not-yet-known not-yet-known not-yet-known unknown This study investigates the representation of near-simultaneous cold and windy extremes in North America and Europe in an ensemble of historical climate model simulations as compared to reanalysis. By leveraging a weather regime perspective, we identify five dynamical pathways for cold spells in three regions of North America. Three of the pathways also engender European wind extremes. The pathways are: (i) A wave train producing central and eastern Canada cold spells, culminating in Scandinavian blocking. (ii) A persistent Atlantic low producing eastern Canada cold spells and wind extremes in the British Isles. (iii) A quasi-stationary wave-2 pattern producing central Canada cold spells and Scandinavian blocking. (iv) An Arctic high producing eastern United States cold spells and wind extremes in Iberia. (v) A wave train producing eastern United States cold spells, culminating in an Atlantic low and wind extremes in Iberia. Models represent well both the frequency and evolution of the pathways compared to reanalysis. However, they under-represent the frequency of pathways (i) and (iii) associated with Scandinavian blocking. The models perform very well in replicating mean surface temperature anomalies during cold spells, though they perform less well on European wind extremes. Typically, the models capture the region and timing of wind extremes associated with Atlantic lows, albeit with some under-representation of occurrence frequency, but fail to adequately capture the wind extremes associated with Arctic highs. This is linked to deficits in how the models reproduce the evolution of the dynamical pathways in the East Atlantic.

The occurrence of cold spells over different regions of North America has been linked to windy extremes over western Europe. These events – termed pan-Atlantic extremes – are mediated by an anomalous state of the North Atlantic storm track. While it is known that the occurrence of European windstorms is modulated by the state of the storm track, the relative contribution of the North American cold spells to European wind extremes is not easy to quantify. In this study, cold spells over two regions of North America are clustered with respect to the evolution of the large-scale circulation over the North Atlantic. The 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 associated with more frequent European wind extremes than expected from the state of the North Atlantic storm track. For the other pathways, on the other hand, the anomalous state of the storm track was able to account alone for the more frequent wind extremes than climatology observed across Europe, with no or little ascertainable contribution from the cold spells. This analysis clarifies the causality of wintertime pan-Atlantic extremes and how these link to different atmospheric dynamical pathways.

Winter cold spells over North America have been correlated with European wind extremes, but the physical mechanisms behind such “pan-Atlantic” compound extremes have not been clarified yet. In this study, we propose that pan–Atlantic cold and windy extremes occur following two possible dynamical pathways. The first one involves the propagation of a Rossby wave train from the Pacific Ocean, associated with windstorms over north-western Europe in the 5-10 days after the cold spell peak. The second is associated with a high-latitude anticyclone over the North Atlantic and an equatorward-shifted jet, leading to windstorms over south-western Europe already in the days preceding the cold spell peak. European windstorms are thus consistently tied to North American cold spells according to the different flow configuration. The analysis underscores that seemingly similar surface extremes may be driven by different processes, and that overlooking these subtleties and conflating them together could lead to misleading conclusions.