Compounding Wet and Cold-Extremes driven by an increasing Pan-Atlantic wave-4-pattern 

Abstract

<p>Wintertime extremes such as cold spells and heavy precipitation events can have severe societal impacts, disrupting critical infrastructures, traffc and affecting human well-being. Here, we relate the occurrence of local and concurrent cold and wet wintertime extremes in North America and Western Europe to a recurrent, quasi-hemispheric wave-4 Rossby wave pattern in the Jetstream. We identify this pattern as a fundamental mode of Northern Hemisphere (NH) winter circulation exhibiting phase-locking behavior as 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 strongest over the pan-Atlantic region, and is associated with an increased probability of extreme cold or wet events by up to 300 % in certain areas of North America and Western Europe. We identify a significant increase in frequency over the past four decades (1979- 2021), which we hypothesise may derive from increased convective activity in the tropical Pacific, from where the pattern originates, while a weakened meridional temperature gradient linked to Arctic warming appears to have no direct effect on its occurrence. 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 Western Europe.</p>