What is the Polar Vortex? (Or, Why Winter 2020-21 Was Crazy)

polar vortex diagram
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Winter 2020-21 didn’t turn out as many thought. Most long-range forecasters thought La Niña would lead to a mild and uneventful winter. My weather station read 62 on Christmas Day in Pennsylvania! Well, the polar vortex had other plans.

While December was the ninth warmest December on record in the U.S., and January mild by mid-winter standards, events were in motion to end Winter 2020-21 on a memorable note, with the much-hyped weather event taking center stage.

We’re sure you have some questions on the polar vortex, and how it affected this winter’s weather. Let’s get started.

What is the polar vortex?

The term “polar vortex” refers to a persistent area (or areas) of low pressure that rotates around the pole. The polar vortex keeps the coldest air locked up in the Arctic but can be disrupted by something known as sudden stratospheric warming (SSW).

What is different about the polar vortex in the winter?

The polar vortex is always present in our troposphere, which is the layer of the atmosphere closest to the ground. More accurately, the polar vortex is a series of vortices (low-pressure systems) moving around the poles.

The same phenomenon forms in the stratosphere as well during the winter. While it’s not completely understood, the interaction between the stratospheric and tropospheric polar vortices plays a part in severe winter weather.

What is a sudden stratospheric warming (SSW) event?

Image showing progression of SSW event

Sudden stratospheric warming (SSW) is a process where the stratosphere warms suddenly, causing the stratospheric polar vortex to weaken. When this happens, the westerly winds that blow around the poles weaken and even reverse direction. The changes, in turn, disrupt the tropospheric polar vortex, dislodging cold air from the poles.

Scientists classify SSW events into two main categories: minor and major.

Minor: Westerly winds weaken but do not completely change direction. The polar vortex is disrupted but not displaced. Weather effects are limited to the higher latitudes. Minor events occur several times a year.

Major: Westerly winds reverse direction to easterly, which completely disrupts the polar vortex. In some cases, the vortex splits and is displaced from the poles. Weather is affected in the mid-latitudes, often featuring bitter cold. Major events happen about every two years.

How does the polar vortex affect the weather?

Our understanding of weather effects when it comes to the polar vortex is basic, and there are differences of opinion on its role in weather patterns. However, during major SSW events, arctic air is pushed into Siberia and often propagating into North America and Northern Europe.

While here in the U.S. it typically means cold and dry conditions, in Northern Europe the cold is often accompanied by increased precipitation and storminess. Places like England and Ireland see increased snowfall. This usually occurs about two to four weeks after the SSW event.

This said, weather models cannot model the stratosphere with the same accuracy they can the troposphere. So our ability to forecast these events is limited. Add to this the fact SSW events were only discovered in 1953, and there is limited data available to truly understand the relationship between the two.

What was different about this winter?

February 16 snow cover US

Winter 2021-22 provided atmospheric scientists with a unique opportunity: several major SSWs in quick succession.

The first showed signs of developing in the models before the Christmas holiday, finally showing up around January 5. But unlike previous years, another split occurred about mid-month, and yet another towards the end of the month.

The result allowed arctic air to plunge deeper and deeper south. The first might have set the stage for the snowstorm across the East Coast in late January and early February, while later disruptions were behind the extreme cold across the Plains, and the major snowstorm across Mexico and Texas late in the month.

But the polar vortex and sudden stratospheric warming weren’t the only reasons why it snowed so much. In a typical La Niña winter pattern, generally mild and wet weather is seen across much of the country.

The SSW events added the cold, but the moisture remained. The combination of the two is the reasons why February was so cold and snowy across the country, including in places you wouldn’t normally expect.

Could it happen again?

Winter 2020-21 features a unique set of circumstances not seen before in the historical record, so it’s unlikely to happen again anytime soon. But the challenges to weather forecasting it caused has once again shined the spotlight on long-range forecasting.

There are atmospheric processes that scientists still don’t understand fully, especially when it comes to our stratosphere’s effects on our weather. Even as our weather models improve, our ability to peer into the “misty, distant future” still is nothing more than an educated guess.

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