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Published June 26, 2023 | Published
Journal Article Open

The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities


Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1–100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Additional Information

© 2023 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. Discussion meeting issue: 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities' organised and edited by Andrew J. S. Meijers, Corinne Le Quéré CBE FRS, Pedro M. S. Monteiro, Jean-Baptiste Sallée. We would like to acknowledge the enormous effort it takes for those at the field sites to deploy, recover and maintain instrumentation and platforms at sea. We also thank the many collective efforts in piloting autonomous ocean platforms during the experiments, many of which last many months at a time. ERA5 data are generated using Copernicus Climate Change Service Information, available online (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5). The authors thank two reviewers, Baylor Fox-Kemper and one anonymous reviewer, for helpful comments to the paper. S.S. is supported by a Wallenberg Academy Fellowship (WAF 2015.0186) and the Swedish Research Council (VR 2019-04400). S.S. and S.-A.N. have been supported by STINT-NRF Mobility Grant and NRF-SANAP grant nos (SNA170522231782, SANAP200324510487). S.S., S.-A.N. and M.D.d.P. have received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 821001 (SO-CHIC). M.D.d.P. is supported by the European Union's Marie Sklodowska Curie Individual Fellowship under Project ID 101032683. K.J.H. is supported by the European Research Council (under H2020-EU.1.1.) through research grant COMPASS (Climate-relevant Ocean Measurements and Processes on the Antarctic continental Shelf and Slope, grant agreement no. 741120). LCB was supported by the Swedish Research Council (VR grant 2020-04281). ST acknowledges institutional support from the CSIR Parliamentary Grant (0000005278, SNA2011112600001) and the Department of Science and Innovation. Authors' contributions. S.S.: conceptualization, funding acquisition, investigation, project administration, resources, writing—original draft, writing—review and editing; M.D.d.P.: visualization, writing—original draft; S.-A.N.: investigation, writing—original draft; P.M.S.M.: investigation, writing—original draft; L.A.D.: writing—original draft; S.T.: writing—original draft; A.F.T.: writing—original draft; L.C.B.: writing—original draft; J.M.E.: visualization, writing—original draft; I.G.: writing—original draft; K.J.H.: writing—original draft; C.L.: writing—original draft; A.M.: writing—original draft; G.S.: data curation, software; R.D.d.S.: writing—original draft. All authors gave final approval for publication and agreed to be held accountable for the work performed therein. Data accessibility. This article has no additional data. We declare we have no competing interests.

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August 22, 2023
August 22, 2023