Kai Kornhuber , Gabriele Messori

Wintertime extremes such as cold spells and heavy precipitation can have severe socioeconomic impacts, disrupting critical infrastructures and affecting human wellbeing. Here, we relate the occurrence of local and concurrent cold or wet wintertime extremes in North America and Europe to a recurrent, quasi-hemispheric wave-4 Rossby wave pattern. We identify this pattern as a fundamental mode of the Northern Hemisphere (NH) winter circulation, since wave-4 is the only wavenumber to exhibit
phase-locking behavior. Thus, the associated atmospheric circulation and surface anomalies re-occur over the same locations when the pattern's wave amplitude is high.
The wave pattern is most pronounced over the pan-Atlantic region, and 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 (1979-2020), which we conjecture may derive from processes in the
tropical Pacific. The identified pattern and its remote forcing might provide pathways for early prediction of local and concurrent cold or wet wintertime extremes in North America and Europe.