Scientists reveal the backstory of what happened last summer in the deadly 2021 Pacific NW heatwave.

From the Daily Kos:

“A new study from the University of Chicago explains a hidden backstory behind the horrific heatwave in late June to early July of 2021 that enveloped the Pacific Northwest. This data from the university should help project future heatwaves as ‘scientists worry that we are approaching—or have already approached—a tipping point in the alteration of the Earth’s atmosphere, after which extreme events become much more likely.’”

“The University of Chicago shares in their presser the work of Professor Noboru Nakamura, who laid out a set of diagnostics to measure the mechanics of sizeable atmospheric pressure events such as this particular event. The National Science Foundation paid for the study. Clare S.Y. Huang, Ph.D., was the other author of the study published by the AGU.“

Pressure changes

The heat wave began on June 26, 2021.

Previous record high temperatures shattered one after the other, by huge margins. Streetcar cables melted in Portland, Oregon; pavement buckled across the region. Before it was over, a town in Canadian British Columbia tied Death Valley for the highest temperature ever recorded in North America – 121 degrees Fahrenheit.

But the conditions had been set in motion weeks before. Using data collected from satellites and on the ground, UChicago scientists set out to re-create the sequence of events.

They found that in the week prior, a cyclone had formed over the Gulf of Alaska. Cyclones are large, spiral-shaped systems that form around a center of low pressure. (Think of the spiral clouds you see during hurricanes.) When clouds form out of water vapor, the process actually releases heat, which accumulated in the atmosphere.  

Then, as the cyclone moved slowly away, it triggered the formation of an anticyclone to the east—a system that rotates slowly around a center of high pressure instead of low. These are known as “blocking” systems because they disrupt the normal eastward movement of weather systems. A blocking anticyclone acts like a blanket, trapping heat in a region.

The result was a warm, stagnant column of air that made it difficult for surface heat to escape to the upper atmosphere as it normally does.

Blocking systems are well known for causing heat waves in the mid-latitudes, explained Emily Neal, a UChicago undergraduate student in environmental science and first author on the paper. “But this was an extraordinarily strong blocking event,” said Neal. “Our analysis showed that the warmth of the air column within the blocking system was in the top 0.01% of all events along the same latitude in the past half a century.”