Did the 2022 strong polar vortex make serial extratropical cyclone clustering more likely? (StratClust)

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Extratropical storms, or cyclones, are the most damaging natural hazard affecting western Europe. Serial clustering of cyclones occurs when several intense storms affect one specific region in a short period, which can lead to large societal impacts such as flooding, damage from strong winds and risk to life. In February 2022, the UK and Ireland were severely impacted by a succession of extratropical storms: Dudley, Eunice and Franklin. These storms caused record breaking wind gusts and sadly led to multiple deaths and injuries. Storm Eunice was described by weather forecasters as a 'once in a decade' storm. Identifying and quantifying the factors that caused this period of intense stormy weather is important for understanding future weather hazards. One potential contributor was the stratospheric polar vortex - a band of strong winds encircling the North pole around 25 km above Earth's surface - which was nearly the strongest on record in February 2022. The polar vortex has been shown to affect surface weather and climate, but it has not previously been connected to clustering of extratropical storms. The hypothesis that StratClust will test is that the unusually strong polar vortex created environmental conditions that favoured the development of multiple intense North Atlantic storms. To address this hypothesis, StratClust will analyse subseasonal forecasts from state-of-the-art numerical prediction systems and perform bespoke 'attribution' experiments to isolate and quantify the impact of the stratosphere on surface weather and separate it from other factors. The outcomes from StratClust will provide the first information about whether the exceptional polar vortex conditions in February 2022 contributed to the extreme weather in the UK and Ireland. StratClust will serve as a pilot study which, through collaboration with an international project as part of the World Climate Research Programme (SNAPSI), will engage other international modelling groups to perform similar attribution experiments, thereby generating a new community dataset of subseasonal forecasts for February 2022 and opportunities for further research.