Meteorologists should watch the tropics rather than the Arctic to predict extreme cold in North America, study says

A satellite image of the continental United States shows the extreme cold weather phenomenon called the polar vortex over the U.S. Midwest and Great Lakes regions, on January 30, 2019. A new study suggests that the polar vortex is not the primary cause of extreme cold events in North America. (Courtesy NOAA/Reuters)
Meteorologists often look at the behaviour of the polar vortex to predict extreme cold events in North America, but the real key may lie further south, according to a new study.

“Despite the most extreme cold snaps experienced in North America often being described as ‘polar vortex outbreaks’, our study suggests vortex strength should not be considered as a cause,” researcher Simon Lee, an atmospheric scientist at the University of Reading in England, said in a news release.

The stratospheric polar vortex is a river of winds that circles the Arctic at heights of 10–50 km, trapping cold air inside.

In Europe and Asia, it is the strength of these winds that creates the episodes of cold winter. When the polar vortex weakens, it releases cold, freezing air from the Arctic to southern regions.

“We know that a weakened polar vortex allows cold air to flood out from the Arctic over Europe and Asia, but we found this is surprisingly not the case the other side of the Atlantic,” Lee said.

“In fact, our work suggests we should actually look south to conditions around the equator, rather than north to the Arctic, for the causes of these widespread freezing conditions in North America.”

The study found that when tropical atmospheric patterns at the equator push high-pressure systems northward to Alaska, they cause the polar vortex and its freezing winds to move towards southern parts of North America.

So, extreme cold episodes are not due to the weakening of the polar vortex as in Europe or Asia, but rather to its move southward in the lower atmosphere.

This phenomenon affects the Alaskan Ridge regime which is associated with the most extreme and widespread cold in North America.

Conditions seen during the Alaskan Ridge regime. Extreme cold across North America is more likely to happen when tropical atmospheric patterns push high pressure systems northward, causing the polar vortex to dip southward. (Simon Lee/University of Reading)
Useful for weather forecasting in North America

Apart from the Alaskan Ridge regime, scientists also looked at the three other winter weather patterns affecting the North American continent—Pacific Trough, Arctic High and Arctic Low.

They found that the strength of the polar vortex has a real influence on them. By looking at the polar vortex, meteorologists could potentially better predict severe weather moving across the continent, for example.

“Our results did reveal that the polar vortex strength provides useful information on the likelihood of most weather patterns over the US and Canada further in advance, including some potentially disruptive temperature changes or heavy rain.” Lee explained.

The four US weather regimes (clockwise from top left): Pacific Trough, Arctic High, Alaskan Ridge, Arctic Low. Red indicates warmer conditions and blue colder conditions. (Simon Lee/University of Reading)

As an example, a strong polar vortex increased the likelihood of extremely cold conditions in the western parts of North America—including Alaska—by up to 11 percent.

On the opposite, it also suggests much milder conditions in the central and eastern parts of the US.

The more accurate information populations have about upcoming changes in weather, the better they can prepare.Simon Lee, lead author

Now, the authors hope their work will inspire scientists to incorporate the polar vortex into their winter weather forecasting models, but also tropical atmospheric models such as El Niño.

The study has been published in the journal Geophysical Research Letters

Related stories from around the North:

Antarctica: Could snow cannons in Antarctica help avert catastrophic sea level rise?, Eye on the Arctic

Canada: Bizarre winter weather in South caused by changes in atmosphere, not sea-ice loss: study, CBC News

Finland: Cooler summer weather has positive effects in Finland, Yle News

Greenland: Evidence of powerful solar storm which occurred 2,600 years ago found in Greenland ice, CBC News

Norway: Arctic summer 2019: record heat, dramatic ice loss and raging wildfires, The Independent Barents Observer

Russia: A hot summer across the Arctic, Russian meteorological institute says, The Independent Barents Observer

Mathiew Leiser, Eye on the Arctic

Mathiew Leiser, Eye on the Arctic

Born in the south of France to an English mother and a French father, Mathiew Leiser travelled the world from an early age. After studying international journalism in London, he quickly acquired different journalistic skills by working as a freelance journalist in various news organisations. From the BBC to Agence France Presse and the UGC agency Newsflare, Mathiew has acquired experience in various fields of journalism. In 2019, he decided to settle in Montreal to face its harsh winters and enjoy its beautiful summers but above all to develop his journalism. He quickly joined Radio Canada International where he strives to give the best of himself within the various teams.

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