Future changes in Mediterranean Heavy Precipitation Events : weather regime frequency and intensity drivers

The Mediterranean region is particularly sensitive to extreme precipitation, with Heavy Precipitation Events (HPEs) predominantly occurring in autumn. These events are typically associated with organised, often quasi-stationary mesoscale convective systems that can produce over 100 mm of rainfall in 24 hours, or even in a few hours. This can lead to major damage to infrastructure and loss of life.

Global climate models (GCMs) show uncertainty regarding the future evolution of Mediterranean HPEs. This uncertainty is primarily driven by inter-model differences in projected large-scale atmospheric circulation, which control the occurrence of weather regimes associated with extreme precipitation. Beyond changes in weather regime occurrence, for a given weather regime, uncertainties exist regarding the role of remote climate drivers, such as sea surface temperature or specific humidity anomalies, in influencing the intensity of Mediterranean HPEs and their future evolution.

In this study, we assess how projected changes in Mediterranean HPEs during autumn can be explained by future changes in the occurrence of weather regimes identified as favourable to HPEs. Using ERA5 reanalysis, we identify four weather regimes that favour the occurrence of Mediterranean HPEs. Analyses based on a CMIP6 multi-model ensemble indicate that three of these four HPE-favourable weather regimes are projected to become less frequent towards the end of the century.

Beyond changes in weather regime frequency, we investigate the role of local and remote climate drivers in explaining the spread in GCM projections of future changes in Mediterranean HPEs within a given weather regime. To provide a physical basis for interpreting this dispersion, we first identify the factors that control the intensity of HPEs in the two weather regimes most favourable to HPEs. Based on ERA5, in the most HPE-favourable weather regime, HPEs intensity is controlled by local Mediterranean moisture availability and upper-level circulation over western Europe. Additional remote influences are associated with Atlantic moisture anomalies and Caribbean sea surface temperature anomalies preceding the events. In the second weather regime, HPEs intensity is primarily driven by local Mediterranean moisture conditions, with remote influences are mainly associated with enhanced moisture over North Africa prior to HPEs occurrence.

These results reveal weather-regime-dependent differences in the role of local and remote drivers controlling the intensity of Mediterranean HPEs. This framework can be used to interpret future changes and uncertainties in GCMs projections, and provides a basis for future storyline-based analyses of Mediterranean HPEs.