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Mixed layer depth seasonality modulates summertime SST variability in the Southern Ocean

Wilson, Earle A. and Bonan, David B. and Thompson, Andrew F. and Armstrong, Natalie and Riser, Stephen C. (2022) Mixed layer depth seasonality modulates summertime SST variability in the Southern Ocean. . (Unpublished)

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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.

Item Type:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription Paper ItemNOAA Optimum Interpolation SST V2 data ItemRoemmich-Gilson Argo product reanalysis ItemModel output from the CESM1-LE Climate Data Record of Passive Microwave Sea Ice Concentration (Version 4) code for carrying out analysis and generating figures
Wilson, Earle A.0000-0003-2329-5115
Bonan, David B.0000-0003-3867-6009
Thompson, Andrew F.0000-0003-0322-4811
Riser, Stephen C.0000-0002-8593-312X
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 The Roemmich-Gilson Argo product was downloaded from ERA5 reanalysis can be accessed at Model output from the CESM1-LE can be downloaded from NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration (Version 4) can be accessed at Python code for carrying out analysis and generating figures is available at
Funding AgencyGrant Number
Caltech Terrestrial Hazard Observation and Reporting (THOR) CenterUNSPECIFIED
NSF Graduate Research FellowshipDGE-1745301
JPL Research and Technology Development FundEarth 2050
National Oceanic and Atmospheric Administration (NOAA)NA20OAR4320271
Record Number:CaltechAUTHORS:20220711-347656000
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115452
Deposited By: George Porter
Deposited On:12 Jul 2022 14:46
Last Modified:12 Jul 2022 14:46

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