Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

International audienceDuring the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500minthe northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedlysampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below themixed layer was regularly observed testifying of important frontal vertical movements. PotentialVorticity dynamics was diagnosed using glider observations and a high resolution realistic modelat 1-km resolution. During down-front wind events in winter, remarkable layers of negative PVwere observed in the upper 100m on the dense side of fronts surrounding the deep convection areaand successfully reproduced by the numerical model. Under such conditions, symmetric instabilitycan grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells.Two important hotpspots for the destruction of PV along the topographically-steered NorthernCurrent undergoing frequent down-front winds have been identified in the western part of Gulf ofLion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter inthe model, whereas localized instability events were found in the open sea, mostly influenced bymesoscale variability. The associated vertical circulations also had an important signature on oxygenand fluorescence, highlighting their under important role for the ventilation of intermediate layers,phytoplankton growth and carbon export