The lethal heat waves that stoked fires and caused transport disruptions in Europe, the United States and China this month have one thing in common: a peculiar shape in the “jet stream” called “wave number 5”. “.
Scientists are racing to understand whether the fast-moving air current that controls the climate in mid-latitudes is changing in a way that makes heat waves more frequent and lasting.
“The jet stream is the main driver of our climate”, explains Paul Williams, professor of atmospheric science at the University of Reading (UK). “The jet stream is like a conveyor belt, bringing us storms one after another.”
It can also generate heat waves when it takes on a U-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 all continents. This pattern, known as wave number 5, can last for weeks, causing hot areas to stay hot for a long time.
In China, more than 900 million people are experiencing heat waves and more than 70 weather stations have set records this month. The US states of Texas and Oklahoma are experiencing record daily temperatures, and more than 20 states have issued heat alerts.
The UK also recorded its highest-ever temperature this week, 40.3°C, as France and Spain battle wildfires after an extreme heat wave that lasted weeks and set temperature records.
“As is often the case in the atmosphere, it’s connected: if we see an extreme event in one place, it could be connected to extreme events in another,” says Stephen Belcher, chief scientist at the UK Meteorological Department (Met Office). “Met Office meteorologists are looking very closely at this wave number 5 pattern to see how long it persists,” he added.
Belcher says three factors contributed to the heat wave in Europe: the wave 5 pattern in the jet stream; the increase in global average temperatures; and dry soils, particularly around the Mediterranean, resulting from prolonged hot weather.
Dim Coumou, a climate scientist at VU Amsterdam, says there are two important patterns in the summer jet stream — with five waves or seven waves — that tend to stay in the same place when they form. “If these wave patterns become stagnant and persist for longer periods, we will typically see simultaneous heat waves.”
A growing research group is trying to answer the question of how exactly the jet stream is being altered by global warming and what that means for future weather patterns. Temperatures have already increased by about 1.1°C since pre-industrial times due to human activity.
The jet stream itself appears to be changing its behavior over the long term and slowing down in the summer, which could increase the likelihood of the “omega block” pattern.
Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center, says the rapid warming of the arctic region appears to be the cause of this slowdown.
“There’s a general decrease in winds in the summer,” says Francis. “The reason there’s a jet stream is because it’s cold in the north and hot in the south, and that temperature difference creates [a condição para a corrente de jato]”, she said.
Because the Arctic is warming much faster than the rest of the planet, there is less temperature difference between these air masses now.
Some of the jet stream’s behavior remains unexplained: “Over the Atlantic, the jet shifted southward in the summer,” said Tim Woollings, author of “Jet Stream” and professor of atmospheric physics at Oxford. “And we expected it to move north in response to climate change.”
The heat wave the UK has recently experienced is “just a small sample” of what has been happening in the rest of Europe, Woollings said. “The real event is over Spain and France.”
The UK experienced two days of extremely high temperatures on Monday (18) and Tuesday (19) before the weather turned cold; Spain and France have seen high temperatures for weeks.
As global average temperatures rise, climate models show that heat waves will get hotter. However, it could be years before researchers know exactly how global warming is influencing these jet stream patterns.
“We need a very extended observation record,” says Williams. “It could take decades, or even a century, before we convincingly detect any changes.”
Translated by Luiz Roberto M. Gonçalves
