Observations of Submesoscale Processes in an Anticyclonic Eddy in the South China Sea

Vertical velocities induced by submesoscale dynamics play a vital role in the oceanic exchanges of tracers within and across the mixed layer, and may constitute an important mechanism for the supply of nutrients to the eutrophic layer. Submesoscale-permitting numerical simulations of the South China Sea (SCS) and Gulf of Mexico suggest that intense ageostrophic circulations are commonly found at the periphery of anticyclonic mesoscale eddies. Here we constrain these model findings and we characterize the submesoscale structure of an anticyclonic eddy observed in the northern SCS using high-frequency sampled ADCP data and CTD profiles. The data resolution is between 1' and 4'. The relative vorticity and the strain field are both enhanced at the periphery of the eddy due to strong lateral velocity shear. The submesoscale ageostrophic circulations that develop in response to the strain-induced divergence contribute to the restratification in the mixed layer and to the vertical pump, particularly at the periphery of the eddy. The observations agree very well with simulations at 1km horizontal resolution. The impact of the submesoscale processes on vertical exchanges in and around anticyclonic mesoscale eddies is quantified by comparing numerical experiments at 1km and at 5 km resolutions in which Lagrangian passive tracers are deployed. This study provides, for the first time, observational evidence for the impact of the submesoscale processes on the oceanic vertical pump and mixed layer restratification associated with mesoscale eddies.