Heavy Precipitations and Strong Winds in Storm Kirk exacerbated by both human-driven climate change and natural variability

Press Summary (First Published 2024/10/10)

• Depressions similar to storm Kirk, which produced floods in France, show increased precipitation along the Atlantic coast (up to 14 mm/day, representing up to 20% more precipitation) and decreased precipitation along the Mediterranean coast (up to 16 mm/day, or up to 40% less precipitation) in the present compared to the past, although the Mediterranean coasts were less affected by Kirk. Depressions similar to Kirk are deeper (by -2 hPa) and produce stronger winds (up to 12 km/h, representing a 12% increase in wind strength) in the present.

• This was a somewhat uncommon event. 

• Natural climate variability likely played a role in driving the pressure pattern and the associated increase in wind and precipitation linked to storm Kirk but human-driven climate change has also contributed.

Event Description

On October 9, the remnant depression of Hurricane Kirk impacted northern Spain and moved into the Bay of Biscay, from where it entered France. Storm Kirk  had enough energy to bring very strong winds, up to 150 km/h, over northern France (affecting Paris as well), Belgium, and western Germany. The storm has made landfall on the western coast of the continent, hitting Galicia with wind gusts over 100 km/h, causing storm surges from Portugal to the western coast of Spain, where intense downpours have also occurred. The intense precipitation associated with the storm spread to western and northern France, dumping 90 mm of rain near Nantes in just a few hours, causing widespread flooding and road closures. Flooding has also been reported in more central regions of France, such as the Centre-Val de Loire. Wind gusts have uprooted 400 trees in Portugal’s Porto region. In the Pyrenees, morning winds reached 177 km/h, with a peak of 211 km/h recorded at 11 a.m. Storm kirk was a very rare case of a post-tropical cyclone hitting Europe. It began on  September 29 as a tropical depression off the coast of the Cape Verde Islands, which then evolved the next day into a tropical storm. After forming a circular eye, it intensified into a Category 1 hurricane by October 1. As it moved northwest, it strengthened further, reaching Category 4 by October 4, halfway between Cape Verde and the Leeward Islands. Kirk then veered northward and eventually turned northeast. As it reached higher latitudes, cooler waters caused the storm to weaken, losing its tropical characteristics and transitioning into an extratropical storm before striking Europe.

The Surface Pressure Anomalies show a large depression system (up to -10 hPa surface pressure anomalies) at the core of storm Kirk centred over the English Channel.. This depression was the main driver of the massive floods. Temperature anomalies show  negative anomalies (up to -2 °C) over Brittanny near the core of the storm and positive anomalies (up to +5°C) elsewhere. The contrast between the cold air and the warmer than average sea surface temperatures of the Atlantic (not shown) determined the extreme precipitation. Indeed, Precipitation data show several areas of accumulation across France, with daily values up to 80 mm. Windspeed data show that most of the domain was affected by moderate to high winds, with values peaking around 100 km/h in some areas.

Climate and Data Background for the Analysis

The  IPCC AR6 WG1 states that the water cycle variability and extremes are projected to increase faster than the average change and in most of the tropical and extratropical regions. In the extratropics during the warmer season, interannual variability of precipitation and runoff are increasing faster than the seasonal changes (Chapter 8). 

The report also underscores the impact of climate change on storminess in Europe, aggravated by rising sea levels and intense precipitation. Anticipated alterations in atmospheric circulation patterns stem from the uneven warming of land and ocean, potentially resulting in diminished continental near-surface relative humidity and localised decreases in precipitation. Extreme precipitation and pluvial flooding are projected to increase at global warming levels exceeding 1.5°C in all regions except the Mediterranean. (high confidence). In a warmer climate, individual midlatitude storms are expected to produce more precipitation, though changes in wind speed remain less certain. Studies, such as Ginesta et al. (2024), found that the intensity of recent storms, including both wind speed and precipitation, is likely to increase in the most impacted regions of Europe.

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 analyze here (see Methodology for more details) how events similar to the low pressure system leading to Storm Kirk changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [-10°E 12°E 40°N 55°N]. Surface Pressure Changes show that similar depressions are deeper than in the past, with atmospheric pressure up to 2 hPa lower in the present than they are in the past in the eastern part of the domain, strictly corresponding to the core of the storm. Temperature Changes show up to -2 °C warmer conditions in the present than in the past over the Iberian Peninsula and some parts of France. Precipitation Changes show significant increasing precipitation in the area affected by the storm experiencing up to 15% (8 mm/day) more precipitation in the present than in the past. Windspeed Changes indicate significant changes up to 12 km/h windier conditions over the French Atlantic coast, close to the core of the storm. We also note that Similar Past Events occur with similar seasonality in the past and present periods, although with a slight increase in November in the present climate. Changes in Urban Areas reveal that Nantes and La Roche-sur-Yon are up to 5 mm/day wetter (up to  15% more precipitation) and 8 km/h windier in the present compared to the past. 

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation may have influenced the changes in this event. This suggests 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 depressions similar to Storm Kirk show increased precipitation along the Atlantic coast (up to 14 mm/day, representing up to 20% more precipitation) and decreased precipitation along the Mediterranean coast (up to 16 mm/day, or up to 40% less precipitation) in the present compared to the past, although the Mediterranean coasts were less affected by Kirk. Depressions similar to Kirk are deeper (by -2 hPa) and produce stronger winds (up to 12 km/h, representing a 12% increase in wind strength) in the present. We interpret Storm Kirk  as an event for which natural climate variability  played a role.

Contact Authors

-Mireia Ginesta, IPSL-CNRS, France 📨Mireia.Ginesta-Fernandez@lsce.ipsl.fr  🗣️Spanish, English

-Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it 🗣️Italian, English

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