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Pacific North American heatwave June 2021

During the last days of July 2021 a region in the Northwestern US and southwestern Canada experienced temperatures that have never been previously observed in this region, with records broken in many places by several degrees (Philip et al. 2021). A new all-time Canadian temperature record of 49.6°C was set in the town of Lytton, British Columbia. The same town was strongly affected and partly destroyed by one of the severe wildfires that occurred during and after the heatwave. The Pacific Northwest heatwave further contributed to a sharp increase in heat-related excess mortality. The Pacific Northwest heat wave is one of a series of record-shattering heat extremes that, based on the previous observational record, may have been deemed impossible. 

Figure1: (left) Temperature anomalies during the 2021 Pacific Northwest heatwave (NASA 2021) and (right) area-average temperatures in 2021 (red) compared to the period 1950-2020 (grey dots) in ERA5 reanalysis (plot by Erich Fischer).

Event definition

XAIDA defines the event intensity as the temperature anomaly averaged over the region (45–52 ºN, 119–123 ºW) used in Philip et al. (2021) including the cities of Seattle, Portland, and Vancouver (with well over 9 million people in their combined metropolitan areas). Different event durations from 1-day to 5-day temperature anomalies are analyzed.


XAIDA aims at improving the understanding of the driving physical mechanisms of this event, at putting the event into a long-term climatological context and at quantifying to what extent human activities have altered the odds of the Pacific Northwest heatwave to occur. Finally, XAIDA addresses the question whether the potential for such a record breaking event could have been foreseen before the event.

In a workshop in early February 2022 co-organized by WP5 and WP6 participants shared their initial results on the above objectives and co-ordinated future tasks in addressing research questions. The workshop helps to coordinate the use of a common event definition for future work. 

WP6 presented a detailed analysis of the processes driving the Pacific Northwest heatwave. The event was driven by an intense anticyclone situated over the heatwave region. A detailed analysis revealed the pathways along which the air masses moved towards the center of the heat anomaly, and quantified the process along these trajectories to the extremely hot air masses that ultimately reached the heatwave region. 

WP5 presented initial analyses demonstrating that in principle the possibility for such an event could have been foreseen based on statistical methods and targeted climate model-based experiments. The statistical analysis of the event is based on a weather generator that resamples extreme states (“empirical importance sampling”), and the climate model experiments are designed to repeatedly simulate a specific extreme event with slightly perturbed initial conditions (“climate model boosting”) to generate different trajectories of this extreme event.

Based on these findings WP5 aims to develop storylines for other record breaking heatwave events unseen in the observational record that have not yet occured. WP5 coordinates the definition of potentially impact-relevant other heatwave events e.g. for the 2024 Summer Olympics in Paris for which heatwave storylines are developed comparing different approaches developed within the XAIDA project.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101003469.