Submesoscale dynamics at the Subantarctic Front in the Scotia Sea: Implications for vertical exchange and water mass transformation

We present new results from the Surface Mixed Layer Evolution at Submesoscales (SMILES) cruise conducted in the Scotia Sea during May 2015. We aimed to (1) measure with submesoscale-resolving resolution the evolution of two different sites within the Subantarctic Front (SAF) with varying lateral buoyancy gradients and strain rates and (2) assess implications for upper ocean vertical exchange and water mass transformation. The first site was a cold filament that extended westward following the northward deflection of the SAF east of the Falkland Islands. Repeated towed CTD sections across the front, which was confined to the surface mixed layer (SML), were made in a Lagrangian reference frame whilst following a drogued drifter. The front oscillated with a near-inertial frequency, repeatedly slumping and returning to the vertical as horizontal current velocities exhibited upward phase propagation consistent with a downward propagating near inertial wave. We discuss the interaction between the wave and front, drawing upon microstructure data acquired over an inertial cycle. The second site was a strongly strained frontal region to the south (57^oS) where the Polar and Subantarctic Fronts converge along the ACC. Intense subduction of surface waters to depths >400 m within a narrow region between the two fronts is observed in chlorophyll fluorescence and dissolved oxygen. Our observations focused on a northward meander in this frontal region and coincided with the precise moment that it evolved into a high chlorophyll, closed core eddy that we tracked post-cruise with drogued drifters and remote sensing for over 2 months. Using data from repeated Seasoar transects that sampled around the entire eddy and the trajectories of three drogued drifter triplets released in a cold water filament that demarcated the inner flank of the eddy periphery, we discuss the eddy generation mechanisms and role of submesoscale instabilities within the front that were apparent in the Seasoar transects and SST images.