistical methods, in order to promote more robust results across a range of models. (orig.)

  10. Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3

    Energy Technology Data Exchange (ETDEWEB)

    Kwon, Young-Oh [Woods Hole Oceanographic Institution, Physical Oceanography Department, Woods Hole, MA (United States); Frankignoul, Claude [Universite Pierre et Marie Curie, LOCEAN/IPSL, Paris (France)

    2012-03-15

    The Atlantic meridional overturning circulation (AMOC) in the last 250 years of the 700-year-long present-day control integration of the Community Climate System Model version 3 with T85 atmospheric resolution exhibits a red noise-like irregular multi-decadal variability with a persistence longer than 10 years, which markedly contrasts with the preceding {proportional_to}300 years of very regular and stronger AMOC variability with {proportional_to}20 year periodicity. The red noise-like multi-decadal AMOC variability is primarily forced by the surface fluxes associated with stochastic changes in the North Atlantic Oscillation (NAO) that intensify and shift northward the deep convection in the Labrador Sea. However, the persistence of the AMOC and the associated oceanic anomalies that are directly forced by the NAO forcing does not exceed about 5 years. The additional persistence originates from anomalous horizontal advection and vertical mixing, which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre. These anomalies are subsequently advected by the mean boundary current into the northern part of the Labrador Sea convection region, reinforcing the density changes directly forced by the NAO. As no evidence was found of a clear two-way coupling with the atmosphere, the multi-decadal AMOC variability in the last 250 years of the integration is an ocean-only response to stochastic NAO forcing with a delayed positive feedback caused by the changes in the horizontal ocean circulation. (orig.)

  11. Variability of the Atlantic meridional overturning circulation in the last millennium and two IPCC scenarios

    Energy Technology Data Exchange (ETDEWEB)

    Ortega, Pablo; Montoya, Marisa; Gonzalez-Rouco, Fidel [Universidad Complutense de Madrid, Ciudad Universitaria, Dpto. Astrofisica y Ciencias de la Atmosfera/Instituto de Geociencias, Facultad de Ciencias Fisicas, Madrid (Spain); Universidad Complutense de Madrid, Ciudad Universitaria, Instituto de Geociencias (UCM-CSIC), Facultad de Ciencias Fisicas, Madrid (Spain); Mignot, Juliette [IPSL/LOCEAN, UPMC/CNRS/IRD/MNHN, Universite Pierre et Marie Curie, Paris Cedex 05 (France); Legutke, Stephanie [Deutsches Klimarechenzentrum (DKRZ), Hamburg (Germany)

    2012-05-15

    The variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Nino-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1 year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs. (orig.)

  12. The impact of multidecadal Atlantic meridional overturning circulation variations on the Southern Ocean

    Science.gov (United States)

    Zhang, Liping; Delworth, Thomas L.; Zeng, Fanrong

    2016-05-01

    The impact of multidecadal variations of the Atlantic meridional overturning circulation (AMOC) on the Southern Ocean (SO) is investigated in the current paper using a coupled ocean-atmosphere model. We find that the AMOC can influence the SO via fast atmosphere teleconnections and subsequent ocean adjustments. A stronger than normal AMOC induces an anomalous warm SST over the North Atlantic, which leads to a warming of the Northern Hemisphere troposphere extending into the tropics. This induces an increased equator-to-pole temperature gradient in the Southern Hemisphere (SH) upper troposphere and lower stratosphere due to an amplified tropical upper tropospheric warming as a result of increased latent heat release. This altered gradients leads to a poleward displacement of the SH westerly jet. The wind change over the SO then cools the SST at high latitudes by anomalous northward Ekman transports. The wind change also weakens the Antarctic bottom water (AABW) cell through changes in surface heat flux forcing. The poleward shifted westerly wind decreases the long term mean easterly winds over the Weddell Sea, thereby reducing the turbulent heat flux loss, decreasing surface density and therefore leading to a weakening of the AABW cell. The weakened AABW cell produces a temperature dipole in the SO, with a warm anomaly in the subsurface and a cold anomaly in the surface that corresponds to an increase of Antarctic sea ice. Opposite conditions occur for a weaker than normal AMOC. Our study here suggests that efforts to attribute the recent observed SO variability to various factors should take into consideration not only local process but also remote forcing from the North Atlantic.

  13. Seasonal to interannual variability in density around the Canary Islands and their influence on the Atlantic meridional overturning circulation at 26°N

    OpenAIRE

    Duchez, Aurelie; Frajka-Williams, Eleanor; Castro, Natalia; Hirschi, Joel; Coward, Andrew

    2014-01-01

    The meridional interior flow obtained from the RAPID array is determined by horizontal density fluctuations at the eastern and western boundary of 26°N. The physical causes of these density variations are responsible for fluctuations in the Atlantic Meridional Overturning Circulation (AMOC) and through it, the meridional heat transport of the Atlantic. In this modelling study, a high resolution ocean model is used to investigate the source and origin of the AMOC variability associated with th...

  14. Effects of Wind and Freshwater on the Atlantic Meridional Overturning Circulation: Role of Sea Ice and Vertical Diffusion

    Science.gov (United States)

    Wang, Kun; Yang, Haijun; Dai, Haijin; Wang, Yuxing; Li, Qing

    2015-04-01

    Effects of wind and fresh water on the Atlantic meridional overturning circulation (AMOC) are investigated in a fully coupled climate model (CESM1.0). The AMOC can change significantly when perturbing either the wind stress or fresh water flux in the northern North Atlantic. This work pays special attention on the wind stress effect. Our model results show that the wind forcing is a crucial element in maintaining the AMOC. When the wind-stress is reduced, the vertical convection and diffusion are weakened immediately, triggering a salt deficit in the northern North Atlantic that prevents the deep water formation there. The salinity advection from the south, however, plays a contrary role to salt the upper ocean. As the AMOC weakens, the sea ice expends southward and melts, freshening the upper ocean that weakens the AMOC further. There is a positive feedback between the sea ice melting and AMOC strength, which eventually determines the AMOC strength in the reduced wind world.

  15. Ocean science: Vagaries of Atlantic overturning

    Science.gov (United States)

    Haine, Thomas W. N.

    2016-07-01

    A weakening of the Atlantic meridional overturning circulation has emerged from noise after years of painstaking measurements. Three independent lines of evidence suggest that an anthropogenic influence on this overturning is not yet detectable.

  16. On the Nordic Seas' role in the Atlantic Meridional Overturning Circulation

    International Nuclear Information System (INIS)

    In the present climate, the North Atlantic thermohaline circulation (THC) plays a fundamental role in the global transport of heat at high latitudes. The response of the North Atlantic Ocean-Nordic Seas THC to surface forcing and basin geometries in an idealized one-hemisphere basin is analyzed to better understand the processes that are fundamental to the modeled circulation. Focusing on the dynamics of the Nordic Seas, analytical and numerical modeling highlight the relevance of a sill (Greenland-Scotland Ridge) in setting the properties of the water masses formed in and exported from a marginal sea. Finally, the influence of the convective activity in the Greenland Sea for the overflow, and thus the overturning, is assessed using hydrographic data (from 1950 to present), a regional ocean model, and a unique tracer release experiment. Greenland Sea Gyre water is estimated to contribute less than 1 Sv, and there is no evidence for causality between changes in the Greenland Sea and the overflow (author)

  17. Further influence of the eastern boundary on the seasonal variability of the Atlantic Meridional Overturning Circulation at 26N

    Science.gov (United States)

    Baehr, Johanna; Schmidt, Christian

    2016-04-01

    The seasonal cycle of the Atlantic Meridional Overturning Circulation (AMOC) at 26.5 N has been shown to arise predominantly from sub-surface density variations at the Eastern boundary. Here, we suggest that these sub-surface density variations have their origin in the seasonal variability of the Canary Current system, in particular the Poleward Undercurrent (PUC). We use a high-resolution ocean model (STORM) for which we show that the seasonal variability resembles observations for both sub-surface density variability and meridional transports. In particular, the STORM model simulation density variations at the eastern boundary show seasonal variations reaching down to well over 1000m, a pattern that most model simulations systematically underestimate. We find that positive wind stress curl anomalies in late summer and already within one degree off the eastern boundary result -through water column stretching- in strong transport anomlies in PUC in fall, coherent down to 1000m depth. Simultaneously with a westward propagation of these transport anomalies, we find in winter a weak PUC between 200 m and 500m, and southward transports between 600m and 1300m. This variability is in agreement with the observationally-based suggestion of a seasonal reversal of the meridional transports at intermediate depths. Our findings extend earlier studies which suggested that the seasonal variability at of the meridional transports across 26N is created by changes in the basin-wide thermocline through wind-driven upwelling at the eastern boundary analyzing wind stress curl anomalies 2 degrees off the eastern boundary. Our results suggest that the investigation of AMOC variability and particular its seasonal cycle modulations require the analysis of boundary wind stress curl and the upper ocean transports within 1 degree off the eastern boundary. These findings also implicate that without high-resolution coverage of the eastern boundary, coarser model simulation might not fully

  18. Hosed vs. unhosed: global response to interruptions of the Atlantic Meridional Overturning, with and without freshwater forcing

    Directory of Open Access Journals (Sweden)

    N. Brown

    2015-10-01

    Full Text Available It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through "hosing" experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we take advantage of a global coupled ocean–atmosphere model that exhibits spontaneous, unhosed oscillations in AMOC strength, in order to examine how the global imprint of AMOC variations depends on whether or not it is the result of external freshwater input. The results imply that, to first order, the ocean–ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. The exception lies in the impact freshwater inputs can have on the strength of other polar haloclines, particularly the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

  19. Glacial climate sensitivity to different states of the Atlantic Meridional Overturning Circulation: results from the IPSL model

    Directory of Open Access Journals (Sweden)

    M. Kageyama

    2009-09-01

    Full Text Available Paleorecords from distant locations on the globe show rapid and large amplitude climate variations during the last glacial period. Here we study the global climatic response to different states of the Atlantic Meridional Overturning Circulation (AMOC as a potential explanation for these climate variations and their possible connections. We analyse three glacial simulations obtained with an atmosphere-ocean coupled general circulation model and characterised by different AMOC strengths (18, 15 and 2 Sv resulting from successive ~0.1 Sv freshwater perturbations in the North Atlantic. These AMOC states suggest the existence of a freshwater threshold for which the AMOC collapses. A weak (18 to 15 Sv AMOC decrease results in a North Atlantic and European cooling. This cooling is not homogeneous, with even a slight warming over the Norwegian Sea. Convection in this area is active in both experiments, but surprisingly stronger in the 15 Sv simulation, which appears to be related to interactions with the atmospheric circulation and sea-ice cover. Far from the North Atlantic, the climatic response is not significant. The climate differences for an AMOC collapse (15 to 2 Sv are much larger and of global extent. The timing of the climate response to this AMOC collapse suggests teleconnection mechanisms. Our analyses focus on the North Atlantic and surrounding regions, the tropical Atlantic and the Indian monsoon region. The North Atlantic cooling associated with the AMOC collapse induces a cyclonic atmospheric circulation anomaly centred over this region, which modulates the eastward advection of cold air over the Eurasian continent. This can explain why the cooling is not as strong over western Europe as over the North Atlantic. In the Tropics, the southward shift of the Inter-Tropical Convergence Zone appears to be strongest over the Atlantic and Eastern Pacific and results from an adjustment of the atmospheric and oceanic heat transports. Finally, the

  20. The influence of glacial ice sheets on Atlantic meridional overturning circulation through atmospheric circulation change under glacial climate

    Science.gov (United States)

    Sherriff-Tadano, Sam; Abe-Ouchi, Ayako; Yoshimori, Masakazu; Oka, Akira; Chan, Wing-Le

    2016-04-01

    Recent coupled modeling studies have shown that the existence of the glacial ice sheets intensifies the Atlantic meridional overturning circulation (AMOC). Since this may play an important role in maintaining a strong AMOC over the last glacial period, which is suggested by recent reconstruction study, it is very important to understand the process by which glacial ice sheets intensify the AMOC. Here, a decoupled simulation is conducted to investigate the effect of wind change due to glacial ice sheets on the AMOC, the crucial region where wind modifies the AMOC and the mechanism, which remained elusive in previous studies. First, from atmospheric general circulation model (AGCM) experiments, the effect of glacial ice sheets on the surface wind is evaluated. Second, from ocean general circulation model (OGCM) experiments, the influence of the wind stress change on the AMOC is evaluated by applying only the changes in the surface wind as a boundary condition, while leaving surface heat and freshwater fluxes unchanged. Moreover, several sensitivity experiments are conducted. Using the AGCM, glacial ice sheets are applied individually. Using the OGCM, changes in the wind are applied regionally or at different magnitudes, ranging from the full glacial to modern levels. These experiments demonstrate that glacial ice sheets intensify the AMOC through an increase in the wind stress curl mainly at the North Atlantic mid-latitudes. This intensification is caused by the increased Ekman upwelling and gyre transport of salt while the change in sea ice transport works as a negative, though minor, feedback.

  1. Coupling of the distribution of silicon isotopes to the meridional overturning circulation of the North Atlantic Ocean

    Science.gov (United States)

    Brzezinski, Mark A.; Jones, Janice L.

    2015-06-01

    The distribution of silicon isotopes within silicic acid, δ30Si(OH)4, was examined along a section in the North Atlantic from the Cape Verde Islands off Africa to Cape Cod, Massachusetts in North America. Surface water displayed elevated δ30Si(OH)4 associated with biological fractionation of Si during silica production. Below 300 m variations in δ30Si(OH)4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis, confirming a tight relationship between δ30Si(OH)4 and the meridional overturning circulation in the Atlantic. A linear relationship between δ30Si(OH)4 and the inverse of silicic acid concentration supported control of Si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the Atlantic. There was a suggestion of a weak local minimum in δ30Si(OH)4 in deep waters above the Trans-Atlantic Geotraverse hydrothermal zone on the mid-Atlantic Ridge consistent with the light δ30Si(OH)4 of hydrothermal waters. The lightest δ30Si(OH)4 values were observed in the deep western and deep eastern basins where Antarctic Bottom Water (AABW) dominated. The heaviest values in subsurface waters occurred in North Atlantic Deep Water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the Arctic Ocean. The concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end-member water masses arise, and how they influence Si isotope distributions. Northern end-member water masses are heavy due to the influence of the Arctic Ocean. Bottom topography prevents light deep waters from entering the Arctic and the further removal of light isotopes through local biological productivity results in extremely heavy δ30Si(OH)4 within the Arctic. Light AABW dominates the southern end member. The Southern Ocean silicic acid trap distills heavier isotopes of Si out of the

  2. An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM

    Directory of Open Access Journals (Sweden)

    P. Lin

    2014-06-01

    Full Text Available In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2. The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 106 m3 s−1. The abrupt signal first occurs at high latitudes (north of 46° N, then shifts gradually to middle latitudes (∼35° N three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.

  3. Revisiting Effect of Ocean Diapycnal Mixing on Atlantic Meridional Overturning Circulation Recovery in a Freshwater Perturbation Simulation

    Institute of Scientific and Technical Information of China (English)

    YU Lei; GAO Yongqi; WANG Huijun; Helge DRANGE

    2008-01-01

    The effects of ocean density vertical stratification and related ocean mixing on the transient response of the Atlantic meridional overturning circulation (AMOC) are examined in a freshwater perturbation simula- tion using the Bergen Climate Model (BCM). The results presented here axe based on the model outputs of a previous freshwater experiment: a 300-year control integration (CTRL), a freshwater integration (FW1) which started after 100 years of running the CTRL with an artificially and continuously threefold increase in the freshwater flux to the Greenland-Iceland-Norwegian (GIN) Seas and the Arctic Ocean throughout the following 150-year simulation. In FW1, the transient response of the AMOC exhibits an initial decreasing of about 6 Sv (1 Sv=106m3 s-1) over the first 50-year integration and followed a gradual recovery during the last 100-year integration. Our results show that the vertical density stratification as the crucial property of the interior ocean plays an important role for the transient responses of AMOC by regulating the convective and diapycnal mixings under the enhanced freshwater input to northern high latitudes in BCM in which the ocean diapyenal mixing is stratification-dependent. The possible mechanism is also investigated in this paper.

  4. High Resolution Model Development to Quantify the Impact of Icebergs on the Stability of the Atlantic Meridional Overturning Circulation

    Energy Technology Data Exchange (ETDEWEB)

    Condron, Alan [Univ. of Massachusetts, Amherst, MA (United States)

    2016-10-18

    In the present-day North Atlantic Ocean, relatively warm and salty water moves northwards from the tropics to the high latitudes, sinks, and returns southward towards the equator as North Atlantic Deep Water, forming the so called Atlantic Meridional Overturning Circulation (AMOC). It has been found that the stability of the AMOC is non-linearly related to the freshwater budget of the North Atlantic. In this way, additional fresh water can be added to the ocean with little impact, until a tipping point is reached that causes the AMOC to suddenly weaken and the Northern Hemisphere to abruptly cool. A great deal of uncertainty still remains over the sensitivity of the AMOC to changes in freshwater discharge as a result of the unrealistic manner in which freshwater has historically been added to climate models. Frequently, freshwater is discharged in ocean models entirely as liquid water, but in reality a large fraction of freshwater entering the ocean is ice calving from marine glaciers (half for Antarctica and two-thirds for Greenland). To more accurately quantify AMOC sensitivity to past and future changes in freshwater input, this project developed a comprehensive iceberg model to more realistically simulate the interaction between the cryosphere and the oceans at high-latitudes. The iceberg model created is written in Fortran90 and designed to scale efficiently on High Performance Computing (HPC) clusters so that tens-of-thousands of icebergs can be simulated at any time. Experiments performed with our model showed that in the Pleistocene there would have been enormous floods of freshwater released into the North Atlantic that would have transported icebergs and meltwater along the entire east coast of the United States, as far south as Florida Keys. In addition, high-resolution, modern-day, model simulations showed that if the Greenland Ice Sheet continues to melt at its current rate then there will be a 6-fold increase in the number of icebergs drifting in the

  5. Climate and vegetation changes around the Atlantic Ocean resulting from changes in the meridional overturning circulation during deglaciation

    Directory of Open Access Journals (Sweden)

    D. Handiani

    2012-07-01

    Full Text Available The Bølling-Allerød (BA, starting ~ 14.5 ka BP is one of the most pronounced abrupt warming periods recorded in ice and pollen proxies. The leading explanation of the cause of this warming is a sudden increase in the rate of deepwater formation in the North Atlantic Ocean and the resulting effect on the heat transport by the Atlantic Meridional Overturning Circulation (AMOC. In this study, we used the University of Victoria (UVic Earth System-Climate Model (ESCM to run simulations, in which a freshwater perturbation initiated a BA-like warming period. We found that under present climate conditions, the AMOC intensified when freshwater was added to the Southern Ocean. However, under Heinrich event 1 (HE1, ~ 16 ka BP climate conditions, the AMOC only intensified when freshwater was extracted from the North Atlantic Ocean, possibly corresponding to an increase in evaporation or a decrease in precipitation in this region. The intensified AMOC led to a warming in the North Atlantic Ocean and a cooling in the South Atlantic Ocean, resembling the bipolar seesaw pattern typical of the last glacial period.

    In addition to the physical response, we also studied the simulated vegetation response around the Atlantic Ocean region. Corresponding with the bipolar seesaw hypothesis, the rainbelt associated with the Intertropical Convergence Zone (ITCZ shifted northward and affected the vegetation pattern in the tropics. The most sensitive vegetation area was found in tropical Africa, where grass cover increased and tree cover decreased under dry climate conditions. An equal but opposite response to the collapse and recovery of the AMOC implied that the change in vegetation cover was transient and robust to an abrupt climate change such as during the BA period, which is also supported by paleovegetation data. The results are in agreement with paleovegetation records from Western tropical Africa, which also show a reduction in forest cover during this

  6. Glacial climate sensitivity to different states of the Atlantic Meridional Overturning Circulation: results from the IPSL model

    Directory of Open Access Journals (Sweden)

    M. Kageyama

    2009-03-01

    Full Text Available Numerous records from the North Atlantic and the surrounding continents have shown rapid and large amplitude climate variability during the last glacial period. This variability has often been associated to changes in the Atlantic Meridional Overturning Circulation (AMOC. Rapid climate change on the same time scales has also been reconstructed for sites far away from the North Atlantic, such as the tropical Atlantic, the East Pacific and Asia. The mechanisms explaining these climatic responses to the state of the AMOC are far from being completely understood, especially in a glacial context. Here we study three glacial simulations characterised by different AMOC strengths: 18, 15 and 2 Sv. With these simulations, we analyse the global climate sensitivity to a weak (18 to 15 Sv and a strong (15 to 2 Sv decrease in the AMOC strength.

    A weak decrease in the AMOC is associated, in our model simulations, to the classical North Atlantic and European cooling, but this cooling is not homogeneous over this region. We investigate the reasons for a lesser cooling (or even slight warming in some cases over the Norwegian Sea and Northwestern Europe. It appears that the convection site in this area is active in both simulations, but that convection is unexpectedly stronger in the 15 Sv simulation. Due to the large variability of the atmosphere, it is difficult to definitely establish what is the origin of this climatic difference, but it appears that the atmospheric circulation anomaly helps sustaining the activity of this convection sites. Far from the North Atlantic, the climatic response is of small amplitude, the only significant change appearing in summer over the tropical Atlantic, where the Inter-Tropical Convergence Zone (ITCZ shifts southward.

    The climate differences between the 15 Sv and 2 Sv simulations are much larger and our analyses focus on three areas: the North Atlantic and surrounding regions, the Tropics and the Indian

  7. Dynamics of the Atlantic meridional overturning circulation and Southern Ocean in an ocean model of intermediate complexity

    Science.gov (United States)

    McCreary, Julian P.; Furue, Ryo; Schloesser, Fabian; Burkhardt, Theodore W.; Nonaka, Masami

    2016-04-01

    A steady-state, variable-density, 2-layer, ocean model (VLOM) is used to investigate basic dynamics of the Atlantic meridional overturning circulation and Southern Ocean. The domain consists of idealized (rectangular) representations of the Atlantic, Southern, and Pacific Oceans. The model equations represent the depth-averaged, layer-1 response (except for one solution in which they represent the depth-integrated flow over both layers). To allow for overturning, water can cross the bottom of layer 1 at the velocity we =wd +wm +wn , the three parts representing: interior diffusion wd that increases the layer-1 thickness h throughout the basin, mixed-layer entrainment wm that ensures h is never less than a minimum value hm , and diapycnal (cooling) processes external to the basin wn that adjust h to hn . For most solutions, horizontal mixing has the form of Rayleigh damping with coefficient ν , which we interpret to result from baroclinic instability through the closure, V∗ = - (ν /f2) ∇P , where ∇P = ∇(1/2 g‧h2) is the depth-integrated pressure gradient, g‧ is the reduced-gravity coefficient, and ν is a mixing coefficient; with this interpretation, the layer-1 flow corresponds to the sum of the Eulerian-mean and eddy-mean (V∗) transport/widths, that is, the "residual" circulation. Finally, layer-1 temperature cools polewards in response to a surface heat flux Q, and the cooling can be strong enough in the Southern Ocean for g‧ = 0 south of a latitude y0 , in which case layer 1 vanishes and the model reduces to a single layer 2. Solutions are obtained both numerically and analytically. The analytic approach splits fields into interior and boundary-layer parts, from which a coupled set of integral constraints can be derived. The set allows properties of the circulation (upwelling-driven transport out of the Southern Ocean M , downwelling transport in the North Atlantic, transport of the Antarctic Circumpolar Current) and stratification (Atlantic

  8. On the Nordic Seas’ role in the Atlantic Meridional Overturning Circulation

    OpenAIRE

    Iovino, Doroteaciro

    2007-01-01

    In the present climate, the North Atlantic thermohaline circulation (THC) plays a fundamental role in the global transport of heat at high latitudes. The response of the North Atlantic Ocean-Nordic Seas THC to surface forcing and basin geometries in an idealized one-hemisphere basin is analyzed to better understand the processes that are fundamental to the modeled circulation. Focusing on the dynamics of the Nordic Seas, analytical and numerical modeling highlight the releva...

  9. Sediment 231Pa/230Th as a recorder of the rate of the Atlantic meridional overturning circulation: insights from a 2-D model

    Directory of Open Access Journals (Sweden)

    S. E. Allen

    2009-11-01

    Full Text Available A two dimensional scavenging-circulation model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.

  10. South Atlantic intermediate water advances into the North-east Atlantic with reduced Atlantic meridional overturning circulation during the last glacial period

    Science.gov (United States)

    Dubois-Dauphin, Quentin; Bonneau, Lucile; Colin, Christophe; Montero-Serrano, Jean-Carlos; Montagna, Paolo; Blamart, Dominique; Hebbeln, Dierk; Van Rooij, David; Pons-Branchu, Edwige; Hemsing, Freya; Wefing, Anne-Marie; Frank, Norbert

    2016-06-01

    The Nd isotopic composition (ɛNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cádiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cádiz over the past 40 ka. ɛNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cádiz reveals relatively constant values (-8.6 to -9.0 and -9.5 to -10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; ɛNd ˜ -9.4) and Mediterranean Sea Water (MSW; ɛNd ˜ -9.9). In contrast, glacial ɛNd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of Cádiz display a higher average value (-8.9 ± 0.4) compared to the present-day value (-11.7 ± 0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic ɛNd values (˜ -8) at 19, 23-24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW ɛNd values remained stable during the last glacial period, it is suggested that these radiogenic ɛNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.

  11. Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

    Directory of Open Access Journals (Sweden)

    P. Heimbach

    2009-11-01

    Full Text Available Daily timeseries of the meridional overturning circulation (MOC estimated from the UK/US RAPID/MOCHA array at 26.5° N in the Atlantic are used to evaluate the MOC as simulated in two global circulation models: (I an 8-member ensemble of the coupled climate model ECHAM5/MPI-OM, and (II the ECCO-GODAE state estimate. In ECHAM5/MPI-OM, we find that the observed and simulated MOC have a similar variability and time-mean within the 99% confidence interval. In ECCO-GODAE, we find that the observed and simulated MOC show a significant correlation within the 99% confidence interval. To investigate the contribution of the different transport components, the MOC is decomposed into Florida Current, Ekman and mid-ocean transports. In both models, the mid-ocean transport is closely approximated by the residual of the MOC minus Florida Current and Ekman transports. As the models conserve volume by definition, future comparisons of the RAPID/MOCHA mid-ocean transport should be done against the residual transport in the models. The similarity in the variance and the correlation between the RAPID/MOCHA, and respectively ECHAM5/MPI-OM and ECCO-GODAE MOC estimates at 26.5° N is encouraging in the context of estimating (natural variability in climate simulations and its use in climate change signal-to-noise detection analyses. Enhanced confidence in simulated hydrographic and transport variability will require longer observational time series.

  12. Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic

    Directory of Open Access Journals (Sweden)

    P. Heimbach

    2009-07-01

    Full Text Available Daily timeseries of the meridional overturning circulation (MOC estimated from the UK/US RAPID/MOCHA array at 26.5° N in the Atlantic are used to evaluate the MOC as simulated in two global circulation models: (i an 8-member ensemble of the coupled climate model ECHAM5/MPI-OM, and (ii the ECCO-GODAE state estimate. In ECHAM5/MPI-OM, we find that the observed and simulated MOC have a similar variability and time-mean within the 99 percent confidence interval. In ECCO-GODAE, we find that the observed and simulated MOC show a significant correlation within the 99 percent confidence interval. To investigate the contribution of the different transport components, the MOC is decomposed into Florida Current, Ekman and upper mid-ocean geostrophic transports. In both models, the mid-ocean transport is closely approximated by the residual of the MOC minus Florida Current and Ekman transports. As the models conserve volume by definition, future comparisons of the RAPID/MOCHA mid-ocean transport should be done against the residual transport in the models. The similarity in the variance and the correlation between the RAPID/MOCHA, and respectively ECHAM5/MPI-OM and ECCO-GODAE MOC estimates at 26.5° N is encouraging in the context of estimating (natural variability in climate simulations and its use in climate change signal-to-noise detection analyses. Enhanced confidence in simulated hydrographic and transport variability will require longer observational time series.

  13. The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5° N

    Directory of Open Access Journals (Sweden)

    M. P. Chidichimo

    2009-11-01

    Full Text Available We study the contribution of eastern-boundary density variations to sub-seasonal and seasonal anomalies of the strength and vertical structure of the Atlantic Meridional Overturning Circulation (AMOC at 26.5° N, by means of the RAPID/MOCHA mooring array between April 2004 and October 2007. The major density anomalies are found in the upper 500 m, and they are often coherent down to 1400 m. The densities have 13-day fluctuations that are apparent down to 3500 m. The two strategies for measuring eastern-boundary density – a tall offshore mooring (EB1 and an array of moorings on the continental slope (EBH – show little correspondence in terms of amplitude, vertical structure, and frequency distribution of the resulting basin-wide integrated transport fluctuations, implying that there are significant transport contributions between EB1 and EBH. Contrary to the original planning, measurements from EB1 cannot serve as backup or replacement for EBH: density needs to be measured directly at the continental slope to compute the full-basin density gradient. Fluctuations in density at EBH generate transport variability of 2 Sv rms in the AMOC, while the overall AMOC variability is 4.9 Sv rms. There is a pronounced deep-reaching seasonal cycle in density at the eastern boundary, which is apparent between 100 m and 1400 m, with maximum positive anomalies in spring and maximum negative anomalies in autumn. These changes drive anomalous southward upper mid-ocean flow in spring, implying maximum reduction of the AMOC, and vice-versa in autumn. The amplitude of the seasonal cycle of the AMOC arising from the eastern-boundary densities is 5.2 Sv peak-to-peak, dominating the 7.0 Sv peak-to-peak seasonal cycle of the total AMOC. Our analysis suggests that the seasonal cycle in density may be forced by the strong near-coastal seasonal cycle in wind stress curl.

  14. Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

    Science.gov (United States)

    Brown, Nicolas; Galbraith, Eric D.

    2016-08-01

    It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through "hosing" experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean-atmosphere model when integrated under a particular background climate state. We compare these unhosed oscillations to hosed oscillations under a range of background climate states in order to examine how the global imprint of AMOC variations depends on whether or not they result from external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with particularly marked variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first-order patterns of response to AMOC interruptions are quite consistent among all simulations, implying that the ocean-sea ice-atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Nonetheless, freshwater addition leads to a more complete shutdown of the AMOC than occurs in the unhosed oscillations

  15. Influence of runoff, high frequency atmospheric forcing and model resolution on deep water mass formation regions and Atlantic Meridional Overturning Circulation, from a numerical model.

    Science.gov (United States)

    Garcia Quintana, Yarisbel; Courtois, Peggy; Hu, Xianmin; Pennelly, Clark; Myers, Paul G.

    2016-04-01

    Water mass formation regions act as windows to the deep ocean where surface waters are transformed to intermediate and deep waters. Within the North Atlantic, Labrador Sea Water (LSW) is convectively produced in the Labrador Sea while in the Nordic Seas the source waters for Denmark Strait Overflow Water (DSOW) and Iceland-Scotland Overflow Water (NEADW) are formed. They are the main components of the North Atlantic Deep Water (NADW) which forms the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). We explore the changes of the LSW formation rates and in AMOC strength as consequence of runoff glacial melt, high frequency atmospheric forcing influence and variations in model's resolution. We use 1/4° resolution Arctic and Northern Hemisphere Atlantic (ANHA4) configuration from the Nucleus for European Modelling of the Ocean (NEMO) model. A nest using ANHA4 and the Adaptive Grid Refinement in FORTRAN (AGRIF) package was used to increase the resolution to 1/12° in the sub-polar gyre. The formation rate is calculated based upon a kinematic subduction approach where the exchange through the dynamic mixed layer base is calculated based on shallowing and deepening in the mixed layer, and convergence of horizontal transport into or out of the mixed layer. Lastly we use a Lagrangian tool (Ariane) to track the path of the DSOW and the NEADW from their formation source.

  16. Abrupt Climate Change and the Atlantic Meridional Overturning Circulation: sensitivity and non-linear response to Arctic/sub-Arctic freshwater pulses. Collaborative research. Final report

    Energy Technology Data Exchange (ETDEWEB)

    Hill, Christopher [MIT (Massachusetts Inst. of Technology), Cambridge, MA (United States)

    2015-06-15

    This project investigated possible mechanisms by which melt-water pulses can induce abrupt change in the Atlantic Meridional Overturning Circulation (AMOC) magnitude. AMOC magnitude is an important ingredient in present day climate. Previous studies have hypothesized abrupt reduction in AMOC magnitude in response to influxes of glacial melt water into the North Atlantic. Notable fresh-water influxes are associated with the terminus of the last ice age. During this period large volumes of melt water accumulated behind retreating ice sheets and subsequently drained rapidly when the ice weakened sufficiently. Rapid draining of glacial lakes into the North Atlantic is a possible origin of a number of paleo-record abrupt climate shifts. These include the Younger-Dryas cooling event and the 8,200 year cooling event. The studies undertaken focused on whether the mechanistic sequence by which glacial melt-water impacts AMOC, which then impacts Northern Hemisphere global mean surface temperature, is dynamically plausible. The work has implications for better understanding past climate stability. The work also has relevance for today’s environment, in which high-latitude ice melting in Greenland appears to be driving fresh water outflows at an accelerating pace.

  17. Pulling the Meridional Overturning Circulation From the South DESC0005100

    Energy Technology Data Exchange (ETDEWEB)

    Cessi, Paola [Univ. of California, San Diego, CA (United States); Wolfe, Christopher L. [Scripps Inst. of Oceanography, San Diego, CA (United States)

    2015-11-25

    This project concerned the Atlantic Meridional Overturning Circulation (AMOC), its stability, variability and sensitivity to atmospheric forcing, both mechanical (wind-stress) and thermodynamical (heat and freshwater surface fluxes). The focus of the study is the interhemispheric cell in the largely adiabatic regime, where the flow is characterized by a descending branch in the high latitudes of the North Atlantic and the upwelling branch in the Antarctic Circumpolar Current (ACC) region of the Southern Ocean. These two end points are connected by shared isopycnals along which the flow takes place. The approach is to systematically study the amplitude and frequency of the AMOC’s response to localized buoyancy with an ocean-only model in both coarse and high-resolution configurations, analyzed with innovative diagnostics, focused on the “residual overturning circulation” (ROC), which is the proper measure of the transport of heat and other tracers.

  18. The impact of horizontal resolution of density field on the calculation of the Atlantic meridional overturning circulation at 34°S

    Science.gov (United States)

    Stepanov, Vladimir N.; Iovino, Doroteaciro; Masina, Simona; Storto, Andrea; Cipollone, Andrea

    2016-06-01

    The Atlantic meridional overturning circulation and meridional heat transport (hereafter the AMOC and MHT) at 34°S as simulated by global 1/16° eddy-rich (henceforth GLOB16) and ¼° eddy-permitting (GLOB4) models are compared with observational estimates. Three different methods are used for calculating the modeled AMOC: the first method (MOCmod) is based on simulated velocity fields, while the second (MOCob) relies on the same assumptions as available observed-derived estimates. The third method (MOCob2) is also based on hydrostatic and geostrophic relationships, but relative to a barotropic circulation instead of the definition of velocity at a specific reference depth. All methods correctly reproduce the time-mean GLOB16 AMOC strength, but the value of the non-Ekman component of the GLOB16 AMOC is only about 75% of the observed-derived estimate. The GLOB16 MHT is also significantly less than observation value (slightly more than 60% of the observed). However, the mean AMOC and MHT values at 34°S obtained with coarser resolution GLOB4 model are comparable with the observed-derived estimates. Possible causes for the differences between the eddy-rich model and observational data are studied. It is shown that the density field from the eddy-rich model has high temporal variability along 34°S with spatial scale of about two hundred km that can be due to mesoscale variations, caused by Agulhas "leakage." This results in the decrease of the mean meridional geostrophic velocity, which leads to smaller values of the AMOC and MHT in GLOB16: subsampling GLOB16 density on ¼° or ½° longitude grid along 34°S for MOCob calculation significantly increases the AMOC values. The findings in this paper provide guidance in understanding AMOC and MHT dissimilarities between observation-based estimates and eddy-rich ocean models at 34°S.

  19. Simulation of the Atlantic meridional overturning circulation in an atmosphere-ocean global coupled model. Part II: weakening in a climate change experiment: a feedback mechanism

    Energy Technology Data Exchange (ETDEWEB)

    Guemas, Virginie [Meteo-France, CNRS, Centre National de Recherches Meteorologiques/Groupe d' Etude de l' Atmosphere Meteorologique (CNRM/GMGEC), Toulouse Cedex (France); CEA-CNRS-UVSQ, Laboratoire des Sciences du Climat et de l' Environnement, UMR 1572, Gif-sur-Yvette (France); Salas-Melia, David [Meteo-France, CNRS, Centre National de Recherches Meteorologiques/Groupe d' Etude de l' Atmosphere Meteorologique (CNRM/GMGEC), Toulouse Cedex (France)

    2008-06-15

    Most state-of-the art global coupled models simulate a weakening of the Atlantic meridional overturning circulation (MOC) in climate change scenarios but the mechanisms leading to this weakening are still being debated. The third version of the CNRM (Centre National de Recherches Meteorologiques) global atmosphere-ocean-sea ice coupled model (CNRM-CM3) was used to conduct climate change experiments for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). The analysis of the A1B scenario experiment shows that global warming leads to a slowdown of North Atlantic deep ocean convection and thermohaline circulation south of Iceland. This slowdown is triggered by a freshening of the Arctic Ocean and an increase in freshwater outflow through Fram Strait. Sea ice melting in the Barents Sea induces a local amplification of the surface warming, which enhances the cyclonic atmospheric circulation around Spitzberg. This anti-clockwise circulation forces an increase in Fram Strait outflow and a simultaneous increase in ocean transport of warm waters toward the Barents Sea, favouring further sea ice melting and surface warming in the Barents Sea. Additionally, the retreat of sea ice allows more deep water formation north of Iceland and the thermohaline circulation strengthens there. The transport of warm and saline waters toward the Barents Sea is further enhanced, which constitutes a second positive feedback. (orig.)

  20. Millennial-Scale Subsurface Temperature Change in the Niger Delta Linked to Atlantic Meridional Overturning Circulation Variability During the Last Deglacial

    Science.gov (United States)

    Schmidt, M. W.; Parker, A. O.; Chang, P.

    2015-12-01

    Understanding how Atlantic Meridional Overturning Circulation (AMOC) variability influenced the abrupt climatic changes that characterized the last deglacial has become a fundamental topic in Paleoceanography. Both modern observations and modeling simulations suggest that subsurface temperatures between 300-600 m across the Tropical North Atlantic warm when AMOC weakens, providing a possible fingerprint for reconstructing past AMOC variability (Schmidt et al., 2012). Furthermore, this AMOC-induced subsurface warming has the potential to impact the West African Monsoon, possibly providing an important high-latitude / tropical climate teleconnection (Chang et al., 2008). Here, we present a new high-resolution Mg/Ca record of subsurface (~300 m water depth) temperature variability across the last deglacial from sediment core Fan 17 (4.81oN, 4.41oE, 1178 m depth) recovered from the Niger Delta based on the deep-dwelling planktonic foraminifera Globorotalia crassaformis. Our G. crassaformis Mg/Ca record suggests an increase in subsurface temperatures up to 4oC from 17.2 to 14.8 kyr, consistent with previously published proxy data suggesting AMOC was in a reduced state during Heinrich Event 1 and the subsequent Mystery Interval (McManus et al., 2004). The G. crassaformis Mg/Ca record then shows a cooling associated with the onset of the Bølling Allerød interstadial, followed by a second warming at the start of the Younger Dryas. These millennial-scale warming events are nearly synchronous with a similar record of subsurface temperature change from the Bonaire Basin in the Southern Caribbean (Schmidt et al., 2012) and may provide important evidence for the rapid adjustment of tropical Atlantic subsurface temperatures in response to a weakened AMOC state.

  1. Effects of reduced vertical mixing under sea ice on Atlantic meridional overturning circulation (AMOC) in a global ice-ocean model

    Science.gov (United States)

    Kim, Sang Yeob; Lee, Ho Jin; Park, Jae-Hun; Kim, Young Ho

    2015-06-01

    Most open ocean and climate models assume a constant background mixing; however, vertical mixing should be reduced under the sea ice in polar region because the sea-ice cover acts as an insulator against the momentum transfer between the atmosphere and ocean. Using a global Ocean General Circulation Model (OGCM), we show that the Atlantic meridional overturning circulation (AMOC) can be substantially affected by reduced vertical mixing under the sea ice. When the background diffusivity under the sea ice is reduced by 1 order less than that in ice-free regions, the volume transport of the AMOC in the upper 3000 m is increased by up to 14% accordingly. The numerical experiment suggests that the reduced background diffusivity makes waters denser in the Arctic Ocean and the denser water is transported into the Nordic Seas to push up the isopycnal surfaces over the Greenland- Iceland-Scotland Ridge. Consequently, the AMOC is enhanced by overflows of the denser water crossing the Denmark Strait.

  2. The North Atlantic anthropogenic carbon conveyor: temporal variability of within-ocean transports and their sensitivity to the meridional overturning circulation

    Science.gov (United States)

    Brown, Peter; McDonagh, Elaine; Sanders, Richard; King, Brian; Smeed, David; Watson, Andrew; Schuster, Ute; Baringer, Molly; Wanninkhof, Rik; Meinen, Chris

    2016-04-01

    The North Atlantic plays a critical role in the global carbon cycle both as a region of substantial air-sea carbon dioxide uptake and as a location for the transfer of CO2 to depth on climatically-important timescales. However, while surface flux variability is relatively well constrained, our understanding of the changing deep carbon distribution is restricted to sub-decadal repeat hydrographic sections, and for anthropogenic carbon (Canth), integrated multi-decadal basin-scale estimates. Here, we present the first observation-derived high-resolution estimate of short-term meridional carbon transport variability and long-term trends across the subtropical North Atlantic. Historical hydrographic data-based estimates of Canth are used to generate predictive regressions that, combined with RAPID mooring and ARGO float-derived transport estimates, create a 10-day frequency interior ocean carbon flux time-series for 2004-2012. The mean net Canth transport across this timeframe is found to be relatively independent of calculation method and robust at 0.18 PgC yr-1 northwards, with poleward advection of high Canthshallow waters outweighing the predominantly southwards transports of low concentrations at depth. Substantial seasonal, sub-annual and interannual transport variability is observed that is highly sensitive to the strength of the overturning circulation. While the recently identified multi-year decrease in MOC strength similarly impacts Canth transports, its full effect is masked by the northwards transport of increasing surface Canth levels. A comparison with historical estimates of the regional carbon sink reveals an intrinsic relationship between air-sea uptake, ocean transport and heat fluxes, which will become more important as the ocean responds to a changing global climate.

  3. The global Meridional Overturning Circulation's response to variable buoyancy forcing

    OpenAIRE

    Butler, Edward D.

    2015-01-01

    The meridional overturning circulation (MOC) is a large-scale circulation throughout the global ocean and plays a significant role in the complex global climate system. However, our traditional understanding of the processes driving the MOC has been questioned in recent years. In particular, it has been suggested that surface buoyancy forcing plays little energetic role in driving the MOC. Furthermore, doubt has also been cast over the relationship between meridional overturning and meridiona...

  4. 20th Century variability of Atlantic Meridional overturning circulation: Planetary wave influences on world ocean surface phosphate utilization and synchrony of small pelagic fisheries

    Science.gov (United States)

    Kamykowski, Daniel

    The Atlantic Meridional Overturning Circulation (AMOC), part of the global Thermohaline Circulation (THC), is variable. In the present analysis, an Atlantic Dipole Phosphate Utilization (ADPU) index, related to the existing Atlantic Dipole Sea Surface Temperature Anomaly (ADSA) index, is used to represent 20th century changes in AMOC strength that are applied to global ecosystem variability. ADPU index cycles set the timing for the calculation of six 2° latitude-longitude resolution world ocean maps depicting higher surface phosphate utilization (SPU) in some regions when AMOC is weaker and in other regions when AMOC is stronger. The average of these six maps yields a summary map with a pattern of alternating latitudinal SPU regions differentiated by AMOC strength that exhibits relationships with ocean bathymetry and wind-driven currents through a consideration of the THC deep and shallow limbs. The latitudinal pattern of SPU regions exhibits conceptual associations with sardine (S) and anchovy (A) population ranges off Japan (J), California (C), Peru (P) and South Africa (B). These sardine and anchovy populations have exhibited apparently synchronous fluctuations on decadal scales through at least part of the 20th century that is summarized in a Regime Indicator Series (RIS=(JS+CS+PS+BA)-(JA+CA+PA+BS)) index. In the present analysis based on Food & Agriculture Organization (FAO) catch data, a revised Regime Indicator Series index formulation (RIS3=(JS+CA+PS+BA)-(JA+CS+PA+BS)), in which CS and CA catches reverse positions, is defined. AMOC variability represented in ADPU is significantly correlated with the RIS3 index (no lag but a significant range of 14 years) and four of eight small pelagic fisheries (JS, PS, BA, and JA). The post-1950 RIS3 index is significantly correlated with seven of eight small pelagic fisheries but not CS. When the regional small pelagic fisheries are considered as normalized species differences (S-A), ADPU has significant positive

  5. Simulated Atlantic Meridional Overturning Circulation in the 20th century with an ocean model forced by reanalysis-based atmospheric data sets

    Science.gov (United States)

    He, Yan-Chun; Drange, Helge; Gao, Yongqi; Bentsen, Mats

    2016-04-01

    Global ocean hindcast simulations for the period 1871-2009 have been run with the ocean-sea ice component of the Norwegian Earth System Model (NorESM-O), forced by an adjusted version of the Twentieth Century Reanalysis version 2 data set (20CRv2 data set), as well as by the commonly used second version of atmospheric forcing data set for the Coordinated Ocean-ice Reference Experiments phase-II (CORE-II) for the period 1948-2007 (hereafter CORE.v2 data set). The simulated Atlantic Meridional Overturning Circulation (AMOC) in the 20CR and the CORE simulations have comparable variability as well as mean strength during the last three decades of the integration. The simulated AMOC undergoes, however, distinctly different evolutions during the period 1948-1970, with a sharply declining strength in CORE but a gradual increase in 20CR. Sensitivity experiments suggest that differences in the wind forcing between CORE and 20CR have major impact on the simulated AMOCs during this period. It is furthermore found that differences in the air temperature between the two data sets do contribute to the differences in AMOC, but to a much lesser degree than the wind. An additional factor for the diverging AMOC in the two decades following 1948 is the inevitable switching of atmospheric forcing fields in 1948 in the CORE.v2-based runs due to the cyclic spin-up procedure of the ocean model. The latter is a fundamental issue for any ocean hindcast simulation. The ocean initial state mainly influence the actual value but to a lesser degree also the temporal evolution (variability) of AMOC. It may take about two decades for the AMOC to adjust to a new atmospheric state during the spin-up, although a dynamically balanced ocean initial state tends to reduce the adjustment time and the magnitude of the deviation, implying that an ocean model run with atmospheric forcing fields extending back in time, like 20CRv2, can be used to extend the reliable duration of CORE-type of simulations.

  6. Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

    NARCIS (Netherlands)

    den Toom, M.; Dijkstra, H.A.

    2011-01-01

    An important expression of the non-linear character of the ocean’s meridional overturning circulation (MOC) is the scaling of its amplitude with the magnitude of the vertical mixing coefficient (diffusivity) of heat and salt. This paper extends recent work that indicated that the Atlantic and Pacifi

  7. Enhancement of the southward return flow of the Atlantic Meridional Overturning Circulation by data assimilation and its influence in an assimilative ocean simulation forced by CORE-II atmospheric forcing

    Science.gov (United States)

    Fujii, Yosuke; Tsujino, Hiroyuki; Toyoda, Takahiro; Nakano, Hideyuki

    2015-08-01

    This paper examines the difference in the Atlantic Meridional Overturning Circulation (AMOC) mean state between free and assimilative simulations of a common ocean model using a common interannual atmospheric forcing. In the assimilative simulation, the reproduction of cold cores in the Nordic Seas, which is absent in the free simulation, enhances the overflow to the North Atlantic and improves AMOC with enhanced transport of the deeper part of the southward return flow. This improvement also induces an enhanced supply of North Atlantic Deep Water (NADW) and causes better representation of the Atlantic deep layer despite the fact that correction by the data assimilation is applied only to temperature and salinity above a depth of 1750 m. It also affects Circumpolar Deep Water in the Southern Ocean. Although the earliest influence of the improvement propagated by coastal waves reaches the Southern Ocean in 10-15 years, substantial influence associated with the arrival of the renewed NADW propagates across the Atlantic Basin in several decades. Although the result demonstrates that data assimilation is able to improve the deep ocean state even if there is no data there, it also indicates that long-term integration is required to reproduce variability in the deep ocean originating from variations in the upper ocean. This study thus provides insights on the reliability of AMOC and the ocean state in the Atlantic deep layer reproduced by data assimilation systems.

  8. Wind Stress Increases Glacial Atlantic Overturning

    Science.gov (United States)

    Muglia, J.; Schmittner, A.

    2015-12-01

    Previous Paleoclimate Model Intercomparison Project (PMIP) simulations of the Last Glacial Maximum (LGM) Atlantic Meridional Overturning Circulation (AMOC) showed ambiguous results on transports and structure. Here we analyze the most recent PMIP3 models, which show a consistent increase (on average by 41%) and deepening (580 m) of the AMOC for all models with respect to pre-industrial control (PIC) simulations (see Figure), in contrast to some reconstructions. Changes in wind stress alone lead to similar AMOC responses in a climate-ocean circulation model, suggesting that atmospheric circulation changes in the North Atlantic due to the presence of ice sheets are an important control in the PMIP3 models' LGM response. These results improve our understanding of the LGM AMOC's driving forces and are relevant for the evaluation of models that are used in the IPCC's Assessment Reports for future climate projections, as well as for the currently ongoing design of the next round of PMIP.

  9. The role of the northward-directed (subsurface limb of the Atlantic Meridional Overturning Circulation during the 8.2 ka Event

    Directory of Open Access Journals (Sweden)

    A. D. Tegzes

    2014-02-01

    Full Text Available The so-called "8.2 ka Event" has been widely regarded as a major climate perturbation over the Holocene. It is most readily identifiable in the oxygen-isotope records from Greenland ice cores as an approximately 160 yr-long cold interval between 8250–8090 yr BP. The prevailing view has been that the cooling over Greenland, and potentially over the northern North Atlantic at least, was triggered by the catastrophic final drainage of the Agassiz-Ojibway proglacial lake as part of the remnant Laurentide Ice Sheet collapsed over Hudson Bay at around 8420 ± 80 yr BP. The consequent freshening of surface waters in the northern North Atlantic Ocean and the Nordic Seas resulted in weaker overturning, hence reduced northward heat transport. Here we present proxy records from site JM97-MD95-2011 on the mid-Norwegian Margin indicating a (sharp decline in the strength of the eastern branch of the Atlantic Inflow into the Nordic Seas immediately following a uniquely large drop in (subsurface ocean temperatures coeval with the lake outbursts. We propose that the final drainage of Lake Agassiz-Ojibway was accompanied by a major iceberg discharge from Hudson Bay, which resulted in the cooling of the northward-directed northern Gulf Stream-North Atlantic Drift-Norwegian Atlantic Current system. Since our current-strength proxy records from the mid-Norwegian Margin do not evidence an exceptionally strong reduction in the main branch of the Atlantic Inflow into the Nordic Seas at the time, we argue that a chilled northward-directed (subsurface-current system and an already colder background climate state could be the main factors responsible for the 8.2 ka climate perturbation.

  10. Simulation of the Atlantic meridional overturning circulation in an atmosphere-ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment

    Energy Technology Data Exchange (ETDEWEB)

    Guemas, Virginie [Meteo-France, CNRS, Centre National de Recherches Meteorologiques/Groupe d' Etude de l' Atmosphere Meteorologique, Toulouse Cedex (France); UMR 1572, CEA-CNRS-UVSQ, Laboratoire des Sciences du Climat et de l' Environnement, Gif-sur-Yvette (France); Salas-Melia, David [Meteo-France, CNRS, Centre National de Recherches Meteorologiques/Groupe d' Etude de l' Atmosphere Meteorologique, Toulouse Cedex (France)

    2008-07-15

    A preindustrial climate experiment was conducted with the third version of the CNRM global atmosphere-ocean-sea ice coupled model (CNRM-CM3) for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). This experiment is used to investigate the main physical processes involved in the variability of the North Atlantic ocean convection and the induced variability of the Atlantic meridional overturning circulation (MOC). Three ocean convection sites are simulated, in the Labrador, Irminger and Greenland-Iceland-Norwegian (GIN) Seas in agreement with observations. A mechanism linking the variability of the Arctic sea ice cover and convection in the GIN Seas is highlighted. Contrary to previous suggested mechanisms, in CNRM-CM3 the latter is not modulated by the variability of freshwater export through Fram Strait. Instead, the variability of convection is mainly driven by the variability of the sea ice edge position in the Greenland Sea. In this area, the surface freshwater balance is dominated by the freshwater input due to the melting of sea ice. The ice edge position is modulated either by northwestward geostrophic current anomalies or by an intensification of northerly winds. In the model, stronger than average northerly winds force simultaneous intense convective events in the Irminger and GIN Seas. Convection interacts with the thermohaline circulation on timescales of 5-10 years, which translates into MOC anomalies propagating southward from the convection sites. (orig.)

  11. Recent increases in Arctic freshwater flux affects Labrador Sea convection and Atlantic overturning circulation

    NARCIS (Netherlands)

    Yang, Qian; Dixon, Timothy H.; Myers, Paul G.; Bonin, Jennifer; Chambers, Don; Van Den Broeke, M. R.

    2016-01-01

    The Atlantic Meridional Overturning Circulation (AMOC) is an important component of ocean thermohaline circulation. Melting of Greenland's ice sheet is freshening the North Atlantic; however, whether the augmented freshwater flux is disrupting the AMOC is unclear. Dense Labrador Sea Water (LSW), for

  12. Effects of Southern Hemisphere Wind Changes on the Meridional Overturning Circulation in Ocean Models.

    Science.gov (United States)

    Gent, Peter R

    2016-01-01

    Observations show that the Southern Hemisphere zonal wind stress maximum has increased significantly over the past 30 years. Eddy-resolving ocean models show that the resulting increase in the Southern Ocean mean flow meridional overturning circulation (MOC) is partially compensated by an increase in the eddy MOC. This effect can be reproduced in the non-eddy-resolving ocean component of a climate model, providing the eddy parameterization coefficient is variable and not a constant. If the coefficient is a constant, then the Southern Ocean mean MOC change is balanced by an unrealistically large change in the Atlantic Ocean MOC. Southern Ocean eddy compensation means that Southern Hemisphere winds cannot be the dominant mechanism driving midlatitude North Atlantic MOC variability.

  13. Mechanisms Determining the Winter Atmospheric Response to the Atlantic Overturning Circulation

    OpenAIRE

    Gastineau, Guillaume; L’Hévéder, B.; Codron, Francis; Frankignoul, Claude

    2016-01-01

    International audience In climate models, an intensification of the Atlantic meridional overturning circulation (AMOC) precedes a warming in the North Atlantic subpolar basin by a few years. In the IPSL-CM5A-LR model, this warming may explain the atmospheric response to the AMOC observed in winter, which resembles a negative phase of the North Atlantic Oscillation (NAO). To firmly establish the causality links between the ocean and the atmosphere and illustrate the underlying mechanisms in...

  14. Dynamical Attribution of Recent Variability in Atlantic Overturning

    Science.gov (United States)

    Pillar, Helen; Heimbach, Patrick; Johnson, Helen; Marshall, David

    2016-04-01

    Attributing observed variability of the Atlantic Meridional Overturning Circulation (AMOC) to past changes in surface forcing is challenging but essential for detecting any influence of anthropogenic forcing and reducing uncertainty in future climate predictions. Here we obtain quantitative estimates of wind and buoyancy-driven AMOC variations at 25N by projecting observed atmospheric anomalies onto model-based dynamical patterns of AMOC sensitivity to surface wind, thermal and freshwater forcing over the preceding 15 years. We show that local wind forcing dominates AMOC variability on short timescales, whereas subpolar heat fluxes dominate on decadal timescales. The reconstructed transport time series successfully reproduces most of the interannual variability observed by the RAPID-MOCHA array. However, the apparent decadal trend in the RAPID-MOCHA time series is not captured, requiring improved model representation of ocean adjustment to subpolar heat fluxes over at least the past two decades, and highlighting the importance of sustained monitoring of the high latitude North Atlantic.

  15. Hydrographic biases in global coupled climate models and their relation to the meridional overturning circulation

    Science.gov (United States)

    Plancherel, Yves

    2015-01-01

    Comparison of the volumetric θ/S distribution of models participating in the Climate Model Intercomparison Project 3 (CMIP3) indicates that these models differ widely in their ability to represent the thermohaline properties of water masses. Relationships between features of the quasi-equilibrium hydrographic mean state of these models and aspects of their overturning circulations are investigated. This is achieved quantitatively with the help of seven diagnostic hydrographic stations. These few stations were specifically selected to provide a minimalist schematic of the global water mass system. Relationships between hydrographic conditions in the North Atlantic measured with a subset of these stations suggest that hydrographic properties in the subpolar North Atlantic are set by the circulation field of each model, pointing towards deficiencies in the models ability to resolve the Gulf Stream-North Atlantic Current system as a major limitation. Since diapycnal mixing and viscosity parameterizations differ across CMIP3 models and exert a strong control on the overturning, it is likely that these architectural differences ultimately explain the main across-model differences in overturning circulation, temperature and salinity in the North Atlantic. The analysis of properties across the quasi-equilibrium states of the CMIP3 models agrees with previously reported relationships between meridional steric height gradients or horizontal density contrasts at depth and the strength of the deep water cell. Robust relationships are also found in the Southern Ocean linking measures of vertical stratification with the strength of the abyssal circulations across the CMIP3 models. Consistent correlations between aspects of the quasi-equilibrium hydrography in the Southern Ocean and the sensitivity of the abyssal cell to increasing radiative forcing by 2100 were found. Using these relations in conjunction with modern hydrographic observations to interpolate the fate of the

  16. "What Controls the Structure and Stability of the Ocean Meridional Overturning Circulation: Implications for Abrupt Climate Change?"

    Energy Technology Data Exchange (ETDEWEB)

    Fedorov, Alexey [Yale University

    2013-11-23

    The central goal of this research project is to understand the properties of the ocean meridional overturning circulation (MOC) – a topic critical for understanding climate variability and stability on a variety of timescales (from decadal to centennial and longer). Specifically, we have explored various factors that control the MOC stability and decadal variability in the Atlantic and the ocean thermal structure in general, including the possibility abrupt climate change. We have also continued efforts on improving the performance of coupled ocean-atmosphere GCMs.

  17. The Shallow Meridional Overturning Circulation in the Northern Indian Ocean and Its Interannual Variability

    Institute of Scientific and Technical Information of China (English)

    HU Ruijin; LIU Qinyu; WANG Qi; J. Stuart GODFREY; MENG Xiangfeng

    2005-01-01

    The shallow meridional overturning circulation (upper 1000 m) in the northern Indian Ocean and its interannual variability are studied, based on a global ocean circulation model (MOM2) with an integration of 10 years (1987-1996). It is shown that the shallow meridional overturning circulation has a prominent seasonal reversal characteristic. In winter, the flow is northward in the upper layer and returns southward at great depth. In summer, the deep northward inflow upwells north of the equator and returns southward in the Ekman layer. In the annual mean, the northward inflow returns through two branches: one is a southward flow in the Ekman layer, the other is a flow that sinks near 10°N and returns southward between 500 m and 1000 m. There is significant interannual variability in the shallow meridional overturning circulation, with a stronger (weaker) one in 1989 (1991) and with a period of about four years. The interannual variability of the shallow meridional overturning circulation is intimately related to that of the surface wind stress. Several indices are proposed to describe the anomaly of this circulation associated with the cross-equatorial part.

  18. Revisiting annual mean and seasonal cycle of deep meridional overturning circulation of the Indian Ocean

    Science.gov (United States)

    Wang, Weiqiang; Xie, Qiang; Li, Sha; Zhu, Xiuhua

    2014-05-01

    The annual mean and seasonal cycle of the deep meridional overturning circulation (MOC) of the Indian Ocean is being revisited here using GECCO synthesis. Resulting from ocean general circulation models, the annual mean deep MOC of the Indian Ocean are generally weak with inflow in the bottom layer and outflow in the intermediate and upper layer mixing with strong Indonesian Throughflow. For seasonal cycle of deep MOC, two significant and seasonal reversed counter-rotating deep cells over full depth of water column, roughly separated by 20S, are revealed during boreal summer and winter. The coincidences of the latitude 20S with where the maximum climatological wind curl for most of seasons reveals intimate relations between the deep meridional overturning and surface winds. Dynamical decompositions on annual mean and complete seasonal cycle of the meridional overturning show varying relative contribution of each dynamical component at different time scale. For annual mean deep MOC, Ekman dynamics is found to be dominant in the region of north of 25S, particularly in upper 3000m, whereas south of 25S external and vertical shear components show remarkable "seamount" features and are compensated with much larger strengths because of topo-modulated strong western boundary topography. At seasonal time scale, dominant role of Ekman dynamics and secondary role of external mode are found in the deep cell north of 20S in January and July. However in transition seasons, vertical shear is responsible for major part of meridional overturning and Ekman dynamics has comparable contribution north of Equator.

  19. Response of the meridional overturning circulation to variable buoyancy forcing in a double hemisphere basin

    Energy Technology Data Exchange (ETDEWEB)

    Lucas, Marc A. [National Oceanography Centre, University of Southampton, School of Ocean and Earth Science, Southampton (United Kingdom); Collecte Localisation Satellite, Ramonville Saint Agne (France); Hirschi, J.J.M. [National Oceanography Centre, University of Southampton, School of Ocean and Earth Science, Southampton (United Kingdom); Marotzke, J. [Max-Planck-Institut fuer Meteorologie, Hamburg (Germany)

    2010-04-15

    We consider how a highly idealized double-hemisphere basin responds to a zonally constant restoring surface temperature profile that oscillates in time, with periods ranging from 0.5 to 32,000 years. In both hemispheres, the forcing is similar but can be either in phase or out of phase. The set-up is such that the Northern Hemisphere always produces the densest waters. The model's meridional overturning circulation (MOC) exhibits a strong response in both hemispheres on decadal to multi-millennial timescales. The amplitude of the oscillations reaches up to 140% of the steady-state maximum MOC and exhibits resonance-like behaviour, with a maximum at centennial to millennial forcing periods. When the forcing is in phase between the Northern and Southern Hemispheres, there is a marked decrease in the amplitude of the MOC response as the forcing period is increased beyond the resonance period. In this case the resonance-like behaviour is identical to the one we found earlier in a single-hemisphere model and occurs for the same reasons. When the forcing is out of phase between the Northern and Southern Hemispheres, the amplitude of the MOC response is substantially greater for long forcing periods (millennial and longer), particularly in the Southern Hemisphere. This increased MOC amplitude occurs because for an out of phase forcing, either the northern or the southern deep water source is always active, leading to generally colder bottom waters and thus greater stratification in the opposite hemisphere. This increased stratification in turn stabilises the water column and thus reduces the strength of the weaker overturning cell. The interaction of the two hemispheres leads to response timescales of the deep ocean at half the forcing period. Our results suggest a possible explanation for the half-precessional time scale observed in the deep Atlantic Ocean palaeo-temperature record. (orig.)

  20. Inferring the zonal distribution of measured changes in the meridional overturning circulation

    Directory of Open Access Journals (Sweden)

    A. de Boer

    2006-10-01

    Full Text Available Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here, we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics, is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region, immediately outside of the Florida Straits.

