A storm-track connection between North American cold extremes and European wet/windy extremes

<p>The occurrence of cold spells over North America is statistically connected with an increased likelihood of extreme wind and precipitation events over Europe. This connection involves the state of the North Atlantic storm track in a nontrivial way. Cold spells over eastern Canada, for instance, are associated with windstorms over the United Kingdom in a configuration of positive North Atlantic Oscillation (NAO). However, cold spells over eastern Canada already tend to occur during the positive phase of the NAO, suggesting a chain of processes in which the internal dynamics of the storm track is responsible of both the cold spell and its European impact. Analogous considerations, although in the context of negative NAO, hold for cold spells hitting central United States and windstorms over the Iberian Peninsula.</p><p>Another key player in the dynamics of North American cold spells are Rossby wave trains propagating from the North Pacific towards North America and then the Atlantic Ocean. These low-frequency wave trains can impact the North Atlantic storm track directly, by reconfiguring the large-scale features defining locally the extratropical waveguide, or indirectly, by promoting anomalies in the genesis or propagation of extratropical cyclones.</p><p>We try to disentangle this complex interplay by analyzing systematically the state of the North Atlantic storm track before and after cold spells over several North American regions. Anomalies of wave activity flux, in the formulation by Takaya and Nakamura (2001), are computed to assess the relevance of Rossby wave trains for cold spells in each region. The main conclusion of this work is a reconsideration of the usual chain of processes connecting cold spells and wet/windy extremes over Europe: in some cases, the systematic connection between the former and the latter is likely mediated by the pre-existing state of the storm track and not simply due to the cold spells alone.</p>