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

Anthony Bosse (1) , Pierre Testor (2) , Pierre Damien (3) , Claude Estournel (4) , Patrick Marsaleix (4) , Laurent Mortier (5, 6) , Louis Prieur (7) , Vincent Taillandier (7)

1 MIO - Institut méditerranéen d'océanologie
2 VOG - Variabilité de l'Océan et de la Glace de mer
3 Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA
4 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
5 ENSTA Paris - École Nationale Supérieure de Techniques Avancées
6 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
7 LOV - Laboratoire d'océanographie de Villefranche

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500min the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can 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 Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

Mots clés: submesoscale dynamics; frontal instability; subduction

Domaines: Océan, Atmosphère

Anthony Bosse, Pierre Testor, Pierre Damien, Claude Estournel, Patrick Marsaleix, et al.. Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea. Fluids, 2021, 6 (3), pp.123. ⟨10.3390/fluids6030123⟩. ⟨hal-03199242⟩