  1. Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation.

    Science.gov (United States)

    Kienast, Markus; Kienast, Stephanie S; Calvert, Stephen E; Eglinton, Timothy I; Mollenhauer, Gesine; François, Roger; Mix, Alan C

    2006-10-19

    Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean-atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of approximately 1 degrees C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of approximately 0.5 degrees C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans' thermohaline circulation, although possibly amplified by tropical ocean-atmosphere interaction as suggested before. PMID:17051216

  2. Recent slowing of Atlantic overturning circulation as a recovery from earlier strengthening

    Science.gov (United States)

    Jackson, Laura C.; Peterson, K. Andrew; Roberts, Chris D.; Wood, Richard A.

    2016-07-01

    The Atlantic meridional overturning circulation (AMOC) has weakened substantially over the past decade. Some weakening may already have occurred over the past century, and global climate models project further weakening in response to anthropogenic climate change. Such a weakening could have significant impacts on the surface climate. However, ocean model simulations based on historical conditions have often found an increase in overturning up to the mid-1990s, followed by a decrease. It is therefore not clear whether the observed weakening over the past decade is part of decadal variability or a persistent weakening. Here we examine a state-of-the-art global-ocean reanalysis product, GloSea5, which covers the years 1989 to 2015 and closely matches observations of the AMOC at 26.5° N, capturing the interannual variability and decadal trend with unprecedented accuracy. The reanalysis data place the ten years of observations--April 2004 to February 2014--into a longer-term context and suggest that the observed decrease in the overturning circulation is consistent with a recovery following a previous increase. We find that density anomalies that propagate southwards from the Labrador Sea are the most likely cause of these variations. We conclude that decadal variability probably played a key role in the decline of the AMOC observed over the past decade.

  3. Low-frequency variability of the shallow meridional overturning circulation in the South China Sea

    Institute of Scientific and Technical Information of China (English)

    YANG Zhitong; LUO Yiyong

    2016-01-01

    The low-frequency variability of the shallow meridional overturning circulation (MOC) in the South China Sea (SCS) is investigated using a Simple Ocean Data Assimilation (SODA) product for the period of 1900-2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells: a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport (LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current (NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon.

  4. Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy

    NARCIS (Netherlands)

    Weijer, W.; Ruijter, W.P.M. de; Sterl, A.; Drijfhout, S.

    2002-01-01

    The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary condition

  5. Simulation of meridional overturning in the upper layer of the South China Sea with an idealized bottom topography

    Institute of Scientific and Technical Information of China (English)

    WANG Dongxiao; LIU Xiongbin; WANG Wenzhi; DU Yan; ZHOU Weidong

    2004-01-01

    The large-scale upper circulations and meridional overturning in the upper layer of the South China Sea (SCS) with idealized bottom topography in winter and summer are investigated. Simulations with the GFDL general circulation model are carried out under the conditions of open or enclosed boundary regarding transport in the Luzon Strait. The intrusion area of Kuroshio, its impact on the meridional overturning in the upper layer of the SCS and seasonal characteristic of this impact are explored, respectively. The model is forced by climatological wind stress and relaxed to monthly mean climatological temperature and salinity. The resultant meridional overturning is non-enclosed, with transporting from north to south in the surface and returning to north at the depth of about 500 m in winter, about 200 m in summer, with amplitudes of 105 m3/s. It shows the transporting path of intermediate water of the SCS and offers an idealized reference for further study on dynamics of wind-driven and thermohaline circulation of the SCS.

  6. Stratification-dependent mixing may increase sensitivity of Atlantic overturning to global warming

    International Nuclear Information System (INIS)

    Full text: We use the earth system model of intermediate complexity CLIMBER-3 to investigate the effect of stratification-dependent mixing on the stability of the Atlantic Meridional Overturning Circulation (AMOC) under an idealized CO2 increase scenario. The vertical diffusivity κ of the ocean is parameterized as κ ∼ N-α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the AMOC to decrease in response to increased CO2 concentrations. The sensitivity of the AMOC is significantly stronger for α ≥ αcr ∼ 1.5, also after stabilization of the CO2 concentration. This threshold behavior is explained by a halt of dense water formation in the subpolar gyre, which is caused by a positive feedback between stratification and mixing anomalies. Our findings indicate that climate models using time-invariant vertical mixing may underestimate the sensitivity of the AMOC to global warming. (author)

  7. The shallow meridional overturning circulation in the South China Sea and the related internal water movement

    Institute of Scientific and Technical Information of China (English)

    ZHANG Ningning; LAN Jian; MA Jie; CUI Fengjuan

    2016-01-01

    The structure of the annual-mean shallow meridional overturning circulation (SMOC) in the South China Sea (SCS) and the related water movement are investigated, using simple ocean data assimilation (SODA) outputs. The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale, which consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow, with a strength of about 1×106 m3/s. The formation mechanisms of its branches are studied separately. The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m. The annual-mean Ekman transport across 18°N is about 1.2×106 m3/s. An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework. An annual subduction rate of about 0.66×106 m3/s is obtained between 17° and 20°N, of which 87% is due to vertical pumping and 13% is due to lateral induction. The subduction rate implies that the subdution contributes significantly to the downwelling branch. The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward. The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents. Significant upwelling mainly occurs off the Vietnam coast in the southern SCS. An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×106 m3/s, of which a large portion is contributed by summer upwelling, with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.

  8. Meridional carbon dioxide transport in the northern North Atlantic

    NARCIS (Netherlands)

    Stoll, M.H.C.; Aken, H.M. van; Baar, H.J.W. de; Boer, C.J. de

    1996-01-01

    Combination of estimated water transport and accurate measurements of total carbon dioxide (TCO2) on a hydrographic section at 58°N allows the assessment of meridional inorganic carbon transport in the northern North Atlantic Ocean. The transport has been decomposed into contributions from the large

  9. The modern and glacial overturning circulation in the Atlantic ocean in PMIP coupled model simulations

    Directory of Open Access Journals (Sweden)

    S. L. Weber

    2007-01-01

    Full Text Available This study analyses the response of the Atlantic meridional overturning circulation (AMOC to LGM forcings and boundary conditions in nine PMIP coupled model simulations, including both GCMs and Earth system Models of Intermediate Complexity. Model results differ widely. The AMOC slows down considerably (by 20–40% during the LGM as compared to the modern climate in four models, there is a slight reduction in one model and four models show a substantial increase in AMOC strength (by 10–40%. It is found that a major controlling factor for the AMOC response is the density contrast between Antarctic Bottom Water (AABW and North Atlantic Deep Water (NADW at their source regions. Changes in the density contrast are determined by the opposing effects of changes in temperature and salinity, with more saline AABW as compared to NADW consistently found in all models and less cooling of AABW in all models but one. In only two models is the AMOC response during the LGM directly related to the response in net evaporation over the Atlantic basin. Most models show large changes in the ocean freshwater transports into the basin, but this does not seem to affect the AMOC response. Finally, there is some dependence on the accuracy of the control state.

  10. Meridional overturning transports at 30°S in the Indian and Pacific Oceans in 2002-2003 and 2009

    Science.gov (United States)

    Hernández-Guerra, Alonso; Talley, Lynne D.

    2016-08-01

    The meridional circulation and transports at 30°S in the Pacific and Indian Oceans for the years 2002-2003 and 2009 are compared, using GO-SHIP hydrographic section data with an inverse box model and several choices of constraints. Southward heat transport across the combined Indian-Pacific sections, reflecting net heating north of these sections, doubled from -0.7 ± 0.2 PW in 2002-2003 to -1.4 ± 0.1 PW in 2009 (negative sign is southward), with the increase concentrated in the Indian Ocean (∼0.6 PW compared with ∼0.2 PW in the Pacific), and was insensitive to model choices for the Indonesian Throughflow. Diagnosed net evaporation also more than doubled in the Indian Ocean, from 0.21-0.27 Sv in 2002-2003 to 0.51-0.58 in 2009, with a smaller but significant increase in net evaporation in the Pacific, from 0.06-0.08 Sv to 0.16-0.32 Sv. These increased heat and freshwater exports coincided with Indian Ocean warming, a shift in the Indian's shallow gyre overturning transport to lower densities, and an increase in southward Agulhas Current transport from 75 Sv in 2002 to 92 Sv in 2009. The Indian's deep overturn weakened from about 11 Sv in 2002 to 7 Sv in 2009. In contrast, the Pacific Ocean overturning circulation was nearly unchanged from 2003 to 2009, independent of model within the uncertainties. The East Australian Current transport decreased only slightly, from -52 Sv to -46 Sv. The southward Pacific Deep Water transport was at a higher density than the southward Indian Deep Water transport in both years and all models, similar to prior results. Estimated diapycnal diffusivity and velocity are strongly enhanced near the ocean bottom and are higher farther up in the water column in the Indian than in the Pacific, likely extending the reach of Indian Ocean overturning up to shallower depths than in the Pacific. The horizontal distribution of transports in the Pacific at all depths changed notably from 2003 to 2009, despite the stability of its meridional

  11. Mechanisms of meridional transport processes in the tropical Atlantic; Mechanismen meridionaler Transportprozesse im tropischen Atlantik

    Energy Technology Data Exchange (ETDEWEB)

    Kroeger, J.

    2001-07-01

    Meridional transport processes of water masses and tracers in the subtropical and tropical Atlantic are investigated using a regional eddy resolving model of the wind driven and thermohaline circulation. Analytical emphasis is on float simulations in the model which, complementary to Eulerian means, represent the Lagrangian view and give further insight into the spreading and pathways of characteristic water masses in this area. In the tropics and subtropics shallow 3-dimensional circulation cells are superimposed on the northward warm water transfer within the deep reaching thermohaline overturning cell (MOC) as part of the global ''Conveyor Belt''. Under present-day climate conditions the model shows that the equatorial thermocline is exclusively ventilated by subsurface flow within the tropical-subtropical cell (STC) of the South Atlantic. Only with a prescribed ''Conveyor-off''-Mode the STC of the North Atlantic contributes to this ventilation process with equal amounts. Throughout the year the interhemispheric transport of surface and central water masses of South Atlantic origin into the Caribbean Sea is dominated by zonal detours to the east as a consequence of the interplay of several retroflection events occuring in the North Atlantic. The eulerian mean flow field in the deep layer postulates the interhemispheric mass transport into the South Atlantic to be confined entirely to the western boundary, whereas Lagrangian means indicate intermittent eastward excursions along the equator, related to seasonally alternating zonal currents due to long Rossby waves. It was suggested that the observed characteristic eastward maximum of tracer concentrations along the equator is a consequence of rectifying effects of single or interacting equatorial waves. The model does not validate this hypothesis. The response to transport anomalies of subpolar origin and long periodicity is subject to different time-scales in both

  12. Holocene North Atlantic Overturning in an atmosphere-ocean-sea ice model compared to proxy-based reconstructions

    Science.gov (United States)

    Blaschek, M.; Renssen, H.; Kissel, C.; Thornalley, D.

    2015-11-01

    Climate and ocean circulation in the North Atlantic region changed over the course of the Holocene, partly because of disintegrating ice sheets and partly because of an orbital-induced insolation trend. In the Nordic Seas, this impact was accompanied by a rather small, but significant, amount of Greenland ice sheet melting. We have employed the EMIC LOVECLIM and compared our model simulations with proxy-based reconstructions of δ13C, sortable silt, and magnetic susceptibility (κ) used to infer changes in past ocean circulation over the last 9000 years. The various reconstructions exhibit different long-term evolutions suggesting changes in either the overturning of the Atlantic in total or of subcomponents of the ocean circulation, such as the overflow waters across the Greenland-Scotland ridge. Thus, the question arises whether these reconstructions are consistent with each other or not. A comparison with model results indicates that δ13C, employed as an indicator of overturning, agrees well with the long-term evolution of the modeled Atlantic meridional overturning circulation (AMOC). The model results suggest that different long-term trends in subcomponents of the AMOC, such as Iceland-Scotland overflow water, are consistent with proxy-based reconstructions and allow some of the reconstructions to be reconciled with the modeled and reconstructed (from δ13C) AMOC evolution. We find a weak early Holocene AMOC, which recovers by 7 kyr B.P. and shows a weak increasing trend of 88 ± 1 mSv/kyr toward present, with relatively low variability on centennial to millennial timescales.

  13. Warm and Saline Events Embedded in the Meridional Circulation of the Northern North Atlantic

    Science.gov (United States)

    Hakkinen, Sirpa; Rhines, Peter B.; Worthen, Denise L.

    2011-01-01

    Ocean state estimates from 1958 to 2005 from the Simple Ocean Assimilation System (SODA) system are analyzed to understand circulation between subtropical and subpolar Atlantic and their connection with atmospheric forcing. This analysis shows three periods (1960s, around 1980, and 2000s) with enhanced warm, saline waters reaching high latitudes, alternating with freshwater events originating at high latitudes. It complements surface drifter and altimetry data showing the subtropical -subpolar exchange leading to a significant temperature and salinity increase in the northeast Atlantic after 2001. The warm water limb of the Atlantic meridional overturning cell represented by SODA expanded in density/salinity space during these warm events. Tracer simulations using SODA velocities also show decadal variation of the Gulf Stream waters reaching the subpolar gyre and Nordic seas. The negative phase of the North Atlantic Oscillation index, usually invoked in such variability, fails to predict the warming and salinization in the early 2000s, with salinities not seen since the 1960s. Wind stress curl variability provided a linkage to this subtropical/subpolar gyre exchange as illustrated using an idealized two ]layer circulation model. The ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal. In this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax; hence, the polar front moves westward, opening an enhanced northward access of the subtropical waters in the eastern boundary current.

  14. Impact of interocean exchange on the Atlantic overturning circulation

    NARCIS (Netherlands)

    Weijer, W.

    2001-01-01

    The awareness that human activity could change climate has greatly raised public and scientific interest in climate. One issue of present-day climate research is the stability of the thermohaline circulation. This overturning circulation, popularly known as the `conveyor belt', redistributes w

  15. Stability of the Atlantic overturning circulation: competition between Bering Strait freshwater flux and Agulhas heat and salt sources

    NARCIS (Netherlands)

    Weijer, W.; Ruijter, W.P.M. de; Dijkstra, H.A.

    2002-01-01

    In this study we examine the role that is played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, the Drak

  16. Fueling primary productivity: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation

    Science.gov (United States)

    Palter, J. B.; Sarmiento, J. L.; Gnanadesikan, A.; Simeon, J.; Slater, D.

    2010-06-01

    In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be catastrophically reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global primary productivity between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, the high preformed nutrients subducted in the SAMW layer are converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

  17. Fueling export production: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation

    Science.gov (United States)

    Palter, J. B.; Sarmiento, J. L.; Gnanadesikan, A.; Simeon, J.; Slater, R. D.

    2010-11-01

    In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be significantly reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global export production between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, nutrients exported in the SAMW layer are utilized and converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

  18. The relation between AMOC, gyre circulation, and meridional heat transports in the North Atlantic in model simulations of the last millennium

    Science.gov (United States)

    Jungclaus, Johann; Moreno-Chamarro, Eduardo; Lohmann, Katja

    2016-04-01

    While it is clear that the Atlantic Meridional Overturning Circulation (AMOC) is responsible for meridional heat transfer from the South Atlantic and the tropics to the North Atlantic, the majority of the heat transport in the northern North Atlantic and the Nordic seas is carried by the gyre system. However, the detailed mechanisms determining the interaction between and the temporal modulation of the components of the northward heat transport system are not clear. Long-term climate records and model simulations can help to identify important processes and to provide background for the changes that are presently observed. Multi-centennial proxy records from the subpolar North Atlantic and the Nordic Seas indicate, for example, an out-of-phase behavior of sea surface temperature and gyre circulation between the two regions with consequences for regional climate. Paleoceanographic evidence from Fram Strait shows a pronounced modulation of heat transfer to the Arctic by the Atlantic Water layer during the last 2000 years and reconstructions from the Subpolar North Atlantic suggest a role of ocean circulation in the transition between the Medieval Climate Anomaly and the Little Ice Age. Here we explore a small ensemble of last millennium simulations, carried out with the Max Planck Institute Earth System Model, and analyze mechanisms connecting the AMOC and gyre circulation and their relation to external forcing. Our results support the important role of the Subpolar Gyre strength and the related meridional mass and temperature fluxes. We find that the modulation of the northward heat transport into the Nordic Seas and the Arctic has pronounced impact on sea-ice distribution, ocean-atmosphere interaction, and the surface climate in Scandinavia and Western Europe.

  19. The Surface-Forced Overturning of the North Atlantic: Estimates from Modern Era Atmospheric Reanalysis Datasets

    Science.gov (United States)

    Grist, Jeremy; Josey, Simon; Marsh, Robert; Kwon, Young-Oh; Bingham, Rory; Blaker, Adam

    2014-05-01

    Estimates of the recent mean and time varying water mass transformation rates associated with North Atlantic surface-forced overturning are presented. The estimates are derived from heat and freshwater surface fluxes and sea surface temperature fields from six atmospheric reanalyses (JRA, NCEP-1, NCEP-2, ERA-I, CFSR and MERRA) together with sea surface salinity fields from two globally gridded data sets (World Ocean Atlas and EN3). The resulting twelve estimates of the 1979-2007 mean surface-forced streamfunction all depict a sub-polar cell, with maxima north of 45oN, near σ = 27.5 kgm-3, and a sub-tropical cell between 20oN and 40oN, near σ = 26.1 kgm-3. The mean magnitude of the sub-polar cell varies between 12-18 Sv, consistent with estimates of the overturning circulation from sub-surface observations. Analysis of the thermal and haline components of the surface density fluxes indicate large differences in the inferred low latitude circulation are largely due to the biases in reanalysis net heat flux fields, which range in the global mean from -13 Wm-2 to 19 Wm-2. The different estimates of temporal variability in the sub-polar cell are well correlated with each other. This suggests the uncertainty associated with the choice of reanalysis product does not critically limit the ability of the method to infer the variability in the sub-polar overturning. In contrast, the different estimates of sub-tropical variability are poorly correlated with each other, and only a subset of them capture a significant fraction of the variability in independently estimated North Atlantic Sub-Tropical Mode Water volume.

  20. Anthropogenic CO2 uptake, transport, storage, and dynamical controls in the ocean imposed by the meridional overturning circulation: A modeling study

    Science.gov (United States)

    Nakano, H.; Ishii, M.; Rodgers, K. B.; Tsujino, H.; Yamanaka, G.

    2015-10-01

    Using an ocean carbon cycle model embedded in an ocean general circulation model, we examine how the budget of anthropogenic CO2 (Cant) is controlled by ocean dynamics. To complement recent studies showing only vertically integrated budgets, we provide a step-by-step description by making use of three different coarse grainings of the full vertical resolution of the ocean model in our budget analysis. For the 11 subdomains of the global ocean, these coarse grainings are (1) a one-layer (vertically integrated) budget, (2) a three-layer budget, and (3) an 11-layer budget. We largely focus on the Pacific circulation. We identify and quantify substantial carbon transport associated with the subtropical cells (STCs), which are dominant contributors to the meridional overturning circulation in the upper ocean in the tropics and subtropics, as playing a fundamental role in governing the ocean interior distribution of Cant. The upper branch of the STCs transports Cant-rich water from the tropics to the subtropics, contributing to the precondition for the high Cant inventory in mode waters. The lower branch of the STCs carries about two thirds of the transported Cant back to the tropics, while it largely excludes Subtropical Mode Waters. This work implies that the reemergence of Cant through recirculation within the STCs may lead to a reduced capacity for further Cant uptake via gas exchange into the surface ocean, potentially contributing to a positive carbon-climate feedback.

  1. An optimally tuned ensemble of the "eb_go_gs" configuration of GENIE: parameter sensitivity and bifurcations in the Atlantic overturning circulation

    Directory of Open Access Journals (Sweden)

    R. Marsh

    2013-02-01

    Full Text Available The key physical parameters of the "eb_go_gs" configuration of GENIE, an Earth system Model of Intermediate Complexity (EMIC, are tuned using a multi-objective genetic algorithm. An ensemble of 90 parameter sets is tuned using two ocean and two atmospheric state variables as targets. These are "Pareto-optimal", representing a range of trade-offs between the four tuning targets. For the leading five parameter sets, simulations are further evaluated alongside a simulation with untuned "default" parameters, comparing selected variables and diagnostics that describe the state of the atmosphere, ocean and sea ice. One of these parameter sets is selected for further analysis of the objective function (error landscape in the vicinity of its tuned values. "Cliffs" along some dimensions motivate closer inspection of corresponding variations in the Atlantic meridional overturning circulation (AMOC. This reveals that bifurcations in the AMOC are highly sensitive to parameters that are not commonly associated with MOC stability. Specifically, the state of the AMOC is sensitive to parameters governing the wind-driven circulation and atmospheric heat transport. Five optimal parameter sets are recommended for future use of GENIE in the configuration presented here.

  2. Oxygen variability and meridional oxygen supply in the tropical North East Atlantic oxygen minimum zone

    Science.gov (United States)

    Hahn, Johannes; Brandt, Peter; Greatbatch, Richard J.; Krahmann, Gerd; Körtzinger, Arne

    2013-04-01

    The oxygen minimum zone (OMZ) of the tropical North East Atlantic (TNEA) is located between the oxygen-rich equatorial region and the Cape Verde Frontal Zone at about 20°N in a depth range of 300 - 700 m. Its horizontal extent is predominantly defined by the North Equatorial Current and by the equatorial zonal current system ventilating the region to the north and south of the OMZ, respectively. The interior of the OMZ is characterized by a sluggish flow regime, where mesoscale eddies play a major role in the ventilation. In this study we focus on the oxygen variability in the TNEA as well as the eddy driven lateral ventilation of the TNEA OMZ across its southern boundary. During recent years an intense measurement program was executed along 23°W cutting meridionally through the TNEA OMZ. Hydrographic and velocity data has been acquired from ship sections and moorings, together covering the latitude range between 6°S and 14°N with particularly high meridional resolution of shipboard and high temporal resolution of moored observations. Based on shipboard data we derived a meridional section of oxygen variance, which reveals numerous local maxima of oxygen variability. Exemplary, strong oxygen variability is observed at the upper (300m, 5° - 12°N) and the southern boundary (400m - 700m, 5°N - 8°N) of the OMZ, whereas the interior of the OMZ is characterized by weak variability. An application of the extended Osborn-Cox model shows that the strong oxygen variability at the southern boundary is mainly generated by mesoscale eddies. The strong variability at the upper boundary is generated by mesoscale eddies as well as microscale turbulence. We apply two methods to estimate the meridional oxygen flux: 1) a flux gradient parameterization and 2) a correlation of oxygen and velocity mooring time series. From the analysis of the 5°N mooring data we find a northward oxygen flux directed towards the OMZ at its core depth, that is mainly due to variability of

  3. Modulation of western North Pacific tropical cyclone activity by the Atlantic Meridional Mode

    Science.gov (United States)

    Zhang, Wei; Vecchi, Gabriel A.; Villarini, Gabriele; Murakami, Hiroyuki; Rosati, Anthony; Yang, Xiaosong; Jia, Liwei; Zeng, Fanrong

    2016-05-01

    This study examines the year-to-year modulation of the western North Pacific (WNP) tropical cyclones (TC) activity by the Atlantic Meridional Mode (AMM) using both observations and the Geophysical Fluid Dynamics Laboratory Forecast-oriented Low Ocean Resolution Version of CM2.5 (FLOR) global coupled model. 1. The positive (negative) AMM phase suppresses (enhances) WNP TC activity in observations. The anomalous occurrence of WNP TCs results mainly from changes in TC genesis in the southeastern part of the WNP. 2. The observed responses of WNP TC activity to the AMM are connected to the anomalous zonal vertical wind shear (ZVWS) caused by AMM-induced changes to the Walker circulation. During the positive AMM phase, the warming in the North Atlantic induces strong descending flow in the tropical eastern and central Pacific, which intensifies the Walker cell in the WNP. The intensified Walker cell is responsible for the suppressed (enhanced) TC genesis in the eastern (western) part of the WNP by strengthening (weakening) ZVWS. 3. The observed WNPTC-AMM linkage is examined by the long-term control and idealized perturbations experiment with FLOR-FA. A suite of sensitivity experiments strongly corroborate the observed WNPTC-AMM linkage and underlying physical mechanisms.

  4. An optimally tuned ensemble of the "eb_go_gs" configuration of GENIE: parameter sensitivity and bifurcations in the Atlantic overturning circulation

    Directory of Open Access Journals (Sweden)

    R. Marsh

    2013-10-01

    Full Text Available The key physical parameters for the "eb_go_gs" configuration of version 2.7.4 of GENIE, an Earth system model of intermediate complexity (EMIC, are tuned using a multi-objective genetic algorithm. An ensemble of 90 parameter sets is tuned using two ocean and two atmospheric state variables as targets. These are "Pareto-optimal", representing a range of trade-offs between the four tuning targets. For the leading five parameter sets, simulations are evaluated alongside a simulation with untuned "default" parameters, comparing selected variables and diagnostics that describe the state of the atmosphere, ocean and sea ice. Further experiments are undertaken with these selected parameter sets to compare equilibrium climate sensitivities and transient climate responses. The pattern of warming under doubled CO2 is strongly shaped by changes in the Atlantic meridional overturning circulation (AMOC, while the pattern and rate of warming under rising CO2 is closely linked to changing sea ice extent. One of the five tuned parameter sets is identified as marginally optimal, and the objective function (error landscape is further analysed in the vicinity of the tuned values of this parameter set. "Cliffs" along some dimensions motivate closer inspection of corresponding variations in the AMOC. This reveals that bifurcations in the AMOC are highly sensitive to parameters that are not typically associated with MOC stability. Specifically, the state of the AMOC is sensitive to parameters governing the wind-driven circulation and atmospheric heat transport. For the GENIE configuration presented here, the marginally optimal parameter set is recommended for single simulations, although the leading five parameter sets may be used in ensemble mode to admit a constrained degree of parametric uncertainty in climate prediction.

  5. Potential for seasonal prediction of the Atlantic sea surface temperatures using the RAPID array at 26°N

    OpenAIRE

    Duchez, Aurélie; Courtois, Peggy; Harris, Elizabeth; Josey, Simon; Kanzow, Torsten; Marsh, Robert; Smeed, David; Hirschi, Joël Jean-Marie

    2015-01-01

    The Atlantic meridional overturning circulation (AMOC) plays a critical role in the climate system and is responsible for much of the meridional heat transported by the ocean. In this paper, the potential of using AMOC observations from the 26 ? N RAPID array to predict North Atlantic sea surface temperatures is investigated for the first time. Using spatial correlations and a composite method, the AMOC anomaly is used as a precursor of North Atlantic sea-surface temperature anomalies (SSTAs...

  6. North Atlantic multidecadal variability in coupled climate models - Mechanisms and responses

    OpenAIRE

    Medhaug, Iselin

    2011-01-01

    Global atmosphere-ocean general circulation models have been used to investigate mechanisms controlling the North Atlantic low-frequency variability, with the focus on the Atlantic Meridional Overturning Circulation, the Subpolar Gyre dynamics and the North Atlantic basin scale sea surface temperatures, called the Atlantic Multidecadal Oscillation. The understanding of the dynamics of, and mechanisms behind the variability in these components of the climate system are of great ...

  7. 北太平洋经向翻转环流和热盐输送研究综述%Investigation into the North Pacific meridional overturning circulation, and heat and salt transport:Progress report

    Institute of Scientific and Technical Information of China (English)

    刘洪伟; 张启龙; 段永亮; 徐永生

    2016-01-01

    The North Pacific meridional overturning circulation (NPMOC) is the general name given to the five North Pacific overturning cells: the subtropical cell (STC), the tropical cell (TC), the subpolar cell (SPC), the deep tropical cell (DTC), and the thermocline cell (THC). These cells are important for the leakage of meridional mate-rial and energy exchange. The NPMOC and its variation play an important role in the upper ocean thermohaline structure and climate change. The structure of the STC, TC, and DTC, and the level and mechanism of variation in these are currently being extensively analyzed, and a preliminary analysis of poleward heat transport by the STC has been completed. Less research has been undertaken on the SPC and THC; however, the three-dimensional structure and mechanism of variation remains unknown. In addition, research into heat and salt transport by the NPMOC is still at an initial stage, with little knowledge at present about the characteristics and the level and mechanism of variation within each cell. Therefore, this requires further investigation in the future.%北太平洋经向翻转环流是北太平洋所有经向翻转环流圈的总称,目前它拥有五个环流圈,即副热带环流圈(the subtropical cell, STC)、热带环流圈(the tropical cell, TC)、副极地环流圈(the subpolar cell, SPC)、深层热带环流圈(the deep tropical cell, DTC)和温跃层环流圈(the thermohaline cell, THC)。这些环流圈是北太平洋经向物质和能量交换的重要通道,它们的变化对海洋上层热盐结构和气候变化皆有重要影响。迄今,人们已对STC、TC和DTC的结构形态、变化特征与机理开展了广泛而深入的研究,并对STC的极向热输送特征也做了一些初步分析。但应指出的是,关于SPC和THC的研究仍较少,迄今尚不清楚这两个环流圈的三维结构和变异机理;而且,对北太平洋经向翻转环流的热盐输送研究尚处于起步阶段,目前对各环流圈

  8. On multidecadal and quasi-decadal North Atlantic variability

    OpenAIRE

    Alvarez-Garcia, F.; Latif, Mojib; Biastoch, Arne

    2008-01-01

    Observed sea surface temperatures (SSTs) in the North Atlantic from 1958 through 2000, as well as data from an ocean model simulation driven with the atmospheric variability observed during the same period, are examined using multichannel singular spectrum analysis. The two leading oscillatory modes are associated with a multidecadal and a quasi-decadal period. The former is connected to a basinwide uniform SST pattern and changes in the deep North Atlantic meridional overturning circulation....

  9. Tropical versus high latitude freshwater influence on the Atlantic circulation

    OpenAIRE

    Goelzer, H.; J. Mignot; Levermann, A; Rahmstorf, S.

    2006-01-01

    We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water for...

  10. Impact of tropical Pacific variability on the mean North Atlantic thermohaline circulation

    OpenAIRE

    Semenov, Vladimir; Latif, Mojib

    2006-01-01

    A series of 500 years long coupled general circulation model simulations has been performed, in which the sea surface temperatures (SSTs) in different tropical oceans have been prescribed from climatology. A statistically significant reduction by about one Sverdrup of the meridional overturning circulation (MOC) in the North Atlantic was found when the tropical Pacific SSTs do not vary interannually. Anomalously low salinities originating in the tropical Atlantic due to increased precipitatio...

  11. The role of the North Atlantic overturning and deep ocean for multi-decadal global-mean-temperature variability

    Directory of Open Access Journals (Sweden)

    C. F. Schleussner

    2014-02-01

    By decomposing the GMT variance into contributions of the AMOC and Northern Hemisphere sea-ice extent using a graph-theoretical statistical approach, we find the AMOC to contribute 8% to GMT variability in the ensemble mean. Our results highlight the importance of AMOC sea-ice feedbacks that explain 5% of the GMT variance, while the contribution solely related to the AMOC is found to be about 3%. As a consequence of multi-decadal AMOC variability, we report substantial variations in North Atlantic deep-ocean heat content with trends of up to 0.7 × 1022 J decade−1 that are of the order of observed changes over the last decade and consistent with the reduced GMT warming trend over this period. Although these temperature anomalies are largely density-compensated by salinity changes, we find a robust negative correlation between the AMOC and North Atlantic deep-ocean density with density lagging the AMOC by 5 to 11 yr in most models. While this would in principle allow for a self-sustained oscillatory behavior of the coupled AMOC–deep-ocean system, our results are inconclusive about the role of this feedback in the model ensemble.

  12. The dynamic characteristics of deep meridional overturning circulation in the Indian Ocean based on six reanalysis datasets%基于多套数据的印度洋深层经向翻转环流的动力特征

    Institute of Scientific and Technical Information of China (English)

    黄旭媚; 王卫强; 刘海龙

    2016-01-01

    The dynamic characteristics of time-mean meridional overturning circulation in the Indian Ocean was examined using six reanalysis datasets. The results showed consistent time-mean features of the deep meridional overturning circulation, which is an anticlockwise overturning cell with inflows in the bottom and deep layers and outflows in the intermediate and upper layers. Dynamic decomposition of meridional overturning circulation was applied to examine the similarities and differences of every dynamic component. The structure of Ekman component is an anticlockwise overturning cell in the South Indian Ocean with maximum strength at ~10°S. In the region south of 10°S, geostrophic and external components show clockwise and anticlockwise overturning cells, respectively, they both reach maximum strengths at~27°S. Based on different products of heat and momentum fluxes used, the dynamic components resulted from the six datasets show some inconsistent features as follows. The overall structures of Ekman component are similar since the wind fields of the six datasets have few differences. The discrepancies of the geostrophic component in the six datasets are due to the strength of the baroclinic flows in the interior ocean and the structure of the western boundary current: the greater the baroclinic flows in the interior ocean, the stronger the strength of the geostrophic component; the wider the western boundary current, the greater impact on the baroclinic flows in the interior ocean, and then on the strength of the geostrophic component. The strength of the external component is affected by the intensity of the western boundary current:the greater the western boundary current, the stronger the strength of the external component.%基于六套模式产品,研究了时间平均状态下印度洋深层经向翻转环流的动力特征。时间平均状态下,印度洋深层经向翻转环流(meridional overturning circulation, MOC)在各套数据中呈现一致的结

  13. A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

    Directory of Open Access Journals (Sweden)

    I. Núñez-Riboni

    2011-02-01

    Full Text Available Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF in the Newfoundland Basin (NFB in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to Labrador Sea Water (LSW volume, atmospheric forcing and intensities of the Subpolar Gyre (SPG and Meridional Overturning Circulation (MOC. The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG, an inter-gyre gyre with a multi-decadal time scale. Contributions of wind stress curl (WSC and LSW volume changes to the inter-gyre gyre are similar between 35 and 55° N (excluding the western boundary current. An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

  14. A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

    Directory of Open Access Journals (Sweden)

    I. Núñez-Riboni

    2012-02-01

    Full Text Available Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF in the Newfoundland Basin (NFB in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to atmospheric forcing, intensities of the subpolar gyre (SPG and Meridional Overturning Circulation (MOC, and Labrador Sea Water (LSW volume. The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG, an inter-gyre gyre with a multi-decadal time scale. A sensitivity experiment indicates that both wind stress curl (WSC and heat fluxes (which match LSW changes contribute to the circulation anomalies in the frontal region and to the SPF displacement. An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB (a "SPF index" could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

  15. Constraining the shallow subtropical overturning circulation with archived radiocarbon records

    Science.gov (United States)

    Rosenheim, B. E.; Swart, P. K.; Thorrold, S. R.; Roberts, M. L.

    2007-12-01

    Archived radiocarbon records in accretionary skeletons can be used to constrain the shallow overturning subtropical cells (STC's) that transport significant amounts of tropical heat poleward in the world's oceans. Radiocarbon values of DIC in the world's oceans reflect a continuum between waters residing on the surface over long periods (high Δ14C due to equilibration with "modern" atmosphere) and waters decoupled from the atmosphere in the abyss (low Δ14C due to radioisotope decay), as well as mixtures between water masses of different ages. Thus, measurements of radiocarbon have demonstrated utility in assessing convective heat tranport such as the Meridional Overturning Circulation that is central to global climate. A prominent radiocarbon gradient is also present between the subsiding subtropical surface waters and the upwelling equatorial surface waters in the world's oceans due to the presence of STC's. These convection cells transport a major proportion of tropical heat in the Pacific and a significant proportion of tropical heat in the Atlantic towards the poles. Archived radiocarbon records in surface corals and subsurface sclerosponges constrain the N. Atlantic STC's on a centennial time scale. Published short records from Cape Verde corals indicate significant changes in radiocarbon content; this is potentially related to migration of the front between upwelled tropical waters and downwelled subtropical waters. An approach is outlined to estimate the proportion of tropical to subtropical waters at Cape Verde using as endmembers high-resolution sclerosponge radiocarbon records from Bahamas subsurface waters and coral radiocarbon records from São Tome and Principe in the Gulf of Guinea. Preliminary data from Bahamas sclerosponges indicate the need for high-resolution subsampling of the skeletons. Initial novel AMS measurements from fine scale laser-decomposition of the skeletons are presented.

  16. Late Glacial – Holocene climate variability and sedimentary environments on northern continental shelves Zonal and meridional Atlantic Water advection

    OpenAIRE

    Sørensen, Steffen Aagaard

    2011-01-01

    The overall objective for this PhD-study was to further advance the understanding of the oceanographic variability and development in the Nordic Seas during the Late Glacial and the Holocene and towards the present. The focus is specifically on the poleward Atlantic Water advection along the continental margins of Norway, into the SW Barents Sea and along the West Spitsbergen slope. Four high resolution sediment cores retrieved from northern continental shelve allowed examination of spatial a...

  17. The North Atlantic Oscillation as a driver of rapid climate change in the Northern Hemisphere

    Science.gov (United States)

    Delworth, Thomas L.; Zeng, Fanrong; Vecchi, Gabriel A.; Yang, Xiaosong; Zhang, Liping; Zhang, Rong

    2016-07-01

    Pronounced climate changes have occurred since the 1970s, including rapid loss of Arctic sea ice, large-scale warming and increased tropical storm activity in the Atlantic. Anthropogenic radiative forcing is likely to have played a major role in these changes, but the relative influence of anthropogenic forcing and natural variability is not well established. The above changes have also occurred during a period in which the North Atlantic Oscillation has shown marked multidecadal variations. Here we investigate the role of the North Atlantic Oscillation in these rapid changes through its influence on the Atlantic meridional overturning circulation and ocean heat transport. We use climate models to show that observed multidecadal variations of the North Atlantic Oscillation can induce multidecadal variations in the Atlantic meridional overturning circulation and poleward ocean heat transport in the Atlantic, extending to the Arctic. Our results suggest that these variations have contributed to the rapid loss of Arctic sea ice, Northern Hemisphere warming, and changing Atlantic tropical storm activity, especially in the late 1990s and early 2000s. These multidecadal variations are superimposed on long-term anthropogenic forcing trends that are the dominant factor in long-term Arctic sea ice loss and hemispheric warming.

  18. Correcting North Atlantic sea surface salinity biases in the Kiel Climate Model: influences on ocean circulation and Atlantic Multidecadal Variability

    Science.gov (United States)

    Park, T.; Park, W.; Latif, M.

    2016-10-01

    A long-standing problem in climate models is the large sea surface salinity (SSS) biases in the North Atlantic. In this study, we describe the influences of correcting these SSS biases on the circulation of the North Atlantic as well as on North Atlantic sector mean climate and decadal to multidecadal variability. We performed integrations of the Kiel Climate Model (KCM) with and without applying a freshwater flux correction over the North Atlantic. The quality of simulating the mean circulation of the North Atlantic Ocean, North Atlantic sector mean climate and decadal variability is greatly enhanced in the freshwater flux-corrected integration which, by definition, depicts relatively small North Atlantic SSS biases. In particular, a large reduction in the North Atlantic cold sea surface temperature bias is observed and a more realistic Atlantic Multidecadal Variability simulated. Improvements relative to the non-flux corrected integration also comprise a more realistic representation of deep convection sites, sea ice, gyre circulation and Atlantic Meridional Overturning Circulation. The results suggest that simulations of North Atlantic sector mean climate and decadal variability could strongly benefit from alleviating sea surface salinity biases in the North Atlantic, which may enhance the skill of decadal predictions in that region.

  19. Decadal predictions of the North Atlantic CO2 uptake.

    Science.gov (United States)

    Li, Hongmei; Ilyina, Tatiana; Müller, Wolfgang A; Sienz, Frank

    2016-01-01

    As a major CO2 sink, the North Atlantic, especially its subpolar gyre region, is essential for the global carbon cycle. Decadal fluctuations of CO2 uptake in the North Atlantic subpolar gyre region are associated with the evolution of the North Atlantic Oscillation, the Atlantic meridional overturning circulation, ocean mixing and sea surface temperature anomalies. While variations in the physical state of the ocean can be predicted several years in advance by initialization of Earth system models, predictability of CO2 uptake has remained unexplored. Here we investigate the predictability of CO2 uptake variations by initialization of the MPI-ESM decadal prediction system. We find large multi-year variability in oceanic CO2 uptake and demonstrate that its potential predictive skill in the western subpolar gyre region is up to 4-7 years. The predictive skill is mainly maintained in winter and is attributed to the improved physical state of the ocean. PMID:27026490

  20. Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean

    Science.gov (United States)

    Böning, Claus W.; Behrens, Erik; Biastoch, Arne; Getzlaff, Klaus; Bamber, Jonathan L.

    2016-07-01

    The Greenland ice sheet has experienced increasing mass loss since the 1990s. The enhanced freshwater flux due to both surface melt and outlet glacier discharge is assuming an increasingly important role in the changing freshwater budget of the subarctic Atlantic. The sustained and increasing freshwater fluxes from Greenland to the surface ocean could lead to a suppression of deep winter convection in the Labrador Sea, with potential ramifications for the strength of the Atlantic meridional overturning circulation. Here we assess the impact of the increases in the freshwater fluxes, reconstructed with full spatial resolution, using a global ocean circulation model with a grid spacing fine enough to capture the small-scale, eddying transport processes in the subpolar North Atlantic. Our simulations suggest that the invasion of meltwater from the West Greenland shelf has initiated a gradual freshening trend at the surface of the Labrador Sea. Although the freshening is still smaller than the variability associated with the episodic `great salinity anomalies', the accumulation of meltwater may become large enough to progressively dampen the deep winter convection in the coming years. We conclude that the freshwater anomaly has not yet had a significant impact on the Atlantic meridional overturning circulation.

  1. Vertical density gradient in the eastern North Atlantic during the last 30,000 years

    Energy Technology Data Exchange (ETDEWEB)

    Rogerson, M.; Ramirez, J. [University of Hull, Geography Department, Hull (United Kingdom); Bigg, G.R. [University of Sheffield, Department of Geography, Sheffield (United Kingdom); Rohling, E.J. [University of Southampton, National Oceanography Centre, School of Ocean and Earth Science, Southampton (United Kingdom)

    2012-08-15

    Past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the Atlantic Meridional Overturning Circulation and consequently climate. However, very little is known about past changes in the vertical density structure of the ocean, even very extensively studied systems such as the North Atlantic. Here we exploit the physical controls on the settling depth of the dense Mediterranean water plume derived from the Strait of Gibraltar to obtain the first robust, observations-based, probabilistic reconstruction of the vertical density gradient in the eastern North Atlantic during the last 30,000 years. We find that this gradient was weakened by more than 50%, relative to the present, during the last Glacial Maximum, and that changes in general are associated with reductions in AMOC intensity. However, we find only a small change during Heinrich Event 1 relative to the Last Glacial Maximum, despite strong evidence that overturning was substantially altered. This implies that millennial-scale changes may not be reflected in vertical density structure of the ocean, which may be limited to responses on an ocean-overturning timescale or longer. Regardless, our novel reconstruction of Atlantic density structure can be used as the basis for a dynamical measure for validation of model-based AMOC reconstructions. In addition, our general approach is transferrable to other marginal sea outflow plumes, to provide estimates of oceanic vertical density gradients in other locations. (orig.)

  2. Dynamics and Variability of the Circulation in the North-Atlantic Subpolar Seas

    OpenAIRE

    Chafik, Léon

    2014-01-01

    This thesis deals with the dynamics and circulation in the northern North Atlantic and the Nordic Seas, processes of crucial importance for the mild climate of Scandinavia and Northern Europe. High-resolution ADCP scans of currents from Greenland to Scotland in the top 400 m demonstrate that the Reykjanes Ridge is a very effective separator of flow towards the Nordic and Labrador Seas, respectively. It was found that the meridional overturning circulation has weakened by ~1.7 Sv (1 Sv = 106 m...

  3. AMOC sensitivity to surface buoyancy fluxes: Stronger ocean meridional heat transport with a weaker volume transport?

    Science.gov (United States)

    Sévellec, Florian; Fedorov, Alexey V.

    2016-09-01

    Oceanic northward heat transport is commonly assumed to be positively correlated with the Atlantic meridional overturning circulation (AMOC). For example, in numerical "water-hosing" experiments, imposing anomalous freshwater fluxes in the northern Atlantic leads to a slow-down of the AMOC and the corresponding reduction of oceanic northward heat transport. Here, we study the sensitivity of the ocean heat and volume transports to surface heat and freshwater fluxes using a generalized stability analysis. For the sensitivity to surface freshwater fluxes, we find that, while the direct relationship between the AMOC volume and heat transports holds on shorter time scales, it can reverse on timescales longer than 500 years or so. That is, depending on the model surface boundary conditions, reduction in the AMOC volume transport can potentially lead to a stronger heat transport on long timescales, resulting from the gradual increase in ocean thermal stratification. We discuss the implications of these results for the problem of steady state (statistical equilibrium) in ocean and climate GCM as well as paleoclimate problems including millennial climate variability.

  4. The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system

    OpenAIRE

    Kandiano, E. S.; H. A. Bauch; K. Fahl; J. P. Helmke; Röhl, U.; Pérez-Folgado, M.; Cacho, I.

    2012-01-01

    Temporal and spatial patterns in eastern North Atlantic sea surface temperatures (SST) were reconstructed for marine isotope stage (MIS) 11c using a submeridional transect of five sediment cores. The SST reconstructions are based on planktic foraminiferal abundances and alkenone indices, and are supported by benthic and planktic stable isotope measurements, as well as by ice-rafted debris content in polar and middle latitudes. Additionally, the larger-scale dynamics of the precipitation regim...

  5. Deep ocean early warning signals of an Atlantic MOC collapse

    CERN Document Server

    Feng, Qing Yi; Dijkstra, Henk A

    2014-01-01

    The Atlantic Meridional Overturning Circulation (MOC) is a crucial part of the climate system because of its associated northward heat transport. The present-day MOC is sensitive to freshwater anomalies and may collapse to a state with a strongly reduced northward heat transport. A future collapse of the Atlantic MOC has been identified as one of the most dangerous tipping points in the climate system. It is therefore crucial to develop early warning indicators for such a potential collapse based on relatively short time series. So far, attempts to use indicators based on critical slowdown have been marginally successful. Based on complex climate network reconstruction, we here present a promising new indicator for the MOC collapse that efficiently monitors spatial changes in deep ocean circulation. Through our analysis of the performance of this indicator we formulate optimal locations of measurement of the MOC to provide early warning signals of a collapse. Our results imply that an increase in spatial reso...

  6. Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic MOC

    Directory of Open Access Journals (Sweden)

    S.-E. Brunnabend

    2014-04-01

    Full Text Available As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic Meridional Overturning Circulation (AMOC are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR strongly eddying version and a low-resolution (LR version in which the effect of eddies are parameterized. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.

  7. The absence of an Atlantic imprint on the multidecadal variability of wintertime European temperature.

    Science.gov (United States)

    Yamamoto, Ayako; Palter, Jaime B

    2016-01-01

    Northern Hemisphere climate responds sensitively to multidecadal variability in North Atlantic sea surface temperature (SST). It is therefore surprising that an imprint of such variability is conspicuously absent in wintertime western European temperature, despite that Europe's climate is strongly influenced by its neighbouring ocean, where multidecadal variability in basin-average SST persists in all seasons. Here we trace the cause of this missing imprint to a dynamic anomaly of the atmospheric circulation that masks its thermodynamic response to SST anomalies. Specifically, differences in the pathways Lagrangian particles take to Europe during anomalous SST winters suppress the expected fluctuations in air-sea heat exchange accumulated along those trajectories. Because decadal variability in North Atlantic-average SST may be driven partly by the Atlantic Meridional Overturning Circulation (AMOC), the atmosphere's dynamical adjustment to this mode of variability may have important implications for the European wintertime temperature response to a projected twenty-first century AMOC decline. PMID:26975331

  8. Abundance, distribution and size structure of zooplankton along a 20° west meridional transect of the northeast Atlantic Ocean in July

    Science.gov (United States)

    Gallienne, C. P.; Robins, D. B.; Woodd-Walker, R. S.

    Zooplankton distribution, abundance and size structure were characterised on leg 2 of the PRIME cruise in the northeast Atlantic during July 1996. Latitudinal sampling was carried out from 60°N to 37°N along a meridian at approximately 20°W. Zooplankton were characterised using an Optical Plankton Counter (OPC), and from traditional depth-integrated net hauls for taxonomy and carbon. The former method allows zooplankton to be described in terms of biovolume and size. There was good agreement between the two methods for abundance and a good correlation between OPC biovolume and carbon analysis. These data also showed compatibility with concurrent acoustic measurements of zooplankton. Results for the cruise compared to recent studies in the region, show that, overall abundance was generally low, although most of the previous studies were closer to spring bloom conditions. The size structure over latitude shows that mean size fluctuated, but there was a general trend of decreasing mean size from the north of the transect to about 47°N. Further statistical treatment of the OPC data, using principal component analysis, showed that zooplankton size structure changed at five positions along the transect. Three of these coincided with changes in physical structure, one each side of the front at 48°N and the third on the outer edge of an eddy. The other two occurred in regions of high-surface chlorophyll concentration.

  9. Reorganization of the North Atlantic Oscillation during early Holocene deglaciation

    Science.gov (United States)

    Wassenburg, Jasper A.; Dietrich, Stephan; Fietzke, Jan; Fohlmeister, Jens; Jochum, Klaus Peter; Scholz, Denis; Richter, Detlev K.; Sabaoui, Abdellah; Spötl, Christoph; Lohmann, Gerrit; Andreae, Meinrat O.; Immenhauser, Adrian

    2016-08-01

    The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.

  10. Surface salinity variability in the northern North Atlantic during recent decades

    Science.gov (United States)

    HäKkinen, Sirpa

    2002-12-01

    The sea surface salinity (SSS) variability in the North Atlantic is investigated using numerical model simulations for the last 50 years based on atmospheric forcing variability from the Comprehensive Atmosphere Ocean Data Set (COADS) and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The focus here is the SSS variability in the subpolar region. The seasonality of the subpolar surface salinity variability is prominent with the maximum standard deviation (SD) occurring in the summer/fall period so that the amplitude of the summer SSS anomalies far exceed those of the wintertime. The interannual SSS variability in the subpolar gyre can be attributed mainly to excess ice melt, while the longer-term (decadal) variability is associated with meridional overturning variability. In these two hindcasts, the deep mixing (which drives overturning changes) is forced by heat flux variability so that weak subpolar heat loss and subsequent weak overturning are usually manifested in fresh surface conditions in the subpolar gyre within 2 years. This is because the role of deep convection is to mix down the net fresh water input received by the high latitudes. Hence, at the lack of widespread deep mixing, a signal of slowing down overturning, the fresh water input that would be expected based on climatology would accumulate at the surface.

  11. Deglacial diatom production in the tropical North Atlantic driven by enhanced silicic acid supply

    Science.gov (United States)

    Hendry, Katharine R.; Gong, Xun; Knorr, Gregor; Pike, Jennifer; Hall, Ian R.

    2016-03-01

    Major shifts in ocean circulation are thought to be responsible for abrupt changes in temperature and atmospheric CO2 during the last deglaciation, linked to variability in meridional heat transport and deep ocean carbon storage. There is also widespread evidence for shifts in biological production during these times of deglacial CO2 rise, including enhanced diatom production in regions such as the tropical Atlantic. However, it remains unclear as to whether this diatom production was driven by enhanced wind-driven upwelling or density-driven vertical mixing, or by elevated thermocline concentrations of silicic acid supplied to the surface at a constant rate. Here, we demonstrate that silicic acid supply at depth in the NE Atlantic was enhanced during the abrupt climate events of the deglaciation. We use marine sediment archives to show that an increase in diatom production during abrupt climate shifts could only occur in regions of the NE Atlantic where the deep supply of silicic acid could reach the surface. The associated changes are indicative of enhanced regional wind-driven upwelling and/or weakened stratification due to circulation changes during phases of weakened Atlantic meridional overturning. Globally near-synchronous pulses of diatom production and enhanced thermocline concentrations of silicic acid suggest that widespread deglacial surface-driven breakdown of stratification, linked to changes in atmospheric circulation, had major consequences for biological productivity and carbon cycling.

  12. Persistent positive North Atlantic oscillation mode dominated the Medieval Climate Anomaly.

    Science.gov (United States)

    Trouet, Valérie; Esper, Jan; Graham, Nicholas E; Baker, Andy; Scourse, James D; Frank, David C

    2009-04-01

    The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña-like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA. PMID:19342585

  13. Sequestration of carbon in the deep Atlantic during the last glaciation

    Science.gov (United States)

    Yu, J.; Menviel, L.; Jin, Z. D.; Thornalley, D. J. R.; Barker, S.; Marino, G.; Rohling, E. J.; Cai, Y.; Zhang, F.; Wang, X.; Dai, Y.; Chen, P.; Broecker, W. S.

    2016-04-01

    Atmospheric CO2 concentrations declined markedly about 70,000 years ago, when the Earth’s climate descended into the last glaciation. Much of the carbon removed from the atmosphere has been suspected to have entered the deep oceans, but evidence for increased carbon storage remains elusive. Here we use the B/Ca ratios of benthic foraminifera from several sites across the Atlantic Ocean to reconstruct changes in the carbonate ion concentration and hence the carbon inventory of the deep Atlantic across this transition. We find that deep Atlantic carbonate ion concentration declined by around 25 μmol kg-1 between ~80,000 and 65,000 years ago. This drop implies that the deep Atlantic carbon inventory increased by at least 50 Gt around the same time as the amount of atmospheric carbon dropped by about 60 Gt. From a comparison with proxy records of deep circulation and climate model simulations, we infer that the carbon sequestration coincided with a shoaling of the Atlantic meridional overturning circulation. We thus conclude that changes in the Atlantic Ocean circulation may have played an important role in reductions of atmospheric CO2 concentrations during the last glaciation, by increasing the carbon storage in the deep Atlantic.

  14. Low-frequency variability of meridional transport in the divergence zone of the North Atlantic subtropical and subpolar gyres. The WOCE section A2; Niederfrequente Variabilitaet meridionaler Transporte in der Divergenzzone des nordatlantischen Subtropen- und Subpolarwirbels. Der WOCE-Schnitt A2

    Energy Technology Data Exchange (ETDEWEB)

    Lorbacher, K.

    2000-07-01

    The subinertial, climate relevant variability of the large-scale ocean circulation in the northern North Atlantic and its integral key parameters such as the advective transports of mass (volume), heat and freshwater are determined from observations alone using the hydrographic data from seven realisations of the so-called '48 N'-section between the English Channel and the Grand Banks of Newfoundland. The data consist of five available sets of the WOCE/A2-section during the Nineties for the years 1993, 1994, 1996, 1997, 1998 and of two previous transatlantic cruises in April of 1957 and 1982. The realisations of the WOCE/A2-section were carried out in the same season (May to July), except for the cruise in October 1994. The '48 N'-section follows the divergence zone of the mainly wind-driven subtropical gyre and the more complex, with respect to the forcing, subpolar gyre. In the central Westeuropean and Newfoundland Basins the section runs a few degrees south of the line of zero wind stress curl (curl{sub z}{tau}). In the West, the WOCE/A2-section turns northwest to cross the boundary current regime perpendicularly. Therefore, this quasi-zonal hydrographic section covers all large-scale circulation elements on the regional scale that contribute essentially to the ocean circulation on the global scale - the Meridional Overturning Circulation (MOC). The transport estimates are given as the sum of the three transport components of a quasi-steady, large-scale ocean circulation: The ageostrophic Ekman-, and the two geostrophic components, the depth-independent, barotropic or Sverdrup- and the baroclinic component. To maintain the mass balance over the plane of the section the compensation of each component is assumed. In the case of the baroclinic component the balance is achieved through a suitable choice for a surface of 'no-motion'. The absolute meridional velocity as a function of the zonal distance along the section and depth is

  15. Mid-Pliocene shifts in ocean overturning circulation and the onset of Quaternary-style climates*

    Science.gov (United States)

    Sarnthein, M.; Prange, M.; Schmittner, A.; Schneider, B.; Weinelt, M.

    2009-01-01

    A major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern Hemisphere Glaciation (NHG) and pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic, that and continue until today. The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3) and 2.7 Ma (glacial MIS G6/4). Various models and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and East Pacific. Each closing event led to enhanced North Atlantic meridional overturning circulation and strengthened the poleward transport of salt and heat (warmings of +2-3°C). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia, which led to enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS), and increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, the closing of CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~1 psu from 3.16-3.00 Ma, right after the first but still reversible attempt of closing the CAS.

  16. Mid-Pliocene shifts in ocean overturning circulation and the onset of Quaternary-style climates

    Science.gov (United States)

    Sarnthein, M.; Bartoli, G.; Prange, M.; Schmittner, A.; Schneider, B.; Weinelt, M.; Andersen, N.; Garbe-Schönberg, D.

    2009-06-01

    A major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG) and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001). The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3) and 2.7 Ma (glacial MIS G6/4) (Lisiecki and Raymo, 2005). Various models (sensu Driscoll and Haug, 1998) and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2-3°C) (Bartoli et al., 2005). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007), which probably led to an enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS), and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~2 psu from 3.25/3.16-3.00 Ma, right after the first but still reversible attempt of closing the CAS.

  17. Mid-Pliocene shifts in ocean overturning circulation and the onset of Quaternary-style climates

    Directory of Open Access Journals (Sweden)

    M. Sarnthein

    2009-06-01

    Full Text Available A major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001. The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3 and 2.7 Ma (glacial MIS G6/4 (Lisiecki and Raymo, 2005. Various models (sensu Driscoll and Haug, 1998 and paleoceanographic records (intercalibrated using orbital age control suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS, deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2–3°C (Bartoli et al., 2005. Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007, which probably led to an enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS, and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC. Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~2 psu from 3.25/3.16–3.00 Ma, right after the first but still reversible attempt of closing the CAS.

  18. Meridional energy transport in the coupled atmosphere-ocean system: Compensation and partitioning

    Science.gov (United States)

    Farneti, R.; Vallis, G. K.

    2012-04-01

    The variability and compensation of the energy transport in the atmosphere and ocean are discussed with a hierarchy of coupled models. A state-of-the-art Coupled Model (GFDL CM2.1), an Intermediate Complexity Climate Model (GFDL ICCM) and a simple Energy Balance Model (EBM) are used in this study. For decadal time scales, a high degree of compensation is found for the transport in the Northern Hemisphere in the Atlantic sector. The variability of the total, or planetary, heat transport (PHT) is much smaller than the variability in either the atmosphere (AHT) or ocean (OHT) alone, a feature sometimes referred to as `Bjerknes compensation'. In the coupled models used, natural decadal variability stems from the Atlantic meridional overturning circulation (AMOC), and variations in the strength of the AMOC tend to lead the variability in the OHT. Furthermore, the PHT is positively correlated with the OHT, implying that the atmosphere is compensating, but imperfectly, for variations in the ocean transport. In the Southern Hemisphere no significant anticorrelation is found between OHT and AHT, consistent with the absence of decadal scale variability in the ocean. For both coupled models, the strongest anticorrelation between transports is found at the period of AMOC variability and decreases as the time scale decreases. Unlike the AHT and AMOC, the AHT and the transport in the oceanic gyres are positively correlated, suggesting that coupling between the wind-driven ocean circulation and the atmosphere militates against long-term variability involving the wind-driven flow. Moisture and sensible heat transports in the atmosphere are also positively correlated at decadal time scales. In the Northern Hemisphere compensation is weaker at low latitudes than at high. This is consistent with the notion that at low latitudes a larger fraction of the oceanic transport is due to the wind-driven warm cell, and the atmospheric and wind-driven oceanic energy transports vary in unison

  19. Summer mean full depth circulation in North Atlantic Ocean along 59.5 N

    Science.gov (United States)

    Gladyshev, Vsevolod; Sokov, Alexey; Gladyshev, Sergey

    2016-04-01

    The large scale oceanic circulation in the North Atlantic is an important part of the climate system. Warm saline upper-ocean waters derived in subtropics release heat into the atmosphere while moving northward as North Atlantic Current and by mixing with colder fresher Arctic waters sink in the subpolar basins therefore originating reverse equatorward flow of cold fresh water. This mechanism, known as Atlantic Meridional Overturning Circulation (MOC) is of fundamental importance in the meridional heat transport. Using data from yearly direct hydrographic measurements at 59.5 N with satellite altimetry data in the period 2009-2015 a mean state of the full-depth summer circulation in the region is estimated. Zonal distribution of the 2009-2015 mean summer velocities across the 59.5 N is obtained using four different data sets from (1) pair of WS 300 kHz LADCPs measurements, (2) ship mounted TRDI OS 38 kHz ADCP measurements, (3) AVISO altimetry data (surface absolute geostrophic velocities), and (4) geostrophic velocities data calculated using CTD measurements. By combining those data mean absolute transport is estimated. Results are compared and analyzed confirming and elaborating previous research. Also assessment of the errors associated with full-depth ADCP profiles is settled. This evaluation allows arguing about certainty of collected data and can be used to improve accuracy of circulation rating.

  20. Tropical Atlantic climate response to different freshwater input in high latitudes with an ocean-only general circulation model

    Science.gov (United States)

    Men, Guang; Wan, Xiuquan; Liu, Zedong

    2016-10-01

    Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation (AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-like SST structure cooling over the North Atlantic and warming over the South Tropical Atlantic in response to the slowdown of the AMOC. Using an ocean-only global ocean model here, an attempt was made to separate the total influence of various AMOC change scenarios into an oceanic-induced component and an atmospheric-induced component. In contrast with previous freshwater-hosing experiments with coupled climate models, the ocean-only modeling presented here shows a surface warming in the whole tropical Atlantic region and the oceanic-induced processes may play an important role in the SST change in the equatorial south Atlantic. Our result shows that the warming is partly governed by oceanic process through the mechanism of oceanic gateway change, which operates in the regime where freshwater forcing is strong, exceeding 0.3 Sv. Strong AMOC change is required for the gateway mechanism to work in our model because only when the AMOC is sufficiently weak, the North Brazil Undercurrent can flow equatorward, carrying warm and salty north Atlantic subtropical gyre water into the equatorial zone. This threshold is likely to be model-dependent. An improved understanding of these issues may have help with abrupt climate change prediction later.

  1. Characteristics of Solar Meridional Flows

    Science.gov (United States)

    Basu, Sarbani; Antia, H. M.

    2011-01-01

    We have done a ring-diagram analysis of MDI full-disc data to determine the properties of solar meridional flow in the outer 2% of the Sun over the solar cycle 23. The meridional flows show a migrating pattern with higher-velocity flows migrating toward the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of the sunspot butterfly diagram and the zonal flows in the shallow layers. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. We also find that the dominant component of the meridional flows during solar maxima was much lower than that during the minima of solar cycles 23 and 24.

