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Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning

Nature Geoscience volume 9, pages 596–601 (2016)Cite this article

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Abstract

Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing1,2,3, where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch4,5,6. While brine rejection from sea ice is thought to contribute to the lower branch7, the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport8,9. Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework10 to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ∼22 × 106 m3 s−1. These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.

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Figure 1: Surface freshwater exchange in the SOSE.
Figure 2: Components of water-mass transformation and formation in SOSE in neutral density (γn) coordinates.
Figure 3: Time series of freshwater transformation over the six-year state estimate, as a function of time (x axis) and neutral density γn (y axis).

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Acknowledgements

R.P.A. acknowledges support from NSF grant OCE-1357133. I.C., L.D.T. and M.M. acknowledge support from NSF grant OCE-1357072. L.D.T. and M.M. acknowledge additional support from NSF grant PLR-1425989, and M.M. from OCE-1234473. I.C. acknowledges additional support from the New International Fellowship Mobility Programme for Experienced Researchers—NEWFELPRO under the European Union’s Seventh Framework Programme for research, technological development and demonstration (FP7-PEOPLE-2011-COFUND-MCA). We acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation, and from the Yeti HPC cluster, provided by Columbia University Research Computing Services. SOSE was produced with support from NSF-XSEDE grant OCE130007.

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Authors and Affiliations

  1. Lamont Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA

    Ryan P. Abernathey

  2. Scripps Institution of Oceanography, La Jolla, California 92093, USA

    Ivana Cerovecki, Matt Mazloff & Lynne D. Talley

  3. British Antarctic Survey, Cambridge CB3 0ET, UK

    Paul R. Holland

  4. University of Washington, Seattle, Washington 98195, USA

    Emily Newsom

Authors
  1. Ryan P. Abernathey
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Contributions

All authors contributed to the interpretation of the results and writing of the manuscript. R.P.A. designed the study, implemented the water-mass calculations, and conducted the validation against ship-based and sea-ice observations. I.C. contributed to the water-mass analysis and validation of SOSE against ARGO observations. P.R.H. contributed to the analysis of sea-ice thermodynamics. E.N. contributed to the water-mass analysis. M.M. developed SOSE and contributed to model validation. L.D.T. contributed to interpretation of the transformation rates.

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Correspondence to Ryan P. Abernathey.

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Abernathey, R., Cerovecki, I., Holland, P. et al. Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning. Nature Geosci 9, 596–601 (2016). https://doi.org/10.1038/ngeo2749

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