Large-Scale Atmospheric Drivers of Compound Cold Spells in North America and Wet or Windy Extremes in Europe

Recent winters have witnessed an ostensibly large number of cold spell occurrences over Eastern North America and wet or windy weather over Western Europe. These extremes have typically been discussed separately. However, their repeated co-occurrence suggests that they may be spatially compounding extremes, namely geographically remote extremes associated with common physical drivers. Here, we analyse the interplay between the large-scale atmospheric circulation and co-occurring cold spells in North America and wet or windy extremes in Europe, which we collectively term compound cold–wet–windy extremes. We leverage a recent approach grounded in dynamical systems theory, which allows to compute the instantaneous coupling between different atmospheric variables, thus providing an analytically and computationally efficient analysis of spatially resolved, multivariate climate extremes. We specifically seek to answer the following questions:

• Do cold spells over Eastern North America and wet or windy extremes over Western Europe individually emerge as events with a particularly strong coupling to recurrent large-scale circulation patterns?
• Is there evidence for recurrent large-scale circulation patterns systematically associated with the co-occurrence of these extremes?

In answer to the first question, we find that there are indeed specific, recurrent large-scale atmospheric circulation patterns systematically associated with the individual cold, wet or windy extremes. We further find that a similar conclusion holds for co-occurring cold–wet–windy anomalies in the two continents. In answer to the second question, evidence for common, recurrent large-scale atmospheric circulation patterns is also found when focusing on compound cold–wet–windy extremes, although with a weaker signal than for the larger group of cold–wet–windy anomalies. We thus hypothesise that the compound extreme occurrences may be linked to multiple dynamical pathways, which may differ from those associated with weaker surface anomalies. The extremes may further be modulated by small-scale effects not reflected in the large-scale atmospheric circulation.