  2. Overturning to the oil sector

    International Nuclear Information System (INIS)

    In only 11 months, the Mines and Energy Sector, especially with concerning to oil politics and to Ecopetrol it has given a total overturn, not only new economic conditions settled down to attract investment in the sector, the politics advanced in the activation of the environmental licenses and the consultations with the community. As for Ecopetrol bottom changes were made that guarantee their passive of pensions, disabling this way a fiscal bomb and the countable system was changed that will make that Ecopetrol counts the raw as a capitalization on the part of the Colombians guaranteeing an appropriate return to the petroleum that is given in administration

  3. Origin and fate of the North Atlantic Current at the Mid-Atlantic Ridge

    Science.gov (United States)

    Breckenfelder, Tilia; Myers, Paul G.; Rhein, Monika; Pennelly, Clark; Hu, Xianmin

    2016-04-01

    Warm, salty tropical and subtropical water is brought into the subpolar gyre by the North Atlantic Current (NAC). The NAC is the northward extension of the Gulf Stream and is part of the upper branch of the Atlantic Meridional Overturning Circulation. The warm, salty water is further transported into the Nordic Seas via the Rockall Trough, into the Denmark Strait and, finally into the Labrador Sea, where it plays an important role in the deep water formation process. On its way into the Labrador Sea the water mass increases its density by dissipating heat to the atmosphere and thereby influencing the local climate. To further understand the processes behind warm water transport towards higher latitudes, we start our investigation at the Mid-Atlantic Ridge (MAR). Here, the NAC flows from the western to eastern basin of the North Atlantic and crosses the MAR via the Charlie-Gibbs, Faraday and Maxwell Fracture Zones. The role of the subpolar and subtropical gyre on the different water masses, and their properties, originating or reaching the MAR is studied using the lagrangian tool ARIANE with the 3D velocity fields taken from a 1/12° AGRIF nest set in a regional NEMO configuration. One result of this investigation is that the majority of particles released at the MAR, distributed over the entire water column, recirculate. Most of the remaining particles make their way into the East Greenland Current or turn in the eastern basin towards the south. The influence of the North Atlantic Oscillation (NAO) is investigated by studying the pathways of the NAC and their properties during different NAO phases.

  4. Enhanced warming of the Northwest Atlantic Ocean under climate change

    Science.gov (United States)

    Saba, Vincent S.; Griffies, Stephen M.; Anderson, Whit G.; Winton, Michael; Alexander, Michael A.; Delworth, Thomas L.; Hare, Jonathan A.; Harrison, Matthew J.; Rosati, Anthony; Vecchi, Gabriel A.; Zhang, Rong

    2016-01-01

    The Intergovernmental Panel on Climate Change (IPCC) fifth assessment of projected global and regional ocean temperature change is based on global climate models that have coarse (˜100 km) ocean and atmosphere resolutions. In the Northwest Atlantic, the ensemble of global climate models has a warm bias in sea surface temperature due to a misrepresentation of the Gulf Stream position; thus, existing climate change projections are based on unrealistic regional ocean circulation. Here we compare simulations and an atmospheric CO2 doubling response from four global climate models of varying ocean and atmosphere resolution. We find that the highest resolution climate model (˜10 km ocean, ˜50 km atmosphere) resolves Northwest Atlantic circulation and water mass distribution most accurately. The CO2 doubling response from this model shows that upper-ocean (0-300 m) temperature in the Northwest Atlantic Shelf warms at a rate nearly twice as fast as the coarser models and nearly three times faster than the global average. This enhanced warming is accompanied by an increase in salinity due to a change in water mass distribution that is related to a retreat of the Labrador Current and a northerly shift of the Gulf Stream. Both observations and the climate model demonstrate a robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm-Temperate Slope Water entering the Northwest Atlantic Shelf. Therefore, prior climate change projections for the Northwest Atlantic may be far too conservative. These results point to the need to improve simulations of basin and regional-scale ocean circulation.

  5. No inter-gyre pathway for sea-surface temperature anomalies in the North Atlantic

    Science.gov (United States)

    Foukal, Nicholas P.; Lozier, M. Susan

    2016-04-01

    Recent Lagrangian analyses of surface drifters have questioned the existence of a surface current connecting the Gulf Stream (GS) to the subpolar gyre (SPG) and have cast doubt on the mechanism underlying an apparent pathway for sea-surface temperature (SST) anomalies between the two regions. Here we use modelled Lagrangian trajectories to determine the fate of surface GS water and satellite SST data to analyse pathways of GS SST anomalies. Our results show that only a small fraction of the surface GS water reaches the SPG, the water that does so mainly travels below the surface mixed layer, and GS SST anomalies do not propagate into the SPG on interannual timescales. Instead, the inter-gyre heat transport as part of the Atlantic Meridional Overturning Circulation must be accomplished via subsurface pathways. We conclude that the SST in the SPG cannot be predicted by tracking SST anomalies along the GS.

  6. Influence of prolonged Anomalies in North Atlantic Sea Surface Temperature on Winter Windstorms

    Science.gov (United States)

    Höschel, Ines; Schuster, Mareike; Grieger, Jens; Ulbrich, Uwe

    2016-04-01

    The focus of this presentation is on decadal scale variations in the frequency and in the intensity of mid-latitude winter windstorms. Projections for the end of the next century are often beyond the time horizon of business, thus there is an increasing interest on decadal prediction, especially for infrastructural planning and in the insurance industry. One source of decadal predictability is the Atlantic multidecadal variability (AMV), a change in the sea surface temperature of the North Atlantic, strongly linked to the meridional overturning circulation. Correlation patterns between annual AMV-indices and annual mean of geopotential height at 500 hPa in reanalysis data show an anti-correlation in the North Atlantic. That is, during AMV warm phases the North Atlantic Oscillation (NAO) is more negative. Consequently, AMV should influence the characteristics of winter windstorms at multi-year scales. For the presented investigations a 10-member ensemble of 38-year-long idealized simulations with the atmosphere model ECHAM6 with lower boundary conditions, representing warm and cool phases of the AMV, is used. In the idealized simulations, the anti-correlation between AMV and NAO is well represented. For the identification of winter windstorms an objective wind tracking algorithm based on the exceedance of the local 98th percentile of 10m wind speed is applied. Storms under AMV-warm and AMV-cool conditions will be compared in terms of storm track density and probability distribution of storm characteristics.

  7. Coherent changes of the circulation in the deep North Atlantic from moored transport arrays

    CERN Document Server

    Frajka-Williams, E; Koelling, J; Send, U

    2016-01-01

    In situ boundary arrays have been installed in the North Atlantic to measure the large-scale ocean circulation. Here, we use measurements at the western edge of the North Atlantic at $16^\\circ$N and $26^\\circ$N to investigate low-frequency variations in deep densities and their associated influence on ocean transports. At both latitudes, deep waters (below 1100 dbar) at the western boundary are becoming fresher and less dense. The associated change in geopotential thickness is about $0.15$ $\\mbox{m}^2\\mbox{s}^{-2}$ between 2004-2009 and 2010-2014, with the shift occurring between 2009-2010 and earlier at $26^\\circ$N than $16^\\circ$N. Without a similar density change on the east of the Atlantic, a mid-depth reduction in water density at the west drives an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at $26^\\circ$N and 3.9 Sv at $16^\\circ$N. While these transport anomalies result in an intensifying tendency in the meridional overturning circulation (MOC) ...

  8. A reversal of climatic trends in the North Atlantic since 2005

    Science.gov (United States)

    Robson, Jon; Ortega, Pablo; Sutton, Rowan

    2016-07-01

    In the mid-1990s the North Atlantic subpolar gyre warmed rapidly, which had important climate impacts such as increased hurricane numbers and changes to rainfall over Africa, Europe and North America. Evidence suggests that the warming was largely due to a strengthening of the ocean circulation, particularly the Atlantic Meridional Overturning Circulation. Since the mid-1990s direct and indirect measurements have suggested a decline in the strength of the ocean circulation, which is expected to lead to a reduction in northward heat transport. Here we show that since 2005 a large volume of the upper North Atlantic Ocean has cooled significantly by approximately 0.45 °C or 1.5 × 1022 J, reversing the previous warming trend. By analysing observations and a state-of-the-art climate model, we show that this cooling is consistent with a reduction in the strength of the ocean circulation and heat transport, linked to record low densities in the deep Labrador Sea. The low density in the deep Labrador Sea is primarily due to deep ocean warming since 1995, but a long-term freshening also played a role. The observed upper ocean cooling since 2005 is not consistent with the hypothesis that anthropogenic aerosols directly drive Atlantic temperatures.

  9. Low-frequency storminess signal at Bermuda linked to cooling events in the North Atlantic region

    Science.gov (United States)

    Hengstum, Peter J.; Donnelly, Jeffrey P.; Kingston, Andrew W.; Williams, Bruce E.; Scott, David B.; Reinhardt, Eduard G.; Little, Shawna N.; Patterson, William P.

    2015-02-01

    North Atlantic climate archives provide evidence for increased storm activity during the Little Ice Age (150 to 600 calibrated years (cal years) B.P.) and centered at 1700 and 3000 cal years B.P., typically in centennial-scale sedimentary records. Meteorological (tropical versus extratropical storms) and climate forcings of this signal remain poorly understood, although variability in the North Atlantic Oscillation (NAO) or Atlantic Meridional Overturning Circulation (AMOC) are frequently hypothesized to be involved. Here we present records of late Holocene storminess and coastal temperature change from a Bermudian submarine cave that is hydrographically circulated with the coastal ocean. Thermal variability in the cave is documented by stable oxygen isotope values of cave benthic foraminifera, which document a close linkage between regional temperature change and NAO phasing during the late Holocene. However, erosion of terrestrial sediment into the submarine cave provides a "storminess signal" that correlates with higher-latitude storminess archives and broader North Atlantic cooling events. Understanding the driver of this storminess signal will require higher-resolution storm records to disentangle the contribution of tropical versus extratropical cyclones and a better understanding of cyclone activity during hemispheric cooling periods. Most importantly, however, the signal in Bermuda appears more closely correlated with proxy-based evidence for subtle AMOC reductions than NAO phasing.

  10. Strong middepth warming and weak radiocarbon imprints in the equatorial Atlantic during Heinrich 1 and Younger Dryas

    Science.gov (United States)

    Weldeab, Syee; Friedrich, Tobias; Timmermann, Axel; Schneider, Ralph R.

    2016-08-01

    We present a benthic foraminiferal multiproxy record of eastern equatorial Atlantic (EEA) middepth water (1295 m) covering the last deglacial. We show that EEA middepth water temperatures were elevated by 3.9 ± 0.5°C and 5.2 ± 1.2°C during Heinrich event 1 (H1) and Younger Dryas (YD), respectively. The radiocarbon content of the EEA middepth during H1 and YD is relatively low and comparable to the values of the pre-H1 episode and Bølling-Allerød, respectively. A transient Earth system model simulation, which mimics the observed deglacial Atlantic Meridional Overturning Circulation (AMOC) history, qualitatively reproduces the major features of the EEA proxy records. The simulation results suggest that fresh water-induced weakening of the AMOC leads to a vertical shift of the horizon of Southern Ocean-sourced water and a stronger influence of EEA sea surface temperatures via mixing. Our findings reaffirm the lack of a distinctive signature of radiocarbon depletion and therefore do not support the notion of interhemispheric exchanges of strongly radiocarbon-depleted middepth water across the tropical Atlantic during H1 and YD. Our temperature reconstruction presents a critical zonal and water depth extension of existing tropical Atlantic data and documents a large-scale and basin-wide warming across the thermocline and middepth of the tropical Atlantic during H1 and YD. Significant difference in the timing and pace of H1 middepth warming between tropical Atlantic and North Atlantic likely points to a limited role of the tropical Atlantic middepth warming in the rapid heat buildup in the North Atlantic middepth.

  11. Southwest Atlantic water mass evolution during the last deglaciation

    Science.gov (United States)

    Lund, D. C.; Tessin, A. C.; Hoffman, J. L.; Schmittner, A.

    2015-05-01

    The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5-17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate 13C-depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13C-depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.

  12. Potential for seasonal prediction of Atlantic sea surface temperatures using the RAPID array at 26°N

    Science.gov (United States)

    Duchez, A.; Courtois, P.; Harris, E.; Josey, S. A.; Kanzow, T.; Marsh, R.; Smeed, D. A.; Hirschi, J. J.-M.

    2016-05-01

    The Atlantic meridional overturning circulation (AMOC) plays a critical role in the climate system and is responsible for much of the meridional heat transported by the ocean. In this paper, the potential of using AMOC observations from the 26°N RAPID array to predict North Atlantic sea surface temperatures is investigated for the first time. Using spatial correlations and a composite method, the AMOC anomaly is used as a precursor of North Atlantic sea-surface temperature anomalies (SSTAs). The results show that the AMOC leads a dipolar SSTA with maximum correlations between 2 and 5 months. The physical mechanism explaining the link between AMOC and SSTA is described as a seesaw mechanism where a strong AMOC anomaly increases the amount of heat advected north of 26°N as well as the SSTA, and decreases the heat content and the SSTA south of this section. In order to further understand the origins of this SSTA dipole, the respective contributions of the heat advected by the AMOC versus the Ekman transport and air-sea fluxes have been assessed. We found that at a 5-month lag, the Ekman component mainly contributes to the southern part of the dipole and cumulative air-sea fluxes only explain a small fraction of the SSTA variability. Given that the southern part of the SSTA dipole encompasses the main development region for Atlantic hurricanes, our results therefore suggest the potential for AMOC observations from 26°N to be used to complement existing seasonal hurricane forecasts in the Atlantic.

  13. An experimental study of the Atlantic variability on interdecadal timescales

    Directory of Open Access Journals (Sweden)

    M. Vincze

    2012-06-01

    Full Text Available A series of laboratory experiments has been carried out to model the basic dynamics of the multidecadal variability observed in North Atlantic sea surface temperature (SST records. According to the minimal numerical sector model introduced by te Raa and Dijkstra (2002, the three key components to excite such a low-frequency variability are rotation, meridional temperature gradient and additive thermal noise in the surface heat forcing. If these components are present, periodic perturbations of the overturning background flow are excited, leading to thermal Rossby mode like propagation of anomalous patches in the SST field. Our tabletop scale setup was built to capture this phenomenon, and to test whether the aforementioned three components are indeed sufficient to generate a low-frequency variability in the system. The results are compared to those of the numerical models, as well as to oceanic SST reanalysis records. To the best of our knowledge, the experiment described here is the very first to investigate the dynamics of the North Atlantic multidecadal variability in a laboratory-scale setup.

  14. Latest Quaternary palaeoceanographic change in the eastern North Atlantic based upon a dinoflagellate cyst event ecostratigraphy.

    Science.gov (United States)

    Harland, Rex; Polovodova Asteman, Irina; Morley, Audrey; Morris, Angela; Harris, Anthony; Howe, John A

    2016-05-01

    The analyses of dinoflagellate cyst records, from the latest Quaternary sediments recovered from DSDP Core 610A taken on the Feni Ridge in the southern Rockall Trough, and part of core MD01-2461 on the continental margin of the Porcupine Seabight in the eastern North Atlantic Ocean, has provided evidence for significant oceanographic change encompassing the Last Glacial Maximum (LGM) and part of the Holocene. This together with other published records has led to a regional evaluation of oceanographic change in the eastern North Atlantic over the past 68 ka, based upon a distinctive dinoflagellate event ecostratigraphy. These changes reflect changes in the surface waters of the North Atlantic Current (NAC), and perhaps the deeper thermohaline Atlantic Meridional Overturning Circulation (AMOC), driving fundamental regime changes within the phytoplanktonic communities. Three distinctive dinoflagellate cyst associations based upon both factor and cluster analyses have been recognised. Associations characterised by Bitectatodinium tepikiense (between 61.1 ± 6.2 to 13.4 ± 1.1 ka BP), Nematosphaeropsis labyrinthus (between 10.5 ± 0.3 and 11.45 ± 0.8 ka. BP), and the cyst of Protoceratium reticulatum (between 8.5 ± 0.9 and 5.2 ± 1.3 ka. BP) indicate major change within the eastern North Atlantic oceanography. The transitions between these changes occur over a relatively short time span (c.1.5 ka), given our sampling resolution, and have the potential to be incorporated into an event stratigraphy through the latest Quaternary as recommended by the INTIMATE (INTegrating Ice core, MArine and TErrestrial records) group. The inclusion of a dinoflagellate cyst event stratigraphy would highlight changes within the phytoplankton of the North Atlantic Ocean as a fully glacial world changed to our present interglacial. PMID:27441285

  15. Tightly linked zonal and meridional sea surface temperature gradients over the past five million years

    Science.gov (United States)

    Fedorov, Alexey V.; Burls, Natalie J.; Lawrence, Kira T.; Peterson, Laura C.

    2015-12-01

    The climate of the tropics and surrounding regions is defined by pronounced zonal (east-west) and meridional (equator to mid-latitudes) gradients in sea surface temperature. These gradients control zonal and meridional atmospheric circulations, and thus the Earth’s climate. Global cooling over the past five million years, since the early Pliocene epoch, was accompanied by the gradual strengthening of these temperature gradients. Here we use records from the Atlantic and Pacific oceans, including a new alkenone palaeotemperature record from the South Pacific, to reconstruct changes in zonal and meridional sea surface temperature gradients since the Pliocene, and assess their connection using a comprehensive climate model. We find that the reconstructed zonal and meridional temperature gradients vary coherently over this time frame, showing a one-to-one relationship between their changes. In our model simulations, we systematically reduce the meridional sea surface temperature gradient by modifying the latitudinal distribution of cloud albedo or atmospheric CO2 concentration. The simulated zonal temperature gradient in the equatorial Pacific adjusts proportionally. These experiments and idealized modelling indicate that the meridional temperature gradient controls upper-ocean stratification in the tropics, which in turn controls the zonal gradient along the equator, as well as heat export from the tropical oceans. We conclude that this tight linkage between the two sea surface temperature gradients posits a fundamental constraint on both past and future climates.

  16. Meridional Circulation in the Sun

    Science.gov (United States)

    Duvall, T. L., Jr.; Hanasoge, S. M.

    2008-01-01

    Measuring the depth variation of the meridional flows is important for understanding the solar cycle, at least according to a number of dynamo models. While attempting to extend the early observations of Giles (1999; Ph. D. thesis, Stanford Univ.) of time-distance measurements of flow, we have stumbled upon some systematic errors that can affect these measurements: 1) the additional distance traveled by radiation coming from points away from disk center causes an apparent 'shrinking' Sun, that is an apparent flow towards the disk center, 2) in measurements away from the central longitude, the rotation signal can leak into meridional flow signals, and 3) in measurements of the north-south mean travel times along the equator, a spurious error of 6 sec travel time is seen. That the signal is spurious is confirmed by observing half the time with the image rotated 180 degrees. Although this is an effect with mean travel times and not differences, it still seems useful to understand it. Attempts to understand and overcome these systematic problems will be presented. Forward modeling has been done using ray theory to test the sensitivity of travel times to various models.

  17. Extratropical North Atlantic SST influence on Sahel rainfall

    Science.gov (United States)

    Liu, Y.; Chiang, J. C.

    2013-12-01

    We present evidence suggesting that the late 1960's Sahel drought was linked to an abrupt cooling in the extratropical North Atlantic, whose influence was then propagated to the Sahel by atmospheric teleconnection. Such linkages have been observed in paleoclimate during abrupt climate changes of the last glacial period. They have also occurred in coupled model simulations of Atlantic meridional overturning circulation (AMOC) slowdown, the latter being the leading cause of said paleoclimate abrupt changes. The AMOC-slowdown simulations show a characteristic global pattern of climate changes, including a northern hemispheric-wide cooling and increased surface pressure, and weakening of the West African and Asian monsoons. We show that an observed northern-hemispheric pattern of changes, resembling the AMOC slowdown, occurred during the late 1960's, co-incident with the Sahel drought. A combined principal component analysis of 20th century surface temperature, sea level pressure and precipitation extracts a leading mode whose spatial pattern closely resemble the impacts of AMOC slowdown. A similar analysis of AMIP-type simulations forced by 20th century observed forcings shows a similar result, suggesting that the origins of the climate change reside in SST changes, in particular over extratropical North Atlantic. Taken together, the results suggests the influence of extratropical North Atlantic cooling on the 20th century Sahel drought, and a teleconnection pathway through surface/tropospheric cooling. Motivated by our observational result, we investigated atmospheric teleconnection mechanisms of extratropical North Atlantic cooling in an atmospheric general circulation model (GCM) coupled with slab ocean. Our results indicate the central role of tropospheric cooling in communicating the influence on the Sahel. We explicitly show this using regional climate model simulation of the Sahel, with air temperature and associated humidity anomalies from the GCM simulation

  18. Meridional Considerations of the Centrifugal Compressor Development

    Directory of Open Access Journals (Sweden)

    C. Xu

    2012-01-01

    Full Text Available Centrifugal compressor developments are interested in using optimization procedures that enable compressor high efficiency and wide operating ranges. Recently, high pressure ratio and efficiency of the centrifugal compressors require impeller design to pay attention to both the blade angle distribution and the meridional profile. The geometry of the blades and the meridional profile are very important contributions of compressor performance and structure reliability. This paper presents some recent studies of meridional impacts of the compressor. Studies indicated that the meridional profiles of the impeller impact the overall compressor efficiency and pressure ratio at the same rotational speed. Proper meridional profiles can improve the compressor efficiency and increase the overall pressure ratio at the same blade back curvature.

  19. Late Circulation Response in the Deep Southwest Atlantic during Termination I

    Science.gov (United States)

    Lund, D. C.; Tessin, A. C.; Schmittner, A.

    2014-12-01

    The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5-17.5 kyr BP) may have been driven by release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean liberates 13C-depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that watermass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of twelve cores from the Brazil Margin. We show that records below 2300 m remained 13C-depleted until 15 kyr BP or later, indicating the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr BP, in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is ill advised on glacial terminations. New data from 2700-3000 m also show the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ13C minima are consistent with conservative behavior and were most likely driven by an abrupt decrease in the δ13C of northern component water. Low δ13C at the Brazil Margin coincided with a ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic Meridional Overturning Circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.

  20. Limited response of peatland CH4 emissions to abrupt Atlantic Ocean circulation changes in glacial climates

    Directory of Open Access Journals (Sweden)

    P. O. Hopcroft

    2013-06-01

    Full Text Available Ice-core records show that abrupt Dansgaard-Oeschger climatic warming events of the last glacial period were accompanied by large increases in the atmospheric CH4 concentration (up to 200 ppbv. These abrupt changes are generally regarded as arising from the effects of changes in the Atlantic Ocean meridional overturning circulation and the resultant climatic impact on natural CH4 sources, in particular wetlands. We use two different ecosystem models of wetland CH4 emissions to simulate northern CH4 sources forced with coupled general circulation model simulations of five different time periods during the last glacial to investigate the potential influence of abrupt ocean circulation changes on atmospheric CH4 levels during D-O events. The simulated warming over Greenland of 7–9 °C in the different time-periods is at the lower end of the range of 11–15 °C derived from ice-cores, but is associated with strong impacts on the hydrological cycle, especially over the North Atlantic and Europe during winter. We find that although the sensitivity of CH4 emissions to the imposed climate varies significantly between the two ecosystem emissions models, the model simulations do not reproduce sufficient emission changes to satisfy ice-core observations of CH4 increases during abrupt events. This suggests that alternative scenarios of climatic change could be required to explain the abrupt glacial CH4 variations.

  1. Meridional Considerations of the Centrifugal Compressor Development

    OpenAIRE

    Xu, C.; Amano, R. S.

    2012-01-01

    Centrifugal compressor developments are interested in using optimization procedures that enable compressor high efficiency and wide operating ranges. Recently, high pressure ratio and efficiency of the centrifugal compressors require impeller design to pay attention to both the blade angle distribution and the meridional profile. The geometry of the blades and the meridional profile are very important contributions of compressor performance and structure reliability. This paper presents some ...

  2. Meridional transport in the Venusian atmosphere

    Science.gov (United States)

    Widemann, Thomas; Mota Machado, Pedro; Peralta, Javier; Marcq, Emmanuel; Helbert, Joern; Smrekar, Suzanne

    2016-10-01

    Atmospheric superrotation on Venus and to a lesser extent, on Titan, is thought to be maintained by opposing transfers of angular momentum between the mean meridional circulation and large-scale planetary waves. The details of this transfer depend on the presence of wave-generating dynamical instabilities and on the strength and direction of the meridional flow. Observational constraints have been gathered over the course of the Venus Express mission. The upper cloud exhibits global meridional motions which are consistent with the upper branch of a Hadley cell circulation (Sánchez-Lavega et al., 2008 ; Hueso et al., 2012). Peralta et al. (2012) determined the meridional structure for the amplitude of the diurnal tide affecting the meridional component of the wind, while VMC cloud-tracked features allowed to detect a diurnal component peaking in the early afternoon (Khatuntsev et al., 2013).New measurements of the meridional flow were simultaneously gathered by VEx/VIRTIS-M and CFHT/ESPaDOnS from the ground with a significant temporal and spatial overlap in April 2014. A symmetrical, poleward meridional Hadley flow is evidenced at cloud top in both hemispheres peaking at v = 22.5 ± 15.5 ms-1 at 9-10am near 40°N-S with a sharp drop poleward of 50° (Machado et al., submitted).The lower cloud meridional motions are less organized with some cloud features moving with intense northwards and southwards motions up to v = ±15 m s-1 but, on average, with almost null global meridional motions at all latitudes. Due to the unfavourable viewing geometry and poor UV contrast of polar clouds, only a fraction of the total wind measurements have been reported for the polar regions. Existing data indicate a circumpolar circulation close to solid-body rotation. The VEM instrument on board VERITAS (Smrekar et al., 2016 ; Helbert et al., 2016) will allow for a comprehensive study of lower cloud climatology spanning at least one Venusian year. Three filter bands of VEM at 1.195, 1

  3. A volcanically triggered regime shift in the subpolar North Atlantic ocean as a possible origin of the Little Ice Age

    Directory of Open Access Journals (Sweden)

    C. F. Schleussner

    2012-12-01

    Full Text Available Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly (MCA, succeeded by the Little Ice Age (LIA are of exceptional importance. The origin of these regional climate anomalies remains however a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully-coupled model Climber 3α forced with stochastically reconstructed wind fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea-ice – ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions leads to a spin-up of the subpolar gyre (SPG and a weakened Atlantic Meridional Overturning Circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for past and future climate.

  4. A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age

    Directory of Open Access Journals (Sweden)

    C. F. Schleussner

    2013-06-01

    Full Text Available Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly succeeded by the Little Ice Age are of exceptional importance. The origin of these regional climate anomalies remains a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully coupled model of intermediate complexity Climber 3α forced with stochastically reconstructed wind-stress fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea ice–ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions in our model leads to a spin-up of the subpolar gyre and a weakened Atlantic meridional overturning circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for understanding the dynamics of past and future climate.

  5. Simulated cold events in the northern North Atlantic during the last millennium

    Science.gov (United States)

    Moreno-Chamarro, Eduardo; Zanchettin, Davide; Lohmann, Katja; Jungclaus, Johann

    2014-05-01

    southward fresh water transport through the Denmark Strait reduce the sea-surface salinity and trigger reduced deep ocean mixing in the Labrador Sea. The consequent weakening of the Atlantic Meridional Overturning Circulation peaks well after the minimum of SST anomalies, allowing us to exclude, in contrast to former suggestions, the overturning circulation as a major triggering factor of such cold events. Cold SSTs and the weakened atmospheric circulation over the Labrador Sea force reduced ocean heat losses in the ocean basin, being the main cause of the cold anomalies over Greenland. The simulated climatic anomaly further entails sea-ice expansion along the eastern coast of Greenland. This would have reduced the trade and fishery in the region, thus increasing the stress on the Norse society during the time of its settlement demise over Greenland.

  6. Meridional circulation in the Sun and stars

    CERN Document Server

    Kitchatinov, L L

    2016-01-01

    Mean-field hydrodynamics advanced to clear explanations for the origin and properties of the global meridional flow in stellar convection zones. Qualitative arguments and analysis of basic equations both show that the meridional circulation is driven by non-conservative centrifugal and buoyancy forces and results from a slight disbalance between these two drivers. The deviations from the thermal wind balance are relatively large near the boundaries of convection zones. Accordingly, the meridional flow attains its largest velocities in the boundary layers and decreases inside the convection zone. This picture, however, is neither supported nor dismissed by the conflicting results of recent helioseismic soundings or 3D numerical experiments. The relevant physics of the differential temperature and its possible relation to the solar oblateness are briefly discussed.

  7. A Decadal-Scale Teleconnection between the North Atlantic Oscillation and Subtropical Eastern Australian Rainfall

    Science.gov (United States)

    Li, J.

    2015-12-01

    The time series of twentieth-century subtropical eastern Australian rainfall (SEAR) shows evident fluctuations over decadal to multidecadal time scales. Using observations from the period 1900-2013, it was found that SEAR is connected to the North Atlantic Oscillation (NAO) over decadal time scales, with the NAO leading by around 15 yr. The physical mechanism underlying this relationship was investigated. The NAO can have a delayed impact on sea surface temperature (SST) fluctuations in the subpolar Southern Ocean (SO), and these SST changes could in turn contribute to the decadal variability in SEAR through their impacts on the Southern Hemisphere atmospheric circulation. This observed lead of the NAO relative to SO SST and the interhemispheric SST seesaw mechanism are reasonably reproduced in a long-term control simulation of an ocean-atmosphere coupled model. The NAO exerts a delayed effect on the variation of Atlantic meridional overturning circulation that further induces seesaw SST anomalies in the subpolar North Atlantic and SO. With evidence that the NAO precedes SEAR decadal variability via a delayed SO bridge, a linear model for SEAR decadal variability was developed by combination of the NAO and Pacific decadal oscillation (PDO). The observed SEAR decadal variability is considerably well simulated by the linear model, and the relationship between the simulation and observation is stable. SEAR over the coming decade may increase slightly, because of the recent NAO weakening and the return of negative PDO phase.The time series of twentieth-century subtropical eastern Australian rainfall (SEAR) shows evident fluctuations over decadal to multidecadal time scales. Using observations from the period 1900-2013, it was found that SEAR is connected to the North Atlantic Oscillation (NAO) over decadal time scales, with the NAO leading by around 15 yr. The physical mechanism underlying this relationship was investigated. The NAO can have a delayed impact on sea

  8. Northern tropical Atlantic climate since late Medieval times from Northern Caribbean coral geochemistry

    Science.gov (United States)

    Kilbourne, K. H.; Xu, Y.

    2015-12-01

    Paleoclimate reconstructions of different global climate modes over the last 1000 years provide the basis for testing the relative roles of forced and unforced variability climate system, which can help us improve projections of future climate change. The Medieval Climate Anomaly (MCA) has been characterized by a combination of persistent La Niña-like conditions, a positive North Atlantic Oscillation (+NAO), and increased Atlantic Meridional Overturning Circulation (AMOC). The northern tropical Atlantic is sensitive to each of these climate patterns, but not all of them have the same regional fingerprint in the modern northern tropical Atlantic. The relative influence of different processes related to these climate patterns can help us better understand regional responses to climate change. The regional response of the northern tropical Atlantic is important because the tropical Atlantic Ocean is a large source of heat and moisture to the global climate system that can feedback onto global climate patterns. This study presents new coral Sr/Ca and δ18O data from the northern tropical Atlantic (Anegada, British Virgin Islands). Comparison of the sub-fossil corals that grew during the 13th and 14th Centuries with modern coral geochemical data from this site indicates relatively cooler mean conditions with a decrease in the oxygen isotopic composition of the water consistent with lower salinities. Similar average annual cycles between modern and sub-fossil Sr/Ca indicate no change in seasonal temperature range, but a difference in the relative phasing of the δ18O seasonal cycles indicates that the fresher mean conditions may be due to a more northerly position of the regional salinity front. This localized response is consistent with some, but not all of the expected regional responses to a La Niña-like state, a +NAO state, and increased AMOC. Understanding these differences can provide insight into the relative importance of advection versus surface fluxes for

  9. New direct estimates of Iceland-Scotland Overflow Water transport through the Charlie-Gibbs Fracture Zone and its relationship to the North Atlantic Current

    Science.gov (United States)

    Bower, Amy; Furey, Heather; Xu, Xiaobiao

    2015-04-01

    Detailed observations of the pathways, transports and water properties of dense overflows associated with the Atlantic Meridional Overturning Circulation (AMOC) provide critical benchmarks for climate models and mixing parameterizations. A recent two-year time series from eight moorings offers the first long-term simultaneous observations of the hydrographic properties and transport of Iceland-Scotland Overflow Water (ISOW) flowing westward through the Charlie-Gibbs Fracture Zone (CGFZ), a major deep gap in the Mid-Atlantic Ridge (MAR) connecting the eastern and western basins of the North Atlantic. In addition, current meters up to 500-m depth and satellite altimetry allow us to investigate the overlying North Atlantic Current (NAC) as a source of ISOW transport variability. Using the isohaline 34.94 to define the ISOW layer, the two year mean and standard deviation of ISOW transport was -1.7 ± 1.5 Sv, compared to -2.4 ± 3.0 Sv reported by Saunders for a 13-month period in 1988-1989 using the same isohaline. Differences in the two estimates are partly explained by limitations of the Saunders array, but more importantly reflect the strong low-frequency variability in ISOW transport through CGFZ (which includes complete reversals). Both the observations and output from a multi-decadal simulation of the North Atlantic using the Hybrid Coordinate Ocean Model (HYCOM) forced with interannually varying wind and buoyancy fields indicate a strong positive correlation between ISOW transport and the strength of the NAC through the CGFZ. This result raises new questions regarding the interaction of the upper and lower limbs of the AMOC, downstream propagation of ISOW transport variability in the Deep Western Boundary Current and alternative pathways of ISOW across the MAR.

  10. Evolution of the deep Atlantic water masses since the last glacial maximum based on a transient run of NCAR-CCSM3

    Science.gov (United States)

    Marson, Juliana M.; Mysak, Lawrence A.; Mata, Mauricio M.; Wainer, Ilana

    2016-08-01

    During the last deglaciation (from approximately 21 to 11 thousand years ago), the high latitudes of the Atlantic Ocean underwent major changes. Besides the continuous warming, the polar and subpolar ocean surface received a large amount of meltwater from the retracting ice sheets. These changes in temperature and salinity affected deep waters, such as the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW), which are formed in the Southern Ocean and in the northern North Atlantic, respectively. In this study, we present the evolution of the physical properties and distribution of the AABW and the NADW since the last glacial maximum using the results of a transient simulation with NCAR-CCSM3. In this particular model scenario with a schematic freshwater forcing, we find that modern NADW, with its characteristic salinity maximum at depth, was absent in the beginning of the deglaciation, while its intermediate version—Glacial North Atlantic Intermediate Water (GNAIW)—was being formed. GNAIW was a cold and relatively fresh water mass that dominated intermediate depths between 60 and 20°N. At this time, most of the deep and abyssal Atlantic basin was dominated by AABW. Within the onset of the Bølling-Allerød period, at nearly 15 thousand years ago (ka), GNAIW expanded southwards when the simulated Meridional Overturning Circulation overshoots. The transition between GNAIW and NADW ocurred after that, when AABW was fresh enough to allow NADW to sink deeper in the water column. When the NADW appears (~11 ka), AABW retracts and is constrained to lie near the bottom.

  11. Atlantic forcing of Amazonian climates in the last ice age

    Science.gov (United States)

    Bush, M. B.; Mosblech, N.; Valencia, B. G.; Hodell, D. A.; Gosling, W. D.; Van Calsteren, P. W.; Thomas, L. E.; Curtis, J. H.

    2011-12-01

    An absence of study sites means that the relative influence of orbitally driven presession cycles and millenial scale variability upon ice-age Amazonian precipitation is unknown. Here we present a continuous isotopic (δO18 and C13) record spanning the period from ~93-8 ka, from the aseasonal forests of Amazonian Ecuador. The variability in δO18 depletion is probably related to the relative strength of evapotranspired moisture (less depleted) and tropical Atlantic moisture carried across the basin by the South American Low Level Jet (more depleted). Times of strengthened South American Low Level Jet probably correspond to increased overall moisture availability and hence elevated precipitation. The occurrence of markedly depleted δO18 signatures during Heinrich events suggests a strong influence of the Atlantic Ocean on this system, and that these northern hemispheric stadials induced wet episodes in western Amazonia. Weakening of the Atlantic Meridional Overturning Circulation (AMOC) has been suggested to strengthen the South American Low Level Jet. The isotopic records reveal strong cohesion with previously published records from southern Brazil. A precessional influence amplifies the north Atlantic signal between c. 93 ka and 50 ka. However, after c. 50 ka the precessional signal weakens, perhaps sugesting that at a critical size the Laurentide ice mass exerted a strong influence on Neotropical climates suppressing the weaker forcing associated with precession. Contrary to long-standing expectation, the Last Glacial Maximum (21 ± 2 ka) does not stand out as time of aridity in this record. However, between c. 35 ka and 18 ka there is a drift toward less depleted rainfall. One hypothesis to account for this observation is that the climate was becoming more seasonal as the Intertropical Convergence Zone (ITCZ) formed further south than its modern location. The resulting weakened influence of the South American Summer Monsoon (SASM) would probably reduce wet

  12. Modelling the Seasonal Overturning Circulation in the Red Sea

    KAUST Repository

    Yao, Fengchao

    2015-04-01

    The overturning circulation in the Red Sea exhibits a distinct seasonally reversing pattern and is studied using 50-year, high-resolution MIT general circulation model simulations. The seasonal water exchange in the Strait of Bab el Mandeb is successfully simulated, and the structures of the intruding subsurface Gulf of Aden intermediate water are in good agreement with summer observations in 2011. The model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the Gulf of Aden resulting from remote winds in the Arabian Sea and an upward surface slope from the Red Sea to the Gulf of Aden set up by local surface winds in the Red Sea. For the winter overturning circulation, the climatological model mean results suggest that the surface inflow intensifies in a western boundary current in the southern Red Sea that switches to an eastern boundary current north of 24°N. The overturning is accomplished through a cyclonic recirculation and a cross-basin overturning circulation in the northern Red Sea, with major sinking occurring along a narrow band of width about 20 km along the eastern boundary and weaker upwelling along the western boundary. The northward pressure gradient force, strong vertical mixing, and horizontal mixing near the boundary are the essential dynamical components in the model\\'s winter overturning circulation.

  13. Construction of the adjoint MIT ocean general circulation model and application to Atlantic heat transport sensitivity

    Science.gov (United States)

    Marotzke, Jochem; Giering, Ralf; Zhang, Kate Q.; Stammer, Detlef; Hill, Chris; Lee, Tong

    1999-12-01

    We first describe the principles and practical considerations behind the computer generation of the adjoint to the Massachusetts Institute of Technology ocean general circulation model (GCM) using R. Giering's software tool Tangent-Linear and Adjoint Model Compiler (TAMC). The TAMC's recipe for (FORTRAN-) line-by-line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of the output of a model to its input. Then, the sensitivity of 1993 annual mean heat transport across 29°N in the Atlantic, to the hydrography on January 1, 1993, is calculated from a global solution of the GCM. The "kinematic sensitivity" to initial temperature variations is isolated, showing how the latter would influence heat transport if they did not affect the density and hence the flow. Over 1 year the heat transport at 29°N is influenced kinematically from regions up to 20° upstream in the western boundary current and up to 5° upstream in the interior. In contrast, the dynamical influences of initial temperature (and salinity) perturbations spread from as far as the rim of the Labrador Sea to the 29°N section along the western boundary. The sensitivities calculated with the adjoint compare excellently to those from a perturbation calculation with the dynamical model. Perturbations in initial interior salinity influence meridional overturning and heat transport when they have propagated to the western boundary and can thus influence the integrated east-west density difference. Our results support the notion that boundary monitoring of meridional mass and heat transports is feasible.

  14. Simplified overturn stability monitoring of agricultural tractors.

    Science.gov (United States)

    Nichol, C I; Sommer, H J; Murphy, D J

    2005-02-01

    Agricultural tractors are the most common source of farm work fatalities in the U.S., with overturns the most common type of incident. For the year 2001, there were 15 tractor-related fatalities in Pennsylvania, 9 of which were due to tractor rollover. A new device using low-cost sensors and microcomputers was developed around a simplified mathematical model of an agricultural tractor to inform the operator of potential tractor instability. This device communicates the current rollover potential, along with a recent history of rollover potential, to the operator of the tractor via a simple bar-graph display. The device uses a single-chip accelerometer to sense the current rollover potential and a small microprocessor to analyze the accelerometer data, compensate for variations due to temperature, and then send this information to a visual display. The use of these low-cost "off the shelf" components enabled the fabrication of a very inexpensive sensor system. Because agricultural tractors have a long service life, it was important to make the device low cost and flexible. This could enable it to be sold as an aftermarket add-on for a variety of tractor models. The device is also capable of interfacing with newer on-board tractor systems via a CAN bus to make it more attractive to tractor manufacturers who may want to incorporate this device into new models. Work is continuing on the development of an improved display to inform the tractor operator of possible instability, including display ergonomic studies, investigation of threshold levels for alerting an operator of potential instability, and investigation into audible warning signals.

  15. Trends of anthropogenic CO2 storage in North Atlantic water masses

    Directory of Open Access Journals (Sweden)

    P. Lherminier

    2010-01-01

    Full Text Available A high-quality inorganic carbon system database spanning over three decades (1981–2006 and comprising 13 cruises has allowed applying the φCT° method and coming up with accurate estimates of the anthropogenic CO2 (Cant stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like Cant, into deeper ocean layers and confer this region an added oceanographic interest from the point of view of air-sea CO2 exchanges. Commonly, a tendency for decreasing Cant storage rates towards the deep layers has been observed. In the Iberian Basin, the deep waters (North Atlantic Deep Water have low Cant values and negligible Cant storage rates, while the North Atlantic Central Water in the upper layers shows the largest Cant concentrations and capacity to increase its storage on a yearly basis (1.13±0.14 μmol kg−1 yr−1. This unmatched Cant storage capacity of the warm upper limb of the Meridional Overturning Circulation weakens towards the Irminger basin (0.68±0.06 μmol kg−1 yr−1 due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather homogeneous Cant storage rates (between 0.33 and 0.45 μmol kg−1 yr−1, whereas in the Iceland basin these layers seem to have been less affected by Cant. The Cant storage rates in the study region during the 1991–1997 high NAO (North Atlantic Oscillation phase are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO2has taken place during the present decade. The changes in deep-water ventilation together with a detrimental renewal of the main water masses are likely the main driving processes causing this weakening of the North Atlantic CO2sink.

  16. Temperature Oscillation from the Western Equatorial Atlantic in the Last 40000 Years

    Science.gov (United States)

    Santos, T. P.; Albuquerque, A.; Barbosa, C. F.; Lessa, D.; Trond, D.; Belem, A. L.

    2011-12-01

    The Atlantic Meridional Overturning Circulation (AMOC), through the North Brazil Current (NBC), has a significant role for the budget of heat and salt of the North Atlantic and changes on its strength are known to have profound impacts on global climate. Studies based on modeling suggested that, from annual to multi-decadal time scales, a slowdown of AMOC causes a deepening of the thermocline in the tropical Atlantic. In this work, we show a surface temperature oscillation reconstruction based on Modern Analog Technique (using the software Paleoanalogs 3.0.) employing planktonic foraminifera assemblage from a multicore collected on Western Equatorial Atlantic for the last 40 kyr. The main species were the shallow and warm dwelling Globigerinoides ruber (pink and white forms), G. sacculifer and Globigerinella siphonifera followed by the deep dwelling Globorotalia truncatulinoides (left and right coiling) and Globorotalia menardii. Our surface temperature reconstruction evidence sea surface temperatures around 2 degree Celsius colder than present with a pronounced warming phase during the Last Glacial Maximum. The productivity-related species Globigerinita glutinata and Neogloboquadrina dutertrei presented a strong reduction in number of individuals at the end of this period as well as at early-Holocene, probably due to stratification of water column. The thermocline-dwelling Globorotalia menardii was poorly represented during the glacial period due to thermocline deepening on the region, but it reached higher percentages drive by early-Holocene oceanic ventilation. This warming is synchronous with increase intensity of SE trade winds, producing more humid conditions in northeast Brazil hinterland and with prolonged retroflection of the North Brazil Current, which reduced the cross-equatorial transport of heat and salinity. Moreover, a warm South Atlantic Ocean resulted in a southward migration of the ITCZ and contributed for the curtailment of cross-equatorial heat

  17. Meridional circulation in turbulent protoplanetary disks

    CERN Document Server

    Fromang, Sebastien; Masset, Frederic

    2011-01-01

    Based on viscous disk theory, a number of recent studies have suggested the existence of a large scale meridional circulation in protoplanetary disks. Such a flow could account for the presence of crystalline silicates, among which Calcium and Aluminium-rich Inclusions (CAIs), at large distances from the sun. This paper aims at examining whether such large scale flows exist in turbulent protoplanetary disks. High resolution global hydrodynamical and magnetohydrodynamical numerical simulations of turbulent protoplanetary disks are used to infer the properties of the flow in such disks. By performing hydrodynamic simulations using explicit viscosity, we demonstrate that our numerical setup does not suffer from any numerical artifact. The aforementioned meridional circulation is readily recovered in viscous and laminar disks. In MHD simulations, the magneto-rotational instability drives turbulence in the disks. Averaging out the turbulent fluctuations over long timescale, the results fail to show any large scale...

  18. Southern Meridional Atmospheric Circulation Associated with IOD

    Institute of Scientific and Technical Information of China (English)

    LIU Na; CHEN Hongxia

    2006-01-01

    Using the monthly wind and sea surface temperature (SST) data, southern meridional atmospheric circulation cells associated with the Indian Ocean Dipole Mode (IOD) events in the Indian Ocean are for the first time described and examined. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. During the four different phases of the positive IOD events, the anomalous meridional Hadley circulation over the western Indian Ocean shows that the air rises in the tropics, flows poleward in the upper troposphere, sinks in the subtropics, and returns back to the tropics in the lower troposphere. The anomalous Hadley circulation over the eastern Indian Ocean is opposite to that over the western Indian Ocean. During positive IOD events, the meridional Hadley circulation over the eastern Indian Ocean is weakened while it is strengthened over the western Indian Ocean. Correlation analysis between the IOD index and the indices of the Hadley cells also proves that, the atmospheric circulation patterns are evident in every IOD event over the period of record.

  19. Modes of winter precipitation variability in the North Atlantic

    Energy Technology Data Exchange (ETDEWEB)

    Zorita, E. [GKSS-Forschungszentrum Geesthacht GmbH (Germany). Inst. fuer Hydrophysik; Saenz, J.; Fernandez, J.; Zubillaga, J. [Bilbao Univ. (Spain)

    2001-07-01

    The modes of variability of winter precipitation in the North Atlantic sector are identified by Empirical Orthogonal Functions Analysis in the NCEP/NCAR global reanalysis data sets. These modes are also present in a gridded precipitation data set over the Western Europe. The large-scale fields of atmospheric seasonal mean circulation, baroclinic activity, evaporation and humidity transport that are connected to the rainfall modes have been also analyzed in order to investigate the physical mechanisms that are causally linked to the rainfall modes. The results indicate that the leading rainfall mode is associated to the North Atlantic oscillation and represents a meridional redistribution of precipitation in the North Atlantic through displacements of the storm tracks. The second mode is related to evaporation anomalies in the Eastern Atlantic that precipitate almost entirely in the Western Atlantic. The third mode seems to be associated to meridional transport of water vapor from the Tropical Atlantic. (orig.)

  20. Decadal-scale sea level rise acceleration along the Florida Atlantic coast and its relations to sea level variability along the Florida Current

    Science.gov (United States)

    Wdowinski, S.; Thompson, P. R.; Mitchum, G. T.; Park, J.

    2015-12-01

    The US Atlantic coast is one of the most vulnerable areas to sea level rise (SLR) due to its low elevation, large population concentrations, and economic importance. Further vulnerability arises from accelerating rates of SLR, which began in the early 2000's and caused a significant increase in flooding frequency in several coastal communities. Several studies have suggested that the accelerating SLR rates are due to the slowing down of the Atlantic Meridional Overturning Circulation, in particular, a weakening of the Gulf Stream (GS). However, there are no direct observations that link the GS conditions and high sea levels along the coast. In this study we use satellite altimetry, tide gauge, and Florida Current (FC) cable data to explore possible relations between the recent SLR rate increase along the Florida Atlantic coast and various dynamical processes in the GS/FC system. Preliminary calculations indicate a good agreement between coastal sea level and nearshore altimetry series (R = 0.76-0.8) suggesting that SSH gradients from altimetry may be useful for assessing the dynamics associated with the coastal sea level change. Here we focus on spatio-temporal SSH changes along the two satellite passes located closest to the Florida Atlantic coast. Our results indicate an intriguing transition in SSH behavior around 2004-5. Prior to 2004, anomalous low coastal SSH events (strong FC) occurred every 3-5 years in correlation with warm ENSO events. After 2004, the strong relationship between ENSO and the gradient across the FC vanishes, while the mean sea level across the current increases. The observed SSH anomaly transition around 2004-5 correlates well with the initiation of accelerated rates of coastal SLR, suggesting that the decadal scale SLR acceleration has occurred during weak FC conditions. However, the forcing of this transition and the role of mean sea level variability, which is of comparable magnitude to variability in the gradient, remain unexplained.

  1. Mixing and diffusion in intermittent overturning turbulence

    Science.gov (United States)

    Redondo, Jose M.; Mahjoub, Otman B.; Gonzalez-Nieto, Pilar L.; Lawry, Andrew

    2014-05-01

    The improvements in experimental methods and high resolution image analysis are nowadays able to detect subtle changes in the structure of the turbulence over a wide range of temporal and spatial scales [1], we compare the scaling shown by different mixing fronts driven by buoyancy that form a Rayleigh-Taylor mixing front. We use PIV and density front tracking in several experimental configurations akin to geophysical overturning [2-7]. We parametrize the role of unstable stratification by means of the Atwood number and compare both the scaling and the multifractal and the maximum local fractal structure functions of the different markers used to visualize the front. Both reactive and passive scalar tracers are used to investigate the mixing structure and the intermittency of the flow. Different initial conditions are compared and the mixing efficiency of the overal turbulent process evaluated [6-7]. An interesting approach, relating the Multi-Fractal dimension spectra, the intermittency and the spectral exponent is to find relationships that may be used to parameterise the sub-grid turbulence in terms of generalized diffusivities [4 ] that take into account the topology and the self-similarity of the Mixing RT and RM flows. As an example, a relationship between the diffusivity, the exponent β, the intermittency μ, and D(i), may be found for the volume fraction or the concentration, at the same time other locally measured parameters such as the enstrophy or the gradient alignment as well as their multi-fractal structures may turn out to be physically relevant indicators of the local turbulence and the mixing. Several methods of deriving local eddy diffusivity and local entrainment should give more realistic estimates of the spatial/temporal non-homogeneities (and intermittencies in the Kolmogorov 62 sense obtained as spatial correlations of the turbulent dissipation, or from structure functions) and these values may be used to parameterise turbulence at a variety

  2. How Synchronous was the Transition into the Younger Dryas across the Euro-Atlantic Region?

    Science.gov (United States)

    Schenk, F.; Muschitiello, F.; Heikkilä, M. P.; Väliranta, M.; Tarasov, L.; Brandefelt, J.; Johansson, A. V.; Naslund, J. O.; Wohlfarth, B.

    2015-12-01

    Observations of a currently weakening subpolar gyre south of Greenland has again increased scientific attention regarding the role of the Atlantic Meridional Overturning Circulation (AMOC) for the regional to global climate. The rapid climate shift of the Younger Dryas (YD, GS-1) cold reversal during the last deglaciation is attributed to an abrupt slowdown or collapse of the AMOC due to a strong meltwater pulse and/or the rapid disintegration of the Laurentide Ice sheet. Although such a dramatic event is not expected for the future, the spatiotemporal climatic response to such a slowdown is an interesting test case. Two recently well dated proxy records around the North Sea region suggest a non-synchronous early cooling/onset of the YD compared to Greenland (NGRIP). Presentation #61803 discusses the hypothesis of a local cooling as a response to increased ice berg calving and/or meltwater from Fenno-Scandinavian Ice Sheet (FIS) during the late Alleröd warm phase (GI-1a). Here we study CCSM3 model output from the quasi-transient atmosphere-ocean simulation (TraCE) where no strong contribution from FIS is considered from the late Alleröd into the YD. We evaluate to which extent the spatiotemporal temperature response to the AMOC slowdown of the simulation is synchronous over the Euro-Atlantic region and how atmospheric teleconnections reorganize during the rapid shift into the YD. In addition, we run time-slice experiments at high spatial resolution of around 100 km with the Community Earth System Model CESM1.0.5 for the late Alleröd and YD to compare spatial climatic differences for both periods taking into account the regional influence from continental ice sheets in more detail.

  3. Optimization of Meridional Flow Channel Design of Pump Impeller

    OpenAIRE

    Miyauchi Sunao; Horiguchi Hironori; Fukutomi Jun-ichirou; Takahashi Akihiro

    2004-01-01

    The meridional flow channel design of a pump impeller affects its performance. However, since so many design parameters exist, a new design method is proposed in which a meridional and blade-to-blade flow channel is designed by the parallel use of the circulation distribution provided by the designer. Thus, an optimization method was used to design an axis-symmetrical meridional flow channel from the circulation distribution. In addition, the inverse design method proposed by Zangeneh et al. ...

  4. Detection of Equatorward Meridional Flow and Evidence of Double-Cell Meridional Circulation inside the Sun

    CERN Document Server

    Zhao, Junwei; Kosovichev, A G; Duvall,, T L; Hartlep, Thomas

    2013-01-01

    Meridional flow in the solar interior plays an important role in redistributing angular momentum and transporting magnetic flux inside the Sun. Although it has long been recognized that the meridional flow is predominantly poleward at the Sun's surface and in its shallow interior, the location of the equatorward return flow and the meridional flow profile in the deeper interior remain unclear. Using the first two years of continuous helioseismology observations from the Solar Dynamics Observatory / Helioseismic Magnetic Imager, we analyze travel times of acoustic waves that propagate through different depths of the solar interior carrying information about the solar interior dynamics. After removing a systematic center-to-limb effect in the helioseismic measurements and performing inversions for flow speed, we find that the poleward meridional flow of a speed of 15 m/s extends in depth from the photosphere to about 0.91 R_sun. An equatorward flow of a speed of 10 m/s is found between 0.82 to 0.91 R_sun in the...

  5. Review experience on overturning prevention of NPP buildings in Switzerland

    International Nuclear Information System (INIS)

    Buildings with high ratios of the building height to the width of their foundations show a great potential for overturning. This problem occurs under large horizontal actions like Safe Shutdown Earthquake (SSE) or Aircraft Impact (AI). There are different concepts to prevent overturning under these actions. During the design process one concept has to be chosen and investigated. Therefore, calculations need to be done, which can be static or dynamic and linear or non-linear. In most concepts the resistance and the activation potential of the foundation soil are important for the overturning stability. Therefore, the definition of nominal values and design values of the soil has to be done carefully. In most of the cases field testing is required due to non-existing data of the soil. Namely the transmission of tensile forces to the underlying soil or rock has to be supported by experimental verification of the system's behaviour. This paper shows in the first place different possibilities of overturning prevention, which were used at NPP buildings in Switzerland. In the second place a case study of a recently investigated NPP site is presented. Thereby, the potentials and the limitations of static and linear dynamic analysis are discussed and special experiences with non-linear dynamic analysis for earthquake and aircraft scenarios are presented. (authors)

  6. Currents, Geostrophic, Aviso, 0.25 degrees, Global, Meridional

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Aviso Meridional Geostrophic Current is inferred from Sea Surface Height Deviation, climatological dynamic height, and basic fluid mechanics.

  7. Acoustic streaming and Sun's meridional circulation

    Science.gov (United States)

    Valverde, Jose Manuel

    2016-09-01

    A vast number of physical processes involving oscillations of a bounded viscous fluid are relevantly influenced by acoustic streaming. When this happens a steady circulation of fluid develops in a thin boundary adjacent to the interface. Some examples are refracted sound waves, a fluid inside a spherical cavity undergoing torsional oscillations or a pulsating liquid droplet. Steady streaming around circular interfaces consists of a hemispherically symmetric recirculation of fluid from the equatorial plane to the polar axes closely resembling the meridional circulation pattern observed in the Sun's convection zone that determines the solar cycle. In this paper, it is argued that the acoustic pulsations exhibited by the Sun would lead to acoustic streaming in the boundary of the convection zone. A simple estimation using a typical dominant frequency of 3 mHz and the observed surface oscillation amplitude yields a steady streaming velocity us ∼ 10 m s‑1, which is on the order of the meridional circulation velocity observed in the Sun's convection zone.

  8. Seasonal overturning circulation in the Red Sea: 2. Winter circulation

    KAUST Repository

    Yao, Fengchao

    2014-04-01

    The shallow winter overturning circulation in the Red Sea is studied using a 50 year high-resolution MITgcm (MIT general circulation model) simulation with realistic atmospheric forcing. The overturning circulation for a typical year, represented by 1980, and the climatological mean are analyzed using model output to delineate the three-dimensional structure and to investigate the underlying dynamical mechanisms. The horizontal model circulation in the winter of 1980 is dominated by energetic eddies. The climatological model mean results suggest that the surface inflow intensifies in a western boundary current in the southern Red Sea that switches to an eastern boundary current north of 24N. The overturning is accomplished through a cyclonic recirculation and a cross-basin overturning circulation in the northern Red Sea, with major sinking occurring along a narrow band of width about 20 km along the eastern boundary and weaker upwelling along the western boundary. The northward pressure gradient force, strong vertical mixing, and horizontal mixing near the boundary are the essential dynamical components in the model\\'s winter overturning circulation. The simulated water exchange is not hydraulically controlled in the Strait of Bab el Mandeb; instead, the exchange is limited by bottom and lateral boundary friction and, to a lesser extent, by interfacial friction due to the vertical viscosity at the interface between the inflow and the outflow. Key Points Sinking occurs in a narrow boundary layer along the eastern boundary Surface western boundary current switches into an eastern boundary current Water exchange in the Strait of Bab el Mandeb is not hydraulically controlled © 2014. American Geophysical Union. All Rights Reserved.

  9. Could massive Arctic sea ice export to the North Atlantic be the real cause of abrupt climate change during the last deglaciation?

    Science.gov (United States)

    Coletti, A. J.; Condron, A.

    2015-12-01

    Using a coupled ocean-sea ice model (MITgcm), we investigate whether the break-up and mobilization of thick, multiyear, Arctic sea ice might have supplied enough freshwater to the Nordic Seas to reduce North Atlantic Deep Water (NADW) formation and weaken the Atlantic Meridional Overturning Circulation (AMOC). Numerical simulations of a Last Glacial Maximum (LGM) environment show the potential for sea ice to grow to ~30m thick, storing ~1.41x105 km3 of freshwater as sea ice in the Arctic (this is ~10 times the volume of freshwater stored in the modern-day Arctic). Releasing this volume of sea ice from the Arctic in 1-yr is equivalent to a high-latitude freshwater forcing of ~4.5 Sv, which is comparable (or larger) in magnitude to most meltwater floods emanating from land-based glacial lakes (e.g. Agassiz) during the last deglaciation. Opening of the Bering Strait and Barents Sea are two plausible mechanisms that may have initiated sea ice mobilization. Opening Bering Strait increases sea ice transport through the Fram Strait by 7% and results in a 22% weakening of AMOC for 2000 years and a >3°C warming in the Arctic basin at 800 m depth. Opening Barents Sea to simulate a collapse of the Fennoscandian ice sheet has little impact on Arctic sea ice and freshwater export to the North Atlantic, but weakens AMOC ~8%. In a simulation with both straits open there is a transition to near-modern sea ice circulation pattern and a 24% reduction in AMOC. Experiments with the Bering Strait open and sea ice artificially capped to 10 m show barely any difference to those when sea ice can grow to ~30m, suggesting that changes in topography have a much greater impact on AMOC than the freshwater forcing from sea ice melting in the Nordic Seas.

  10. Extreme air-sea interaction over the North Atlantic subpolar gyre during the winter of 2013-2014 and its sub-surface legacy

    Science.gov (United States)

    Grist, Jeremy P.; Josey, Simon A.; Jacobs, Zoe L.; Marsh, Robert; Sinha, Bablu; Van Sebille, Erik

    2016-06-01

    Exceptionally low North American temperatures and record-breaking precipitation over the British Isles during winter 2013-2014 were interconnected by anomalous ocean evaporation over the North Atlantic subpolar gyre region (SPG). This evaporation (or oceanic latent heat release) was accompanied by strong sensible heat loss to the atmosphere. The enhanced heat loss over the SPG was caused by a combination of surface westerly winds from the North American continent and northerly winds from the Nordic Seas region that were colder, drier and stronger than normal. A distinctive feature of the air-sea exchange was that the enhanced heat loss spanned the entire width of the SPG, with evaporation anomalies intensifying in the east while sensible heat flux anomalies were slightly stronger upstream in the west. The immediate impact of the strong air-sea fluxes on the ocean-atmosphere system included a reduction in ocean heat content of the SPG and a shift in basin-scale pathways of ocean heat and atmospheric freshwater transport. Atmospheric reanalysis data and the EN4 ocean data set indicate that a longer-term legacy of the winter has been the enhanced formation of a particularly dense mode of Subpolar Mode Water (SPMW)—one of the precursors of North Atlantic Deep Water and thus an important component of the Atlantic Meridional Overturning Circulation. Using particle trajectory analysis, the likely dispersal of newly-formed SPMW is evaluated, providing evidence for the re-emergence of anomalously cold SPMW in early winter 2014/2015.

  11. Estimating ventilation time scales using overturning stream functions

    Science.gov (United States)

    Thompson, Bijoy; Nycander, Jonas; Nilsson, Johan; Jakobsson, Martin; Döös, Kristofer

    2014-06-01

    A simple method for estimating ventilation time scales from overturning stream functions is proposed. The stream function may be computed using either geometric coordinates or a generalized vertical coordinate, such as potential density (salinity in our study). The method is tested with a three-dimensional circulation model describing an idealized semi-enclosed ocean basin ventilated through a narrow strait over a sill, and the result is compared to age estimates obtained from a passive numerical age tracer. The best result is obtained when using the stream function in salinity coordinates. In this case, the reservoir-averaged advection time obtained from the overturning stream function in salinity coordinates agrees rather well with the mean age of the age tracer, and the corresponding maximum ages agree very well.

  12. Creating a National Coalition to Address Tractor Overturn Fatalities.

    Science.gov (United States)

    Tinc, P J; Ayers, P D; May, J J; Purschwitz, M A; Sorensen, J A

    2015-04-01

    Tractor overturns continue to be the leading cause of death on U.S. farms. While rollover protective structures (ROPS) are effective in preventing these fatalities, they are underutilized due to a number of barriers. Past programs in the U.S. and abroad have targeted this area of agricultural safety; however, a national program is not yet in place for U.S. farmers. This study seeks to build a national partnership to address tractor overturn fatalities by increasing the number of tractors with ROPS. A diverse, multisector steering committee has been organized and is working together using Whole System in a Room methods. This method brings together partners from nine stakeholder groups to identify and commit to a collaborative solution to the issue. PMID:26204786

  13. A spatial cluster analysis of tractor overturns in Kentucky from 1960 to 2002

    Science.gov (United States)

    Saman, D.M.; Cole, H.P.; Odoi, A.; Myers, M.L.; Carey, D.I.; Westneat, S.C.

    2012-01-01

    Background: Agricultural tractor overturns without rollover protective structures are the leading cause of farm fatalities in the United States. To our knowledge, no studies have incorporated the spatial scan statistic in identifying high-risk areas for tractor overturns. The aim of this study was to determine whether tractor overturns cluster in certain parts of Kentucky and identify factors associated with tractor overturns. Methods: A spatial statistical analysis using Kulldorff's spatial scan statistic was performed to identify county clusters at greatest risk for tractor overturns. A regression analysis was then performed to identify factors associated with tractor overturns. Results: The spatial analysis revealed a cluster of higher than expected tractor overturns in four counties in northern Kentucky (RR = 2.55) and 10 counties in eastern Kentucky (RR = 1.97). Higher rates of tractor overturns were associated with steeper average percent slope of pasture land by county (p = 0.0002) and a greater percent of total tractors with less than 40 horsepower by county (ptractor overturns exist in Kentucky and identifies factors associated with overturns. This study provides policymakers a guide to targeted county-level interventions (e.g., roll-over protective structures promotion interventions) with the intention of reducing tractor overturns in the highest risk counties in Kentucky. ?? 2012 Saman et al.

