Heavy precipitation in the Cevennes floods likely enhanced by both human-driven climate change and natural variability  

Press Summary (First published 2023/09/17, Updated 2023/11/22)

• Low pressure systems similar to that producing September 2023 Cevennes Floods are 2 to 6 mm/day wetter in the Cevennes and in Nouvelle Aquitaine than they would have been in the past. 

• September 2023 Cevennes floods was similar to other events in the past

• We mostly ascribe the heavier precipitation of September 2023 Cevennes floods to human driven climate change and natural climate variability likely played a modest role.

Event Description

In mid September 2023, the French region of the Cévennes was affected by exceptionally intense rainfall. On September 16, a trough approaching Europe from the North Atlantic caused the deepening of a depression just to the west of Portugal. The southerly winds on the eastern side of the depression transported warm, unstable and humid air from the Mediterranean sea towards Spain and France. This unstable flow interacted with the Cévennes, a mountain range located in southern France, approximately between Montpellier and Valence. Forcing the air to lift, the mountains act as a trigger for intense, long-lived, stationary thunderstorms, which take the name "épisode cévenol". While the existence of this nomenclature suggests that this is a recurring phenomenon over this region, this episode was particularly intense. On September 16th, cumulative rainfall reached an astounding 550 mm in Roqueredonde, Hérault, setting a new record and causing widespread flooding. On September 17th 2023, the low-pressure system moved across France colliding with warm, moist air and enhancing the wind shear. The resulting instability fueled powerful thunderstorms, including a few supercells, that produced large hail, tornadoes, and strong wind gusts, causing significant damage in several regions. Notably, hailstones up to 10 cm in diameter were reported in some areas, a rare occurrence for autumn in France. The widespread and intense storms made September 17th the stormiest September day in almost 15 years, with more than 35000 detected lightning strikes. Surface Pressure Anomalies show a deep depression offshore Portugal with intense anomalies reaching up to -20 hPa. The Precipitation Data show that large rain amount have fallen in France as well as in Portugal, Britanny, Wales. 

Climate and Data Background for the Analysis

Chapter 11 of the IPCC AR6 report underscores the challenge of evaluating climate trends and their connection to intense rain events. The inherent variability in rain amount definitions and the constraints of long-term observations render it difficult to derive precise conclusions, particularly concerning thunderstorms that result in flooding in regions close to the Mediterranean sea. Similarly, Chapter 12 of the IPCC AR6 report refrains from providing explicit statements about historical trends in extreme precipitation specific to the Mediterranean. However, within the distinctive context of the Cévennes region, Ribes et al.'s research unveils a noteworthy +22% surge in extreme rainfall intensity spanning from 1961 to 2015. This implies the potential influence of human-induced climate factors on these trends. Furthermore, Drobinsky et al. contribute valuable insights by demonstrating a consistent association between temperature and extreme precipitation throughout the Mediterranean, pinpointing a shift in this correlation at approximately 20°C. Notably, Vautard et al.'s findings explicitly emphasize the heightened intensity of daily fall precipitation in the Cévennes mountains. Their research indicates that the likelihood of experiencing precipitation levels akin to those observed in 2014 has likely tripled since 1950, albeit with considerable uncertainties. Collectively, these studies, along with the IPCC AR6 report, illuminate the evolving climate trends and their ramifications for extreme weather events. They offer indispensable insights into the amplification of extreme precipitation in the Cévennes region and the broader Mediterranean context, despite the intricacies inherent in such assessments.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For the Cévennes Floods we have high confidence in the robustness of our approach given the available climate data, as the event is very similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to low the low pressure systems leading to the recent Cévennes 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 [-3°E  20°E 40°N 52°N]. The Surface Pressure Changes show that  low pressure systems leading to Cevennes floods are less intense by about 2 hPa in the present than they were in the past. Precipitation Changes show that similar events produce larger (between 2 and 6 mm/day) amounts of precipitation in Nouvelle Aquitaine and in the Cévennes. Considering the affected urban areas, Bordeaux and Montpellier see an increase in precipitation in the present (1-5 mm/day) while Valence see no changes. We also find that Similar Past Events have become less frequent in September, and slightly more common in November, while no change in the frequency of similar depressions is observed in October and December. 

Finally, we find that sources of natural climate variability, notably the El Nino Southern Oscillation, may have partly influenced the event. This suggests that the changes we see in the event compared to the past may be mostly due to human driven climate change, with a secondary contribution from natural variability.

Conclusion

Based on the above, we conclude that low pressure systems leading to Cévennes floods similar to that observed in September 2023 are 2 and 6 mm/day wetter in the Cévennes and in Nouvelle Aquitaine than they would have been in the past. We interpret the Cévennes Floods as an event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr  🗣️French, Italian, English

Flavio Pons, IPSL-CNRS, France  📨flavio.pons@lsce.ipsl.fr  🗣️French, Italian, English