North American Cold Spells Modulate North Atlantic Extratropical Cyclones and Extreme Weather in Europe

Recent research has established a statistical link between North American cold spells (CS) and concurrent wet or windy extremes in Europe. Here, we investigate whether such a link can be related to changes in the characteristics of north Atlantic extratropical cyclones (hereafter cyclones). Despite large regions of anomalous baroclinicity during periods of North American CS, the number of cyclones across the north Atlantic as a whole is comparable to climatology. We however find that CS over different North American regions are associated with different large-scale atmospheric configurations, that enhance the jet stream in the vicinity of the CS but result in different upper level wind anomalies in the east Atlantic. These in turn modulate the regional distributions and characteristics of cyclones in the north Atlantic and Europe. For eastern Canada CS, the north Atlantic jet extends over northern Europe, with the Atlantic storm track as a whole experiencing more intense cyclones which then affect France, the British Isles and Northern Europe. For eastern United States CS, the jet is displaced equatorward and extends over southern Europe, with a significantly heightened number of cyclones affecting Iberia, France and Southern Europe. For central Canada CS, the jet is displaced north of climatology and only partly extends over Europe, resulting in an anomalously high cyclone density in the eastern Atlantic and a higher number of cyclones affecting Iberia only. For eastern Canada and eastern United States CS, we observe a significantly heightened occurrence of cyclones undergoing explosive deepening. Our results provide a dynamical explanation for previous statistical findings on the concurrence of north American CS and wet or windy extremes over Europe.