  14. Risk analysis of tractor overturns on catfish farms.

    Science.gov (United States)

    Stephens, Walter B; Ibendahl, Gregory A; Myers, Melvin L; Cole, Henry P

    2010-10-01

    Fatal and nonfatal injuries occur on catfish farms as a result of tractor overturns, but these injuries can be greatly mitigated when a tractor is equipped with a rollover protective structure (ROPS) and seatbelt. This study analyzed the proportion of tractors on catfish farms in Mississippi not equipped with ROPS and the cost of retrofitting those tractors with a ROPS and seatbelt as compared to the expected benefits gained from adding the protection. To determine if farmers have the financial incentive to retrofit older tractors, a net present value framework was used because the expected benefits occur over a number of years. The ROPS retrofit is a one-time cost that occurs immediately, and thus the present value does not need to be calculated, i.e., time period 0. According to this study adding ROPS will provide a net benefit of $22,877 in the event of an overturn. When dealing with small farms (tractor is used for approximately twice as many hours compared to an average tractor on large farms, the extra hours worked by a single tractor resulted in total benefits of retrofitting ROPS of $9.45 per year, whereas for large farms the net benefit is $4.70 (=125 acres). Given that the least expensive retrofit is $147, it would take 15.5 years of tractor use on the small farms for the expected benefits to outweigh the cost. Given the large range of costs that could occur due to an overturn, there could be a significant premium for not having ROPS protection. In other words, there likely is a smaller range of costs from an overturn with ROPS than without ROPS because ROPS reduces the more serious incidents. PMID:20954036

  15. The global warming in the North Atlantic Sector and the role of the ocean

    Science.gov (United States)

    Hand, R.; Keenlyside, N. S.; Greatbatch, R. J.; Omrani, N. E.

    2014-12-01

    This work presents an analysis of North Atlantic ocean-atmosphere interaction in a warming climate, based on a long-term earth system model experiment forced by the RCP 8.5 scenario, the strongest greenhouse gas forcing used in the climate projections for the 5th Assessement report of the Intergovernmental Panel on Climate Change). In addition to a global increase in SSTs as a direct response to the radiative forcing, the model shows a distinct change of the local sea surface temperature (SST hereafter) patterns in the Gulf Stream region: The SST front moves northward by several hundred kilometers, likely as a response of the wind-driven part of the oceanic surface circulation, and becomes more zonal. As a consequence of a massive slowdown of the Atlantic Meridional Overturning Circulation, the northeast North Atlantic only shows a moderate warming compared to the rest of the ocean. The feedback of these changes on the atmosphere was studied in a set of sensitivity experiments based on the SST climatology of the coupled runs. The set consists of a control run based on the historical run, a run using the full SST from the coupled RCP 8.5 run and two runs, where the SST signal was deconstructed into a homogenous mean warming part and a local pattern change. In the region of the precipitation maximum in the historical run the future scenario shows an increase of absolute SSTs, but a significant decrease in local precipitation, low-level convergence and upward motion. Since warmer SSTs usually cause the opposite, this indicates that the local response in that region is connected to the (with respect to the historical run) weakened SST gradients rather than to the absolute SST. Consistently, the model shows enhanced precipitation north of this region, where the SST gradients are enhanced. However, the signal restricts to the low and mid-troposphere and does not reach the higher model levels. There is little evidence for a large-scale response to the changes in the Gulf

  16. Reconstructing Links between AMOC and Surface Temperature Variability in the North Atlantic

    Science.gov (United States)

    Borchert, Leonard; Fischer, Matthias; Müller, Wolfgang; Baehr, Johanna

    2016-04-01

    Recent studies found an impact of the Atlantic Meridional Overturning Circulation (AMOC) through sea surface temperatures (SSTs) and ocean-atmosphere surface heat fluxes (SHFs) on North Atlantic (NA) climate on interannual time scales. Since fluctuations in SSTs and SHFs as well as AMOC and oceanic heat transport (OHT) are highly model dependent and cannot be assumed to be independent of the mean state of the model in use. By using the Max Planck Institute Ocean Model (MPIOM) forced with the Twentieth Century Reanalysis (20CR, Compo et al (2011)), we confirm earlier studies and reconstruct for the governing period 1871-2010, that cold SSTs emerge in the Gulf Stream region and warm SSTs emerge in the NA Subpolar Gyre after strong AMOC anomalies at 50° N. The MPIOM in use has an average 1.5° horizontal resolution and 40 vertical non-equidistant depth levels. The model is forced by fluxes of heat, momentum, and freshwater at the air sea boundary through bulk formulas as described in Müller et al (2014). A positive density anomaly in the NA (= higher salinity / lower temperatures) is followed by an intensification of the AMOC and subsequently OHT. The proposed mechanism is examined in more detail studying correlations between AMOC, OHT, SSTs, and SHFs, as well as composite means of SSTs and SHFs in the Atlantic focusing on particularly strong and weak AMOC and OHT states at 50° N. High SSTs are shown to mostly appear simultaneously with upward SHFs and vice versa. The characteristic AMOC anomaly pattern appears in both correlation analysis and composite mean analysis over strong AMOC states after 2-6 years, and seems to occur favorably in winter (DJF). We further demonstrate that a similar, stronger pattern arises from OHT anomalies on similar time scales. Literature: Compo, GP, JS Whitaker, PD Sardeshmukh, N Matsui, RJ Allan, X Yin, BE Gleason, RS Vose, G Rutledge, P Bessemoulin, S Brönnimann, M Brunet, RJ Crouthamel, AN Grant, PY Groisman, PD Jones, MC Kruk, AC

  17. Mass, nutrient and oxygen budgets for the northeastern Atlantic Ocean

    Directory of Open Access Journals (Sweden)

    G. Maze

    2012-10-01

    Full Text Available The northeast Atlantic is a key horizontal and vertical crossroads region for the meridional overturning circulation, but basic nutrient and oxygen fluxes are still poorly constrained by observations in the region. A surface to bottom northeast Atlantic Ocean budget for mass, nutrients (nitrate and phosphate and oxygen is determined using an optimization method based on three surveys of the OVIDE transect (from Greenland to Portugal completed with the World Ocean Atlas 2009. Budgets are derived for two communicating boxes representing the northeastern European basin (NEEB and the Irminger Sea.

    For the NEEB (Irminger box, it is found that 30% of the mass import (export across the OVIDE section reach (originate from the Nordic Seas, while 70% are redistributed between both boxes through the Reykjanes Ridge (9.3 ± 0.7 × 109 kg s−1.

    Net biological source/sink terms of nitrate point to both the Irminger and NEEB boxes as net organic matter production sites (consuming nitrate at a rate of –7.8 ± 6.5 kmol s−1 and –8.4 ± 6.6 kmol s−1, respectively. Using a standard Redfield ratio of C : N = 106 : 16, nitrate consumption rates indicate that about 40 TgC yr−1 of carbon is fixed by organic matter production between the OVIDE transect and the Greenland–Scotland Ridge. Nutrient fluxes also induce a net biological production of oxygen of 73 ± 60 kmol s−1 and 79 ± 62 kmol s−1 in the Irminger and NEEB boxes, which points to the region as being autotrophic.

    The abiotic air–sea oxygen flux leads to an oceanic oxygen uptake in the two regions (264 ± 66 kmol s−1 in the north and 443 ± 70 kmol s−1 in the south. The abiotic flux is partitioned into a mixing and a thermal component. It is found that the Irminger Sea oceanic oxygen uptake is driven by an air–sea heat flux cooling increasing the ocean surface

  18. Interannual to Decadal Climate Predictability in the North Atlantic: A Multi-Model-Ensemble Study

    OpenAIRE

    Collins, M.; Hadley Centre, Met Office, Exeter, United Kingdom; Botzet, M.; Max-Planck-Institut für Meteorologie, Hamburg, Germany; Carril, A. F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Drange, H.; Nansen Environmental and Remote Sensing Center, and Bjerknes Centre for Climate Research, Bergen, Norway; Jouzeau, A.; CERFACS, Toulouse, France; Latif, M.; Max-Planck-Institut für Meterologie, Hamburg, and Leibniz-Institut für Meereswissenschaften, Kiel, Germany; Masina, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Otteraa, O. H.; Nansen Environmental and Remote Sensing Center, and Bjerknes Centre for Climate Research, Bergen, Norway; Pohlmann, H.; Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada; Sorteberg, A.; Bjerknes Centre for Climate Research, Bergen, Norway; Sutton, R.; Centre for Global Atmospheric Modelling, Reading, United Kingdom; Terray, L.; CERFACS, Toulouse, France

    2006-01-01

    Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surfac...

  19. Spatial and Temporal Variability of Temperature and Salinity in the Deep and Abyssal Layers of the Subpolar North Atlantic

    Science.gov (United States)

    Yashayaev, I.; Bacon, S.; de Jong, F.; Dye, S.; Fischer, J.; Holliday, N. P.; Kieke, D.; Quadfasel, D. R.; Rhein, M.; Sarafanov, A.; Valdimarsson, H.; van Aken, H. M.

    2010-12-01

    The dense water overflows crossing the Denmark Strait and Faroe-Shetland Channel form the Denmark Strait Overflow Water (DSOW) and Northeast Atlantic Deep Water, respectively. Collectively with the convectively-formed Labrador Sea Water (LSW), these water masses form the deep limb of the Atlantic Meridional Overturning Circulation and hence are important components of the global climate system. Recent variability in the properties of the intermediate and deep water masses will be described by using hydrographic, moored, profiling float, altimeter and tracer data from several programs. We will show that the variability of intermediate-depth water is strongly influenced by the strength and duration of winter convection in the Labrador Sea on the western side, and the advection of warmer more saline intermediate waters from the lower latitudes on the eastern side. Four variations of LSW produced in different years were identified in the 2010 annual survey of the Labrador Sea. While gradually transforming in time, these waters have been preserved in different ranges of density and depth because of gradual weakening of winter convection since 2008, and are still distinguishable by their unique signatures in temperature, salinity and chemical tracers. The fate of each individual LSW class can now be followed by combining profiles from Argo floats and hydrographic data from several institutes. We will show how an international array of hydrographic and tracer sections supported by moored, profiling float and satellite measurements has resolved the downstream propagation of some interesting events formed in the subpolar or Arctic seas. In particular, we document strong fresh and cold anomalies in DSOW, first observed in the Irminger Sea in 1999, 2004 and 2009, and then with a year delay in the abyssal Labrador Sea.

  20. Technical Note: Is radiation important for the high amplitude variability of the MOC in the North Atlantic?

    Directory of Open Access Journals (Sweden)

    D. Nof

    2007-09-01

    Full Text Available Radiation is of fundamental importance to climate modeling and it is customary to assume that it is also important for the variability of North Atlantic Deep Water (NADW formation and the meridional overturning cell (MOC. Numerous articles follow this scenario and incorporate radiation into the calculation. Using relatively old heat-flux maps based on measurements taken in the nineteen sixties, Sandal and Nof (2007 recently suggested that, even though the radiation terms are of the same order as the other heat-flux terms, they are not important for the variability of the NADW and the MOC. They proposed that only sensible and latent heat fluxes are important for the long-term variability of the convection, i.e., for processes such as Heinrich events, which supposedly correspond to turning convection on-and-off in the Atlantic.

    Here, we place this suggestion on a firmer ground by presenting new and accurate up-to-date heat flux maps that also suggest that the radiation is of no major consequence to the NADW variability. Also, we attribute the relative importance of sensible and latent heat fluxes and the contrasting negligible role of radiation to the fact that the latent and sensible heat fluxes are primarily proportional to the difference between the sea surface and the air temperature whereas the radiation is primarily proportional to the sea surface temperature, i.e., radiation is approximately independent of the atmospheric temperature. Due the small heat capacity ratio of air/water (1/4, the difference between the ocean temperature and the air temperature varies dramatically between the state of active and inactive MOC, whereas the ocean temperature by itself varies very modestly between a state of active and inactive convection.

  1. Meridional Transport in the Atmospheres of Earth and Mars

    CERN Document Server

    Soto, Alejandro

    2015-01-01

    As we continue to discover terrestrial exoplanets, many with orbital and planetary characteristics drastically different from anything encountered in our solar system, we are likely to encounter 'exotic' atmospheric transport processes. As an example, we show an analysis of meridional transport from simulations Mars. These simulations provide insight into the differences in meridional transport between Earth and Mars, particularly through the role of a condensation flow. The differences between Earth and Mars are a reminder that there may be a wide variety of meridional transport processes at work across the range of observed terrestrial planets.

  2. Zonal drifts of irregularities imparted by meridional winds.

    Science.gov (United States)

    Waldman, H.; Da Rosa, A. V.

    1973-01-01

    In a uniform ionosphere, meridional winds cause only meridional motions of irregularities. It is shown, however, that, if F-region irregularities are considered in a real ionosphere in which there is a highly conductive E-layer, zonal motions occur. During the day a substantial westward drift takes place, while at night the drift is eastward but smaller, owing to the much smaller E-layer conductivity. Thus, the effect of meridional winds is to impart a net westward drift to small irregularities in the ionization, provided such irregularities persist long enough.

  3. Regime shifts in the Arctic North Atlantic during the Neoglacial revealed by seabirds and precipitation isotopes on Bjørnøya, Svalbard

    Science.gov (United States)

    D'Andrea, William J.; Hormes, Anne; Bakke, Jostein; Nicolaisen, Line

    2016-04-01

    The northeastern North Atlantic Ocean, and the Norwegian and Greenland Seas are subject to large hydrographic changes. These variations can influence oceanic heat transport to the Arctic, meridional overturning circulation, and atmospheric circulation patterns and thereby impact global climate patterns. Marine records suggest that numerous large-scale changes in the hydrography of the northern North Atlantic took place during the middle to late Holocene. We report a record of nitrogen and hydrogen isotope measurements from a lake sediment core from Bjørnøya, Svalbard (74.38°N, 19.02°E) that documents major regime shifts in the climate of the northern North Atlantic during the past 6,000 years. Bjørnøya is the nesting ground for one of the largest seabird populations in the North Atlantic. As top predators in the marine ecosystem, seabirds (and their guano) are enriched in 15N; during spring and summer months they deliver isotopically enriched nitrogen to nesting areas. We developed a record of seabird population changes on Bjørnøya based on the nitrogen isotope composition of sediments in a core collected from lake Ellasjøen. The record reveals multiple multicentennial scale changes in δ15N values (varying between ~8-12‰) that track past changes in the size of seabird populations. From the same sediment core, we also developed a record of δD of precipitation, using δD values of sedimentary n-alkanes. Past intervals with the largest inferred bird populations correspond with the most enriched δD of precipitation, which we interpret to represent a more Atlantic climate. Periods with reduced seabird populations correspond with intervals with more negative δD of precipitation and representing a more Arctic climate. Together, the nitrogen and hydrogen isotope records signify regime shifts in the oceanography, marine ecosystem, and atmospheric circulation of the northern North Atlantic that are related to variations in the strength of the subpolar gyre.

  4. On the relationship between the Meridional Mode and the equatorial SST anomalies

    Science.gov (United States)

    Martín-Rey, Marta; Lazar, Alban

    2016-04-01

    The tropical Atlantic climate is dominated by two modes of variability at inter-annual time scales, the Equatorial Mode (EM) and the Meridional Mode (MM). They are characterized by specific Sea Surface Temperature anomalies (SSTA) distributions, respectively with a maximum over the central-eastern equator during boreal summer, and with an inter-hemispheric gradient during boreal spring. Although their structures, air-sea interactions and impacts have been widely studied, the processes associated to their development, and their connections, still remain a challenge. In the present work, we present a classification of MM events, regarding to their connections with the successive equatorial summer SSTA, associated with an EM or neutral conditions. The MM-I events display SSTA in North Tropical Atlantic (NTA) followed by a same sign summer equatorial SSTA. The MM-II events are related to NTA SSTA and successive equatorial summer SSTA of opposite sign. For both types, the spring north-eastern trades anomalies could generate SSTA impacting on the equatorial SSTA. Nevertheless, it is evidenced that the anomalous wind pattern shown along the equatorial band and South Tropical Atlantic is crucial to give rise, or not, to an EM. In order to further analyse the air-sea interactions and oceanic processes at work in the two different MM types, sensitivity experiments with different wind forcings are performed with the NEMO OGCM.

  5. Impact of the seasonal cycle on the decadal predictability of the North Atlantic volume and heat transport under global warming

    Science.gov (United States)

    Fischer, Matthias; Müller, Wolfgang A.; Domeisen, Daniela I. V.; Baehr, Johanna

    2016-04-01

    The North Atlantic ocean circulation is projected to change considerably with future climate change. Here, we investigate whether changes in the North Atlantic meridional overturning circulation (AMOC) and the meridional heat transport (OHT) result in changes in their decadal predictability. In MPI-ESM-LR, we generate two hindcast ensembles with 20 start dates and 10 ensemble members per start date for (i) the present climate state in the CMIP5 historical simulation extended with RCP4.5 starting in 1995 and (ii) a future climate state in RCP4.5 starting in 2045. These two hindcast ensembles are compared against the historical simulation and RCP4.5 as control simulation, respectively, using the anomaly correlation coefficient (ACC) and the Brier skill score (BSS) decomposition in combination with reliability diagrams. The analysis is performed for yearly means and multiyear seasonal means of the AMOC and the OHT. Our results show a decrease in predictability of the AMOC and the OHT from the present climate state to the future climate state in RCP4.5. Both, changes in the AMOC and the OHT decadal predictability are largest at latitudes where the mean seasonal cycle of both AMOC and OHT is projected to change. For example around 25°N, the AMOC shows a reduction in the seasonal amplitude of about 0.5 Sv and a shift of up to 5 months in concert with a reduction in predictable lead times from up to 10 years to 2 years in the ACC. For the OHT, we find a reduction in the seasonal amplitude of about 0.1 PW and a shift of up to 5 months in concert with a reduction in predictable lead time from up to 4 years to 2 years in the ACC around 25°N. Similarly, the BSS and reliability diagrams show a reduction in skill from the present climate state to the future climate state. For multiyear seasonal means, summer months dominate the predictability in the present and future climate. Even though the changes in the decadal predictability of AMOC and OHT are small in general, their

  6. Characteristics of Solar Meridional Flows during Solar Cycle 23

    Science.gov (United States)

    Basu, Sarbani; Antia, H. M.

    2010-07-01

    We have analyzed available full-disk data from the Michelson Doppler Imager on board SOHO using the "ring diagram" technique to determine the behavior of solar meridional flows over solar cycle 23 in the outer 2% of the solar radius. We find that the dominant component of meridional flows during solar maximum was much lower than that during the minima at the beginning of cycles 23 and 24. There were differences in the flow velocities even between the two minima. The meridional flows show a migrating pattern with higher-velocity flows migrating toward the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of sunspot butterfly diagram and the zonal flows in the shallow layers. A high-latitude band in meridional flow appears around 2004, well before the current activity minimum. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. The different components of the flow have different time dependences, and the dependence is different at different depths.

  7. Characteristics of solar meridional flows during solar cycle 23

    CERN Document Server

    Basu, Sarbani

    2010-01-01

    We have analyzed available full-disc data from the Michelson Doppler Imager (MDI) on board SoHO using the "ring diagram" technique to determine the behavior of solar meridional flows over solar cycle 23 in the outer 2% of the solar radius. We find that the dominant component of meridional flows during solar maximum was much lower than that during the minima at the beginning of cycles 23 and 24. There were differences in the flow velocities even between the two minima. The meridional flows show a migrating pattern with higher-velocity flows migrating towards the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of sunspot butterfly diagram and the zonal flows in the shallow layers. A high latitude band in meridional flow appears around 2004, well before the current activity minimum. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that...

  8. The Circum-Pacific Teleconnection Pattern in Meridional Wind in the High Troposphere

    Institute of Scientific and Technical Information of China (English)

    WANG Huijun

    2005-01-01

    The Circum-Pacific Teleconnection Pattern (CPTP) is revealed in the meridional wind in the high troposphere via an emprirical orthogonal function (EOF) and correlation analysis on NCEP/NCAR reanalysis data. The CPTP is found to be composed of the North Pacific-North American teleconnection pattern (PNA), the South Pacific-South American teleconnection pattern (PSA), and the teleconnection patterns over the tropical western Pacific and the tropical eastern Pacific (or, Central America, or, tropical Atlantic). There is substantial interannual variability of the CPTP and a typical CPTP can be detected in some years. It is speculated that the zonal wind anomalies over the equatorial region in the western and eastern sides of the Pacific may play a role in linking the two hemispheres. The anomalous convection activities in the Tropics are plausible triggering factors for the zonal wind anomalies that are responsible for the composition of the CPTP.

  9. Agulhas ring injection into the South Atlantic during glacials and interglacials

    Directory of Open Access Journals (Sweden)

    V. Zharkov

    2008-09-01

    Full Text Available Recent proxies suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA. This brought about a dramatic increase in the salt-influx (from the Indian Ocean into the SA helping re-start the then-collapsed meridional overturning cell (MOC, leading to the termination of the Younger Dryas (YD. Here, we propose a mechanism through which large variations in ring production take place.

    Using nonlinear analytical solutions for eddy shedding, we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings.

    Simple process-oriented numerical simulations (using the Bleck and Boudra model are in satisfactory agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC was similar to that of the modern East Australian Current (EAC, for which the coastline slant is supercritical.

  10. Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

    Science.gov (United States)

    Fernandez, Alvaro; Lapen, Thomas J.; Andreasen, Rasmus; Swart, Peter K.; White, Christopher D.; Rosenheim, Brad E.

    2015-07-01

    We present coral- and sclerosponge-based reconstructions of the 14C content in North Atlantic dissolved inorganic carbon (DIC) during the last ~100 years from the subtropical cells (STCs). These waters are sensitive to the dynamics of the shallow overturning meridional circulation that transports heat and water masses from the subtropics to the tropics. We use these records to investigate the circulation patterns of the off-equatorial upwelling regions of the STCs, which are not well understood. Coral and sclerosponge skeletons provide long time series of ocean DIC 14C content, a tracer of oceanic circulation, effectively extending the observational record back in time. Sclerosponge data from the Bahamas were used to extend the existing subtropical 14C time series to the 21st century. Coral 14C data from the Cape Verde Islands (1890-2002) captured the 14C signature of water brought to the surface in the off-equatorial regions of the STC present near the West African coast. We observe a unique postbomb trend at Cape Verde that is similar to the upwelling regions in the Pacific, and we interpret this trend as the result of the slow penetration of bomb 14C into the interior ocean as part of the STC circulation. Using a multibox mixing model we constrain the time history of bomb 14C in the eastern tropical Atlantic, and we estimate a 20 year time scale for ventilation of the thermocline in this area of the ocean. The similarity between the Atlantic and Pacific 14C-based records of upwelling suggests that both are caused by bomb 14C penetration rather than more complex explanations that invoke changes in thermocline depth (e.g., related to El Niño-Southern Oscillation variability) or changes in the strength of the subtropical cells. Our results offer constraints for models of tropical ocean circulation and anthropogenic CO2 uptake that attempt to reproduce the characteristics of the shallow wind-driven circulation in the Atlantic.

  11. Rapid bottom-water circulation changes during the last glacial cycle in the coastal low-latitude NE Atlantic

    NARCIS (Netherlands)

    Gallego-Torres, D.; Romero, O.E.; Martínez-Ruiz, F.; Kim, J.-H.; Donner, B.; Ortega-Huertas, M.

    2014-01-01

    Previous paleoceanographic studies along the NW African margin focused on the dynamics of surface and intermediate waters, whereas little attention has been devoted to deep-water masses. Currently, these deep waters consist mainly of North Atlantic Deep Waters as part of the Atlantic Meridional Over

  12. Passive tracers and active dynamics - a model study of hydrography and circulation in the northern North Atlantic

    OpenAIRE

    Mauritzen, C.; Hjøllo, Solfrid Sætre; Sandø, Anne Britt

    2006-01-01

    A long-standing problem in oceanography has been to understand the relationship between what can be measured in the ocean, such as hydrography, and what cannot, such as the strength and structure of the complete Meridional Overturning Circulation (MOC) of the world oceans, commonly considered the main oceanic long-term modifier of Earth’s climate. With the aid of a 50 year simulation from a numerical ice-ocean model we have investigated this relationship in the area of the...

  13. Centennial-scale links between Atlantic Ocean dynamics and hydroclimate over the last 4400 years: Insights from the northern Gulf of Mexico

    Science.gov (United States)

    Thirumalai, K.; Quinn, T. M.; Okumura, Y.; Richey, J. N.; Partin, J. W.; Poore, R. Z.

    2015-12-01

    Surface circulation in the Atlantic Ocean is an important mediator of global climate and yet its variability is poorly constrained on centennial timescales. Changes in the Atlantic meridional overturning circulation (AMOC) have been implicated in late Holocene climate variability in the Western Hemisphere, although the relationship between AMOC variability and hydroclimate is uncertain due to the lack of sufficiently highly resolved proxy records. Here we present a replicated reconstruction of sea-surface temperature (SST) and salinity (SSS) from the Garrison Basin in the northern Gulf of Mexico (NGOM) spanning the last 4,400 years to better constrain past sea-surface conditions. We generated time series of paired Mg/Ca (SST proxy) and δ18O (SST and SSS proxy) variations in planktic foraminifer Globigerinoides ruber (white variety) from three multi-cores collected in 2010. Using a Monte Carlo-based technique we produce a stacked record from the three multi-cores and constrain analytical, calibration, chronological, and sampling uncertainties. We apply this technique to existing paired Mg/Ca- δ18O studies in the Gulf of Mexico and Atlantic Ocean to facilitate comparison between time-uncertain proxy reconstructions. The Garrison Basin stack exhibits large centennial-scale variability (σSST~0.6°C; δ18Osw~0.17‰) and indicates a substantially cool (0.9±0.5°C) and fresh (0.26±0.1‰) Little Ice Age (LIA; 1450-1850 A.D.), corroborating extant records from the Gulf of Mexico. Focusing on the last millennium, we analyze a suite of oceanic and terrestrial proxy records to demonstrate a centennial-scale link between salt advection in the Atlantic Ocean, a diagnostic parameter of ocean circulation, and hydroclimate in the adjacent continents. The ensuing multiproxy relationships seem to be consistent with spatial field correlations of limited salinity and rainfall instrumental/reanalysis data, which suggest that NGOM salinity varies with large-scale Atlantic Ocean

  14. OPTIMIZATION METHOD ON IMPELLER MERIDIONAL CONTOUR AND 3D BLADE

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    An optimization method for 3D blade and meridional contour of centrifugal or mixed-flow impeller based on the 3D viscous computational fluid dynamics (CFD) analysis is proposed. The blade is indirectly parameterized using the angular momentum and calculated by inverse design method. The design variables are separated into two categories: the meridional contour design variables and the blade design variables. Firstly, only the blade is optimized using genetic algorithm with the meridional contour remained constant. The artificial neural network (ANN) techniques with the training sample data schemed according to design of experiment theory are adopted to construct the response relation between the blade design variables and the impeller performance. Then, based on the ANN approximated relation between the meridional contour design variables and impeller performance, the meridional contour is optimized. Fewer design variables and less calculation effort is required in this method that may be widely used in the optimization of three-dimension impellers. An optimized impeller in a mixed-flow pump, where the head and the efficiency are enhanced by 12.9% and 4.5% respectively, confirms the validity of this newly proposed method.

  15. Reduced North Atlantic deep water coeval with the glacial Lake Agassiz freshwater outburst.

    Science.gov (United States)

    Kleiven, Helga Kikki Flesche; Kissel, Catherine; Laj, Carlo; Ninnemann, Ulysses S; Richter, Thomas O; Cortijo, Elsa

    2008-01-01

    An outstanding climate anomaly 8200 years before the present (B.P.) in the North Atlantic is commonly postulated to be the result of weakened overturning circulation triggered by a freshwater outburst. New stable isotopic and sedimentological records from a northwest Atlantic sediment core reveal that the most prominent Holocene anomaly in bottom-water chemistry and flow speed in the deep limb of the Atlantic overturning circulation begins at approximately 8.38 thousand years B.P., coeval with the catastrophic drainage of Lake Agassiz. The influence of Lower North Atlantic Deep Water was strongly reduced at our site for approximately 100 years after the outburst, confirming the ocean's sensitivity to freshwater forcing. The similarities between the timing and duration of the pronounced deep circulation changes and regional climate anomalies support a causal link. PMID:18063758

  16. Estimates of volume, heat and freshwater budgets for the Arctic Mediterranean and North Atlantic in relation to the main physical processes: Insight from the EU-NACLIM observations

    Science.gov (United States)

    Rudels, Bert; Hansen, Bogi; Karstensen, Johannes; McCarthy, Gerard; Quadfasel, Detlef

    2016-04-01

    The EU NACLIM (North Atlantic Climate) project aims to understand the forcing of the North Atlantic circulation and its importance for the climate of northwestern Europe. NACLIM comprises extensive modelling studies of the atmosphere, ocean and climate, but here mainly the oceanographic observations are presented. The core observation areas are the North Atlantic Subpolar Gyre and the Greenland-Scotland Ridge, separating the North Atlantic from the Arctic Mediterranean Sea. These are the areas, where the waters of the lower limb of the Meridional Overturning Circulation (MOC) are formed and sink into the deep North Atlantic to return southward, mainly in the Deep Western Boundary Current (DWBC). The exchanges across the Greenland-Scotland Ridge, both the northward flowing Atlantic and the returning dense waters, have been monitored over decades, as have the circulation in the Subpolar gyre and the convection and mode water formation in the Labrador Sea. These studies are extended southward to the RAPID array located in the Subtropical gyre at 26oN to capture the MOC further south, and northward into the Arctic Mediterranean Sea and the formation area of the densest water in the DWBC. In the Subtropical gyre the ocean circulation is mainly forced by the wind, while in the Subpolar gyre the atmospheric influence, in addition to wind forcing, also has a large thermodynamic component, changing the characteristics of the water masses and the density structure of the gyre. The importance of cooling and freshwater input increases in the Arctic Mediterranean Sea. Variability and a recent declining trend of the MOC strength have been observed in the Subtropical gyre at the RAPID array. By contrast, both the northward flow across the Greenland-Scotland Ridge and the overflows have remained steady during the observation periods. An increased atmospheric freshwater flux does not appear to affect the dense water formation in the Arctic Mediterranean, mainly because the low

  17. Whatever happened to the 'mad, bad' scientist? Overturning the stereotype.

    Science.gov (United States)

    Haynes, Roslynn D

    2016-01-01

    The cluster of myths relating to the pursuit of knowledge has perpetuated the archetype of the alchemist/scientist as sinister, dangerous, possibly mad and threatening to society's values. Shelley's Frankenstein provided imagery and a vocabulary universally invoked in relation to scientific discoveries and technological innovation. The reasons for the longevity of this seemingly antiquated, semiotic imagery are discussed. In the twenty-first century, this stereotype has been radically revised, even overturned. Scientists are now rarely objects of fear or mockery. Mathematicians, both real-life and fictional, are discussed here as being representative of scientists now depicted empathically. This article examines possible sociological reasons for this reversal; what the revisionist image suggests about society's changed attitudes to science; and what might be the substitute fears and sources of horror. PMID:24916194

  18. North Atlantic Deep Water Production during the Last Glacial Maximum

    Science.gov (United States)

    Howe, Jacob N. W.; Piotrowski, Alexander M.; Noble, Taryn L.; Mulitza, Stefan; Chiessi, Cristiano M.; Bayon, Germain

    2016-06-01

    Changes in deep ocean ventilation are commonly invoked as the primary cause of lower glacial atmospheric CO2. The water mass structure of the glacial deep Atlantic Ocean and the mechanism by which it may have sequestered carbon remain elusive. Here we present neodymium isotope measurements from cores throughout the Atlantic that reveal glacial-interglacial changes in water mass distributions. These results demonstrate the sustained production of North Atlantic Deep Water under glacial conditions, indicating that southern-sourced waters were not as spatially extensive during the Last Glacial Maximum as previously believed. We demonstrate that the depleted glacial δ13C values in the deep Atlantic Ocean cannot be explained solely by water mass source changes. A greater amount of respired carbon, therefore, must have been stored in the abyssal Atlantic during the Last Glacial Maximum. We infer that this was achieved by a sluggish deep overturning cell, comprised of well-mixed northern- and southern-sourced waters.

  19. Meridional Circulation in Solar and Stellar Convection Zones

    CERN Document Server

    Featherstone, Nicholas A

    2015-01-01

    We present a series of 3-D nonlinear simulations of solar-like convection, carried out using the Anelastic Spherical Harmonic (ASH) code, that are designed to isolate those processes that drive and shape meridional circulations within stellar convection zones. These simulations have been constructed so as to span the transition between solar-like differential rotation (fast equator/slow poles) and ``anti-solar' differential rotation (slow equator/fast poles). Solar-like states of differential rotation, arising when convection is rotationally constrained, are characterized by a very different convective Reynolds stress than anti-solar regimes, wherein convection only weakly senses the Coriolis force. We find that the angular momentum transport by convective Reynolds stress plays a central role in establishing the meridional flow profiles in these simulations. We find that the transition from single-celled to multi-celled meridional circulation profiles in strong and weak regimes of rotational constraint is lin...

  20. Regime shifts in the northern North Atlantic during the past 6,000 years: A record of seabird population size and precipitation isotopes on Bjørnøya, Svalbard

    Science.gov (United States)

    D'Andrea, W. J.; Hormes, A.; Bakke, J.; Nicolaisen, L.

    2015-12-01

    The northeastern North Atlantic Ocean, and the Norwegian and Greenland Seas are subject to large hydrographic changes. These variations can influence oceanic heat transport to the Arctic, meridional overturning circulation, and atmospheric circulation patterns and thereby impact global climate patterns. Marine records suggest that numerous large-scale changes in the hydrography of the northern North Atlantic took place during the middle to late Holocene. Here, we report a record of nitrogen and hydrogen isotope measurements from a lake sediment core from Bjørnøya, Svalbard (74.38°N, 19.02°E) that documents major regime shifts in the climate of the northern North Atlantic during the past 6,000 years. Bjørnøya is the nesting ground for one of the largest seabird populations in the North Atlantic. As top predators in the marine ecosystem, seabirds (and their guano) are enriched in 15N; during spring and summer months they deliver this isotopically enriched nitrogen to their nesting area. We developed a record of seabird population changes on Bjørnøya based on the bulk nitrogen isotope composition of sediments in a core collected from lake Ellasjøen. The record reveals multiple multicentennial scale changes in δ15N values (varying between ~8-12‰) that track past changes in the size of seabird populations. From the same sediment core, we also developed a record of δD of precipitation, by measuring δD values of sedimentary n-alkanes. Past intervals with the largest inferred bird populations correspond with the most enriched δD of precipitation, which we interpret to represent a more Atlantic climate. Periods with reduced seabird populations correspond with intervals having more negative δD of precipitation and representing a more Arctic climate. Together, the nitrogen and hydrogen isotope records signify regime shifts in the oceanography, marine ecosystem, and atmospheric circulation of the northern North Atlantic that are related to variations in the

  1. An overturn-cyclic regime of the thermochemical two-mantle evolution

    Science.gov (United States)

    Kotelkin, V.; Lobkovsky, L.

    2009-04-01

    , spreads over the entire surrounding convective cell. In other words, an avalanche or a plume does not have own outer boundary, and its role is played by boundary of the surrounding convective cell. Thus, plume tectonics is incorporated into plate tectonics via common outer boundaries. On the contrary, during slow convection, chemical processes slow down or, if the temperature is insufficient, stop altogether and thermal convection is maintained solely. So chemical processes lead to a nonlinear impulsive regime of convection, they significantly affect the intermittent pattern of mantle convection, facilitating the overcoming of the 670-km endothermic phase barrier. The numerical experiment shows that regional avalanches are observed with the frequency of the geological Bertrand cycles (175 Myr). This result explains the modern seismic tomography data. The main result of our thermochemical modeling is the phenomena of mantle overturn. In case of critical density stratification between upper and lower mantle a new phenomena of self-organization of mantle convection was observed in numerical experiments which can be described as a one global mantle sink overturn flow (superavalanche). As the result this single sink leads to closure of oceans and assembling of continents. The stabilization of sink positions explains the asymmetry of the Earth and fixed placement (opposite sinks) of the Pacific Ocean. The cyclicity of sink formation results in migration of oceans of the atlantic type and supercontinent constructions, it explains the Wilson cycles. To reach critical stratification and realize the overturn-cyclic regime special condition of initial unstable equilibrium state of the mantle is necessary. Such initial state corresponds to new astrophysical data (Hf-W chronometry), i.e. the short time of hot planet accumulation. From the unstable initial state the mantle convection begins with power mantle overturn, and it repeats several times. A considerable time (650-900 Myr) is

  2. Frequency and severity of injuries to operators during overturns of farm tractors.

    Science.gov (United States)

    Cole, H P; Myers, M L; Westneat, S C

    2006-05-01

    Previous estimates of operator death from farm tractor overturn events range from 0.03 to 0.68. This study provided population-based estimates of the denominator of total farm tractor overturns and the frequency of six classes of overturn injury outcomes for tractors with and without rollover protective structures (ROPS). A 40-item telephone survey collected information from a random sample of 6,063 (8.0%) Kentucky farms about each farm's most recent overturn. A total of 551 (9.1%) farms reported overturns, and 5,512 (90.1%) reported no overturns. Injury outcomes for 443 overturns of non-ROPS tractors and 89 ROPS tractors were distributed as follows: no or minor injury (non-ROPS: 70.43%; ROPS: 82.02%), outpatient treatment (non-ROPS: 21.90%; ROPS: 9.00%), hospital admission (non-ROPS: 15.35%; ROPS: 3.37%), temporary disability (non-ROPS: 13.54%; ROPS: 14.61%), permanent disability (non-ROPS: 3.16%; ROPS: 0.00%), and death (non-ROPS: 5.42%; ROPS: 1.12%). (Percents total to more than 100 because some operators treated as outpatients were subsequently hospitalized, disabled, or died.) The observed 0.054 probability of death from overturn of non-ROPS tractors in this sample was corrected for the proportion of farms that went out of business prior to the survey and thus were excluded from the sampling frame. The adjusted 0.08 probability of death from overturn of a non-ROPS tractor is five times smaller than the NIOSH estimate of 0.40. The discrepancy lies in the much larger denominator of all non-fatal and fatal overturns than assumed previously. PMID:16724789

  3. Unipolar outflows and global meridional circulations in rotating accretion flows

    CERN Document Server

    Igumenshchev, I V

    1999-01-01

    Using two-dimensional simulations of non-radiative viscous rotating black hole accretion flows, we show that the flows with alpha~0.1-0.3 self-organize to form stationary unipolar or bipolar outflows accompanied by global meridional circulations. The required energy comes, with efficiency implications are discussed.

  4. Meridional circulation and the lithium abundance gap in F stars

    International Nuclear Information System (INIS)

    The effect of meridional circulation on the time evolution of superficial abundances of helium, lithium, and beryllium in F stars is calculated in detail. It is shown that, as long as the presence of convection zones is assumed not to modify global meridional circulation patterns, the maximum equatorial rotational velocity allowing the settling of Li and He decreases rapidly with T(eff), going from 50 km/s at 7250 K to only 5 km/s at 6400 K. It is also shown that, for stars of the age of the Hyades with T(eff) less than 7000 K and rotational velocities larger than about 25 km/s, meridional circulation is rapid enough to bring to the surface matter that originally was deep enough to have been depleted of its Li through nuclear burning. Observational tests are suggested to distinguish between this scenario and the gravitational settling model or turbulent diffusion model. It is shown how the Li/Be ratio varies in the presence of transport by meridional circulation. 48 references

  5. The water mass structure and transports in the Atlantic Subpolar Gyre

    Science.gov (United States)

    García-Ibáñez, Maribel I.; Pardo, Paula C.; Carracedo, Lidia I.; Mercier, Herlé; Lherminier, Pascale; Ríos, Aida F.; Pérez, Fiz F.

    2014-05-01

    The water mass structure, mixing and spreading in the North Atlantic Subpolar Gyre (SPG) were analyzed by means of an extended Optimum MultiParameter (eOMP) approach over the six repeats of the WOCE A25 hydrographic line located at the southern boundary of this gyre. The data includes the Fourex (4x) line taken in 1997 and the five repeat sections of the OVIDE line taken every other year from 2002 to 2010. We proposed 10 water masses, defined by their thermohaline properties (potential temperature and salinity), oxygen and nutrients (nitrate, phosphate and silicate), to resolve the water mass structure of the SPG. The eOMP enables to decompose the transports by water mass quantitatively. Our model provides water mass distributions that are able to reproduce the input data of potential temperature, salinity and silicate with r2>0.997 and of oxygen, nitrate and phosphate with r2>0.96. By combining the velocity field and the water mass structure across each section we provide the relative contribution of each water mass to the Meridional Overturning Circulation (MOC) and we evaluate the water mass transformation in the North Atlantic. The MOC upper limb during OVIDE (2002-2010) is constituted by the northward transports of the central waters (9.4 Sv; 1 Sv = 106 m3 s-1), the Subarctic Intermediate Water (SAIW, 2.8 Sv) and the Subpolar Mode Water (SPMW) of the Iceland Basin (2.1 Sv). The MOC lower limb is constituted by the southward transports of the Iceland-Scotland Overflow Water (ISOW, 2.9 Sv), the Denmark Strait Overflow Water (DSOW, 2.5 Sv), the Polar Intermediate Water (PIW, 0.8 Sv), the Labrador Sea Water (LSW, 3.6 Sv) and the SPMW of the Irminger Sea (4.7 Sv). These results contrast with those obtained for the 1997, cruise developed after a period of high NAO index. The greater MOC strength in 1997 resulted in greater northward transports of central waters (17.5 Sv), while the SAIW transports remained approximately unchanged. The increase of the northward

  6. Meridional flow velocities on solar-like stars with known activity cycles

    CERN Document Server

    Baklanova, Dilyara

    2014-01-01

    The direct measurements of the meridional flow velocities on stars are impossible today. To evaluate the meridional flow velocities on solar-like stars with stable activity periods, we supposed that during the stellar Hale cycle the matter on surfaces of stars passes the meridional way equivalent to $2\\pi R_\\star$. We present here the dependence of the mean meridional flow velocity on Rossby number, which is an effective parameter of the stellar magnetic dynamo.

  7. A flux-transport dynamo with a multi-cell meridional circulation

    Science.gov (United States)

    Bonanno, A.; Elstner, D.; Belvedere, G.; Rüdiger, G.

    2005-04-01

    We discuss the effect of a non-trivial meridional circulation pattern on a flux-transport type of solar dynamo. The critical value of the turbulent helicity and the periods are calculated as a function of the meridional flow strength. We found that the dynamo mechanism is mainly determined by the global topology of the meridional flow. In particular the equatorwards migration in the butterfly diagram can be easily obtained by the combined action of two cells of meridional circulation.

  8. Alleviating α quenching by solar wind and meridional flows

    Science.gov (United States)

    Mitra, D.; Moss, D.; Tavakol, R.; Brandenburg, A.

    2011-02-01

    Aims: We study the ability of magnetic helicity expulsion to alleviate catastrophic α-quenching in mean field dynamos in two-dimensional spherical wedge domains. Methods: Motivated by the physical state of the outer regions of the Sun, we consider α^2Ω mean field models with a dynamical α quenching. We include two mechanisms which have the potential to facilitate helicity expulsion, namely advection by a mean flow ("solar wind") and meridional circulation. Results: We find that a wind alone can prevent catastrophic quenching, with the field saturating at finite amplitude. In certain parameter ranges, the presence of a large-scale meridional circulation can reinforce this alleviation. However, the saturated field strengths are typically below the equipartition field strength. We discuss possible mechanisms that might increase the saturated field.

  9. Alleviating alpha quenching by solar wind and meridional flow

    CERN Document Server

    Mitra, Dhrubaditya; Tavakol, Reza; Brandenburg, Axel

    2010-01-01

    We study the ability of magnetic helicity expulsion to alleviate catastrophic $\\alpha$-quenching in mean field dynamos in two--dimensional spherical wedge domains. Motivated by the physical state of the outer regions of the Sun, we consider $\\alpha^2\\Omega$ mean field models with a dynamical $\\alpha$ quenching. We include two mechanisms which have the potential to facilitate helicity expulsion, namely advection by a mean flow (``solar wind'') and meridional circulation. We find that a wind alone can prevent catastrophic quenching, with the field saturating at finite amplitude. In certain parameter ranges, the presence of a large-scale meridional circulation can reinforce this alleviation. However, the saturated field strengths are typically below the equipartition field strength. We discuss possible mechanisms that might increase the saturated field.

  10. Toric and toric multifocal IOLs in meridional amblyopia

    Institute of Scientific and Technical Information of China (English)

    Kirithika; Muthusamy; Charles; Claoué

    2015-01-01

    <正>Dear Sir,M eridional amblyopia is sometimes put forward as a reason for not implanting Toric intraocular lenses(IOLs).It has been noted that patients with high levels of childhood astigmatism(>3 DC)can develop persistent orientation-dependent visual deficits despite optical correction.Studies by Mitchell et al[1]demonstrated that meridional visual deprivation during the critical period of visual development results in permanently reduced response to stimuli in those orientations.This phenomenon was termed

  11. Chores at Times of Fatal or Serious Injuries Associated with Tractor Overturns with and without Rollover Protection

    Directory of Open Access Journals (Sweden)

    Henry P. Cole

    2016-09-01

    Full Text Available This study describes chores when farmers were either fatally or seriously injured and required emergency medical treatment as a result of overturns of tractors with or without rollover protective structures (ROPS. Data from the 2002 Kentucky Farm Tractor Overturn Survey were used for this study. The data were collected by a telephone survey of a population-based random sample of 6063 (7.98% of Kentucky’s 76,017 farm operators as listed in the Kentucky Agricultural Statistics Service database. Of farm operators interviewed, 551 (9.1% reported 603 overturns and 5512 (90.9% reported no overturns in the history of their farm, covering a period from 1925 to February 2002. Only the latest overturn was considered to improve recall accuracy. In addition, since the 1925 to 1959 time period had only 49 (8.1% of the overturns reported, (14 farmers did not provide the year of most recent overturn; only data from the 1960 to 2002 period (approximately 41 years were used. After making these adjustments, incidents evaluated included 25 cases (one fatal and four serious nonfatal injuries that involved ROPS-equipped tractor overturns and 88 cases (24 fatal and 64 serious nonfatal injuries that involved non-ROPS tractor overturns. Chores at highest risk for tractor overturns were identified for which educational and ROPS retrofit interventions could be emphasized. The highest frequency of overturn-related fatalities and nonfatal injuries were associated with hay harvesting, rotary mowing, and on-farm travel chores. These three chores represented 68.2% of fatal events and 50.0% of permanent and 56.6% of temporary disability overturn incidents. Tragically, in countries such as India and China with emerging mechanization, a large majority of tractors are produced without ROPS that can be expected to result in the same overturn-related epidemic of deaths experienced in highly mechanized countries, despite evidence of the protection provided by ROPS.

  12. Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade

    Science.gov (United States)

    Hellmer, Hartmut H.; Rhein, Monika; Heinemann, Günther; Abalichin, Janna; Abouchami, Wafa; Baars, Oliver; Cubasch, Ulrich; Dethloff, Klaus; Ebner, Lars; Fahrbach, Eberhard; Frank, Martin; Gollan, Gereon; Greatbatch, Richard J.; Grieger, Jens; Gryanik, Vladimir M.; Gryschka, Micha; Hauck, Judith; Hoppema, Mario; Huhn, Oliver; Kanzow, Torsten; Koch, Boris P.; König-Langlo, Gert; Langematz, Ulrike; Leckebusch, Gregor C.; Lüpkes, Christof; Paul, Stephan; Rinke, Annette; Rost, Bjoern; van der Loeff, Michiel Rutgers; Schröder, Michael; Seckmeyer, Gunther; Stichel, Torben; Strass, Volker; Timmermann, Ralph; Trimborn, Scarlett; Ulbrich, Uwe; Venchiarutti, Celia; Wacker, Ulrike; Willmes, Sascha; Wolf-Gladrow, Dieter

    2016-09-01

    In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program `Antarctic Research' (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals

  13. Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade

    Science.gov (United States)

    Hellmer, Hartmut H.; Rhein, Monika; Heinemann, Günther; Abalichin, Janna; Abouchami, Wafa; Baars, Oliver; Cubasch, Ulrich; Dethloff, Klaus; Ebner, Lars; Fahrbach, Eberhard; Frank, Martin; Gollan, Gereon; Greatbatch, Richard J.; Grieger, Jens; Gryanik, Vladimir M.; Gryschka, Micha; Hauck, Judith; Hoppema, Mario; Huhn, Oliver; Kanzow, Torsten; Koch, Boris P.; König-Langlo, Gert; Langematz, Ulrike; Leckebusch, Gregor C.; Lüpkes, Christof; Paul, Stephan; Rinke, Annette; Rost, Bjoern; van der Loeff, Michiel Rutgers; Schröder, Michael; Seckmeyer, Gunther; Stichel, Torben; Strass, Volker; Timmermann, Ralph; Trimborn, Scarlett; Ulbrich, Uwe; Venchiarutti, Celia; Wacker, Ulrike; Willmes, Sascha; Wolf-Gladrow, Dieter

    2016-11-01

    In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program `Antarctic Research' (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals

  14. The Investigation of EM Scattering from the Time-Varying Overturning Wave Crest Model by the IEM

    Directory of Open Access Journals (Sweden)

    Xiao Meng

    2016-01-01

    Full Text Available Investigation of the electromagnetic (EM scattering of time-varying overturning wave crests is a worthwhile endeavor. Overturning wave crest is one of the reasons of sea spike generation, which increases the probability of false radar alarms and reduces the performance of multitarget detection in the environment. A three-dimensional (3D time-varying overturning wave crest model is presented in this paper; this 3D model is an improvement of the traditional two-dimensional (2D time-varying overturning wave crest model. The integral equation method (IEM was employed to investigate backward scattering radar cross sections (RCS at various incident angles of the 3D overturning wave crest model. The super phenomenon, where the intensity of horizontal polarization scattering is greater than that of vertical polarization scattering, is an important feature of sea spikes. Simulation results demonstrate that super phenomena may occur in some time samples as variations in the overturning wave crest.

  15. Instability of Meridional Axial System in f(R) Gravity

    CERN Document Server

    Sharif, M

    2015-01-01

    We analyze dynamical instability of non-static reflection axial stellar structure by taking into account generalized Euler's equation in metric $f(R)$ gravity. Such an equation is obtained by contracting Bianchi identities of usual anisotropic and effective stress-energy tensors, which after using radial perturbation technique gives modified collapse equation. In the realm of $R+\\epsilon R^n$ gravity model, we investigate instability constraints at Newtonian and post-Newtonian approximations. We find that instability of meridional axial self-gravitating system depends upon static profile of structure coefficients while $f(R)$ extra curvature terms induce stability to the evolving celestial body.

  16. The Meridional Flux of Sensible Heat over the Northern Hemisphere

    OpenAIRE

    White, Robert M.

    2011-01-01

    The meridional eddy flux of sensible heat is examined by means of finite difference integration methods from northern hemisphere charts for the winter 1945-46. The eddy sensible heat flux is shown to occur principally in the lowest layers of the atmosphere and to decrease with elevation. The magnitude of the eddy sensible heat transports are compared with the total poleward energy flux required by the radiation balance, and are shown to comprise a very large fraction of this flux.DOI: 10.1111...

  17. Paisatge i conflictes territorials a les comarques meridionals de Catalunya

    OpenAIRE

    Saladié Gil, Sergi

    2016-01-01

    Aquesta tesi doctoral analitza el paper del paisatge en els conflictes territorials que els darrers quinze anys han tingut lloc a les comarques meridionals de Catalunya, i reflexiona sobre la seva capacitat com a condicionador dels conflictes. Per assolir aquest objectiu, la investigació s’ha centrat en diversos aspectes clau. Primer, la tesi fa un repàs teòric als conceptes de conflicte territorial i paisatge, així com a les polítiques de paisatge desplegades a Catalunya els darrers anys....

  18. On the secondary meridional circulation associated with the quasi-biennial oscillation

    OpenAIRE

    Choi, Wookap; Lee, Hyunah; Grant, William B.; Park, Jae H.; HOLTON, JAMES R.; Lee, Kwang-Mog; Naujokat, Barbara

    2002-01-01

    Concentrations and distributions of stratospheric aerosol, hydrogen fluoride and ozone from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) are used to investigate features associated with transport by the secondary meridional circulation induced by the quasi-biennial oscillation (QBO). The points of maxima in the divergence and convergence of the QBO-induced meridional velocity at the equator are identified from the meridional gradients of the tra...

  19. Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?

    OpenAIRE

    Hazra, Gopal; Karak, Bidya Binay; Choudhuri, Arnab Rai

    2013-01-01

    The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still r...

  20. The role of meridional motions for the solar dynamo

    Science.gov (United States)

    Elstner, D.; Rüdiger, G.

    The dynamo equation is solved for the solar convection zone with the given (“observed”) rotation law and positive α-effect. If the latter exists in the entire convection zone the resulting dynamo shows strong toroidal field belts in the polar region migrating equatorwards. The same happens for α concentrated at the bottom of the convection zone but then we get too many belts with higher amplitude. The cycle period is always too short. Including meridional circulation which is directed equatorwards at the bottom of the convection zone (where the eddy diffusivity is reduced), the amplitude of the toroidal field grows and the butterfly diagram reaches low-latitudes. The cycle time approaches the solar value. The dynamo regime is highly sensitive to the interplay between flow and diffusivity at the bottom of the convection zone. Stationary solutions are not very seldom. For less active stars a slight increase of the cycle period with the rotation period is observed in agreement with the decrease of the meridional flow for faster rotation.

  1. Evidence for a poleward meridional flow on the sun

    Energy Technology Data Exchange (ETDEWEB)

    Topka, K.; Moore, R. (California Inst. of Tech., Pasadena (USA). Big Bear Solar Observatory); LaBonte, B.J.; Howard, R. (Mount Wilson Observatory, Pasadena, CA (USA))

    1982-08-01

    We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episode polar field formation and the meridional flow.

  2. Measuring Photospheric Rotational and Meridional Flows Using Magnetic Feature Tracking

    Science.gov (United States)

    Lamb, Derek

    2016-05-01

    Long-lived rotational and meridional flows are important ingredients of the solar cycle. Using magnetic field images to measure these flows on the solar surface has typically been performed by cross-correlating thin longitudinal strips or square patches across sufficiently long time gaps. Here, I use one month of SDO/HMI line-of-sight magnetic field observations, combined with the SWAMIS magnetic feature tracking algorithm to measure the motion of individual features in these magnetograms. By controlling for perturbations due to short-lived flows and due to false motions from feature interactions, I effectively isolate the long-lived flows traced by the magnetic features. This allows me to produce high-fidelity differential rotation measurements with well-characterized variances and covariances of the fit parameters. I also produce medium-fidelity measurements of the much weaker meridional flow that is broadly consistent with previous results, showing a peak flow of 16.7 m/s at 45 degrees latitude. This work shows that measuring the motions of individual features in photospheric magnetograms can produce high precision results in relatively short time spans, which suggests that high resolution non-longitudinally averaged photospheric velocity residual measurements could be produced to compare with coronal results, and to provide other diagnostics of the solar dynamo. This work has been partially supported by NASA Grants NNX11AP03G and NNX14AJ67G.

  3. Supergranules as Probes of the Sun's Meridional Circulation

    CERN Document Server

    Hathaway, David H

    2012-01-01

    Recent analysis revealed that supergranules (convection cells seen at the Sun's surface) are advected by the zonal flows at depths equal to the widths of the cells themselves. Here we probe the structure of the meridional circulation by cross-correlating maps of the Doppler velocity signal using a series of successively longer time lags between maps. We find that the poleward meridional flow decreases in amplitude with time lag and reverses direction to become an equatorward return flow at time lags > 24 hours. These cross-correlation results are dominated by larger and deeper cells at longer time lags. (The smaller cells have shorter lifetimes and do not contribute to the correlated signal at longer time lags.) We determine the characteristic cell size associated with each time lag by comparing the equatorial zonal flows measured at different time lags with the zonal flows associated with different cell sizes from a Fourier analysis. This association gives a characteristic cell size of ~50 Mm at a 24 hour ti...

  4. What caused the 2009 cold event in the Atlantic cold tongue region?

    Science.gov (United States)

    Burmeister, Kristin; Brandt, Peter; Lübbecke, Joke F.

    2016-04-01

    The tropical Atlantic (TA) exhibits sea surface temperature (SST) variability on seasonal to inter-annual time scales. This variability is associated with changes of atmospheric dynamics, linking it to severe flooding or droughts in South America and West Africa. This study investigates processes in the TA that might have caused the extreme cold event in the Atlantic cold tongue (ACT) region in 2009. During boreal spring, a strong negative Atlantic meridional mode event developed in the TA associated with northwesterly wind anomalies along the equator. Contrary to what would be expected from ENSO-like dynamics, these wind anomalies did not lead to a warming in the eastern equatorial Atlantic in boreal summer. Instead, from May to August 2009, an abrupt cooling took place in the ACT region resulting in the coldest August ACT SST on record. In the literature, two processes - equatorial wave reflection and meridional advection of subsurface temperatures - are discussed as potential causes of such an event. Whereas previous studies are mainly based on satellite data, reanalysis products and model output, we here use in situ measurements (data from Argo floats, PIRATA buoys, and TACE moorings, as well as CTD data of various ship cruises) in addition to satellite and reanalysis products to investigate the contribution of both processes to the strong surface cooling in the ACT region in 2009. Results based on the Argo float data confirm previous findings that equatorial wave reflection contributed to the cold event in the ACT region in 2009. They further indicate that higher baroclinic mode waves played an important role. The analysis of in situ and reanalysis temperature and velocity data does not suggest a significant contribution of meridional advection of subsurface temperatures for the onset of the 2009 cold event. The results indicate an asymmetry in the importance of meridional advection for non-ENSO-like cold and warm events with warm events more strongly affected

  5. Light transmission for polymer fibers using skew and meridional rays

    Science.gov (United States)

    Tekelioglu, Murat

    This dissertation is concerned with the development of a light transmission model for polymer optical fibers having application in hybrid solar lighting (HSL) systems. Conceptually, the HSL system consists of a solar collector/receiver that focuses concentrated visible solar light onto a polymer optical fiber (up to 10-m-long) that transports the solar light to an interior space. For this study, the polymer optical fiber is a large-core (0.2-40 mm diameter) plastic optical fiber (POF) comprised of various lengths of straight and bent sections. Although there has been extensive research on the transmission of monochromatic light through optical fibers for the communications industry, there are relatively few publications for visible light transmission through POFs. These publications were critically reviewed in this research. It is shown that the light transmission can be described with either skew or meridional rays. For each ray type, the HSL system light transmission was determined as a function of fiber geometrical properties (core and cladding radii, bend radius, bend angle, and fiber length) and optical properties (core and clad refractive indices, absorption and scattering coefficients, core-clad rms roughness height, and core-clad interface defects loss coefficient). To do this, first, models were developed for separate straight and bent sections for multiple skew and meridional rays. Second, the straight and bent models were combined into a FORTRAN simulation program for an arbitrary fiber with a combination of arbitrary straight and bent sections. The input condition of the rays (arrangement of rays, incident angle, and intensity profile) is user-defined. Third, the simulation results were experimentally validated using two different POFs, twelve configurations of straight and bent fiber subsystems, and two ray types. These experimental comparisons show that the transmission model using meridional rays is slightly better than that with the skew rays. But

  6. Neural network analysis of overturning response under near-fault type excitation

    Institute of Scientific and Technical Information of China (English)

    Nikos Gerolymos; Marios Apostolou; George Gazetas

    2005-01-01

    Under strong seismic excitation, a rigid block will uplift from its support and undergo rocking oscillations which may lead to (complete) overturning. Numerical and analytical solutions to this highly nonlinear vibration problem are first highlighted in the paper and then utilized to demonstrate how sensitive the overturning behavior is not only to the intensity and frequency content of the base motion, but also to the presence of strong pulses, to their detailed sequence, and even to their asymmetry. Five idealised pulses capable of representing "rupture-directivity" and "fling" affected ground motions near the fault, are utilized to this end: the one-cycle sinus, the one-cycle cosinus, the Ricker wavelet, the truncated (T)-Ricker wavelet, and the rectangular pulse "Overturning-Acceleration Amplification" and "Rotation" spectra are introduced and presented. Artificial neural network modeling is then developed as an alternative numerical solution. The neural network analysis leads to closed-form expressions for predicting the overturning failure or survival of a rigid block,as a function of its geometric properties and the characteristics of the excitation time history. The capability of the developed neural network modeling is validated through comparisons with the numerical solution. The derived analytical expressions could also serve as a tool for assessing the destructiveness of near-fault ground motions, for structures sensitive to rocking with foundation uplift.

  7. Seasonal overturning circulation in the Red Sea: 1. Model validation and summer circulation

    KAUST Repository

    Yao, Fengchao

    2014-04-01

    The overturning circulation in the Red Sea exhibits a distinct seasonally reversing pattern and is studied using high-resolution MIT general circulation model simulations. In the first part of this study, the vertical and horizontal structure of the summer overturning circulation and its dynamical mechanisms are presented from the model results. The seasonal water exchange in the Strait of Bab el Mandeb is successfully simulated, and the structures of the intruding subsurface Gulf of Aden intermediate water are in good agreement with summer observations in 2011. The model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the Gulf of Aden resulting from remote winds in the Arabian Sea and an upward surface slope from the Red Sea to the Gulf of Aden set up by local surface winds in the Red Sea. In addition, during late summer two processes associated, respectively, with latitudinally differential heating and increased salinity in the southern Red Sea act together to cause the reversal of the contrast of the vertical density structure and the cessation of the summer overturning circulation. Dynamically, the subsurface northward pressure gradient force is mainly balanced by vertical viscosity resulting from the vertical shear and boundary friction in the Strait of Bab el Mandeb. Unlike some previous studies, the three-layer summer exchange flows in the Strait of Bab el Mandeb do not appear to be hydraulically controlled.

  8. Instability of meridional axial system in f(R) gravity

    Energy Technology Data Exchange (ETDEWEB)

    Sharif, M.; Yousaf, Z. [University of the Punjab, Department of Mathematics, Lahore (Pakistan)

    2015-05-15

    We analyze the dynamical instability of a non-static reflection axial stellar structure by taking into account the generalized Euler equation in metric f(R) gravity. Such an equation is obtained by contracting the Bianchi identities of the usual anisotropic and effective stress-energy tensors, which after using a radial perturbation technique gives a modified collapse equation. In the realm of the R + εR{sup n} gravity model, we investigate instability constraints at Newtonian and post-Newtonian approximations. We find that the instability of a meridional axial self-gravitating system depends upon the static profile of the structure coefficients, while f(R) extra curvature terms induce the stability of the evolving celestial body. (orig.)

  9. An analytical framework for understanding tropical Meridional Modes

    CERN Document Server

    Martinez-Villalobos, Cristian

    2016-01-01

    A theoretical framework is developed for understanding the transient growth and propagation characteristics of thermodynamically coupled, meridional mode-like structures in the tropics. The model consists of a Gill-Matsuno type steady atmosphere under the longwave approximation coupled via a wind-evaporation-sea surface temperature (WES) feedback to a "slab" ocean model. When projected onto basis functions for the atmosphere the system simplifies to a non-normal set of equations that describes the evolution of individual sea surface temperature (SST) modes, with clean separation between symmetric and anti-symmetric modes. The following major findings result from analysis of the system: (i) a transient growth process exists whereby specific SST modes propagate toward lower order modes at the expense of the higher-order modes; (ii) the same dynamical mechanisms govern the evolution of symmetric and anti-symmetric SST modes except for the lowest-order wave number, where for symmetric structures the atmospheric K...

