A plot of the stratospheric polar vortex showing where strong winds encircle a depression of frigid air. (Hannah Attard)

It was a weird winter in the Arctic, and even stranger in the United States and Europe, where wintry weather was mostly absent. A near-record strong polar vortex bottled up frigid air in polar regions, while mild weather frequently flooded the mid-latitudes. Europe posted its warmest winter on record.

As far removed as it is, the polar vortex had dramatic implications on our winter weather, and this could continue to heading into spring.

Some springs, abrupt changes in the polar vortex can favor wild temperature swings and severe weather over the Lower U.S. 48. Other years, a more gradual transition of the vortex from winter into springtime mode blends together the seasons more gingerly.

What’s in store this year? Scientists are unsure: After such a bizarre winter for the polar vortex, research scientists suggest to expect the unexpected.

What is the polar vortex?

A positive Arctic Oscillation (left) is associated with a strong, stable polar vortex whereas a negative Arctic Oscillation (right) is associated with weak, unstable vortex. (NOAA)

Each hemisphere has not one, but two polar vortexes: one in the troposphere, and one in the stratosphere.

The troposphere is the region of the atmosphere in which we live. It’s where our weather happens. At the equator, the troposphere might extend up to 60,000 feet high; in polar regions, sometimes less than 20,000 feet.

Above that lays the stratosphere. It’s a region of the atmosphere where temperatures warm with altitude — rather than decrease. That’s where a narrow, but more intense, polar vortex devilishly whirls round and round.

The polar vortex — in either section of the atmosphere — is caused by the frigid temperatures near the pole. Since cold air contracts and sinks, a sort of void is left in the upper troposphere and stratosphere. That void draws air spiraling inwards, where it too cools, and a vortex is born.

The tropospheric polar vortex is more meandering, wobbling north and south during different times of the year. It wears the jet stream as a necklace, the turbulent river of winds separating frigid air to the north and milder air south.

But above the tropospheric vortex is the stratospheric polar vortex. It’s only around in the wintertime, and usually decays once sunlight returns in full force by March or April.

How the stratospheric polar vortex disintegrates at the end of the season has bearings on spring at the lower latitudes. Does the vortex go out with a bang? Or fade quietly into the ether? Experts say this season might fall somewhere in between, but emphasize the enormous predictive challenges we’re up against.

What will the vortex do?

The European model simulates the stratospheric polar vortex remaining strong and perhaps meandering towards the U.S. late in March, eventually beginning to weaken. (WeatherBell)

Since mid-February, the stratosphere polar vortex has remained near “record strength,” according to Hannah Attard, a professor of atmospheric sciences at the University of Albany. “It’s been a really interesting season,” she said.

A strong polar vortex is colder, more concentrated and symmetrically-shaped than a weak one, revolving about the pole at breakneck speed.

But time is running out as increasing daylight and warmer temperatures weaken and eventually destroy the vortex. Weeks ago, there were signs that pointed to an abrupt dissipation, and a shock wave-like chain reaction. Now, that’s not looking as probable.

“The forecast varies every day, so it’s a little hard to say,” said Amy Butler, a NOAA research scientist specializing in polar vortex dynamics. “The general forecast from most models I’ve seen shows more of a gradual change, but there have been some forecasts recently from [the NOAA] model that do suggest a more abrupt change happening towards the end of March.”

“What’s forecasted now is a deceleration [of the vortex],” said Attard. She explained that some “upward wave flux” of energy from the troposphere could make it into the stratosphere around March 20 to 22. It could be enough to disrupt the vortex, slowing it a bit as we inch towards its inevitable demise.

However, even then the stratospheric polar vortex’s strength could keep gyrating at speeds well above normal — in the 90th percentile. Signs point to the vortex slowing little by little for the next several weeks, with an outside chance of a more abrupt decay at the end of March.

Potential effects where we live

Storms tower to the east on May 18, 2019 viewed from Fort Supply, Oklahoma. (Matthew Cappucci)

If the vortex decays gradually, impacts at the mid-latitudes aren’t likely to be as dramatic as they could be. But Butler said current forecasts suggest the predicted vortex behavior will help somewhat reduce “this crazy warmth in the mid-latitudes.”

According to Butler, Europe and Asia, especially at high latitudes, are more likely than North America to see significant springtime effects. She added polar vortex impacts are much predictable in Eurasia compared to North America.

Some models suggest the vortex could still break down abruptly, increasing the chance of extreme weather in both Europe and the Lower 48 states.

An abrupt weakening of the vortex "can lead to an increase of high latitude blocking,” explained Judah Cohen, an atmospheric scientist at Atmospheric Environmental Researching. That means high pressure over the Arctic could shunt colder than normal air down towards the East Coast.

That same pattern can make the jet stream more wavy too and push it farther south. Some seasons, like last year, this can have big impacts.

“A southward displaced jet stream can also contribute to an increase in severe weather, as there is more energy in the atmosphere and often strong [differences] in temperature near storms and cold fronts,” wrote Cohen.

“We had a dynamic final [stratospheric] warming in April [of last year] that contributed to a cold spring and an active tornado season in May.”

Effects beyond the weather

A look at average temperatures in the stratospheric polar vortex. Note that temperatures are forecast to be near or below record minima in late March. That could form polar stratospheric clouds. (NASA)

The unusual longevity and intensity of the vortex mean the polar stratosphere is unusually cold, which has implications for its chemistry.

“The polar air [it contains] and the mid-latitude air just aren’t mixing," said Butler. "And that can create polar stratospheric clouds.”

Polar stratospheric clouds require frigid temperatures to form, typically below minus-110 Fahrenheit. They’re pretty, but that beauty comes at a price. The chemical reactions they feature, catalyzed by sunlight, are responsible for the destruction of ozone.

Ivan Riley's photograph of incredible nacreous clouds shot while landing in Tromsø, Norway on December 29. (Ivan Riley via Spaceweather.com)

“We’ve now seen Arctic ozone depletion occurring,” explained Butler. “Last time it was very large was March of 2011. It doesn’t happen very often.”

As winter turns to spring and lingering bitter cold is met with growing daylight, the formation of these ozone-depleting clouds may increase.

Although it doesn’t have enormous societal implications according to Butler, stratospheric ozone "was the lowest ever February over the polar cap, and we’re expecting it considerably low in March as well.”