The Typicality of Regimes Associated with Northern Hemisphere Heatwaves

Abstract We study the hemispheric- to continental-scale regimes that lead to summertime heatwaves in the Northern Hemisphere. By using a powerful data mining methodology—archetype analysis—we identify the characteristic spatial patterns consisting of blocking high pressure systems embedded within a meandering upper atmosphere circulation that is longitudinally modulated by coherent Rossby wave packets. Periods when these atmospheric regimes are strongly expressed correspond to large increases in the likelihood of extreme surface temperature. Most strikingly, these regimes are shown to be typical of surface extremes and frequently reoccur. Three well-publicized heatwaves are studied in detail—the June–July 2003 western European heatwave, the August 2010 “Russian” heatwave, and the June 2021 “heatdome” event across western North America, and are shown to be driven by blocking high pressure systems linked to stalled Rossby wave packets. We discuss the implications of our work for long-range prediction or early warning, climate model assessment, and postevent diagnosis.