  10. Is the Butterfly diagram due to meridional motions?

    Science.gov (United States)

    Rüdiger, G.; Elstner, D.

    2002-07-01

    The dynamo equation is solved for the solar convection zone with the given (``observed'') rotation law and positive \\alf-effect. If the latter exists in the entire convection zone the resulting dynamo shows strong toroidal field belts in the polar region migrating equatorwards. The same happens for alpha concentrated at the bottom of the convection zone but then we get too many belts with higher amplitude. The cycle period is always too short. Including meridional circulation which is directed equatorwards at the bottom of the convection zone (where the eddy diffusivity is reduced), the amplitude of the toroidal field grows and the butterfly diagram reaches low-latitudes. The cycle time approaches the solar value. The dynamo regime is highly sensitive to the interplay between flow and diffusivity at the bottom of the convection zone. Stationary solutions are not very seldom.

  11. Evidence for a poleward meridional flow on the sun

    Science.gov (United States)

    Topka, K.; Moore, R.; Labonte, B. J.; Howard, R.

    1982-01-01

    Two subsets of all polar zone filaments, designated polemost filament and polar filament bands, are defined for observational study of their behavior, which is compared with the evolution of the polar magnetic field over Howard and LaBonte's (1981) activity cycle. The magnetic data show that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions originating in active region latitudes and subsequently drifting to the poles. F-polarity regions are carried poleward by a meridional flow, rather than by diffusion. It is noted that the mean latitude of the polemost filaments tracks the boundary of the polar field cap, and undergoes an equatorward dip during each arrival of additional polar field, and that the polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m/sec.

  12. Meridional Circulation From Differential Rotation in an Adiabatically Stratified Solar/Stellar Convection Zone

    CERN Document Server

    Dikpati, Mausumi

    2013-01-01

    Meridional circulation in stellar convection zones is not generally well observed, but may be critical for MHD dynamos. Coriolis forces from differential rotation (DR) play a large role in determining what the meridional circulation is. Here we consider whether a stellar DR that is constant on cylinders concentric with the rotation axis can drive a meridional circulation.Conventional wisdom says that it can not. Using two related forms of governing equations that respectively estimate the longitudinal components of the curl of meridional mass flux and the vorticity, we show that such DR will drive a meridional flow. This is because to satisfy anelastic mass conservation, non-spherically symmetric pressure contours must be present for all DRs, not just ones that depart from constancy on cylinders concentric with the rotation axis. Therefore the fluid is always baroclinic if DR is present, because, in anelastic systems, the perturbation pressure must satisfy a Poisson type equation, as well as an equation of st...

  13. Pulsation-driven mean zonal and meridional flows in rotating massive stars

    CERN Document Server

    Lee, Umin; Neiner, Coralie

    2015-01-01

    Zonal and meridional axisymmetric flows can deeply impact the rotational and chemical evolution of stars. Therefore, momentum exchanges between waves propagating in stars, differential rotation, and meridional circulation must be carefully evaluated. In this work, we study axisymmetric mean flows in rapidly and initially uniformly rotating massive stars driven by small amplitude non-axisymmetric $\\kappa$-driven oscillations. We treat them as perturbations of second-order of the oscillation amplitudes and derive their governing equations as a set of coupled linear ordinary differential equations. This allows us to compute 2-D zonal and meridional mean flows driven by low frequency $g$- and $r$-modes in slowly pulsating B (SPB) stars and $p$-modes in $\\beta$ Cephei stars. Oscillation-driven mean flows usually have large amplitudes only in the surface layers. In addition, the kinetic energy of the induced 2-D zonal rotational motions is much larger than that of the meridional motions. In some cases, meridional f...

  14. Meridional Circulation in Solar and Stellar Convection Zones

    Science.gov (United States)

    Featherstone, Nicholas A.; Miesch, Mark S.

    2015-05-01

    We present a series of 3D nonlinear simulations of solar-like convection, carried out using the Anelastic Spherical Harmonic code, that are designed to isolate those processes that drive and shape meridional circulations (MCs) within stellar convection zones. These simulations have been constructed so as to span the transition between solar-like differential rotation (DR; fast equator/slow poles) and “anti-solar” DR (slow equator/fast poles). Solar-like states of DR, which arise when convection is rotationally constrained, are characterized by a very different convective Reynolds stress (RS) than anti-solar regimes, wherein convection only weakly senses the Coriolis force. We find that the angular momentum transport by convective RS plays a central role in establishing the meridional flow profiles in these simulations. We find that the transition from single-celled to multi-celled MC profiles in strong and weak regimes of rotational constraint is linked to a change in the convective RS, which is a clear demonstration of gyroscopic pumping. Latitudinal thermal variations differ between these different regimes, with those in the solar-like regime conspiring to suppress a single cell of MC, whereas the cool poles and warm equator established in the anti-solar states tend to promote single-celled circulations. Although the convective angular momentum transport becomes radially inward at mid-latitudes in anti-solar regimes, it is the MC that is primarily responsible for establishing a rapidly rotating pole. We conclude with a discussion of how these results relate to the Sun, and suggest that the Sun may lie near the transition between rapidly rotating and slowly rotating regimes.

  15. Variations in the Strength of the North Atlantic Bottom water during Holocene.

    Science.gov (United States)

    Kissel, C.; Van Toer, A.; Michel, E.; Cortijo, E.

    2012-04-01

    centered at about 8.4 kyr (cal. ages) is marked by a pronounced minima in magnetic content and the smaller mean sortable silt sizes, typical for an abrupt reduction in deep flow speed. At the same time, the benthic delta13C values which could be obtained from Cib. wuellerstorfi reach significantly negative values (-0.5‰) providing evidence of a significant change to a major downwelling limb of the Atlantic meridional overturning circulation. This event is in phase with the meltwater outbursts from the final drainage of the proglacial lakes associated with the decaying Laurentide Ice Sheet margin. In addition, all through the Holocene, a series of short-term events of lower bottom flow speed and weaker ISOW always illustrated by minima mean size of the sortable silt and in magnetic concentration respectively are observed with a periodicity of 300-600 years between 6 and 2 kyr. These results are compared to those we obtained from other cores located along the Gardar Drift (P.I.C.A.S.S.O cruise in 2003) and the Eirik drift and with recently published results.

  16. Variations in the Holocene North Atlantic Bottom Current Strength in the Charlie Gibbs Fracture Zone

    Science.gov (United States)

    Kissel, C.; Van Toer, A.; Cortijo, E.; Turon, J.

    2011-12-01

    speed. Although not exactly at the same age, we note that the pattern in the same as the one observed by Ellison et al. (2006) further north along the Gardar drift with a gradual decrease in the mean sortable silt size followed by a two steps rather fast increase. At the same time, the benthic delta13C values which could be obtained from the few Cib. wuellerstorfi present in the sediment reach significantly negative values (-0.5%) providing evidence of a significant change to a major downwelling limb of the Atlantic meridional overturning circulation. This event is in phase with the meltwater outbursts from the final drainage of the proglacial lakes associated with the decaying Laurentide Ice Sheet margin. In addition, all through the Holocene, a series of short-term events of lower bottom flow speed always illustrated by minima in magnetic concentration and mean size of the sortable silt are observed with a periodicity of 900 years between 6 and 2 kyr.

  17. Great Basin hydrology, paleoclimate, and connections with the North Atlantic: A speleothem stable isotope and trace element record from Lehman Caves, NV

    Science.gov (United States)

    Cross, Mellissa; McGee, David; Broecker, Wallace S.; Quade, Jay; Shakun, Jeremy D.; Cheng, Hai; Lu, Yanbin; Edwards, R. Lawrence

    2015-11-01

    uncertainties with the abrupt strengthening of the East Asian summer monsoon, which marks the end of both the Weak Monsoon Interval and Heinrich Stadial 11. Our records demonstrate a link between North Atlantic climate and Great Basin moisture during this time that is consistent with the interpretation of data from the last glacial period and may result from abrupt shifts in Atlantic Meridional Overturning Circulation affecting the strength of the Aleutian Low.

  18. Coccolithophore paleoproductivity and ecology response to deglacial and Holocene changes in the Azores Current System

    DEFF Research Database (Denmark)

    Schwab, C.; Kinkel, Hanno; Weinelt, M.;

    2012-01-01

    In order to test the sensitivity of marine primary productivity in the midlatitude open ocean North Atlantic to changes in the Atlantic Meridional Overturning Circulation (AMOC), we investigated two spliced sediment cores from a site south of the Azores Islands at the northern rim of the North At...

  19. Numerical simulation of the sensitivity of the Pacific subtropical-tropical meridional cell to global warming

    Institute of Scientific and Technical Information of China (English)

    ZHOU Tianjun; YU Yongqiang; LIU Hailong; LI Wei; YU Rucong

    2006-01-01

    Sensitivity of the Pacific subtropical-tropical meridional cell to global warming is examined by using a global ocean-atmosphere coupled model developed at LASG/IAP. Results indicate that associated with the increasing of atmospheric carbon dioxide, the most prominent signals of global warming locate at high latitudes, and the change of middle and low latitudes, in particular the surface wind, is relatively weak, which leads to a weak response of the Pacific subtropical-tropical meridional cell. At the time of atmospheric carbon dioxide doubling, the change of the meridional cell strength is smaller than the amplitude of natural variability.

  20. Loki, Io: New groundbased observations and a model describing the change from periodic overturn

    CERN Document Server

    Rathbun, J A; Rathbun, Julie A.; Spencer, John R.

    2006-01-01

    Loki Patera is the most powerful volcano in the solar system. We have obtained measurements of Loki's 3.5 micron brightness from NASA's Infrared Telescope Facility (IRTF) and have witnessed a change from the periodic behavior previously noted. While Loki brightened by a factor of several every 540 days prior to 2001, from 2001 through 2004 Loki remained at a constant, medium brightness. We have constructed a quantitative model of Loki as a basaltic lava lake whose solidified crust overturns when it becomes buoyantly unstable. By altering the speed at which the overturn propagates across the patera, we can match our groundbased brightness data. In addition, we can match other data taken at other times and wavelengths. By slowing the propagation speed dramatically, we can match the observations from 2001-2004. This slowing may be due to a small change in volatile content in the magma.

  1. The Sliding and Overturning Analysis of Spent Fuel Storage Rack Based on Dynamic Analysis Model

    OpenAIRE

    Liu, Yu; Lu, Daogang; Wang, Yuanpeng; LIU, HONGDA

    2016-01-01

    Spent fuel rack is the key equipment for the storage of spent fuel after refueling. In order to investigate the performance of the spent fuel rack under the earthquake, the phenomena including sliding, collision, and overturning of the spent fuel rack were studied. An FEM model of spent fuel rack is built to simulate the transient response under seismic loading regarding fluid-structure interaction by ANSYS. Based on D’Alambert’s principle, the equilibriums of force and momentum were establis...

  2. FEM for Stability Analysis Against Overturning of Portal Water Injection Sheet Pile

    Institute of Scientific and Technical Information of China (English)

    LIU Lingyun; GUO Haiyan; SUN Qi

    2006-01-01

    Portal water injection sheet pile (PWISP), as a retaining wall, appeared in seashore engineering in 2000.Although there have been many systematic methods addressing the issue, there are very few focusing on the new structure because of the difficulties in defining the earth pressure between the two piles.A new method is proposed in this paper to obtain the earth pressure between the PWISPs.Stability analysis against overturning follows as a consequence.Using Finite Element Analysis (FEA) software ANSYS, both the nonlinear characteristics of the soil and those of the contact elements are taken into account to obtain the earth pressure distribution on the contact surface.Based on the results of the FEA, Rankin's theory and the slip plane theory, the formula of the earth pressure on the inner surfaces between the piles is given.Assuming the PWISP as the analysis object and the earth pressure as an outside force acting upon it, the equation of stability against overturning of the PWISP is presented.Finally, some parameters are discussed about the stability of the PWISP against overturning, such as the embedded depth of the front pile, the distance between the two rows of piles, the internal friction angle and the cohesion of the earth.The results show that the increase of the cohesion and the internal friction angle will decrease the distance and the embedded depth, and therefore enhance the stability against overturning.Specifically, when the distance is 1/3-2/3 of the maximal excavation depth, the two rows of piles give the best performance in stability.

  3. Meridional equatorial electrojet current in the American sector

    Directory of Open Access Journals (Sweden)

    R. G. Rastogi

    Full Text Available Huancayo is the only equatorial electrojet station where the daytime increase of horizontal geomagnetic field (H is associated with a simultaneous increase of eastward geomagnetic field (Y. It is shown that during the counter electrojet period when ∆H is negative, ∆Y also becomes negative. Thus, the diurnal variation of ∆Y at equatorial latitudes is suggested to be a constituent part of the equatorial electrojet current system. Solar flares are known to increase the H field at an equatorial station during normal electrojet conditions (nej. At Huancayo, situated north of the magnetic equator, the solar flare effect, during nej, consists of positive impulses in H and Y and negative impulse in Z field. During counter electrojet periods (cej, a solar flare produces a negative impulse in H and Y and a positive impulse in Z at Huancayo. It is concluded that both the zonal and meridional components of the equatorial electrojet in American longitudes, as in Indian longitudes, flows in the same, E region of the ionosphere.

    Key words. Geomagnetism and paleomagnetism (dynamo theories · Ionosphere (equatorial ionosphere; ionosphere disturbances

  4. The Sliding and Overturning Analysis of Spent Fuel Storage Rack Based on Dynamic Analysis Model

    Directory of Open Access Journals (Sweden)

    Yu Liu

    2016-01-01

    Full Text Available Spent fuel rack is the key equipment for the storage of spent fuel after refueling. In order to investigate the performance of the spent fuel rack under the earthquake, the phenomena including sliding, collision, and overturning of the spent fuel rack were studied. An FEM model of spent fuel rack is built to simulate the transient response under seismic loading regarding fluid-structure interaction by ANSYS. Based on D’Alambert’s principle, the equilibriums of force and momentum were established to obtain the critical sliding and overturning accelerations. Then 5 characteristic transient loadings which were designed based on the critical sliding and overturning accelerations were applied to the rack FEM model. Finally, the transient displacement and impact force response of rack with different gap sizes and the supporting leg friction coefficients were analyzed. The result proves the FEM model is applicable for seismic response of spent fuel rack. This paper can guide the design of the future’s fluid-structure interaction experiment for spent fuel rack.

  5. A global hybrid coupled model based on Atmosphere-SST feedbacks

    CERN Document Server

    Cimatoribus, Andrea A; Dijkstra, Henk A

    2011-01-01

    A global hybrid coupled model is developed, with the aim of studying the effects of ocean-atmosphere feedbacks on the stability of the Atlantic meridional overturning circulation. The model includes a global ocean general circulation model and a statistical atmosphere model. The statistical atmosphere model is based on linear regressions of data from a fully coupled climate model on sea surface temperature both locally and hemispherically averaged, being the footprint of Atlantic meridional overturning variability. It provides dynamic boundary conditions to the ocean model for heat, freshwater and wind-stress. A basic but consistent representation of ocean-atmosphere feedbacks is captured in the hybrid coupled model and it is more than ten times faster than the fully coupled climate model. The hybrid coupled model reaches a steady state with a climate close to the one of the fully coupled climate model, and the two models also have a similar response (collapse) of the Atlantic meridional overturning circulati...

  6. THE ROLE OF MERIDIONAL WIND STRESS IN THE TROPICAL UNSTABLE AIR-SEA INTERACTION

    Institute of Scientific and Technical Information of China (English)

    房佳蓓; 杨修群

    2003-01-01

    With a simple tropical coupled ocean-atmosphere model, this paper presents an analysis aiming to understand the relative role of the meridional and zonal wind stresses in the tropical unstable air-sea interaction. The roles of thezonal wind stress, the meridional wind stress and the both are considered respectively into the coupled system. It is demonstrated that the meridional component of the wind stress does not lead to any instability under the local thermal balance assumption, but it does lead to a weak instability under the sea surface temperature advection assumption. Unstable air-sea interaction is dominated by the zonal component of the wind stress, suggesting that ignoring the meridional wind stress is approximately feasible in studying the tropical unstable air-sea interaction.

  7. Wind Stress, QuikSCAT SeaWinds, 0.25 degrees, Global, Science Quality, Meridional

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — NOAA CoastWatch distributes science quality wind stress data in zonal, meridional, modulus, and wind stress curl sets. This data begins with wind velocity...

  8. Wind Diffusivity Current, METOP ASCAT, 0.25 degrees, Global, Near Real Time, Meridional

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — NOAA CoastWatch distributes near real time Ekman current (in zonal, meridional, and modulus sets) and Ekman upwelling data. This data begins with wind velocity...

  9. Currents, HF Radio-derived, Monterey Bay, 1 hr, Meridional, EXPERIMENTAL

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The data is the 1 hour average of the meridional component of ocean surface currents derived from High Frequency Radio-derived measurements. THIS IS AN EXPERIMENTAL...

  10. Currents, HF Radio-derived, SF Bay Outlet, 25 hr, Meridional, EXPERIMENTAL

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The data is the 25 hour running average of the meridional component of ocean surface currents derived from High Frequency Radio-derived measurements. THIS IS AN...

  11. Wind Stress, METOP ASCAT, 0.25 degrees, Global, Near Real Time, Meridional

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — NOAA CoastWatch distributes near real time wind stress data in zonal, meridional, modulus, and wind stress curl sets. This data begins with wind velocity...

  12. Currents, HF Radio-derived, Monterey Bay, 25 hr, Meridional, EXPERIMENTAL

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The data is the 25 hour running average of the meridional component of ocean surface currents derived from High Frequency Radio-derived measurements. THIS IS AN...

  13. Currents, HF Radio-derived, Ano Nuevo, Normal Model, Meridional, EXPERIMENTAL

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The data is the meridional component of ocean surface currents derived from High Frequency Radio-derived measurements, with missing values filled in by a normal...

  14. Currents, HF Radio-derived, Ano Nuevo, 1 hr, Meridional, EXPERIMENTAL

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The data is the 1 hour average of the meridional component of ocean surface currents derived from High Frequency Radio-derived measurements. THIS IS AN EXPERIMENTAL...

  15. The effect of transient eddy on interannual meridional displacement of summer East Asian subtropical jet

    Science.gov (United States)

    Xiang, Yang; Yang, Xiuqun

    2012-05-01

    Using ERA-40 reanalysis daily data for the period 1958-2002, this study investigated the effect of transient eddy (TE) on the interannual meridional displacement of summer East Asian subtropical jet (EASJ) by conducting a detailed dynamical diagnosis. The summer EASJ axis features a significant interannual coherent meridional displacement. Associated with such a meridional displacement, the TE vorticity forcing anomalies are characterized by a meridional dipole pattern asymmetric about the climatological EASJ axis. The TE vorticity forcing anomalies yield barotropic zonal wind tendencies with a phase meridionally leading the zonal wind anomalies, suggesting that they act to reinforce further meridional displacement of the EASJ and favor a positive feedback in the TE and time-mean flow interaction. However, The TE thermal forcing anomalies induce baroclinic zonal wind tendencies that reduce the vertical shear of zonal wind and atmospheric baroclinicity and eventually suppress the TE activity, favoring a negative feedback in the TE and time-mean flow interaction. Although the two types of TE forcing tend to have opposite feedback roles, the TE vorticity forcing appears to be dominant in the TE effect on the time-mean flow.

  16. The Effect of Transient Eddy on Interannual Meridional Displacement of Summer East Asian Subtropical Jet

    Institute of Scientific and Technical Information of China (English)

    XIANG Yang; YANG Xiuqun

    2012-01-01

    Using ERA-40 reanalysis daily data for the period 1958-2002.this study investigated the effect of transient eddy (TE) on the interannual meridional displacement of summer East Asian subtropical jet (EASJ)by conducting a detailed dynamical diagnosis.The summer EASJ axis features a significant interannual coherent meridional displacement.Associated with such a meridional displacement,the TE vorticity forcing anomalies are characterized by a meridional dipole pattern asymmetric about the climatological EASJ axis.The TE vorticity forcing anomalies yield barotropic zonal wind tendencies with a phase meridionally leading the zonal wind anomalies,suggesting that they act to reinforce further meridional displacement of the EASJ and favor a positive feedback in the TE and time-mean flow interaction.However,The TE thermal forcing anomalies induce baroclinic zonal wind tendencies that reduce the vertical shear of zonal wind and atmospheric baroclinicity and eventually suppress the TE activity,favoring a negative feedback in the TE and time-mean flow interaction.Although the two types of TE forcing tend to have opposite feedback roles,the TE vorticity forcing appears to be dominant in the TE effect on the time-mean flow.

  17. Interpreting the implied meridional oceanic energy transport in AMIP

    International Nuclear Information System (INIS)

    The Atmospheric Model Intercomparison Project (AMIP) was outlined in Paper No. CLIM VAR 2.3 (entitled open-quote The validation of ocean surface heat fluxes in AMIP') of these proceedings. Preliminary results of AMIP subproject No. 5 were also summarized. In particular, zonally averaged ocean surface heat fluxes resulting from various AMIP simulations were intercompared, and to the extent possible they were validated with uncertainties in observationally-based estimates of surface heat fluxes. The intercomparison is continued in this paper by examining the Oceanic Meridional Energy Transport (OMET) implied by the net surface heat fluxes of the AMIP simulations. As with the surface heat fluxes of the AMIP simulations. As with the surface heat fluxes, the perspective here will be very cursory. The annual mean implied ocean heat transport can be estimated by integrating the zonally averaged net ocean surface heat flux, Nsfc, from one pole to the other. In AGCM simulations (and perhaps reality), the global mean Nsfc is typically not in exact balance when averaged over one or more years. Because of this, an important assumption must be made about changes in the distribution of energy in the oceans. Otherwise, the integration will yield a non-zero transport at the endpoint of integration (pole) which is not physically realistic. Here the authors will only look at 10-year means of the AMIP runs, and for simplicity they assume that any long term imbalance in the global averaged Nsfc will be sequestered (or released) over the global ocean. Tests have demonstrated that the treatment of how the global average energy imbalance is assumed to be distributed is important, especially when the long term imbalances are in excess of 10 W m-2. However, this has not had a substantial impact on the qualitative features of the implied heat transport of the AMIP simulations examined thus far

  18. Influence of the Atlantic thermohaline circulation on neodymium isotopic composition at the Last Glacial Maximum – a modelling sensitivity study

    OpenAIRE

    O. Marti; Jeandel, C.; Alkama, R.; Lacan, F.; Kageyama, M.; J.-C. Dutay; Arsouze, T.

    2008-01-01

    The oceanic neodymium isotopic composition (hereafter expressed as ε Nd) is modeled for the Last Glacial Maximum (LGM) using the coarse resolution Ocean Global Circulation Model NEMO–ORCA2°. This study focuses on the impact of changes in the overturning cell and circulation patterns between LGM and Holocene on ε Nd in the Atlantic basin. Three different LGM freshwater forcing experiments are performed to test the variability in ε Nd oceanic distribution as a ...

  19. Assessing Agulhas leakage

    OpenAIRE

    Van Sebille, E.

    2009-01-01

    Agulhas leakage, the water that flows from the Indian Ocean to the Atlantic Ocean, plays an important role in the circulation of the Atlantic Ocean. The magnitude of this flux of warm and saline Indian Ocean water into the much colder and fresher Atlantic Ocean can be related to the strength of the Atlantic meridional overturning circulation, both in numerical ocean models and in paleoceanographic records. However, estimating the magnitude of this Agulhas leakage is not easy. The Agulhas regi...

  20. Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current

    Directory of Open Access Journals (Sweden)

    A. M. Treguier

    2007-12-01

    Full Text Available An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC in a streamline-following framework. Previous model-based estimates of the meridional circulation were calculated using zonal averages: this method leads to a counter-intuitive poleward circulation of the less dense waters, and underestimates the eddy effects. We show that on the contrary, the upper ocean circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. Two model simulations, in which the buoyancy forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface buoyancy flux is not as straightforward as assumed by the simplest theoretical models: the sign of the residual circulation cannot be inferred from the surface buoyancy forcing only. Among the other processes that likely play a part in setting the meridional circulation, our model results emphasize the complex three-dimensional structure of the ACC (probably not well accounted for in streamline-averaged, two-dimensional models and the distinct role of temperature and salinity in the definition of the density field. Heat and salt transports by the time-mean flow are important even across time-mean streamlines. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.

  1. Statistical Aspects of Tropical Cyclone Activity in the North Atlantic Basin, 1945-2010

    Science.gov (United States)

    Wilson, Robert M.

    2012-01-01

    Examined are statistical aspects of the 715 tropical cyclones that formed in the North Atlantic basin during the interval 1945-2010. These 715 tropical cyclones include 306 storms that attained only tropical storm strength, 409 hurricanes, 179 major or intense hurricanes, and 108 storms that struck the US coastline as hurricanes. Comparisons made using 10-year moving average (10-yma) values between tropical cyclone parametric values and surface air and ENSO-related parametric values indicate strong correlations to exist, in particular, against the Armagh Observatory (Northern Ireland) surface air temperature, the Atlantic Multi-decadal Oscillation (AMO) index, the Atlantic Meridional Mode (AMM) index, and the North Atlantic Oscillation (NAO) index, in addition to the Oceanic Ni o index (ONI) and Quasi-Biennial Oscillation (QBO) indices. Also examined are the decadal variations of the tropical cyclone parametric values and a look ahead towards the 2012 hurricane season and beyond.

  2. Searching for overturning convection in penumbral filaments: slit spectroscopy at 0.2 arcsec resolution

    CERN Document Server

    Rubio, L R Bellot; Langhans, K

    2010-01-01

    Recent numerical simulations of sunspots suggest that overturning convection is responsible for the existence of penumbral filaments and the Evershed flow, but there is little observational evidence of this process. Here we carry out a spectroscopic search for small-scale convective motions in the penumbra of a sunspot located 5 deg away from the disk center. The position of the spot is very favorable for the detection of overturning downflows at the edges of penumbral filaments. Our analysis is based on measurements of the Fe I 709.0 nm line taken with the Littrow spectrograph of the Swedish 1 m Solar Telescope under excellent seeing conditions. We compute line bisectors at different intensity levels and derive Doppler velocities from them. The velocities are calibrated using a nearby telluric line, with systematic errors smaller than 150 m/s. Deep in the photosphere, as sampled by the bisectors at the 80%-88% intensity levels, we always observe blueshifts or zero velocities. The maximum blueshifts reach 1.2...

  3. Overturning instability in the mesosphere and lower thermosphere: analysis of instability conditions in lidar data

    Directory of Open Access Journals (Sweden)

    L. Hurd

    2009-07-01

    Full Text Available Resonant sodium lidar measurements from the transition region between the mesosphere and lower thermosphere have revealed frequently-occurring overturning events characterized by vertical scales of ~3–6 km and timescales of several hours. Larsen et al. (2004 proposed that a convective roll instability, similar to that found in the planetary boundary layer, is the likely mechanism responsible for the events. This type of instability requires an inflection point in the background winds near the center of the vortex roll with a low static stability region capped by an inversion. The earlier paper argued that the conditions required to support the instability are common in the altitude range where the features are found. In this paper, we use data from the University of Illinois sodium lidar that was located at the Starfire Optical Range near Albuquerque, New Mexico, and from the Maui/MALT Lidar Facility in Hawaii and present several cases that are used to examine the behavior of the inflection point in detail as a function of time during the evolution of the overturning event. In addition, we examine the background static stability conditions using the temperature data from the lidar.

  4. Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning

    Science.gov (United States)

    Abernathey, Ryan P.; Cerovecki, Ivana; Holland, Paul R.; Newsom, Emily; Mazloff, Matt; Talley, Lynne D.

    2016-08-01

    Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing, where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch. While brine rejection from sea ice is thought to contribute to the lower branch, the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport. Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ~22 × 106 m3 s-1. These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.

  5. Relations between overturning length scales at the Spanish planetary boundary layer

    Science.gov (United States)

    López, Pilar; Cano, José L.

    2016-04-01

    We analyze the behavior of the maximum Thorpe displacement (dT)max and the Thorpe scale LTat the atmospheric boundary layer (ABL), extending previous research with new data and improving our studies related to the novel use of the Thorpe method applied to ABL. The maximum Thorpe displacements vary between -900 m and 950 m for the different field campaigns. The maximum Thorpe displacement is always greater under convective conditions than under stable ones, independently of its sign. The Thorpe scale LT ranges between 0.2 m and 680 m for the different data sets which cover different stratified mixing conditions (turbulence shear-driven and convective regions). The Thorpe scale does not exceed several tens of meters under stable and neutral stratification conditions related to instantaneous density gradients. In contrast, under convective conditions, Thorpe scales are relatively large, they exceed hundreds of meters which may be related to convective bursts. We analyze the relation between (dT)max and the Thorpe scale LT and we deduce that they verify a power law. We also deduce that there is a difference in exponents of the power laws for convective conditions and shear-driven conditions. These different power laws could identify overturns created under different mechanisms. References Cuxart, J., Yagüe, C., Morales, G., Terradellas, E., Orbe, J., Calvo, J., Fernández, A., Soler, M., Infante, C., Buenestado, P., Espinalt, Joergensen, H., Rees, J., Vilà, J., Redondo, J., Cantalapiedra, I. and Conangla, L.: Stable atmospheric boundary-layer experiment in Spain (Sables 98). A report, Boundary-Layer Meteorology, 96, 337-370, 2000. Dillon, T. M.: Vertical Overturns: A Comparison of Thorpe and Ozmidov Length Scales, J. Geophys. Res., 87(C12), 9601-9613, 1982. Itsweire, E. C.: Measurements of vertical overturns in stably stratified turbulent flow, Phys. Fluids, 27(4), 764-766, 1984. Kitade, Y., Matsuyama, M. and Yoshida, J.: Distribution of overturn induced by internal

  6. A statistical-dynamical scheme for reconstructing ocean forcing in the Atlantic. Part II: methodology, validation and application to high-resolution ocean models

    Energy Technology Data Exchange (ETDEWEB)

    Minvielle, Marie; Cassou, Christophe; Terray, Laurent; Najac, Julien [CERFACS/CNRS, Climate Modelling and Global Change Team, Toulouse (France); Bourdalle-Badie, Romain [CERFACS/CNRS, Climate Modelling and Global Change Team, Toulouse (France); MERCATOR Parc Technologique du Canal, Ramonville St Agne (France)

    2011-02-15

    A novel statistical-dynamical scheme has been developed to reconstruct the sea surface atmospheric variables necessary to force an ocean model. Multiple linear regressions are first built over a so-called learning period and over the entire Atlantic basin from the observed relationship between the surface wind conditions, or predictands, and the anomalous large scale atmospheric circulations, or predictors. The latter are estimated in the extratropics by 500 hPa geopotential height weather regimes and in the tropics by low-level wind classes. The transfer function further combined to an analog step is then used to reconstruct all the surface variables fields over 1958-2002. We show that the proposed hybrid scheme is very skillful in reproducing the mean state, the seasonal cycle and the temporal evolution of all the surface ocean variables at interannual timescale. Deficiencies are found in the level of variance especially in the tropics. It is underestimated for 2-m temperature and humidity as well as for surface radiative fluxes in the interannual frequency band while it is slightly overestimated at higher frequency. Decomposition in empirical orthogonal function (EOF) shows that the spatial and temporal coherence of the forcing fields is however very well captured by the reconstruction method. For dynamical downscaling purposes, reconstructed fields are then interpolated and used to carry out a high-resolution oceanic simulation using the NATL4 (1/4 ) model integrated over 1979-2001. This simulation is compared to a reference experiment where the original observed forcing fields are prescribed instead. Mean states between the two experiments are virtually undistinguishable both in terms of surface fluxes and ocean dynamics estimated by the barotropic and the meridional overturning streamfunctions. The 3-dimensional variance of the simulated ocean is well preserved at interannual timescale both for temperature and salinity except in the tropics where it is

  7. On the seasonal cycles and variability of Florida Straits, Ekman and Sverdrup transports at 26° N in the Atlantic Ocean

    Directory of Open Access Journals (Sweden)

    C. P. Atkinson

    2010-10-01

    Full Text Available Since April 2004 the RAPID array has made continuous measurements of the Atlantic Meridional Overturning Circulation (AMOC at 26° N. Two key components of this system are Ekman transport zonally integrated across 26° N and western boundary current transport in the Florida Straits. Whilst measurements of the AMOC as a whole are somewhat in their infancy, this study investigates what useful information can be extracted on the variability of the Ekman and Florida Straits transports using the decadal timeseries already available. Analysis is also presented for Sverdrup transports zonally integrated across 26° N.

    The seasonal cycles of Florida Straits, Ekman and Sverdrup transports are quantified at 26° N using harmonic analysis of annual and semi-annual constituents. Whilst Sverdrup transport shows clear semi-annual periodicity, calculations of seasonal Florida Straits and Ekman transports show substantial interannual variability due to contamination by variability at non-seasonal frequencies; the mean seasonal cycle for these transports only emerges from decadal length observations. The Florida Straits and Ekman mean seasonal cycles project on the AMOC with a combined peak-to-peak seasonal range of 3.5 Sv. The combined seasonal range for heat transport is 0.40 PW.

    The Florida Straits seasonal cycle possesses a smooth annual periodicity in contrast with previous studies suggesting a more asymmetric structure. No clear evidence is found to support significant changes in the Florida Straits seasonal cycle at sub-decadal periods. Whilst evidence of wind driven Florida Straits transport variability is seen at sub-seasonal and annual periods, a model run from the 1/4° eddy-permitting ocean model NEMO is used to identify an important contribution from internal oceanic variability at sub-annual and interannual periods. The Ekman transport seasonal cycle possesses less symmetric structure, due in part to different seasonal transport

  8. Calculation of meridional neutral winds in the middle latitudes from the Irkutsk incoherent scatter radar

    Science.gov (United States)

    Shcherbakov, A. A.; Medvedev, A. V.; Kushnarev, D. S.; Tolstikov, M. V.; Alsatkin, S. S.

    2015-12-01

    This paper presents a consistent technique for velocity determination of meridional neutral winds from the Irkutsk incoherent scatter radar (IISR.) We calculated plasma drift velocity based on phase analysis of an autocorrelation function of an incoherent scatter signal. We also preliminary tested the described technique by determining several low-orbit satellite velocities. Midlatitude meridional neutral winds were calculated using a "three-beam" technique from the IISR velocity with taking into account motions due to electric fields across magnetic field lines in both meridional and zonal directions. It has been shown that an underestimated impact of the motions generated by electric fields can seriously interfere in determining wind velocities. The results obtained were compared with the modeled wind values.

  9. Monthly Climatology of Thermospheric Zonal and Meridional Winds Obtained from a Kalman Filter Model

    Science.gov (United States)

    Scherliess, L.; Lomidze, L.

    2015-12-01

    Knowledge of the thermospheric neutral wind and its meridional and zonal components is critical for an improved understanding of the low- and mid-latitude F-region dynamics and morphology. To date, the reliable estimation of the wind and its components remains a challenge because of difficulties in both measurement and modeling. Previous methods that use ionospheric measurements to deduce winds provide their values only in the direction of the magnetic meridian. We will present the monthly climatology of the zonal and meridional components of thermospheric neutral wind at low and mid-latitudes obtained by a Kalman Filter technique. First, the climatology of the magnetic meridional wind is obtained by assimilating monthly maps of F-region ionosphere peak parameters (NmF2 and hmF2), obtained from COSMIC radio occultation data, into the Global Assimilation of Ionospheric Measurements Full Physics (GAIM-FP) model. The model provides the 3-D electron density throughout the ionosphere, together with the magnetic meridional wind. Next, the estimation of the global zonal and meridional wind components is performed using the newly developed Thermospheric Wind Assimilation Model (TWAM). TWAM combines magnetic meridional wind data obtained from GAIM-FP with a physics-based 3-D thermospheric neutral wind model using an implicit Kalman Filter technique. The ionospheric drag and ion diffusion velocities, needed for the wind calculation, are also taken from the GAIM-FP model. We present the monthly climatology of our wind estimation and compare individual horizontal wind components to their corresponding empirical model values and to measurements made by interferometers.

  10. Effects of the Observed Meridional Flow Variations since 1996 on the Sun's Polar Fields

    Science.gov (United States)

    Hathaway, David H.; Upton, Lisa

    2013-01-01

    The cause of the low and extended minimum in solar activity between Sunspot Cycles 23 and 24 was the small size of Sunspot Cycle 24 itself - small cycles start late and leave behind low minima. Cycle 24 is small because the polar fields produced during Cycle 23 were substantially weaker than those produced during the previous cycles and those (weak) polar fields are the seeds for the activity of the following cycle. The polar fields are produced by the latitudinal transport of magnetic flux that emerged in low-latitude active regions. The polar fields thus depend upon the details of both the flux emergence and the flux transport. We have measured the flux transport flows (differential rotation, meridional flow, and supergranules) since 1996 and find systematic and substantial variation in the meridional flow alone. Here we present experiments using a Surface Flux Transport Model in which magnetic field data from SOHO/MDI and SDO/HMI are assimilated into the model only at latitudes between 45-degrees north and south of the equator (this assures that the details of the active region flux emergence are well represented). This flux is then transported in both longitude and latitude by the observed flows. In one experiment the meridional flow is given by the time averaged (and north-south symmetric) meridional flow profile. In the second experiment the time-varying and north-south asymmetric meridional flow is used. Differences between the observed polar fields and those produced in these two experiments allow us to ascertain the effects of these meridional flow variations on the Sun s polar fields.

  11. Improvements in launchings and recipients of PIG in Malha de Gasodutos Nordeste Meridional; Melhorias nos lancamentos e recebedores de PIG da Malha de Gasodutos Nordeste Meridional

    Energy Technology Data Exchange (ETDEWEB)

    Maciel, Jairo A.C.; Lemos, Francisco A.C.; Lima, Artur W.R. de S. [PETROBRAS Transporte S.A. (TRANSPETRO), Rio de Janeiro, RJ (Brazil)

    2012-07-01

    This paper describes the study that resulted in the preparation of a project for improvement of launchers and receivers of PIG in facilities maintained and operated by PETROBRAS S.A. (TRANSPETRO) in the Malha de Gasodutos Nordeste Meridional (Malha NEM). The improvements are part of the component in cleaner production system and reduce the risks of accidents and the costs of hazardous waste management.

  12. Angular momentum fluxes caused by Lambda-effect and meridional circulation structure of the Sun

    CERN Document Server

    Pipin, V V

    2016-01-01

    Using mean-field hydrodynamic models of the solar angular momentum balance we show that the non-monotonic latitudinal dependence of the radial angular momentum fluxes caused by Lambda-effect can affect the number of the meridional circulation cells stacking in radial direction in the solar convection zone. In particular, our results show the possibility of a complicated triple-cell meridional circulation structure. This pattern consists of two large counterclockwise circulation cells (the N-hemisphere) and a smaller clockwise cell located at low latitudes at the bottom of the convection zone.

  13. North Atlantic warming: patterns of long-term trend and multidecadal variability

    Energy Technology Data Exchange (ETDEWEB)

    Polyakov, Igor V.; Alexeev, Vladimir A.; Zhang, Xiangdong [University of Alaska Fairbanks, International Arctic Research Center, Fairbanks, AK (United States); Bhatt, Uma S. [University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK (United States); Polyakova, Evgenia I. [Stanford University, Department of Geological and Environmental Studies, Stanford, CA (United States)

    2010-02-15

    Climate fluctuations in the North Atlantic Ocean have wide-spread implications for Europe, Africa, and the Americas. This study assesses the relative contribution of the long-term trend and variability of North Atlantic warming using EOF analysis of deep-ocean and near-surface observations. Our analysis demonstrates that the recent warming over the North Atlantic is linked to both long-term (including anthropogenic and natural) climate change and multidecadal variability (MDV, {proportional_to}50-80 years). Our results suggest a general warming trend of 0.031 {+-} 0.006 C/decade in the upper 2,000 m North Atlantic over the last 80 years of the twentieth century, although during this time there are periods in which short-term trends were strongly amplified by MDV. For example, MDV accounts for {proportional_to}60% of North Atlantic warming since 1970. The single-sign basin-scale pattern of MDV with prolonged periods of warming (cooling) in the upper ocean layer and opposite tendency in the lower layer is evident from observations. This pattern is associated with a slowdown (enhancement) of the North Atlantic thermohaline overturning circulation during negative (positive) MDV phases. In contrast, the long-term trend exhibits warming in tropical and mid-latitude North Atlantic and a pattern of cooling in regions associated with major northward heat transports, consistent with a slowdown of the North Atlantic circulation as evident from observations and confirmed by selected modeling results. This localized cooling has been masked in recent decades by warming during the positive phase of MDV. Finally, since the North Atlantic Ocean plays a crucial role in establishing and regulating the global thermohaline circulation, the multidecadal fluctuations discussed here should be considered when assessing long-term climate change and variability, both in the North Atlantic and at global scales. (orig.)

  14. 2005 Atlantic Hurricanes Poster

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The 2005 Atlantic Hurricanes poster features high quality satellite images of 15 hurricanes which formed in the Atlantic Basin (includes Gulf of Mexico and...

  15. South Atlantic Shrimp System

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The SEFSC, in cooperation with the South Atlantic states, collects South Atlantic shrimp data from dealers and fishermen. These data are collected to provide catch,...

  16. The effect of mechanical stirring on horizontal convection

    OpenAIRE

    R. Tailleux; Rouleau, L

    2010-01-01

    An important experimental result, as yet poorly understood, is that mechanical stirring can significantly enhance the strength of horizontal convection. A contentious issue is whether this necessarily implies that the mechanical stirring replaces the buoyancy forcing as the main source of energy driving the observed overturning circulation, as has been suggested for the Atlantic meridional overturning circulation (AMOC). In this paper, rigorous energetics considerations and idealized numerica...

  17. A flow law for ilmenite in dislocation creep: Implications for lunar cumulate mantle overturn

    Science.gov (United States)

    Dygert, Nick; Hirth, Greg; Liang, Yan

    2016-01-01

    We present results from new deformation experiments and a dislocation creep flow law for synthetic ilmenite. The flow law predicts an effective viscosity more than 3 orders of magnitude lower than dry olivine at mantle stresses and temperatures. Using the flow law, we predict that lunar ilmenite-bearing cumulates (IBC) will be weakened by the presence of low-viscosity ilmenite. Dense, low-viscosity IBC are expected to flow into the lunar interior by a process known as cumulate mantle overturn. Low-viscosity IBC that sink to the core-mantle boundary may be dynamically stable with respect to upwelling. A hot, stable layer of IBC surrounding the lunar core would suppress the development of a core dynamo. A layer of partially molten IBC can also explain the inferred zone of seismic attenuation around the lunar core, as well as a low-viscosity layer suggested by tidal dissipation.

  18. Zonal overturning circulation and heat flux induced by heaving modes in the world oceans

    Institute of Scientific and Technical Information of China (English)

    TAN Wei; HUANG Rui Xin; WANG Weiqiang; WANG Xin

    2015-01-01

    Zonal overturning circulation (ZOC) and its associated zonal heat flux (ZHF) are important components of the oceanic circulation and climate system, although these conceptions have not received adequate attentions. Heaving induced by inter-annual and decadal wind stress perturbations can give rise to anomalous ZOC and ZHF. Based on a simple reduced gravity model, the anomalous ZOC and ZHF induced by idealized heaving modes in the world oceans are studied. For example, in a Pacific-like model basin intensified equatorial easterly on decadal time scales can lead to a negative ZOC with a non-negligible magnitude (–0.3×106 m3/s) and a considerable westward ZHF with an amplitude of –11.2 TW. Thus, anomalous ZOC and ZHF may consist of a major part of climate signals on decadal time scales and thus play an important role in the oceanic circulation and climate change.

  19. Modulation of extremes in the Atlantic region by modes of climate variability/change: A mechanistic coupled regional model study

    Energy Technology Data Exchange (ETDEWEB)

    Saravanan, Ramalingam

    2015-01-09

    During the course of this project, we have accomplished the following: 1) Explored the parameter space of component models to minimize regional model bias 2) Assessed the impact of air-sea interaction on hurricanes, focusing in particular on the role of the oceanic barrier layer 3) Contributed to the activities of the U.S. CLIVAR Hurricane Working Group 4) Assessed the impact of lateral and lower boundary conditions on extreme flooding events in the U.S. Midwest in regional model simulations 5) Analyzed the concurrent impact of El Niño-Southern Oscillation and Atlantic Meridional Mode on Atlantic Hurricane activity using observations and regional model simulations

  20. Atmospheric and Surface Contributions to Planetary Albedo and their Relationship to the Total Meridional Energy Transport

    Science.gov (United States)

    Donohoe, A.; Battisti, D. S.

    2010-12-01

    The meridional distribution of incident solar radiation and planetary albedo both contribute to the equator-to-pole gradient in absorbed solar radiation (ASR) in the observed climate system. While the former component is determined by the Earth-Sun geometry and composes 60% of the equator-to-pole gradient in ASR, the latter component makes a significant (40%) contribution to the ASR gradient and is potentially a function of climate state due to its dependence on both atmospheric and surface albedo. In turn, the equator-to-pole gradient in planetary albedo is found to be primarily (86% -89%) dictated by atmospheric albedo with meridional gradients in surface albedo playing a much smaller role in forcing the climate system on the equator-to-pole scale. Simulations of the pre-industrial climate system using the CMIP3 coupled models show large differences in the equator-to-pole gradient in planetary albedo which are mainly due to differences in the simulated cloud distribution, with surface processes playing a much smaller role. The inter-model spread in total meridional heat transport is also primarily (85% of the inter-model spread) due to differences in the simulated cloud distribution. Further model simulations demonstrate that the surface albedo changes associated with moving from the present climate to an ice free climate have a small effect on the equator-to-pole gradient of ASR as compared to the uncertainty in simulated cloud distributions, and hence a small effect on the meridional heat transport.

  1. Importance of Meridional Circulation in Flux Transport Dynamo: Possibility of Maunder-like Grand Minimum

    CERN Document Server

    Karak, Bidya Binay

    2010-01-01

    Meridional circulation is an important ingredient in flux transport dynamo model. We have studied its importance on the period, amplitude of solar cycle and also on producing Maunder-like grand minima in this model. First, we model the periods of last 23 sunspot cycles by varying the meridional circulation speed. We find that most of the cycle amplitudes also get modeled up to some extent when the dynamo is in diffusion-dominated regime. Next, we propose that at the beginning of the Maunder minimum the amplitude of meridional circulation dropped to a low value and then after a few years it increased again. Several independent studies also favor this assumption. With this assumption, a diffusion-dominated dynamo is able to reproduce many important features of Maunder minimum remarkably well. If the dynamo is in diffusion-dominated regime, then the slower meridional circulation means that the poloidal field gets more time to diffuse away from the tachocline, making the dynamo weaker. This consequence helps to m...

  2. Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?

    CERN Document Server

    Hazra, Gopal; Choudhuri, Arnab Rai

    2014-01-01

    The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as proper butterfly diagram, proper phase relation between the toroidal and poloidal fields). We consider additional cells of the meridional circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. Howeve...

  3. 40 CFR 142.311 - What procedures allow the Administrator to object to a proposed small system variance or overturn...

    Science.gov (United States)

    2010-07-01

    ... Administrator to object to a proposed small system variance or overturn a granted small system variance for a... REGULATIONS IMPLEMENTATION Variances for Small System Epa Review and Approval of Small System Variances § 142.311 What procedures allow the Administrator to object to a proposed small system variance or...

  4. Overturned folds in ice sheets: Insights from a kinematic model of traveling sticky patches and comparisons with observations

    Science.gov (United States)

    Wolovick, Michael J.; Creyts, Timothy T.

    2016-05-01

    Overturned folds are observed in regions of the Greenland ice sheet where driving stress is highly variable. Three mechanisms have been proposed to explain these folds: freezing subglacial water, traveling basal slippery patches, and englacial rheological contrasts. Here we explore how traveling basal sticky patches can produce overturned folds. Transitions from low to high stress cause a tradeoff in ice flow between basal slip and internal deformation that deflects ice stratigraphy vertically. If these transitions move, the slip-deformation tradeoff can produce large folds. Those folds record the integrated effects of time-varying basal slip. To understand how dynamic changes in basal slip influence ice sheet stratigraphy, we develop a kinematic model of ice flow in a moving reference frame that follows a single traveling sticky patch. The ice flow field forms a vortex when viewed in the moving reference frame, and this vortex traps ice above the traveling patch and produces overturned folds. Sticky patches that travel downstream faster produce larger overturned folds. We use the model as an interpretive tool to infer properties of basal slip from three example folds. Our model suggests that the sticky patches underneath these folds propagated downstream at rates between one half and the full ice velocity. The regional flow regime for the smaller two folds requires substantial internal deformation whereas the regime for the largest fold requires substantially more basal slip. The distribution and character of stratigraphic folds reflect the evolution and propagation of individual sticky patches and their effects on ice sheet flow.

  5. On the configurations of the Atlantic Niño phenomenon under negative AMO phases

    Science.gov (United States)

    Martin-Rey, Marta; Rodríguez de Fonseca, Belen; Polo, Irene; Losada, Teresa; Lazar, Alban

    2016-04-01

    An air-sea coupled mode of inter-annual variability akin to ENSO emerges in the tropical Atlantic basin, named as Atlantic Niño. The teleconnections of the Atlantic Niño phenomenon have changed during recent decades, coinciding with an alteration of its spatial configuration. Previous studies have suggested that the background state could favour particular atmospheric forcings and could also contribute to generate different variability modes. Here, we demonstrate that two different Atlantic Niño patterns coexist in the tropical Atlantic basin during certain decades, coinciding with a negative phase of the Atlantic Multidecadal Oscillation (AMO). The leading mode, Basin-Wide (BW) Atlantic Niño, is characterized by positive SST anomalies covering the entire tropical Atlantic and the second mode, Dipolar (D) Atlantic Niño, presents an equatorial warming flanked by negative SST anomalies in north and south Tropical Atlantic. These modes are driven by different wind patterns, controlled by the Subtropical High Pressure Systems. The BW-Atlantic Niño is preceded by a weakening of both Azores and Sta Helena High, which induces a general reduction of the tropical trades and anomalous wind convergence in the equatorial band. On the other hand, the D-Atlantic Niño is associated with a strengthening of Azores High and a weakening of Sta Helena High, given rise to a meridional Sea Level Pressure (SLP) gradient that intensifies the subtropical trades and generate anomalous trans-equatorial winds along the equatorial band. Both modes seem to be forced by an ENSO-like signal emanating from the Pacific, but with different atmospheric response over the Atlantic. It could be attributed to the changes in the mean state during negative AMO phases. For these decades, shallower thermocline conditions, together with an increase of the oceanic variability (SST and thermocline) in the tropical Atlantic could contribute to the generation of both Atlantic Niño modes. Furthermore, a

  6. Weak response of the Atlantic thermohaline circulation to an increase of atmospheric carbon dioxide in IAP/LASG Climate System Model

    Institute of Scientific and Technical Information of China (English)

    ZHOU Tianjun; YU Rucong; LIU Xiying; GUO Yufu; YU Yongqiang; ZHANG Xuehong

    2005-01-01

    Response of the Atlantic thermohaline circula- tion (THC) to global warming is examined by using the climate system model developed at IAP/LASG. The evidence indicates that the gradually warming climate associated with the increased atmospheric carbon dioxide leads to a warmer and fresher sea surface water at the high latitudes of the North Atlantic Ocean, which prevents the down-welling of the surface water. The succedent reduction of the pole-to- equator meridional potential density gradient finally results in the decrease of the THC in intensity. When the atmospheric carbon dioxide is doubled, the maximum value of the Atlantic THC decreases approximately by 8%. The associated poleward oceanic heat transport also becomes weaker. This kind of THC weakening centralizes mainly in the northern part of the North Atlantic basin, indicating briefly a local scale adjustment rather than a loop oscillation with the whole Atlantic "conveyor belt" decelerating.

  7. Aerosol interactions with African/Atlantic climate dynamics

    Science.gov (United States)

    Hosseinpour, F.; Wilcox, E. M.

    2014-07-01

    Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSAL) and transient changes in the dynamics of Western Africa and the tropical Atlantic Ocean. This study provides evidence of possible interactions between dust in the OSAL region and African easterly jet-African easterly wave (AEJ-AEW) system in the climatology of boreal summer, when easterly wave activity peaks. Synoptic-scale changes in instability and precipitation in the African/Atlantic intertropical convergence zone are correlated with enhanced aerosol optical depth (AOD) in the OSAL region in response to anomalous 3D overturning circulations and upstream/downstream thermal anomalies at above and below the mean-AEJ level. Upstream and downstream anomalies are referred to the daily thermal/dynamical changes over the West African monsoon region and the Eastern Atlantic Ocean, respectively. Our hypothesis is that AOD in the OSAL is positively correlated with the downstream AEWs and negatively correlated with the upstream waves from climatological perspective. The similarity between the 3D pattern of thermal/dynamical anomalies correlated with dust outbreaks and those of AEWs provides a mechanism for dust radiative heating in the atmosphere to reinforce AEW activity. We proposed that the interactions of OSAL dust with regional climate mainly occur through coupling of dust with the AEWs.

  8. Aerosol interactions with African/Atlantic climate dynamics

    International Nuclear Information System (INIS)

    Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSAL) and transient changes in the dynamics of Western Africa and the tropical Atlantic Ocean. This study provides evidence of possible interactions between dust in the OSAL region and African easterly jet–African easterly wave (AEJ–AEW) system in the climatology of boreal summer, when easterly wave activity peaks. Synoptic-scale changes in instability and precipitation in the African/Atlantic intertropical convergence zone are correlated with enhanced aerosol optical depth (AOD) in the OSAL region in response to anomalous 3D overturning circulations and upstream/downstream thermal anomalies at above and below the mean-AEJ level. Upstream and downstream anomalies are referred to the daily thermal/dynamical changes over the West African monsoon region and the Eastern Atlantic Ocean, respectively. Our hypothesis is that AOD in the OSAL is positively correlated with the downstream AEWs and negatively correlated with the upstream waves from climatological perspective. The similarity between the 3D pattern of thermal/dynamical anomalies correlated with dust outbreaks and those of AEWs provides a mechanism for dust radiative heating in the atmosphere to reinforce AEW activity. We proposed that the interactions of OSAL dust with regional climate mainly occur through coupling of dust with the AEWs. (paper)

  9. The role of the Gulf Stream in European climate.

    Science.gov (United States)

    Palter, Jaime B

    2015-01-01

    The Gulf Stream carries the warm, poleward return flow of the wind-driven North Atlantic subtropical gyre and the Atlantic Meridional Overturning Circulation. This northward flow drives a significant meridional heat transport. Various lines of evidence suggest that Gulf Stream heat transport profoundly influences the climate of the entire Northern Hemisphere and, thus, Europe's climate on timescales of decades and longer. The Gulf Stream's influence is mediated through feedback processes between the ocean, atmosphere, and cryosphere. This review synthesizes paleoclimate archives, model simulations, and the instrumental record, which collectively suggest that decadal and longer-scale variability of the Gulf Stream's heat transport manifests in changes in European temperature, precipitation, and storminess. Given that anthropogenic climate change is projected to weaken the Atlantic Meridional Overturning Circulation, associated changes in European climate are expected. However, large uncertainty in the magnitude of the anticipated weakening undermines the predictability of the future climate in Europe.

  10. Strong winter cooling over the Irminger Sea in winter 2014–2015, exceptional deep convection, and the emergence of anomalously low SST

    NARCIS (Netherlands)

    de Jong, M.F.; de Steur, L.

    2016-01-01

    Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation,even though observational evidence for the link remains inconclusive. Modeling studies have suggested thatconvection will weaken as a result of enhanced freshwater input. The emergence of anomalously lo

  11. Thermohaline feedbacks in ocean-climate models of varying complexity

    NARCIS (Netherlands)

    den Toom, M.

    2013-01-01

    The Atlantic Meridional Overturning Circulation (AMOC) is considered an important component of the climate system, because of its significant contribution to the heat budget of the Northern Hemisphere. Theoretical models indicate that the AMOC has non-linear dynamics, which result in a strong sensit

  12. Titan's meridional wind profile and Huygens' orientation and swing inferred from the geometry of DISR imaging

    Science.gov (United States)

    Karkoschka, Erich

    2016-05-01

    The altitude and zonal motion of the Huygens probe descending through Titan's atmosphere was determined early under the assumption of no meridional motion (Bird et al. [2005]. Nature 438, 800-802). By comparing images taken during the descent, Karkoschka et al. (Karkoschka et al. [2007]. Planet. Space Sci. 55, 1895-1935) determined the meridional motion of Huygens, which was generally much smaller than its zonal motion. Here, we present a comprehensive geometrical analysis of all images taken during the descent that is four times more accurate than the previous study. The result is a meridional wind profile across Titan's troposphere with northward winds by up to 0.4 m/s with an average of 0.1 m/s above 1 km altitude, and southward winds below, peaking at 0.9 m/s near 0.4 km altitude. The imaging data extend down to 0.22 km altitude, although additional information came from the horizontal impact speed near 0.8 m/s southward (Schröder et al. [2012]. Planet. Space Sci. 73, 327-340). There is a region between 5 and 8 km altitude with no significant meridional wind. In the stratosphere, the average meridional wind was 1.2 ± 1.5 m/s northward, and zero meridional motion is possible down to 15 km altitude. We present the difference between the zonal speeds of Huygens and the wind that was ignored in previous publications and amounts to up to 7 m/s. We determined the three rotational angles of Huygens for the times of each exposure that showed surface features. During 26 exposures, the swing speed of Huygens was fast enough to smear images. Inferred swing speeds were up to 20°/s during the calm phase of the descent, consistent with up to 40°/s swings reported before during the rough phase. The improved geometric calibration of images allowed identification of many features also seen in Cassini radar images. This comparison yields the location of the Huygens LandingSite as 192.34 ± 02° West and 10.47 ± 0.02° South.

  13. A Review of ENSO Influence on the North Atlantic. A Non-Stationary Signal

    Directory of Open Access Journals (Sweden)

    Belén Rodríguez-Fonseca

    2016-06-01

    Full Text Available The atmospheric seasonal cycle of the North Atlantic region is dominated by meridional movements of the circulation systems: from the tropics, where the West African Monsoon and extreme tropical weather events take place, to the extratropics, where the circulation is dominated by seasonal changes in the jetstream and extratropical cyclones. Climate variability over the North Atlantic is controlled by various mechanisms. Atmospheric internal variability plays a crucial role in the mid-latitudes. However, El Niño-Southern Oscillation (ENSO is still the main source of predictability in this region situated far away from the Pacific. Although the ENSO influence over tropical and extra-tropical areas is related to different physical mechanisms, in both regions this teleconnection seems to be non-stationary in time and modulated by multidecadal changes of the mean flow. Nowadays, long observational records (greater than 100 years and modeling projects (e.g., CMIP permit detecting non-stationarities in the influence of ENSO over the Atlantic basin, and further analyzing its potential mechanisms. The present article reviews the ENSO influence over the Atlantic region, paying special attention to the stability of this teleconnection over time and the possible modulators. Evidence is given that the ENSO–Atlantic teleconnection is weak over the North Atlantic. In this regard, the multidecadal ocean variability seems to modulate the presence of teleconnections, which can lead to important impacts of ENSO and to open windows of opportunity for seasonal predictability.

  14. Meridional Flow in the Solar Convection Zone II: Helioseismic Inversions of GONG Data

    CERN Document Server

    Jackiewicz, J; Kholikov, S

    2015-01-01

    Meridional flow is thought to play a very important role in the dynamics of the solar convection zone; however, because of its relatively small amplitude, precisely measuring it poses a significant challenge. Here we present a complete time-distance helioseismic analysis of about two years of ground-based GONG Doppler data to retrieve the meridional circulation profile for modest latitudes, in an attempt to corroborate results from other studies. We use an empirical correction to the travel times due to an unknown center-to-limb systematic effect. The helioseismic inversion procedure is first tested and reasonably validated on artificial data from a large-scale numerical simulation, followed by a test to broadly recover the solar differential rotation found from global seismology. From GONG data, we measure poleward photospheric flows at all latitudes with properties that are comparable with earlier studies, and a shallow equatorward flow about $65$\\,Mm beneath the surface, in agreement with recent findings f...

  15. Angular momentum fluxes caused by Λ -effect and meridional circulation structure of the Sun

    Science.gov (United States)

    Pipin, Valery V.; Kosovichev, Alexander G.

    2016-10-01

    Using mean-field hydrodynamic models of the solar angular momentum balance we show that the non-monotonic latitudinal dependence of the radial angular momentum fluxes caused by Λ -effect can affect the number of the meridional circulation cells stacking in the radial direction in the solar convection zone. In particular, our results show the possibility of a complicated triple-cell meridional circulation structure. This pattern consists of two large counterclockwise circulation cells (the N-hemisphere) and a smaller clockwise cell located at low latitudes at the bottom of the convection zone. Similar triple-cell circulation patterns were previously earlier found in a number of 3D global simulations models. The effect is demonstrated for the first time using the mean-field parametrization of the Λ -effect.

  16. Impacts of non-canonical El Niño patterns on Atlantic hurricane activity

    Science.gov (United States)

    Larson, S.; Lee, S.; Wang, C.; Chung, E.; Enfield, D. B.

    2012-12-01

    The impact of non-canonical El Niño patterns, typically characterized by warmer than normal sea surface tempera- tures (SSTs) in the central tropical Pacific, on Atlantic tropical cyclone (TC) is explored by using composites of key Atlantic TC indices and tropospheric vertical wind shear over the Atlantic main development region (MDR). The highlight of our major findings is that, while the canonical El Niño pattern has a strong suppressing influence on Atlantic TC activity, non-canonical El Niño patterns con- sidered in this study, namely central Pacific warming, El Niño Modoki, positive phase Trans-Niño, and positive phase Pacific meridional mode, all have insubstantial impact on Atlantic TC activity. This result becomes more conclu- sive when the impact of MDR SST is removed from the Atlantic TC indices and MDR wind shear by using the method of linear regression. Further analysis suggests that the tropical Pacific SST anomalies associated with the non- canonical El Niño patterns are not strong enough to cause a substantial warming of the tropical troposphere in the Atlantic region, which is the key factor that increases the wind shear and atmospheric static stability over the MDR. During the recent decades, the non-canonical El Niños have been more frequent while the canonical El Niño has been less frequent. If such a trend continues in the future, it is expected that the suppressing effect of El Niño on Atlantic TC activity will diminish and thus the MDR SST will play a more important role in controlling Atlantic TC activity in the coming decades.

  17. Topside plasma bubbles, seen as He+ density depletions, and thermosphere meridional wind influence

    International Nuclear Information System (INIS)

    Complete text of publication follows. The study deals with the evaluation of the possible contribution of the thermosphere meridional/transequatorial winds in the diurnal occurrence probability of the equatorial and low-latitudinal plasma bubbles. It is suggested, that the plasma bubbles, produced by Rayleigh-Taylor instability at the bottomside of ionosphere and transported up by buoyancy to the topside ionosphere and plasmasphere, could be strong affected by meridional (poleward) wind during a generation due to inhibiting the growth of Rayleigh-Taylor instability and flux tube integrated conductivity. He+ density depletions, considered as originating from equatorial plasma bubbles phenomena, or as possible fossil bubble signatures, are studied here. He+ density depletions (or subtroughs) are usually observed during a high solar activity at the topside ionospheric altitudes deeply inside the plasmasphere (L∼1.3-3). The diurnal He+ density depletion statistics, obtained from ISS-b satellite data (1978-79) for the different months and averaged for the periods around the solstices and equinoxes, was compared with the velocity variations of the meridional thermosphere wind, calculated in the different models. The best amplitude correlation was found in the longitudinal region of 0 deg - 60 deg W for winter solstice (R=0.72), when the model calculation of Maruyama (1996) was used. The magnetic meridional component of the thermosphere wind was calculated in this model for declination angle of 20 deg. He+ density depletion distribution as function of latitude-longitude, longitudinal statistics and the map of the magnetic declination angle, calculated from IGRF 1975, were involved for further analyses. It was concluded that the significant modulation effect is determined by season and declination angle of the earth magnetic field in the equatorial region.

