Understanding future projections for a high-impact Mediterranean storm track

Mediterranean cyclones often cause severe damage due to extreme winds and intense precipitation. Two notable examples are the November 1966 storm that flooded Florence and Storm Adrian in October 2018. These cyclones caused significant damage in central-northern Italy and share common characteristics in their mature stages, including a south-to-north trajectory, rapid intensification, and the presence of a deep upper-level precursor trough.

In this study, we identify cyclones with similar dynamics using reanalysis data (ERA5) and an ensemble of EC-Earth3 coupled climate simulations of present and future climate conditions. Then we apply a storlyine approach, whereby the probability of an extreme precipitation event over Italy is decomposed into the product of three terms: the probability of the occurrence of the large-scale trough precursor, the probability of cyclogenesis resembling the Adrian/1966 track given the large-scale trough, and the probability of extreme precipitation events in Italy, given an Adrian-like cyclone track.  We first compare reanalysis data with model outputs to assess the representation of such cyclones in the EC-Earth3 historical ensemble. Next, we extend our analysis to EC-Earth future projections in the SSP5-8.5 scenario. We evaluate the probability of the three terms composing the storyline in both the historical and future simulations, and then we estimate the risk ratio, which quantifies how each component has contributed to the changes in the probability of this class of precipitation extremes. 

Preliminary results show that EC-Earth3 has a satisfactory representation of the North Atlantic and Mediterranean storm-tracks in Autumn, although it seems unable to simulate storms with a trajectory and intensity matching storm Adrian. Our aim is now to identify the probability of such events in the EC-Earth3 ensemble, and to decompose the probability in the product of the three terms. Based on previous literature, a possibile outcome is that while the probability of the large scale trough occurrence, and of a developing cyclone decreases under climate change - due to large scale circulation changes - the probability of high-impact precipitation increases.  Our analysis will enable to understand the relative importance of these processes in triggering precipitation extremes via this Mediterranean storm-track. Future applications of this approach to different climate models will further enable to explore uncertainties and to develop physical climate storylines based on plausible future changes in large-scale atmospheric dynamics, Mediterranean cyclone development and moisture environment.