A dead end between mountains and sea traps moist air over some regions for a long time, causing record floods. As in Emilia Romagna in 2023.
Sea, mountains and extreme phenomena made more probable by climate change can build an interaction that favors extreme floods: a study by the Euro-Mediterranean Center on Climate Change (CMCC Foundation) reveals for the first time how a specific conformation of the mountains, a cyclone and humidity coming from the Adriatic Sea contributed to the devastating floods that occurred in Emilia Romagna in May 2023. And how those same conditions could pave the way for similar phenomena, in a Mediterranean strongly marked by climate change. The research was published on Scientific Reports.
The flood in Emilia Romagna
Between 2 and 17 May 2023, a series of very heavy rainfalls fueled by a Mediterranean cyclone generated persistent rain in Emilia Romagna, accompanied by flooding, overflows and landslides. 17 people died and the damage in the area was at least 8.5 billion euros.
According to the study, coordinated by Enrico Scoccimarro, senior scientist at CMCC, the international center for climate studies, where he directs the research team that deals with climate analysis, its variability and predictability, these record floods would have been favored by a “cul-de-sac” effect, or dead end: the mountains would have blocked the humidity coming from the Adriatic, trapping the rain for a long time on the region. The result was a flood of the kind one would expect to see once every 500 years.
Three actors
«The atmospheric dynamics underlying the event described considers three main actors», explains Scoccimarro a Evidence Network.it. «First actor: a stationary cyclone in a certain position (in the specific case over Central Italy), which generates constant winds that blow on the sea surface for a long time (specifically sirocco winds over the Adriatic).
Second Actor: the sea which, lapped by these constant winds, loads the column of air above with water which will then be carried in the same direction as the winds (in this case towards Emilia Romagna).
Third actor: the mountains (Alps and Apennines in this case) which ensure that the flow of water-filled air remains trapped in the Emilia Romagna region, creating convergence, i.e. accumulation.
Without mountains, this flow could continue towards the west-northwest without creating convergence: a lot of water would enter the region from the “right” and the same amount would exit from the “left”, without converging (accumulating), and without therefore making itself available to then fall on the region itself”.
Inexhaustible rains
«Under such conditions, intense precipitation events, which usually tend to empty the column of water in a few hours, can persist for a long time, because the overlying column of air is constantly supplied with water by these flows coming from the Adriatic region.
Until the persistent cyclone moves from this particular position, the supply of water vapor over the Emilia Romagna region continues, and the presence of the mountains that block it creates convergence of water, which then becomes available for incessant precipitation.”
Risks also in other regions
With climate change warming the Mediterranean – the almost closed and shallow Mare Nostrum is warming faster than any other sea – the same “dead end” effect could also occur in other areas with similar orographic characteristics.
«These particular conditions can occur in other parts of the Mediterranean domain, for example on the coasts of Albania, in the case of stationary cyclones over central Italy, or even in Occitania and Provence (France) or Valencia or Catalonia (Spain) in the case of stationary cyclones in the western part of the Mediterranean basin. In these regions the three actors mentioned can appear on stage together and give rise to a similar, sad, show”, adds Scoccimarro.
Improve forecasts
In light of what has been observed, the authors of the study propose and introduce a new parameter, the “persistence of cyclone density”, to improve forecasts of extreme precipitation events. «These extreme events can become more intense in a warmer climate context such as that of the Mediterranean, due to the greater capacity of hot air to retain more water compared to cold air: this determines a greater availability of water at the time of the intense rain event, which tends to empty the entire column of air. Furthermore, the higher temperature of our sea facilitates evaporative processes that supply the atmosphere with water.”