  18. Meridional flow in the solar convection zone. I. Measurements from gong data

    International Nuclear Information System (INIS)

    Large-scale plasma flows in the Sun's convection zone likely play a major role in solar dynamics on decadal timescales. In particular, quantifying meridional motions is a critical ingredient for understanding the solar cycle and the transport of magnetic flux. Because the signal of such features can be quite small in deep solar layers and be buried in systematics or noise, the true meridional velocity profile has remained elusive. We perform time-distance helioseismology measurements on several years worth of Global Oscillation Network Group Doppler data. A spherical harmonic decomposition technique is applied to a subset of acoustic modes to measure travel-time differences to try to obtain signatures of meridional flows throughout the solar convection zone. Center-to-limb systematics are taken into account in an intuitive yet ad hoc manner. Travel-time differences near the surface that are consistent with a poleward flow in each hemisphere and are similar to previous work are measured. Additionally, measurements in deep layers near the base of the convection zone suggest a possible equatorward flow, as well as partial evidence of a sign change in the travel-time differences at mid-convection zone depths. This analysis on an independent data set using different measurement techniques strengthens recent conclusions that the convection zone may have multiple 'cells' of meridional flow. The results may challenge the common understanding of one large conveyor belt operating in the solar convection zone. Further work with helioseismic inversions and a careful study of systematic effects are needed before firm conclusions of these large-scale flow structures can be made.

  19. Interannual Meridional Displacement of the East Asian Upper-tropospheric Jet Stream in Summer

    Institute of Scientific and Technical Information of China (English)

    LIN Zhongda; LU Riyu

    2005-01-01

    On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS's meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-JulyAugust, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

  20. Meteorological conditions, physiochemical properties, thermal-oxygen stratification, water overturn and water balance of Lake Gardno on Wolin Island

    Directory of Open Access Journals (Sweden)

    Tylkowski Jacek

    2015-09-01

    Full Text Available The main research problem of the paper is aimed at determining the proper functioning of Lake Gardno within the period 2012-2014 considered as hydrological years in reference to the physiochemical properties of its waters, water balance, thermal regime and water overturn. Lake Gardno is a representative of non-run-off lake geo-eco-systems; it is situated within the Southern Baltic Sea Coastland at the cliff shore of Wolin Island. The paper analyses how weather conditions affect the specifics of water supplies provided to the lake and seasonal dynamics of its waters, their chemical, thermal and aerobic properties. It also specifies their overturn and balance with a particular emphasis on their supplies together with fog deposits.

  1. Flux Transport Solar Dynamos with Shallow Meridional Flow and Turbulent Pumping

    Science.gov (United States)

    Nandi, Dibyendu; Hazra, Soumitra

    2016-07-01

    The large-scale solar magnetic cycle is sustained by a dynamo mechanism in which the induction of the toroidal component of the magnetic field by differential rotation and the regeneration of the poloidal component are crucial processes. In the Sun, the Babcock-Leighton mechanism, i.e., the (near-surface) redistribution of the flux of tilted bipolar sunspot pairs is thought to be the main source of the solar poloidal field. The poloidal field so generated must be transported to the solar interior where the toroidal field is generated and stored - presumably near the base of the solar convection zone. Traditionally, flux transport dynamo models have relied on a deep meridional circulation to achieve this transport. However, recent observations claim that the meridional circulation could be much shallower that previously thought. We explore the question whether flux transport dynamos can function with a shallow meridional flow and present an alternative paradigm for flux transport dynamics in solar-stellar interiors sustained by turbulent pumping.

  2. Meridional Flow in the Solar Convection Zone I: Measurements from GONG Data

    CERN Document Server

    Kholikov, Shukur; Jackiewicz, Jason

    2014-01-01

    Large-scale plasma flows in the Sun's convection zone likely play a major role in solar dynamics on decadal timescales. In particular, quantifying meridional motions is a critical ingredient for understanding the solar cycle and the transport of magnetic flux. Because the signal of such features can be quite small in deep solar layers and be buried in systematics or noise, the true meridional velocity profile has remained elusive. We perform time-distance helioseismology measurements on several years worth of GONG Doppler data. A spherical harmonic decomposition technique is applied to a subset of acoustic modes to measure travel-time differences to try to obtain signatures of meridional flows throughout the solar convection zone. Center-to-limb systematics are taken into account in an intuitive, yet ad hoc manner. Travel-time differences near the surface that are consistent with a poleward flow in each hemisphere and are similar to previous work are measured. Additionally, measurements in deep layers near th...

  3. Causes and impacts of changes in the stratospheric meridional circulation in a chemistry-climate model

    Energy Technology Data Exchange (ETDEWEB)

    Garny, Hella

    2011-05-13

    The stratospheric meridional circulation is projected to be subject to changes due to enhanced greenhouse-gas concentrations in the atmosphere. This study aims to diagnose and explain long-term changes in the stratospheric meridional circulation using the chemistry-climate model E39CA. The diagnosed strengthening of the circulation is found to be driven by increases in tropical sea surface temperatures which lead to a strengthening and upward shift of the subtropical jets. This enables enhanced vertical propagation of large scale waves into the lower stratosphere, and therefore stronger local wave forcing of the meridional circulation in the tropical lower stratosphere. The impact of changes in transport on the ozone layer is analysed using a newly developed method that allows the separation of the effects of transport and chemistry changes on ozone. It is found that future changes of mean stratospheric ozone concentrations are largely determined by changes in chemistry, while changes in transport of ozone play a minor role. (orig.)

  4. Agrobiodiversity of cactus pear (Opuntia, Cactaceae in the Meridional Highlands Plateau of Mexico

    Directory of Open Access Journals (Sweden)

    Juan Antonio Reyes-Agüero

    2011-08-01

    Full Text Available Mexico is characterized by a remarkable richness of Opuntia, mostly at the Meridional Highlands Plateau; it is also here where the greatest richness of Opuntia variants occurs. Most of these variants have been maintained in homegardens; however, the gathering process which originated these homegardens has been disrupted over the past decades, as a result of social change and the destruction of large wild nopaleras. If the variants still surviving in homegardens are lost, these will be hard to recover, that is, the millenary cultural heritage from the human groups that populated the Mexican Meridional Highland Plateau will be lost forever. This situation motivated the preparation of a catalogue that records the diversity of wild and cultivated Opuntia variants living in the meridional Highlands Plateau. To this end, 379 samples were obtained in 29 localities, between 1998 and 2003. The information was processed through Twinspan. All specimens were identified and preserved in herbaria. Botanical keys and descriptions were elaborated. The catalogue includes information on 126 variants comprising 18 species. There were species with only one variant (Opuntia atropes, O. cochinera, O. jaliscana, O. leucotricha, O. rzedowskii and O. velutina, two (O. durangensis, O. lindheimeri, O. phaeacantha and O. robusta, five (O. joconostle and O. lasiacantha, seven (O. chavena, 12 (O. hyptiacantha and O. streptacantha, 15 (O. ficus-indica, 22 (O. albicarpa, and up to 34 (O. megacantha. Additionally, 267 common cactus pear names were related to those variants.

  5. Wind Diffusivity Current, QuikSCAT SeaWinds, 0.25 degrees, Global, Science Quality, Meridional

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — NOAA CoastWatch distributes science quality Ekman current (in zonal, meridional, and modulus sets) and Ekman upwelling data. This data begins with wind velocity...

  6. A frozen record of density-driven crustal overturn in lava lakes: The example of Kilauea Iki 1959

    Science.gov (United States)

    Stovall, W.K.; Houghton, B.F.; Harris, A.J.L.; Swanson, D.A.

    2009-01-01

    Lava lakes are found at basaltic volcanoes on Earth and other planetary bodies. Density-driven crustal foundering leading to surface renewal occurs repeatedly throughout the life of a lava lake. This process has been observed and described in a qualitative sense, but due to dangerous conditions, no data has been acquired to evaluate the densities of the units involved. Kilauea Iki pit crater in Hawai'i houses a lava lake erupted during a 2 month period in 1959. Part of the surface of the Kilauea Iki lake now preserves the frozen record of a final, incomplete, crustal-overturn cycle. We mapped this region and sampled portions of the foundering crust, as well as overriding and underlying lava, to constrain the density of the units involved in the overturn process. Overturn is driven by the advance of a flow front of fresh, low-density lava over an older, higher density surface crust. The advance of the front causes the older crust to break up, founder, and dive downwards into the lake to expose new, hot, low-density lava. We find density differences of 200 to 740 kg/m3 between the foundering crust and over-riding and under-lying lava respectively. In this case, crustal overturn is driven by large density differences between the foundering and resurfacing units. These differences lead, inevitably, to frequent crustal renewal: simple density differences between the surface crust and underlying lake lava make the upper layers of the lake highly unstable. ?? Springer-Verlag 2008.

  7. Two-dimensional model for an αΩ-dynamo with meridional circulation and an associated Hamilton-Jacobi equation

    Science.gov (United States)

    Popova, E. P.

    2015-08-01

    A two-dimensional model for an αΩ-dynamo is constructed, taking into account meridional flows. A Hamilton-Jacobi equation for the resulting system of magnetic-field generation equatons is constructed using an asymptotic method analogous to the WKB method. This equation makes it possible to analytically study the influence of meridional flows on the duration of the solar magnetic-activity cycle and the evolution of magnetic waves.

  8. Critical embedment depth of a rigid retaining wall against overturning in unsaturated soils considering intermediate principal stress and strength nonlinearity

    Institute of Scientific and Technical Information of China (English)

    张常光; 陈新栋; 范文

    2016-01-01

    The overturning stability is vital for the retaining wall design of foundation pits, where the surrounding soils are usually unsaturated due to water draining. Moreover, the intermediate principal stress does affect the unsaturated soil strength; meanwhile, the relationship between the unsaturated soil strength and matric suction is nonlinear. This work is to present closed-form equations of critical embedment depth for a rigid retaining wall against overturning by means of moment equilibrium. Matric suction is considered to be distributed uniformly and linearly with depth. The unified shear strength formulation for unsaturated soils under the plane strain condition is adopted to characterize the intermediate principal stress effect, and strength nonlinearity is described by a hyperbolic model of suction angle. The result obtained is orderly series solutions rather than one specific answer; thus, it has wide theoretical significance and good applicability. The validity of this present work is demonstrated by comparing it with a lower bound solution. The traditional overturning designs for rigid retaining walls, in which the saturated soil mechanics neglecting matric suction or the unsaturated soil mechanics based on the Mohr-Coulomb criterion are employed, are special cases of the proposed result. Parametric studies about the intermediate principal stress, matric suction and its distributions along with two strength nonlinearity methods on a new defined critical buried coefficient are discussed.

  9. North Atlantic Temperature Anomaly

    OpenAIRE

    Vukcevic, M.A.

    2009-01-01

    The author postulates the existence of a high correlation between North Atlantic Temperature Anomaly and the variations of magnetic field over the Hudson Bay region. Post-glacial uplift and convection in the underlying mantle uplift (as reflected in changes of the area's magnetic intensity) are making significant contribution to the Atlantic basin climate change.

  10. Ocean and atmosphere feedbacks affecting AMOC hysteresis in a GCM

    Science.gov (United States)

    Jackson, L. C.; Smith, R. S.; Wood, R. A.

    2016-10-01

    Theories suggest that the Atlantic Meridional Overturning Circulation (AMOC) can exhibit a hysteresis where, for a given input of fresh water into the north Atlantic, there are two possible states: one with a strong overturning in the north Atlantic (on) and the other with a reverse Atlantic cell (off). A previous study showed hysteresis of the AMOC for the first time in a coupled general circulation model (Hawkins et al. in Geophys Res Lett. doi: 10.1029/2011GL047208, 2011). In this study we show that the hysteresis found by Hawkins et al. (2011) is sensitive to the method with which the fresh water input is compensated. If this compensation is applied throughout the volume of the global ocean, rather than at the surface, the region of hysteresis is narrower and the off states are very different: when the compensation is applied at the surface, a strong Pacific overturning cell and a strong Atlantic reverse cell develops; when the compensation is applied throughout the volume there is little change in the Pacific and only a weak Atlantic reverse cell develops. We investigate the mechanisms behind the transitions between the on and off states in the two experiments, and find that the difference in hysteresis is due to the different off states. We find that the development of the Pacific overturning cell results in greater atmospheric moisture transport into the North Atlantic, and also is likely responsible for a stronger Atlantic reverse cell. These both act to stabilize the off state of the Atlantic overturning.

  11. Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean.

    Science.gov (United States)

    Meckler, A N; Sigman, D M; Gibson, K A; François, R; Martínez-García, A; Jaccard, S L; Röhl, U; Peterson, L C; Tiedemann, R; Haug, G H

    2013-03-28

    Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning

  12. Linking The Atlantic Gyres: Warm, Saline Intrusions From Subtropical Atlantic to the Nordic Seas

    Science.gov (United States)

    Hakkinen, Sirpa M.; Rhines, P. B.

    2010-01-01

    Ocean state estimates from SODA assimilation are analyzed to understand how major shifts in the North Atlantic Current path relate to AMOC, and how these shifts are related to large scale ocean circulation and surface forcing. These complement surface-drifter and altimetry data showing the same events. SODA data indicate that the warm water limb of AMOC, reaching to at least 600m depth, expanded in density/salinity space greatly after 1995, and that Similar events occurred in the late 1960s and around 1980. While there were large changes in the upper limb, there was no immediate response in the dense return flow, at least not in SODA, however one would expect a delayed response of increasing AMOC due to the positive feedback from increased salt transport. These upper limb changes are winddriven, involving changes in the eastern subpolar gyre, visible in the subduction of low potential vorticity waters. The subtropical gyre has been weak during the times of the northward intrusions of the highly saline subtropical waters, while the NAO index has been neutral or in a negative phase. The image of subtropical/subpolar gyre exchange through teleconnections within the AMOC overturning cell will be described.

  13. Response of the Water Cycle of West Africa and Atlantic to Radiative Forcing by Saharan Dust

    Science.gov (United States)

    Lau, K. M.; Kim, Kyu-Myong; Sud, Yogesh C.; Walker, Gregory L.

    2010-01-01

    The responses of the atmospheric water cycle and climate of West Africa and the Atlantic to radiative forcing of Saharan dust are studied using the NASA finite volume general circulation model (fvGCM), coupled to a mixed layer ocean. We find evidence in support of the "elevated heat pump" (EHP) mechanism that underlines the responses of the atmospheric water cycle to dust forcing as follow. During the boreal summer, as a result of large-scale atmospheric feed back triggered by absorbing dust aerosols, rainfall and cloudiness are enhanced over the West Africa/Easter Atlantic ITCZ, and suppressed over the West Atlantic and Caribbean. region. Shortwave radiation absorption by dust warms the atmosphere and cools the surface, while long wave has the opposite response. The elevated dust layer warms the air over Nest Africa and the eastern Atlantic. The condensation heating associated with the induced deep convection drives and maintains an anomalous large-scale east-west overturning circulation with rising motion over West Africa/eastern Atlantic, and sinking motion over the Caribbean region. The response also includes a strengthening of the West African monsoon, manifested in northward shift of the West Africa precipitation over land, increased low-level westerlies flow over West Africa at the southern edge of the dust layer, and a near surface energy fluxes, resulting in cooling of the Nest African land and the eastern Atlantic, and a warming in the West Atlantic and Caribbean. The EHP effect is most effective for moderate to highly absorbing dusts, and becomes minimized for reflecting dust with single scattering albedo at 0.95 or higher.

  14. Evidence for cooler European summers during periods of changing meltwater flux to the North Atlantic.

    Science.gov (United States)

    Heiri, Oliver; Tinner, Willy; Lotter, André F

    2004-10-26

    We analyzed fossil chironomids (nonbiting midges) and pollen in two lake-sediment records to reconstruct and quantify Holocene summer-temperature fluctuations in the European Alps. Chironomid and pollen records indicate five centennial-scale cooling episodes during the early- and mid-Holocene. The strongest temperature declines of approximately 1 degrees C are inferred at approximately 10,700-10,500 and 8,200-7,600 calibrated 14C years B.P., whereas other temperature fluctuations are of smaller amplitude. Two forcing mechanisms have been presented recently to explain centennial-scale climate variability in Europe during the early- and mid-Holocene, both involving changes in Atlantic thermohaline circulation. In the first mechanism, changes in meltwater flux from the North American continent to the North Atlantic are responsible for changes in the Atlantic thermohaline circulation, thereby affecting circum-Atlantic climate. In the second mechanism, solar variability is the cause of Holocene climatic fluctuations, possibly triggering changes in Atlantic thermohaline overturning. Within their dating uncertainty, the two major cooling periods in the European Alps are coeval with substantial changes in the routing of North American freshwater runoff to the North Atlantic, whereas quantitatively, our climatic reconstructions show a poor agreement with available records of past solar activity. Thus, our results suggest that, during the early- and mid-Holocene, freshwater-induced Atlantic circulation changes had stronger influence on Alpine summer temperatures than solar variability and that Holocene thermohaline circulation reductions have led to summer-temperature declines of up to 1 degrees C in central Europe. PMID:15492214

  15. Air-sea interactions and oceanic processes in the development of different Atlantic Niño patterns

    Science.gov (United States)

    Martin-Rey, Marta; Polo, Irene; Rodríguez-Fonseca, Belén; Lazar, Alban

    2016-04-01

    Atlantic Niño is the leading mode of inter-annual variability of the tropical Atlantic basin at inter-annual time scales. A recent study has put forward that two different Atlantic Niño patterns co-exist in the tropical Atlantic basin during negative phases of the Atlantic Multidecadal Oscillation. The leading mode, Basin-Wide (BW) Atlantic Niño is characterized by an anomalous warming extended along the whole tropical basin. The second mode, the Dipolar (D) Atlantic Niño presents positive Sea Surface Temperature (SST) anomalies in the central-eastern equatorial band, surrounded by negative ones in the North and South tropical Atlantic. The BW Atlantic Niño is associated with a weakening of both Azores and Sta Helena High, which reduces the tropical trades during previous autumn-winter. On the other hand, the D-Atlantic Niño is related to a strengthening of the Azores and a weakening of Helena High given rise to a meridional Sea Level Pressure (SLP) gradient that originates an intensification of the subtropical trades and anomalous westerlies along the equatorial band. This different wind forcing suggests that different oceanic processes could act in the development of the BW and D Atlantic Niño patterns. For this reason, an inter-annual simulation with the ocean NEMO model has been performed and the heat budget analysis has been analysed for each Atlantic Niño mode. The results suggest that the two Atlantic Nino configurations have different timing. The heat budget analysis reveals that BW Atlantic Nino SST pattern is due to anomalous air-sea heat fluxes in the south tropical and western equatorial Atlantic during the autumn-winter, while vertical processes are responsible of the warming in the central and eastern part of the basin during late-winter and spring. For the D-Atlantic Nino, the subtropical cooling is attributed to turbulent heat fluxes, the equatorial SST signal is mainly forced by vertical entrainment. The role of the oceanic waves in the

  16. Remote influences on freshwater flux variability in the Atlantic warm pool region

    Science.gov (United States)

    Zhang, Liping; Wang, Chunzai

    2012-10-01

    The understanding of freshwater flux variability is both scientifically and socially important. Local freshwater flux response to a large Atlantic warm pool (AWP) is excessive freshwater or negative Evaporation minus Precipitation (EmP) anomalies, whereas the response is deficient to a small AWP. However, the EmP anomalies in the AWP region are also influenced by the SST anomalies in the tropical eastern Pacific and in the tropical South Atlantic. These remote influences operate through the inter-basin mode represented by the SST gradient between the tropical North Atlantic and eastern Pacific and the Atlantic meridional mode (AMM) defined as the SST gradient between the tropical North and South Atlantic. When either of these two modes is in the negative phase, the EmP and sea surface salinity anomalies in the AWP region can be positive although the AWP is large. This indicates that the remote influences of the inter-basin mode and/or the AMM can overwhelm the local effect and induce an opposite freshwater response. Additionally, although ENSO and the AMM sometimes coincide with AWP variability, an El Niño in the preceding winter or a positive AMM in the spring does not necessarily follow a large AWP in the summer.

  17. SUBGLACIAL VOLCANO IN ATLANTIC

    OpenAIRE

    Nakamura, Shigehisa

    2010-01-01

    This is a note to satellite monitoring of the subglacial volcano in the Atlantic. The satellite data are obtained by EUMETSAT and NASA An eruption of a subglacial volcano in Iceland had issued volcanic ash, and the winds transferred the ash to the European Union in April 2010. This volcano is one of the volcanoes in the Atlantic. There are volcanoes in the north and south Atlantic. Some of them are in Azores Islands, in Canary Islands and Cape Verde Islands. Iceland is located on the zone of ...

  18. Tierra y familia en la España meridional, siglos XIII-XIX.

    OpenAIRE

    Igor Goicovic Donoso

    2011-01-01

    Las siete contribuciones recopiladas por Francisco García Gon-zález en el texto Tierra y familia en la España meridional, siglos XIII-XIX, nos sitúan frente a la historia social a partir de dos inflexiones epistemológicas íntimamente relacionadas en la historia: tierra y familia. Efectivamente, la diada tierra-familia, durante el Antiguo Régimen europeo, se convirtió en un factor explicativo de buena parte de los procesos de articulación de la sociedad. Así, la relación de los sujetos co...

  19. Atlantic Salmon Telemetry Monitoring

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Annual telemetry data are collected as part of specific projects (assessments within watersheds) or as opportunistic efforts to characterize Atlantic salmon smolt...

  20. Probabilistic safety verification for the overturning of a service water pump house against 'over pressure wave'

    International Nuclear Information System (INIS)

    The Atucha II Nuclear Power Plant is being built in a riverside site located about 100 km north of the city of Buenos Aires. When completed it will become Argentina's third operating station. The Service Water Pump House of the 740 MW Natural Uranium NPP is supplied directly by the Parana de las Palmas River, which is a navigable waterway with heavy fluvial traffic that links the River Plate Estuary with ports along the Parana River, including Rosario, Argentina's third city. Although dolphins were built to protect the water intake structures from direct impact from ships or barges, an accidental escape of gas or flammable material outside the protected area that may result in an explosion with a subsequent pressure wave, could not be precluded. Thus, the water intake structures had to be designed against this event. In addition, concern with the margin of safety against foundation instability due to the overturning moment associated to the induced pressures, made an evaluation of the conditional probability of failure for this loading condition, desirable. The task placed still uncommon demands on the Project Engineers, who were required to provide estimates of the Pump House reliability. In the process, it became necessary to introduce assumptions concerning the unspecified variability of the loads which, according to current international practice, were defined in the form of 'deterministic' design criteria. It seems appropriate to underline these deficiencies in current standards, as well as in aspects of the dynamic of soils that are responsible for a quite large model uncertainty. On the basis of available information, it may be established that the probability of occurrence of a pressure wave at the site is less than 10-3 per year. Accordingly, the probability of failure of the foundation due to a pressure wave does not exceed 1.5 -8 per year, value that is considered admissible by current standards. The use of triangular probability density functions for

  1. Woman Swims Atlantic

    Institute of Scientific and Technical Information of China (English)

    贾庆文

    2009-01-01

    Jennifer Figge pressed her toes into the Caribbean sand, excited and exhausted as she touched land this week for the first time in almost a month. Reaching a beach in Trinidad, she became the first woman on record to s,Mm across the Atlantic Ocean-a dream she'd had since the early 1960s, when a stormy trans-Atlantic flight got her thinking she could wear a life vest and swim the rest of the way if needed.

  2. Overturning anti-miscegenation laws: news media coverage of the Lovings' legal case against the state of Virginia.

    Science.gov (United States)

    Hoewe, Jennifer; Zeldes, Geri Alumit

    2012-01-01

    This study fills a gap in scholarship by exploring historical news coverage of interracial relationships. It examines coverage by The New York Times, Washington Post and Times-Herald, and Chicago Tribune of the progression of the landmark civil rights case of Loving v. Virginia, in which the Supreme Court overturned Virginia's anti-miscegenation law, which prohibited marriage between any White and non-White person. An analysis of the frames and sources used in these publications' news stories about the case indicate all three publications' coverage favored the Lovings.

  3. On the possibility of convective overturning in the Słupsk Furrow overflow of the Baltic Sea

    Directory of Open Access Journals (Sweden)

    Victor Zhurbas

    2011-09-01

    Full Text Available Alosely spaced CTD transects across the Slupsk Furrow displayed a "downward-bending" of salinity contours below the salinity interface on the southern flank due to a transverse circulation in the saline water overflow. Numerical simulation of a gravity current in an idealized channel with geometry, dimensions and initial density stratification all much the same as in the Slupsk Furrow was applied to verify whether the downward-bending could be transformed into an inverted density stratification. Some arguments in favour of the possibility of convective overturning due to the differential transverse advection beneath the gravity current, brought on by the numerical simulations, are discussed.

  4. Characteristics of winter-time meridional thermospheric winds over Tromsø during solar minimum

    Science.gov (United States)

    Cai, Hongtao; Zhan, Weijia; Huang, Dingjuan; Li, Fei; Zhou, Kangjun; Shen, Ge; Willian McCrea, Ian; Ma, Shuying

    2015-04-01

    The background of the winter-time thermospheric wind over Tromsø (69 °N, 19 °E) were focused on in this paper. The meridional component of the neutral wind in F-region were derived from the field-aligned ion velocity detected by the European incoherent scattering (EISCAT) radar. In order to eliminate possible influences from solar activity variances and geomagnetic disturbance, only measurements accomplished under geomagnetically quiet conditions (with maximum Kp ≤ 3) around the winter solstice during solar minimum (2008-2009) were chosen in present work. Two major characteristics of the radar derived winds are revealed. The first feature is the vertical variations of the meridional winds. Magnitudes of the equatorward winds observed show a hint of increasing with altitudes during nighttime. The second one is the persistent equatorward winds at altitudes higher than 280 km height during daytime, especially around local noon, whilst the prevailing poleward winds appear at lower altitudes. Thus, significant shears of horizontal winds are expected in the vertical direction. Detail comparisons with models and discussions of the possible driving forces for the day-time equatorward winds will be presented in the report.

  5. On the role of meridional flows in flux transport dynamo models

    CERN Document Server

    Jouve, L

    2007-01-01

    The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We are using 2-D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation pro...

  6. Meridional motions and Reynolds stress from SDO/AIA coronal bright points data

    CERN Document Server

    Sudar, Davor; Skokić, Ivica; Beljan, Ivana Poljančić; Brajša, Roman

    2016-01-01

    Context. It is possible to detect and track coronal bright points (CBPs) in SDO/AIA images. Combination of high resolution and high cadence provides a wealth of data that can be used to determine velocity flows on the solar surface with very high accuracy. Aims. We derived a very accurate solar rotation profile and investigated meridional flows, torsional oscillations and horizontal Reynolds stress based on $\\approx$6 months of SDO/AIA data. Methods. We used a segmentation algorithm to detect CBPs in SDO/AIA images. We also used invariance of the solar rotation profile with central meridian distance (CMD) to determine the height of CBPs in 19.3 nm channel. Results. Best fit solar rotation profile is given by $\\omega(b)=(14.4060\\pm0.0051 + (-1.662\\pm0.050)\\sin^{2}b + (-2.742\\pm0.081)\\sin^{4}b)${\\degr} day$^{-1}$. Height of CBPs in SDO/AIA 19.3 nm channel was found to be $\\approx$6500 km. Meridional motion is predominantly poleward for all latitudes, while solar velocity residuals show signs of torsional oscill...

  7. Meridional lenticular astigmatism associated with bilateral concurrent uveal metastases in renal cell carcinoma

    Directory of Open Access Journals (Sweden)

    Priluck JC

    2012-11-01

    Full Text Available Joshua C Priluck, Sandeep Grover, KV ChalamDepartment of Ophthalmology, University of Florida College of Medicine, Jacksonville, FL, USAPurpose: To demonstrate a case illustrating meridional lenticular astigmatism as a result of renal cell carcinoma uveal metastases.Methods: Case report with images.Results: Clinical findings and diagnostic testing of a patient with acquired meridional lenticular astigmatism are described. The refraction revealed best-corrected visual acuity of 20/20–1 OD (−2.50 + 0.25 × 090 and 20/50 OS (−8.25 + 3.25 × 075. Bilateral concurrent renal cell carcinoma metastases to the choroid and ciliary body are demonstrated by utilizing ultrasonography, ultrawidefield fluorescein angiography, and unique spectral-domain optical coherence tomography.Conclusions: Metastatic disease should be included in the differential of acquired astigmatism. Spectral-domain optical coherence tomography, ultrawidefield fluorescein angiography, and ultrasonography have roles in delineating choroidal metastases.Keywords: astigmatism, metastasis, optical coherence tomography, renal cell carcinoma

  8. Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum.

    Science.gov (United States)

    Felis, Thomas; McGregor, Helen V; Linsley, Braddock K; Tudhope, Alexander W; Gagan, Michael K; Suzuki, Atsushi; Inoue, Mayuri; Thomas, Alexander L; Esat, Tezer M; Thompson, William G; Tiwari, Manish; Potts, Donald C; Mudelsee, Manfred; Yokoyama, Yusuke; Webster, Jody M

    2014-06-17

    Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and δ(18)O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1-2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.

  9. On th meridional surface profile of the Gulf Stream at 55 deg W

    Science.gov (United States)

    Hallock, Zachariah R.; Teague, William J.

    1995-01-01

    Nine-month records from nine inverted echo sounders (IESs) are analyzed to describe the mean baroclinic Gulf Stream at 55 deg W. IES acoustic travel times are converted to thermocline depth which is optimally interpolated. Kinematic and dynamic parameters (Gulf Stream meridional position, velocity, and vorticity) are calculated. Primary Gulf Stream variabiltiy is attributed to meandering and and changes in direction. A mean, stream-coordinate (relative to Gulf Stream instantaneous position and direction) meridional profile is derived and compared with results presented by other investigators. The mean velocity is estimated at 0.84 m/s directed 14 deg to the right eastward, and the thermocline (12 c) drops 657 m (north to south), corresponding to a baroclinic rise of the surface of 0.87 m. The effect of Gulf Stream curvature on temporal mean profiles is found to be unimportant and of minimal importance overall. The derived, downstream current profile is well represented by a Gaussian function and is about 190 km wide where it crosses zero. Surface baroclinic transport is estimated to be 8.5 x 10(exp 4) sq m/s, and maximum shear (flanking the maximum) is 1.2 x 10(exp -5). Results compare well with other in situ observational results from the same time period. On the other hand, analyses (by others) of concurrent satellite altimetry (Geosat) suggest a considerable narrower, more intense mean Gulf Stream.

  10. Verification of the helioseismic Fourier-Legendre analysis for meridional flow measurements

    CERN Document Server

    Roth, Markus; Hartlep, Thomas

    2016-01-01

    Measuring the Sun's internal meridional flow is one of the key issues of helioseismology. Using the Fourier-Legendre analysis is a technique for addressing this problem. We validate this technique with the help of artificial helioseismic data. The analysed data set was obtained by numerically simulating the effect of the meridional flow on the seismic wave field in the full volume of the Sun. In this way, a 51.2-hour long time series was generated. The resulting surface velocity field is then analyzed in various settings: Two $360^\\circ \\times 90^\\circ$ halfspheres, two $120^\\circ \\times 60^\\circ$ patches on the front and farside of the Sun (North and South, respectively) and two $120^\\circ \\times 60^\\circ$ patches on the northern and southern frontside only. We compare two possible measurement setups: observations from Earth and from an additional spacecraft on the solar farside, and observations from Earth only, in which case the full information of the global solar oscillation wave field was available. We ...

  11. Meridional-Flow Measurements from 15 Years of GONG Spherical-Harmonic Time Series

    International Nuclear Information System (INIS)

    We present results of meridional-flow measurements for 1995-2009, using travel-time differences from velocity images reconstructed using GONG spherical harmonic (SH) coefficients after applying phase-velocity and low-m filters. This filtering technique increases the signal-to-noise ratio and thus extends travel-time measurements to relatively high latitudes and deep into the convection zone. Preliminary analyses shows a strong one-year periodicity presumably due to solar pole misalignment and B0-angle artifacts, which makes it difficult to see underlying temporal variations. Removing a simple one-year-period sine wave fit reveals long-term temporal variations of the flow on top of this yearly periodicity. High-latitude measurements are affected more stronger by foreshortening and B0 -angle artifacts. We analyze different B0-angle intervals separately, so in each hemisphere better high-latitude visibility comes six months apart. This approach suggests why at high latitudes travel-time measurements of meridional flow shows a tendency to change sign instead of continuing towards the poles.

  12. Variability and trends of the meridional energy transports and implications for the Hadley circulation

    Science.gov (United States)

    Boschat, Ghyslaine; Simmonds, Ian

    2016-04-01

    The atmosphere and the ocean play a critical role in the Earth energy balance by transporting heat from the equator to the poles. In the atmosphere, this net meridional energy transport is the sum of many atmospheric processes on very different scales in space and time: in the tropics most of the energy is transported poleward by the Hadley circulation, whereas eddies become the principal agency of heat transport in higher latitudes. However these synoptic systems are also believed to interact in a quite complex manner with the Hadley circulation. The aim of this study is to perform a detailed and updated analysis of the meridional heat transports by the atmosphere and determine the relative role of the Hadley circulation and eddy activity in this energy redistribution. Using 6 hourly ERA-Interim data from 1979 to 2014, we examine the spatial contrasts in the transport of sensible heat, latent heat and potential energy components, and diagnose how these may have been changing on seasonal to inter-annual timescales. These contributions are further partitioned into mean circulations and (stationary and transient) eddies' activities, to explore the complementary variations occurring between baroclinic synoptic systems and mean tropical cells. Finally, this "unifying conceptual framework" provides new insights into the variability and trends of the Hadley circulation, which help us better understand the factors controlling the behavior of the cell both in the present climate and under a global warming scenario.

  13. On the relationship between east equatorial Atlantic SST and ISM through Eurasian wave

    Science.gov (United States)

    Yadav, Ramesh Kumar

    2016-03-01

    The dominant mode of July-August (JA) seasonal variability of Indian summer monsoon rainfall (ISMR) are obtained by performing empirical orthogonal function (EOF) analysis. The first dominant mode of ISMR and its relationships with the sea surface temperature (SST), pressure level wind and geopotential height (GPH) fields are examined using gridded datasets for the period 1979-2014. The principal component of the first leading mode (PC1) obtained in the EOF analysis of JA rainfall over Indian landmass is highly correlated with north-west and central India rainfall, and anti-correlated with east-equatorial Atlantic SST (EEASST). The positive EEASST anomaly intensifies the inter-tropical convergence zone over Atlantic and west equatorial Africa which generates stationary wave meridionally, as meridional transfer of energy is strong, as the influence of background jet-streams are minimal over North Africa and Europe. The anomalous positive and negative GPH are generated over sub-tropics and extra-tropics, respectively, due to the stationary wave. This increases the climatological background steep pressure gradient between sub-tropics and extra-tropics consisting of anomalous negative GPH field over north-west (NW) Europe and vice versa for negative EEASST anomaly. The anomalous positive GPH over NW Europe acts as center of action for the propagation of a Rossby wave train to NW India via Europe consisting of anomalous high over NW of India. This intensifies the Tibetan High westward which reinforces the outbreak of monsoon activities over central and NW India.

  14. Research on Decreasing Width of Side Overturning of Stainless Steel Cold Plate%减小不锈钢冷板侧翻宽度的研究

    Institute of Scientific and Technical Information of China (English)

    刘满

    2013-01-01

    The paper was introduced formation mechanism of side overturning of stainless steel cold plate, and designed a program of billet side concave pilot. Then, the paper was obtained the reasons of leading side overturning of slab edge por-tion forming due to the slab surface metal roll contact friction which caused by surface and internal the metal poor deforma-tion. The tests indicated that the billet side overturning could reduce rollover occurs regional. Adjusting mould taper and pa-rameters of cooling water could achieve the purpose of billet produce side overturning, and effectively reduce the overturning value.%  介绍了不锈钢冷板侧翻的形成原理,设计了连铸坯侧凹的试验方案,得出了在热连轧粗轧过程中,由于板坯表面金属与轧辊接触产生摩擦力,造成表面和内部金属的变形差,最终导致板坯边部形成侧翻。试验表明连铸坯侧凹可以减少侧翻发生区域。调整连铸机结晶器锥度、冷却水等参数可以达到使连铸坯产生侧凹的目的,并有效降低侧翻值。

  15. Substrate Factors Determine Roadside Vegetation Structure and Species Richness: A Case Study Along a Meridional Gradient in Fennoscandia.

    Science.gov (United States)

    Jaźwa, Małgorzata; Heise, Waldemar; Klimek, Beata

    2016-10-01

    This study assessed the effects of road-related alteration of substrate, including increased salinity, on vegetation along a meridional gradient in Fennoscandia. Vegetation community composition were surveyed in 29 randomly selected 1-m(2) sized roadside plots. Number of plant species and plant cover (%) on the plots were positively interrelated (p CCA) indicated that roadside vegetation diversity and composition were most related to the importance of the road (i.e. its size and traffic intensity) and substrate pH. Road importance affects plant dispersal, whereas substrate pH was found to be a factor limiting growth. CCA indicated also that vegetation composition was affected by the meridional gradient and by the substrate salinity; both substrate salinity pH and salinity were not related to meridional gradient. Our results indicate that roadside vegetation diversity and composition is driven by natural and anthropogenic factors. PMID:27485008

  16. Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution ocean model

    Science.gov (United States)

    Topper, R. P. M.; Meijer, P. Th.

    2015-02-01

    A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations of the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation). A series of experiments with different sill depths in the Atlantic-Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is shallower and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin, a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic-Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth ≤10 m, is a plausible scenario for the second stage of the Messinian Salinity Crisis during which

  17. Importance of Meridional Circulation in Flux Transport Dynamo: The Possibility of a Maunder-like Grand Minimum

    OpenAIRE

    Karak, Bidya Binay

    2010-01-01

    Meridional circulation is an important ingredient in flux transport dynamo models. We have studied its importance on the period, the amplitude of the solar cycle, and also in producing Maunder-like grand minima in these models. First, we model the periods of the last 23 sunspot cycles by varying the meridional circulation speed. If the dynamo is in a diffusion-dominated regime, then we find that most of the cycle amplitudes also get modeled up to some extent when we model the periods. Next, w...

  18. North Atlantic Circulation

    Science.gov (United States)

    Molinari, R.; Bryan, K.; Schott, F.

    The intensity of the North Atlantic winddriven and thermohaline circulation and the close proximity of many oceanographic installations make the North Atlantic a particularly favored region of the world ocean from the standpoint of research in ocean circulation. Recent increases in available data and advances in numerical modeling techniques served as the impetus to convene a joint workshop of modelers and observers working on the North Atlantic with the Scientific Committee on Oceanic Research (SCOR) Working Group (WG) 68 (“North Atlantic Circulation”). Goals of the workshop were to provide an update on data sets and models and to discuss the poleward heat flux problem and possible monitoring strategies. The joint Workshop/SCOR WG-68 meeting was convened by F. Schott (chairman of the working group; Rosenstiel School of Marine and Atmospheric Science, Miami, Fla.), K. Bryan (National Oceanic and Atmospheric Administration/ Geophysical Fluid Dynamics Laboratory (NOAA/GFDL)), and R. Molinari (NOAA/Atlantic Oceanographic and Meteorological Laboratory (NOAA/AOML)).

  19. Atlantic menhaden adult tagging study

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Atlantic menhaden are a schooling forage fish species, which are subject to a large commercial purse seine fishery. Atlantic menhaden are harvested for reduction...

  20. The Aqua-planet Experiment (APE): Response to Changed Meridional SST Profile

    Science.gov (United States)

    Williamson, David L.; Blackburn, Michael; Nakajima, Kensuke; Ohfuchi, Wataru; Takahashi, Yoshiyuki O.; Hayashi, Yoshi-Yuki; Nakamura, Hisashi; Ishiwatari, Masaki; Mcgregor, John L.; Borth, Hartmut; Wirth, Volkmar; Frank, Helmut; Bechtold, Peter; Wedi, Nils P.; Tomita, Hirofumi; Satoh, Masaki; Zhao, Ming; Held, Isaac M.; Suarez, Max J.; Lee, Myong-In; Watanabe, Masahiro; Kimoto, Masahide; Liu, Yimin; Wang, Zaizhi; Molod, Andrew; RajenDran, Kavirajan; Kitoh, Akio; Stratton, Rachel

    2013-01-01

    This paper explores the sensitivity of Atmospheric General Circulation Model (AGCM) simulations to changes in the meridional distribution of sea surface temperature (SST). The simulations are for an aqua-planet, a water covered Earth with no land, orography or sea- ice and with specified zonally symmetric SST. Simulations from 14 AGCMs developed for Numerical Weather Prediction and climate applications are compared. Four experiments are performed to study the sensitivity to the meridional SST profile. These profiles range from one in which the SST gradient continues to the equator to one which is flat approaching the equator, all with the same maximum SST at the equator. The zonal mean circulation of all models shows strong sensitivity to latitudinal distribution of SST. The Hadley circulation weakens and shifts poleward as the SST profile flattens in the tropics. One question of interest is the formation of a double versus a single ITCZ. There is a large variation between models of the strength of the ITCZ and where in the SST experiment sequence they transition from a single to double ITCZ. The SST profiles are defined such that as the equatorial SST gradient flattens, the maximum gradient increases and moves poleward. This leads to a weakening of the mid-latitude jet accompanied by a poleward shift of the jet core. Also considered are tropical wave activity and tropical precipitation frequency distributions. The details of each vary greatly between models, both with a given SST and in the response to the change in SST. One additional experiment is included to examine the sensitivity to an off-equatorial SST maximum. The upward branch of the Hadley circulation follows the SST maximum off the equator. The models that form a single precipitation maximum when the maximum SST is on the equator shift the precipitation maximum off equator and keep it centered over the SST maximum. Those that form a double with minimum on the equatorial maximum SST shift the double

  1. Relation between the meridional distribution of potential vorticity and the Lagrangian mean circulation in the troposphere

    Science.gov (United States)

    Satoh, Masaki; Damtp

    1999-10-01

    The meridional distribution of potential vorticity (PV) in the troposphere is examined in terms of the Lagrangian transport by using an idealistic general circulation model. A zonally uniform forcing and uniform boundary conditions are applied to the model to particularly examine the PV structure in the mid-latitudes and the subtropics. Trajectories of air parcels released from each grid point of the model and Lagrangian changes in PV are calculated for a period of 60days. Values of PV of each parcel are changing along the Lagrangian motions due to the diabatic effect, the frictional effect and the mixing effect which has smaller scales than those resolvable in the model. Both diabatic and frictional effects are dominant in the lower layers, and the mixing effect is larger in the other regions. It is found that the zonal mean PV changes have different characteristics between the "Underworld" in which isentropes intersect the ground and the "Middleworld" in which isentropes are above the ground and intersect the tropopause. In the Underworld, the zonal mean PV changes are determined by the equatorial flow in the lower layers. In particular, the PV changes are negative in the lower layers of the low- and the mid-latitudes. (The sign of PV tendency is for the northern hemisphere. The southern hemispheric tendency is opposite as in the followings.) This negative tendency is due to the diabatic effect near the surface. In the Middleworld, there remain positive and negative tendency regions, which are resulted from the isentropic mixing. In general, if a parcel moves poleward in the mid-latitudes, the value of PV increases, whereas the value of PV decreases if a parcel moves equatorward. The sign of the Lagrangian mean change in PV corresponds to whether the Lagrangian mean motions cross the PV contours equatorward or poleward in the meridional plane. In particular, the contour of no change in PV has a similar shape to that of meridional distribution of PV in the mid

  2. Study on mechanism of interdecadal Atlantic thermohaline circulation variability Ⅲ.Influences of inter-decadal variations by ocean-atmosphere elements%大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

    Institute of Scientific and Technical Information of China (English)

    牟林; 陈学恩; 宋军; 李欢; 李琰; 姜晓轶; 李海; 陈幸荣

    2011-01-01

    Using the advanced climate ocean-atmosphere coupled model described in the first paper of this series of studies, response analyses of inter-decadal variations of the Atlantic thermohaline circulation by ocean-atmosphere elements were deliberated. For studying the relationship between the ocean elements of the North Atlantic and inter-decadal variations of Atlantic thermohaline circulation, subpolar sea-surface-density index (SSI) and North-Atlantic-Current Strength Index (NSI) were defined and analyzed. The results of analyses indicate that SSI changes 7 years earlier than Atlantic meridional overturning circulation (MOO while NSI changing 4 years earlier than MOC. The strength of water overflows from Greenland-Scotland sea ridge (including Denmark Strait and Faro Strait), which is an important water resource ofNorth Atlantic deep water, changes 3 years ahead MOC. The inter-decadal variations of Atlantic thermohaline circulation THC are influenced by North Atlantic atmosphere elements significantly. 2 years before subpolar current and North Atlantic Current (NAC) reaching their full strength, High-latitude atmosphere is abnormal in the pattern of cyclonic circulation and middle-latitude atmosphere is abnormal in the pattern of anticyclonic circulation. Meanwhile, the sea surface heat flux in Atlantic subpolar sea area is negative anomaly. All of those abnormal conditions enhance the strength of subpolar current and NAC. More water with higher salinity intruding into Greenland-Icelandic-Norwegian Sea (GIN) brings higher sea surface density of GIN. This weakens the stability of water stratification and increases the probability of occurrence of deep convection. Meanwhile, the variation of atmosphere influences the deep water generating process in GIN by the curl of wind stress and sea surface heat flux. Then the strength of water overflows from Greenland-Scotland sea ridge is strengthened.%基于该系列文章前文研究中构建的海气耦合气候模式和所

  3. Variability of Atlantic Ocean heat transport and its effects on the atmosphere

    Directory of Open Access Journals (Sweden)

    R. T. Sutton

    2003-06-01

    Full Text Available The variability of the Atlantic meridional Ocean Heat Transport (OHT has been diagnosed from a simulation of a coupled ocean-atmosphere general circulation model, and the mechanisms responsible for this variability have been elucidated. It has been demonstrated that the interannual variability in Atlantic OHT is dominated by windstress-driven Ekman fluctuations. In contrast, the decadal and multidecadal variability is associated with the fluctuations of the Thermohaline Circulation (THC, driven by the fluctuations in deep convection over the Greenland-Iceland-Norwegian (GIN Sea. The fluctuations of OHT induce Ocean Heat Content (OHC, and Sea Surface Temperature (SST anomalies over the tropical and subtropical North Atlantic. The SST anomalies, in turn, have an impact on the atmosphere. The lead-lag relationships between the fluctuations of THC-related OHT and those of OHC and SST raise the possibility that a knowledge of OHT fluctuations could be used to predict variations in Atlantic Sea surface temperatures, and perhaps aspects of climate, several years in advance. A comparison of results from a second, independent, coupled model simulation is also presented, and similar conclusions reached.

  4. The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin.

    Science.gov (United States)

    van Hengstum, Peter J; Donnelly, Jeffrey P; Fall, Patricia L; Toomey, Michael R; Albury, Nancy A; Kakuk, Brian

    2016-01-01

    Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval. PMID:26906670

  5. Relation between sea surface temperature anomaly in the Atlantic and summer precipitation over the Northeast China

    Institute of Scientific and Technical Information of China (English)

    白人海

    2001-01-01

    Based on global monthly average data set of sea surface temperature (SST) during 1950 -1992 and global monthly average 500 hPa height during 1950 - 1997 offered by NCAR/NCEP, the feature of SST anomaly in the Atlantic and its relation with summer precipitation over the Northeast China are analyzed. The results show that, the second eigenvector of the SST′s empirical orthogonal expanssion in winter season over the North Atlantic suggests that distribution of SST anomaly has unusual meridional difference; The location of its center is basically identical to center of significant correlation region between summer precipitation over the Northeast China and winter SST in the Atlantic. When winter SST in the North Atlantic is hot in south and cold in north, the blocking situation is stronger in the middle- high latitude. Correspondingly, the blocking high pressure in the northern North Pacific is also getting stronger,the westerlies circulation index in East Asia in next summer would be lower, asa result, more precipitation in the summer would be experienced over Northeast China and vice versa.

  6. Seasonal Variability of South Atlantic Central Water

    Science.gov (United States)

    Nobrega Passos, E.; de Freitas Assad, L.; Landau, L.

    2013-05-01

    The South Atlantic Ocean (SAO) is constituted by different density water masses. Among these, the South Atlantic Central Water (SACW) is formed on the Brazil-Falkland Confluence region (BFC) and once formed, it becomes part of the Subtropical Gyre. When approaching again the Brazilian coast, this water mass bifurcates next to São Tomé Cape and part of it flows to the North and part to the South. There is another bifurcation that formed the sub-gyre of the SAO and occurs near 30°S. This work aims to analyze the seasonal variability associated to the SACW trajectory on the SAO basin. To achieve this goal, ECCO2 project's time series of prognostic fields were analysed. The parameters evaluated were temperature, salinity and the zonal and meridional velocity components in averaged monthly fields between January 1992 and November 2010. First a climatological year was calculated and was composed by means from all time series for each month. And second, it was estimate seasonally means for the south hemisphere to summer, autumn, winter and springer. For the analysis, the SACW was separated from the rest of the water masses by isolating it for its temperature, salinity and density index. Then the volume transport (VT) was calculated for seven different sections: A (10°S and 36°W-30°W), B (35°S and 55°W-45°W), C (40°W and 37°S-43°S), D (34°S and 7°E-20°E), E (20°E and 34°S-45°S), F (20°W and 27°S-33°S) and G (10°W and 20°S-25°S). The VT integrated on the water column occupied by SACW was calculated from the zonal and meridional velocities. The analysis showed that the VT balance between the sections is consistent with the climatologic analysis, according to scientific references. The analysis of the climatological VT showed that the VT field integrated in SAWC levels is also consistent with scientific reference. On the seasonal analysis, the sections A and F show a stronger VT during autumn. Since section A is formed from part of the flux of section

  7. Calculation of a residual mean meridional circulation for a zonal-mean tracer transport model

    Energy Technology Data Exchange (ETDEWEB)

    Choi, W.K.; Rotman, D.A.; Wuebbles, D.J. [Lawrence Livermore National Lab., CA (United States). Global Climate Research Div.

    1995-04-01

    Because of their computational advantages, zonally-averaged chemical-radiative-transport models are widely used to investigate the distribution of chemical species and their change due to the anthropogenic chemicals in the lower and middle atmosphere. In general, the Lagrangian-mean formulation would be ideal to treat transport due to the zonal mean circulation and eddies. However, the Lagrangian formulation is difficult to use in practical applications. The most widely-used formulation for treating global atmospheric dynamics in two-dimensional models is the transformed Eulerian mean (TEM) equations. The residual mean meridional circulation (RMMC) in the TEM system is used to advect tracers. In this study, the authors describe possible solution techniques for obtaining the RMMC in the LLNL two-dimensional chemical-radiative-transport model. In the first section, the formulation will be described. In sections 3 and 4, possible solution procedures will be described for a diagnostic and prognostic case, respectively.

  8. Zonal superrotation above Venus' cloud base induced by the semidiurnal tide and the mean meridional circulation

    Science.gov (United States)

    Hou, Arthur Y.; Goody, Richard M.; Fels, Stephen B.

    1990-01-01

    The equilibrium zonal wind structure resulting from the interaction of the semidiurnal tide and the mean meridional circulation driven by the zonally averaged solar heating above the Venus cloud base were calculated. The results show that the tidal mechanism proposed by Fels and Lindzen (1974) can account for a substantial fraction (and possibly all) of the increase of the equatorial wind speed above the cloud base. Above the cloud tops, tidal deceleration may be too small to produce the zonal wind decrease with height inferred from thermal data. Tidal forcing does not explain the superrotation below the clouds, and additional eddy sources are needed to account for the zonal wind structure at mid and high latitudes.

  9. The concentration of the large-scale solar magnetic field by a meridional surface flow

    Science.gov (United States)

    Devore, C. R.; Boris, J. P.; Sheeley, N. R., Jr.

    1984-01-01

    Analytical and numerical solutions to the magnetic flux transport equation in the absence of new bipolar sources of flux are calculated for several meridional flow profiles and a range of peak flow speeds. It is found that a poleward flow with a broad profile and a nominal 10 m/s maximum speed concentrates the large-scale field into very small caps of less than 15 deg half-angle, with average field strengths of several tens of gauss, contrary to observations. A flow which reaches its peak speed at a relatively low latitude and then decreases rapidly to zero at higher latitudes leads to a large-scale field pattern which is consistent with observations. For such a flow, only lower latitude sunspot groups can contribute to interhemispheric flux annihilation and the resulting decay and reversal of the polar magnetic fields.

  10. Meridional trapping and zonal propagation of inertial waves in a rotating fluid shell

    CERN Document Server

    Rabitti, Anna

    2014-01-01

    Inertial waves propagate in homogeneous rotating fluids, and constitute a challenging and simplified case study for the broader class of inertio-gravity waves, present in all geophysical and astrophysical media, and responsible for energetically costly processes as diapycnal and angular momentum mixing. However, a complete analytical description and understanding of internal waves in arbitrarily shaped enclosed domains, such as the ocean, or a planet liquid core, is still missing. In this work, the inviscid, linear inertial wave field is investigated by means of three dimensional ray tracing in spherical shell domains, having in mind possible oceanographic applications. Rays are here classically interpreted as representative of energy paths. But in contrast with previous studies, they are now launched with a non-zero initial zonal component allowing for a more realistic, localized forcing, and the development of azimuthal inhomogeneities. We find that meridional planes generally act in the shell geometry as a...

  11. The meridional variation of the eddy heat fluxes by baroclinic waves and their parameterization

    Science.gov (United States)

    Stone, P. H.

    1974-01-01

    The meridional and vertical eddy fluxes of sensible heat produced by small-amplitude growing baroclinic waves are calculated using solutions to the two-level model with horizontal shear in the mean flow. The results show that the fluxes are primarily dependent on the local baroclinicity, i.e., the local value of the isentropic slopes in the mean state. Where the slope exceeds the critical value, the transports are poleward and upward; where the slope is less than the critical value, the transports are equatorward and downward. These results are used to improve an earlier parameterization of the tropospheric eddy fluxes of sensible heat based on Eady's model. Comparisons with observations show that the improved parameterization reproduces the observed magnitude and sign of the eddy fluxes and their vertical variations and seasonal changes, but the maximum in the poleward flux is too near the equator.

  12. Solar rotation measurements at Mount Wilson. III - Meridional flow and limbshift

    Science.gov (United States)

    Labonte, B. J.; Howard, R.

    1982-01-01

    It is shown that the use of a two-parameter limbshift and a meridional flow velocity fits solar velocity data better than the standard analysis defined by Howard et al. (1980). The data used are the coarse residual velocity arrays, with 34 equal intervals in both sine latitude and sine longitude. There are a total of 2899 full-disk observations between January 1, 1967, and December 12, 1980. The original velocity fields are reconstructed by adding into the residual arrays the large-scale patterns that were measured and removed on a daily basis by a standard reduction. Tests of this reconstituded data set show that no significant errors are introduced in the analysis of large-scale velocity fields. The results of the analysis presented here imply that the study of solar velocity pattern at the level of a few m/s requires that magnetic regions be treated separately from nonmagnetic regions.

  13. Tierra y familia en la España meridional, siglos XIII-XIX.

    Directory of Open Access Journals (Sweden)

    Igor Goicovic Donoso

    2011-10-01

    Full Text Available Las siete contribuciones recopiladas por Francisco García Gon-zález en el texto Tierra y familia en la España meridional, siglos XIII-XIX, nos sitúan frente a la historia social a partir de dos inflexiones epistemológicas íntimamente relacionadas en la historia: tierra y familia. Efectivamente, la diada tierra-familia, durante el Antiguo Régimen europeo, se convirtió en un factor explicativo de buena parte de los procesos de articulación de la sociedad. Así, la relación de los sujetos con la propiedad y usufructo de la tierra, y las formas de articulación familiar desplegadas por los mismos, tienden a imbricarse recurrentemente en complejos procesos de reproducción social.

  14. Control of Meridional Flow by a Non-Uniform Rotational Magnetic Field

    Science.gov (United States)

    Mazuruk, Konstantin; Ramachandran, Narayanan

    1999-01-01

    The diffusive mass transfer of species during crystal growth in vertical ampoules is significantly affected by fluid flow in the liquid mother phase (melt). For electrically conductive melts, an elegant way of remotely inducing and controlling this flow is by utilizing a uniform rotational magnetic field (RMF) in the transverse direction. It induces an azimuthal flow which tends to homogenize the thermal and solutal fields. The rotating field also reduces the diffusion boundary layer, stabilizes temperature fluctuations, and promotes better overall crystal growth. For moderate strengths of the applied magnetic field (2-20 m Tesla) with frequencies of up to 400 Hz, the induced secondary meridional flow becomes significant. It typically consists of one roll at the bottom of the liquid column and a second roll (vortex) at the top. The flow along the centerline (ampoule axis) is directed from the growing solid (interface) towards the liquid (melt). In case of convex interfaces (e.g. in floating zone crystal growth) such flow behavior is beneficial since it suppresses diffusion at the center. However, for concave interfaces (e.g. vertical Bridgman crystal growth) such a flow tends to exacerbate the situation in making the interface shape more concave. It would be beneficial to have some control of this meridional flow- for example, a single recirculating cell with controllable direction and flow magnitude will make this technique even more attractive for crystal growth. Such flow control is a possibility if a non-uniform PNE field is utilized for this purpose. Although this idea has been proposed earlier, it has not been conclusively demonstrated so far. In this work, we derive the governing equations for the fluid dynamics for such a system and obtain solutions for a few important cases. Results from parallel experimental measurements of fluid flow in a mercury column subjected to non-uniform RMF will also be presented.

  15. Cloud Effects on Meridional Atmospheric Energy Budget Estimated from Clouds and the Earth's Radiant Energy System (CERES) Data

    Science.gov (United States)

    Kato, Seiji; Rose, Fred G.; Rutan, David A.; Charlock, Thomas P.

    2008-01-01

    The zonal mean atmospheric cloud radiative effect, defined as the difference of the top-of-atmosphere (TOA) and surface cloud radiative effects, is estimated from three years of Clouds and the Earth's Radiant Energy System (CERES) data. The zonal mean shortwave effect is small, though it tends to be positive (warming). This indicates that clouds increase shortwave absorption in the atmosphere, especially in midlatitudes. The zonal mean atmospheric cloud radiative effect is, however, dominated by the longwave effect. The zonal mean longwave effect is positive in the tropics and decreases with latitude to negative values (cooling) in polar regions. The meridional gradient of cloud effect between midlatitude and polar regions exists even when uncertainties in the cloud effect on the surface enthalpy flux and in the modeled irradiances are taken into account. This indicates that clouds increase the rate of generation of mean zonal available potential energy. Because the atmospheric cooling effect in polar regions is predominately caused by low level clouds, which tend to be stationary, we postulate that the meridional and vertical gradients of cloud effect increase the rate of meridional energy transport by dynamics in the atmosphere from midlatitude to polar region, especially in fall and winter. Clouds then warm the surface in polar regions except in the Arctic in summer. Clouds, therefore, contribute in increasing the rate of meridional energy transport from midlatitude to polar regions through the atmosphere.

  16. Atlantic City memories.

    Science.gov (United States)

    Epstein, Franklin H

    2008-04-01

    Fifty years ago, the Atlantic City meetings, held the first week in May of every year, were attended by all the elite of American academic medicine and all who wanted to join that group. Part of the magic of those meetings was that professors and neophytes took each other seriously and talked to each other. PMID:18382726

  17. Latitudinal distribution of Trichodesmium spp. and N2 fixation in the Atlantic Ocean

    Directory of Open Access Journals (Sweden)

    E. Marañón

    2010-10-01

    Full Text Available We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1, and community N2 fixation rates (mean = 60 μmol m−2 d−1 occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1 but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations results in conservative estimates for total N2 fixation of ∼6 TgN yr−1 in the North Atlantic (0–40° N and ~1.2 TgN yr−1 in the South Atlantic (0–40° S.

  18. Latitudinal distribution of Trichodesmium spp. and N2 fixation in the Atlantic Ocean

    Directory of Open Access Journals (Sweden)

    E. Marañón

    2010-03-01

    Full Text Available We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1 and community N2 fixation rates (mean = 60 μmol m−2 d−1 occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1 but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations results in conservative estimates for total N2 fixation of ~6 TgN yr−1 in the North Atlantic (0–40° N and 1.2 TgN yr−1 in the South Atlantic (0–40° S.

  19. Rotational atmospheric circulation during North Atlantic-European winter: the influence of ENSO

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Serrano, J. [UCM, Departamento de Geofisica y Meteorologia, Madrid (Spain); Institut Catala de Ciencies del Clima (IC3), Barcelona (Spain); Rodriguez-Fonseca, B.; Zurita-Gotor, P.; Camara, A. de la [UCM, Departamento de Geofisica y Meteorologia, Madrid (Spain); Blade, I. [UB, Departament d' Astronomia i Meteorologia, Barcelona (Spain)

    2011-11-15

    The dominant variability modes of the North Atlantic-European rotational flow are examined by applying a principal component analysis (PCA/EOF) to the 200 hPa streamfunction mid-winter anomalies (Jan-Feb monthly means). The results reveal that, when this norm is used, the leading mode (EOF1) does not correspond to the traditional North Atlantic Oscillation (NAO, which appears in our analysis as the second leading mode, EOF2) but is the local manifestation of the leading hemispheric streamfunction EOF. The regression of this regional mode onto the global SST field exhibits a clear El Nino signature, with no signal over the Atlantic, while the associated upper height anomalies resemble the Tropical/Northern Hemisphere (TNH) pattern. East of North America, this TNH-like wavetrain produces a meridional dipole-like pattern at lower levels. Although in some ways this pattern resembles the NAO (EOF2), the dynamics of these two modes are very different in that only EOF2 is associated with a latitudinal shift of the North Atlantic stormtrack. Thus, the choice of the streamfunction norm in the EOF analysis allows the separation of two different phenomena that can produce similar dipolar surface pressure anomalies over the North Atlantic but that have different impact on European climate. These two modes also differ on their contribution to variability at lower levels: while NAO-EOF2 is mostly confined to the North Atlantic, TNH-EOF1 has a more annular, global character. At upper levels NAO-EOF2 also produces a global pattern but with no annular structure, reminiscent of the ''circumglobal'' teleconnection. (orig.)

  20. A GCM Study of Responses of the Atmospheric Water Cycle of West Africa and the Atlantic to Saharan Dust Radiative Forcing

    Science.gov (United States)

    Lau, K. M.; Kim, K. M.; Sud, Y. C.; Walker, G. K.

