Those unusual frozen Februaries in Texas may not be so unusual anymore.
Early winter has been warming across North America, but late winter is another story. Scientists have documented a cooling trend over more than 40 Februaries, marked by dangerous and increasingly common intrusions of Arctic air deep into the United States.
“December has certainly been warming if you look at the U.S.,” Judah Cohen, a climatologist at the Massachusetts Institute of Technology, says in a recent video. But “February, going back to 1979—so quite a few years now—we're actually seeing in the center of the U.S. a very distinctive cooling trend.”
Cohen published a paper in the wake of the 2021 Texas Freeze—an event that also froze everything north of Texas—in which he links changes in Arctic snow cover and sea ice to these February intrusions of cold air.
“We tend to get more severe winter weather when the polar vortex is weak and more milder rainy weather when the polar vortex is strong, and we've seen a decrease in the strong state of the polar vortex and an increase and the weak state of the polar vortex,” Cohen said in an interview with Peter Sinclair, a videographer for Yale Climate Connections who posts pithy snippets of interviews on his excellent Youtube channel, greenmanbucket.
Sinclair said he first learned of the February trend from Martha Shulski, the state climatologist for Nebraska.
“One thing that we have seen is that we're warming during the early winter and then really cooling down in the last 30 years during February,” Shulski tells him. “We have a tendency to be getting more Arctic air, more polar air outbreaks for this part of the country during February, during late winter.
“There's a lot of research going on in this area, but what it seems to be linked to is those strong changes that we're seeing in the Arctic—loss of sea ice for certain regions of the Arctic—that's resulting in the wavy jet stream pattern and more of these polar air outbreaks.”
The National Weather Service’s outlook for February does not forecast an icy intrusion this February, but that’s not unusual.
Released Jan. 19, the outlook forecasts above-normal temperatures for the Southeastern United States and below-normal temperatures from the Pacific Northwest to the Northern Plains. But “Equal chances of above, near, and below normal temperatures are indicated across the southwestern CONUS (Continental United States), Central Plains, much of the Great Lakes, parts of the Northeast, and much of Mainland Alaska due to weak or conflicting dynamical and statistical guidance.”
Cohen and his had team set out to investigate why climate models did not anticipate the observed episodes of Arctic air intruding on central North America.
Some scientists have pushed back on Cohen’s paper, saying the February pattern could be random, but Cohen said he doubts that a 43-year trend is random. Other scientists disagree that there’s a trend.
“Observations and models strongly suggest that Arctic change is reducing, not increasing, the risk of winter cold extremes over the United States,” two Canadian and one UK scientist wrote in a letter to Science. “The mechanism proposed by Cohen et al. may be relevant for year-to-year variability, but the evidence does not support a long-term increase in severe winter weather.”
Cohen agrees that climate change is indeed reducing cold extremes overall, but notes that the central U.S. is experiencing an increase in “stretched polar vortices” like the 2021 event that froze Texas.
“If the number of stretched polar vortex days is increasing, it follows that the overall warming trend in the colder months across the United States will be dampened.”
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January 29, 2023 at 12:14PM
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