A monthlong snow blitz was underway in Philadelphia, with as much as 4 feet accumulating in nearby suburbs.
Texas, which long has endured devastating and destructive hurricane damage, was experiencing an all-out disaster of a different sort, a pipe-bursting cold spell that short-circuited the state’s power grid and darkened millions of homes and businesses.
At one point, nearly three-quarters of the contiguous United States was snow-covered in what turned out to be the coldest February in more than 30 years.
And with the 2021-22 snow season about to begin, Northern Hemisphere snow cover would be above normal in October and November if trends continue.
This is global warming? Yes, say polar scientist Judah Cohen and other researchers, this is the handiwork of climate change; more specifically, what they call “Arctic change.”
In 2020 worldwide temperatures were about 1.74 degrees Fahrenheit higher than the 20th-century average, according to the National Centers for Environmental Prediction. The Arctic region has warmed at double that rate.
Rising temperatures up that way have been melting snow and ice that in turn have liberated previously frozen waters.
Changes in the Northern Hemisphere’s vital cold-air pantry might be conspiring to disrupt the upper atmosphere in such a way as to cause powerful pulses of polar air to spill into the lower latitudes more frequently via the polar vortex, Cohen and his coauthors say in a paper published in the journal Science last month.
“I’m not arguing that winters are getting colder,” Cohen, with Atmospheric and Environmental Research Inc., said in a phone interview. But climate models generally have underrated winters’ ferocity, and ”expecting a decrease in severe winter weather might be hazardous.”
Other atmospheric scientists, such as National Oceanic and Atmospheric Administration researcher Amy Butler, agree that Arctic warmth may well be related to episodes of severe winter weather in the United States. However, they expressed reservations about the limited period of observations and said it was unclear whether the authors had identified a trend.
About the polar vortex
Ten to 30 miles above the surface, during winter the polar vortex circulates around the Arctic with winds at times greater than 150 mph.
When it’s howling away it confines vast pools of frigid air, perhaps 100 degrees below zero Fahrenheit, over the sunless North Pole. Don’t ask about the wind chill.
On occasion disturbances at lower levels of the atmosphere disrupt the stratosphere in such a way that the vortex weakens, exporting Arctic air as it oozes southward.
In ways not fully understood, the cold air in the high atmosphere interacts with the lower-level jet stream winds that ignite and transport winter storms.
With 40 years of observations, Cohen and his team conducted computer-modeling experiments using previously identified Arctic air-pressure patterns that have preceded polar-vortex disruptions.
They included data to simulate the loss of Arctic sea ice and recent increases in autumn snow cover in Eurasia — which they attributed to the additional moisture available from the freed waters. The modeling results strongly showed a “physical link” between Arctic warming and the polar-vortex weakening that leads to cold-air spillage, they wrote, leading to severe winter outbreaks in Philadelphia and elsewhere.
Butler, a research scientist with NOAA’s Chemical Sciences Laboratory, said that the team’s concept is more than a stretch. Its “hypothesis is plausible,” she said, but “I don’t think there is any convincing evidence of a long-term trend in the strength of the polar vortex.”
But any changes in its location or shape could also influence weather, she said.
Paul Pastelok, the veteran seasonal forecaster at AccuWeather Inc., said the team’s findings were promising, but the data were limited.
“We need measurements to go back to the ’60s, that are more accurate,” he said. “We don’t have true data for all of the Arctic, only partially, and that’s just not good enough.”
The future in forecasting
Nothing happens in a vacuum in the atmosphere, and Pastelok said he had no doubt that the Arctic is a powerful influence on winter weather and lower latitudes.
He said he is an admirer of Cohen’s papers, which have included correlating October snow cover in Siberia with the subsequent North American winter, and has consulted them in developing his outlooks.
Regarding the findings in the Science paper, Pastelok said: “I think it’s going to take another 20 years to really secure whether or not this can be used as a forecasting tool.”
Cohen says they could be of short-term use. “I would argue that the Arctic can precondition the atmosphere for these events so you might anticipate when such events are more likely in the upcoming winter,” he said.
And in the meantime he holds that it would be a mistake to conclude that a warmer world will mean gentler U.S. winters.
See: Texas, February 2021.