A recurrent pan-Atlantic wave pattern driving concurrent wintertime extremes

Wintertime extremes such as cold spells and heavy precipitation can have severe socioeconomic impacts, disrupting critical infrastructures and affecting human well-being. In this context, recurrent and persistent atmospheric patterns that favour the occurrence of extremes in specific regions are of particular interest for timely extreme event prediction and for understanding the underlying physical mechanisms. Furthermore, if these atmospheric patterns span large spatial scales, they may lead to concurrent weather extremes at geographically remote locations. Such spatially compounding extremes are of particular interest due to their potentially enhanced impacts compared to extremes occurring in isolation. We identify a quasi-hemispheric wave-4 pattern in the Northern Hemisphere winter atmosphere, which is most pronounced over the pan-Atlantic region and coincides with local and concurrent cold or wet wintertime extremes in North America and Europe. Wave-4 is the only wavenumber to exhibit recurrent and phase-locked behavior, such that the associated atmospheric circulation and surface anomalies re-occur over the same locations when the pattern's wave amplitude is high. Specifically, the wave-4 pattern we identify increases the probability of extreme cold or wet events by up to 300 % in certain areas of North America and Europe, as well as favouring their concurrence at different locations. High-amplitude wave-4 events have increased significantly in frequency over the past four decades (1979–2021). Wave-4 amplitude does not seem to be directly linked to changes in the hemispheric-scale meridional temperature gradient, but shows moderate correlations with some modes of variability in the Pacific. We conclude that the identified wave-4 pattern could provide useful insights for both statistical forecasting of regional and concurrent pan-Atlantic wintertime extremes and for an improved understanding of their future changes.