    2009-01-01

    The responses of the atmospheric water cycle and climate of West Africa and the Atlantic to radiative forcing of Saharan dust are studied using the NASA finite volume general circulation model (fvGCM), coupled to a mixed layer ocean. We find evidence of an "elevated heat pump" (EHP) mechanism that underlines the responses of the atmospheric water cycle to dust forcing as follow. During the boreal summerr, as a result of large-scale atmospheric feedback triggered by absorbing dust aerosols, rainfall and cloudiness are ehanIed over the West Africa/Eastern Atlantic ITCZ, and suppressed over the West Atlantic and Caribbean region. Shortwave radiation absorption by dust warms the atmosphere and cools the surface, while longwave has the opposite response. The elevated dust layer warms the air over West Africa and the eastern Atlantic. As the warm air rises, it spawns a large-scale onshore flow carrying the moist air from the eastern Atlantic and the Gulf of Guinea. The onshore flow in turn enhances the deep convection over West Africa land, and the eastern Atlantic. The condensation heating associated with the ensuing deep convection drives and maintains an anomalous large-scale east-west overturning circulation with rising motion over West Africa/eastern Atlantic, and sinking motion over the Caribbean region. The response also includes a strengthening of the West African monsoon, manifested in a northward shift of the West Africa precipitation over land, increased low-level westerlies flow over West Africa at the southern edge of the dust layer, and a near surface westerly jet underneath the dust layer overr the Sahara. The dust radiative forcing also leads to significant changes in surface energy fluxes, resulting in cooling of the West African land and the eastern Atlantic, and warming in the West Atlantic and Caribbean. The EHP effect is most effective for moderate to highly absorbing dusts, and becomes minimized for reflecting dust with single scattering albedo at0

  1. Low-latitude control of interhemispheric sea-surface temperature contrast in the tropical Atlantic over the past 21 kyears: the possible role of SE trade winds

    Energy Technology Data Exchange (ETDEWEB)

    Kim, J.H.; Schneider, R.R. [Fachbereich Geowissenschaften, Universitaet Bremen, Klagenfurter Strasse, 28359 Bremen (Germany)

    2003-09-01

    To stimulate a discussion on the role of tropical atmospheric circulation versus thermohaline circulation changes for tropical Atlantic sea-surface temperature (SST) variations, we present a record of the SST contrast ({delta}SST) between the tropical northwest and southeast Atlantic from the Last Glacial Maximum to the Late Holocene. The {delta}SST was calculated from two alkenone-derived SST records; one from the Caribbean Sea and the other from the Angola Basin. Changes in the cross-equatorial {delta}SST were then compared with an abundance record of Florisphaera profunda from the equatorial Atlantic, which is indicative of SE trade-wind induced variations in thermocline depth in the equatorial divergence zone. This comparison implies that the Last Glacial Maximum, the Younger Dryas, and the Mid to Late Holocene were periods of strong SE trade winds, which led to an intense upwelling-related cooling in the southeast Atlantic and concurrently enhanced advection of warm tropical South Atlantic waters into the western tropical Atlantic. Accordingly, a coupled ocean-atmospheric process has probably created a dipole-like SST distribution pattern in the tropical Atlantic during these three distinct climatic periods. In contrast, Heinrich Event 1, the Boelling-Alleroed, and the Early Holocene were intervals of weakened SE trade winds, causing a warming in the southeast Atlantic. However, synchronous warming in both regions during Heinrich Event 1 can be partially attributed to a weakening of thermohaline overturning which caused a reduced northward heat transport from the low-latitude to the high-latitude North Atlantic. (orig.)

  2. 75 FR 44938 - Atlantic Coastal Fisheries Cooperative Management Act Provisions; Atlantic Coastal Shark Fishery

    Science.gov (United States)

    2010-07-30

    ... National Oceanic and Atmospheric Administration RIN 0648-XX28 Atlantic Coastal Fisheries Cooperative Management Act Provisions; Atlantic Coastal Shark Fishery AGENCY: National Marine Fisheries Service (NMFS... Atlantic Coastal Fisheries Cooperative Management Act (Atlantic Coastal Act), based on the...

  3. Establishment of mathematical model of overturned collar structure%连翻领结构设计数学模型的建立

    Institute of Scientific and Technical Information of China (English)

    邹平; 吴世刚

    2011-01-01

    To make the overturned collar overturn accurately and simplify its structure to realize free overturning and achieve an attractive static shape, through direct drawing method, 84 samples of overturned collar were made by using different fabrics, different neck widths, and different necklines, taking Japanese culture bodice prototype as garment body prototype. The samples are tried on the standard mannequin, respectively, and with optimium static shape as the standard, neck width is measured. The measured data are analyzed by factor analysis, correlation analysis, and regression analysis, and SPSS statistics software is used to analyze the rule of the structural change of the overturned collar. A mathematical model of overturned collar structure-neck width vs. fabric physical property and neck width vs. neckline is developed, realizing fast and accurate drawing of the structure of overturned collars.%为使连翻领翻折准确,结构简化,实现连翻领翻折自如,静态造型美观,通过直接作图方法,以日本文化式女装原型为衣身原型,采用不同的面料、不同的领宽和领凹,绘制连翻领.制作连翻领样品84件,制作样品在标准人体模台上试穿,以静态造型最佳为标准,测量翻领宽的数值,运用SPSS统计软件分析连翻领结构变化规律,对测量的数据进行因子分析、相关分析和回归分析,建立连翻领的翻领宽结构与面料物理性能、连翻领宽与领凹的数学模型,实现连翻领结构的快捷、准确、独立绘制.

  4. Application of Overturning Technology in Invert Block Prefabrication%空中翻转技术在仰拱预制块生产中的应用

    Institute of Scientific and Technical Information of China (English)

    刘清泉

    2011-01-01

    Vacuum suction type overturning machine is ing method of invert block overturning on the ground is not applicable to the overturning of invert blocks, and the exist- laggard and has many disadvantages. New hydraulic type overturning machine is used in the construction of West Qinling tunnel, which is the first time in China. A set of proved overturning technology is summarized on basis of practice and improvement in the site. Some key points of the new technology are presented. The comparison and contrast between the new method and the existing method show that the new method is superior to the existing one.%为解决长期以来真空吸吊翻转机不适用于仰拱预制块翻转、地面翻滚仰拱预制块技术落后且弊端明显的问题,以适应TBM快速高效的施工节奏,西秦岭隧道全国首次引入仰拱预制块液压起吊翻转机。通过现场实践和改进,总结出一套比较成熟的空中翻转技术,并介绍新技术引入后的一些注意事项。通过方法对比,人员配置及作业时间等均优于地面翻滚法4倍左右,并可有效消除掉块和开裂损伤,对于确保预制块生产进度和生产质量意义重大。

  5. Impacts of Indian and Atlantic oceans on ENSO in a comprehensive modeling framework

    Science.gov (United States)

    Terray, Pascal; Masson, Sébastien; Prodhomme, Chloé; Roxy, Mathew Koll; Sooraj, K. P.

    2016-04-01

    run. These results may be explained by modulations of the Walker circulation induced directly or indirectly by the Indian and Atlantic SSTs. Another interesting result is that decoupling the Atlantic or Indian oceans change the timing of ENSO onset and the relative role of other ENSO atmospheric precursors such as the extra-tropical Pacific Meridional Modes or the Western North Pacific SSTs.

  6. Vertical Structure Variability In The Tropical Atlantic From A High-resolution Ogcm Experiment In 1980-2000

    Science.gov (United States)

    Illig, S.; Dewitte, B.; Ayoub, N.; de Mey, P.; Du Penhoat, Y.

    We investigate the vertical structure variability in the Tropical Atlantic based on the CLIPPER project high-resolution Ocean General Circulation Model (OGCM) simula- tion for the 1980-2000 period. Near the equator the linear vertical modes are estimated at each grid point of the OGCM simulation. The contribution of baroclinic modes to surface zonal current and sea level anomalies is analyzed for interannual frequen- cies. The second baroclinic mode is the most energetic mode. The first and the third modes contribute with comparable amplitude but with different spatial distribution in the equatorial wave guide. The first mode exhibits a variability peak in the western part of the basin, where the maximum in zonal wind stress variability is observed, whereas the energy of the third baroclinic mode is confined in the east, where the largest zonal gradients in the density field and in the vertical mode characteristics are found. The summed-up contribution of the high-order baroclinic modes variability (4 to 10) is as energetic as the gravest modes and is the largest in the east. Kelvin and meridional Rossby components are derived for each of the gravest baroclinic mode contributions by projecting onto the associated meridional structures. The effects of boundaries close to the equator is taken into consideration, and shown to modify wave coefficients near the meridional boundaries as compared to the classical calculation assuming infinite meridional extension structures. We have found evidence of equato- rial Kelvin and Rossby wave propagation with phase speed values close to theoretical ones. The results are compared to equivalent ones of a multi-mode linear simulation using the projection coefficients and phase speed values derived from the OGCM sim- ulation. This, in particular, allows for separating locally wind-forced waves from free waves and deriving the boundary reflection efficiency. The analysis also reveals verti- cal propagation of the energy in the sub

  7. Energetics of IPCC4AR Climate Models: Energy Balance and Meridional Enthalpy Transports

    CERN Document Server

    Lucarini, Valerio

    2009-01-01

    We consider the climate simulations performed using pre-industrial and SRESA1B scenarios and analyse the outputs of the state-of-the-art models included in IPCC4AR. For control simulations, large energy biases are present for several models both when global climate budgets and when energy budgets of the atmospheric, oceanic, and land subdomains are considered. The energy biases depend on the imperfect closure of the energy cycle in the fluid components of the climate system and on issues in the treatment of phase transitions and heat fluxes over land. Additionally, the consequence of a positive global energy bias, which is what most models feature, is the underestimation of the thermodynamic emission temperature of the planet and of the globally averaged surface temperature. This may help explaining the cold bias of climate models. Models agree on the representation of meridional enthalpy transports in terms of location of the peaks of the total and atmospheric transports, whereas quantitative disagreements o...

  8. MAPEAMENTO DE UNIDADES DE RELEVO NA MÉDIA SERRA DO ESPINHAÇO MERIDIONAL - MG

    Directory of Open Access Journals (Sweden)

    Éric Andrade Rezende

    2011-04-01

    Full Text Available This paper proposes a mapping of landforms units for the southern portion of the middle third of the Serra do Espinhaço Meridional. The landforms units were delineated from the integrated analysis of thematic maps and satellite images that allowed the observation of changes in morphology, altimetry, slope, lithostructure and organization of the drainage network. The step of digital mapping, processed in the software ArcGIS 9.2, was based in the use of SRTM products (Shuttle Radar Topography Mission. The methodological procedures also included field work and literature research. The taxonomy of geomorphological mapping proposed by IBGE (2009 was applied , through which fourteen Geomorphologycal Units were individualized. The units are distributed amongst five different compartments as follows: two plateaux, two escarpments, six depressions, three sets of ridges and one set of intermediate levels. It was observed that the general layout of the units reflects the strong lithostructural control forced to regional relief. The differentiated resistance of the various lithologies in relation to denudational processes and the influence of neotectonic on landscape evolution are noteworthy.

  9. Sub-surface Meridional Flow Results from MWO, GONG, and MDI during Solar Cycle 23

    Science.gov (United States)

    Pinkerton, Stephen; Rhodes, Edward J.; Bogart, Richard S.

    2014-06-01

    Time series of full-disk Dopplergrams were acquired at the 60-Foot Solar tower of the Mount Wilson Observatory every year between 1987 and 2009. Analysis of this archive revealed that the focal plane of the Tower did experience a small amount of systematic rotation, which suggested that the alignment of the optics had changed slightly over the years since its construction in 1907. This has caused some of the initial daily flow maps to possess a so-called “washing machine” effect similar to the pattern that was seen in raw GONG flow maps. We have incorporated a systematic program of ring-diagram analysis in which we have tracked the raw solar images using five differing assumed instrumental rotation rates. We have then gone on to compute synoptic maps of the horizontal flow vectors at several different depths over much of Solar Cycle 23 in order to study how such an instrumental rotation might affect both the zonal and meridional flows as functions of latitude, depth, and time. We compare these results with GONG and MDI flow measurements to empirically determine the regime within which the MWO results are reliable and extend our analysis into Solar Cycle 22.

  10. Solar activity variations of nocturnal thermospheric meridional winds over Indian longitude sector

    Science.gov (United States)

    Madhav Haridas, M. K.; Manju, G.; Arunamani, T.

    2016-09-01

    The night time F-layer base height information from ionosondes located at two equatorial stations Trivandrum (TRV 8.5°N, 77°E) and Sriharikota (SHAR 13.7°N, 80.2°E) spanning over two decades are used to derive the climatology of equatorial nocturnal Thermospheric Meridional Winds (TMWs) prevailing during High Solar Activity (HSA) and Low Solar Activity (LSA) epochs. The important inferences from the analysis are 1) Increase in mean equatorward winds observed during LSA compared to HSA during pre midnight hours; 25 m/s for VE (Vernal Equinox) and 20 m/s for SS (Summer Solstice), AE (autumnal Equinox) and WS (Winter Solstice). 2) Mean wind response to Solar Flux Unit (SFU) is established quantitatively for all seasons for pre-midnight hours; rate of increase is 0.25 m/s/SFU for VE, 0.2 m/s/SFU for SS and WS and 0.08 m/s/SFU for AE. 3) Theoretical estimates of winds for the two epochs are performed and indicate the role of ion drag forcing as a major factor influencing TMWs. 4) Observed magnitude of winds and rate of flux dependencies are compared to thermospheric wind models. 5) Equinoctial asymmetry in TMWs is observed for HSA at certain times, with more equatorward winds during AE. These observations lend a potential to parameterize the wind components and effectively model the winds, catering to solar activity variations.

  11. The implication of radiative forcing and feedback for meridional energy transport

    Science.gov (United States)

    Huang, Yi; Zhang, Minghong

    2014-03-01

    The distributions of radiative forcing and feedback in the Coupled Model Intercomparison Project phase 5 abrupt4xCO2 and Historical experiments are diagnosed, with a focus on their effects on the zonal mean structure of the top-of-the-atmosphere radiation anomalies and implications for the meridional energy transport. It is found that because the greenhouse gas longwave forcing peaks in the low latitudes, it reinforces the equator-to-pole net radiation gradient and accounts for the increase in the poleward energy transport in both hemispheres under global warming. The shortwave forcing by aerosol, ozone, etc. peaks in the Northern Hemisphere and instead implies an interhemispheric energy transport. Although the water vapor feedback also reinforces the equator-to-pole gradient of the net radiation, the temperature and albedo feedback act against it. The feedback tend to offset the zonal mean radiation anomaly caused by the forcing, although the overall feedback effect on the energy transport is rather uncertain, mainly due to the uncertainty in the cloud feedback.

  12. A meridional dipole in pre-monsoon Bay of Bengal tropical cyclone activity induced by ENSO

    Energy Technology Data Exchange (ETDEWEB)

    Balaguru, Karthik; Leung, Ruby L.; Lu, Jian; Foltz, Gregory R.

    2016-06-30

    Analysis of Bay of Bengal tropical cyclone (TC) track data for the month of May during 1980-2013 reveals a meridional dipole in TC intensification: TC intensification rates increased in the northern Bay and decreased in the southern Bay. The dipole was driven by an increase in low-level vorticity and atmospheric humidity in the northern Bay, making the environment more favorable for TC intensification, and enhanced vertical wind shear in the southern Bay, tending to reduce TC development. These environmental changes were associated with a strengthening of the monsoon circulation for the month of May, driven by a La Nin˜a-like shift in tropical Pacific SSTs andassociated tropical wave dynamics. Analysis of a suite of climate models fromthe CMIP5 archive for the 150-year historical period shows that most models correctly reproduce the link between ENSO and Bay of Bengal TC activity through the monsoon at interannual timescales. Under the RCP 8.5 scenario the same CMIP5 models produce an El Nin˜o like warming trend in the equatorial Pacific, tending to weaken the monsoon circulation. These results suggest

  13. Meridional thermal field of a coupled ocean-atmosphere system: a conceptual model

    Science.gov (United States)

    Ou, Hsien-Wang

    2006-05-01

    This paper constitutes the author's continuing effort in the construction of a minimal theory of the earth's climate. In an earlier paper published in the Journal of Climate in 2001, this author has derived the global-mean fields of an aquatic planet forced by the solar insolation, which provide the necessary constraints for the present derivation of the meridional thermal field. The model closure invokes maximized entropy production (MEP), a thermodynamic principle widely used in turbulence and climate studies. Based on differing convective regimes of the ocean and atmosphere, both fluids are first reduced two thermal masses with aligned fronts, consistent with a minimal description of the observed field. Subjected to natural bounds, a robust solution is then found, characterized by an ice-free ocean, near-freezing cold fluid masses, mid-latitude fronts, and comparable ocean and atmosphere heat transports. The presence of polar continents, however, sharply reduces the ocean heat transport outside the tropics, but leaves the thermal field largely unchanged. Given the limitation of an extremely crude model, the deduced thermal field nonetheless seems sensible, suggesting that the model has captured the physics for a minimal account of the observed field. Together with the above-mentioned paper, the model reinforces the pre-eminent role of the triple point of water in stabilizing the surface temperature - against changing external condition. Such internal control is made possible by the turbulent nature of the climate fluids, which necessitates a selection rule based on extremization.

  14. ENSO and meridional modes: A null hypothesis for Pacific climate variability

    Science.gov (United States)

    Di Lorenzo, E.; Liguori, G.; Schneider, N.; Furtado, J. C.; Anderson, B. T.; Alexander, M. A.

    2015-11-01

    Pacific low-frequency variability (timescale > 8 years) exhibits a well-known El Niño-like pattern of basin-scale sea surface temperature, which is found in all the major modes of Pacific decadal climate. Using a set of climate model experiments and observations, we decompose the mechanisms contributing to the growth, peak, and decay of the Pacific low-frequency spatial variance. We find that the El Niño-like interdecadal pattern is established through the combined actions of Pacific meridional modes (MM) and the El Niño-Southern Oscillation (ENSO). Specifically, in the growth phase of the pattern, subtropical stochastic excitation of the MM energizes the tropical low-frequency variance acting as a red noise process. Once in the tropics, this low-frequency variance is amplified by ocean-atmospheric feedbacks as the pattern reaches its peak phase. At the same time, atmospheric teleconnections distribute the variance from the tropics to the extratropics, where the pattern ultimately decays. In this stochastic red noise model of Pacific climate, the timescale of the extra-tropical/tropical interactions (1-2 years) permits the stochastic excitation of the ENSO-like pattern of decadal and interdecadal variance.

  15. Millennial meridional dynamics of Indo-Pacific Warm Pool during the last termination

    Directory of Open Access Journals (Sweden)

    L. Lo

    2014-08-01

    Full Text Available To develop an in-depth understanding of the natural dynamics of the Indo-Pacific Warm Pool (IPWP during the last deglaciation, stacked North- (N- and South-IPWP (S-IPWP thermal and hydrological records over the past 23–10.5 thousand years (ka were built using planktonic foraminiferal geochemical data from a new core, MD05-2925 (9.3° S, 151.5° E, water depth 1661 m in the Solomon Sea and eleven previous sites. Ice-volume corrected seawater δ18O (δ18OSW-IVC stacks show that S-IPWP δ18OSW-IVC values are indistinguishable from their northern counterpart through glacial time. The N-IPWP SST stacked record features an increasing trend of 0.5 °C ka−1 since 18 ka. Its S-IPWP counterpart shows an earlier onset of temperature increase at 19 ka and a strong teleconnection to high-latitude climate in the Southern Hemisphere. Meridional SST gradients between N- and S-IPWP were 1.5 °C during the Bølling/Allerød period and < 0.5 °C during both Heinrich event 1 and the Younger Dryas due to a warmer S-IPWP. A warm S-IPWP during the cold events snapshots may possibly weaken the southern hemispheric convection branch of the Hadley Cell and reduce precipitation in the Asian Monsoon region.

  16. Two Distinct Roles of Atlantic SSTs in ENSO Variability: North Tropical Atlantic SST and Atlantic Nino

    Science.gov (United States)

    Ham, Yoo-Geun; Kug, Jong-Seong; Park, Jong-Yeon

    2013-01-01

    Two distinct roles of the Atlantic sea surface temperatures (SSTs), namely, the North Tropical Atlantic (NTA) SST and the Atlantic Nino, on the El Nino-Southern Oscillation (ENSO) variability are investigated using the observational data from 1980 to 2010 and coupled model experiments. It appears that the NTA SST and the Atlantic Nino can be used as two independent predictors for predicting the development of ENSO events in the following season. Furthermore, they are likely to be linked to different types of El Nino events. Specifically, the NTA SST cooling during February, March, and April contributes to the central Pacific warming at the subsequent winter season, while the negative Atlantic Nino event during June, July, and August contributes to enhancing the eastern Pacific warming. The coupled model experiments support these results. With the aid of a lagged inverse relationship, the statistical forecast using two Atlantic indices can successfully predict various ENSO indices.

  17. Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution parallel ocean model

    Science.gov (United States)

    Topper, R. P. M.; Meijer, P. Th.

    2014-07-01

    A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations on the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation). A series of experiments with different sill depths in the Atlantic-Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is less and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic-Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth ≤10 m, is a plausible scenario for the second stage of the Messinian Salinity Crisis during which

  18. Interhemispheric Changes in Atlantic Ocean Heat Content and Their Link to Global Monsoons

    Science.gov (United States)

    Lopez, H.; Lee, S. K.; Dong, S.; Goni, G. J.

    2015-12-01

    This study tested the hypothesis whether low frequency decadal variability of the South Atlantic meridional heat transport (SAMHT) influences decadal variability of the global monsoons. A multi-century run from a state-of-the-art coupled general circulation model is used as basis for the analysis. Our findings indicate that multi-decadal variability of the South Atlantic Ocean plays a key role in modulating atmospheric circulation via interhemispheric changes in Atlantic Ocean heat content. Weaker SAMHT produces anomalous ocean heat divergence over the South Atlantic resulting in negative ocean heat content anomaly about 15 years later. This, in turn, forces a thermally direct anomalous interhemispheric Hadley circulation in the atmosphere, transporting heat from the northern hemisphere (NH) to the southern hemisphere (SH) and moisture from the SH to the NH, thereby intensify (weaken) summer (winter) monsoon in the NH and winter (summer) monsoon in the SH. Results also show that anomalous atmospheric eddies, both transient and stationary, transport heat northward in both hemispheres producing eddy heat flux convergence (divergence) in the NH (SH) around 15-30°, reinforcing the anomalous Hadley circulation. The effect of eddies on the NH (SH) poleward of 30° is opposite with heat flux divergence (convergence), which must be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream and mean storm track. The mechanism described here could easily be interpreted for the case of strong SAMHT, with the reverse influence on the interhemispheric atmospheric circulation and monsoons. Overall, SAMHT decadal variability leads its atmospheric response by about 15 years, suggesting that the South Atlantic is a potential predictor of global climate variability.

  19. Impact of the North Atlantic Sea Surface Temperature Tripole on the East Asian Summer Monsoon

    Institute of Scientific and Technical Information of China (English)

    ZUO Jinqing; LI Weijing; SUN Chenghu; XU Li; REN Hong-Li

    2013-01-01

    A strong (weak) East Asian summer monsoon (EASM) is usually concurrent with the tripole pattern of North Atlantic SST anomalies on the interannual timescale during summer,which has positive (negative)SST anomalies in the northwestern North Atlantic and negative (positive) SST anomalies in the subpolar and tropical ocean.The mechanisms responsible for this linkage are diagnosed in the present study.It is shown that a barotropic wave-train pattern occurring over the Atlantic-Eurasia region likely acts as a link between the EASM and the SST tripole during summer.This wave-train pattern is concurrent with geopotential height anomalies over the Ural Mountains,which has a substantial effect on the EASM.Diagnosis based on observations and linear dynamical model results reveals that the mechanism for maintaining the wave-train pattern involves both the anomalous diabatic heating and synoptic eddy-vorticity forcing.Since the North Atlantic SST tripole is closely coupled with the North Atlantic Oscillation (NAO),the relationships between these two factors and the EASM are also examined.It is found that the connection of the EASM with the summer SST tripole is sensitive to the meridional location of the tripole,which is characterized by large seasonal variations due to the north-south movement of the activity centers of the NAO.The SST tripole that has a strong relationship with the EASM appears to be closely coupled with the NAO in the previous spring rather than in the simultaneous summer.

  20. Effect of sporadic destratification, seasonal overturn, and artificial mixing on CH4 emissions from a subtropical hydroelectric reservoir

    Science.gov (United States)

    Guérin, Frédéric; Deshmukh, Chandrashekhar; Labat, David; Pighini, Sylvie; Vongkhamsao, Axay; Guédant, Pierre; Rode, Wanidaporn; Godon, Arnaud; Chanudet, Vincent; Descloux, Stéphane; Serça, Dominique

    2016-06-01

    Inland waters in general and freshwater reservoirs specifically are recognized as a source of CH4 into the atmosphere. Although the diffusion at the air-water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured temperature and O2 and CH4 concentrations every 2 weeks for 3.5 years at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We compared diffusive emissions with ebullition from Deshmukh et al. (2014) and aerobic methane oxidation and downstream emissions from Deshmukh et al. (2016). In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependent on the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. Concentration and storage decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry (CD) season. The sharp decreases in CH4 storage were concomitant with extreme diffusive fluxes (up to 200 mmol m-2 d-1). These sporadic emissions occurred mostly in the inflow region in the WW season and during overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these extreme CH4 emissions (> 5 mmol m-2 d-1) contributed up to 50 % of total annual emissions by diffusion. During the transition between the WD and WW seasons, a new emission hotspot was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m-2 d-1 in 2010 down to 200 mmol m-2 d-1 in 2012. The hotspot was attributed to the mixing induced by the water intakes (artificial mixing). Emissions from this area contributed 15-25 % to total annual emissions

  1. A Babcock-Leighton solar dynamo model with multi-cellular meridional circulation in advection- and diffusion-dominated regimes

    CERN Document Server

    Belucz, Bernadett; Forgacs-Dajka, Emese

    2015-01-01

    Babcock-Leighton type solar dynamo models with single-celled meridional circulation are successful in reproducing many solar cycle features. Recent observations and theoretical models of meridional circulation do not indicate a single-celled flow pattern. We examine the role of complex multi-cellular circulation patterns in a Babcock-Leighton solar dynamo in advection- and diffusion-dominated regimes. We show from simulations that presence of a weak, second, high-latitude reverse cell speeds up the cycle and slightly enhances the poleward branch in butterfly diagram, whereas the presence of a second cell in depth reverses the tilt of butterfly wing to an anti-solar type. A butterfly diagram constructed from middle of convection zone yields a solar-like pattern, but this may be difficult to realize in the Sun because of magnetic buoyancy effects. Each of the above cases behaves similarly in higher and lower magnetic diffusivity regimes. However, our dynamo with a meridional circulation containing four cells in...

  2. Meridional circulation in the solar convection zone: time-distance helioseismic inferences from four years of HMI/SDO observations

    CERN Document Server

    Rajaguru, S P

    2015-01-01

    We present and discuss results from time-distance helioseismic measurements of meridional circulation in the solar convection zone using 4 years of Doppler velocity observations by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Using an in-built mass conservation constraint in terms of the stream function we invert helioseismic travel times to infer meridional circulation in the solar convection zone. We find that the return flow that closes the meridional circulation is possibly beneath the depth of $0.77 R_{\\odot}$. We discuss the significance of this result in relation to other helioseismic inferences published recently and possible reasons for the differences in the results. Our results show clearly the pitfalls involved in the measurements of material flows in the deep solar interior given the current limits on signal-to-noise and our limited understanding of systematics in the data. We also discuss the implications of our results for the dynamics of solar interi...

  3. Variability of the North Atlantic summer storm track: mechanisms and impacts on European climate

    International Nuclear Information System (INIS)

    The summertime variability of the extratropical storm track over the Atlantic sector and its links to European climate have been analysed for the period 1948–2011 using observations and reanalyses. The main results are as follows. (1) The dominant mode of the summer storm track density variability is characterized by a meridional shift of the storm track between two distinct paths and is related to a bimodal distribution in the climatology for this region. It is also closely related to the Summer North Atlantic Oscillation (SNAO). (2) A southward shift is associated with a downstream extension of the storm track and a decrease in blocking frequency over the UK and northwestern Europe. (3) The southward shift is associated with enhanced precipitation over the UK and northwestern Europe and decreased precipitation over southern Europe (contrary to the behaviour in winter). (4) There are strong ocean–atmosphere interactions related to the dominant mode of storm track variability. The atmosphere forces the ocean through anomalous surface fluxes and Ekman currents, but there is also some evidence consistent with an ocean influence on the atmosphere, and that coupled ocean–atmosphere feedbacks might play a role. The ocean influence on the atmosphere may be particularly important on decadal timescales, related to the Atlantic Multidecadal Oscillation (AMO). (letter)

  4. The long-term changes in solar meridional circulation as the cause for the long-term changes in the correlation between solar and geomagnetic activity

    CERN Document Server

    Georgieva, K

    2007-01-01

    Since the beginning of the 20th century, the correlation in the 11-year solar cycle between the sunspot number and geomagnetic aa-index has been decreasing, while the lag between the two has been increasing. We show how this can be used as a proxy for the solar meridional circulation, and investigate the long-term changes in the meridional circulation and their role for solar activity and terrestrial climate.

  5. Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution ocean model

    Directory of Open Access Journals (Sweden)

    R. P. M. Topper

    2015-02-01

    Full Text Available A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations of the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation. A series of experiments with different sill depths in the Atlantic–Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is shallower and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin, a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic–Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth ≤10 m, is a plausible scenario for the second stage of the Messinian

  6. Inter-comparison of stratospheric mean-meridional circulation and eddy mixing among six reanalysis datasets

    Directory of Open Access Journals (Sweden)

    K. Miyazaki

    2015-10-01

    Full Text Available The stratospheric mean-meridional circulation (MMC and eddy mixing are compared among six meteorological reanalysis datasets: NCEP-NCAR, NCEP-CFSR, ERA-40, ERA-Interim, JRA-25, and JRA-55 for the period 1979–2012. The reanalysis datasets produced using advanced systems (i.e., NCEP-CFSR, ERA-Interim, and JRA-55 generally reveal a weaker MMC and stronger eddy mixing in the Northern Hemisphere (NH compared with those produced using older systems (i.e., NCEP/NCAR, ERA-40, and JRA-25. In the NH lower stratosphere, the stronger eddy mixing is attributed to stronger planetary-scale mixing in the new datasets, whereas small-scale mixing is weaker in the new datasets. Conventional data assimilation techniques introduce analysis increments without maintaining physical balance, which may have caused an overly strong MMC and spurious small-scale eddies in the old datasets. At the NH mid-latitudes, only ERA-Interim reveals a weakening MMC trend in the deep branch of the Brewer–Dobson Circulation (BDC. The relative importance of the eddy mixing compared with the mean transport in the subtropical lower stratosphere is considered to be important in controlling mean Age-of-Air (AoA variations above, which showed increasing trends in ERA-Interim and JRA-55; this together with the weakened MMC in the deep branch may imply an increasing AoA trend in the NH middle stratosphere in ERA-Interim. Overall, discrepancies between the different variables and trends therein as derived from the different reanalyses are still relatively large, suggesting that more investments into these products are needed in order to obtain a consolidated picture of observed changes in the BDC and the mechanisms that drive them.

  7. Estudios sobre la vegetación del estado de Paraná (Brasil meridional

    Directory of Open Access Journals (Sweden)

    de Bolòs, Oriol

    1991-12-01

    Full Text Available Contribution to knowledge about the vegetation of the state of Paraná in southern Brazil (Serra do Mar, Planaltos, Iguaçu Valley. Numerous plant associations are described and grouped together in the following classes: Pistio-Eichhornietea (communities of floating cormophytes Xyrido-Typhetea (helophytic herbaceous vegetation Polypodio-Tillandsietea (epiphytic and comophytic vegetation Ruderali-Manihotetea (ruderal and segetal vegetation Andropogono-Baccharidetea (savanoid vegetation Rhizophoretea (mangroves Lantano-Chusqueetea (woody marginal communities of the forest Cedrelo-Ocoteetea (rain and mesophilous forest. Special attention is paid to the study of the physiognomy, structure and dynamism of the vegetation and its biogeographical significance.

    Aportación al conocimiento de la vegetación del estado de Paraná en el Brasil meridional (Serra do Mar, Planaltos, valle del Iguaçu. Se describen numerosas asociaciones vegetales agrupadas en las clases siguientes: Pistio-Eichhornietea (comunidades de cormófitos flotantes. Xyrido-Typhetea (vegetación herbácea helofítica. Polypodio-Tillandsietea (vegetación epifítica y comofítica. Ruderali-Manihotetea (vegetación ruderal, viaria y arvense. Andropogono-Baccharidetea (vegetación sabanoide. Rhizophoretea (manglar. Lantano-Chusqueetea (manto marginal leñoso de la selva. Cedrelo-Ocoteetea (selva pluvial y mesófila. Se dedica atención especial al estudio de la fisionomía, estructura y dinamismo de la vegetación y a su significación biogeográfica.

  8. El registro biológico humano de la costa meridional de Santa Cruz

    Directory of Open Access Journals (Sweden)

    Suby, Jorge A.

    2009-01-01

    Full Text Available La costa patagónica constituye un sector de riesgo para el registro arqueológico, asociado a factores naturales y antrópicos. Al mismo tiempo, la conservación de las colecciones bioarqueológicas, destacándose la pérdida de restos óseos e información asociada por escasez de recursos, desconocimiento o falta de atención especializada, representa escenarios de riesgo para el registro. Una de las áreas para las cuales no se dispone hasta el momento de información bioarqueológica es la región que comprende la costa meridional de la provincia de Santa Cruz. Considerando esta ausencia de información, el objetivo de este trabajo es presentar y discutir los primeros resultados sobre restos óseos humanos hallados en la región costera próxima a la desembocadura del Río Santa Cruz (Santa Cruz, Argentina. Los estudios incluyen el análisis de las situaciones de hallazgo y riesgo de los restos recuperados en acciones de rescate, el reconocimiento y puesta en valor de materiales depositados en el Museo Regional "Carlos Borgialli" (Puerto Santa Cruz, Santa Cruz, Argentina y estudios paleopatológicos. Al mismo tiempo se informan y analizan resultados cronológicos e isotópicos. Los resultados brindan evidencias claras de la ocupación de la región costera al menos durante los últimos 2000 años, consumo de recursos predominantemente terrestres y un estilo de vida que favorece el desarrollo de lesiones articulares, con escasos indicios de estrés sistémico que coinciden con los resultados reportados para la región continental del estrecho de Magallanes.

  9. Predicting Atlantic seasonal hurricane activity using outgoing longwave radiation over Africa

    Science.gov (United States)

    Karnauskas, Kristopher B.; Li, Laifang

    2016-07-01

    Seasonal hurricane activity is a function of the amount of initial disturbances (e.g., easterly waves) and the background environment in which they develop into tropical storms (i.e., the main development region). Focusing on the former, a set of indices based solely upon the meridional structure of satellite-derived outgoing longwave radiation (OLR) over the African continent are shown to be capable of predicting Atlantic seasonal hurricane activity with very high rates of success. Predictions of named storms based on the July OLR field and trained only on the time period prior to the year being predicted yield a success rate of 87%, compared to the success rate of NOAA's August outlooks of 53% over the same period and with the same average uncertainty range (±2). The resulting OLR indices are statistically robust, highly detectable, physically linked to the predictand, and may account for longer-term observed trends.

  10. Oceanic heat advection to the Arctic in the last Millennium

    OpenAIRE

    Spielhagen, Robert F.; Werner, Kirstin; Aagaard-Sørensen, Steffen; Zamelczyk, Katarzyna; Kandiano, Evguenia; Budeus, Gereon; Husum, Katrine; Marchitto, Thomas M.; Hald, Morten

    2011-01-01

    EGU2011-8738 At present, the Arctic is responding faster to global warming than most other areas on earth, as indicated by rising air temperatures, melting glaciers and ice sheets and a decline of the sea ice cover. As part of the meridional overturning circulation which connects all ocean basins and influences global climate, northward flowing Atlantic Water is the major means of heat and salt advection towards the Arctic where it strongly affects the sea ice distribution. Records of its ...

  11. Competition between global warming and an abrupt collapse of the AMOC in Earth’s energy imbalance

    OpenAIRE

    Sybren Drijfhout

    2015-01-01

    A collapse of the Atlantic Meridional Overturning Circulation (AMOC) leads to global cooling through fast feedbacks that selectively amplify the response in the Northern Hemisphere (NH). How such cooling competes with global warming has long been a topic for speculation, but was never addressed using a climate model. Here it is shown that global cooling due to a collapsing AMOC obliterates global warming for a period of 15–20 years. Thereafter, the global mean temperature trend is reversed an...

  12. Response of the AMOC to reduced solar radiation – the modulating role of atmospheric-chemistry

    OpenAIRE

    Muthers, Stefan; Raible, Christoph C.; Thomas F Stocker

    2016-01-01

    The influence of reduced solar forcing (grand solar minimum or geoengineering scenarios like solar radiation management) on the Atlantic meridional overturning circulation (AMOC) is assessed in an ensemble of atmosphere-ocean-chemistry-climate model simulations. Ensemble sensitivity simulations are performed with and without interactive chemistry. Without chemistry-climate interaction the AMOC is intensified in the course of the solar radiation reduction (SRR), which is ...

  13. Influence of assimilating transports and in situ data from the Rapid-MOCHA array into the GECCO2 ocean synthesis

    OpenAIRE

    Köhl, A.

    2015-01-01

    By assimilating information required for the estimation of the Atlantic meridional overturning circulation (AMOC) by the Rapid-MOCHA array, we investigate how transports should be constrained. For the period 2004–2011, we find that even the large adjustments in Florida Strait transport (FST) imposed by assimilating FST data do not impact the AMOC strength at 26.5° N while the AMOC away from this section changes due to the baroclinic response. Moreover, the high correlation betw...

  14. Air-sea interactions during glacial Heinrich events

    OpenAIRE

    Krebs, Uta

    2006-01-01

    'Heinrich events' - massive iceberg discharges from Northern Hemisphere ice sheets during the last ice age - coincided with cold periods and subsequent abrupt warmings on the Northern Hemisphere. Climate reconstructions suggest that the associated freshwater pulses caused a temporary collapse of the Atlantic Meridional Overturning Circulation (AMOC) by stabilizing stratification in the regions of deep water formation. In the present work a coupled atmosphere-ocean-sea ice model is employed u...

  15. Interannual variability of the January meridional heat transport by planetary waves in the northern latitudes

    International Nuclear Information System (INIS)

    Evidence of large temporal and spatial variability in the eddy fluxes of sensible heat in the lower troposphere (100-50 kPa thickness layer) in January, from 1946 to 1988, is presented. The spatial distribution of the standing eddy heat flux is dominated by three main features, or centers of action: (1) a region north of Korea (extreme eastern Siberia), (2) northeastern Atlantic Ocean, and (3) the Gulf of Alaska. Even though the center just north of Korea is the most active heat transport area, most of the interannual variability of the January standing eddy heat flux is associated with the heat transport centers over the northeastern Atlantic and the Gulf of Alaska, correlated with the positions of the Icelandic Low and the Aleutian Low, respectively. This year-to-year variability in these two geographical locations is due to interannual variability in the planetary waves, and a significant role of the air-sea interaction in this respect cannot be ruled out

  16. New nutrients exert fundamental control on dissolved organic carbon accumulation in the surface Atlantic Ocean

    Science.gov (United States)

    Romera-Castillo, Cristina; Letscher, Robert T.; Hansell, Dennis A.

    2016-09-01

    The inventories of carbon residing in organic matter dissolved in the ocean [dissolved organic carbon (DOC)] and in the atmosphere as CO2 are of the same order of magnitude, such that small changes in the DOC pool could have important consequences in atmospheric carbon and thus climate. DOC in the global ocean is largely formed in the sunlit euphotic zone, but identifying predictable controls on that production is an important yet unrealized goal. Here, we use a testable and causative correlation between the net production of DOC and the consumption of new nutrients in the euphotic zone of the Atlantic Ocean. We demonstrate that new nutrients introduced to the euphotic zone by upwelling in divergence zones and by winter convective overturn of the water column, and the primary production associated with those nutrients, are the ultimate driver of DOC distributions across the Atlantic basins. As new nutrient input will change with a changing climate, the role of DOC in the ocean’s biological pump should likewise be impacted.

  17. New nutrients exert fundamental control on dissolved organic carbon accumulation in the surface Atlantic Ocean.

    Science.gov (United States)

    Romera-Castillo, Cristina; Letscher, Robert T; Hansell, Dennis A

    2016-09-20

    The inventories of carbon residing in organic matter dissolved in the ocean [dissolved organic carbon (DOC)] and in the atmosphere as CO2 are of the same order of magnitude, such that small changes in the DOC pool could have important consequences in atmospheric carbon and thus climate. DOC in the global ocean is largely formed in the sunlit euphotic zone, but identifying predictable controls on that production is an important yet unrealized goal. Here, we use a testable and causative correlation between the net production of DOC and the consumption of new nutrients in the euphotic zone of the Atlantic Ocean. We demonstrate that new nutrients introduced to the euphotic zone by upwelling in divergence zones and by winter convective overturn of the water column, and the primary production associated with those nutrients, are the ultimate driver of DOC distributions across the Atlantic basins. As new nutrient input will change with a changing climate, the role of DOC in the ocean's biological pump should likewise be impacted.

  18. Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal

    Indian Academy of Sciences (India)

    D Shankar; S R Shetye; P V Joseph

    2007-10-01

    We use daily satellite estimates of sea surface temperature (SST)and rainfall during 1998 –2005 to show that onset of convection over the central Bay of Bengal (88-92°E, 14-18°N)during the core summer monsoon (mid-May to September)is linked to the meridional gradient of SST in the bay.The SST gradient was computed between two boxes in the northern (88-92°E, 18-22°N) and southern (82-88°E, 4-8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current.Convection over central bay followed the SST difference between the northern and southern bay ( ) exceeding 0.75°C in 28 cases.There was no instance of exceeding this threshold without a burst in convection.There were,however,five instances of convection occurring without this SST gradient.Long rainfall events (events lasting more than a week)were associated with an SST event ( ≥ 0.75°C);rainfall events tended to be short when not associated with an SST event.The SST gradient was important for the onset of convection, but not for its persistence:convection often persisted for several days even after the SST gradient weakened.The lag between exceeding 0.75°C and the onset of convection was 0-18 days,but the lag histogram peaked at one week.In 75% of the 28 cases,convection occurred within a week of exceeding the threshold of 0.75°C. The northern bay SST, contributed more to but it was a weaker criterion for convection than the SST gradient.A sensitivity analysis showed that the corresponding threshold for was 29°C. We hypothesise that the excess heating (∼1° C above the threshold for deep convection)required in the northern bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient.

  19. 75 FR 22103 - Atlantic Coastal Fisheries Cooperative Management Act Provisions; Atlantic Coastal Shark Fishery

    Science.gov (United States)

    2010-04-27

    ... National Oceanic and Atmospheric Administration RIN 0648-XV13 Atlantic Coastal Fisheries Cooperative Management Act Provisions; Atlantic Coastal Shark Fishery AGENCY: National Marine Fisheries Service (NMFS...-compliance; Declaration of a moratorium. SUMMARY: In accordance with the Atlantic Coastal...

  20. Coherency of late Holocene European speleothem δ18O records linked to North Atlantic Ocean circulation

    Science.gov (United States)

    Deininger, Michael; McDermott, Frank; Mudelsee, Manfred; Werner, Martin; Frank, Norbert; Mangini, Augusto

    2016-09-01

    Speleothem δ18O records provide valuable information about past continental environmental and climatic conditions, although their interpretation is often not straightforward. Here we evaluate a compilation of late Holocene speleothem δ18O records using a Monte Carlo based Principal Component Analysis (MC-PCA) method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each δ18O record. The MC-PCA approach permits not only the identification of temporally coherent changes in speleothem δ18O; it also facilitates their graphical depiction and evaluation of their spatial coherency. The MC-PCA method was applied to 11 Holocene speleothem δ18O records that span most of the European continent (apart from the circum-Mediterranean region). We observe a common (shared) mode of speleothem δ18O variability that suggests millennial-scale coherency and cyclicity during the last 4.5 ka. These changes are likely caused by variability in atmospheric circulation akin to that associated with the North Atlantic Oscillation, reflecting meridionally shifted westerlies. We argue that these common large-scale variations in European speleothem δ18O records are in phase with changes in the North Atlantic Ocean circulation indicated by the vigour of the Iceland Scotland Overflow Water (ISOW), the strength of the subpolar gyre (SPG) and an ocean stacked North Atlantic ice rafted debris (IRD) index. Based on a recent modelling study, we conclude that these changes in the North Atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies. However, the mechanisms that ultimately force the westerlies remain unclear.

  1. Atlantic Offshore Seabird Dataset Catalog

    Data.gov (United States)

    U.S. Geological Survey, Department of the Interior — Several bureaus within the Department of Interior compiled available information from seabird observation datasets from the Atlantic Outer Continental Shelf into a...

  2. Atlantic Flyway Sea Duck Survey

    Data.gov (United States)

    US Fish and Wildlife Service, Department of the Interior — The Atlantic Flyway Sea Duck Survey, conducted from 1991 to 2002 by the U.S. Fish and Wildlife Service, was established to record sea duck numbers using near shore...

  3. Virginia Atlantic Coast Recreational Use

    Data.gov (United States)

    Virginia Department of Environmental Quality — As a member of the Mid-Atlantic Regional Council on the Ocean (MARCO), Virginia, through its Coastal Zone Management (CZM) Program, collected information on how the...

  4. VA Atlantic Coast Recreational Use

    Data.gov (United States)

    Virginia Department of Environmental Quality — As a member of the Mid-Atlantic Regional Council on the Ocean (MARCO), Virginia, through its Coastal Zone Management (CZM) Program, collected information on how the...

  5. Atlantic Flyway Breeding Waterfowl Survey

    Data.gov (United States)

    US Fish and Wildlife Service, Department of the Interior — The Atlantic Flyway Technical Section initiated this breeding waterfowl survey in 11 northeast states ranging from New Hampshire to Virginia.

  6. Measurement and Interpretation of Travel-Time Shifts in the context of Time-Distance Helioseismic Detection of Meridional Flows in the Solar Convection Zone

    Science.gov (United States)

    Chakraborty, S.; Duvall, T. L., Jr.; Hanasoge, S.; Hartlep, T.; Larson, T. P.; Kholikov, S.

    2014-12-01

    The role of meridional flow in maintaining the solar dynamo and differential rotation in the solar convection zone is not well understood and is currently under scrutiny. The traditional flux-transport dynamo models have posited the well known single-cell meridional flow with poleward flow at the photosphere and equatorward flow near the base of the convection zone. However, recent investigations seem to be revealing a different picture of meridional flow which is double celled in the radial direction with poleward flow at the photosphere and equatorward flow at a much shallower level in the convection zone. In this work time-distance helioseismology is used to probe the solar convection zone to accurately determine the structure of meridional circulation. Helioseismology uses the photospherically visible aspect of (acoustic, surface-gravity) waves, that propagate and interfere throughout the Sun to form standing oscillation modes, as probes to make inferences about the structure and flows on the solar surface and interior. Time-distance helioseismology is based on measuring the travel-times of wave-packets moving between distinct points on the solar surface. Travel-time shifts obtained by calculating the difference in the travel-times of counter-propagating waves between the same points on the solar surface yield information about flows throughout the solar convection zone. In this work time-distance techniques are applied on artificial and solar Doppler velocity images to detect travel-time shifts due to meridional flow. Modifications are suggested to enhance the signal-to-noise ratio of travel-time shift measurements. The artificial data is constructed by embedding various meridional flow models in 3D acoustic simulators, which is then used to discuss the interpretation of travel-time shifts, so that in the future an inversion procedure may be designed to calculate meridional flow velocities with greater accuracy. The solar data is obtained from the Helioseismic

  7. Centennial-scale teleconnection between North Atlantic sea surface temperatures and the Indian summer monsoon during the Holocene

    Science.gov (United States)

    Zhang, Xiaojian; Jin, Liya; Jia, Wanna

    2016-05-01

    Proxy records have shown that abrupt changes in the Indian summer monsoon (ISM) are closely linked to cold events in the North Atlantic at centennial timescales during the Holocene. However, mechanisms for these co-occurring phenomena are not fully understood. This study uses simulation results from a coupled atmosphere-ocean-sea-ice general circulation model forced by astronomical variations to investigate how summer (June, July, August and September) North Atlantic sea surface temperatures (SSTs) may have influenced the ISM at centennial timescales during the Holocene (9.5-0 ka BP). Our analyses identified an intimate relationship between the North Atlantic tripole SST (NATS) mode and the ISM. The NATS mode can affect the ISM in several ways. First, air-sea interactions over the tropical Atlantic can induce negative tropospheric temperature (TT) anomalies over the Indian Ocean, resulting in a strengthened meridional TT gradient favorable to a prolonged monsoonal rainy season. Second, a positive NATS mode tends to induce closed zonal vertical circulation over the tropical Atlantic, North Africa and the tropical Indian Ocean, creating anomalous convergence over India, and hence an enhanced ISM. Third, westerly surface wind anomalies, related to the NATS mode and coursing over the Arabian Sea, can increase moisture delivery to the monsoon region, causing enhanced rainfall in India. This mechanism resembles a decadal-scale mechanism that operates in the present-day climate. We also compared the Atlantic multidecadal oscillation (AMO), an alternative North Atlantic SST mode, with the NATS mode to determine their relationships to the ISM. A Holocene transient simulation indicates that the AMO's trend has diverged from that of the ISM since 5.5 ka BP, due to inverse SST trends over the tropical and extratropical North Atlantic. This latter trend leads to a much weaker relationship between the AMO and the ISM, relative to that observed between the NATS mode and the ISM

  8. The joint influence of topography and atmosphere on the meridional transport fluctuations in the Southern Ocean and its link with the ENSO events

    CERN Document Server

    Stepanov, V N

    2007-01-01

    This article deals with the investigation of the influence of topography and coastlines on the dynamics of the depth averaged Antarctic Circumpolar Current (ACC), driven by wind and atmospheric pressure. This is achieved with the help of a global barotropic circulation model, under idealized and real atmospheric conditions. It is shown that the variability of meridional mass fluxes due to the atmospheric conditions over the ACC can induce short-period density anomalies in the Southern Ocean to the north in the vicinity of 47S, which can be transferred to low latitudes by the wave mechanism described by Ivchenko et al. 2004, that could have significance with respect to rapid extra-tropical oceanic links. The results of the barotropic modeling demonstrate that changes in wind strength over the ACC, together with the effect of bottom topography and coastlines, induce some meridional flow variability in the Southern Ocean, and this meridional flux variability in the Pacific Ocean is anticorrelated with the streng...

  9. Meridional motions of sunspots from 1947.9 to 1985.0. II - Latitude motions dependent on SPOT type and phase of the activity cycle

    Science.gov (United States)

    Lustig, G.; Hanslmeier, A.

    1987-01-01

    The dependence of the meridional motions of sunspots on sunspot-type and phase in the solar activity cycle for the time interval 1947.9-1985.0 is examined; this was done also with the sunspot data from the solar-observatory Kanzelhoehe. For the total time interval, investigations for each cycle were carried out only for the elder or long lasting G, H, and J sunspot groups and distinctions between similar sunspot types AB, C, D, EF, GHJ (Zuerich-classification). The meridional motions about the different activity maxima were also examined. In all investigations in the period from 1947.9 to 1985.0 a tendency to a southdrift can be observed on both hemispheres of the sun, but the mean meridional motions are between the error-bars not very significantly different from zero.

  10. Meridional Variation of the 1955-2003 Sea Level Anomalies in the Tropical Pacific Ocean Associated with El Ni(n)o Events

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    The Sea Level Anomaly-Torque (SLAT, relative to a reference location in the Pacific Ocean), which means the total torque of the gravity forces of sea waters with depths equal to the Sea Level Anomaly (SLA) in the tropical Pacific Ocean, is defined in this study. The time series of the SLAT from merged altimeter data (1993-2003) had a great meridional variation during the 1997-1998 El Ni(n)o event. By using historical upper layer temperature data (1955-2003) for the tropical Pacific Ocean, the temperature-based SLAT is also calculated and the meridional variation can be found in the historical El Ni(n)o events (1955-2003), which suggests that the meridional shifts of the sea level anomaly are also intrinsic oscillating modes of the El Ni(n)o cycles like the zonal shifts.

  11. Physical and remineralization processes govern the cobalt distribution in the deep western Atlantic ocean

    Directory of Open Access Journals (Sweden)

    G. Dulaquais

    2013-10-01

    Full Text Available The distributions of the bio-essential trace element dissolved Co (DCo and the apparent particulate Co (PCo are presented along the GEOTRACES-A02 deep section from 64° N to 50° S in the West Atlantic Ocean. PCo was determined as the difference between total cobalt (TCo, unfiltered samples and DCo. DCo concentrations ranged from 14.7 pM to 94.3 pM, and PCo concentrations from undetectable values to 18.8 pM. The lowest DCo concentrations were observed in the subtropical domains, and the highest in the low-oxygenated Atlantic Central Waters (ACW that appeared to be the major reservoir of DCo in the West Atlantic. In the Antarctic Bottom Waters, the enrichment in DCo with ageing of the water-mass can be related to suspension and redissolution of bottom sediments a well as diffusion of DCo from abyssal sediments. Mixing and dilution of deep water-masses, rather than scavenging of DCo onto settling particles, generated the meridional decrease of DCo along the southward large-scale circulation in the deep West Atlantic. Furthermore the apparent scavenged profile of DCo observed in the deep waters likely resulted from the persistence of relatively high concentrations in intermediate waters and low DCo concentrations in underlaying bottom waters. We suggested that the 2010 Icelandic volcanic eruption can be a source of DCo that could have been transported in the core of the North-East Atlantic Deep Waters. At intermediate depths, the high concentrations of DCo recorded in the ACW linearly correlated with the apparent utilization of oxygen (AOU, indicating that remineralization of DCo can be significant (representing up to 29% of the DCo present. Furthermore the preferential remineralization of phosphate (P compared to Co in these low-oxygenated waters suggested a decoupling between the deep cycles of P and Co. The vertical diffusion of DCo from the ACW appeared to be a significant source of DCo into the surface waters of the equatorial domain

  12. Propagation of Convective Complexes Observed by TRMM in the Eastern Tropical Atlantic

    Science.gov (United States)

    Druyan, Leonard M.; Fulazeka, Matthew

    2012-01-01

    Precipitation maxima during the West African summer monsoon propagate generally westward in tandem with African easterly waves. A heretofore unreported, repeating pattern of northward drift of precipitation maxima is detected on Tropical Rainfall Measurement Mission (TRMM satellite) time-latitude distributions of daily accumulations over the eastern tropical Atlantic. Corresponding 3-hourly TRMM accumulations show that the northward drifting envelopes of precipitation during August 2006 are often comprised of individual swaths propagating towards the southwest, presumably as mesoscale squall lines. The implied northward drift on the time-latitude distribution is a component of a resultant northwestward movement. The study examines the entire available record of TRMM precipitation observations, 1998-2010, to summarize TRMM maxima propagation over the eastern tropical Atlantic. Meridional displacements of precipitation maxima are most prevalent in June-September 2006, occurring less frequently during other summers. An investigation of geopotential and circulation fields, limited to two case studies, suggests mechanisms to explain some of the observed propagation of TRMM maxima. In one event, northward drift of the precipitation envelope is consistent with the corresponding displacement of the intertropical convergence zone trough, although the southwest propagation of individual mesoscale convection maxima does not correspond to any synoptic feature on reanalysis circulation or reanalysis downscaled by a regional model. One speculation is that southwestward propagation of precipitation maxima could be caused by regeneration of convection at outflow boundaries of mature thunderstorms.

  13. Oceanic source strength of carbon monoxide on the basis of basin-wide observations in the Atlantic.

    Science.gov (United States)

    Park, Keyhong; Rhee, Tae Siek

    2016-01-01

    We measured the carbon monoxide (CO) concentrations in the marine boundary layer and the surface waters of the Atlantic Ocean from 50°N to 50°S during the UK Atlantic Meridional Transect expedition (AMT-7) in October 1998, covering the open ocean and coastal regions. Throughout the cruise track, atmospheric CO concentrations continually decreased southwards in the northern hemisphere with sporadic low and high concentrations encountered. South of the intertropical convergence zone (ITCZ) atmospheric CO was enhanced by ∼10 ppb compared to north of the ITCZ due likely to biomass burning emissions prevailing in the tropical continents. The remainder of the southern hemisphere remains nearly invariable except for the vicinity of Rio de la Plata. The surface seawater was supersaturated everywhere along the track and its saturation anomaly oscillated up to 90, exhibiting a typical diurnal cycle. The maximal dissolved CO concentration in the diurnal cycle appeared 2-5 hours behind the local maximum of solar insolation in the open ocean and the time lag further increased in the coastal region. The global ocean flux of CO to the atmosphere was estimated to be 14 Tg(CO) a(-1) within the range of 4-24 Tg(CO) a(-1). This is within uncertainty almost identical to what was estimated on the basis of the basin-wide observations in the Pacific and the Atlantic, but more than ∼4 times lower than the values appeared in the Intergovernmental Panel on Climate Change (IPCC) reports.

  14. Anomalous mid-twentieth century atmospheric circulation change over the South Atlantic compared to the last 6000 years

    Science.gov (United States)

    Turney, Chris S. M.; Jones, Richard T.; Lister, David; Jones, Phil; Williams, Alan N.; Hogg, Alan; Thomas, Zoë A.; Compo, Gilbert P.; Yin, Xungang; Fogwill, Christopher J.; Palmer, Jonathan; Colwell, Steve; Allan, Rob; Visbeck, Martin

    2016-06-01

    Determining the timing and impact of anthropogenic climate change in data-sparse regions is a considerable challenge. Arguably, nowhere is this more difficult than the Antarctic Peninsula and the subantarctic South Atlantic where observational records are relatively short but where high rates of warming have been experienced since records began. Here we interrogate recently developed monthly-resolved observational datasets from the Falkland Islands and South Georgia, and extend the records back using climate-sensitive peat growth over the past 6000 years. Investigating the subantarctic climate data with ERA-Interim and Twentieth Century Reanalysis, we find that a stepped increase in precipitation across the 1940s is related to a change in synoptic atmospheric circulation: a westward migration of quasi-permanent positive pressure anomalies in the South Atlantic has brought the subantarctic islands under the increased influence of meridional airflow associated with the Amundsen Sea Low. Analysis of three comprehensively multi-dated (using 14C and 137Cs) peat sequences across the two islands demonstrates unprecedented growth rates since the mid-twentieth century relative to the last 6000 years. Comparison to observational and reconstructed sea surface temperatures suggests this change is linked to a warming tropical Pacific Ocean. Our results imply ‘modern’ South Atlantic atmospheric circulation has not been under this configuration for millennia.

  15. Oceanic source strength of carbon monoxide on the basis of basin-wide observations in the Atlantic.

    Science.gov (United States)

    Park, Keyhong; Rhee, Tae Siek

    2016-01-01

    We measured the carbon monoxide (CO) concentrations in the marine boundary layer and the surface waters of the Atlantic Ocean from 50°N to 50°S during the UK Atlantic Meridional Transect expedition (AMT-7) in October 1998, covering the open ocean and coastal regions. Throughout the cruise track, atmospheric CO concentrations continually decreased southwards in the northern hemisphere with sporadic low and high concentrations encountered. South of the intertropical convergence zone (ITCZ) atmospheric CO was enhanced by ∼10 ppb compared to north of the ITCZ due likely to biomass burning emissions prevailing in the tropical continents. The remainder of the southern hemisphere remains nearly invariable except for the vicinity of Rio de la Plata. The surface seawater was supersaturated everywhere along the track and its saturation anomaly oscillated up to 90, exhibiting a typical diurnal cycle. The maximal dissolved CO concentration in the diurnal cycle appeared 2-5 hours behind the local maximum of solar insolation in the open ocean and the time lag further increased in the coastal region. The global ocean flux of CO to the atmosphere was estimated to be 14 Tg(CO) a(-1) within the range of 4-24 Tg(CO) a(-1). This is within uncertainty almost identical to what was estimated on the basis of the basin-wide observations in the Pacific and the Atlantic, but more than ∼4 times lower than the values appeared in the Intergovernmental Panel on Climate Change (IPCC) reports. PMID:26648555

  16. 78 FR 59878 - Atlantic Highly Migratory Species; Commercial Atlantic Aggregated Large Coastal Shark (LCS...

    Science.gov (United States)

    2013-09-30

    ... management groups is closed, even across fishing years. On July 3, 2013 (78 FR 40318), NMFS announced the... Species; Commercial Atlantic Aggregated Large Coastal Shark (LCS), Atlantic Hammerhead Shark, Atlantic Blacknose Shark, and Atlantic Non-Blacknose Small Coastal Shark (SCS) Management Groups AGENCY:...

  17. Atlantic menhaden processing plant test tagging data

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Atlantic menhaden are a schooling forage fish species, which are subject to a large commercial purse seine fishery. Atlantic menhaden are harvested for reduction...

  18. Atlantic NAD 83 OCS Planning Areas

    Data.gov (United States)

    Bureau of Ocean Energy Management, Department of the Interior — This data set contains BOEM Planning Area outlines in ESRI shapefile format for the BOEM Atlantic Region. The old Atlantic Planning Area outlines were changed as of...

  19. The North Atlantic Oscillation in the Atlantic-European SLP*

    OpenAIRE

    GLOWIENKA-HENSE, RITA

    2011-01-01

    An analysis of the signature of the North Atlantic Oscillation (NAO) in the Atlantic-European sea level pressure (SLP) is presented for observed (German Weather Service) and ECMWF T21 model data. The former time series consists of 1881–1984 January to December fields and the latter of 42 monthly fields from 3 permanent January simulations. The NAO is shown to be one of the dominant eigenmodes of SLP for all calendar months. A very similar NAO anomaly pattern is filtered from the T21 model dat...

  20. Atlantic Seaduck Project

    Science.gov (United States)

    Perry, M.C.; Hanson, Alan; Kerekes, Joseph; Paquet, Julie

    2006-01-01

    Atlantic Seaduck Project is being conducted to learn more about the breeding and moulting areas of seaducks in northern Canada and more about their feeding ecology on wintering areas, especially Chesapeake Bay. Satellite telemetry is being used to track surf scoters wintering in Chesapeake Bay, Maryland and black scoters on migrational staging areas in New Brunswick, Canada to breeding and moulting areas in northern Canada. Various techniques used to capture the scoters included mist netting, night-lighting, and net capture guns. All captured ducks were transported to a veterinary hospital where surgery was conducted following general anaesthesia procedures. A PTT100 transmitter (39 g) manufactured by Microwave, Inc., Columbia, Maryland was implanted into the duck?s abdominal cavity with an external (percutaneous) antenna. Eight of the surf scoters from Chesapeake Bay successfully migrated to possible breeding areas in Canada and all 13 of the black scoters migrated to suspected breeding areas. Ten of the 11 black scoter males migrated to James Bay presumably for moulting. Updated information from the ARGOS Systems aboard the NOAA satellites on scoter movements was made accessible on the Patuxent Website. Habitat cover types of locations using GIS (Geographical Information Systems) and aerial photographs (in conjunction with remote sensing software) are currently being analyzed to build thematic maps with varying cosmetic layer applications. Many factors related to human population increases have been implicated in causing changes in the distribution and abundance of wintering seaducks. Analyses of the gullet (oesophagus and proventriculus) and the gizzard of seaducks are currently being conducted to determine if changes from historical data have occurred. Scoters in the Bay feed predominantly on the hooked mussel and several species of clams. The long-tailed duck appears to select the gem clam in greater amounts than other seaducks, but exhibits a diverse diet of

  1. Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution parallel ocean model

    Directory of Open Access Journals (Sweden)

    R. P. M. Topper

    2014-07-01

    Full Text Available A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations on the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation. A series of experiments with different sill depths in the Atlantic–Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is less and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth. The model results are used to interpret the sedimentary record of the Late Miocene preceding and during the Messinian Salinity Crisis. In the western basin a correlation exists between sill depth and rate of refreshment of deep water. On the other hand, because sill depth has little effect on the overturning and deep water formation in the eastern basin, the model results do not support the notion that restriction of the Atlantic–Mediterranean connection may cause lower oxygenation of deep water in the eastern basin. However, this discrepancy may be due to simplifications in the surface forcing and the use of a bathymetry different from that in the Late Miocene. We also tentatively conclude that blocked outflow, as found in experiments with a sill depth ≤10 m, is a plausible scenario for the second stage of the Messinian Salinity

  2. Atlantic CFC data in CARINA

    Directory of Open Access Journals (Sweden)

    R. Steinfeldt

    2010-01-01

    Full Text Available Water column data of carbon and carbon-relevant parameters have been collected and merged into a new database called CARINA (CARbon IN the Atlantic. In order to provide a consistent data set, all data have been examined for systematic biases and adjusted if necessary (secondary quality control (QC. The CARINA data set is divided into three regions: the Arctic/Nordic Seas, the Atlantic region and the Southern Ocean. Here we present the CFC data for the Atlantic region, including the chlorofluorocarbons CFC-11, CFC-12 and CFC-113 as well as carbon tetrachloride (CCl4. The methods applied for the secondary quality control, a crossover analyses, the investigation of CFC ratios in the ocean and the CFC surface saturation are presented. Based on the results, the CFC data of some cruises are adjusted by a certain factor or given a "poor'' quality flag.

