The unprecedented Pacific Northwest heat-wave of June 2021

The Pacific Northwest (PNW) experienced an unprecedented heatwave from June 25th to July 2nd, 2021, affecting British Columbia and Alberta in Canada, and Washington and Oregon in the United States. Near-surface air temperature anomalies reached 16–20 °C above normal, exceeding previous all-time maximum temperature records by more than 5°C in many locations. The Canadian national temperature record was broken by 4.6 °C, reaching 49.6 °C in Lytton, BC. This event was significantly more intense than comparable heatwaves in Europe (2003) and Russia (2010), in terms of record exceedance and maximum temperature anomalies, though shorter in duration. The study aims to provide a comprehensive analysis of this extreme event, its meteorological causes, forecasting accuracy, and wide-ranging impacts across various sectors.

Previous research has documented the devastating effects of heatwaves on human health, ecosystems, and infrastructure. Studies on the 2003 European and 2010 Russian heatwaves highlighted the significant mortality associated with such events. The role of upstream diabatic heating in heatwave formation has been established, along with the impact of blocking high-pressure systems. Research on subseasonal forecasts has shown increasing accuracy in predicting extreme weather events with longer lead times. The study references existing literature to contextualize and interpret findings regarding the unprecedented PNW heatwave.

The study uses various data sources, including daily maximum 2 m temperatures (TX) from weather stations and ERA5 reanalysis data. TX anomalies relative to a 1981–2020 climatology were calculated, along with a 3-day running mean. Record temperature exceedances were analyzed, comparing the 2021 PNW heatwave to the 2003 European and 2010 Russian heatwaves. Synoptic conditions were analyzed using geopotential height, wind speed, and other atmospheric fields. Backwards air parcel trajectories from GFS forecast data were used to quantify the roles of diabatic and adiabatic heating processes. Operational weather forecasts from the North American Ensemble Forecast System (NAEFS) were examined to evaluate forecast accuracy and communication. Subseasonal forecasts from the ECMWF S2S project were analyzed to assess longer-term prediction capabilities. Impacts across human health, marine life, wildfires, agriculture, glacier/snow melt, and landslides were examined using various sources including government reports, satellite imagery, and streamflow data. The study also utilized the Berkeley Earth temperature anomaly dataset to assess regional temperature trends and PAMIP simulations to explore the potential role of Arctic amplification.

The analysis revealed that upstream diabatic heating, within frontal zones over the eastern Pacific and in orographic clouds over the Alaska Panhandle, played a major role in the heatwave's intensity. Adiabatic warming due to subsidence under strong high-pressure ridging and diabatic heating over land also contributed. Approximately 78% of the net temperature change of trajectory parcels resulted from diabatic processes. Operational weather forecasts provided advanced notice of the heatwave, although the magnitude of the maximum temperatures was slightly underestimated. Subseasonal forecasts showed an increased likelihood of a heat extreme with lead times of 10–20 days. The heatwave resulted in approximately 868 deaths across the PNW, extensive marine life mortality, increased wildfire activity, substantial agricultural yield losses, rapid glacier and snow melt leading to river flooding, and numerous landslides. The impacts were disproportionately felt by vulnerable populations, including First Nations communities and those in socially and economically deprived neighborhoods. Analysis of PAMIP simulations showed no robust evidence of changes in atmospheric waviness due to Arctic amplification, though further investigation is needed.

The findings highlight the critical role of upstream diabatic heating in the extreme intensity of the 2021 PNW heatwave, a factor often overlooked in simplified "heat dome" explanations. The study demonstrates that despite reasonably accurate forecasting, the unprecedented nature of the event led to significant human and ecological consequences. The findings underscore the need for improved early warning systems and heatwave preparedness, including investments in research and collaborative communication between forecast providers and users. Disproportionate impacts on vulnerable populations highlight the importance of climate justice. The lack of clear evidence from PAMIP simulations for Arctic amplification's role does not negate other potential climate change impacts on atmospheric circulation, such as altered soil moisture, tropical Rossby wave sources, or changes in atmospheric waveguides.

The 2021 PNW heatwave stands as an example of the devastating impacts of climate change, particularly emphasizing the importance of considering upstream diabatic heating in forecasting and mitigation strategies. The study underscores the critical need for improved forecasting capabilities, enhanced warning systems, and comprehensive preparedness measures to protect vulnerable populations. Further research is needed to better understand the interplay of various factors driving extreme heat events and to refine effective mitigation strategies.

The study acknowledges limitations, such as the difficulty of definitively attributing all deaths to the heatwave, challenges in precisely quantifying total marine life mortality, and complexities in isolating the heatwave's impact on annual crop yields. While the study included analysis of two PAMIP models, this may not fully represent the range of potential model responses. Further studies utilizing higher resolution models and more comprehensive data might provide better insights into complex interactions within the climate system.