Cooler than observed sea surface could have reduced impacts of storm Alex and induced mediterranean heavy precipitation event in France

This study utilizes convection-permitting regional climate modeling to analyze the intense Mediterranean Heavy Precipitation Event (HPE) in October 2020, which affected the south-eastern French and north-western Italian Alps following extra-tropical storm Alex. The CNRM-AROME model successfully reproduces the characteristics of both the HPE and storm Alex, including the observed sequence of events and impacts. Sensitivity experiments using initial-condition large ensembles are performed to evaluate the influence of sea surface temperatures (SSTs) on the HPE and to assess the role of internal variability in shaping extremes locally. Idealized, uniform 2 K warming and cooling patterns are applied to the SSTs over the Mediterranean Sea solely, and over the entire domain, while keeping all other forcings representative of observed conditions. We find that sea surface warming intensifies the Mediterranean HPE, while cooling reduces the intensity of extreme precipitation and local impacts. Our findings consistently show the key role of regional SSTs in shaping the HPE’s intensity in the western Alps, with an eastward shift of heavy precipitation with higher Mediterranean SSTs. Changes in wind convergence, advection of moister air, and increased instability upstream are important drivers of the displacement and intensification of extreme precipitation. By contrast, changes in storm Alex’s characteristics with higher and lower SSTs in the Atlantic have limited impact on the regional Mediterranean circulation and HPE. A storyline investigating the effects of storm Alex occurring two years later, in 2022, with warmer Mediterranean Sea surface suggests milder precipitation-related impacts in the French Alpes-Maritimes region but increased ones in Italy.