Typicality of the 2021 Western North America summer heatwave

Elucidating the statistical properties of extreme meteo-climatic events and capturing the physical processes responsible for their occurrence are key steps for improving our understanding of climate variability and climate change and for better evaluating the associated hazards. It has recently become apparent that large deviation theory (LDT) is very useful for investigating persistent extreme events, and specifically, for flexibly estimating long return periods and for introducing a notion of dynamical typicality. Using a methodological framework based on LDT and taking advantage of long simulations by a state-of-the-art Earth system model, we investigate the 2021 Western North America summer heatwave. Indeed, our analysis shows that the 2021 event can be seen as an unlikely but possible manifestation of climate variability, whilst its probability of occurrence is greatly amplified by the ongoing climate change. We also clarify the properties of spatial coherence of the 2021 heatwave and elucidate the role played by the Rocky Mountains in modulating hot, dry, and persistent extreme events in the Western Pacific region of North America.

Automated summary

Understanding the statistical properties of extreme weather events and their underlying physical processes is crucial for evaluating climate variability, climate change, and associated hazards. Recent studies have shown that large deviation theory (LDT) is useful for investigating persistent extreme events and estimating long return periods. In this study, we utilize LDT and a state-of-the-art Earth system model to analyze the 2021 Western North America summer heatwave. We find that the occurrence of the heatwave can be attributed to climate variability, but its probability is greatly amplified by ongoing climate change. We also examine the spatial coherence and the role of the Rocky Mountains in influencing extreme events in the Western Pacific region of North America.