Process-based classification of Mediterranean cyclones using potential vorticity

Mediterranean cyclones govern extreme weather across the basin and into the continents around it, affecting the lives of hundreds of millions. Reliable model prediction and future projections of Mediterranean cyclones (MCs) remain a significant challenge, partly attributed to the large variability among MCs. To this end, past classifications distinguished MCs by their geographical and seasonal occurrence, however, there has been no consideration of a dynamics-based classification, focusing on cyclone genesis and deepening mechanisms. The processes governing cyclogenesis and evolution include diabatic and adiabatic processes, topographic influences, and surface heat anomalies. Here we aim to classify MCs throughout the basin and year-round, according to the potential vorticity (PV) distribution. Based on a combined (‘best tracks’) MC dataset derived from ECMWF ERA5 from 1979-2020, we classify the tracks based on the upper-level isentropic PV structures relative to the cyclone center at its peak intensity, using the Self Organizing Map (SOM) algorithm. The SOM analysis reveals 9 classes of Mediterranean cyclones, each attributed to a distinct Rossby wave signature. Though classified by upper-level PV, each class shows different PV signatures also in the lower troposphere and different surface anomalies. Each class has distinct cyclone characteristics, associated hazards, and observed trends. Unique large-scale, thermal, dynamical, seasonal, and geographical features indicate dominant processes in the evolution of each Mediterranean cyclone subset. Furthermore, the tropopause-surface coupling is explored and reveals the importance of topographically-induced Rossby wave breaking to the generation of the most extreme Mediterranean cyclones. These results enhance our understanding of Mediterranean cyclones' predictability, by linking the relatively predictable Rossby wave formations and life cycles to under-predicted cyclonic variability and impact.