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Simultaneous heatwaves and flooding in swaths of the US, Europe and Asia are being fuelled by a specific jet stream pattern that creates a series of “heat domes” which in turn drive up temperatures, scientists say.
The fast-moving band of air that is a key driver of global weather systems has been locked for weeks in a pattern characterised by five large U-bend shapes, dubbed “wavenumber 5”.
Scientists say such a pattern was also behind the extreme weather seen around the world last year.
Heat domes form in the regions south of the U-bends, raising temperatures in places within the dome as hot air is trapped and warms as it is squashed downwards.
Many regions are “suffering mightily” under these heat domes that have been “parked in place” for about a month, said Jennifer Francis, a scientist at the Woodwell Climate Research Center.
Scientists have been attempting to understand whether the band of fast-moving air that controls weather in the mid-latitudes is changing in a way that makes heatwaves and rainfall more frequent and persistent.
Following the month of June as the hottest on record globally, the World Meteorological Organisation has said preliminary data indicated that the start of July was the “hottest week on record”.
National meteorological agencies said records had been set in places including Xinjiang in China, which hit 52.2C last week, and Rome in Italy, which hit 41.8C.
Scientists are clear that extreme weather events will become more frequent and intense as climate change accelerates. The world has already warmed by at least 1.1C since pre-industrial times.
How climate change is changing the behaviour of the jet stream is less clear. Researchers are trying to untangle the relationship between the two phenomena and what it might mean for future weather patterns.
“It’s entirely normal for the jet stream to have this variability and get stuck in certain positions,” said Cathryn Birch, professor of meteorology and climate at the University of Leeds in the UK. “It’s not really possible to say whether climate change has influenced that.”
Although heatwaves can occur in the absence of a wavenumber 5 pattern, the presence of the naturally occurring phenomenon can drive simultaneous heatwaves since it creates a series of heat domes.
Heat domes can be self-reinforcing, since dry hot ground can heat up faster as a result of the lack of moisture that might otherwise evaporate and help cool the atmosphere.
Human-induced climate change exacerbates the conditions, given that average global temperatures are higher than they would otherwise be.
At present, marine heatwaves in the Pacific and Atlantic oceans were also “dumping heat into the atmosphere”, said Francis.
“It’s really remarkable what’s going on [in the oceans] right now and very disturbing,” she said. “When we get wavenumber 5 as well it accentuates the heat that’s being dumped into the atmosphere.”
Scientists have warned that this year and next are likely to be unusually hot as a result of the developing El Niño weather phenomenon, which is associated with a warmer Pacific Ocean surface temperature.
However, most concur that El Niño’s consequences are more likely to be felt in coming months as it strengthens.
Looking ahead, researchers said the wavy jet stream pattern was expected to continue into August, although it could shift pattern or location, which would alter which regions are affected by heat domes and rainfall.
The wavy jet stream was in a slightly different position in June, for example, causing the UK to experience unusually hot temperatures and Europe to endure heavy rainfall.
Regions that are between heat domes — such as parts of the US and South Korea in July — may experience sudden rainfall and flash flooding.
“You need something quite big to kick [a wavenumber 5 pattern] out of shape,” such as a large amount of energy from tropical thunderstorms, said Paul Davies, chief meteorologist at the UK’s Met Office.
The pattern is less common in winter in the northern hemisphere, as there is more energy in the atmosphere created by weather such as storms, which allows the jet stream to plough through potential waves.
There were indications the jet stream pattern might be shifting and changing further, forming into a wavenumber 6, which would mean an additional U-bend, said Davies.