The ocean mixed layer under Southern Ocean sea-ice: Seasonal cycle and forcing

Type Article
Date 2017-02
Language English
Author(s) Pellichero Violaine1, Sallee Jean-Baptiste1, Schmidtko Sunke2, Roquet Fabien3, Charrassin Jean-Benoit1
Affiliation(s) 1 : UPMC Univ Paris 06, Sorbonne Univ, UMR 7159, LOCEAN,IPSL, F-75005 Paris, France.
2 : GEOMAR Helmholtz Ctr Ocean Res Kiel, Kiel, Germany.
3 : Stockholm Univ, Dept Meteorol, Stockholm, Sweden.
Source Journal Of Geophysical Research-oceans (2169-9275) (Amer Geophysical Union), 2017-02 , Vol. 122 , N. 2 , P. 1608-1633
DOI 10.1002/2016JC011970
WOS© Times Cited 87
Keyword(s) Southern Ocean, mixed layer, sea-ice, elephant seals, salt budget, heat budget
Abstract The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea-ice plays a key role for water mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal-derived, ship-based, and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large-scale water mass formation and transformation with far reaching consequences for ocean ventilation.
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