More frequent European heatwaves linked to jet stream changes
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Heatwaves over Europe are growing three to four times faster than the rest of the northern mid-latitudes, including the U.S. and Canada. Why?exist a new study, an international team of scientists have shown that the increase is related to changes in the jet stream, a fast flow of air flowing from west to east about 10 kilometers above the surface of the northern hemisphere. The period during which the jet splits into two branches—the so-called double-jet regime—has gotten longer. According to the authors, these twin jet regimes explain nearly all of the rise in heatwaves in Western Europe, and about 30% of the rise across Europe.The study has just been published in the journal Nature Communications.
“While summer heatwaves are not a new phenomenon, what is new is the increased frequency and intensity of extreme heat events in Europe over the past few years,” says lead author Efi Rousi from the Potsdam Institute for Climate Impact Research (PIK). Recently, Europe had an extremely hot summer in 2018, 2019 and 2020; it has now seen unusually warm weather.
For their analysis, the scientists defined a sustained heatwave as at least six consecutive days during which the maximum temperature exceeded a threshold of 10 percent of the hottest days in a given location. They examined daily climate data for two of Europe’s hottest months in 42 years, July and August.
“We found that the jet stream generally has three states, one of which is a dual jet state, consisting of two jet streams with increasing wind speeds, one in the south and one in northern Eurasia,” said the co-authors. Kay CohenhuberAdjunct Fellow at Columbia Climate Institute Lamont-Doherty Earth Observatory. While the number of double-spray events varies little from year to year, they become more persistent.
Western Europe coincides with storm tracks entering the continent from the North Atlantic, and these weather systems often have a cooling effect. But during the twin jet state, these weather systems shift north. Outcome: Persistent heatwaves can develop in Western Europe.This increased persistence operates on top of a more intense temperature increase from anthropogenic warming to fuel heat wave.
Research shows that the persistent increase in twin jets explains about 30% of heatwave trends across Europe, but almost 100% in the west. The authors say this is in stark contrast to other European regions, such as the Mediterranean and Eastern Europe, where dry soils may be more important to the development of persistent heatwaves.
“Our new results underscore the importance of understanding atmospheric dynamical processes to predict future risk of extreme heat and to identify global hotspots,” Kornhuber said.
Adapted from a press release from the Potsdam Institute for Climate Impact Research.
