A persistent kink or shift in the jet stream can trap warm air over Europe and lead to heatwaves lasting two weeks or more. Recognising this could help improve forecasting of prolonged hot spells.
Heatwaves of at least a week are deadlier than ones spanning just a few days because people鈥檚 bodies are under stress for longer. Infrastructure like hydroelectric dams and agriculture also come under pressure. An early warning that a long heatwave is coming can give people, businesses and governments a chance to get ready. For instance, grid operators can ask power companies to prepare to generate more electricity to cope with a spike in demand due to air conditioning.
鈥淭he models can be improved if we understand these events better,鈥� says at the University of Bern in Switzerland.
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快猫短视频s only have daily atmospheric data for about 20 long heatwaves, so Pappert and his colleague , also at the University of Bern, analysed 1900 plausible hot spells simulated by a state-of-the-art climate model. They found that these periods tended to be driven by two types of atmospheric patterns. In both, Rossby waves 鈥� chains of weather systems that transport energy around the globe 鈥� alter the polar jet stream, the high-altitude belt of strong winds at roughly 60掳 north.
In the type I pattern, the jet stream gets wavier, kinking to the north around the UK and Scandinavia before veering south again to form a shape like the Greek letter omega. A persistent ridge of high pressure forms within this so-called omega blocking, setting the stage for a long-lasting heat dome over western Europe. Rather than breaking up the ridge, storms that come across the Atlantic Ocean reinforce the low-pressure cyclonic circulation at the beginning of the omega sign, which pulls hot subtropical air northwards as it spins.
In type II, the jet stream shifts polewards, again moving north of the UK but this time in a straighter pattern, and a ridge forms over Europe. The storm track then follows the jet stream, diverting low-pressure weather systems north of the continent, as warm subtropical air floods in from the south.
鈥淚t鈥檚 [always] the ridge that鈥檚 creating the heat 鈥� but how the atmosphere is setting up the situation for the ridge to be maintained, that鈥檚 the difference,鈥� says Pappert.
In both cases, the outcome is a relentless heat dome in western Europe. At Earth鈥檚 surface, air flows away from the high-pressure system to lower-pressure areas. As dry air descends from above to replace it, it is compressed and heats up, much like how pumping air into a bicycle tyre generates heat. Moist air isn鈥檛 rising to create clouds, so the sky is clear and constant sunshine further heats the lower atmosphere. It also heats the land, drying the soil and reducing evaporation, which heats the land even more in a feedback loop.
The two types are averaged tendencies, not hard categories. The heat dome behind the record heatwave in June, which killed an estimated 20,000 people in Europe, was a type II before morphing into a type I, says Pappert. Europe鈥檚 current heatwave largely resembles a type I, with a trough of low pressure to the west of a high-pressure ridge over Europe, he says.
鈥淭hese patterns should help forecasters,鈥� says at the University of Oxford. 鈥淭his really clearly lays out the structure and evolution of these events, so people know what to look for.鈥�
Heatwaves are getting hotter, more frequent and longer-lasting because global warming is raising background temperatures. But it is harder to say whether heat-inducing atmospheric patterns like these jet stream shifts are becoming more frequent. Some research has suggested that high-pressure blocking related to the jet stream will become more and over Europe, while other modelling has .
The jet stream is starting to shift northwards in general, which could bring drier conditions to southern Europe, says Woollings. If these type I and type II patterns did become more frequent or longer-lasting, it would be extremely damaging to economies and human health, he says.
鈥淚t is a concern that we don鈥檛 know,鈥� says Woollings. 鈥淲e don鈥檛 have a good enough understanding of these events really to know how their duration is changing.鈥�
Environmental Research Letters
