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Time-Dependent Response of the Overturning Circulation and Pycnocline Depth to Southern Ocean Surface Wind Stress Changes

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  • 1 a Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois
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Abstract

Changes in the Southern Ocean (SO) surface wind stress influence both the meridional overturning circulation (MOC) and stratification not only in the SO but in the global oceans, which can take multiple millennia to fully equilibrate. We use a hierarchy of models to investigate the time-dependent response of the MOC and low-latitude pycnocline depth (which quantifies the stratification) to SO wind stress changes: a two-layer analytical theory, a multicolumn model (PyMOC), and an idealized general circulation model (GCM). We find that in both the GCM and PyMOC, the MOC has a multidecadal adjustment time scale while the pycnocline depth has a multicentennial time scale. The two-layer theory instead predicts the MOC and pycnocline depth to adjust on the same, multidecadal time scale. We argue that this discrepancy arises because the pycnocline depth depends on the bulk stratification, while the MOC amplitude is sensitive mostly to isopycnals within the overturning cell. We can reconcile the discrepancy by interpreting the “pycnocline depth” in the theory as the depth of a specific isopycnal near the maximum of the MOC. We also find that SO stationary eddies respond very quickly to a sudden wind stress change, compensating for most of the change in the Ekman-driven MOC. This effect is missing in the theory, where the eddy-induced MOC only follows the adjustment of the pycnocline depth. Our results emphasize the importance of depth dependence in the oceans’ transient response to changes in surface boundary conditions, and the distinct role played by stationary eddies in the SO.

Significance Statement

Our work resolves the question of why previous theories predict the ocean density structure to adjust to a change in the winds over the Southern Ocean within centuries, while climate models indicate that this adjustment takes thousands of years. The question is important because it is related to our understanding of how the ocean responds to potential climate change scenarios. Our results emphasize the importance of depth dependence in the density response (i.e., the upper ocean adjusts faster than the deep ocean), suggesting that future theoretical advancement should be made with careful considerations of the ocean’s vertical structure. Our results also highlight the role of stationary meanders in the Southern Ocean’s Antarctic Circumpolar Current, whose influence has not been included in the existing theories.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Hailu Kong, hlkong.uchicago@gmail.com

Abstract

Changes in the Southern Ocean (SO) surface wind stress influence both the meridional overturning circulation (MOC) and stratification not only in the SO but in the global oceans, which can take multiple millennia to fully equilibrate. We use a hierarchy of models to investigate the time-dependent response of the MOC and low-latitude pycnocline depth (which quantifies the stratification) to SO wind stress changes: a two-layer analytical theory, a multicolumn model (PyMOC), and an idealized general circulation model (GCM). We find that in both the GCM and PyMOC, the MOC has a multidecadal adjustment time scale while the pycnocline depth has a multicentennial time scale. The two-layer theory instead predicts the MOC and pycnocline depth to adjust on the same, multidecadal time scale. We argue that this discrepancy arises because the pycnocline depth depends on the bulk stratification, while the MOC amplitude is sensitive mostly to isopycnals within the overturning cell. We can reconcile the discrepancy by interpreting the “pycnocline depth” in the theory as the depth of a specific isopycnal near the maximum of the MOC. We also find that SO stationary eddies respond very quickly to a sudden wind stress change, compensating for most of the change in the Ekman-driven MOC. This effect is missing in the theory, where the eddy-induced MOC only follows the adjustment of the pycnocline depth. Our results emphasize the importance of depth dependence in the oceans’ transient response to changes in surface boundary conditions, and the distinct role played by stationary eddies in the SO.

Significance Statement

Our work resolves the question of why previous theories predict the ocean density structure to adjust to a change in the winds over the Southern Ocean within centuries, while climate models indicate that this adjustment takes thousands of years. The question is important because it is related to our understanding of how the ocean responds to potential climate change scenarios. Our results emphasize the importance of depth dependence in the density response (i.e., the upper ocean adjusts faster than the deep ocean), suggesting that future theoretical advancement should be made with careful considerations of the ocean’s vertical structure. Our results also highlight the role of stationary meanders in the Southern Ocean’s Antarctic Circumpolar Current, whose influence has not been included in the existing theories.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Hailu Kong, hlkong.uchicago@gmail.com
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