Several miles above the North Pole, an atmospheric chain reaction may soon initiate that sets off important changes to weather patterns across the Northern Hemisphere in the coming weeks.
Most of the Eastern U.S. and Europe have escaped winter’s wrath so far, except for some very short-lived Arctic blasts. With signs of spring emerging — from barren ski slopes to budding blooms — it’s natural to wonder whether winter is already over. But there’s some chance this chain reaction could shut down the movement toward spring.
It all begins in the stratosphere, where computer models are coming into agreement that temperatures may leap 60 degrees in the coming week, signaling what’s known as a “sudden stratospheric warming” event.
Although such a spike in temperatures may not sound like something that will bring winter to life, this phenomenon is known for disrupting the polar vortex. When the vortex is knocked off kilter, it sometimes results in significant changes to the prevailing weather patterns over North America, Europe and Asia in the weeks and months that follow.
“Often following SSWs [sudden stratospheric warmings], more severe wintry weather becomes more widespread across the Northern Hemisphere,” Judah Cohen, a seasonal weather forecaster with the firm Atmospheric and Environmental Research, wrote in a blog post this week.
Such warming events occur every winter or two, on average, but seldom do they play out the same way. Some result in only mild alterations to the prevailing weather pattern. But others bring about radical change.
Consider 2018. Up until mid-February, it had been a mild winter. But then the stratosphere suddenly warmed, the polar vortex fractured and weather mayhem unfolded over much of Europe and eastern Northern America. In late February, a punishing blast of Siberian air — dubbed the “beast from the east” — invaded Europe, delivering the coldest weather in years. By March, frigid air arrived in eastern North America and triggered four blockbuster storms in the Northeast. The cold didn’t retreat until late April in some areas.
Experts who study the stratosphere and the polar vortex say there’s no reason to expect a repeat of 2018 this year, but they agree that a vortex disruption has a chance to shake up weather patterns.
Inside a disruption of the polar vortex and the chance it happens
There are actually two polar vortexes over the North Pole at any given time — a meandering, wavy one in the troposphere, or lowest level of the atmosphere, and a more concentrated whirlpool of frigid air in the stratosphere above.
The stratospheric polar vortex is a buzzsaw-like maelstrom of cold air, low pressure and spin that gyrates and whirs over the North Pole. The Southern Hemisphere has its own polar vortex over Antarctica.
At present, most major computer models are hinting that a sudden stratospheric warming event could occur next week, which would disturb the vortex.
Simon Lee, an atmospheric scientist at Columbia University, said in an email it “would be the first major sudden stratospheric warming” since Jan. 5, 2021, if it occurs, and the fourth in the past six winters.
A marker of a sudden stratospheric warming event is a reversal in the prevailing polar vortex winds, wrote Amy Butler, an atmospheric scientist with the National Oceanic and Atmospheric Administration, which models are generally simulating.
Even if the winds of the polar vortex don’t fully reverse, models “agree on a strong deceleration” wrote Daniela Domeisen, a professor at the Institute for Atmospheric and Climate Science in Zurich.
Models are generally calling for the sudden stratospheric warming to occur around Feb. 15-17, Lee said. Butler noted that’s within the 10-day time frame in which these events “can be skillfully predicted,” but she cautioned “there is still considerable uncertainty in how things will evolve.”
It is increasingly likely that a major sudden stratospheric warming -- defined as a reversal of the zonal winds at 10 hPa and 60°N -- will take place, probably between 14 and 17 February. Cautionary note that the GFS seems to be on the extreme end of ensemble solutions, however. pic.twitter.com/limSM0FiEi— Dr Simon Lee (@SimonLeeWx) February 6, 2023
Lee likened the situation to a “boxing match” between “planetary waves,” or large-scale weather systems nestled within dips and crests of the jet stream and the vortex itself.
“An earlier minor warming,” analogous to a smaller punch from a jet stream disturbance, Lee said, “seems to have left the vortex in a more vulnerable state.” That happened at the end of January.
“There is pretty high certainty that the polar vortex will get strongly disturbed by mid-February, but less certainty that it will be disturbed enough to influence the surface below,” Butler said.
What the polar vortex disruption could mean
Under ordinary circumstances, the stratospheric polar vortex spins like an unperturbed top, minding its business and keeping cold air bottled up high in the Arctic. But if something interrupts the flow of the vortex, it sometimes allows bitterness to slosh down to the mid-latitudes.
But how much cold air is dislodged and where is still up in the air.
“Saying anything about what happens afterward to our weather remains quite challenging,” Lee said. “What we do know for sure is that a sudden stratospheric warming increases the risk of a prolonged negative [North Atlantic Oscillation],” or a weather pattern that favors insurgences of colder air at lower latitudes in Europe and sometimes the Eastern United States.
Sudden stratospheric warmings also often help establish or reinforce “Greenland blocks,” or stagnant domes of high pressure over Greenland. That can result in an atmospheric logjam that maintains prolonged periods of unsettled weather over the U.S. Northeast and East Coast, like what occurred in March 2018.
“Such a configuration can bring colder weather to North America, but it’s actually typically far more noteworthy for bringing cold to northwest Europe,” Lee said.
That said, changes in the lower atmosphere aren’t instantaneous.
“Any direct significant impacts on the weather are typically delayed following the warming by a week or two as the signal propagates into the lower stratosphere, so we’d be looking at early March,” Lee said.
It will probably be at least another week or so before any potential impacts at the mid-latitudes can be ironed out.
“Usually we can make the first more reliable predictions only starting around the time when the sudden stratospheric warming happens,” Domeisen said. “For North America, these predictions are even more difficult than for Europe.”