Deadly heat waves that have fueled fires and caused transportation disruptions in Europe, the United States and China this month have one thing in common: a peculiar jet shape dubbed “wave number 5.”
Scientists are trying to understand whether the band of fast-moving air that controls mid-latitude weather is changing in ways that make heat waves more frequent and persistent.
“The jet stream is the main driver of our weather,” said Paul Williams, a professor of atmospheric sciences at the University of Reading in the UK. “The jet stream is like a conveyor belt that brings us storms one after another.”
It can also create heat waves when it forms into a U-bend shape called an “omega block” because it resembles the shape of the Greek letter omega.
Right now, a global pattern of five big waves is circling the world, causing simultaneous heat waves on continents. This pattern, known as wave number 5, can persist for weeks, causing hot areas to stay hot for a long time.
In China, more than 900 million people are feeling the heat, and more than 70 weather stations have broken records this month. In the US, Texas and Oklahoma saw record high daily temperatures, and heat warnings were issued in more than 20 states.
The UK also recorded its hottest temperature on record this week at 104.5F, while France and Spain battle wildfires after weeks of record-breaking heatwaves.
“As is often the case in the atmosphere, it’s linked: if we see an extreme event in one place, it can be associated with extreme events elsewhere,” said Stephen Belcher, chief scientist at the UK Met Office. “The Met Office forecasters are looking very closely at this wave number 5 pattern to see how long it persists,” he added.
Belcher said three factors contributed to the heat over Europa: a wavenumber 5 pattern in the jet stream; increase in average global temperatures; and dry soils, especially around the Mediterranean, as a result of long hot weather.
Diem Koumu, a climate scientist at the University of Amsterdam, said that in summer there are two important patterns in the jet stream — with five or seven waves — that tend to stay in the same place when they form. “When these waves become stagnant and persist for longer periods, then we usually see simultaneous heat waves,” he said.
A growing body of research is trying to answer the question of exactly how global warming is changing the jet stream and what that means for future weather. Temperatures have already risen about 2 degrees Fahrenheit since pre-industrial times due to human activity.
The jet stream itself appears to change its behavior in the long term and slows down in the summer, which may make the “omega block” picture more likely.
Jennifer Francis, an atmospheric scientist at the Woodwell Center for Climate Research in Falmouth, Mass., said the reason for this slowdown is the rapid warming of the Arctic region.
“In the summer, there is a general decrease in winds,” Francis said. “The reason for the jet stream is that the north is cold and the south is warm, and this temperature difference creates [the condition for the jet stream],” she said.
Because the Arctic is warming much faster than the rest of the planet, the temperature difference between these air masses is now smaller.
Some of the behavior of the jet stream is still unexplained: “Over the Atlantic, the jet shifted south during the summer,” said Tim Woolings, author Jet stream and Professor of Atmospheric Physics at Oxford. “Whereas we expected it to shift northward in response to climate change.”
The recent heatwave Britain has experienced is “just a small taster” of what the rest of Europe has experienced, Woollings said. “A real event over Spain and France,” he said.
The UK experienced two days of extremely hot temperatures on 18 and 19 July before the weather turned colder; Spain and France have seen high temperatures for several weeks.
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As global average temperatures rise, climate models show that heat waves will become hotter. However, it could be years before researchers know exactly how global warming is affecting these jet patterns.
“We need very long records of observations,” Williams said. “It could be decades or even centuries before we conclusively detect any changes.”
This story originally appeared in the July 21, 2022 issue of The Financial Times.
Copyright 2022 The Financial Times Limited
Reprinted with permission.