scholarly journals Ocean Basin Geometry and the Salinification of the Atlantic Ocean

2013 ◽  
Vol 26 (16) ◽  
pp. 6163-6184 ◽  
Author(s):  
Johan Nilsson ◽  
Peter L. Langen ◽  
David Ferreira ◽  
John Marshall
Keyword(s):  
Atlantic Ocean ◽  
Ocean Circulation ◽  
Deep Water ◽  
World Ocean ◽  
Ocean Basin ◽  
Salt Transport ◽  
Western Boundary ◽  
Deep Water Formation ◽  
Water Formation ◽  
Basin Geometry

Abstract A coupled atmosphere–sea ice–ocean model is used in an aqua-planet setting to examine the role of the basin geometry for the climate and ocean circulation. The basin geometry has a present-day-like topology with two idealized northern basins and a circumpolar ocean in the south. A suite of experiments is described in which the southward extents of the two (gridpoint wide) “continents” and the basin widths have been varied. When the two basins have identical shapes, the coupled model can attain a symmetric climate state with northern deep-water formation in both basins as well as asymmetric states, where the deep-water formation occurs only in one of the basins and Atlantic–Pacific-like hydrographic differences develop. A difference in the southward extents of the land barriers can enhance as well as reduce the zonal asymmetries of the atmosphere–ocean circulation. This arises from an interplay between the basin boundaries and the wind-driven Sverdrup circulation, which controls the interbasin exchange of heat and salt. Remarkably, when the short “African” continent is located near or equatorward of the zero wind line in the Southern Hemisphere, the deep-water formation becomes uniquely localized to the “Atlantic”-like basin with the long western boundary. In this case, the salinification is accomplished primarily by a westward wind-routed interbasin salt transport. Furthermore, experiments using geometries with asymmetries in both continental extents and basin widths suggest that in the World Ocean these two fundamental basin asymmetries should independently be strong enough for uniquely localizing the Northern Hemisphere deep-water formation to the Atlantic Ocean.

scholarly journals North Atlantic deep water formation and AMOC in CMIP5 models

Ocean Science ◽  
2017 ◽  
Vol 13 (4) ◽  
pp. 609-622 ◽  
Author(s):  
Céline Heuzé
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Deep Water ◽  
Deep Convection ◽  
The Arctic ◽  
Subpolar Gyre ◽  
Deep Water Formation ◽  
Nordic Seas ◽  
Water Formation

Abstract. Deep water formation in climate models is indicative of their ability to simulate future ocean circulation, carbon and heat uptake, and sea level rise. Present-day temperature, salinity, sea ice concentration and ocean transport in the North Atlantic subpolar gyre and Nordic Seas from 23 CMIP5 (Climate Model Intercomparison Project, phase 5) models are compared with observations to assess the biases, causes and consequences of North Atlantic deep convection in models. The majority of models convect too deep, over too large an area, too often and too far south. Deep convection occurs at the sea ice edge and is most realistic in models with accurate sea ice extent, mostly those using the CICE model. Half of the models convect in response to local cooling or salinification of the surface waters; only a third have a dynamic relationship between freshwater coming from the Arctic and deep convection. The models with the most intense deep convection have the warmest deep waters, due to a redistribution of heat through the water column. For the majority of models, the variability of the Atlantic Meridional Overturning Circulation (AMOC) is explained by the volumes of deep water produced in the subpolar gyre and Nordic Seas up to 2 years before. In turn, models with the strongest AMOC have the largest heat export to the Arctic. Understanding the dynamical drivers of deep convection and AMOC in models is hence key to realistically forecasting Arctic oceanic warming and its consequences for the global ocean circulation, cryosphere and marine life.

scholarly journals The global ocean circulation on a retrograde rotating earth

2011 ◽  
Vol 7 (2) ◽  
pp. 487-499 ◽  
Author(s):  
V. Kamphuis ◽  
S. E. Huisman ◽  
H. A. Dijkstra
Keyword(s):  
Ocean Circulation ◽  
Deep Water ◽  
North Pacific ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Global Ocean Circulation ◽  
The North Pacific

