Linking compound weather extremes to Mediterranean cyclones, fronts, and airstreams

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. The large-scale characteristics of a Mediterranean cyclone, the properties of the associated airflows and temperature fronts, the interaction with the Mediterranean Sea and with the topography around the basin, and the season of occurrence all contribute to determining its surface impacts. Here, we take these factors into account to interpret the statistical links between Mediterranean cyclones and compound extremes of two types, namely co-occurring rain-wind and wave-wind extremes. Compound extremes are attributed to a cyclone if they fall within a specially defined Mediterranean cyclone impact area. Our results show that the majority of Mediterranean rain-wind and wave-wind extremes occur in the neighbourhood of a Mediterranean cyclone, with local peaks exceeding 80 %. The fraction of compounds happening within a cyclone's impact area is highest when considering transition seasons and for rain-wind events compared with wave-wind events. Winter cyclones, matching with the peak occurrence of large and distinctively baroclinic cyclones, are associated with the highest compound frequency. A novel deconstruction of cyclones' impact areas based on the presence of objectively identified airstreams and fronts reveals a high incidence of both types of compound extremes below warm conveyor belt ascent regions and of wave-wind extremes below regions of dry intrusion outflow.