Past, Present and Future precipitation extremes driven by Mediterranean cyclones in a regional large ensemble

Extreme event attribution aims to understand the role of climate change in impactful weather events. One method to achieve this consists in comparing two sets of weather instances selected as ‘circulation analogues’ of a given event, one representing the past and the other the present climate (or present and future). Differences between climate periods show how climate change affects the hazards associated with a specific circulation.
In this work, we focus on a selection of high-impact, cyclone-driven precipitation extremes that have affected Italy during the recent climatological period. These events are selected to span a range of distinct synoptic conditions known to produce extreme precipitation [1]. Using regional large-ensemble climate projections from the CRCM5 model under the RCP8.5 scenario, we identify, for each observed event, its best analogue in the model (BAM). The BAMs are then used to conduct attribution analyses within a perfect model approach, that is, by searching for model analogues of simulated rather than observed events.  In this framework, individual ensemble members are treated as pseudo-observations, allowing us to assess the robustness of analogue-based attribution studies based on single realization datasets, such as reanalyses or observational records. Furthermore, the use of a large ensemble enables a comprehensive assessment of present and future changes in cyclone-driven precipitation over the central Mediterranean region, capturing variations in the mean and tails of the precipitation distribution.
Our results provide new insights into the influence of anthropogenic climate change on cyclone-driven precipitation in Italy, and into the methodological robustness of attribution studies in complex regional settings such as the Mediterranean. We find that attribution outcomes vary substantially depending on the specific event considered. In particular, intense cyclones in the northern Mediterranean show an increase in associated precipitation under climate change, whereas weaker cyclones at lower latitudes become shallower in the future climate and are, on average, associated with reduced precipitation. The most extreme events linked to these lower-latitude cyclones show no change in intensity. Finally, by examining the sensitivity of our results to ensemble size, we find that robust precipitation signals typically emerge when each climate period is represented by approximately 100 years of data.

[1] Iacomino, C., S. Pascale, G. Zappa, et al. 2025. “ A Classification of High-Risk Atmospheric Circulation Patterns for Italian Precipitation Extremes.” International Journal of Climatology e70118. https://doi.org/10.1002/joc.70118.