Weather News: Amplitude of up to 15°C in two weeks: how to explain the temperature changes this winter? – METEO CONSULT – DETAILED 15-DAY WEATHER FORECASTS

From the beginning of December, our winter in France is hot and cold. These changes were important and, above all, surprisingly regulated by two weeks. It seems that this alternative pattern continues in February. How can these variations be explained?

So far, our winter in France has not been cold, +0.6°C on average in December and +0.9°C for this month of January. But these numbers hide very important differences. Changes of about 15°C were observed both in December and January. In this atypical situation, you can find an explanation in the behavior of high-altitude winds, the jet stream.

It’s the turn of a cold and very sweet two weeks

With the arrival of December, a bewildered a persistent cold in the first two weeks. Temperature anomalies ranging from -2 to -3°C above average did not allow us to speak of a cold wave, but nevertheless we recorded peaks of -18°C in Lorraine, which does not happen every year. This persistent situation even suggested that December could finish below average for the season, possibly the coldest month since 2010. However, a rapid and strong thaw occurred on the 18th and will continue for a long time with temperatures rising between +4 and +. Above 5°C. The last decade of December was even The sweetest observed since 1947 with many monthly notes.

The same pattern was repeated in January, but in reverse. Great mildness was maintained from the 1st to the 16th, with the highest temperature being +4.5°C above normal. Then, starting on the 16th, a distinct cooling set in, with snowy episodes up to the plains before a slight thaw at the end of the month. Note that each time the soft anomalies were stronger than the cold anomalies.

Talk: jet stream fluctuations and polar vortex stalling

Part of the reason for these meteorological blockages lies in human behavior jet stream, this high-altitude wind that circulates across the Northern Hemisphere from west to east. The faster it rotates, the more troughs it passes over the mid-latitudes, including western Europe and France. Conversely, some years are characterized by a slower jet stream, or in any case more irregular. When it loses its speed, it begins to undulate like the meanders of a river. Its descent to the south causes a flow of cold air, while its ascent to the north causes large waves of mildness. On the other hand, we also witness those who drop out of school polar vortex. It is a freezing air mass that surrounds the arctic zone. As this cold air breaks away from the polar zone, the jet stream descends to the south, causing intense cold. Due to the influence of connecting basins, the descent of cold air causes the rise of mild air in other areas. historical coldness, Western Europe broke records for softness. After these air masses are well located, high pressure areas are created and called anticyclones. These anticyclones have the characteristic of being very broad and very strong, and then slowly dislodged.

February: anticyclonic blockages may continue

These anticyclones, consisting of temperate or cold air depending on their origin (subtropical or continental), lead to constant stable weather conditions, lasting for two weeks, and thus explain the rhythm of our winter. Until today, after a brief mild spell at the end of January the first decade of February a priori, it will again be under the influence of an exaggerated anticyclone over Central Europe. This configuration will create easterly winds that will bring cold air to France. This situation should last for a good ten days with dry and cold weather. Looking ahead, it seems plausible that the second half of February will be mild again. It should be noted that these anticyclonic turns do not allow the disturbed ocean flow to stop for a long time, which explains the relative lack of precipitation in certain regions and does not allow proper replenishment of water layers under the earth.

At the end, it should be noted that these blocked meteorological conditions are occurring more frequently in the current decade, whereas the 1990s and 2000s were marked by the recurrence of disturbed flows leading to storms and floods. Currently, we are witnessing the opposite, very anticyclonic weather conditions causing drought. According to certain studies (1), global warming may contribute to this type of situation due to the slowing down of the jet stream and the predominance of very strong anticyclonic belts where depressions have to circulate more.

(1) A study published in 2012 already highlighted this possibility: Do global models predict more anticyclonic blockage in Europe for the future?

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