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Zonally asymmetric response of the Southern Ocean mixed-layer depth to the Southern Annular Mode

Abstract

Interactions between the atmosphere and ocean are mediated by the mixed layer at the ocean surface. The depth of this layer is determined by wind forcing and heating from the atmosphere. Variations in mixed-layer depth affect the rate of exchange between the atmosphere and deeper ocean, the capacity of the ocean to store heat and carbon and the availability of light and nutrients to support the growth of phytoplankton. However, the response of the Southern Ocean mixed layer to changes in the atmosphere is not well known. Here we analyse temperature and salinity data from Argo profiling floats to show that the Southern Annular Mode (SAM), the dominant mode of atmospheric variability in the Southern Hemisphere, leads to large-scale anomalies in mixed-layer depth that are zonally asymmetric. From a simple heat budget of the mixed layer we conclude that meridional winds associated with departures of the SAM from zonal symmetry cause anomalies in heat flux that can, in turn, explain the observed changes of mixed-layer depth and sea surface temperature. Our results suggest that changes in the SAM, including recent and projected trends attributed to human activity, drive variations in Southern Ocean mixed-layer depth, with consequences for air–sea exchange, ocean sequestration of heat and carbon, and biological productivity.

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Figure 1: Seasonal cycle of MLD.
Figure 2: MLD anomaly associated with the SAM.
Figure 3: Mixed-layer heat forcing associated with the SAM.
Figure 4: Observed and expected MLD and temperature anomaly during a SAM event.
Figure 5: Averaged summer chlorophyll-concentration response to MLD anomaly.

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Acknowledgements

We acknowledge the role of Jeff Dunn in developing and making available the loess fitting method. K.G.S. received support from NSF OCE-0822075, OCE-0612167 and OCE-0622670. J.B.S. was supported by a CSIRO Office of the Chief Executive (OCE) postdoctoral fellowship. S.R.R. was supported by the Australian Government’s Cooperative Research Centres Programme through the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE-CRC). J.B.S. and S.R.R. were also supported by the CSIRO Wealth from Oceans National Research Flagship. This study has benefited from discussions with R. Matear, A. Lenton, N. Cassar and P. Boyd.

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J.B.S. directed the analysis of the several datasets used in this study and shared responsibility for writing the manuscript. K.G.S. and S.R.R. participated in the data analysis and shared responsibility for writing the manuscript. All authors contributed to the final version of the manuscript.

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Correspondence to J. B. Sallée.

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Sallée, J., Speer, K. & Rintoul, S. Zonally asymmetric response of the Southern Ocean mixed-layer depth to the Southern Annular Mode. Nature Geosci 3, 273–279 (2010). https://doi.org/10.1038/ngeo812

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