Increasing Risk of Concurrent Heatwaves in the Northern Hemisphere Mid- to High‑Latitudes

Abstract:

Simultaneous heatwaves across multiple regions pose coinciding and compounding threats to natural and human systems. While global heating is increasing heatwave risk across most regions, interactions between the pattern of heating and changes in atmospheric circulation patterns that yield concurrent heatwaves have not yet been investigated. Here, we quantify long-term (1979-2019) trends in warm-season (May-September) concurrent heatwaves across the Northern Hemisphere mid‑ to high‑latitudes. We use Self-Organising Maps to identify characteristic atmospheric circulation patterns in the mid-latitudes and examine the relative contributions of changes in the frequency of these patterns and thermodynamics in driving concurrent heatwave trends. We find a significant increase of ~46% in the mean spatial extent of concurrent heatwaves, ~17% increase in their maximum intensity, and ~7-fold increase in their frequency. We identify significant increases in circulation patterns associated with concurrent heatwave hotspots over eastern North America, eastern Europe, western Asia, eastern Asia, north-eastern Africa, the Barents and Kara Seas, the north-east Pacific, and the north Atlantic. Further, we show that changes in specific circulation patterns have a greater dynamical influence on concurrent heatwave frequency than others. Our results highlight the increasing risk of concurrent heatwaves in a warming world and show how thermodynamic and dynamic contributions interact to generate global hotspots at risk of simultaneous heatwaves. These findings are indispensable for evaluating projected climate risks on interconnected societal systems and fostering regional preparedness to extreme weather.