The observational record shows a substantial 40-yr upward trend in summertime westerly winds over the Southern Ocean, as characterized by the southern annular mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downward, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.
Bibliographical noteFunding Information:
This project received grant funding from the Australian Government as part of the Antarctic Science Collaboration Initiative program. The Australian Antarctic Program Partnership is led by the University of Tasmania, and includes the Australian Antarctic Division, CSIRO Oceans and Atmosphere, Geoscience Australia, the Bureau of Meteorology, the Tasmanian State Government, and Australia’s Integrated Marine Observing System. Climate modeling at NASA–GISS is supported by the NASA Modeling, Analysis, and Prediction program. Computational resources for the E2.1 simulations in this study were provided by the NASA High-End Computing Program through the NASA Center for Climate Simulation (NCCS) at Goddard Space Flight Center.
Acknowledgments. We are grateful to three anonymous reviewers for their feedback and guidance. Their input has greatly improved this manuscript. We thank Alex Haumann for helpful comments on an earlier draft. E. Doddridge acknowledges support from the NSF’s Antarctic program. J. Marshall acknowledges support from the MIT-GISS collaborative agreement, the NASA Physical Oceanography Program, and the NSF Polar Antarctic Program. H. Song is supported by Yonsei University Research Fund (2018-22-0053) and National Research Foundation of Korea (NRF) grant funded by Korea government (MSIST) (NRF-2019R1C1C1003663).
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All Science Journal Classification (ASJC) codes
- Atmospheric Science