Explaining Pan-Atlantic Cold and Windy Extremes Using an Analog-Based Approach

The occurrence of cold spells over different regions of North America has been previously linked to windy extremes over Western Europe. These so-called pan-Atlantic extremes are necessarily mediated by the North Atlantic circulation. It is known that the Atlantic storm track modulates European windstorm occurrence, but it is unclear whether the American cold spells directly influence the storm track, or whether the cooccurrence of extremes is indirect—a result of a common large-scale driver. In this study, cold spells over both central North America and northeast Canada are clustered with respect to the evolution of the large-scale circulation over the North Atlantic. The direct contribution of cold spells to the European wind extremes is then ascertained using circulation analogs, so that different states of the North Atlantic storm track can be compared for days with and without cold spells. Consistent with previous work, two main pathways emerge from the analysis, called “zonal” and “wavy” for simplicity. For a wavy pathway, North American cold spell occurrence is directly associated with more frequent European wind extremes than expected from the Euro-Atlantic flow, as a result of Rossby wave trains. For the other pathways, the common driver of storm track variability linked to the anomalous Atlantic circulation was sufficient to explain more frequent wind extremes across Europe, with no or little ascertainable contribution from the cold spells. This analysis clarifies that the causality of wintertime pan-Atlantic extremes is flow-dependent—either direct or indirect depending on the active dynamical pathway.