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

TitleWater-mass transformation by sea ice in the upper branch of the Southern Ocean overturning
Publication TypeJournal Article
Year of Publication2016
AuthorsAbernathey RP, Cerovecki I, Holland P.R, Newsom E., Mazlo M., Talley LD
JournalNature Geoscience
Date Published2016/08
Type of ArticleArticle
ISBN Number1752-0894
Accession NumberWOS:000382137900015
Keywordscirculation; closure; driven; enhancement; evolution; flux; mixed-layer; model; transport; ventilation

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 mixing(1-3), where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch(4-6). While brine rejection from sea ice is thought to contribute to the lower branch(7), 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 transport(8,9). 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 framework(10) 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 similar to 22 x 10(6) m(3) 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.

Short TitleNat. Geosci.
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