What caused the massive Pacific Northwest heatwave?

In June 2021, the Pacific Northwest experienced one of its worst heatwaves in history, with Canada and the states of Washington and Oregon setting historical records of extremely high temperatures. To better understand what factors made it so severe, a team of climate scientists from the Portland State University (PSU) has provided a detailed meteorological analysis of the progression of this heatwave, and compared the atmospheric conditions associated with the 2021 event with other similar historical events.

“It’s like taking the symptoms and diagnosing what it was that happened to cause something so severe to happen to the system,” said study lead author Paul Loikith, an associate professor of Geography and director of PSU’s Climate Science Lab. “We looked into a bunch of different factors that have been known to drive heat waves in the region to see if they were there with this heatwave. Were they normal but way more severe, or did something unique happen that we haven’t seen before?”

The analysis revealed that, although this heatwave largely shared the same meteorological features and processes as other heatwaves in the Pacific Northwest, each aspect of it was stronger. Its main driver was a large ridge of high pressure – known as a heat dome – in which air sinks under areas of high pressure, preventing clouds from forming and maximizing solar heating during the day. One of the reasons this ridge was so strong was that a storm with an atmospheric river (a long, narrow band of water vapor which releases massive amounts of moisture) moved into Alaska a week before. Then, water vapor condensed, releasing heat that accumulated in the atmosphere and caused the ridge to further intensify.

“This was more like a wintertime ridge in its physical structure and characteristics, which are bigger than summertime ridges, but occurring just after the summer solstice, we had maximum solar heating and the ability for the air to get really, really hot,” Loikith explained.

After reaching peak strength over southern British Columbia when it was 10,000 to 15,000 feet above the surface, the ridge warmed at a rate of 10 degrees Celsius per kilometer while descending to Seattle, Portland, and Pendleton. Although the air aloft was cool by human standards, it was quite hot for that altitude, and, when it was brought down to the surface, it turned extremely hot. In addition, stronger offshore winds blowing from the land to the coast further increased the air temperatures. Interestingly, the heatwave ended abruptly when a surge of ocean air displaced the heat and moved the system eastward.

Better understanding the complexity of factors coming together to cause such extreme phenomena could help forecast similar – or even more severe – future events. “These features coming together in this way will continue to be really rare, but in the future as the climate continues to warm, you will not need as strong of a weather system to make it extremely hot by historical standards,” Loikith concluded.

The study is published in the journal Monthly Weather Review.

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By Andrei Ionescu, Earth.com Staff Writer

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