  3. Gradual onset and recovery of the Younger Dryas abrupt climate event in the tropics.

    Science.gov (United States)

    Partin, J W; Quinn, T M; Shen, C-C; Okumura, Y; Cardenas, M B; Siringan, F P; Banner, J L; Lin, K; Hu, H-M; Taylor, F W

    2015-01-01

    Proxy records of temperature from the Atlantic clearly show that the Younger Dryas was an abrupt climate change event during the last deglaciation, but records of hydroclimate are underutilized in defining the event. Here we combine a new hydroclimate record from Palawan, Philippines, in the tropical Pacific, with previously published records to highlight a difference between hydroclimate and temperature responses to the Younger Dryas. Although the onset and termination are synchronous across the records, tropical hydroclimate changes are more gradual (>100 years) than the abrupt (10-100 years) temperature changes in the northern Atlantic Ocean. The abrupt recovery of Greenland temperatures likely reflects changes in regional sea ice extent. Proxy data and transient climate model simulations support the hypothesis that freshwater forced a reduction in the Atlantic meridional overturning circulation, thereby causing the Younger Dryas. However, changes in ocean overturning may not produce the same effects globally as in Greenland.

  4. Atlantic versus Indo-Pacific influence on Atlantic-European climate

    OpenAIRE

    Pohlmann, Holger; Latif, Mojib

    2005-01-01

    The influence of the Atlantic and Indo-Pacific oceans on Atlantic-European climate is investigated by analyzing ensemble integrations with the atmospheric general circulation model ECHAM4 forced by anomalous sea surface temperature and sea ice conditions restricted to the Atlantic (AOGA) and Indo-Pacific (I+POGA) oceans. The forcing from both the Indo-Pacific and Atlantic oceans are important for the generation of the sea level pressure (SLP) variability in the Atlantic region in the boreal w...

  5. Modeling spatial and temporal variations in temperature and salinity during stratification and overturn in Dexter Pit Lake, Tuscarora, Nevada, USA

    International Nuclear Information System (INIS)

    , convective overturn, shear, and eddy diffusion). Inputs to the model include the size and shape of the lake, daily meteorological data (short wave radiation, long wave radiation or cloud cover, air temperature, vapor pressure, wind speed, and rainfall), rates for water inputs and outputs, the composition of inflowing water, and initial profiles of temperature and salinity. Predicted temperature profiles, which are influenced by seasonal changes in the magnitude of solar radiation, are in good agreement with observations and show the development of a strong thermocline in the summer, erosion of the thermocline during early fall, and turnover in late fall. Predicted salinity profiles are in reasonable agreement with observations and are affected by the hydrologic balance, particularly inflow of surface and groundwater and, to a lesser degree, evaporation. Defining the hydrodynamics model for Dexter pit lake is the first step in using a coupled physical - biogeochemical model (Dynamic Reservoir Simulation Model-Computational Aquatic Ecosystem Dynamics Model or DYRESM-CAEDYM) to predict the behavior of non-conservative elements (e.g., dissolved O2, Mn, and Fe) and their effect on water quality in this system

  6. Mean zonal and meridional accelerations and mean heating induced by solar tides for equinox and solstice conditions

    International Nuclear Information System (INIS)

    Evaluations are presented of the momentum and energy flux divergences of the diurnal and semidiurnal tidal fields calculated by Forbes from 0 to 400 km altitude. Results are presented in the form of meridional cross-sections from 0 to 780N or S latitude with a 60 latitude interval. Comparisons are made with evaluations of the momentum flux divergences of the diurnal tide by Miyahara and good agreement is obtained in the lower thermosphere (below about 130 km) but a large disparity arises in the upper thermosphere. In the lower thermosphere momentum flux divergences of the semidiurnal tide are comparable with those of the diurnal tide and should be included in general circulation calculations of the 90-120 km region. (author)

  7. Spectral characteristics and meridional variations of energy transformations during the first and second special observation periods of FGGE

    Science.gov (United States)

    Kung, E. C.; Tanaka, H.

    1984-01-01

    The global features and meridional spectral energy transformation variations of the first and second special observation periods of the First Global GARP Experiment (FGGE) are investigated, together with the latitudinal distribution of the kinetic energy balance. Specific seasonal characteristics are shown by the spectral distributions of the global transformations between (1) zonal mean and eddy components of the available potential energy, (2) the zonal mean and eddy components of the kinetic energy, and (3) the available potential energy and the kinetic energy. Maximum kinetic energy production is found to occur at subtropical latitudes, with a secondary maximum at higher middle latitudes. Between these two regions, there is another region characterized by the adiabatic destruction of kinetic energy above the lower troposphere.

  8. Inter-comparison of stratospheric mean-meridional circulation and eddy mixing among six reanalysis data sets

    Science.gov (United States)

    Miyazaki, Kazuyuki; Iwasaki, Toshiki; Kawatani, Yoshio; Kobayashi, Chiaki; Sugawara, Satoshi; Hegglin, Michaela I.

    2016-05-01

    The stratospheric mean-meridional circulation (MMC) and eddy mixing are compared among six meteorological reanalysis data sets: NCEP-NCAR, NCEP-CFSR, ERA-40, ERA-Interim, JRA-25, and JRA-55 for the period 1979-2012. The reanalysis data sets produced using advanced systems (i.e., NCEP-CFSR, ERA-Interim, and JRA-55) generally reveal a weaker MMC in the Northern Hemisphere (NH) compared with those produced using older systems (i.e., NCEP/NCAR, ERA-40, and JRA-25). The mean mixing strength differs largely among the data products. In the NH lower stratosphere, the contribution of planetary-scale mixing is larger in the new data sets than in the old data sets, whereas that of small-scale mixing is weaker in the new data sets. Conventional data assimilation techniques introduce analysis increments without maintaining physical balance, which may have caused an overly strong MMC and spurious small-scale eddies in the old data sets. At the NH mid-latitudes, only ERA-Interim reveals a weakening MMC trend in the deep branch of the Brewer-Dobson circulation (BDC). The relative importance of the eddy mixing compared with the mean-meridional transport in the subtropical lower stratosphere shows increasing trends in ERA-Interim and JRA-55; this together with the weakened MMC in the deep branch may imply an increasing age-of-air (AoA) in the NH middle stratosphere in ERA-Interim. Overall, discrepancies between the different variables and trends therein as derived from the different reanalyses are still relatively large, suggesting that more investments in these products are needed in order to obtain a consolidated picture of observed changes in the BDC and the mechanisms that drive them.

  9. Data Assimilation in a Solar Dynamo Model Using Ensemble Kalman Filters: Sensitivity and Robustness in Reconstruction of Meridional Flow Speed

    Science.gov (United States)

    Dikpati, Mausumi; Anderson, Jeffrey L.; Mitra, Dhrubaditya

    2016-09-01

    We implement an Ensemble Kalman Filter procedure using the Data Assimilation Research Testbed for assimilating “synthetic” meridional flow-speed data in a Babcock-Leighton-type flux-transport solar dynamo model. By performing several “observing system simulation experiments,” we reconstruct time variation in meridional flow speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that flow speed is reconstructed best if observations of near-surface poloidal fields from low latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for ≤slant 40 % error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low-latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals \\lt 5 days do not produce faithful reconstructions of flow speed, because the system requires a minimum time to develop dynamics to respond to flow variations. Reconstruction also deteriorates if an assimilation interval \\gt 45 days is used, because the system’s inherent memory interferes with its short-term dynamics during a substantially long run without updating.

  10. La heterogeneidad interna del español meridional o atlántico: variación diasistemática vs. pluricentrismo

    Directory of Open Access Journals (Sweden)

    Araceli López Serena

    2013-07-01

    Full Text Available ResumenEste trabajo se propone mostrar la idoneidad del marco teórico de la Lingüística de las variedades desarrollado por la Escuela de Friburgo para abordar el problema de la heterogeneidad interna del llamado español meridional o atlántico. Dicha heterogeneidad concierne a las diferencias de estatus que unos mismos rasgos lingüísticos manifiestan en las variedades andaluzas y americanas de la lengua; de ahí que su abordaje precise un marco de análisis que permita describir el funcionamiento discursivo de los distintos fenómenos de variación que afloran en el español hablado en Andalucía y en América desde una perspectiva dinámica, respetuosa con la extraordinaria complejidad de la interacción que, en el discurso, tiene lugar entre hechos de variación de distinta naturaleza (diatópica, diastrática o diafásica. Gracias a la modelización del comportamiento lingüístico diferenciado de los hablantes en situaciones de comunicación diversas que permite llevar a cabo el aparato conceptual de la Lingüística de las variedades, podremos dirimir cómo, por encima de la posible identidad material de algunos hechos lingüísticos compartidos por andaluz, canario y español de América, es necesario atribuir a fenómenos, en principio equivalentes, un estatus, sin embargo, muy desigual: diasistemáticamente marcado en unas comunidades, pero no marcado o estándar en otras, en las que el rasgo lingüístico en cuestión tenga, por tanto, el rango de norma diferenciada en el seno de una lengua policéntrica como es la española. AbstractThis paper claims that the inner heterogeneity of the so-called Southern or Atlantic Spanish can be best analyzed under the theoretical framework commonly known as German Varieties Linguistics developed by the Fribourg School. This heterogeneity is shown in the diverse status of materially identical linguistic facts in the Andalusian and American varieties of the Spanish language. The paper argues

  11. North and South Atlantic Bidecadal SL variability: Rossby Waves, AMOC fingerprints and Regime Shifts

    Science.gov (United States)

    Vianna, M. L.; Menezes, V. V.

    2014-12-01

    The relationship between the North and South Atlantic bidecadal sea level (SL) oscillations in the twentieth century (1908-2008) is investigated using SODA 2.2.4 reanalysis and 102 monthly mean SL time series from TG stations provided by PSMSL. Bidecadal SL signal extraction was done using our method of subjectively choosing groupings of space-time data into non-overlapping period bands by use of Singular Spectrum Analysis (SSA)/ Maximum Entropy Method (MEM) analysis. A CEOF analysis of the SODA bidecadal band shows dominance of 22-24 year periods. Propagating bidecadal mode expansions were then determined through this analysis, which gives two principal modes. The first mode is dominant from 1915 to 1965 and the second from 1970 onward. The amplitude variabilities obtained suggests the presence of regime shifts, which coincide in time with shifts actually observed in European climate and South African lobster fisheries. The first mode is characterized by states with North and South Atlantic subtropical gyres in phase, while tropical and subpolar regions are in opposite phase relative to them. The second mode is characterized by the subpolar gyre and North subtropical gyre almost in quadrature, with North and South subtropical gyres out of phase. The evolution mechanism of this latter mode is related to east-west density contrasts and westward propagating Rossby waves. These waves show phase speeds dominated by geostrophic self-advection of density anomalies relative to the mean meridional density gradient as known from previous studies, but are shown to be additionally influenced by bottom topography. The oscillations caused by these waves are shown to have phase differences (quadrature in the North Atlantic) with regional indices of bidecadal AMOC oscillations. A number of SL-AMOC fingerprints are also reviewed, and a new eastern equatorial fingerprint is proposed.

  12. The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model

    Energy Technology Data Exchange (ETDEWEB)

    Schiller, A. [Max-Planck-Institut fuer Meteorologie, Hamburg (Germany); Mikolajewicz, U. [Max-Planck-Institut fuer Meteorologie, Hamburg (Germany); Voss, R. [Deutsches Klimarechenzentrum (DKRZ), Hamburg (Germany)

    1996-02-01

    The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and slowly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the Atlantic intertropical convergence zone is displaced southward and the westerlies in the northern hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface waters in high northern latitudes, which allows them to accumulate more precipitation and runoff from the continents, which results in an increased stability in the North Atlantic.

  13. Analysis of overturning resistant performance of base-isolated structures subjected to near-fault earthquake%近断层地震作用下基础隔震结构抗倾覆性能的分析

    Institute of Scientific and Technical Information of China (English)

    杜永峰; 徐超; 李慧

    2012-01-01

    The overturning resistant performance of base-isolated structures subjected to near-fault earth quake was analyzed. The formula of the ratio of anti-over turning moment to overturning moment was established first and then five records of near-fault pulse-like ground motion and far-field earthquake were chosen, respectively, to set up the analysis model of framed anti-vibration structure. The influence of the earthquake intensity and the pulse effect of near-fault ground motion action on the overturning resistant performance of base-isolated structure was analyzed. The numerical computation result showed that base-isolated structure would exhibit worse overturning resistant performance with larger seismic peak acceleration. The ratio of anti-overturning moment to overturning moment decreased by more than 40% under rare earthquake of scale 7 when compared to that of scale 9. The pulse effect of near-fault ground motion action on overturning resistant performance of base-isolated structure was worse compared with far-field earthquake. Much more disciplinary action must be taken against the overturning resistant property of base isolated buildings during the design in near-fault area.%分析基础隔震结构在近断层脉冲型地震作用下的抗倾覆性能.首先建立隔震结构抗倾覆比的计算公式;然后选取近断层脉冲型地震动和远场非脉冲型地震动记录各5条,建立框架隔震结构分析模型,分析地震烈度及近断层地震动脉冲效应对基础隔震结构抗倾覆性能的影响.数值计算结果表明,地震动加速度峰值越大,基础隔震结构的抗倾覆性能越差,9度罕遇地震时的隔震结构抗倾覆比值较7度罕遇地震下减小40%以上;与远场非脉冲型地震相比,近断层脉冲型地震动对基础隔震结构抗倾覆性能的影响更为不利;近断层地区隔震结构设计时对抗倾覆性能应该采取更为严格的标准.

  14. Surface mapping, organic matter and water stocks in peatlands of the Serra do Espinhaço meridional - Brazil

    Directory of Open Access Journals (Sweden)

    Márcio Luiz da Silva

    2013-10-01

    Full Text Available Peatlands are soil environments that store carbon and large amounts of water, due to their composition (90 % water, low hydraulic conductivity and a sponge-like behavior. It is estimated that peat bogs cover approximately 4.2 % of the Earth's surface and stock 28.4 % of the soil carbon of the planet. Approximately 612 000 ha of peatlands have been mapped in Brazil, but the peat bogs in the Serra do Espinhaço Meridional (SdEM were not included. The objective of this study was to map the peat bogs of the northern part of the SdEM and estimate the organic matter pools and water volume they stock. The peat bogs were pre-identified and mapped by GIS and remote sensing techniques, using ArcGIS 9.3, ENVI 4.5 and GPS Track Maker Pro software and the maps validated in the field. Six peat bogs were mapped in detail (1:20,000 and 1:5,000 by transects spaced 100 m and each transect were determined every 20 m, the UTM (Universal Transverse Mercator coordinates, depth and samples collected for characterization and determination of organic matter, according to the Brazilian System of Soil Classification. In the northern part of SdEM, 14,287.55 ha of peatlands were mapped, distributed over 1,180,109 ha, representing 1.2 % of the total area. These peatlands have an average volume of 170,021,845.00 m³ and stock 6,120,167 t (428.36 t ha-1 of organic matter and 142,138,262 m³ (9,948 m³ ha-1 of water. In the peat bogs of the Serra do Espinhaço Meridional, advanced stages of decomposing (sapric organic matter predominate, followed by the intermediate stage (hemic. The vertical growth rate of the peatlands ranged between 0.04 and 0.43 mm year-1, while the carbon accumulation rate varied between 6.59 and 37.66 g m-2 year-1. The peat bogs of the SdEM contain the headwaters of important water bodies in the basins of the Jequitinhonha and San Francisco Rivers and store large amounts of organic carbon and water, which is the reason why the protection and preservation

  15. Hydrography and through-flow in the north-eastern North Atlantic Ocean: the NANSEN project

    Science.gov (United States)

    van Aken, H. M.; Becker, G.

    The circulation and hydrography of the north-eastern North Atlantic has been studied with an emphasis on the upper layers and the deep water types which take part in the thermohaline overturning of the Oceanic Conveyor Belt. Over 900 hydrographic stations were used for this study, mainly from the 1987-1991 period. The hydrographic properties of Subpolar Mode Water in the upper layer, which is transported towards the Norwegian Sea, showed large regional variation. The deep water mass was dominated by the cold inflow of deep water from the Norwegian Sea and by a cyclonic recirculation of Lower Deep Water with a high Antarctic Bottom Water content. At intermediate levels the dominating water type was Labrador Sea Water with only minor influence of Mediterranean Sea Water. In the permanent pycnocline traces of Antarctic Intermediate Water were found. Geostrophic transports have been estimated, and these agreed in order of magnitude with the local heat budget, with current measurements, with data from surface drifters, and with the observed water mass modification. A total of 23 Sv of surface water entered the region, of which 20 Sv originated from the North Atlantic Current, while 3 Sv entered via an eastern boundary current. Of this total, 13 Sv of surface water left the area across the Reykjanes Ridge, and 7 Sv entered the Norwegian Sea, while 3 Sv was entrained by the cold overflow across the Iceland-Scotland Ridge. Approximately 1.4 Sv of Norwegian Sea Deep Water was involved in the overflow into the Iceland Basin, which, with about 1.1 Sv of entrained water and 1.1 Sv recirculating Lower Deep Water, formed a deep northern boundary current in the Iceland Basin. At intermediate depths, where Labrador Sea Water formed the dominant water type, about 2 Sv of entrained surface water contributed to a saline water mass which was transported westwards along the south Icelandic slope.

  16. Cluster analysis of explicitly and downscaled simulated North Atlantic tropical cyclone tracks

    Science.gov (United States)

    Daloz, A.; Camargo, S. J.; Kossin, J. P.; Emanuel, K.

    2013-12-01

    The response of tropical cyclone (TC) activity to climate change is a question of major interest. In order to address this crucial issue, several types of models have been developed in the past, such as Global Climate Models (GCMs). However, the horizontal resolution of those models usually leads to some difficulties in resolving the inner core of TCs and then to properly simulate TC activity. In order to avoid this problem, an alternative tool has been developed by Emanuel (2005). This downscaling technique uses tracks that are initiated by randomly seeding large areas of the tropics with weak vortices. Then the survival of the tracks is based on large-scale environmental conditions produced by GCMs in our case. Here we compare the statistics of TC tracks simulated explicitly in four GCMs to the results of the downscaling technique driven by the four same GCMs in the present and future climates over the North Atlantic basin. Simulated tracks are objectively separated into four groups using a cluster technique (Kossin et al. 2010). The four clusters form zonal and meridional separations of tracks as shown in Figure 1. The meridional separation largely captures the separation between hybrid or baroclinic storms (clusters 1 and 2) and deep tropical systems (clusters 3 and 4), while the zonal separation segregates Gulf of Mexico and Cape Verde storms. Except for the seasonality, the downscaled simulations better capture the general characteristics of the clusters (mean duration of the tracks, intensity...) compared with the explicit simulations, which present strong biases. In the second part of this study, we use three different scenarios to examine the possible future changes of the clusters from the downscaled simulations. We explored the role of a warming of the SST, an increase in carbon dioxide and a combination of both ones. The results show that the response to each scenario is highly varying depending on the simulation examined. References - Kossin, J. P., S

  17. CARINA oxygen data in the Atlantic Ocean

    OpenAIRE

    Stendardo, I.; Gruber, N.; A. Körtzinger

    2009-01-01

    In the CARINA (Carbon dioxide in the Atlantic Ocean) project, a new dataset with many previously unpublished hydrographic data from the Atlantic, Arctic and Southern Ocean was assembled and subjected to careful quality control (QC) procedures. Here, we present the dissolved oxygen measurements in the Atlantic region of the dataset and describe in detail the secondary QC procedures that aim to ensure that the data are internally consistent. This is achieved by a cross-over analysis, i.e. the c...

  18. 77 FR 69596 - Atlantic Highly Migratory Species; Advisory Panel for Atlantic Highly Migratory Species Southeast...

    Science.gov (United States)

    2012-11-20

    ... but not limited to data and models, used in stock assessments for oceanic sharks in the Atlantic Ocean... assessments for oceanic sharks in the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. While the SEDAR Pool was created specifically for Atlantic oceanic sharks, it may be expanded to include other HMS,...

  19. 76 FR 65700 - Atlantic Highly Migratory Species; Advisory Panel for Atlantic Highly Migratory Species Southeast...

    Science.gov (United States)

    2011-10-24

    ..., used in stock assessments for oceanic sharks in the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea... sharks in the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. While the SEDAR Pool was created specifically for Atlantic oceanic sharks, it may be expanded to include other HMS, as needed. The...

  20. 75 FR 57698 - Atlantic Highly Migratory Species; Atlantic Billfish Management, White Marlin (Kajikia albidus

    Science.gov (United States)

    2010-09-22

    ... western Atlantic Ocean, white marlin and blue marlin from the North Atlantic Ocean, and longbill spearfish... published an interim rule (63 FR 14030) that increased the minimum size limits for Atlantic blue marlin and... Consolidated HMS FMP (71 FR 58058), which, among other things, included the annual recreational 250 blue...

  1. Influence of the Atlantic thermohaline circulation on neodymium isotopic composition at the Last Glacial Maximum – a modelling sensitivity study

    Directory of Open Access Journals (Sweden)

    O. Marti

    2008-03-01

    Full Text Available The oceanic neodymium isotopic composition (hereafter expressed as ε Nd is modeled for the Last Glacial Maximum (LGM using the coarse resolution Ocean Global Circulation Model NEMO–ORCA2°. This study focuses on the impact of changes in the overturning cell and circulation patterns between LGM and Holocene on ε Nd in the Atlantic basin. Three different LGM freshwater forcing experiments are performed to test the variability in ε Nd oceanic distribution as a function of ocean circulation. Highly distinct representations of ocean circulation are generated in the three simulations, which drive significant differences in ε Nd, particularly in deep waters of the western part of the basin. However, mean Atlantic LGM ε Nd values are remain half a unit more radiogenic than for the modern control run. A fourth experiment shows that changes in Nd sources and bathymetry drive a shift in the ε Nd signature of Northern end-members (NADW or GNAIW glacial equivalent that is sufficient to explain the shift in mean ε Nd during our LGM simulations. None of our three LGM circulation scenarios gives a better agreement with the existing ε Nd paleo-data, as the model fails in reproducing the dynamical features of the area. Therefore, this study cannot indicate the likelihood of a given LGM oceanic circulation scenario. Rather, our modeling results highlight the need for data from western Atlantic deep waters, where the ε Nd gradient in the three LGM scenarios is the most important (up to 3 ε Nd. This would also aid more precise conclusions concerning the north end-member ε Nd signature evolution, and thus the potential use of ε Nd as a tracer of past oceanic circulation.

  2. 2D photochemical modeling of Saturn's stratosphere. Part I: Seasonal variation of atmospheric composition without meridional transport

    CERN Document Server

    Hue, Vincent; Dobrijevic, Michel; Hersant, Franck; Greathouse, Thomas K

    2015-01-01

    Saturn's axial tilt of 26.7{\\deg} produces seasons in a similar way as on Earth. Both the stratospheric temperature and composition are affected by this latitudinally varying insolation along Saturn's orbital path. A new time dependent 2D photochemical model is presented to study the seasonal evolution of Saturn's stratospheric composition. This study focuses on the impact of the seasonally variable thermal field on the main stratospheric C2 hydrocarbon chemistry (C2H2 and C2H6) using a realistic radiative climate model. Meridional mixing and advective processes are implemented in the model but turned off in the present study for the sake of simplicity. The results are compared to a simple study case where a latitudinally and temporally steady thermal field is assumed. Our simulations suggest that, when the seasonally variable thermal field is accounted for, the downward diffusion of the seasonally produced hydrocarbons is faster due to the seasonal compression of the atmospheric column during winter. This ef...

  3. Análisis de restos bioarqueológicos de la costa meridional de Santa Cruz

    Directory of Open Access Journals (Sweden)

    Suby, Jorge A.

    2007-01-01

    Full Text Available En este trabajo se presentan análisis bioarqueológicos y paleopatológicos de los restos óseos humanos hallados en el marco del proyecto “Variabilidad del registro arqueológico y bioantropológico de la costa atlántica meridional patagónica” (PIP 5576. Fueron estudiados dos esqueletos humanos hallado en la región costera de amortiguación del Parque Nacional Monte León. Los materiales corresponden a un adulto y un niño, sobre los que se analizaron aspectos paleopatológicos a través de estudios macroscópicos y radiológicos. A su vez se estudiaron variables tafonómicas y de conservación. Los restos óseos mostraron, en el caso del individuo adulto, importantes marcadores de stress ocupacional en su columna vertebral, así como signos de traumatismos costales. En los restos pertenecientes al niño no pudieron identificarse alteraciones patológicas significativas. Se discuten los resultados hallados en relación a los antecedentes correspondientes al registro bioarqueológico de la región. Al mismo tiempo, en el marco del proyecto mencionado, se analizan las condiciones de conservación de los restos óseos tratados.

  4. NAO-ocean circulation interactions in a coupled general circulation model

    Energy Technology Data Exchange (ETDEWEB)

    Bellucci, A. [Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna (Italy); Gualdi, S.; Navarra, A. [Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna (Italy); Istituto Nazionale di Geofisica e Vulcanologia, Bologna (Italy); Scoccimarro, E. [Istituto Nazionale di Geofisica e Vulcanologia, Bologna (Italy)

    2008-12-15

    The interplay between the North Atlantic Oscillation (NAO) and the large scale ocean circulation is inspected in a twentieth century simulation conducted with a state-of-the-art coupled general circulation model. Significant lead-lag covariance between oceanic and tropospheric variables suggests that the system supports a damped oscillatory mode involving an active ocean-atmosphere coupling, with a typical NAO-like space structure and a 5 years timescale, qualitatively consistent with a mid-latitude delayed oscillator paradigm. The two essential processes governing the oscillation are (1) a negative feedback between ocean gyre circulation and the high latitude SST meridional gradient and (2) a positive feedback between SST and the NAO. The atmospheric NAO pattern appears to have a weaker projection on the ocean meridional overturning, compared to the gyre circulation, which leads to a secondary role for the thermohaline circulation in driving the meridional heat transport, and thus the oscillatory mode. (orig.)

  5. 22 CFR 120.31 - North Atlantic Treaty Organization.

    Science.gov (United States)

    2010-04-01

    ... DEFINITIONS § 120.31 North Atlantic Treaty Organization. North Atlantic Treaty Organization (NATO) is..., France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, The Netherlands,...

  6. Systematic Center-To-Limb Variation in Measured Helioseismic Travel Times and Its Effect on Inferences of Solar Interior Meridional Flows

    Science.gov (United States)

    Zhao, Junwei; Nagashima, Kaori; Bogart, R. S.; Kosovichev, Alexander; Duvall, T. L., Jr.

    2012-01-01

    We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m s-1 slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior and needs to be better understood.

  7. SYSTEMATIC CENTER-TO-LIMB VARIATION IN MEASURED HELIOSEISMIC TRAVEL TIMES AND ITS EFFECT ON INFERENCES OF SOLAR INTERIOR MERIDIONAL FLOWS

    International Nuclear Information System (INIS)

    We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m s–1 slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior and needs to be better understood.

  8. On the Penetration of Meridional Circulation below the Solar Convection Zone II: Models with Convection Zone, the Taylor-Proudman constraint and Applications to Other Stars

    CERN Document Server

    Garaud, P

    2009-01-01

    The solar convection zone exhibits a strong level of differential rotation, whereby the rotation period of the polar regions is about 25-30% longer than the equatorial regions. The Coriolis force associated with these zonal flows perpetually "pumps" the convection zone fluid, and maintains a quasi-steady circulation, poleward near the surface. What is the influence of this meridional circulation on the underlying radiative zone, and in particular, does it provide a significant source of mixing between the two regions? In Paper I, we began to study this question by assuming a fixed meridional flow pattern in the convection zone and calculating its penetration depth into the radiative zone. We found that the amount of mixing caused depends very sensitively on the assumed flow structure near the radiative--convective interface. We continue this study here by including a simple model for the convection zone "pump", and calculating in a self-consistent manner the meridional flows generated in the whole Sun. We fin...

  9. Internal valves in populations of Meridion circulare from the A'er Mountain region of northeastern China: Implications for taxonomy and systematics

    Institute of Scientific and Technical Information of China (English)

    J.P. KOCIOLEK; Yan LIU; Quanxi WANG

    2011-01-01

    A population of Meridion circulare var.circulare (Greville) C.A.Agardh from Inner Mongolia was found to produce Innenschalen or internal spores.Examination of this population with light and scanning electron microscopy showed morphological differentiation between vegetative and spore morphologies.Vegetative valves typically bear costae and one rimoportula at the headpole.Spores lack costae and have two rimoportulae,one at the headpole and the other at the footpole.There is plasticity in the production of valve morphologies,and a variety of vegetative valve and spore combinations are evident.This population of M.circulare var.circulare has initial valves of over 90μm in length,and all of the initial cells encountered are acostate and bear two rimoportulae.These observations suggest that either spores are the product of sexual reproduction,or that initial valves may be produced parthenogenetically in Meridion.Spores as products of the sexual process have not been reported in diatoms previously,and parthenogenesis in Meridion was reported previously but discounted in other published reports.The plasticity of valve morphologies expressed in M.circulare var.ciculare,between vegetative valves and spores (and back) across a short temporal period suggests that diatoms can alter their cell wall structure dramatically and quickly in response to external variables.

  10. The North Atlantic circulation in the early 1980s - An estimate from inversion of a finite-difference model

    Science.gov (United States)

    Martel, Francoise; Wunsch, Carl

    1993-01-01

    A finite-difference model of the North Atlantic is constructed for the purpose of making an estimate of the circulation through an inverse calculation. The data base is eclectic, and includes hydrography, oxygen, nutrients, current meter and float records, atmospheric momentum, heat and water vapor transfers, as well as estimates of certain integral fluxes. Owing to the available hydrographic database, the model resolution is restricted to 1 deg at best, and is much coarser in many aspects. This limited resolution is a major obstacle to accurate estimates of climatological fluxes. In its final form, there are about 9000 constraints in 29,000 formal unknowns plus 9000 noise unknowns. The system is solved as a tapered least-squares system by a sparse conjugate gradient algorithm. With the exception of a few float velocities, all constraints are found to be consistent within error estimates. The model produces estimates of large-scale fluxes and flux divergences for all conventional properties including heat and nutrients as well as carbon dioxide and alkalinity. Meridional fluxes of carbon are found to be indistinguishable from zero, whereas the North Atlantic tends to export nutrients to the south, but carry heat to the north. Traditional oceanographic depictions of the circulation through combination of nonsynoptic data into steady models may have reached their useful limit in the present calculation, as the conflicts between the data and physical requirements become quantitatively apparent.

  11. Tropical Cyclone Activity in the North Atlantic Basin During the Weather Satellite Era, 1960-2014

    Science.gov (United States)

    Wilson, Robert M.

    2016-01-01

    ) oceanic Nino index (); (2) Atlantic multi-decadal oscillation () index; (3) Atlantic meridional mode () index; (4) global land-ocean temperature index (); and (5) quasi-biennial oscillation () index. Lastly, the associational aspects (using both linear and nonparametric statistical tests) between selected tropical cyclone parameters and the climatic factors are examined based on their 10-year moving average trend values.

  12. Effects of Drake Passage on a strongly eddying global ocean

    CERN Document Server

    Viebahn, Jan P; Bars, Dewi Le; Dijkstra, Henk A

    2015-01-01

    The climate impact of ocean gateway openings during the Eocene-Oligocene transition is still under debate. Previous model studies employed grid resolutions at which the impact of mesoscale eddies has to be parameterized. We present results of a state-of-the-art eddy-resolving global ocean model with a closed Drake Passage, and compare with results of the same model at non-eddying resolution. An analysis of the pathways of heat by decomposing the meridional heat transport into eddy, horizontal, and overturning circulation components indicates that the model behavior on the large scale is qualitatively similar at both resolutions. Closing Drake Passage induces (i) sea surface warming around Antarctica due to changes in the horizontal circulation of the Southern Ocean, (ii) the collapse of the overturning circulation related to North Atlantic Deep Water formation leading to surface cooling in the North Atlantic, (iii) significant equatorward eddy heat transport near Antarctica. However, quantitative details sign...

  13. Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

    Science.gov (United States)

    Schütte, Florian; Brandt, Peter; Karstensen, Johannes

    2016-05-01

    Coherent mesoscale features (referred to here as eddies) in the tropical northeastern Atlantic Ocean (between 12-22° N and 15-26° W) are examined and characterized. The eddies' surface signatures are investigated using 19 years of satellite-derived sea level anomaly (SLA) data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo-Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST) and sea surface salinity (SSS) anomalies are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are identified by an elevation/depression of SLA and enhanced/reduced SST and SSS in their cores. However, about 20 % of all anticyclonically rotating eddies show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs). About 146 ± 4 eddies per year with a minimum lifetime of 7 days are identified (52 % cyclones, 39 % anticyclones, 9 % ACMEs) with rather similar mean radii of about 56 ± 12 km. Based on concurrent in situ temperature and salinity profiles (from Argo float, shipboard, and mooring data) taken inside of eddies, distinct mean vertical structures of the three eddy types are determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland regions and carry South Atlantic Central Water supplied by the northward flow within the Mauretanian coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d-1) is confined to distinct zonal corridors with a small meridional deflection dependent on the eddy type (anticyclones - equatorward, cyclones - poleward, ACMEs - no deflection). Heat and salt fluxes out of the coastal region and across the Cape Verde Frontal Zone, which separates the shadow zone from

  14. Air-sea CO2 fluxes in the Atlantic as measured during the FICARAM cruises

    Directory of Open Access Journals (Sweden)

    A. F. Ríos

    2009-06-01

    Full Text Available A total of fourteen hydrographic cruises spanning from 2000 to 2008 were conducted during the spring and autumn seasons between Spain and the Southern Ocean, under the framework of the Spanish research project FICARAM. The performed underway measurements are processed and analysed to describe the meridional air-sea CO2 fluxes (FCO2 along the Atlantic Ocean. The data was organised into different biogeochemical oceanographic provinces, according mainly to the thermohaline characteristics. The obtained spatial and temporal distributions of FCO2 follow the generally expected patterns and annual trends. The Subtropical regions in both hemispheres alternated the CO2 source and sink nature from autumn to spring, respectively. On the other hand, Tropical waters and the Patagonian Sea clearly behaved as sinks of atmospheric CO2 like the waters of the Drake Passage during autumn. The obtained results during the cruises also revealed significant long-term trends, such as the warming of equatorial waters (0.11±0.03°C yr−1 and the decrease of surface salinity (−0.16±0.01 yr−1 in tropical waters caused by the influence of the Amazon River plume. This reduction in surface salinity appears to have a direct influence over the CO2 storage rates, fostering the uptake capacity of atmospheric CO2 (−0.09±0.03 mol m−2 yr−1. An analysis of the biogeochemical forcing on the CO2 fugacity (fCO2 variability performed from an empirical algorithm highlighted the major role of the Amazon River input in the tropical North Atlantic fluxes. In addition, it has provided a quantitative measure of the importance of the thermodynamic control of FCO2 at temperate latitudes.

  15. The Two Branches of the Recirculation of Atlantic Water in Fram Strait

    Science.gov (United States)

    von Appen, Wilken-Jon; Schauer, Ursula; Hattermann, Tore; Albretsen, Jon

    2016-04-01

    The Fram Strait between Greenland and Svalbard is one of the two gateways by which warm Atlantic Water enters the Arctic Ocean providing oceanic heat. The West Spitsbergen Current advects the warm water northward in the eastern Fram Strait. However, only some of this water stays in the boundary current and enters the Arctic Ocean. Another part leaves the boundary current and flows westward across Fram Strait before turning southward in the East Greenland Current. This recirculation of Atlantic Water corresponds with the ice edge in Fram Strait and the two likely depend on each other. Here we present results from a high resolution regional numerical model that shows the recirculation to consist of two branches. The northern branch depends on eddy fluxes while the southern branch exhibits less high frequency variability. We also present a compilation of different observational data in the center of Fram Strait around 0°EW that give insight into the structure of the southern recirculation branch near the ice edge. A glider section resolves the small horizontal scale over which the geostrophic flow occurs. Several meridional CTD sections capture the differences and similarities between different summers. Moorings and Argo floats provide information in winter as well. These observations are compared to the representation of the recirculation in the numerical model. We show that the southern recirculation occurs over a small horizontal distance of about 20km in the vicinity of 79°N and is significantly stronger in winter than in summer. While there is cold freshwater at the surface north of the front, the temperature down to 500m is much higher in the recirculation than further south.

  16. Occurrence and characteristics of mesoscale eddies in the tropical northeast Atlantic Ocean

    Directory of Open Access Journals (Sweden)

    F. Schütte

    2015-12-01

    Full Text Available Coherent mesoscale features (referred to here as eddies in the tropical northeast Atlantic (between 12–22° N and 15–26° W are examined and characterised. The eddies' surface signatures are investigated using 19 years of satellite derived sea level anomaly (SLA data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo–Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST and salinity (SSS are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are associated with elevation/depression of SLA and enhanced/reduced SST and SSS patterns. However, about 20 % of all detected anticyclones show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs. About 146 ± 4 eddies per year are identified (52 % cyclones, 39 % anticylones, 9 % ACMEs with rather similar mean radii of about 56 ± 12 km. Based on concurrent in-situ temperature and salinity profile data (from Argo float, shipboard and mooring data inside of the three eddy types, their distinct differences in vertical structure is determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland region and carry South Atlantic Central Water that originates from the northward transport within the Mauretania coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d−1 is confined to distinct corridors with a small meridional deflection dependent on the eddy type (anticyclones – equatorward, cyclones – poleward, ACMEs – no deflection. Heat and salt flux out of the coastal region and across the Cap Verde Frontal Zone, which separates the shadow zone from the ventilated gyre, are calculated.

  17. Occurrence and characteristics of mesoscale eddies in the tropical northeast Atlantic Ocean

    Science.gov (United States)

    Schütte, F.; Brandt, P.; Karstensen, J.

    2015-12-01

    Coherent mesoscale features (referred to here as eddies) in the tropical northeast Atlantic (between 12-22° N and 15-26° W) are examined and characterised. The eddies' surface signatures are investigated using 19 years of satellite derived sea level anomaly (SLA) data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo-Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST) and salinity (SSS) are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are associated with elevation/depression of SLA and enhanced/reduced SST and SSS patterns. However, about 20 % of all detected anticyclones show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs). About 146 ± 4 eddies per year are identified (52 % cyclones, 39 % anticylones, 9 % ACMEs) with rather similar mean radii of about 56 ± 12 km. Based on concurrent in-situ temperature and salinity profile data (from Argo float, shipboard and mooring data) inside of the three eddy types, their distinct differences in vertical structure is determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland region and carry South Atlantic Central Water that originates from the northward transport within the Mauretania coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d-1) is confined to distinct corridors with a small meridional deflection dependent on the eddy type (anticyclones - equatorward, cyclones - poleward, ACMEs - no deflection). Heat and salt flux out of the coastal region and across the Cap Verde Frontal Zone, which separates the shadow zone from the ventilated gyre, are calculated.

  18. Spatiotemporal variations in phytoplankton biomass and community structure in a meridional transect of the East/Japan Sea

    Science.gov (United States)

    Kwak, J. H.; Kang, C. K.; Kang, H.

    2015-12-01

    To better understand the variations in phytoplankton biomass and community composition associated with water-column structure and environmental conditions in the East/Japan Sea (EJS), three cruses were carried out along a meridional transect in May 2007 (spring), July 2009 (summer), and October 2012 (fall). The subpolar front (SPF) was formed between warm and cold water mass (37-40 °N). Chlorophyll a concentration and phytoplankton community composition were studied using HPLC pigment and CHEMTAX analysis and the results showed no significant differences between warm and cold water masses. These results reject our initial hypothesis that different water masses between southern and northern parts of the EJS may lead to different phytoplankton community structure. During the study periods, isotherm layers (≤ 12 °C) fluctuated over 50 m depth between warm and cold water masses on the basis of the SPF. In contrast, the nitracline (i.e. 2.5 μM nitrate isopleths) depth was recorded within the limited depths ranged 20-40 m, 30-50 m, and 40-60 m in spring, summer, and fall, respectively. The chlorophyll a concentrations at the subsurface chlorophyll maxima (SCM) in spring and summer (356 ± 233 and 270 ± 182 ng L-1, respectively) were significantly higher than those in fall (117 ± 89 ng L-1). The relative contributions of phytoplankton groups to total chlorophyll a concentration reflected phytoplankton community composition in the SCM layer with showing a dominance of diatoms (58 ± 19, 48 ± 11, and 30 ± 20 % in spring, summer, and fall, respectively). High contribution of diatoms to total biomass may enhance the efficiency of biological pump in the EJS. In addition, canonical correspondence analysis revealed a clear distribution of phytoplankton groups associated with temperature and nutrient concentration which mean prevalence of vertical variation. Finally, our findings suggested that phytoplankton biomass and groups are regulated by surface mixed layer depth

  19. Pedological and isotopic relations of a highland tropical peatland, Mountain Range of the Espinhaço Meridional (Brazil

    Directory of Open Access Journals (Sweden)

    Ingrid Horák

    2011-02-01

    Full Text Available The evolution of organic matter sources in soil is related to climate and vegetation dynamics in the past recorded in paleoenvironmental Quaternary deposits such as peatlands. For this reason, a Histosol of the mineralotrophic peatland from the Pau-de-Fruta Special Protection Area - SPA, Espinhaço Meridional, State of Minas Gerais, was described and characterized to evidence the soil constituent materials and properties as related to changes in environmental conditions, supported by the isotopic and elementary characterization of soil C and N and 14C ages. Samples were collected in a depression at 1,350 m asl, where Histosols are possibly more developed due to the great thickness (505 cm. Nowadays, the area is colonized by vegetation physiognomies of the Cerrado Biome, mainly rocky and wet fields (Campo Rupestre and Campo Úmido, aside from fragments of Semidecidual Seasonal Forest, called Capões forests. The results this study showed that early the genesis of the analyzed soil profile showed a high initial contribution of mostly herbaceous organic matter before 8,090 ± 30 years BP (14C age. In the lower-mid Holocene, between 8,090 ± 30 years AP (14C age to ± 4,100 years BP (interpolated age, the vegetation gradually became more woody, with forest expansion, possibly due to increased humidity, suggesting the existence of a more woody Cerrado in the past than at present. Drier climate conditions than the current were concluded ± 2,500 years BP (interpolated age and that after 430 years BP (14C age the forest gave way to grassland, predominantly. After the dry season, humidity increased to the current conditions. Due to these climate fluctuations during the Holocene, three decomposition stages of organic matter were observed in the Histosols of this study, with prevalence of the most advanced (sapric, typical of a deposit in a highly advanced stage of pedogenetic evolution.

  20. A meridional structure of static stability and ozone vertical gradient around the tropopause in the Southern Hemisphere extratropics

    Directory of Open Access Journals (Sweden)

    Y. Tomikawa

    2010-08-01

    Full Text Available An analysis of the static stability and ozone vertical gradient in the ozone tropopause based (OTB coordinate is applied to the ozonesonde data at 10 stations in the Southern Hemisphere (SH extratropics. The tropopause inversion layer (TIL with a static stability maximum just above the tropopause shows similar seasonal variations at two Antarctic stations, which are latitudinally far from each other. Since the sunshine hour varies with time in a quite different way between these two stations, it implies that the radiative heating due to solar ultraviolet absorption of ozone does not contribute to the seasonal variation of the TIL. A meridional section of the static stability in the OTB coordinate shows that the static stability just above the tropopause has a large latitudinal gradient between 60° S and 70° S in austral winter because of the absence of the TIL over the Antarctic. It is accompanied by an increase of westerly shear with height above the tropopause, so that the polar-night jet is formed above this latitude region. This result suggests a close relationship between the absence of the TIL and the stratospheric polar vortex in the Antarctic winter. A vertical gradient of ozone mixing ratio, referred to as ozone vertical gradient, around the tropopause shows similar latitudinal and seasonal variations with the static stability in the SH extratropics. In a height region above the TIL, a small ozone vertical gradient in the midlatitudes associated with the Antarctic ozone hole is observed in a height region of the subvortex but not around the polar vortex. This is a clear evidence of active latitudinal mixing between the midlatitudes and subvortex.

  1. Experimental Study of Transient Thermal Convection Following a Catastrophic Lithospheric Overturn: Applications to the Tectonic Style, Thermal Evolution and Topography of Venus

    Science.gov (United States)

    Robin, C. M.; Thayalan, V.; Jellinek, A. M.

    2004-05-01

    The coexistence of dynamically supported highlands and coronae is difficult to reconcile with models of Venusian mantle convection. Coronae are well explained by transient discrete mantle upwellings (thermals), which are characteristic of weak cooling due to the stagnant lid style of convection expected in one-plate planets. In contrast, topographic highlands such as Atla, Beta and Themis Regio are better explained by persistent axisymmetric plumes more typical of the Earth's mantle, which is cooled strongly by subduction and plate tectonics (i.e. mobile lid convection). From the observed crater distribution, it is also inferred that the surface of Venus has a mean age of ~700 Ma. One explanation for this young surface age is that it is a result of a recent and catastrophic resurfacing event. We test a hypothesis that the occurrence of highlands and coronae are a consequence of the style of transient mantle convection driven by sudden overturn of the lithosphere. A series of stagnant lid and mobile lid convection control experiments at thermal equilibrium under conditions appropriate for Venus' mantle are first performed. The mobile lid regime is achieved using a conveyor belt at the cold boundary. Next, we investigate the thermal and temporal characteristics of transitions from steady-state stagnant lid to mobile lid, and from mobile lid to stagnant lid regimes. Using a combination of time-lapse video, shadowgraphs and analyses of time-series of temperature and heat flux data, we identify the qualitative changes in convective regime, the quantitative changes in the heat transfer characteristics of the flows, and the characteristic time scales over which transitions occur. Three regimes are observed: (i) steady-state stagnant lid mode characterized by time-dependent hot (rising) and cold (sinking) thermals; (ii) steady-state mobile-lid mode characterized by active stirring and long-lived plumes; and (iii) a transient mixed mode characterized by the coexistence of

  2. Effect of sporadic destratification, seasonal overturn and artificial mixing on CH4 emissions at the surface of a subtropical hydroelectric reservoir (Nam Theun 2 Reservoir, Lao PDR

    Directory of Open Access Journals (Sweden)

    F. Guérin

    2015-07-01

    Full Text Available Inland waters in general and specifically freshwater reservoirs are recognized as source of CH4 to the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured fortnightly CH4 concentrations and physico-chemical parameters at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR. Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We also compared emissions with aerobic methane oxidation calculated from Deshmukh et al. (2015. In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependant of the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD when the reservoir was stratified. They decreased during the warm wet (WW season and reached its minimum after the reservoir overturned in the cool dry season (CD. The sharp decreases of the CH4 storage were concomitant with sporadic extreme diffusive fluxes (up to 200 mmol m−2 d−1. These hot moments of emissions occurred mostly in the inflow region in the WW season and during the overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these CH4 extreme emissions (> 5 mmol m−2 d−1 contributed up to 50 % of total annual emissions by diffusion. Based on our fortnightly monitoring, we determined that accurate estimation of the emissions can be determined from measurements made at least at a monthly frequency. During the transition between the WD and WW seasons, a new hotspot of emissions was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m−2 d−1 in 2010 down to 200 mmol m−2 d−1 in 2012. In the CD season, diffusive

  3. Atlantic hurricane response to geoengineering

    Science.gov (United States)

    Moore, John; Grinsted, Aslak; Ji, Duoying; Yu, Xiaoyong; Guo, Xiaoran

    2015-04-01

    Devastating Atlantic hurricanes are relatively rare events. However their intensity and frequency in a warming world may rapidly increase - perhaps by a factor of 5 for a 2°C mean global warming. Geoengineering by sulphate aerosol injection preferentially cools the tropics relative to the polar regions, including the hurricane main development region in the Atlantic, suggesting that geoengineering may be an effective method of controlling hurricanes. We examine this hypothesis using 6 Earth System Model simulations of climate under the GeoMIP G3 and G4 schemes that use aerosols to reduce the radiative forcing under the RCP4.5 scenario. We find that although temperatures are ameliorated by geoengineering, the numbers of storm surge events as big as that caused the 2005 Katrina hurricane are only slightly reduced compared with no geoengineering. As higher levels of sulphate aerosol injection produce diminishing returns in terms of cooling, but cause undesirable effects in various regions, it seems that stratospheric aerosol geoengineering is not an effective method of controlling hurricane damage.

  4. 分层土中水泥土围护结构抗倾覆验算方法的改进%Improved method for calculating anti-overturning safety factor of cement-soil retaining wall in layered soil

    Institute of Scientific and Technical Information of China (English)

    李卫超; 熊巨华; 杨敏

    2011-01-01

    An improved anti-overturning safety factor calculation method of the cement-soil retaining wall in layered soil is presented Based on the improved method, the influence of different soil structures on the anti-overturning safety factor is analyzed. When the soft soil is over the hard soil the anti-overturning safety factor increases with increasing of the width and the embedded depth of the wall respectively. Similarly the anti-overturning safety factor also increases with increasing of the width of the wall but has less change with increasing of the embedded depth of the wall when the hard soil is over the soft soil. The reason for that is the anti-overturning safety of the cement-soil retaining wall relies mainly on the passive earth pressure; that is to say, the anti-overturning safety factor is influenced by the lower soil's strength mainly. If treating the layered soil parameters according to the thickness weighted as the homogeneous soil, the anti-overturning safety factor is smaller which inclines to conservative when the soft soil is over the hard soil. Unlike that the anti-overturning safety factor is bigger which has the security risk when the hard soil is over the soft soil. Compared with the conventional calculation method, the improved method presents a more reasonable and safe result%推导了分层土中水泥土围护结构抗倾覆稳定系数计算公式,提出了改进的计算方法,研究了土层分层结构对水泥土围护结构抗倾覆稳定系数的影响.土层上软下硬时,水泥土围护结构抗倾覆稳定系数分别随着墙体宽度与入土深度的增加而增加.土层上硬下软时,增加墙体宽度对于提高抗倾覆稳定系数具有较好的效果,而增加墙体入土深度对于提高抗倾覆稳定系数作用不大,因为分层土中,水泥土围护结构抗倾覆稳定性主要依靠下部土体的被动土压力来维持,即抗倾覆稳定性主要受下层土体强度的影响.如将分层土参数按厚度

  5. Strong winter cooling over the Irminger Sea in winter 2014-2015, exceptional deep convection, and the emergence of anomalously low SST

    Science.gov (United States)

    Jong, Marieke Femke; Steur, Laura

    2016-07-01

    Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation, even though observational evidence for the link remains inconclusive. Modeling studies have suggested that convection will weaken as a result of enhanced freshwater input. The emergence of anomalously low sea surface temperature in the subpolar North Atlantic has led to speculation that this process is already at work. Here we show that strong atmospheric forcing in the winter of 2014-2015, associated with a high North Atlantic Oscillation (NAO) index, produced record mixed layer depths in the Irminger Sea. Local mixing removed the stratification of the upper 1400 m and ventilated the basin to middepths resembling a state similar to the mid-1990s when a positive NAO also prevailed. We show that the strong local atmospheric forcing is predominantly responsible for the negative sea surface temperature anomalies observed in the subpolar North Atlantic in 2015 and that there is no evidence of permanently weakened deep convection.

  6. Air-Sea CO2 fluxes in the Atlantic as measured during boreal spring and autumn

    Directory of Open Access Journals (Sweden)

    A. F. Ríos

    2010-05-01

    Full Text Available A total of fourteen hydrographic cruises from 2000 to 2008 were conducted during the spring and autumn seasons between Spain and the Southern Ocean under the framework of the Spanish research project FICARAM. The underway measurements were processed and analysed to describe the meridional air-sea CO2 fluxes (FCO2 in the covered sector of the Atlantic Ocean. The data has been grouped into different biogeochemical oceanographic provinces based on thermohaline characteristics. The spatial and temporal distributions of FCO2 followed expected distributions and annual trends reproducing the recent climatological ΔfCO2 estimations with a mean difference of −3 ± 18 μatm (Takahashi et al., 2009. The reduction in the CO2 saturation along the meridional FICARAM cruises represented an increase of 0.02 ± 0.14 mol m−2 yr−1 in the ocean uptake of atmospheric CO2. The subtropical waters in both Hemispheres acted as a sink of atmospheric CO2 during the successive spring seasons and as a source in autumn. The coarse reduction of the ocean uptake of atmospheric CO2 observed in the North Atlantic Ocean was linked to conditions of negative phase of the North Atlantic Oscillation that prevailed during the FICARAM period. Surface waters in the North Equatorial Counter Current revealed a significant long-term decrease of sea surface salinity of −0.16 ± 0.01 yr−1 coinciding with a declination of −3.5 ± 0.9 μatm yr−1 in the air–sea disequilibrium of CO2 fugacity and a rise of oceanic CO2 uptake of −0.09 ± 0.03 mol m−2 yr−1. The largest CO2 source was located in the equatorial upwelling system. These tropical waters that reached emissions of 0.7 ± 0.5 and 1.0 ± 0.7 mol m−2 y−1 in spring and autumn, respectively, showed an interannual warming of 0.11 ± 0.03 °C yr−1 and a wind speed decrease of −0.58 ± 0.14 m s−1 yr−1 in spring cruises which suggest the weakening of upwelling events associated with warm El Niño – Southern

  7. Climatic trends in the North Atlantic region during the last 2,000 years in an orbitally forced AOGCM simulation

    Science.gov (United States)

    Wagner, S.; Zorita, E.

    2013-12-01

    The global coverage of temporal highly resolved proxy-based climate reconstructions is extending to cover the last 2,000 years. It is thus important to fully understand the effect of the orbital forcing at these time scales, as the imprint of the orbital forcing becomes clearer when analyzing climate on time scales longer than the last 1,000 years. The slow-varying orbital parameters affect the seasonal distribution of the incoming solar radiation. Although changes are not as pronounced compared to the mid-Holocene, still distinct differences exist, with lower insolation between February and May and higher insolation between July and October over the mid- and high northern latitudes 2,000 years ago compared to present. Here, we analyze a simulation with the coupled climate model ECHO-G forced only with changes in orbital variations for the last 2,000 years. Other factors such as solar activity and greenhouse gas changes are set to constant pre-industrial values. The modeled near-surface temperature trends reflect the expected orbitally induced insolation trends over the northern hemispheric continents and the Arctic, with increased temperatures during May and reduced temperatures during October. Over the North Atlantic Ocean, however SST trends are not directly consistent to changes in orbital forcing throughout the year, mostly showing little or slight uniform cooling trends. The strength of the maximum overturning circulation in the North Atlantic Ocean also shows no clear-cut trends that can be linked to changes in external forcings. Other variables related to oceanic convection and surface heat fluxes indicate, however, spatially heterogeneous trend patterns. For example, regions south of Greenland and off Labrador show increases in convection that compensate the decreases over the Labrador and the Norwegian Sea. This pattern varies in intensity and spatial extent between the different winter half year months. Changes in oceanic convection and surface heat

  8. Temperature fluctuations in the Atlantic Ocean

    International Nuclear Information System (INIS)

    The article discusses the temperature fluctuations in connection with drought in Africa, the climate in North America, the European heat waves and the frequent tropical hurricanes in the Atlantic Ocean. Problems with climate modelling and some pollution aspects are mentioned

  9. Atlantic Marine Mammal Assessment Vessel Surveys

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — These data sets are a compilation of large vessel surveys for marine mammal stock assessments in South Atlantic (Florida to Maryland) waters from 1994 to the...

  10. Atlantic energy and the strategic outlook

    Directory of Open Access Journals (Sweden)

    Paul Isbell

    2013-09-01

    Full Text Available Sweeping changes are beginning to transform energy scenarios around the world. The gas revolution, a renaissance in petroleum technology and exploration, and a chaotic but powerful movement toward the goal of low-carbon economies are three of the principal energy trends currently interacting with structural changes in the geo-economics of the Atlantic world to present new perspectives and opportunitiesfor the diverse actors in the ‘Atlantic Basin’. This article explores how changes in the energy landscape are contributing to a reassessment of the strategic horizon. The potential impacts of the shale revolution, deep-offshore oil, biofuels and other modern renewable energies on the geopolitics of the Atlantic Basin will be assessed, and the hypothesis that an Atlantic Basin energy system is now taking shape will be evaluated, along with an analysis of anticipated impacts.

  11. Northwest Atlantic Regional Climatology (NCEI Accession 0155889)

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — To provide an improved oceanographic foundation and reference for multi-disciplinary studies of the Northwest Atlantic Ocean, NCEI Regional Climatology Team...

  12. Atlantic NAD 83 Continental Shelf Boundary (CSB)

    Data.gov (United States)

    Bureau of Ocean Energy Management, Department of the Interior — This data set contains Continental Shelf Boundary (CSB) lines in ESRI shapefile format for the BOEM Atlantic Region. The CSB defines the seaward limit of federally...

  13. Atlantic NAD 83 SLA Baseline Tangents

    Data.gov (United States)

    Bureau of Ocean Energy Management, Department of the Interior — This data set contains baseline tangent lines in ArcGIS shapefile format for the BOEM Atlantic Region. Baseline tangent lines are typically bay or river closing...

  14. Atlantic NAD 83 SLA Baseline Points

    Data.gov (United States)

    Bureau of Ocean Energy Management, Department of the Interior — This data set contains baseline points in ArcGIS shapefile format for the BOEM Atlantic Region. Baseline points are the discrete coordinate points along the...

  15. Atlantic Sharpnose Shark Reproductive Biology Data

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Reproductive data from Atlantic sharpnose sharks were collected from specimens captured throughout the northern Gulf of Mexico on various research vessels. Data...

  16. North Atlantic Coastal Plain aquifer system

    Data.gov (United States)

    U.S. Geological Survey, Department of the Interior — This data set represents the extent of the North Atlantic Coastal Plain aquifer system in North Carolina, Virginia, Maryland, Deleware, and New Jersey.

  17. Atlantic Surfclam and Ocean Quahog Survey

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The standardized NEFSC Atlantic Surfclam and Ocean Quahog Survey has covered an area from Cape Hatteras to Georges Bank. The survey was conducted every two or three...

  18. Radiative budget and cloud radiative effect over the Atlantic from ship-based observations

    Directory of Open Access Journals (Sweden)

    J. Kalisch

    2012-10-01

    Full Text Available The aim of this study is to determine cloud-type resolved cloud radiative budgets and cloud radiative effects from surface measurements of broadband radiative fluxes over the Atlantic Ocean. Furthermore, based on simultaneous observations of the state of the cloudy atmosphere, a radiative closure study has been performed by means of the ECHAM5 single column model in order to identify the model's ability to realistically reproduce the effects of clouds on the climate system.

    An extensive database of radiative and atmospheric measurements has been established along five meridional cruises of the German research icebreaker Polarstern. Besides pyranometer and pyrgeometer for downward broadband solar and thermal radiative fluxes, a sky imager and a microwave radiometer have been utilized to determine cloud fraction and cloud type on the one hand and temperature and humidity profiles as well as liquid water path for warm non-precipitating clouds on the other hand.

    Averaged over all cruise tracks, we obtain a total net (solar + thermal radiative flux of 144 W m−2 that is dominated by the solar component. In general, the solar contribution is large for cirrus clouds and small for stratus clouds. No significant meridional dependencies were found for the surface radiation budgets and cloud effects. The strongest surface longwave cloud effects were shown in the presence of low level clouds. Clouds with a high optical density induce strong negative solar radiative effects under high solar altitudes. The mean surface net cloud radiative effect is −33 W m−2.

    For the purpose of quickly estimating the mean surface longwave, shortwave and net cloud effects in moderate, subtropical and tropical climate regimes, a new parameterisation was created, considering the total cloud amount and the solar zenith angle.

    The ECHAM5 single column model provides a surface net cloud effect that is more

  19. Radiative budget and cloud radiative effect over the Atlantic from ship based observations

    Directory of Open Access Journals (Sweden)

    J. Kalisch

    2012-03-01

    Full Text Available The aim of this study is to determine cloud-type resolved cloud radiative budgets and cloud radiative effects from surface measurements of broadband radiative fluxes over the Atlantic Ocean. Furthermore, based on simultaneous observations of the state of the cloudy atmosphere a radiative closure study has been performed by means of the ECHAM5 single column model in order to identify the models ability to realistically reproduce the effects of clouds on the climate system.

    An extensive data base of radiative and atmospheric measurements has been established along five meridional cruises of the German research icebreaker POLARSTERN. Besides pyranometer and pyrgeometer for downward broadband solar and thermal radiative fluxes, a sky imager and a microwave radiometer have been utilized to determine cloud fraction and cloud type on the one hand and temperature and humidity profiles as well as liquid water path for warm non-precipitating clouds on the other hand.

    Averaged over all cruise tracks we obtain a total net (solar + thermal radiative flux of 144 W m−2 that is dominated by the solar component. In general, the solar contribution is large for cirrus clouds and small for stratus clouds. No significant meridional dependencies were found for the surface radiation budgets and cloud effects. The strongest surface longwave cloud effects were shown in the presence of low level clouds. Clouds with a high optical density induce strong negative solar radiative effects under high solar altitudes. The mean surface net cloud radiative effect is −34 W m−2.

    For the purpose of quickly estimating the mean surface longwave, shortwave and net cloud effects in moderate, subtropical and tropical climate regimes a new parameterisation was created, considering the total cloud amount and the solar zenith angle.

    The ECHAM5 single column model provides a surface net cloud effect that is more cooling by 16 W

  20. The Coupled Mars Dust and Water Cycles: Understanding How Clouds Affect the Vertical Distribution and Meridional Transport of Dust and Water.

    Science.gov (United States)

    Kahre, M. A.

    2015-01-01

    The dust and water cycles are crucial to the current Martian climate, and they are coupled through cloud formation. Dust strongly impacts the thermal structure of the atmosphere and thus greatly affects atmospheric circulation, while clouds provide radiative forcing and control the hemispheric exchange of water through the modification of the vertical distributions of water and dust. Recent improvements in the quality and sophistication of both observations and climate models allow for a more comprehensive understanding of how the interaction between the dust and water cycles (through cloud formation) affects the dust and water cycles individually. We focus here on the effects of clouds on the vertical distribution of dust and water, and how those vertical distributions control the net meridional transport of water. For this study, we utilize observations of temperature, dust and water ice from the Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO) combined with the NASA ARC Mars Global Climate Model (MGCM). We demonstrate that the magnitude and nature of the net meridional transport of water between the northern and southern hemispheres during NH summer is sensitive to the vertical structure of the simulated aphelion cloud belt. We further examine how clouds influence the atmospheric thermal structure and thus the vertical structure of the cloud belt. Our goal is to identify and understand the importance of radiative/dynamic feedbacks due to the physical processes involved with cloud formation and evolution on the current climate of Mars.