Meridional contrasts of the Southern Ocean mixed layer in response to summer forcing

Marcel du Plessis, University of Cape Town, Department of Oceanography, Cape Town, South Africa, Sebastiaan Swart, University of Gothenburg, Department of Marine Sciences, Gothenburg, Sweden, Andrew F Thompson, California Institute of Technology, Pasadena, United States, Pedro M. S. Monteiro, CSIR, Cape Town, South Africa, Louise C Biddle, University of Gothenburg, Department of Marine Science, Gothenburg, Sweden and Sarah-Anne Nicholson, Council for Scientific and Industrial Research, Cape Town, South Africa
Abstract:
The mixed layer provides a gateway for the exchange of heat, freshwater and momentum fluxes between the ocean and the atmosphere, where ocean properties are set prior to being subducted into the deep ocean. As around 75% of the global ocean was last in contact with the atmosphere in the Southern Ocean, air-sea interactions that set the mixed layer depth and its variability are key for water mass transformation and the long term trends in the carbon cycle. However, the response of the seasonal mean state of mixed layer, which sets the water mass characteristics, to forcing at synoptic time scales, is still not well known. An additional challenge is that the Southern Ocean is divided into different dynamical regimes that have contrasting physical properties and experience different atmospheric forcing. Here we analyze three-months of summer high-resolution glider observations along the GoodHope Line from three dynamical zones of the Southern Ocean (Subtropical, Antarctic Polar and the Seasonal Ice Zone). Each zone experiences different atmospheric heat, freshwater and momentum forcing which influences the seasonal progression of the mixed layer buoyancy. This allows us to examine the response of buoyancy forcing to the varying mixed layer heat and freshwater budgets and subsequently contrast them to the evolution of upper ocean stability. Distinct differences are observed in both the mean properties and the variability of lateral and vertical gradients across zones. Stronger mesoscale stirring in the SAZ enhances lateral density gradients in the mixed layer and thus supports a more active submesoscale field. However, the vertical stratification at the base of the mixed layer is nearly an order of magnitude weaker in the APZ as compared to regions to the north and south. The impact of this spatial structure on ventilation properties will be examined.