# Western Mediterranean Precipitation Extremes, the Result of Quasi-Resonant Sea-Atmosphere Feedbacks
The variation of sea surface temperature in certain areas caused significant changes in rainfall amount and its spatial distribution. The authors concluded the existence of areas that to a greater extent favor air-sea interaction, leading to the development of torrential rainfall. These results could be extended to the whole Mediterranean basin to look for such potential recharge areas, showing that SST could be a relevant indicator for forecasting and/or monitoring EREs. However, like cyclonic systems forming at mid-latitudes, the most intense Mediterranean events result from quasi-resonant phenomena. They simultaneously involve the clustering of mesoscale depressions, the availability of a latent heat flux over warm seas whose surface temperature is peaking, the deepening of a cut-off low, and a relative stability of the atmospheric blocking circulation [19]. The simultaneity of these phenomena is not fortuitous. It necessarily presupposes their concerted development during the slow maturation processes of the cyclonic system. This maturation phase occurring in a particular frequency band, although broad, presents a characteristic of quasi-resonance. As such, the genesis of extreme Mediterranean events has similarities with tropical cyclones when they landfall while migrating poleward. Here again, the initiation of the cyclonic system over the warm sea does not necessarily occur when the SST reaches a maximum. On the other hand, the available latent heat flux must be relayed by the surrounding seas for the cyclone to develop, which occurs when the SST anomalies peak there in the period range of quasi-resonance. In this way, SST is not only an indicator of torrential rain recharge areas when measured in the western Mediterranean, but its monitoring in the central and eastern Mediterranean as well as in the Black Sea, the Baltic Sea, the North Sea, and the Atlantic Ocean shows that the recharge involves much larger areas than what was commonly accepted until now. The complexity of dynamic systems that occur in the characteristic period ranges result from the conjunction of interactions that occur in a large range of period scales, when they lead to a very heavy large-scale rainfall on an almost-stationary convergence zone. The characteristic periods, expressed from the available data of precipitation in harmonic modes with respect to the declination of the sun, generally range from 1/12 year for the uniformity of the phase of SST anomalies to 1/24 year for the clustering of lows and to 1/384 year for the deep convective system [19]. In some cases, phase homogenization of SST anomalies occurs in the same harmonic mode as the clustering of lows, that is 1/24 year. These two modes are distinct in the case of Mediterranean events. The methodology followed in this study was developed in two previous studies focusing on mid-latitude EREs. It consists of breaking down the precipitation height and SST anomalies into characteristic period ranges; their coherence and their phase highlight the different dynamic systems involved in the genesis of EREs. However, the Mediterranean has two peculiar characteristics. On one hand, the height of precipitation can reach ex tremely high values--an event reaching 817 mm in one day is documented [20]--and on the other hand, the sea-atmosphere interactions imply distant closed or semi-closed seas whose role is essential in the development of EREs. Large scale moisture transport is a more important factor than evaporation over local sources [15]. From the observation of 160 extreme precipitation events with an atmospheric model enabled for state-of-the-art moisture tracking, the authors demonstrated that more than 50% of the average precipitation fraction comes from distant source regions. This particularity indicates direct connections of the Mediterranean basin with multiple locations on the western, central, and eastern Europe and a global scale energy redistribution.