Submesoscale In-Situ Observations Across the ACC during the Formation of a Mesoscale Eddy during SMILES

We present here observations from the Surface Mixed Layer Evolution at Submesoscales (SMILES) cruise conducted in May 2015 that constitute the first in-situ, submesoscale-resolving measurements of the formation and evolution of a closed core eddy within the ACC. Our observations, primarily consisting of towed CTD (Seasoar), vessel-mounted ADCP, and drogue drifters, concentrated on a prominent northward meander of the ACC within the strongly strained frontal region where the Subantarctic and Polar fronts converge to the east of Drake Passage. Our strategy consisted of following a drifter triplet in a cross-frontal pattern whilst towing Seasoar down to 200-m water depth, yielding a horizontal resolution of 2 km. Repeated sections of water velocity, temperature, conductivity, and dissolved oxygen data, are presented and analysed as a Lagrangian evolution of the front. The Seasoar survey coincided precisely with the moment that the meander separated from the ACC and formed a closed, cold-core mesoscale eddy. The ship-based measurements encompassed the entire eddy and thus provide insight into the variations in the strain and shear fields in different sectors of the eddy and their role in instigating the evolution from a meander to a closed core eddy. The resolution of in-situ observations and the perturbations to the mean drifter trajectories reveal the presence of submesoscale variability and subsurface intrusions across the front. We further highlight the close correspondence of the drifter trajectories during the 20 days and 4 circulations of eddy. Drifters were ejected from the eddy in a cold-water streamer which is visible in a rare cloud-free SST image. Streamlines derived from geostrophic surface currents computed from sea surface height anomaly agree well with observed drifter trajectories.