Group seminar on 8. November, 14:15 CET

Extra-Tropical Cyclone Clustering in Recent and Future Winter Climates: Insights from ERA5 and CESM2-LE

Alexia Karwat

Extra-tropical cyclones that appear as groups or storm sequences, a phenomenon known as clustering, are more common over specific regions, for example, the eastern North Atlantic and western Europe. In the past, clustered cyclones, such as the storm triplet of Dudley, Eunice, and Franklin of February 2022, have led to large socio-economic impacts. However, it is still unclear how future cyclone clustering can be quantified and how the properties of clustered cyclones, for example, their intensity and life time, will change in a warmer climate. It is therefore crucial to investigate the extent of clustering that Europe and other regions may experience under global warming. Here, we perform a cyclone cluster analysis using the ERA5 reanalysis to characterize clustering in the extratropics during 1980-2020. In addition, we use Large Ensemble simulations from the Community Earth System Model version 2 of the SSP3-7.0 scenario to compare clustering during 2060-2100 to 1980- 2020 and 1850-1890, respectively. Our model simulations show significant increases in cyclone clustering over Europe for 2 cyclones within 7 (14) days and for 3 cyclones within 14 days in the future: clustering is increasing up to 28.2% on average during 2060-2100. As the overdispersion, a statistical measure for clustering, is projected to increase, clustered cyclones are intensifying more in the future due to lower minimum pressure and larger radii and depths compared to non-clustered events. In contrast to Europe, cyclone clustering decreases along the west coast of the US and Canada by up to 24.3% towards the end of the 21st century. Furthermore, the number of cyclones that pass across the Aleutian Island chain and over Alaska are also decreasing by up to 10.1% on average during 2060-2100 in comparison with 1980-2020. Our findings suggest that dynamic changes in relation to anthropogenic greenhouse gas emissions are an important factor in determining the response of cyclone clustering to global warming.