fluids Article 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 2,5 , Louis Prieur 6 and Vincent Taillandier 6   Citation: Bosse, A.; Testor, P.; Damien, P.; Estournel, C.; Marsaleix, P.; Mortier, L.; Prieur, L.; Taillandier, V. Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea. Fluids 2021, 6, 123. https://doi.org/10.3390/ fluids6030123 Academic Editors: Jonathan Gula, James C. McWilliams and Leif N. Thomas Received: 19 January 2021 Accepted: 11 March 2021 Published: 17 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Mediterranean Institute of Oceanography, Aix Marseille University, Université de Toulon, CNRS, IRD, MIO UM 110, F-13288 Marseille, France 2 Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques, Sorbonne Universités, UPMC Université Paris 06, IPSL, CNRS, IRD, MNHN, LOCEAN UMR 182, F-75252 Paris, France; testor@locean-ipsl.upmc.fr (P.T.); laurent.mortier@ensta-paristech.fr (L.M.) 3 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90025, USA; pdamien@ucla.edu 4 Laboratoire d’Études en Géophysique et Océanographie Spatiales, CNRS, CNES, IRD, Université de Toulouse, LEGOS UMR 5566, F-31400 Toulouse, France; claude.estournel@legos.obs-mip.fr (C.E.); patrick.marsaleix@legos.obs-mip.fr (P.M.) 5 ENSTA-Paristech, F-91120 Palaiseau, France 6 Laboratoire d’Océanographie de Villefranche, Sorbonne Universités, CNRS, IMEV-LOV UMR 7093, F-06230 Villefranche-sur-Mer, France; prieur@obs-vlfr.fr (L.P.); taillandier@obs-vlfr.fr (V.T.) * Correspondence: anthony.bosse@mio.osupytheas.fr Abstract: During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in 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 100 m 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. Keywords: deep convection; submesoscale dynamics; frontal instability; subduction 1. Introduction The Gulf of Lion in the northwestern Mediterranean Sea (NW Mediterranean) is one of the few oceanic regions where intense atmospheric forcing and oceanic preconditionning allow vertical mixing to reach large depths (>500 m to the seafloor at 2500 m [1]). This phenomenon, known as “open-ocean deep convection” (see Marshall and Schott [2] for a review), is a key process of the ocean thermohaline circulation. Deep convection is characterized by localized and intense diapycnal mixing events [3] important for the ocean ventilation [4–6], subsequent intense phytoplankton spring blooms [7–9] and carbon sequestration [10]. In accordance with idealized results [11], the net downwelling induced by deep convection however occurs along the boundary circulation where a large amount Fluids 2021, 6, 123. https://doi.org/10.3390/fluids6030123 https://www.mdpi.com/journal/fluids