Abstract. To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterised by an Atlantic Meridional Overturning Circulation (MOC) with a deep water formation at northern latitudes and the absence of such a deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the Atlantic as a whole is a basin with net evaporation, while the Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed, but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is relatively weak but intermittently strong deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model the stability properties of the Atlantic MOC on a retrograde rotating earth are also investigated, showing a similar regime of multiple equilibria as in the present-day case. These results indicate that the present-day asymmetry in surface freshwater flux is not the most important factor setting the Atlantic-Pacific salinity difference and, thereby, the asymmetry in the global MOC.

scholarly journals Early Eocene vigorous ocean overturning and its contribution to a warm Southern Ocean

2020 ◽  
Author(s):  
Yurui Zhang ◽  
Thierry Huck ◽  
Camille Lique ◽  
Yannick Donnadieu ◽  
Jean-Baptiste Ladant ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Heat Transport ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Deep Water ◽  
Atmospheric Co2 ◽  
Early Eocene ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Water Formation

Abstract. The early Eocene (~ 55 Ma) is the warmest period, and most likely characterized by the highest atmospheric CO2 concentrations, of the Cenozoic era. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 coupled climate model set up with paleogeographic reconstructions of this period from the DeepMIP project, with different levels of atmospheric CO2, and compare them with simulations of the modern conditions. This allows us to explore the changes of the ocean circulation and the resulting ocean meridional heat transport. At a CO2 level of 840 ppm, the Early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60º S), fed by deep water formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the build-up of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with the subpolar gyres, sustains the poleward advection of saline subtropical water to the convective region in the Southern Ocean, maintaining deep-water formation. This salt-advection feedback mechanism works similarly in the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3–0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20° S, and contributing to maintain the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO2 levels show that the ocean circulation and heat transport are relatively insensitive to CO2-doubling.

scholarly journals The Atlantic Subtropical Front/Current Systems of Azores and St. Helena

2007 ◽  
Vol 37 (11) ◽  
pp. 2573-2598 ◽  
Author(s):  
Manuela F. Juliano ◽  
Mário L. G. R. Alves
Keyword(s):  
Atlantic Ocean ◽  
Ocean Circulation ◽  
Large Scale ◽  
World Ocean ◽  
Circulation Model ◽  
Western Boundary ◽  
Mode Water ◽  
Subtropical Front ◽  
St Helena ◽  
Current Systems

Abstract A large-scale climatic ocean circulation model was used to study the Atlantic Ocean circulation. This inverse model is an extension of the β-spiral formulation presented in papers by Stommel and Schott with a more complete version of the vorticity equation, including relative vorticity in addition to planetary vorticity. Also, a more complete database for hydrological measurements in the Atlantic Ocean was used, including not only the National Oceanographic Data Center database but also World Ocean Circulation Experiment data and cruises near the Azores, Angola, and Guinea-Bissau. A detailed analysis of the Northern Hemisphere Azores Current and Front shows that this new database and the model results were able to capture all major features reported previously. In the Southern Hemisphere, the authors have identified fully and described the subtropical front that is the counterpart to the Azores Current, which they call the St. Helena Current and Front. Both current systems of both hemispheres have similar intensities, depth penetration, volume transports, and zonal flow. Both have associated subsurface adjacent countercurrent flows, and their main cores flow at similar latitudes (∼34°N for the Azores Current and 34°S for the St. Helena Current). It is argued that both current systems and associated fronts are the poleward 18°C Mode Water discontinuities of the two Atlantic subtropical gyres and that both originate at the corresponding hemisphere western boundary current systems from which they penetrate into the open ocean interior. Thus, both currents should have a similar forcing source, and their origin should not be linked to any geographical peculiarities.

