Regional mesoscale air–sea coupling impacts and extreme meteorological events role on the Mediterranean Sea water budget

The Mediterranean Sea water budget (MWB) is a key parameter, as it controls the Mediterranean Sea water loss and thus the Atlantic Water inflow and the Mediterranean general circulation. More accurately, the MWB controls the net flow through the Strait of Gibraltar, which implies both inflow and outflow. Generally considered at the basin scale and over long-term periods, the MWB is in fact characterized by a large variability in space and time, induced by the complex topography of the region, mesoscale processes and (short) intense events in the ocean and atmosphere compartments. In this study, we use an ocean-atmosphere coupled system at mesoscale able to represent such phenomena, to evaluate the MWB atmospheric components: Evaporation (E) and Precipitation (P). We compare two companion regional simulations: an uncoupled atmospheric run using the ERA-interim Sea Surface Temperature (SST) reanalysis and a coupled run using the MORCE system with the two-way coupling between the NEMO-MED12 eddy-resolving ocean model and the non-hydrostatic Weather Research and Forecasting atmospheric model. We first evaluate the SST validity against satellite data and evidence the coupled system ability in representing SST mesoscale structures, characteristics of the Mediterranean circulation and of small-scale ocean processes, despite a colder mean value and a lower amplitude of the annual cycle. Then, the comparison aims to examine the coupled processes effects (meaning the impacts of the interactive high-resolution and high-frequency SST) on E and P and on their variability. The comparison highlights that the SST is the controlling factor for E and P budgets, with reduction by 6 and 3 % in the coupled run compared to the uncoupled run, respectively. The modifications propagate until 750 km inland far from the Mediterranean coast, as towards the Atlantic Ocean and the Black Sea. This indicates that coupling plays a major role in distributing water at mesoscale. The coupling directly modifies the seasonal variability. It also significantly decreases extreme evaporation and precipitation occurences and intensities. This extreme events mitigation in the coupled run contributes for ~50 % to the decrease in the E and P budgets.