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A new study challenges the idea that climate change is behind the erratic wintertime behavior of the polar jet stream, the massive current of Arctic air that regulates weather for much of the Northern Hemisphere.

Large waves in the jet stream observed since the 1990s have, in recent years, driven abnormally frigid temperatures and devastating winter storms deep into regions such as the southern United States. Scientists fear that a warming atmosphere brought on by climate change is fueling these wild undulations, causing long troughs of bitter-cold air to drop down from the Arctic.

But Dartmouth researchers report in that the jet stream's volatility actually may not be that unusual. Instead, the jet stream appears to have undergone natural鈥攖hough sporadic鈥攑eriods of "waviness" since before the effects of climate change were considered significant, the researchers report.

In the first study of its kind, the researchers compiled a record of the jet stream's wintertime variability since 1901 by using machine learning to analyze long鈥恡erm climate records. Nearly all prior studies of the jet stream focus on the period since 1979, when data on weather and climate systems began being collected by satellites.

The researchers found that the jet stream is the latest of several wavy periods that have occurred over the past 125 years. The jet stream was even more volatile during many of these phases than it is today, says Jacob Chalif, the study's first author and a graduate student in the lab of senior author Erich Osterberg, an associate professor of earth sciences.

"The jet stream was frequently just as wavy as it is today, if not more so, before, climate change would have been a significant influence," Chalif says. "This calls into question whether climate change is causing the jet stream to be more erratic now."

While climate change is undeniably amplifying extreme winter weather, the new study shows that it is likely not doing so by making the jet stream wavier, says Osterberg, who directs the Ice, Climate, and Environment Lab at Dartmouth.

"Our research shows that the jet stream isn't doing anything unusual that would cause the recent increase in intense storms. Climate change is intensifying these storms through a different process," he says.

The study may allow scientists to change focus to more direct links between global warming and severe weather, Osterberg says, such as the fact that a warmer atmosphere holds more moisture and that leads to bigger storms.

"For me, these findings fundamentally change the way I approach this problem," says Osterberg, who co-authored a series of earlier studies identifying the jet stream as a possible cause of intensifying storms.

"If the jet stream is not this critical link between climate change and more severe storms, then we need to focus our attention on different explanations for why we're seeing more extreme weather events," he says.

The high-altitude wind of the polar jet stream modulates weather in Europe, Asia, and North America. In North America, it flows roughly along the United States' border with Canada. Big waves in the jet stream cause Arctic air to cut deeper into the balmy subtropics. This can lead to unusual cold snaps and severe winter storms in areas where the warm and cold air collide.

Dramatic waves in the jet stream can also usher south the polar vortex, which is the raft of subzero air circling the North Pole that has become a popular term for bouts of dangerously cold temperatures.

Several scientific studies have suggested that the current waviness of the jet stream is a result of climate change. It coincides with record-high concentrations of greenhouse gases in the atmosphere, a clear increase in the intensity of extreme weather, and the decline in sea ice in the Arctic since the 1990s especially, Chalif says.

But the Dartmouth team found that the last pronounced wavy period started to peak around 1979. That means satellite observation of the jet stream began as the system was entering a more normal period, so wavy periods after 1979 appeared to be abnormal, Osterberg says.

"The jet stream seemed like a direct link between global climate change and extreme weather with big storms, but we didn't really know what happened before 1979," he says. "It created this appearance of an unusual trend, but when you see the full picture, you see that it isn't that unusual. There were periods that were wavier than what we're seeing now."

The researchers found that an even stronger wavy period lasting from the 1960s through the 1980s was the main driver of the "warming hole," a 30-year period of abnormally cool winter temperatures in the United States. Centered in the southeastern United States, the warming hole caused average winter temperatures to drop more than 2 degrees Fahrenheit (1.3 degrees Celsius) beginning around 1958. Colder-than-expected winters persisted until the late 1980s.

Trevor Partridge, a co-author of the study who completed his Ph.D. in earth sciences at Dartmouth, first reported in a 2018 paper he led with Osterberg as a co-author that this volatile phase coincided with the warming hole.

Partridge, now a physical scientist at the U.S. Geological Survey, suggested in that study that waviness in the jet stream caused the warming hole to park over the South in winter. This would have created a glide path for Arctic air to routinely flow into the region.

The new AGU Advances study confirms that pronounced waves in the jet stream contributed to two-thirds of warming hole cooling from 1958 to 1988. During that time, the researchers report, periodic changes in the jet stream's waviness led directly to fluctuations of the average winter temperature in the Southeast.

"We found there was a link between the warming hole and jet-stream variability decades before climate change would have been a significant influence," Chalif says. "It comes back to the idea of climate chaos鈥攖here's so much that influences our weather day-to-day and year-to-year."

"That's why developing this 125-year record is so helpful," Osterberg continues. "When you look at just a thin slice of the record like we were doing before, it's hard to fully understand what's happening."