【】

Meteorologists descended upon Boston in mid-January for the 100th annual meeting of the American Meteorological Society. January is Boston's coldest month, but during the weather conference, meteorologists experienced 75-degree temperatures.

"I was in shorts and a T-shirt," said Jeff Weber, a research meteorologist at the University Corporation for Atmospheric Research.

Overall, winter across the East Coast this year has been pretty pathetic.

A major reason why is the polar vortex — which is a circulating stream of air that forms over the Arctic during winter — has stayed rigid and in place, keeping its cold air mostly locked up north. Consequently, little of this exceptionally chilly air has dumped down into the U.S. this winter.

The result? Mild winter temperatures and often low snowfall.

Arctic blasts, like those in January 2019, happen when the polar vortex becomes unstable and wobbly, allowing frigid air to spill south. But this winter, "instead of a weak polar vortex, we've had strong polar vortex conditions," said Andrea Lang, an atmospheric scientist at the University of Albany who researches changes in seasonal winter weather.

Tweet may have been deleted

The polar vortex has stayed strong because other weather systems, sometimes originating over the Pacific Ocean, haven't come along and knocked the spinning vortex out of place.

Mashable Light SpeedWant more out-of-this world tech, space and science stories?Sign up for Mashable's weekly Light Speed newsletter.By signing up you agree to our Terms of Use and Privacy Policy.Thanks for signing up!

A frequent culprit in disturbing the polar vortex is a powerful stream of air traveling some 30,000 to 35,000 up in the atmosphere (where jets fly) which we commonly call the jet stream. The jet stream shoots west to east around Earth at speeds of 120 to 250 miles per hour, but can become dramatically wavier and meander when it's disturbed by potent storm activity or encounters masses of hot or cold air.

But this winter season, the meandering jet stream has not knocked the polar vortex off its pole, explained Lang. Instead, the jet stream has bent dramatically south, right through the heart of the central U.S., as seen below.

"This is an extremely amplified wave," noted UCAR's Weber.

A meandering jet stream.Credit: SFSU

That big downward bend in the jet stream, called a trough, has still allowed some frigid air to swoop down from Canada (but not from the polar vortex) and chill places like Iowa and Kansas, explained Weber. Meanwhile, the super-kinked jet stream — which acts like a wall separating cold northern air from warmer southern air, has allowed warm air to flow up the East Coast, into New England.

"This has been the dominating pattern we’ve seen [this winter]," said Weber.

This pattern may not budge much this winter, which would mean a continued weak winter for the East coast, he added. That's because there's a robust region of high air pressure in the western Atlantic, acting as a barrier to keep that big jet stream wave locked in place.

SEE ALSO:Even the 'optimistic' climate change forecast is catastrophic

Yet, there are "hints" that the polar vortex could become disturbed this February, said Lang. Atmospheric scientists are watching how weather patterns develop over the Pacific to see if something, like a meandering jet stream, might perturb this winter's robust polar vortex.

Surges of Arctic air from the polar vortex, though, are pretty common today. The polar vortex has become regularly disturbed over the last couple decades. Between 1989 and 1998, the vortex never got so thrown out of whack that it split in winter — like it did in 2019. (This caused the polar vortex to wobble over into both the U.S. and Europe, bringing extreme cold.)

Atmospheric scientists are deeply interested in why a perturbed polar vortex has become more frequent. There's evidence that the incessantly warming Arctic — the fastest-warming place on Earth — has led to a weaker and less stable polar vortex, which means more outbreaks of freezing Arctic air.

Just not this winter — so far, anyway.

(责任编辑：休閑)