# Western Mediterranean Precipitation Extremes, the Result of Quasi-Resonant Sea-Atmosphere Feedbacks
## 5 Conclusions  
Based on the decomposition in the period range of 11.4 to 45.7 days of precipitation depth in the western Mediterranean basin and SST from European seas as well as the Atlantic Ocean, 13 case studies of extreme rainfall events made it possible to categorize the convective systems according to the amplitude of the anomalies as well as their phase relative to the occurrence of the extreme event. Two low-pressure systems are highlighted, tropical-like and extratropical cyclones. Extratropical cyclones result from the co-evolution of the low-pressure system and the SST anomalies in the Atlantic off the coasts of France and Spain and the enclosed and semi-enclosed seas of Europe, mainly the western Mediterranean, the Adriatic, the western Black Sea, the Baltic, and the North Sea off the Scandinavian peninsula. Quasi-resonant atmospheric water-SST feedbacks occur, resulting in the uniformization of the phases of the precipitation and SST anomalies, concomitantly with the extreme event, which reflects the co-evolution of the clustering of lows and the harmonization of thermocline depths. However, the development until its term of the extreme event supposes a relative stability in the atmospheric blocking circulation. Otherwise, the event does not occur, even in the presence of coherent SST anomalies. When SST anomalies in the western Mediterranean are greater than 0.5 \({}^{\circ}\)C, the deep convective systems are centered over the southeast of France and the Mediterranean off the French coast. On the other hand, when SST anomalies in the western Mediterranean are weaker, deep convective systems can develop in different patterns, depending on SST anomalies in the peripheral seas. They can move towards northern Italy or even produce a low-pressure system extending from the Pyrenees to southern Italy or Sicily when the SST anomaly in the western Mediterranean is in phase opposition with EREs. In some cases, partial clustering of Atlantic and Mediterranean low-pressure systems occurs, producing a large cyclonic system. Tropical-like cyclones may develop in the absence of significant SST anomalies in the period range of 11.4 to 45.7 days, both in the western Mediterranean and the peripheral seas. Like extratropical cyclones, tropical-like cyclones occur in autumn or even winter, when the thermal gradient between the sea surface and the upper atmosphere is greatest but, this way, non-resonantly. Such convective systems are frequent and the return period is around 2 to 3 years. However, to the increase in the temperature of the western Mediterranean in summer resulting from global warming can lead to an ERE, as happened on 21 January 2020. To summarize, the classification of EREs according to the SST anomalies observed in the period range of 11.4 to 45.7 days shows that Mediterranean events can behave either as extratropical cyclones or as tropical-like cyclones. The latter occurring in a non-resonant way, EREs appear in autumn or at the beginning of winter when the subsurface temperature of the western Mediterranean is still high, generally exceeding 30 \({}^{\circ}\)C, without however revealing any localized SST anomaly. Such cyclones remain confined to the \begin{table}
\begin{tabular}{c c c} \hline  & \(\Delta\)**SST** (\({}^{\circ}\)**C**) & \(\Delta\)**Moisture (\%)** \\ \hline Western Mediterranean & 1.46 & 10\% \\ Adriatic & 2.16 & 15\% \\ Black Sea & 1.34 & 9\% \\ Baltic & 1.73 & 12\% \\ \hline \end{tabular}
\end{table} 
Table 2: Increase in SST and moisture during the observation period (1996–2022). Moisture variations are deduced from \(\Delta\)SST by using the Clausius–Clapeyron relationship. western Mediterranean and can very severely impact the eastern Spanish coast and/or the southeastern French coast. Extratropical cyclones are more frequent and more extensive. They happen in October-November resulting from an SST anomaly in the western Mediterranean, coherent with EREs.