Lagrangian pathways of upwelling in the Southern Ocean

The Southern Ocean is one of the key regions in which surface waters are subducted and deep waters ventilated. Gebbie and Huybers (2011) estimated that 15% of the surface area of the ocean, clustered around the Southern Ocean and the North Atlantic, is responsible for filling 85% of its interior volume. Once surface waters are brought down into the deep ocean, their properties are highly conserved, since they are no longer subject to direct forcing. Thus, a thorough understanding of the mechanisms and time scales of subduction and upwelling is critical to modeling the transport of dissolved gases and nutrients on a global scale. Of particular importance is the uptake of carbon dioxide by the ocean. The ocean is able to mitigate the effects of climate change by absorbing 25-30% of anthropogenically-released carbon dioxide annually. With more than 40% of this carbon entering the ocean south of 40°S (Sallée et al., 2012), it is clear that understanding the pathways of subduction in the Southern Ocean is key to modeling future climate change.

In this study, we focused on identifying the sources of upwelling waters and the pathways connecting subduction and upwelling sites by simulating the advection of Lagrangian drifters in an eddy-resolving ocean model. Deployments were run on three different density classes (σ = 27.0, 27.1, 27.2) in multiple locations, with a particular focus on regions near topographic features. We observed a strong influence of bathymetry on isopycnal upwelling. There is also significant seasonal dependence of the mixed layer depth, affecting which isopycnals outcrop at the surface, which impacts mixed layer residence times and ventilation rates. In addition to the seasonal cycle, there is inter-annual variation of the mixed layer depth. From this work, we have begun to identify localized regions of upwelling around the ACC. Future work will introduce drifters with time-dependent tracer concentrations such as carbon content or 13C values. By allowing these values to equilibrate with the atmosphere while they are in the mixed layer, we can begin to provide insight on the storage of carbon in the deep ocean and ocean/atmosphere disequilibrium and how these may evolve over time.