WeatherBlog 2 2020/21 | Weather services report: Winter is getting milder

These are not groundbreaking new findings, but the cross-national message once again clarifies what we basically already know: Higher temperatures mean rising snowfall limits, means less snow, especially at low altitudes. It generally snows less (it rains instead) and snow that has fallen melts again more quickly. Phases with continuous snow cover at low altitudes start later and end earlier. The number of days with snow cover in Vienna, Innsbruck and Graz has decreased by around 30% in the last 90 years or so. The figures are similar on the Swiss Plateau. In Munich, too, there are on average around 20 fewer days with snow today than in the 1950s. While the temperature determines the amount of snow at low altitudes, the amount of precipitation is decisive at high altitudes. Although it is also getting warmer in the high mountains, it is usually still cold enough for snow instead of rain.

The weather services emphasize that the temperature and, above all, winter precipitation fluctuate greatly from year to year and that long-term trends are not always easy to identify, as they are overlaid by regionally different, short-term effects. Despite long-term warming, cooler winters can occur in the meantime. Similarly, there can also be a lot of snow at low altitudes - it just doesn't happen as often.

If emissions remain high, it can be assumed that snow cover in Austria will decrease by 90% at low altitudes by 2100, and by 50% at around 1500m. In Switzerland, it is estimated that snow cover below 1000m will decrease by 80% by 2060, and by 30 to 50% above 1500m. But we don't have to give up because everything is lost anyway: climate protection measures can still counteract this development. If the goals of the Paris Climate Agreement are achieved, the expected decrease in snow cover would be roughly halved, according to the weather services.

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Sonnblick (A), Hohenpeißenberg (D) and Säntis (CH): Warm winters have become more frequent in recent decades, cold winters less frequent. Comparison with the climate reference period 1961-1990. The dotted line marks the increase in warming (linear trend).

DWD/ZAMG/MeteoSchweiz

And what about the polar vortex and sea ice?

For several years now, there have been repeated reports that the wave patterns of the jet stream are changing as sea ice decreases, which could lead to more blocking weather patterns and consequently to cooler winters, particularly in Europe and neighboring Asia. The ideas behind this stem from the fact that sea ice extent in the Arctic is undisputedly declining sharply. At the same time, parts of Eurasia repeatedly experienced very cold winters from the 1980s to the early 2010s. The winter temperatures of the mid-latitudes and the sea ice extent as well as the "Arctic amplification" - the increased warming in the Arctic compared to other regions - correlate relatively strongly during this period. The theoretical reasoning behind this is (greatly abbreviated) that the winter polar vortex may not be able to form in a round shape if there is a lack of sea ice and cold, but instead oscillates more strongly.

The whole thing was and is fraught with uncertainties. This has always been emphasized in the relevant scientific publications. In many model studies, the effect mentioned is not found at all or only very ambiguously (e.g. here). According to a recently published study, the correlation between the continuing decline in sea ice and Arctic amplification with mid-latitude winter temperatures has also decreased significantly in recent years. The last few years no longer fit the supposed pattern (see here). The previously observed correlation was therefore perhaps simply the result of normal fluctuations inherent in the climate system. This is supported by the patterns of the last few winters and some fairly convincing model studies.

Satellite image (Sentinel 2) from 21.11.2020, Andorra. — Satellite image (Sentinel 2) from 21.11.2020, Andorra
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On the other hand, the same still applies: More data and more research are needed to really understand the links between climatic changes in the Arctic and mid-latitude weather. Moreover, the answer to the question "Can it happen like this?" is not necessarily the same as the answer to the question "Has it happened like this in recent decades?" And, as is so often the case, everything gets more complicated the deeper you delve into the details. If you want to do this, you will find a good starting point in the publications linked above, or in the links in this Twitter thread. In any case, you shouldn't be lulled into a false sense of security and rely on the lurching polar vortex if you want cold winters despite climate change!

And what about the weather now?

Well, it's staying more or less as it is: Warm and sunny in most of the Alpine region. There will be a bit of precipitation in the south-west of the Alps at the weekend and probably a bit more cloud in the eastern Alps, but otherwise there's little to report. Of course, this is just the weather at the moment. It fits symbolically with the aforementioned study of winter trends, but at some point there will be PowderAlert again, for sure! And in areas where the sun can no longer reach, at least the frost formations are becoming more and more beautiful!