Increase in heatwaves in western Europe linked to changes in the jet stream

• Heatwaves over Europe have increased 3-4 times faster than in the rest of northern mid-latitudes, especially in western Europe
• These heat extremes are linked to double jet streams and their increasing persistence
  
• This amplified heatwave trend is unique to Europe compared to other regions in the Northern Hemisphere

“While summer heatwaves are not a new phenomenon, what is new is that extreme heat events in Europe have been occurring with greater frequency and intensity in the past years. Just think about the hot and dry summers of 2018, 2019, 2020 and the current heatwave in Europe– and this is expected to get worse,” says Efi Rousi from the Potsdam Institute for Climate Impact Research (PIK) and lead author of the study to be published in Nature Communications. “Our study shows that these heat extremes in Europe are linked to double jet streams and their increasing persistence over the Eurasian sector.“

In the study, the scientists looked at how the jet stream – a fast-flowing air current that travels west-to-east around the globe’s Northern hemisphere at around 10 km height – might have contributed to the observed heatwave trends. To conduct the analysis, the scientists defined persistent heatwaves as at least six consecutive days during which the maximum air temperature exceeded the threshold of the 10 percent hottest days in a given location. They examined daily climate data for the two hottest European months, July and August, over a period of 42 years.

An important role for atmosphere dynamics in fueling western European mega-heatwaves

“We found that there are typically three states of the jet stream, one of them being the double jet state, consisting of two jet stream branches with increased wind, one over Southern and one over Northern Eurasia,“ co-author Kai Kornhuber, scientist at Columbia University in New York and PIK, explains. While the number of double jet events per year did not change much, the double jet events became longer and thus more persistent. This increased persistence then acts on top of the temperature increases from human-made warming to fuel more intense heatwaves. Kornhuber explains: “Our new results highlight the importance of understanding atmosphere dynamical processes to anticipate future risks of extreme heat and to identify global hotspots such as western Europe.”                        

The increasing persistence of double jet streams is especially relevant for western Europe, the researchers found. “Our study shows that the increasing persistence of double jets explains about 30 percent of heatwave trends for the whole of Europe. Yet, if we only look at the smaller western European region, it explains almost 100 percent”, says Efi Rousi. “In this region, which coincides with the exit of the storm track coming from the North Atlantic towards Europe, weather systems normally originate from the Atlantic and therefore have a cooling effect – during double jet states the weather systems get diverted northwards and persistent heatwaves can develop over western Europe.” This is in contrast to other European regions like the Mediterranean and Eastern Europe where dry soils are likely more important for the development of persistent heatwaves.

What is favouring the increased persistence of double jet events?

“Double jets can be triggered by a variety of reasons including chaotic variability in the atmosphere,” explains co-author Dim Coumou, researcher at the Institute for Environmental Studies (IVM) at VU Amsterdam and the Royal Netherlands Meteorological Institute (KNMI). “But the interesting question is what makes double jets more persistent? A possible explanation is the enhanced warming of high-latitudes, in particular over land regions like Siberia, Northern Canada and Alaska. In summer those regions have warmed much faster than the Arctic ocean, as over the ocean the excess energy is used to melt sea ice.” The land surrounding the Arctic ocean has seen very rapid warming in summer associated with rapid retreat in late spring snow cover. ” This increasing temperature difference between land and ocean favours the persistence of double jet states in summer,” says Coumou. Kornhuber adds: “Climate models tend to underestimate extreme weather risks. Thus future research needs to assess to what degree the identified relationships are captured by the models, as projections of extreme heat under continued greenhouse gas emissions might be too conservative.“

Rousi concludes: “While this needs further research, one thing is clear: Double jet streams and their growing persistence are key to understand current and future heatwave risks over western Europe.”