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

On 1 October 2020, the intense extra-tropical storm Alex impacted the north-west coast of France, producing unusually strong wind gusts for the season. On 2 October, the storm triggered record-breaking rainfall over the south-eastern French Alps and north-western Italian Alps. In France, this Heavy Precipitation Event (HPE) caused severe flooding and land­slides, resulting in casualties, and over 1 billion euros in economic losses.

We used convection-permitting regional climate modeling with a spa­tial resolution of 2.5 km to investigate these observed events. Simulations were conducted over September-October 2020 on an extensive domain centered on France. Our model successfully reproduces the characteristics of both the HPE and storm Alex, including the observed sequence of events and impacts (Bador et al., 2025).

We then explored how the observed 2020 Mediterranean HPE could have been differ­ent had it occurred 2 years later, in 2022, where warmer sea surface was recorded in the western Mediterranean Sea. This storyline analysis suggested reduced precipitation impacts over the south-eastern French Alps but enhanced impacts in Italy. Additional sensitivity experiments confirmed the key role of regional sea surface temperatures (SSTs) in shaping the HPE’s intensity in the western Alps, with an eastward shift of heavy precipitation with higher Mediterranean SSTs. Our simulations consistently show that sea surface warming can further intensify the Mediterranean HPE, while cooling reduces the intensity of extreme precipitation and local impacts. In contrast, modifications to the Atlantic SSTs affecting storm Alex itself have a limited influence on the regional Mediterranean circulation and the HPE.

All simulations were performed using initial-condition large ensembles to assess the role of internal variability in shaping local extremes. We highlighted variations among ensemble members in both local rainfall extremes and in gustiness. As impact sectors increasingly rely on km-scale climate modelling to inform local climate change assessments, our results underscore the importance of the ensemble-based approaches to fully capture the range of possible outcomes for extreme events locally.