scholarly journals Effects of Salt Compensation on the Climate Model Response in Simulations of Large Changes of the Atlantic Meridional Overturning Circulation*

2007 ◽  
Vol 20 (24) ◽  
pp. 5912-5928 ◽  
Author(s):  
Thomas F. Stocker ◽  
Axel Timmermann ◽  
Manuel Renold ◽  
Oliver Timm
Keyword(s):  
Southern Ocean ◽  
Climate Model ◽  
Temperature Response ◽  
Coupled Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Salt Flux ◽  
Overturning Circulation ◽  
Meridional Overturning

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.

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

2015 ◽  
Vol 28 (23) ◽  
pp. 9250-9257 ◽  
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.

scholarly journals Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

2018 ◽  
Vol 31 (12) ◽  
pp. 4727-4743 ◽  
Author(s):  
Wei Liu ◽  
Jian Lu ◽  
Shang-Ping Xie ◽  
Alexey Fedorov
Keyword(s):  
Southern Ocean ◽  
Heat Transport ◽  
Climate Models ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
The Other ◽  
Anthropogenic Heat ◽  
Overturning Circulation ◽  
Ocean Heat Uptake ◽  
Meridional Overturning

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

scholarly journals Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

2020 ◽  
Vol 6 (26) ◽  
pp. eaaz4876 ◽  
Author(s):  
Wei Liu ◽  
Alexey V. Fedorov ◽  
Shang-Ping Xie ◽  
Shineng Hu
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Arctic Sea Ice ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Overturning Circulation ◽  
Meridional Overturning

While the Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down under anthropogenic warming, the exact role of the AMOC in future climate change has not been fully quantified. Here, we present a method to stabilize the AMOC intensity in anthropogenic warming experiments by removing fresh water from the subpolar North Atlantic. This method enables us to isolate the AMOC climatic impacts in experiments with a full-physics climate model. Our results show that a weakened AMOC can explain ocean cooling south of Greenland that resembles the North Atlantic warming hole and a reduced Arctic sea ice loss in all seasons with a delay of about 6 years in the emergence of an ice-free Arctic in boreal summer. In the troposphere, a weakened AMOC causes an anomalous cooling band stretching from the lower levels in high latitudes to the upper levels in the tropics and displaces the Northern Hemisphere midlatitude jets poleward.

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

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 881-891 ◽  
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.

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

2010 ◽  
Vol 7 (3) ◽  
pp. 4045-4088 ◽  
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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