Typicality of the 2021 Western North America Summer Heatwave

TL;DR

This paper uses large deviation theory and Earth System Model simulations to analyze the 2021 Western North America heatwave, revealing its statistical rarity, physical mechanisms, and influence of climate change and geography.

Contribution

It introduces a novel application of large deviation theory to characterize the typicality and probability of the 2021 heatwave using high-resolution climate simulations.

Findings

The 2021 heatwave is an unlikely but possible climate variability event.

Climate change significantly increased the probability of such extreme events.

The Rocky Mountains influence the spatial coherence and persistence of the heatwave.

Abstract

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 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 large deviation theory and taking advantage of long simulations by a state-of-the-art Earth System Model, we investigate the 2021 North America 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.