Deep water formation in the North Atlantic Ocean during the last ice age

Nature ◽  
1980 ◽  
Vol 286 (5772) ◽  
pp. 479-482 ◽  
Author(s):  
Jean-Claude Duplessy ◽  
J. Moyes ◽  
C. Pujol
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
North Atlantic Ocean ◽  
Ice Age ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Last Ice Age ◽  
The North Atlantic

scholarly journals Transport across 48°N in the Atlantic Ocean

2008 ◽  
Vol 38 (4) ◽  
pp. 733-752 ◽  
Author(s):  
Rick Lumpkin ◽  
Kevin G. Speer ◽  
K. Peter Koltermann
Keyword(s):  
Atlantic Ocean ◽  
Ocean Circulation ◽  
World Ocean ◽  
Inverse Model ◽  
Reference Level ◽  
Western Boundary ◽  
North Atlantic Current ◽  
Data Set ◽  
Freshwater Fluxes ◽  
World Ocean Circulation Experiment

Abstract Transports across 48°N in the Atlantic Ocean are estimated from five repeat World Ocean Circulation Experiment (WOCE) hydrographic lines collected in this region in 1993–2000, from time-varying air–sea heat and freshwater fluxes north of 48°N, and from a synthesis of these two data sources using inverse box model methods. Results from hydrography and air–sea fluxes treated separately are analogous to recently published transport variation studies and demonstrate the sensitivity of the results to either the choice of reference level and reference velocities for thermal wind calculations or the specific flux dataset chosen. In addition, flux-based calculations do not include the effects of subsurface mixing on overturning and transports of specific water masses. The inverse model approach was used to find unknown depth-independent velocities, interior diapycnal fluxes, and adjustments to air–sea fluxes subject to various constraints on the system. Various model choices were made to focus on annually averaged results, as opposed to instantaneous values during the occupation of the hydrographic lines. The results reflect the constraints and choices made in the construction of the model. The inverse model solutions show only marginal, not significantly different temporal changes in the net overturning cell strength and heat transport across 48°N. These small changes are similar to seasonally or annually averaged numerical model simulations of overturning. Significant variability is found for deep transports and air–sea flux quantities in density layers. Put another way, if one ignores the details of layer exchanges, the model can be constrained to produce the same net overturning for each repeat line; however, constraining individual layers to have the same transport for each line fails. Diapycnal fluxes are found to be important in the mean but are relatively constant from one repeat line to the next. Mean air–sea fluxes are modified slightly but are still essentially consistent with either the NCEP data or the National Oceanography Centre, Southampton (NOC) Comprehensive Ocean–Atmosphere Data Set (COADS) within error. Modest reductions in air–sea flux uncertainties would give these constraints a much greater impact. Direct transport estimates over broader regions than the western boundary North Atlantic Current are needed to help resolve circulation structure that is important for variability in net overturning.

scholarly journals Paleogeographic controls on the evolution of Late Cretaceous ocean circulation

2020 ◽  
Author(s):  
Jean-Baptiste Ladant ◽  
Christopher J. Poulsen ◽  
Frédéric Fluteau ◽  
Clay R. Tabor ◽  
Kenneth G. MacLeod ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Late Cretaceous ◽  
Deep Water ◽  
Climate Model ◽  
Deep Water Formation ◽  
Geologic History ◽  
Water Formation ◽  
History Of ◽  
Considerable Impact ◽  
Climate Model Simulations

Abstract. Understanding of the role of ocean circulation on climate during the Late Cretaceous is contingent on the ability to reconstruct its modes and evolution. Geochemical proxies used to infer modes of past circulation provide conflicting interpretations for the reorganization of the ocean circulation through the Late Cretaceous. Here, we present climate model simulations of the Cenomanian (100.5–93.9 Ma) and Maastrichtian (72.1–66.1 Ma) stages of the Cretaceous with the CCSM4 earth system model. We focus on intermediate (500–1500 m) and deep (> 1500 m) ocean circulation, and show that while there is continuous deep-water production in the southwest Pacific, major circulation changes occur between the Cenomanian and Maastrichtian. Opening of the Atlantic and Southern Ocean, in particular, drives a transition from a mostly zonal circulation to enhanced meridional exchange. Using additional experiments to test the effect of deepening of major ocean gateways in the Maastrichtian, we demonstrate that the geometry of these gateways likely had a considerable impact on ocean circulation. We further compare simulated circulation results with compilations of εNd records and show that simulated changes in Late Cretaceous ocean circulation are reasonably consistent with inferences from this proxy. In our simulations, consistency with the geologic history of major ocean gateways and absence of shift in areas of deep-water formation suggest that the Late Cretaceous trend in εNd values in the Atlantic and southern Indian Oceans was caused by the subsidence of volcanic provinces and opening of the Atlantic and Southern Oceans rather than changes in deep-water formation areas and/or reversal of deep-water fluxes. However, the complexity in interpreting Late Cretaceous εNd values underscores the need for new records as well as specific εNd modeling to better discriminate between the various plausible theories of ocean circulation change during this period.

