Every winter in the Northern Hemisphere, a cold wind circles the North Pole like water around a drain. It's an annual weather pattern meteorologists keep an anxious eye on – any significant changes could suggest Europe is in for a serious cold snap. Right now, that wind is ripping in two.
Researchers from the Universities of Bristol, Exeter, and Bath have come up with a new way to predict the knock-on effects of various changes to this major air current high up in the stratosphere, 10 to 50 kilometres (6 to 30 miles) overhead.
Ironically, the cause of this chill is a sudden burst of heat seeping into the whirling currents over a window of just 24 to 48 hours.
With its temperature surging by as much as 40 degrees Celsius, the vortex undergoes some rapid changes, changing course or dramatically breaking apart into daughter vortices that shove against surrounding atmosphere.
The results can be devastating. Just a few years ago, a sudden stratospheric warming (SSW) event nudged frigid polar air from Siberia into Europe, delivering a snow-laden cell of high pressure the media dubbed The Beast from the East.
Centred over Scandinavia, the shock of icy weather cast a frozen pall as far west as the UK, contributing to transport chaos and even a number of deaths.
That said, not all shifts in this polar vortex end in freezing conditions. Two years ago, warming of stratospheric polar winds preceded one of the warmest winter days in United Kingdom's recorded history.
Knowing which deviations are portents of winter fury, and which will fizzle, will go a long way in making weather forecasting more accurate.
Surprisingly, such stratospheric warming events themselves aren't exactly rare, with records suggesting an average of around half a dozen of them occur in the Arctic's polar vortex every decade.
"While an extreme cold weather event is not a certainty, around two thirds of SSWs have a significant impact on surface weather," says Richard Hall, University of Bristol meteorologist and lead author of the new study.
Observations dating back more than six decades have provided the researchers with 40 such examples of wobbles and splits in the northern stratospheric polar vortex, which inform a tracking algorithm that attempts to predict the impact each kind of change will have on weather systems across the northern hemisphere.
The results suggest any time the polar vortex splits into two smaller winds we can expect more severe cooling events, compared with other SSW anomalies.
It's a timely result, with forecast changes to the air currents appearing over the weekend.
"As predicted, atmospheric observations are now showing that the Arctic stratosphere is undergoing a sudden warming event associated with a weakening stratospheric polar vortex," says Adam Scaife, head of long-range prediction at the UK Met Office.
What's more, the change has all the hallmarks of the more dangerous kind of SSW, meaning there's a good chance that the predicted drop in temperature will be significant.
Having informed climate models certainly helps improve the odds of knowing what to expect. But while modelling on this scale benefits from improved algorithms, there's still room for plenty of uncertainty when it comes to nailing down the precise details in coming days.
Oddly, it might even turn out that Europe sweats instead of shivers.
The UK experienced record-setting winter warmth after a SSW in February 2019 after all, so the Met Office doesn't rule out the possibility of a similar swelter in coming weeks.
"Although the prolonged cold spell and snow events in February and March of 2018 – dubbed the 'Beast from the East' by the UK media – were linked to a sudden stratospheric warming, the record warm spell that occurred in February 2019 also followed such an event," says meteorologist Matthew Lehnert.
We've got some way to go before we can promise with confidence which way the weather will go in the wake of these polar changes.
But tools like this new algorithm will improve the odds of guessing, and continue to do so the more we learn about our atmosphere.
"Despite this advance many questions remain as to the mechanisms causing these dramatic events, and how they can influence the surface, and so this is an exciting and important area for future research," says mathematician William Seviour from the University of Exeter.
This research was published in JGR Atmospheres.