Something is happening in the stratosphere over the polar region, another "major warming" is looming. With a delay of a few weeks, this could also have an impact on our weather patterns. We are therefore taking a cautious look at the long-term models for the coming spring.
The polar vortex disintegrates
Until the end of the year, the stratospheric polar vortex appeared healthy and intact, lying fairly centrally over the polar region with temperatures of around -70 degrees in its central area. In the course of January, however, a pronounced displacement occurred for the first time and the polar vortex was pushed away – over the North Pole temperatures rose significantly at altitudes of 25 to 30 kilometers. This was followed by a certain recovery until the beginning of February, but temperatures did not fall back to their previous level.
Fig. 1: Observed and forecast temperatures at 10 hPa between 60° and 90° north; Source: NOAA
This is the second time in a year that a major warming has occurred, which is very rare. This has only happened seven times since 1960. The warming that is now imminent also appears to be of a sustained nature.
Fig. 2: Temperatures on the 10 hPa surface (approx. at an altitude of 28 km) on Wednesday, February 14, 2024; Source: meteociel.fr
Fig. 3: Temperatures on the 10 hPa surface (approx. at an altitude of 28 km) on Sunday, February 25, 2024; Source: meteociel.fr
Fig. 4: Temperature on the 10 hPa surface (approx. at an altitude of 28 km) on Friday, March 1, 2024; Source: meteociel.fr
The remaining cold air is being pushed southwards over the Atlantic, Europe and Asia, but will also warm up over the next few days. By the end of February to the beginning of March, only cold air remains and there is no longer a polar vortex. The westerly wind circulation comes to a standstill at these heights, with easterly wind components (see Figure 5) playing an increasing role. From this high level, this process may increasingly affect the upper troposphere in the coming weeks, increasing the probability of blocking weather conditions. However, it is not possible to derive detailed long-term forecasts for Switzerland from this. As soon as there are any signs of this, we will of course inform you accordingly!
Fig. 5: Forecast index of the mean zonal wind at the 10 hPa level. Positive for westerly winds, negative for easterly winds; Source: ECMWF
Blocking weather conditions
As mentioned above, the tendency for blocking weather patterns increases after the polar vortex breaks up. Where it is cold, it stays cold for longer – The same applies in the opposite case. However, what actually happens on the ground depends on the distribution of the high and low pressure areas and the resulting flow pattern! In many cases, the blocking high lies over the North Atlantic near Iceland or the British Isles (negative NAO index), in which case cool or cold air masses can push far south on its eastern flank. The result could once again be a kind of March winter. However, if the high lies over Eastern Europe, mild air will continue to reach us. Whatever happens, the flow pattern could tend to last longer.
The new seasonal forecasts from ECMWF and NOAA
Please note: This is not a specific weather forecast, but a large-scale and long-term assessment of flow and pressure patterns. This is done on a global and continental scale for the deviation of various parameters from the climatic mean. This is only a rough trend! The trend for the winter months published last November has so far been very accurate (low pressure, too wet and too mild). So here is a comparison of the latest data from ECMWF and NOAA for the anomalies in air pressure, precipitation and temperature for the months of March, April and May (i.e. the meteorological spring).
No clear trend in air pressure
The two models do not entirely agree on the prevailing distribution of air pressure, but the differences for Central Europe remain small. The ECMWF sees more frequent areas of low pressure over the Atlantic, while the CFSv2 is neutral in this respect with a tendency towards lower air pressure in the Mediterranean region. (The comparison period is slightly different: basis for ECMWF 1993-2016, for NOAA data 1984-2009).
Fig. 6: Deviation of air pressure from the long-term average in Europe for the months of March, April and May (ECMWF); Source: ECMWF
Fig. 7: Deviation of air pressure from the long-term average in Europe for the months of March, April and May (CFSv2, NOAA); Source: tropicaltidbits.com
Normal precipitation, possibly too wet in the south
This distribution is also reflected in the precipitation. According to the ECMWF, the precipitation for the months of March to May would be within the long-term norm, while the CFSv2 indicates that it is too wet on the southern side of the Alps and in Italy.
Fig. 8: Deviation of precipitation from the long-term average in Europe for the months of March, April and May (ECMWF); Source: ECMWF
Fig. 9: Deviation of precipitation from the long-term average in Europe for the months of March, April and May (CFSv2, NOAA); Source: tropicaltidbits.com
Too mild/warm
On the other hand, there is an astonishingly high degree of consistency in the temperatures! According to this, this 3-month period in the Alpine region will be significantly too mild/warm. Both models see above-average temperatures in the Mediterranean region and less positive deviations in the northwest. Scandinavia remains the "cold pole", which was already the case for most of the winter.
Fig. 10: Temperature deviation from the long-term average in Europe for the months of March, April and May (ECMWF); Source: ECMWF
Fig. 11: Temperature deviation from the long-term average in Europe for the months of March, April and May (CFSv2, NOAA); Source: tropicaltidbits.com
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