Jet stream is a critical link in climate disasters worldwide
Devastating floods destroyed towns in Germany and Belgium. A ruthless heat wave broiled the Western U.S. and Canada. Heavy rains paralyzed a Chinese industrial hub home to 10 million people. These recent weather phenomena are being intensified by the changing climate.
But the link between these far-flung extremes goes beyond warming global temperatures. All of these events are touched by jet streams, strong and narrow bands of westerly winds blowing above the Earth’s surface. The currents are generated when cold air from the poles clashes against hot air from the tropics, creating storms and other phenomena such as rain and drought.
“Jet streams are the weather — they create it and they steer it,” said Jennifer Francis, a senior scientist at the Woodwell Climate Research Center. “Sometimes the jet stream takes on a very convoluted pattern. When we see it taking big swings north and big dips southward we know we’re going to see some unusual weather conditions.”
Meteorologists worry whenever those swings and dips form curves that look like waves. When that happens, warm air travels further north and cold air penetrates further south. The result is a succession of unusually hot and cold weather systems along the same latitude.
“It’s just like when waves in the ocean get to a beach, overturn and break,” said Tess Parker, a research fellow at the ARC Centre for Excellence for Climate Extremes at Monash University in Melbourne, Australia. “That can happen in the atmosphere as well, and if that happens, you tend to catch a high- or low-pressure system that will become stationary.”
That’s what sunk parts of Germany into floods earlier this month, as a low pressure system became pinned above the country’s western region. Heavy rains soaked the terrain for the first two days, followed by a few hours of even more intense precipitation that caused rivers to overflow. Water and mudslides overran houses and roads, killing more than 170 people and leaving hundreds missing. Heavy rains also swamped parts of Luxembourg, Belgium and the Netherlands.
“We were immensely shocked,” said Stefan Heydt, a spokesperson for the German armed forces in the state of North Rhine-Westphalia, which suffered severe damage in the recent floods. His home state, Rhineland-Palatinate, also was badly affected.
“Whole existences were destroyed from one moment to the next,” Heydt said.
When movements in the jet stream coincide with climate-driven extremes — heat, drought, intense rainfall — the consequences can be catastrophic.
That may be what happened in China this week. A record rainstorm brought a year’s worth of precipitation in three days to Zhengzhou, the world’s biggest manufacturing base for iPhones and a major hub in central China for food production. At least 33 people died, and as many as 380,000 had to be evacuated.
Scientists at the China Meteorological Administration attributed the storm to strong and sustained highpressure blocks that, along with Typhoon In-fa approaching from the southeast, pushed water vapor in from the sea. That dense air hit the mountains surrounding Henan, the province where Zhengzhou lies, converged and then shot upwards, where it cooled to form the destructive rains.
The question for scientists is to what extent climate change affected those disturbances in the jet stream. It takes time to do that kind of analysis. A rapid attribution study of the Europe floods has already begun, led by the German weather service, with results expected by mid-August.
The heat waves that struck western U.S. and Canada in late June were so unprecedented that researchers were able to conclude by early July that climate change had made them at least 150 times more likely. A high-pressure system, typically linked to hot and dry weather, was made worse by the fact that the land below it was extremely dry.
Some extreme weather may have nothing to do with the jet stream at all.
Still, understanding the jet stream is becoming more pressing as warming temperatures drive more frequent extreme weather events.
“We need to think more about the way weather systems will change with the changing climate, rather than just how the climate will change,” said Parker from Monash University.