Strong winds in storms Babet & Aline likely strengthened by both human-driven climate change and natural variability  

Press Summary (First published 2023/10/22, updated 2023/11/22)

• Storms similar to Babet & Aline are 3 hPa to 5 hPa deeper, 5 km/h to 15 km/h windier and 5-20 mm/day wetter on the Atlantic coasts  in the present than they would have been in the past.

• Storms Babet &Aline were somewhat uncommon events.

• Natural climate variability likely played a role in driving the pressure pattern and the associated increase in precipitation linked to storms Babet & Aline .

Event Description

On October 20th 2023, two storms, Storm Babet and Storm Aline, brought chaos in many countries of Europe with strong winds, persistent rain and floods. Storm Babet caused high rainfall and severe winds in eastern Scotland, while low pressure from Storm Aline moved across western France and pushed torrential rain into southern England and central Spain. Scotland had a rare red weather warning, while amber and yellow warnings were in place for parts of northern England. River levels in Scotland rose to extreme levels. High winds with gusts of up to 80mph were expected in North East Scotland, and heavy rain was forecasted for northern, central, and eastern England, raising the risk of flooding. In Spain, Storm Aline caused red and amber warnings to be issued for coastal flooding and strong winds. Madrid recorded 108 mm of precipitation within a 24-hour period, the highest in the past 163 years. The Libeccio and Sirocco winds brought by the storm caused damage in Central and Southern Italy, affecting Tuscany, Sardinia and Sicily.  In this analysis, we refer to the event as "storm Babet & Aline". 

The Surface Pressure Anomalies show a large negative (cyclonic) anomaly over Western Europe. This configuration, typical of Atlantic extratropical storms, is prone to advect wet and warm air from Western Africa to central Europe contributing to strong winds. Temperature data show warm anomalies over a large part of the European continent during the storms. Precipitation Data show high amounts of precipitaiton over Northern Spain, Scotland, the Benelux and the Alps. Windspeed data shows large areas of Europe with wind above 50 km/h locally peaking to 100 km/h.

Climate and Data Background for the Analysis

The Coupled Model Intercomparison Project (CMIP) phases 3, 5, and 6 generally agree on the spatial signature of the projected changes in storminess in the North Atlantic (Harvey et al., 2020, Priestley et al, 2020). Summers are expected to see reduced storm activity, while winters should bring increased storms in the British Isles and reduced storms in the Mediterranean and Norwegian seas. According to the IPCC AR6 report, changes in the dynamical intensity (e.g. wind speeds) of storms will be small (medium confidence), “although changes in the location of storm tracks can lead to substantial changes in local extreme wind speeds ”. The IPCC report further states that “there is high confidence that the precipitation associated with extratropical cyclones will increase in the future”.  Ginesta et al 2023 found that Storm Alex, similar in the meteorological structure, impacts and timing to Storm Aline, has shown an increase in persistence, as well as higher levels of precipitation and stronger wind speeds in the recent period compared to the past.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past.  For this event 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 2023 Babet & Aline storms 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 [-20°E 20°E 30°N 60°N]. Surface pressure changes indicate that storms resembling Babet & Aline exhibit pressure anomalies deeper by 3 to 5 hPa in the present than in the past. Additionally, changes in Windspeed  changes reveal that these storms are now 5-18 mm/day wetter  5 to 20 km/h windier on the Altantic European coasts in the present than in the past. A similar signal is found for  the analysis in the urban areas of Cork (Ireland), Glasgow (Scotland) and Oristano (Sardinia, Italy). These cities are getting windier and wetter in the present than in the past during storms similar to Babet & Aline. We also note that Similar Past Events have become more common in the months of October and November, while they previously occurred in the extended autumn season. Among similar past events we find indeed Alex storm, which caused widespread destruction in South of France and Northern Italy.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation, may have partly influenced the event. This means that the changes we see in the event compared to the past may be due to human driven climate change, with a contribution from natural variability.

Conclusion

Based on the above, we conclude that windstorms similar to Babet & Aline are 3 hPa to 5 hPa deeper, 5 km/h to 15 km/h windier and 5-20 mm/day wetter on the Atlantic coasts  in the present than they would have been in the past.  We interpret Storms Babet & Aline as unusual events whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

Mireia Ginesta, IPSL-CNRS, France  📨mireia.ginesta-fernandez@lsce.ipsl.fr  🗣️Catalan, Spanish, English

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