By Kendra Pierre-Louis
Find your long johns, break out the thick socks and raid the supermarket. After a month of relatively mild winter weather, the Midwest and the East Coast are bracing for what is becoming a seasonal right of passage: the polar vortex.
The phrase has become synonymous with frigid temperatures that make snowstorms more likely. A blast of arctic air is expected to herald the vortex’s arrival Monday.
If it seems as if these polar freezes are happening more often, you’re right. “They are definitely becoming more common,” said Jennifer Francis, a senior scientist at the Woods Hole Research Center. “There have been a couple of studies that have documented that.”
The cold snap may feel especially shocking after an unusually warm few weeks. Colder temperatures have been arriving later in winter over the past few years, according to Judah Cohen, a climatologist at Atmospheric and Environmental Research, a weather risk assessment firm. But because of changes to the polar vortex, when wintry weather does arrive, it’s often more intense — witness the four back-to-back nor’easters last year.
“I’ve been making that argument that winter is shortening, but you’re getting these more intensive periods in that shorter winter,” Cohen said.
A major snowstorm has already pummeled parts of California and is heading east, blanketing the Plains with blizzardlike conditions. Over the weekend, snow is expected across the Midwest, the East Coast and as far south as Arkansas. But once the storm clears out, the effects of a dipping polar vortex will arrive. Monday’s high temperature in New York City is predicted to reach just 16 degrees, 20 degrees below average, according to the forecasting service Weather Underground.
“We’re going into what looks like a pretty unusually cold period, even for late January, punctuated by some winter storms,” said Bob Henson, a meteorologist with Weather Underground.
Scientists are still trying to figure out why these intense arctic chills are flooding southward more frequently. To understand their thinking, it helps to understand the nature of the polar vortex. While the expression became broadly popular during an unusually cold winter in 2014, the vortex was known to meteorologists long before that.
The term refers to circular bands of winds near the poles that are strongest in wintertime and well above the jet stream in the stratosphere. The stratosphere is an atmospheric layer that extends roughly 7 to 31 miles above the earth.
Usually, those circular bands act as walls that keep the teeth-chattering cold air locked at the poles. But, every so often, the winds break down and allow the cold air to escape. That’s what happened at the beginning of this month, when the polar vortex split into three separate bands.
It’s this escaping polar air that is dropping temperatures in the Midwest and the East — there’s a lag time between the atmospheric event and when we experience the effects. The broken vortex is also sending icy temperatures to much of Europe in what some call the “Beast From the East.”
Some researchers, including Francis and Cohen, say they suspect that the more frequent polar vortex breakdowns can be tied to climate change.
While climate change is warming the earth, not all parts of the earth are warming at the same rate; the Arctic is warming at a rate twice as fast as the world average. That warming has led to historically low levels of sea ice in the region. The melting sea ice, particularly in an area near the Barents and Kara Seas off Siberia, may be linked to the changes in the polar vortex.
“When we lose a lot of ice in that particular area in the summer, it absorbs a lot of extra heat from the sun,” Francis said. This is because the darker open ocean absorbs more heat than reflective ice. “And so we see a very persistent, hot spot there in terms of temperature differences from what they should be.”
Research suggests that the hot spot, along with changes in the jet stream driven by climate change, cause the polar vortex to break down in mid- to late winter.
“As the Arctic gets warmer and warmer, the severe weather picks up,” Cohen said.
Whether the polar vortex directly caused the snowstorm is a tricky question, Henson said. There have always been major winter storms, of course. In this case, the polar vortex, the El Niño pattern and the jet stream flowing from the Pacific all play a role, he said.
In October, the National Oceanic and Atmospheric Administration predicted a milder than average winter, but that is not necessarily at odds with the coming chill.
“There’s a difference between some seasonal outlooks such as NOAA’s that look at the whole three-month period and others that may be breaking it down month by month,” Henson said. “It’s quite possible the winter will average warm for December through February. But that may well manifest as the extreme warmth we’ve seen over the last month followed by some much colder and colder than average conditions into February.”