Explaining the unexplainable: leveraging extremal dependence to characterize the 2021 Pacific Northwest heatwave

TL;DR

This paper develops a spatial extremes model to analyze the 2021 Pacific Northwest heatwave, revealing the significant influence of climate factors and improving prediction of extreme temperatures beyond single-station methods.

Contribution

It introduces a flexible spatial extremes model that accounts for extremal dependence across multiple stations, enhancing understanding of extreme temperature events.

Findings

Model explains over 96% of 2021 high temperatures.

Greenhouse gases, drought, and atmospheric variability significantly impact heat extremes.

Spatial analysis outperforms single-station approaches.

Abstract

In late June, 2021, a devastating heatwave affected the US Pacific Northwest and western Canada, breaking numerous all-time temperature records by large margins and directly causing hundreds of fatalities. The observed 2021 daily maximum temperature across much of the U.S. Pacific Northwest exceeded upper bound estimates obtained from single-station temperature records even after accounting for anthropogenic climate change, meaning that the event could not have been predicted under standard univariate extreme value analysis assumptions. In this work, we utilize a flexible spatial extremes model that considers all stations across the Pacific Northwest domain and accounts for the fact that many stations simultaneously experience extreme temperatures. Our analysis incorporates the effects of anthropogenic forcing and natural climate variability in order to better characterize time-varying changes in the distribution of daily temperature extremes. We show that greenhouse gas forcing, drought conditions and large-scale atmospheric modes of variability all have significant impact on summertime maximum temperatures in this region. Our model represents a significant improvement over corresponding single-station analysis, and our posterior medians of the upper bounds are able to anticipate more than 96% of the observed 2021 high station temperatures after properly accounting for extremal dependence.