The circulation of the Southern Ocean is unique due to the lack of meridional boundaries at the latitudes of Drake Passage. Westerly winds drive the Antarctic Circumpolar Current (ACC), linking the major ocean basins and facilitating inter-basin exchange of properties. Additionally, the steeply tilted isopycnals in the Southern Ocean allow interaction between the deep ocean and the atmosphere, and as a result the Southern Ocean has an outsized contribution to the global uptake and redistribution of heat, carbon and nutrients. Complex topography and eddies make this circulation fundamentally three-dimensional, but many features and associated mechanisms of this three-dimensional circulation are not well understood.

The objective of this thesis is to use the 1/6°, data-assimilating Southern Ocean State Estimate (SOSE), along with other high-resolution ocean models and available observations, to describe aspects of the three-dimensional structure of the upper cell of the Southern Ocean overturning circulation. First, we diagnose the upper ocean heat budget in the Southern Ocean (Chapter 2), and determine that a strong zonal asymmetry in the air-sea heat flux over the Southern Ocean is associated with large-scale meander the ACC mean path and associated asymmetry in geostrophic heat advection. Second, we use Lagrangian particle release experiments to show, for the first time, the full three-dimensional upwelling pathways of deep water from 30°S to the surface of the Southern Ocean (Chapter 3). We find that deep water moves south in narrow paths along the western and eastern boundaries of each ocean basin, then within the ACC upwelling is concentrated at hotspots associated with high eddy activity at major topographic features. Next, we quantify the water mass transformation along the upwelling pathways from Chapter 3, and find that although the upwelling in the ocean interior is largely along isopycnals, there is significant transformation just below the mixed layer and homogenization of deep water mass properties due to isopycnal mixing (Chapter 4). Finally, we highlight a newly identified poleward pathway of deep water along the eastern boundary of the Indian Ocean and describe the structure and variability of this pathway (Chapter 5).