Submesoscale processes observed by high-frequency float in the western South China sea

Abstract

Using in situ high-frequency data obtained from a new type of float (Navis-SL1) with a thermal engine, this study documents that steep isopycnals result in submesoscale processes during a strong wind event in the western South China Sea. The mixing induced by strong wind promoted the appearance of vertical isopycnals and weakened stratification in the mixed layer. The background isopycnals did not have a traditional frontal structure with a single inclined direction (i.e., a cold dense filament configuration). Submesoscale characteristics induced by the mixed-layer instability (MLI) were found in the mixed layer. By using the parameterization of streamfunction of MLI, two ageostrophic overturning circulation cells formed. In the center of the two cells, the vertical velocity was downward, while it was upwelling on both sides. The horizontal velocity diagnosis showed convergence/divergence at the top/bottom of the mixed layer. The vertical equivalent heat flux induced by MLI was compared with the daily averaged surface net heat flux and was found to have an important impact on the buoyancy budget in the mixed layer.