Did El Niño cause an early breakdown in the polar vortex?

The polar vortex has suffered an unprecedented early breakdown, threatening Europe with cold and snow, and some scientists think the record super El Niño is to blame.

The polar vortex, a high-altitude band of westerly winds circling the Arctic that bottles up cold air over the north pole, typically strengthens through the winter and weakens in the spring.

The El Niño climate phase, which has already warmed surface waters in the tropical Pacific to record temperatures and continues to heat up, tends to keep the vortex strong in autumn and early winter. Then in late winter it often weakens the vortex, allowing that cold air to spill southward into Europe and sometimes North America.

But on 10 November the polar vortex weakened to the point that it broke down, with the winds reversing direction to blow weakly from the east. While these collapses happen every few years, they almost never occur in November. This is the earliest it’s happened since weather balloon records began in the 1950s, beating the old record by 18 days.

This may have been due to random fluctuations in the atmosphere. It’s also possible, however, that the extremely strong El Niño could have sped up the weakening of the polar vortex that it typically causes later in the winter, though that is not proven, according to Adam Scaife at the Met Office, the UK’s weather service.

“There probably is some kind of El Niño influence on it,” Scaife says. “That would be my suspicion, but we don’t know that for sure.”

As east-to-west trade winds weaken during El Niñno and the warm water they built up in the western Pacific washes back eastward, a low-pressure centre of warm rising air moves eastward with it. This causes extra waviness in the jet stream, another band of strong westerly winds that circles the globe, except further south and lower in the atmosphere than the polar vortex. The jet stream meanders further southward around Alaska, deepening a low-pressure system there called the Aleutian low.

But the El Nino-linked wave doesn’t just propagate side-to-side in the jet stream. It can also propagate upward into the stratosphere, where the polar vortex is. As the air gets thinner, the wave’s amplitude gets bigger until it crashes onto the polar vortex like a wave crashing onto a beach, Scaife says. And while the polar vortex is westerly, these waves carry easterly momentum.

“When they dump that momentum, they push the polar vortex against its flow, and they weaken it,” he explains.

But another reason for the polar vortex breakdown could be a high-pressure “blocking” system that developed over Scandinavia in recent weeks, which blocked the jet stream and caused large meanders in it. This waviness could also have weakened the polar vortex through the same wave-breaking process, regardless of El Niño.

“I would be wary of saying it was caused by El Niño,” says Andrew Charlton-Perez at the University of Reading, UK. “I would say the blocking high is probably more important.”

The reversal of the polar vortex is not related to the current cold weather in the UK. But it could bring even more cold, snowy weather in the coming weeks.

A polar vortex breakdown is also known as a sudden stratospheric warming event. As the winds weaken, high-level air rushes in over the north pole, increasing atmospheric pressure and raising temperatures, just as pumping up a bike tyre warms it up.

Meanwhile, over the following weeks, cold surface-level air can descend from the Arctic into northern Europe. A polar vortex breakdown preceded the “Beast from the East” cold snap in 2018, when blizzards covered much of the UK in snow and killed 10 people.

While the polar vortex is expected to strengthen toward the end of November, bringing warmer weather, it could break down again this winter, Scaife warns.

“The risk of cold easterlies this winter is increasing,” he says.