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Published July 12, 2022 | Submitted
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Mixed layer depth seasonality modulates summertime SST variability in the Southern Ocean


In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss---a stark reversal of sea ice expansion and surface cooling trends that prevailed over preceding decades. The most dramatic changes occurred in the austral spring of 2016 when Antarctic sea-ice extent (SIE) reached a record minimum as sea surface temperatures (SST) climbed to a near-record high. In late 2019, another circumpolar surface warming event spanned the Southern Ocean, albeit with no appreciable decline in Antarctic SIE. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. The latter effect amplified the surface warming effect of air-sea heat fluxes during months of peak solar insolation. More generally, summertime SST across the Southern Ocean is sensitive to the timing of the springtime shoaling of the mixed layer, which is controlled by the strength and temporal variance of the circumpolar westerlies. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annual Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. Thus, future Southern Ocean surface warming extremes will depend on the evolution of regional mixed layer depths and interannual SAM variability.

Additional Information

License: Attribution-NonCommercial-NoDerivatives 4.0 International. E.A.W. acknowledges support from Caltech's Terrestrial Hazard Observations and Reporting Center. D.B.B. was supported by the National Science Foundation Graduate Research Fellowship Program (NSF Grant DGE-1745301). A.F.T. received support from NSF award OCE-1756956 and the Internal Research and Technology Development program (Earth 2050), Jet Propulsion Laboratory, California Institute of Technology. E.A.W. and S.C.R. received support through the SOCCOM Project, funded by the National Science Foundation, Division of Polar Programs (NSF PLR-1425989 and OPP-1936222). E.A.W. and S.C.R. also received funding from NOAA as part of the US Argo Program via grant NA20OAR4320271 to the University of Washington. Data availability statement. All data and reanalysis products used in this study are sourced from publicly accessible repositories. NOAA Optimum Interpolation SST V2 data were retrieved from https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.html. The Roemmich-Gilson Argo product was downloaded from https://sio-argo.ucsd.edu/RG_Climatology.html. ERA5 reanalysis can be accessed at https:doi.org/10.24381/cds.f17050d7. Model output from the CESM1-LE can be downloaded from https://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html. NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration (Version 4) can be accessed at https://doi.org/10.7265/efmz-2t65z. Python code for carrying out analysis and generating figures is available athttps://doi.org/10.5281/zenodo.6588645.

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August 22, 2023
October 23, 2023