Cold weather in February? 24/01/2023
This is a term that has been widely used by the media in recent days: the “polar vortex” will threaten France for February 2023. What is it really? Weather Cities reckons.
What is polar vortex and stratospheric warming?
Before tackling this topic, it is important to define what a polar vortex is. The latter describes a broad and robust low that circulates at the level of the poles. This is associated with a large pocket of cold air extending into the stratosphere (between 10 and 50 km altitude). It decreases in summer and increases in winter. The jet stream – a corridor of high winds that rotates from west to east in the northern hemisphere – rotates around the polar vortex at a higher speed during the winter season. In the classic case, the polar vortex – as the name suggests – is located above the polar regions. However, it is not always stable and can move.
Diagram explaining the polar vortex phenomenon – Le Parisien infographic
It may be that in the middle of winter there is a decrease in the activity of the polar vortex. This begins with a spectacular rise in stratospheric temperature, which can rise by 30-50°C in a matter of days. This phenomenon is called sudden stratospheric warming. When this happens, the vortex becomes weaker and more mobile. This means that it can move to lower latitudes, bringing with it very cold air. However, its movement remains random and often difficult to predict. The map below shows stratospheric warming for the end of January 2023. In certain areas of Asia, the temperature in the stratosphere drops from -60°C to 0°C in a matter of days!
Animation of this late January 2023 stratospheric warming – via meteociel.fr
A cold spell that guarantees everything
Many media have started talking about the possible arrival of the polar vortex in France, which will bring a cold weather in February 2023. However, we are still a long way from it! Indeed, the effects of stratospheric warming are variable because they sometimes have difficulty propagating into the lower atmosphere. In the case of a polar vortex, the results are radically different depending on the area where it sinks. But according to recent projections, the current warming of the stratosphere will have only a limited effect and cause the polar vortex to migrate towards the North Atlantic. If this were the case, we would only see an acceleration of ocean currents.
Polar vortex modeling for Monday, February 6, 2023 – via meteociel.fr
Moreover, modeling of the North Atlantic Oscillation Index (NAO) also goes in this direction. To simplify, the more positive the number, the more low pressure activity is noted over the North Atlantic, which emphasizes ocean flow over Western Europe, including France. In the chart below, we can clearly see that almost all scenarios suggest a bullish trend for early February 2023. In other words, the polar vortex over the North Atlantic will favor the disturbed ocean over France for easier passage of mild air. than cold weather.
Modeling of the North Atlantic Oscillation Index through early February 2023 – via NOAA
The latest forecasts for February 2023 from the American model also count for a very mild month with an anomaly of around +2°C in France and a good part of Europe. They also find a wetter-than-normal trend over the northern half of France and the British Isles, witnessing a mild and disturbed ocean current.
Modeling of temperature and precipitation anomalies in February 2023 – US model via NOAA
Even if these multi-week forecasts are to be taken with a grain of salt, they justify the sensationalist articles claiming that February 2023 will be cold in France. shortest path…