Every year in winter, a current of cold air circulates around the North Pole. The phenomenon, which takes place high in the stratosphere, is closely monitored by meteorologists: an atypical behavior of this cold wind may indeed suggest that Europe is preparing to undergo a major cold wave. However, currently, this current of air seems to disperse …
Paradoxically, this intense cold wave is due to a sudden increase in temperature: in just 24 to 48 hours, hot currents infiltrate the polar vortex, whose temperature then increases by nearly 40 ° C. This sudden increase causes rapid changes in the cold current: it suddenly changes direction or segments into several smaller currents. The consequences of this phenomenon, called “sudden stratospheric warming” (or sudden stratospheric warming, or SSW in English), can be devastating.
Europe has experienced it not so long ago: in February 2018, freezing cold from Siberia – nicknamed “the beast from the east” by European media – was shot down over all of Europe for several days, in particular over eastern countries and Scandinavia; the temperatures recorded were 10 to 20 ° C below normal. This cold snap had also largely impacted the United Kingdom by wreaking havoc on transport for a time, leading to the closure of thousands of schools and causing several deaths. The Guardian then reported “worst weather conditions in years”.
Characteristic signs of future cooling
Note, however, that any change in the polar vortex does not necessarily lead to freezing conditions. So in 2019 it was quite the opposite: an SSW preceded one of the hottest winter days in UK history. In the community of Trawsgoed in Wales a temperature of 20.6 ° C was recorded in the middle of February, a value which is rarely reached in the region, even in summer! Several other localities had recorded temperatures above 20 ° C.
For experts, therefore, it is essential to understand which changes in the polar vortex are responsible for an intense cold snap, and which others should not be of particular concern. Especially since SSWs are not uncommon: they occur approximately every two years. In any case, the phenomenon is not without consequences, as Richard Hall, meteorologist at the University of Bristol explains: “ Although an extremely cold weather event is not a certainty, around two-thirds of SSWs have a significant impact on surface weather conditions “.
Hall and his collaborators therefore worked on the development of new prediction models, making it possible to determine the effects of various changes in this major cold air current. Their study, published in the Journal of Geophysical Research: Atmospheres, is based on observations recorded for more than six decades; the researchers were thus able to retain 40 examples of oscillations and segmentations at the level of the north stratospheric polar vortex to develop their predictive algorithm.
The results suggest that whenever the polar vortex splits into two smaller winds, it results in more severe cooling events compared to other anomalies in the SSW. However, this is precisely the type of behavior observed by meteorologists over the weekend.
A cold snap … or abnormal heat
” As expected, atmospheric observations now show that the arctic stratosphere is experiencing sudden warming associated with a weakening stratospheric polar vortex Confirms Adam Scaife, head of long-range forecasting at the UK Met Office. Should we therefore expect the next drop in temperatures? It seems so. According to experts, the behavior of the vortex has all the characteristics of the most formidable type of SSW. In other words, Europe can expect a very likely and significant drop in temperatures.
As part of their study, the researchers found that while the difference in oscillation anomalies between the divisions and displacements of the polar vortex is small, the surface temperature anomalies over northwestern Europe and the northern Eurasia are significantly cooler for division events, especially in the UK, just before the surface impact date. In addition, displacement events are on average wetter in north-western Europe at the time of surface impact.
Nevertheless, there is still hope. Even if the modeling proposed by the researchers is based on solid algorithms, it also involves certain uncertainties. It could be that the predicted scenario does not go exactly as planned. Meteorologist Matthew Lehnert believes that, as in winter 2019, Europe could on the contrary experience temperatures exceptionally above seasonal norms in the coming weeks. In any case, the Met Office does not exclude this possibility.
As the mathematician William Seviour of the University of Exeter, who participated in the design of the algorithm, points out, more research will be needed to benefit from a completely reliable predictive system for the behavior of the north polar vortex: ” Despite this advance, many questions remain as to the mechanisms behind these dramatic events, and how they may influence the surface, and therefore this is an exciting and important area for future research. “. In the meantime, the new algorithm created by Seviour and his colleagues is already making it possible to benefit from improved predictions, and will undoubtedly be more and more precise as scientists increase their knowledge of the atmosphere.