The polar vortex appears to be on the move. That's because stratospheric warming is occurring at high altitudes above the North Pole, resulting in a spike in temperatures.
That, in turn, could result in bitter cold air pushing southward into the United States within a couple of weeks, though where exactly that Arctic air will swoop down -- and for how long -- remains uncertain.
The polar vortex is simply a low pressure system that swirls cold air around the polar regions of the globe. But the system can sometimes move off the North Pole. In doing so, it releases cold air much farther south in regions such as North America and Europe.
The polar vortex is located in the stratosphere, about 18 miles above Earth's surface, which is well above the jet stream, where planes fly, and where most weather occurs.
But agitations and disruptions to the flow and location of the polar vortex, like what we are seeing to the start of 2021, can influence the movement of air and weather below it. And scientists are closely watching current events for clues as to when and where the impacts will occur.
What is the polar vortex?
Although the term "polar vortex" has become trendy on social media in recent years, the concept is not new. It is a staple for the polar regions every year.
"Some winters, it remains fairly unperturbed. Other winters (like this one), it can be severely disrupted/weakened," Jason Furtado, an assistant professor at the University of Oklahoma's School of Meteorology, said via email. "Understanding what is happening with the polar vortex is one of several features in our climate that helps meteorologists understand what to expect for winter weather over the next 2-6 weeks."
When the polar low pressure system is strong, it keeps the jet stream traveling around Earth in a very circular path and keeps Arctic air bundled up close to the Pole. But when that system is weakened, parts of the vortex break off and become elongated, resulting in cold air shifting southward.
When that low pressure system is weaker, the jet stream also does not have enough strength to maintain its usual path. It is that disruption in the jet stream that has a direct correlation to our weather closer to the surface.
"When the stratospheric polar vortex is strong, the jet stream tends to move further north, which keeps the cold air in the Arctic and allows relatively milder conditions across much of the United States and Eurasia," Furtado said.
A common reason the polar vortex leaves its usual location is due to a sudden shift of hotter air, known as a sudden stratospheric warming, or SSW. This weakens the polar vortex and allows it to move.
"When the polar vortex is weak, or an SSW event occurs, then the jet stream will tend to weakened, move further south, and become 'wavier,'" Furtado said. "The effect of these changes is for warmer than normal air to move into the Arctic, colder weather to enter North America and Europe/Asia, and more extreme weather and storms overall in the middle latitudes (e.g., snowstorms)."
Impact on the US
The jet stream is the main storm track across the middle latitudes of the Northern Hemisphere and divides colder air to the north and warmer air to the south.
Because the polar vortex is disconnected physically from where most weather occurs, it often has more of an indirect impact on daily weather.
Cold air is more dense, so it sinks, allowing the hot air from the SSW to remain in the stratosphere and the colder air to sink down into the lower levels near the surface. That downward sinking air results in sections of the polar vortex pushing down into lower latitudes of North America, Europe, and Asia.
Recently, a SSW caused the polar vortex to begin shifting away from the North Pole. But where that cold air will end up over the next week or two remains a question. What makes this SSW so special is its intensity, which increases the chances of impacting the Northern Hemisphere.
It's important to note that a disruption in the polar vortex does not garner immediate changes to weather. Instead, its effects are typically delayed.
"In the stratosphere, the polar vortex typically recovers in strength within a couple of weeks of the peak of the event," Furtado said. "However, in the troposphere, the effects of the SSW event (e.g., a further south jet stream, cold and stormy weather) can last for up to 8 weeks. So, these events, which can evolve quickly, can have lasting impacts on the winter weather patterns in the troposphere."
Another thing to note: Even with strong SSW events, there is still no certainty that the US will see direct impacts.
"We actually had two events in the recent past that shows this contrast. There was a major SSW event in February 2018 and one in January 2019, and the after effects of each were very different," Furtado said.
Furtado says we had cold and stormy weather for much of the central and eastern regions of North America following the 2018 event, but minimal effects were felt in North America following the 2019 event.
So, no need to panic about this recent event. But it's certainly something to watch over the next seven to 14 days.
How climate change plays a role
If you have a warming Earth, then it would only make sense for more frequent sudden stratospheric warming events to occur, right? The answer is complex, mostly because no one lives at the North Pole -- other than Santa -- which makes long term historical weather data very hard to come by.
But this doesn't mean climate change isn't impacting the polar vortex.
"We know from observations that the Arctic region is warming at a much faster rate than other parts of the globe (we call this Arctic Amplification)," Furtado said. "The impact of Arctic Amplification is twofold. First, since the Arctic is getting much warmer, when cold air outbreaks occur in North America and Europe/Asia, they aren't as cold as they were decades ago. The second effect (though currently debated in the science community) is that a warmer Arctic is also making the stratospheric polar vortex weaker on the average."
This is why, in theory, a weaker polar vortex should be easier to disrupt, allowing for more frequent SSW events.
The takeaway? Even though a warming planet may mean the overall number of snowstorms across the globe may decrease, the ones that do happen could produce much larger snowfall accumulations.
Brandon Miller contributed to this story