Anthropogenic contributions to the 2021 Pacific Northwest heatwave

Abstract

On 26-29 June 2021, an unprecedented heatwave affected the Pacific Northwest (PNW) of the United States and western Canada. Temperature records were shattered, with all-time highs of 116°F (47°C) in Portland, Oregon, 108°F (42°C) in Seattle, Washington, and 121°F (49°C) in Lytton, British Columbia (Di Liberto, 2021). Heatwaves, characterized by prolonged periods of excessive heat, can have dangerous impacts on human health, infrastructure, and the environment (Campbell et al., 2018;McEvoy et al., 2009;Perkins-Kirkpatrick & Alexander, 2013;Ruffault et al., 2020), and the PNW heatwave was no exception. Over 500 deaths were attributed to the heatwave (Popovich & Choi-Schagrin, 2021), and the environment and infrastructure throughout the affected region were strained and damaged, with crops ruined and roads buckled due to the excessively hot temperatures (Baker & Sergio, 2021). The devastating and large-scale impacts of the PNW heatwave were exacerbated by the lack of adaptability of a region unaccustomed to such extreme high temperatures, with the observational record suggesting that this was a highly anomalous event (Figure 1).The meteorological conditions of the PNW heatwave were similar to previous deadly heatwaves in Europe and Russia (Black et al., 2004;Dole et al., 2011). These events are associated with atmospheric blocking patterns, which are known to cause extreme heatwaves (

How Unexpected Was the 2021 Pacific Northwest Heatwave?

“…The record‐breaking 2021 PNW heatwave raised many questions for the climate science community that we are only now beginning to answer. In this work, motivated by limitations in estimating the probability of the event using either statistical (Philip et al., 2021) or climate modeling (Bercos‐Hickey et al., 2022; Lin et al., 2022; Pendergrass et al., 2021) methods, we focus on the role of non‐normality in increasing the probability of the heat event beyond what would be expected in the case of a normal distribution. In particular, the magnitude of climatological skewness at weather stations across the PNW region is found to be a good predictor of the standardized magnitude of the maximum temperature during the 2021 heat wave.…”

“…However, despite the GEV fitting the 1950–2020 data well, the 2021 event was predicted to have a probability of zero. Second, initial analyses of subseasonal forecasting (Bercos‐Hickey et al., 2022; Lin et al., 2022) and climate (Pendergrass et al., 2021) model ensembles tend to find that dynamical models cannot produce temperature anomalies as large as observed in advance of peak summer. Given the record‐breaking nature of the heatwave, as well as the high likelihood that it was an unusual event even given historic climate change, we turn to simulated data in order to produce a data set sufficiently large to capture very extreme events.…”

How unexpected was the 2021 Pacific Northwest heatwave?

“…The record-breaking 2021 PNW heatwave raised many questions for the climate science community that we are only now beginning to answer. In this work, motivated by limitations in estimating the probability of the event using either statistical (Philip et al, 2021) or climate modeling (Bercos-Hickey et al, 2022;Lin et al, 2022;Pendergrass et al, 2021) methods, we focus on the role of non-normality in increasing the probability of the heat event beyond what would be expected in the case of a normal distribution. In particular, the magnitude of climatological skewness at weather stations across the PNW region is found to be a good predictor of the standardized magnitude of the maximum temperature during the 2021 heat wave.…”

“…However, despite the GEV fitting the 1950-2020 data well, the 2021 event was predicted to have a probability of zero. Second, initial analyses of subseasonal forecasting (Bercos-Hickey et al, 2022;Lin et al, 2022) and climate (Pendergrass et al, 2021) model ensembles tend to find that dynamical models cannot produce temperature anomalies as large as observed in advance of peak summer. Given the record-breaking nature of the heatwave, as well as the high likelihood that it was an unusual event even given historic climate change, we turn to simulated data in order to produce a data set sufficiently large to capture very extreme events.…”

Simulating the West Pacific Heatwave of 2021 with Analog Importance Samping

“…There is also evidence that extreme heating may have been partly due to the interaction between the Omega blocking over the PNW and an atmospheric river (Mo et al, 2022;Lin et al, 2022;Bercos-Hickey et al, 2022) excited by the East Asian Summer Monsoon (EASM). Qian et al (2022) and Bartusek et al (2022) argue that subseasonal variations of the EASM and of the jetstream may have contributed to the intensification of a Rossby wave train crossing the Pacific and in phase-locking configuration with the PNW anticyclonic blocking.…”