For the past several months, an El Niño of “super,” “mega,” and “monster” proportions has been making headlines. Climate scientists say this event will likely lead to an overall mild winter for the U.S., but a look at the polar vortex reminds us that El Niño won’t be the only factor shaping the season.
Forecast models show that the polar vortex could undergo an “unprecedentedly” early large disruption in the first weeks of November, according to Judah Cohen, a climatologist at the Massachusetts Institute of Technology and a leading expert on this phenomenon. He tracks the polar vortex and issues forecasts via his blog, and his latest outlook suggests it could soon make history.
“This level of polar vortex disruption is most common in January and February,” Cohen told Gizmodo in an email. “Occasionally it has happened in December, though none so far this century.”
In fact, there are no polar vortex disruptions of this magnitude in November in the satellite record, and there are only two documented in the pre-satellite record, which is much less reliable, Cohen said. Now, “the models are predicting one almost three weeks earlier than the earliest-known large polar vortex disruption, which is a remarkable record in my opinion.”
The effect this will have on U.S. weather remains uncertain. While other forecasters agree that a polar vortex disruption—also known as a “sudden stratospheric warming” (SSW) event—is likely on the way, that doesn’t necessarily mean we’ll get an early taste of winter.
A “major” disruption expected
The polar vortex is a large current of low pressure and cold air that spins counterclockwise around the North Pole. It forms in the stratosphere each fall and has a major influence over winter weather in the northern hemisphere.
When the polar vortex is strong, it maintains a tight flow around the North Pole, keeping cold temperatures locked up in the Arctic. But when it weakens, the vortex can wobble and stretch, allowing frigid air to meander south into the mid-latitudes. Forecasters call this an “Arctic outbreak” or, colloquially, an “Arctic blast.”
So what causes the polar vortex to weaken? That would be large-scale atmospheric waves that travel from the troposphere up into the stratosphere. When these waves “break” on top of the polar vortex, they weaken it. And if they’re strong enough, the vortex can weaken so much that its winds temporarily reverse direction, causing cold air to sink and stratospheric temperatures to rapidly rise.
When this happens, it’s considered a “major” sudden stratospheric warming event. As of today, a whopping 86% of forecast models run by the European Center for Medium-Range Weather Forecasts (ECMWF) predict that the polar vortex will reverse direction by November 10. The ECMWF notes that no major SSW has occurred this early in the year since its record keeping began in 1959.
Major SSWs are more likely to produce Arctic outbreaks, but they’re still not guaranteed. The ECMWF forecast shows the polar vortex stretching into an oval nearly three times as long as it is wide around November 1, suggesting Arctic air eventually could make its way into the United States. Even if it does, mid-latitude weather impacts will lag behind the polar vortex disruption, according to Cohen.
While experts widely believe that it takes about two weeks for a large polar vortex disruption to influence U.S. weather, he said it often takes longer than that. “I wouldn’t expect to see the impacts until sometime in December,” Cohen said.
What’s behind this unusual timing?
The main reason why forecasters are expecting a sudden stratospheric warming event so early in the year is the presence of strong Ural/Scandinavian blocking, according to Cohen. Essentially, a high-pressure system has parked itself over northwestern Russia and Scandinavia and is pushing large atmospheric waves up into the stratosphere.
By enhancing this upward propagation, Ural/Scandinavian blocking can weaken the polar vortex, which is why forecast models are pointing to a sudden stratospheric warming event within the next few weeks. But Cohen emphasized that predictions are not certainties. The models have never seen an event as early as this, so “they’re going to struggle with it,” he said.
No matter how this particular event plays out, we can expect the polar vortex to play a key role in shaping weather across the U.S. this winter, even amid a record-strong El Niño. Cohen points to the winter of 2009 and 2010 as an example—despite El Niño tipping the scales toward mild conditions, a very weak polar vortex led to a colder season.
“Is there a point where the El Niño is so strong that it can deflect any cold thrown at us by the polar vortex? I don’t know, and I argue that no one else does either,” he said.
