Dynamics of Southern Ocean Mixed Layers

Abstract

While it is often conceptualized in a spatially and/or temporally averaged sense, the mixed layer depth of the global ocean exhibits significant variability in both space and time. The mixed layer plays a key role in controlling the exchange of heat and gases between the atmosphere and the ocean interior; an inaccurate portrayal of mixed layer depths can be a major source of error in global climate models. In particular, the Southern Ocean, or the waters around Antarctica, take up a significant portion of anthropogenically released carbon dioxide and subduct it into the deep ocean, affecting global climate on both relatively short and glacial timescales. Variability in the mixed layer also affects the formation and subduction of mode waters, the partitioning of waters between the upper and lower overturning cells, and biological productivity. The stratification of the mixed layer is significantly modified by submesoscale dynamics, which are not resolved in current state-of-the-art climate models. The parameterization of these dynamics represents a large source of uncertainty, and better observations and a better understanding of the submesoscale can be used to improve climate predictions. In this work, the variability of Southern Ocean mixed layers is examined using both numerical and observational methods. General circulation model output is combined with a simple advection scheme to examine upwelling pathways, mixed layer residence times, and air-sea equilibrium in the Southern Ocean. Virtual Lagrangian drifters are released around the basin and tracked as they outcrop into the mixed layer, where they can exchange properties with the atmosphere. These studies are combined with high-resolution observations of mesoscale and submesoscale dynamics in the Southern Ocean, which play a leading order role in setting the stratification of the mixed layer. Seaglider data are used to construct potential vorticity fields, which are used to identify possible instances of different submesoscale instabilities in Drake Passage. Seasonal and zonal mixed layer variability are also examined using these observations. A second set of Seaglider observations are used to diagnose changes in ventilation and eddy stirring on sub-seasonal timescales at the Polar Front, one of the major fronts of the Southern Ocean. This thesis aims to expand current knowledge of mixed layer dynamics, especially at the submesoscale, and examine their implications for global circulation and climate.