scholarly journals Paleogeographic controls on the evolution of Late Cretaceous ocean circulation

2020 ◽  
Vol 16 (3) ◽  
pp. 973-1006 ◽  
Author(s):  
Jean-Baptiste Ladant ◽  
Christopher J. Poulsen ◽  
Frédéric Fluteau ◽  
Clay R. Tabor ◽  
Kenneth G. MacLeod ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Late Cretaceous ◽  
Deep Water ◽  
Climate Model ◽  
Deep Water Formation ◽  
Geologic History ◽  
Water Formation ◽  
History Of ◽  
Considerable Impact ◽  
Climate Model Simulations

Abstract. Understanding of the role of ocean circulation on climate during the Late Cretaceous is contingent on the ability to reconstruct its modes and evolution. Geochemical proxies used to infer modes of past circulation provide conflicting interpretations for the reorganization of the ocean circulation through the Late Cretaceous. Here, we present climate model simulations of the Cenomanian (100.5–93.9 Ma) and Maastrichtian (72.1–66.1 Ma) stages of the Cretaceous with the CCSM4 earth system model. We focus on intermediate (500–1500 m) and deep (> 1500 m) ocean circulation and show that while there is continuous deep-water production in the southwestern Pacific, major circulation changes occur between the Cenomanian and Maastrichtian. Opening of the Atlantic and Southern Ocean, in particular, drives a transition from a mostly zonal circulation to enhanced meridional exchange. Using additional experiments to test the effect of deepening of major ocean gateways in the Maastrichtian, we demonstrate that the geometry of these gateways likely had a considerable impact on ocean circulation. We further compare simulated circulation results with compilations of εNd records and show that simulated changes in Late Cretaceous ocean circulation are reasonably consistent with proxy-based inferences. In our simulations, consistency with the geologic history of major ocean gateways and absence of shift in areas of deep-water formation suggest that Late Cretaceous trends in εNd values in the Atlantic and southern Indian oceans were caused by the subsidence of volcanic provinces and opening of the Atlantic and Southern oceans rather than changes in deep-water formation areas and/or reversal of deep-water fluxes. However, the complexity in interpreting Late Cretaceous εNd values underscores the need for new records as well as specific εNd modeling to better discriminate between the various plausible theories of ocean circulation change during this period.

scholarly journals North Atlantic deep water formation and AMOC in CMIP5 models

2017 ◽  
Author(s):  
Céline Heuzé
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Deep Water ◽  
Deep Convection ◽  
The Arctic ◽  
Subpolar Gyre ◽  
Deep Water Formation ◽  
Nordic Seas ◽  
Water Formation

Abstract. Deep water formation in climate models is indicative of their ability to simulate future ocean circulation, carbon and heat uptake, and sea level rise. Present-day temperature, salinity, sea ice concentration and ocean transport in the North Atlantic subpolar gyre and Nordic Seas from 23 CMIP5 (Climate Model Intercomparison Project, phase 5) models are compared with observations to assess the biases, causes and consequences of North Atlantic deep convection in models. The majority of models convect too deep, over too large an area, too often, and too far south. Deep convection occurs at the sea ice edge and is most realistic in models with accurate sea ice extent, mostly those using the CICE model. Half of the models convect in response to local cooling or salinification of the surface waters; only a third have a dynamic relationship between freshwater coming from the Arctic and deep convection. The models with the most intense deep convection have the warmest deep waters, due to a redistribution of heat through the water column. For the majority of models, the variability of the Atlantic Meridional Overturning Circulation (AMOC) is explained by the volumes of deep water produced in the subpolar gyre and Nordic Seas up to 2 years before. In turns, models with the strongest AMOC have the largest heat export to the Arctic. Understanding the dynamical drivers of deep convection and AMOC in models is hence key to realistically forecast Arctic oceanic warming and its consequences on the global ocean circulation, cryosphere and marine life.

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