Extratropical cyclones play a key role in modulating the precipitation and wind in mid-latitudes and can be responsible for extreme wet and windy events. With global warming, the dynamics and thermodynamics associated with these atmospheric systems are being affected. However, due to the complexity of modelling the climate system, a challenge arises when evaluating the influence that climate change might have had in specific observed extreme cyclones. An effort towards this direction has been made by Extreme Event Attribution, a recent research field that aims to analize how global warming has modified the characteristics, such as frequency, intensity and duration, of an extreme event. A recent study led by Mireia Ginesta presents a methodology for attributing the changes in the underlying dynamics of extreme extratropical cyclones to ongoing climate change. They illustrate the methodology on storm Alex, a severe storm that affected southwestern Europe at the beginning of October 2020. Southern France and Northern Italy received numerous record-breaking precipitation amounts, such as 630 mm in just 24 hours in Sambughetto. Storm Alex caused more than 20 fatalities and an estimated economic loss of at least 2 and a half billion euros.
The study uses climate observations of the last seven decades to define two different climate periods: a past period from 1950 to 1984, and a more recent period, from 1986 to 2021. The latter represents a climate largely influenced by anthropogenic emissions, while in the former, the human influence on climate is weaker. They first identified 30 storms similar to Alex, which they named analogues, by selecting those that resemble Alex in terms of sea level pressure in each period. They then compared the characteristics of the storms of the past and present periods. They found that Alex-like storms in the more recent period have more meridional atmospheric pressure patterns, and they are more persistent. Storms in the present period become more common in autumn, when Alex took place. In terms of impacts, there is an increase in precipitation and wind gusts in Southern France and Northern Italy, increasing the probability of severe flooding events. Therefore, those changes collectively point to Alex-like storms more impactful and common in a warmer climate.
The toolkit presented in the study may be applied with profit to other severe extra-tropical and tropical cyclones, and provide new insights on the influence of climate change on extreme weather phenomena. To that end, the authors have made the code developed and applied in the project available at: https://github.com/mireiaginesta/analogues-cyclones.
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