# Western Mediterranean Precipitation Extremes, the Result of Quasi-Resonant Sea-Atmosphere Feedbacks
## 4 Discussion  
### Typology  
These 13 cases studies (EREs 1 to 11 occurring in the southeast of France, ERE 12 occurring in Italy, and ERE 13 occurring in Spain) highlighted the driving role of SST anomalies in the western Mediterranean as to the evolution of EREs in southwestern Europe. The discriminating criterion is the amplitude and the phase relative to the ERE of the SST anomaly in the period range of 11.4 to 45.7 days. A quasi-resonance phenomenon occurs when the SST anomaly is in phase with EREs, whatever its amplitude. In this case, an ERE is the outcome of co-evolution of the low-pressure system, SST anomalies of the enclosed and semi-enclosed seas of Europe, as well as the deepening of a cut-off low over the western Mediterranean off the western coasts of Italy.  
Atmospheric water-SST feedbacks in the period range of 11.4 to 45.7 days lead to concerted evolution of the clustering of lows and the homogenization of SST anomalies, in phase with EREs. This supposes a relative stability of the atmospheric blocking circulation. Otherwise, the maturation time of the clustering processes of mesoscale depressions, as well as the harmonization of the depth of the thermoclines, is not sufficient to allow the formation of an ERE. The western Mediterranean, the enclosed and semi-enclosed seas of Europe, and possibly the narrow strip of the Atlantic along the southwest coast of France are concerned. The interruption of such a budding rainfall event has occurred three times during the 22 years of observation (events 1\({}^{\prime}\), 2\({}^{\prime}\), and 3\({}^{\prime}\) in Figure 2b,c).  
Partial clustering of Atlantic and Mediterranean low-pressure systems may occur, in phase with EREs in the period range of 11.4 to 45.7 days. A transfer of cloud/water  
Figure 7: Precipitation depth and SST anomalies related to ERE 13 occurring on 21 January 2020. The common time reference is the daily precipitation time series at 40.9\({}^{\circ}\)N, 1.7\({}^{\circ}\)E (precipitation height is 428 mm/day). (**a–f**) same conventions as in Figure 3.  
vapor occurs between the two low-pressure systems, independent of SST anomalies in the Atlantic off the French or Spanish coasts. Such SST anomalies may however be involved in cyclogenesis over the western Mediterranean as well as the enclosed or semi-enclosed seas when they are in phase with EREs, as for EREs 5 and 11 (Table 1).  
EREs may develop wherein the SST anomaly of the western Mediterranean is out of phase in the period range of 11.4 to 45.7 days, whether it is ahead or behind the ERE. This shows that the Mediterranean is not resonantly involved in cyclogenesis. Such events occur mainly through the contribution of the peripheral enclosed or semi-enclosed seas, so much so that the depression is shifted eastward, in the direction of the Aegean and the Black seas. The deep convective system may even extend from the Pyrenees or northeastern Spain towards Italy within 2 to 3 days. This happened twice during the observation period (EREs 5, 11). The daily rainfall amount of ERE 5 reached an unequaled amplitude both in Italy and in Croatia.  
Finally, some EREs may occur non-resonantly, similar to tropical cyclones. When no SST anomaly is in phase with EREs, the low-pressure system remains confined to the western Mediterranean. Most Mediterranean events of this type occur in late summer, with a return period of a few years. However, ERE 13 with a return period much greater than the observation period occurred on 21/01/2020 (Figure 7). Mainly the northeast of Spain was impacted near the coast.