Evolution of the Drivers of Stratification of the Gulf of Lion in a Changing Climate

Abstract

Deep convection, or open-ocean convection, occurs in the higher latitude regions of the world and is an important ocean circulation process. It is formed when the stable density gradient along the ocean column is eroded by surface buoyancy loss, leading to an overturning that can span the entire depth of the column. In the western basin of the Mediterranean Sea (Med. Sea), this process can occur in the Gulf of Lion (GOL) and assists in the thermohaline circulation of the sea by forming the Western Mediterranean Deep Water (WMDW), which then laterally spreads throughout the western basin. Significant deep convection events occur every few years in the GOL, driven by the Mistral and Tramontane winds and the seasonal atmospheric change. However, in the future these events are expected to stop altogether due to increasing stratification. In this presentation, the changes in atmospheric forcing and stratification in the GOL are examined to determine the driving factor behind the collapse of the deep convection process in the region. This task is completed using NEMO simulations driven by the CMIP6 results of Météo France's RCSM6 regional model. The years from 2015 to 2100 are studied, under the SSP5 8.5 scenario (the worst case SSP scenario). Two sets of simulations are presented, a control and seasonal set. The seasonal set was forced with filtered atmospheric forcing to remove the effect of the Mistral and Tramontane. Comparing the two sets allows to determine the effects of the Mistral and atmospheric forcing separately. Results show an evolution in atmospheric forcing that effectively leads to no net change over time in the energy fluxes at the surface of the ocean. However, the stratification in the gulf increases, driven by advected stratification, with temperature as the primary advected quantity increasing stratification.