Impact of the ocean-atmosphere coupling on Mediterranean cyclones

The Mediterranean basin is well recognized as one of the main climate change hotspots; besides, this region is one of the most active cyclogenetic area of the Northern Hemisphere with a large number of intense cyclones occurring every year. Intense Mediterranean cyclones are often responsible for extreme precipitation and strong wind events leading to severe socio-economic and environmental impacts especially over densely populated coastal areas. Complex feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the variability in and extremes of the regional climate system.

This study aims to investigate the impact of the ocean-atmosphere coupling on the regional climate during intense Mediterranean cyclones. To this end, two simulations are performed using the ENEA-REG regional earth system model at 12 km atmospheric horizontal resolution over the Med-CORDEX domain, both driven by ERA5 reanalysis. The first experiment uses the mesoscale WRF model with prescribed ERA5 Sea Surface Temperature (SST), while the second is coupled to the MITgcm ocean model at horizontal resolution of 1/12°. Cyclones are tracked by applying a Lagrangian algorithm to the mean sea level pressure field. The 500 most intense cyclones mainly occur in winter over the Thyrrenian, Adriatic, Ionian and Aegean Sea. They are similarly reproduced between WRFs and ERA5 in terms of seasonal and spatial distribution, due to the same large-scale atmospheric conditions. The coupled simulation is compared with the standalone WRF in terms of sub-daily fields, such as evaporation, precipitation and wind speed, during the mature stage of the cyclones. The different SST distribution between the models appears to be the main controlling factor for the differences in the atmospheric properties affecting not only the surface, but also the entire atmospheric boundary layer (ABL) and its height, due to the mixing of the turbulent processes, enhanced during intense cyclones. A statistically significant higher specific humidity and wind speed are found in the coupled model from the surface to the top of the ABL, as well as higher precipitation over sea and coastal areas. These results are consequences of higher turbulent heat and moisture fluxes in the coupled model that destabilize the ABL and provide higher moisture content available for convection.

We conclude that the use of the coupled model is crucial for a more realistic representation of the energy redistribution in both the ocean mixed layer and the ABL during intense Mediterranean cyclones. This highlights the importance of the coupled model to study the influence of climate change on intense Mediterranean cyclones and associated impacts under different future scenarios.