The polar vortex is acting weird and the US is paying the price this winter

CNN

21-02-2025

It's really, really cold again — as the US shivers through at least the eighth blast of air from the Arctic this winter.
Winter, which is warming faster than any other season for much of the US, seems to be making a comeback for the first time in years; this January was the coldest in the Lower 48 since 1988.
But the US is an outlier, and so is this winter. January was the warmest on record for the globe and, in a vast expanse of global warmth, the US sticks out like a cold, sore thumb.
Scientists say it's being caused by a misbehaving polar vortex combining with a key weather pattern that seems to be stuck in place.
Some scientists say these factors and this winter could be examples of how extreme cold behaves in a warming world. Others argue it doesn't paint a complete picture and further research is needed.
What they do have consensus on is that winter is getting warmer as the planet warms because of fossil fuel pollution, so this Arctic blast from the past feels more like a relic of a bygone era.
'We're definitely shifting the goal posts on what winter looks like,' Zeke Hausfather, a research scientist with the University of California, Berkeley, said, noting that when we experience a season that's actually chilly, it's somewhat jarring.
'There's no location in the US where the coldest day of the year has gotten colder over the last 50 years,' Hausfather continued. 'Our memories are short as to what a normal winter is.'
A few atmospheric factors — including the polar vortex — have come together to make the US the epicenter of cold this winter.
One is a weather pattern around the Arctic Circle that has emerged more frequently than usual this winter and is driving this week's cold, according to Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research.
It features a large, long-lasting area of high pressure known as a blocking high because it blocks cold air and reroutes it south via a large dip the jet stream — the river of air over the US that storms flow through –— that also separates cold air from warm air.
This high has been stuck over Alaska and northwest Canada, which is something that generally happens more often during La Niña winters and forces the cold to spill into parts of the Lower 48. The end result has been one of the warmest winters to-date in Alaska and an unusually cold one in the Lower 48.
The frequent emergence of this pattern this winter could be a sign of things to come in a warming world. A 2023 study found blocking highs in the Arctic Circle similar to this year's would become more frequent as the Arctic warmed and weakened the jet stream, allowing more cold to spill south.
It's part of a growing body of research linking the rapidly warming Arctic to changes in jet stream behavior and extreme cold. Other scientists, including Hausfather, think additional research is needed.
Whether or not it's connected to climate change, the pattern is still rearing its head this winter and it's working in tandem with the polar vortex.
The jet stream and frigid air just could not make it quite as far south without also having help from the polar vortex, according to Cohen. That's because the polar vortex doesn't cause US cold air outbreaks but instead amplifies them, Jennifer Francis, a senior scientist at the Woodwell Climate Research Center said.
The polar vortex is area of fast-moving winds well above the Earth's surface and the jet stream that circle the Arctic during the Northern Hemisphere's coldest months. When it's strong, it keeps brutally cold air trapped in the Arctic, like a figure skater doing a spin with their arms tight to their body. When it's weak, the cold air frequently spills south.
The polar vortex has been 'considerably stronger' than usual this year, according to Laura Ciasto, a meteorologist with NOAA's Climate Prediction Center, which should keep record-breaking cold out of the US. But it hasn't because the polar vortex has been frequently been stretching into weird shapes Cohen said.
A strong polar vortex is circular like a rubber band when it rests on a surface untouched. But energy circling the atmosphere can sometimes smash into the polar vortex, like two hands trying to fling a rubber band, stretching it into something more oblong than circular. That's what's happening now.
Now stretched, the polar vortex can then shift the jet stream even farther south than just the blocking high could do on its own. This allows more cold air to spill into the US, and further south, too, Cohen explained.
See, for example, the below-zero wind chills Dallas endured Wednesday morning, or when New Orleans was buried under a record-breaking 8 inches of snow in January.
The polar vortex has been snapping back and forth from a normal to a stretched state with unusual frequency this year, hence all of the cold snaps, Cohen said. At least 10 of these stretched polar vortex events have occurred this winter, including the ongoing event, according to Cohen: four in December, four in January and two in February.
The polar vortex is usually 'like an aircraft carrier, it doesn't turn around quickly and isn't very nimble,' Cohen said. 'I've really never seen anything like it.'
These polar vortex stretches are happening more frequently as the world — and especially the Arctic — warms, a 2021 paper published in the journal Science, also co-authored by Cohen, demonstrated.
And it's having a huge impact. A stretched polar vortex event played a significant role in the Arctic outbreak that froze Texas in February 2021, killing more than 200 people, according to a 2020 study.
The blocking pattern and stretched polar vortex are two factors at the forefront of a still-active and often highly debated area of research into both why and how frequently extreme cold outbreaks reach the US in a warming world.
'There are multiple ways that human caused climate change is having an influence on the jet stream, but it's never clear which factor is the most important one in any given event, like the cold spell happening now,' Francis explained. 'It's always a combination (of factors), and it's always complicated.'
There could be other yet to be discovered influences, and confidence will grow as research continues, but scientists know extreme bouts of cold like what's happening this winter will still happen even as temperatures keep rising globally.
'These extreme cold events (will) perhaps happen more often, even though they probably won't be quite as cold over time as the air generally warms,' Francis concluded. But when they do, 'they're going to be just as disruptive.'