What are the polar vortex and stratospheric warming?
A frost as early as the first few days of November? It’s too early to say.
Key points
The bitter cold in Italia could arrive as early as the first few days of November. But bear in mind that this is not yet certain: we will need to monitor how the stratospheric polar vortex develops.
What are the polar vortex and stratwarming?
Winter, the Arctic. A large circulation of westerly winds rotates from west to east around the polar regions: this is the stratospheric polar vortex, which moves within the stratosphere – the atmospheric layer where most of the weather conditions experienced at ground level develop. When this vortex is compact, it helps to keep the air colder at high latitudes, that is, in the areas near the poles. However, if the polar vortex is disrupted, the wind circulation can change – slowing down, shifting or losing its shape. And this could happen as early as the first few days of November: the latest projections from the European Centre for Medium-Range Weather Forecasts (ECMWF) show a sharp slowdown in the winds, verging on a collapse, and even a possible reversal, with the return of easterly winds. The phenomenon currently underway is known as ‘stratwarming’ and consists of a rapid warming – exceeding 25°C in a week – of the polar stratosphere, which can weaken, distort or even split the polar vortex.
Precociousness
Forecasts suggest that the weather event could occur as early as the first ten days of November, with an approximate date of 4 November. If these projections prove accurate, it would be the first time it has happened so early. In fact, to date, stratospheric warming on this scale has never been recorded before 28 November. However, the event is only recognised when the observed atmospheric data meet the criteria used to identify it: a condition which, at present, has not been met.
What does this mean for Italia and for Europe
The weakening of the polar vortex can, in some cases, lead to cold air moving downwards towards parts of Europe, Asia or North America. However, this is neither an immediate nor an automatic consequence: one or more weeks may elapse between stratowarming at high altitude and any change in weather at ground level, provided that the phenomenon does not remain confined to the upper atmosphere.
It is therefore incorrect to automatically interpret a change in wind direction as a forecast of frost or snow for Italia and Europe. The degree of uncertainty is very high: some events are followed by marked cold spells, whilst others have modest effects or vary from one region to another. The season must also be taken into account: at the start of November, the continent and the Arctic air have not yet reached the temperatures typical of the middle of winter. Consequently, the cold is not comparable to the major cold spells of January or February.
