Submesoscale variability in the recently melted Antarctic Seasonal Ice Zone

The ocean surface boundary layer in the Southern Ocean plays a critical role in heat and carbon exchange with the atmosphere. Submesoscale flows have been found to be important in setting mixed layer variability in the Antarctic Circumpolar Current (ACC). However, sparsity in observations, particularly south of the ACC in the Antarctic Seasonal Ice Zone (SIZ) where the horizontal density structure of the mixed layer is influenced by sea ice melt/formation and mesoscale stirring, brings into question the ability of climate models to correctly resolve mixed layer variability. We present three months of novel fine-scale observations of the activity of submesoscale variability in the ice-free Antarctic SIZ using underwater gliders. Salinity-dominated density fronts of O(1)km are observed less than a week after the winter ice melts, associated with strong buoyancy gradients. While we do not observe strong restratification fluxes by mixed layer eddies, intermittent wind events associated with Ekman buoyancy fluxes show equivalent heat fluxes reaching up to 600 W/m2, even under low wind strength conditions (Wind stress ~0.1N/m2) indicating that variability at submesoscales is impacting the mixed layer. The continuous spring-summer glider time series shows a gradual mixed layer deepening (from ~40m to 60m) together with decreasing horizontal buoyancy gradients, suggesting a seasonal cycle in submesoscale flows. We postulate the underlying mechanisms that may be at play (e.g. vertical entrainment and lateral stirring) in setting the observed surface layer properties.