Every winter in the northern hemisphere, a cold wind circulates the North Pole like water around a drain. It is an annual weather pattern that meteorologists keep an eye on – any significant change could suggest that Europe is about to suffer a severe cold snap. Now, this wind is breaking in two.
Researchers at the Universities of Bristol, Exeter and Bath have discovered a new way to predict the indirect effects of various changes in this large high air current in the stratosphere, 10 to 50 kilometers (6 to 30 miles) above.
Ironically, the cause of this cold is a sudden explosion of heat that seeps into the whirling currents in a window that lasts just 24 to 48 hours.
With its temperature rising by up to 40 degrees Celsius, the vortex undergoes some rapid changes, changing course or dividing dramatically into child vortexes that push against the surrounding atmosphere.
The results can be devastating. Just a few years ago, a sudden stratospheric warming (SSW) event pushed icy polar air from Siberia to Europe, releasing a cell laden with high-pressure snow that the media dubbed The Eastern Beast.
Focused on Scandinavia, the shock of icy weather launched a frozen shroud to the west of the United Kingdom, contributing to transport chaos and even several deaths.
That said, not all changes in this polar vortex end in freezing conditions. Two years ago, the warming of stratospheric polar winds preceded one of the warmest winter days in UK history.
Knowing which deviations are portents of winter’s fury, and which will fail, will go a long way in making the weather forecast more accurate.
Surprisingly, these stratospheric warming events themselves are not exactly rare, with records suggesting that an average of about half a dozen of them occur in the Arctic’s polar vortex every decade.
“Although an extreme cold weather event is not a certainty, about two-thirds of SSWs have a significant impact on the surface climate,” said Richard Hall, a meteorologist at the University of Bristol and lead author of the new study.
Observations dating back more than six decades have provided researchers with 40 examples of oscillations and divisions in the northern stratospheric polar vortex, which inform a tracking algorithm that attempts to predict the impact that each type of change will have on meteorological systems in the northern hemisphere.
The results suggest that whenever the polar vortex divides into two smaller winds, we can expect more severe cooling events compared to other SSW anomalies.
It is a timely result, with changes in forecasting drafts appearing over the weekend.
“As predicted, atmospheric observations now show that the Arctic stratosphere is experiencing a sudden warming event associated with a weakened stratospheric polar vortex,” said Adam Scaife, head of long-range forecasts at the UK Met Office.
Furthermore, the change has all the characteristics of the most dangerous type of SSW, which means that there is a good chance that the predicted drop in temperature will be significant.
Having informed climate models certainly helps to increase the chances of knowing what to expect. But while modeling at this scale benefits from improved algorithms, there is still room for many uncertainties when it comes to defining the precise details in the coming days.
Strangely, it may even happen that Europe sweats instead of shivers.
After all, the UK experienced record heat in the winter after an SSW in February 2019, so the Met Office does not rule out the possibility of similar heat in the coming weeks.
“Although the prolonged periods of cold and snow in February and March 2018 – dubbed the ‘East Beast’ by the UK media – were linked to a sudden stratospheric warming, the record period of heat that occurred in February 2019 also occurred after such an event, “says meteorologist Matthew Lehnert.
We have a long way to go before we can confidently promise which way the climate will go in the wake of these polar changes.
But tools like this new algorithm will improve the chances of guessing and will continue to do so the more we learn about our atmosphere.
“Despite this advance, many questions remain as to the mechanisms that cause these dramatic events and how they can influence the surface, so this is an important and exciting area for future research,” said mathematician William Seviour, from the University of Exeter.
This research was published in JGR atmospheres.