Supercharged heat waves, like Washington’s deadliest, will hit harder and more often, study finds

The 2021 North American heat wave was the deadliest weather disaster in Washington, leaving more than 100 people dead in the evergreen state and many more in neighboring areas. Scientists not only suggest that such heatwaves will become more intense and strike more often – in new work published in npj Climate and atmospheric sciencethey reveal the mechanism underlying these enhanced heat waves.

“The scientific community has suggested for some time that heat waves will intensify with climate change,” said lead author Ziming Chen, a postdoctoral researcher at the Department of Energy’s Pacific Northwest National Laboratory. .

“But we haven’t understood exactly what’s happening in Earth’s atmosphere that intensifies these heat waves, or what’s causing them to occur more frequently. We can now see what specific changes in atmospheric circulation patterns are likely to be the cause of these changes.

The authors of the new study sought to better understand how global warming affects thermal domes like the one in summer 2021. The authors present their work this week at the American Geophysical Union’s 2023 fall meeting in San Francisco.

Previous work by PNNL scientists suggests that current climate commitments by countries around the world are still insufficient to limit global warming to 1.5 degrees Celsius, leading to the risk of persistent extreme weather. COP28 – the global climate conference at which world leaders manage their emissions commitments in a bid to limit warming – ends today.

“Our new work suggests that if current emissions trends continue,” said Ruby Leung, co-author and atmospheric scientist, “we expect these changes to increase heat wave risk throughout the Northwest region in the coming decades. Residents of California, Idaho and Oregon Washington, Washington and other neighboring states could experience particularly strong heat waves. By better understanding conditions leading up to these events, we can all be better prepared.

What causes heat domes and why do they get worse?

The process begins with a warmer sea surface in the Pacific Ocean, heated by warming greenhouse gases and further enhanced by El Niño-like warming.

This heat gives way to convection: the air above the warmer sea surface begins to mix as warm air rises and cool, dense air sinks. This convective mixing transfers heat upwards.

This convection releases heat aloft, which then gives way to what scientists call atmospheric Rossby waves. Just as ocean waves move through the sea, atmospheric Rossby waves travel through Earth’s atmosphere to affect weather patterns far downstream.

Rossby waves can strongly influence Earth’s climate and even intensify extreme weather conditions. In this new work, Rossby waves play a direct role in preparing the Pacific Northwest for a thermal dome-like effect, threatening the region with sweltering heat.

Heat domes occur when high-pressure atmospheric conditions lock onto a region, sometimes refusing to move for weeks. High pressure conditions create clear skies, reducing cloud cover or rain that cools the area. By simulating Earth’s atmospheric processes, the authors discover that Rossby waves could become stationary more often over the Pacific Northwest in the future, producing a heat dome-like effect.

They also find that these Rossby standing waves could almost double in magnitude over the next few decades.

“Although the amplitude of the heat dome standing wave over the Pacific Northwest would be about twice as large in a high emissions scenario near the end of the century,” Chen said, “ the intensity of heat waves would increase much more than that of the thermal dome itself due to the contribution of background heating and land-atmosphere interactions.”

Such interactions, such as the amount of soil moisture available to help cool the atmosphere through evaporation, can significantly affect local climate. In the example of the summer 2021 heat wave, Chen said, overly dry ground contributed additional degrees of heat to an already intense heat wave. Changes in wind patterns could also push Rossby waves further and further toward North America, causing them to strike more often.

Previous research by PNNL scientists has shown that warmer summer nights can increase urban heat stress and that residents of many U.S. cities experience these effects disproportionately. The authors of the new work hope their findings will inform strategies to help communities better prepare for extreme weather. Understanding changes related to extreme weather is also key to building a more resilient power grid.