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

Abstract Freshwater hosing experiments with a comprehensive coupled climate model and a coupled model of intermediate complexity are performed with and without global salt compensation in order to investigate the robustness of the bipolar seesaw. In both cases, a strong reduction of the Atlantic meridional overturning circulation is induced, and a warming in the South Atlantic results. When a globally uniform salt flux is applied at the surface in order to keep the global mean salinity constant, this causes additional widespread warming in the Southern Ocean. It is shown that this warming is mainly due to heat transport anomalies that are induced by the specific parameterization in ocean models to represent eddy mixing. Surface salt fluxes tend to move outcropping isopycnals equatorward. As the density perturbation originates at the surface, changes in isopycnal slopes are generated that lead to anomalies in the bolus velocity field. The associated bolus heat flux convergence creates a warming enhancing the bipolar seesaw response, particularly in the Southern Ocean. The importance of this mechanism is illustrated in coupled model simulations in which this parameterization in the ocean model component is switched on or off. Additional experiments in which the same total amount of freshwater is delivered at rates 10 times smaller show that the effect of the global salt compensation is not important in this case, but that the eddy-mixing parameterization is still responsible for a substantial temperature response in the Southern Ocean.

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The Connection between Southern Ocean Winds, the Atlantic Meridional Overturning Circulation, and Indo-Pacific Upwelling

Journal of Climate ◽

10.1175/jcli-d-15-0263.1 ◽

2015 ◽

Vol 28 (23) ◽

pp. 9250-9257 ◽

Cited By ~ 12

Author(s):

M. Jochum ◽

Carsten Eden

Keyword(s):

Southern Ocean ◽

Bottom Water ◽

Dynamic Effect ◽

Atlantic Meridional Overturning Circulation ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

Antarctic Bottom Water ◽

Overturning Circulation ◽

Ocean Winds ◽

Meridional Overturning

Abstract Coupled GCM simulations are analyzed to quantify the dynamic effect of Southern Ocean (SO) winds on transports in the ocean. It is found that the closure for skew diffusivity in the non-eddy-resolving ocean model does not allow for a realistic eddy saturation of the zonal transports in the SO in response to the wind changes and that eddy compensation of the meridional transports in the SO is underestimated too. Despite this underestimated eddy compensation in the SO, however, and in contrast to previous suggestions, the Atlantic meridional overturning circulation (AMOC) strength is almost insensitive to SO winds. In the limit of weak SO winds the AMOC waters upwell not in the SO but rather in the tropical Indo-Pacific. Through their effect on sea ice, weaker SO winds also lead to less production of Antarctic Bottom Water and therefore a deeper and stronger AMOC.

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Characteristic evolution of the Atlantic Meridional Overturning Circulation from 1990 to 2015: An eddy-resolving ocean model study

Deep Sea Research Part I Oceanographic Research Papers ◽

10.1016/j.dsr.2019.06.002 ◽

2019 ◽

Vol 149 ◽

pp. 103056 ◽

Cited By ~ 3

Author(s):

Zeliang Wang ◽

David Brickman ◽

Blair J.W. Greenan

Keyword(s):

Atlantic Meridional Overturning Circulation ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Model Study ◽

Meridional Overturning ◽

Characteristic Evolution

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Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

Ocean Science ◽

10.5194/os-10-881-2014 ◽

2014 ◽

Vol 10 (6) ◽

pp. 881-891 ◽

Cited By ~ 8

Author(s):

S.-E. Brunnabend ◽

H. A. Dijkstra ◽

M. A. Kliphuis ◽

B. van Werkhoven ◽

H. E. Bal ◽

...

Keyword(s):

North Atlantic ◽

Atlantic Meridional Overturning Circulation ◽

Sea Surface Height ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

Sea Surface ◽

Sea Level Changes ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning

Abstract. 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 is parameterised. 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.

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

Biogeosciences Discussions ◽

10.5194/bgd-7-4045-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 4045-4088 ◽

Cited By ~ 3

Author(s):

J. B. Palter ◽

J. L. Sarmiento ◽

A. Gnanadesikan ◽

J. Simeon ◽

D. Slater

Keyword(s):

Southern Ocean ◽

Primary Productivity ◽

Deep Ocean ◽

Ocean Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Nutrient Return ◽

Mode Water ◽

Meridional Overturning

Abstract. 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.

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

Ocean Science Journal ◽

10.1007/s12601-015-0012-6 ◽

2015 ◽

Vol 50 (2) ◽

pp. 155-161 ◽

Cited By ~ 1

Author(s):

Sang Yeob Kim ◽

Ho Jin Lee ◽

Jae-Hun Park ◽

Young Ho Kim

Keyword(s):

Sea Ice ◽

Vertical Mixing ◽

Atlantic Meridional Overturning Circulation ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning

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

Biogeosciences ◽

10.5194/bg-7-3549-2010 ◽

2010 ◽

Vol 7 (11) ◽

pp. 3549-3568 ◽

Cited By ~ 47

Author(s):

J. B. Palter ◽

J. L. Sarmiento ◽

A. Gnanadesikan ◽

J. Simeon ◽

R. D. Slater

Keyword(s):

Southern Ocean ◽

Deep Ocean ◽

Ocean Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Nutrient Return ◽

Mode Water ◽

Export Production ◽

Meridional Overturning

Abstract. 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.

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Glacial mode shift of the Atlantic meridional overturning circulation by warming over the Southern Ocean

Communications Earth & Environment ◽

10.1038/s43247-021-00226-3 ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Akira Oka ◽

Ayako Abe-Ouchi ◽

Sam Sherriff-Tadano ◽

Yusuke Yokoyama ◽

Kenji Kawamura ◽

...

Keyword(s):

Southern Ocean ◽

Southern Hemisphere ◽

General Circulation ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Sea Surface ◽

Surface Cooling ◽

Thermal Threshold ◽

Overturning Circulation ◽

Meridional Overturning

AbstractAbrupt climate warming events, known as Dansgaard-Oeschger events, occurred frequently during glacial periods, and are thought to be linked to changes in the Atlantic meridional overturning circulation. However, the mechanism responsible is not fully understood. Here, we present numerical simulations with a sea-ice coupled ocean general circulation model that systematically investigate the thermal threshold where deep water formation, and hence the overturning circulation, shift abruptly when the sea surface cools or warms sufficiently. Specifically, in our simulations where the magnitude of the sea surface cooling is changed separately or simultaneously in the Northern and Southern Hemispheres, a prominent threshold is identified when the Southern Hemisphere is slightly warmer than during glacial maxima. Abrupt mode changes of the Atlantic Meridional Overturning Circulation, like those during Dansgaard-Oeschger events, occur past a threshold in a transient simulation where the Southern Hemisphere is gradually warmed. We propose that the Southern Ocean plays a role in controlling the thermal threshold of the Atlantic Meridional Overturning Circulation in a glacial climate and that Southern Ocean warming may have triggered Dansgaard-Oeschger events which occurred with long interval.

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