2024/06/23-24 Genoa Low Summer Floods
Heavy precipitation in Emilia Romagna and the French Alps likely influenced by human-driven climate change
Contact Authors
-Alice Portal, University of Bern, Switzerland 📨alice.portal@unibe.ch 🗣️ Italian, English
-Marco Reale, OGS, Italy 📨mreale@ogs.it 🗣️Italian, English
-Davide Faranda, IPSL-CNRS, France 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English
Press Summary (First Published 2024/06/27)
Depressions similar to those producing the heavy precipitation in Emilia Romagna and the Alps are locally up to 5 mm/day (up to 12%) heavier in the present climate than in the past.
Genoa-Low Summer Floods was a somewhat uncommon event.
The increase in precipitation associated with Genoa-Low Summer Floods is mostly ascribed to human driven climate change while natural climate variability likely played a minor role.
Event Description
On the 23rd and 24th of June 2024, Emilia Romagna experienced unusually heavy rainfall for this period of the year, with 48-hour rain accumulation close to 200 mm in some areas of the Emilian Apennines. The event was caused by the presence of an extratropical depression centered over the Ligurian Sea (known as Genoa Low).
The depression, remaining stationary over the Ligurian Sea for more than two days, was associated with heavy rain and thunderstorms events from Bologna to Piacenza. Many towns in the Emilian Apennines were flooded and there were several landslides, with road and rail transport heavily affected. On June 25th the flood risk shifted towards the larger towns in the Po Valley. Heavy rains and flooding also affected the Vésubie valley (Alpes-Maritimes region, France).
Surface Pressure Anomalies show a depression with values reaching -5 hPa near Corsica and Near Surface Temperature anomalies close to -3°C over land in the region around the center of the system. Precipitation Data show values locally above 30 mm/day. Windspeed Data reached values up to 40 km/h over the sea.
Climate and Data Background for the Analysis
According to the IPCC AR6 report (WG1, Chapter 12), there is high consensus on the fact that Mediterranean depressions will decrease in frequency in the future, while their severity in terms of precipitation and wind hazards is expected to increase. The local increase in Mediterranean temperatures can lead to enhanced precipitation intensity associated with extratropical depressions.
Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For Genoa-Low Summer Floods, we have medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.
ClimaMeter Analysis
We analyse here (see methodology for more details) how events similar to Genoa-Low Summer Floods have changed in the present (2001-2022) compared to what they would have looked like if they had occurred in the past (1979-2000) in the region [0°E 17°E 34°N 48°N]. Surface Pressure Changes show that the low-pressure system is of similar intensity in the present compared to the past climate, with weak positive Wind Changes along the western coast of Italy. Precipitation Changes show that similar events produce daily rainfall which can be up to 5 mm more abundant in the present climate than in the past, with associated Temperature Changes exceeding 2 degrees Celsius.
This has resulted in precipitation over Nice and Parma having are up to 2 mm/day (up to 10%) heavier in the present than it would have been in the past. No significant changes in seasonality have been observed with respect to Similar Past events. Finally, we find that sources of natural climate variability did not significantly influence the event. The changes we see in the present event may be mostly due to human driven climate change.
Conclusion
Based on the above, we conclude that Depressions similar to those producing the heavy precipitation in Emilia Romagna and the Alps have become locally up to 5 mm/day wetter (up to 12% wetter) in the present than in the past. We interpret Genoa-Low Summer Floods as an event whose changes can mostly be ascribed to human driven climate change.
Additional Information : Complete Output of the Analysis
NB1: The following output is specifically intended for scientists and contain details that are fully understandable only by reading the methodology described in Faranda, D., Bourdin, S., Ginesta, M., Krouma, M., Noyelle, R., Pons, F., Yiou, P., and Messori, G.: A climate-change attribution retrospective of some impactful weather extremes of 2021, Weather Clim. Dynam., 3, 1311–1340, https://doi.org/10.5194/wcd-3-1311-2022, 2022.
NB2: Colorscales may vary from the ClimaMeter figure presented above.
The figure shows the average of surface pressure anomaly (msl) (a), average 2-meter temperatures anomalies (t2m) (e), cumulated total precipitation (tp) (i), and average wind-speed (wspd) in the period of the event. Average of the surface pressure analogs found in the counterfactual [1979-2000] (b) and factual periods [2001-2022] (c), along with corresponding 2-meter temperatures (f, g), cumulated precipitation (j, k), and wind speed (n, o). Changes between present and past analogues are presented for surface pressure ∆slp (d), 2 meter temperatures ∆t2m (h), total precipitation ∆tp (i), and windspeed ∆wspd (p): color-filled areas indicate significant anomalies with respect to the bootstrap procedure. Violin plots for past (blue) and present (orange) periods for Quality Q analogs (q), Predictability Index D (r), Persistence Index Θ (s), and distribution of analogs in each month (t). Violin plots for past (blue) and present (orange) periods for ENSO (u), AMO (v) and PDO (w). Number of the Analogues occurring in each subperiod (blue) and linear trend (black). Values for the peak day of the extreme event are marked by a blue dot. Horizontal bars in panels (q,r,s,u,v,w) correspond to the mean (black) and median (red) of the distributions.