A once fairly boring way to describe the behavior of cold air at our planet’s poles has quickly become a part of the winter lexicon: the polar vortex. The mere mention of the word on the evening news might bring a sense of dread to those not fans of the cold. But there’s also a lot of hype surrounding this topic, so we wanted to write this blog to explain this wintertime event.
The most notable recent polar vortex event was the storm that passed across the country the week before Christmas. Behind an unusually strong cold front, a portion of the vortex dove southward starting on December 21, causing temperatures to crash in some areas by as much as 40-50 degrees in an hour, and helped trigger one of the worst lake effect blizzards in Buffalo in at least four decades.
The polar vortex also turned Winter 2020-21 into a memorable season, even though my weather station read 62 on Christmas Day 2020. in Pennsylvania. So let’s learn a bit more about our Arctic visitor.
What is a polar vortex?
The term “polar vortex” refers to a persistent area (or areas) of low pressure that rotates around the pole. It keeps the coldest air locked up in the Arctic but can be disrupted by “sudden stratospheric warming (SSW).”
What is a sudden stratospheric warming (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 higher latitudes. Minor events occur several times a year.
Major: Westerly winds reverse direction to easterly, completely disrupting 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.
Can SSWs occur after one another? Does it make the cold and snow worse?
Yes, but it’s very rare. Winter 2021-22 allowed atmospheric scientists to monitor 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, 2022. 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 2022, 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 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 why February 2022 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 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” is still an educated guess.
Does the polar vortex happen every year?
Yes. The polar vortex is always at the poles. However, it isn’t always disrupted every winter.
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, it 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.
How does the polar vortex affect the weather?
Our understanding of weather effects regarding 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, often propagating into North America and Northern Europe.
While here in the US typically means cold and dry conditions, the cold is often accompanied by increased precipitation and storminess in Northern Europe. Places like England and Ireland see increased snowfall. This usually occurs about two to four weeks after the SSW event.
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.
Is a polar vortex caused by global warming?
No. The polar vortex is a naturally occurring weather phenomenon. However, the events that lead to a disruption of the polar vortex causing it to spill cold air southward, don’t always occur every winter, or at least in a way that affects weather in populated areas.