North Atlantic seasonal climate variability significantly modulates extreme winter Euro-Atlantic extratropical cyclone hazards

Abstract. North Atlantic extratropical cyclones (ETCs) cause significant financial losses in Europe, particularly in winter. Previous work has shown seasonal relationships between ETC hazards and modes of North Atlantic atmospheric variability, including the North Atlantic Oscillation (NAO; PC1) and East Atlantic Pattern (EAP; PC2). This study examines the relationship between the most extreme ETC hazards experienced at a given location in a winter season with the NAO and EAP, focusing on the winter maximum 10 metre wind gust and coastal wave swell height and the maximum daily total precipitation. We examine compound effects where PC1 or PC2 have signals in multiple hazard types at the same location. Positive PC1 exhibits coincident increases in winter maximum wind gust and wave swell hazards around most coastal regions in northern Europe. Positive PC2 exhibits coincident increases in winter maximum wind gust and daily total precipitation hazards over land areas in southern UK, Portugal and Spain, with an additional compound effect from increased wave swell near southern UK, northern France and Spain coasts. We also consider compound effects where PC1 and PC2 show coincident signals in the same hazard at a given location, potentially indicating an elevated hazard likelihood when circulation anomalies project onto both modes concurrently. PC1 and PC2 have coincident signals for wind gusts in southern Ireland, southern UK, Portugal and Scandinavian coast. For wave swell height, PC1 and PC2 have coincident signals around the Scandinavian, southern UK and Ireland and Northern Portugal coasts. This study shows that large-scale modes of seasonal North Atlantic climate variability modulate the exposure to extreme ETC hazards in many parts of Europe. The results have the potential to be combined with skilful seasonal climate forecasts of PC1 and PC2 to inform the insurance sector.