Every winter in the Northern Hemisphere, a cold wind revolves around the North Pole like water around a drain. It’s an annual weather pattern that meteorologists are watching anxiously – any significant change could indicate that Europe is heading for a serious cold snap. Right now that wind is tearing 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 several changes in this large air stream high in the stratosphere, 10 to 50 kilometers (6 to 30 miles) overhead. .
Ironically, the cause of this chill is a sudden burst of heat that seeps into the swirling currents over a window of just 24 to 48 hours.
With its temperature rise of a whopping 40 degrees Celsius, the vortex undergoes some rapid changes, changes course or breaks up dramatically into daughter vortices sliding against the surrounding atmosphere.
The results can be devastating. Just a few years ago, a sudden stratospheric warming (SSW) brought icy polar air from Siberia to Europe, delivering a high-pressure snow-laden cell that the media dubbed The Beast from the East.
Centered over Scandinavia, the shock of the icy weather threw a frozen pall as far west as the UK, adding to transport chaos and even some deaths.
That said, not all shifts in this polar vortex end in freezing cold. Two years ago, the warming of the stratospheric polar winds preceded one of the warmest winter days in the recorded history of the United Kingdom.
Knowing which deviations are omens of winter rage and which ones will hiss will go a long way in making weather forecasts more accurate.
Surprisingly, such stratospheric warming events themselves aren’t exactly rare, with data suggesting that, on average, about half a dozen of them occur every decade in the Arctic’s arctic vortex.
“While an extremely cold weather event is not a certainty, about two-thirds of SSWs have a significant impact on surface weather,” said 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 wobble and splitting in the northern stratospheric polar vortex informing a tracking algorithm that attempts to predict the impact each type of change will have on Northern Hemisphere weather systems. . .
The results suggest that every time the polar vortex splits into two smaller winds, we can expect more severe cooling, compared to other SSW anomalies.
It is a current result, with expected changes in air currents appearing over the weekend.
“As predicted, atmospheric observations now show that the Arctic stratosphere is undergoing a sudden warming associated with a weakening of the stratospheric polar vortex,” said Adam Scaife, head of long-term forecasting at the UK Met Office.
In addition, the change has all the characteristics of the more dangerous kind of SSW, which means that the predicted temperature drop is likely to be significant.
Having informed climate models certainly helps increase the odds of knowing what to expect. But while models on this scale benefit from improved algorithms, there is still room for a lot of uncertainty when it comes to capturing the precise details in the coming days.
Strangely enough, it may even turn out that Europe is sweating instead of shivering.
After all, the UK experienced record-breaking winter heat after an SSW in February 2019, so the Met Office doesn’t rule out the possibility of a similar warming in the coming weeks.
While the protracted cold snap and snow events in February and March 2018 – dubbed the ‘Beast from the East’ by the British media – were linked to a sudden warming of the stratosphere, the record warmth that occurred in February 2019 also followed some event, ‘says meteorologist Matthew Lehnert.
We still have some way to go before we can confidently promise which way things will go again in the wake of these polar changes.
But tools like this new algorithm will increase the odds of guessing, and continue to do so as we learn more about our atmosphere.
“Despite these advances, many questions remain about the mechanisms that trigger these dramatic events and how they may affect the surface, and so this is an exciting and important area for future research,” said mathematician William Seviour of the University of Exeter.
This research is published in JGR Atmospheres.