Linking compound weather extremes to Mediterranean cyclones, fronts and air-streams

Abstract

Abstract. Mediterranean cyclones are the primary driver of many types of surface weather extremes in the Mediterranean region, the association with extreme rainfall being the most established. Although smaller in size compared to Atlantic cyclones, they share a similar synoptic structure organised in distinct air streams, such as the warm conveyor belt and the dry intrusion, and are associated with low-level temperature fronts. The large-scale characteristics of a Mediterranean cyclone, the properties of the associated airflows, the interaction with the topography around the Mediterranean basin, and the season of occurrence, all contribute in determining its surface impacts. Here, we take these factors into account to establish statistical links between Mediterranean cyclones and weather compounds of two types, namely co-occurring rain-wind and wave-wind extremes. Specifically, compound extremes are attributed to a cyclone if they fall within the system's impact area, using a definition that is expressly tested on Mediterranean cyclones and on the compound selection. Our results show that the majority of Mediterranean compound rain-wind and wave-wind extremes occur in the neighbourhood of a Mediterranean cyclone, with peaks exceeding 80 %; the proportion of cyclone-related compounds is highest when considering transition seasons, and rain-wind events. Winter cyclones show highest compound frequency, matching with the peak winter occurrence of distinctively baroclinic cyclones. A novelty of this work, the de-construction of the cyclones' impact areas based on the presence of objectively-identified air streams and fronts, reveals a high incidence of both types of compound extremes below warm conveyor belt ascent regions, of wave-wind extremes below dry intrusions.