Large-scale winds at 5 to 10 kilometer height, the so-called jet stream, are changing over Eurasia. Periods during which the jet stream is split into two branches – so called double jet states – have become longer lasting. These double jet states explain almost all of the upward trend in heatwaves in western Europe, and around 30 percent over the larger European domain. The results were published in Nature Communications.
In the study, the scientists looked at how the jet stream 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.
“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 and 2020 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. “Our study shows that these heat extremes in Europe are linked to double jet streams and their increasing persistence over the Eurasian sector.“
An important role for atmosphere dynamics
“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. “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”, says Kornhuber.
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 makes double jets more persistent?
“Double jets can be triggered by a variety of reasons including chaotic variability in the atmosphere,” explains co-author Dim Coumou, Professor of Climate Extremes at the 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.