October was one of the warmest since measurements began. Will the trend continue in the coming winter months? In view of the high energy costs, that would be nice to know. But it’s not that easy with long-term forecasts.
If October is warm and fine, a sharp winter is coming. If only long-term climate predictions were that easy. Right now the question arises as to whether energy will be scarce in the cold season. The Helmholtz Vice President for Energy, Holger Hanselka, recently predicted that there would probably be no bottlenecks if the winter was mild. Which phenomena in Central Europe have an influence on winter, how seasonal forecasts work and how (un)certain they are.
What influences the course of winter in Central Europe?
The general weather situation is largely responsible for this, i.e. the distribution of highs and lows, what long-term constellations arise and which air masses can make their way towards us. “One big player in this winter roulette is the polar vortex,” explains ntv meteorologist Björn Alexander. If this phenomenon is strong in the stratosphere above the North Pole, the polar vortex forms a barrier against cold and freezing air masses from northern and eastern latitudes. Then westerly to south-westerly and correspondingly mild air masses predominate. On the other hand, if it is weak, the chances of polar, arctic or even Siberian air masses increase.
“When the polar vortex is unstable – which happens regularly in winter – it disrupts the stability of the westerly wind circulation. This increases the likelihood of cold snaps,” says climate scientist Klaus Pankatz from the German Weather Service (DWD). The good news: “At the moment, the polar vortex is very strong and stable for the foreseeable future.” In addition, the snow cover in Siberia and Central Asia has an influence on the course of the next winter. The climate scientist explains: “A high snow cover in early winter strengthens the Siberian cold high pressure area. It can spread further west in late winter and bring us cold eastern regions as a result.” In this general weather situation, continental air flows from the east to Central Europe. Ultimately, according to Pankatz, a “bouquet of phenomena” determines what winter will be like.
How are seasonal forecasts made?
For seasonal forecasts, computers use a huge amount of data to simulate various possible weather scenarios based on special climate models. In this way, they calculate the probability that the coming season will be wetter, drier, warmer or colder than the long-term average. Relative statements are always made and absolute values ??such as specific temperatures are never given. “Seasonal forecasts are climate forecasts, not weather forecasts,” explains DWD climate researcher Andreas Paxian.
What are the differences to the weather forecast?
Seasonal forecasts work with the “memory” of long-term processes, says Pankatz. The scientists’ simulations relate to recurring and longer-lasting climatic patterns that extend over large areas. “When forecasting the weather, you look at individual days, for example their maximum and minimum temperatures. But as soon as you go beyond the period of ten days to two weeks, you have to start summarizing, averaging,” describes the climate scientist.
In the case of the seasonal forecast, it is a three-month average that is compared with that of the reference period. “On the seasonal forecast scale, there’s no point in looking at individual days.” And that’s exactly what’s “important for the statement,” emphasizes Pankatz. “If we say: There is a tendency for warmer conditions for the three-month mean, then individual days, weeks or even a whole month can be cold in this period.”
Which data is evaluated?
For seasonal forecasts, the scientists work with an Earth system model. For this purpose, the earth is divided into three-dimensional grid boxes, in which the state of the atmosphere, the ocean, the land surface and the sea ice is described. “I have to have observation points all over the globe with a certain spatial resolution at the level of the atmosphere and the depth of the ocean,” describes Paxian. On the one hand, these are measured data and, on the other hand, calculated values ??for those parts of the earth system for which there are no measured data. In addition, assumptions about the concentration of greenhouse gases in the atmosphere are incorporated throughout the simulation.
How reliable are the results?
The scientists incorporate an enormous number of values ??into their simulation, some of which are based on extrapolations. “Of course there is a lot of uncertainty,” says Paxian. For example, if the researchers only know the range in which a starting value lies, they let the model calculate with different values. Another difficulty: We are far from familiar with all the processes and interactions between the atmosphere, ocean, land surface and sea ice. These are therefore not included in the current simulations. In addition, computers quickly reach their capacity limits when performing complex calculations. Paxian summarizes: “The quality of the prediction depends on which period, location and variable is considered.”
This is also confirmed by ntv meteorologist Alexander: “Summer thunderstorms are very inaccurate, even shortly before the events. A cold front passage or storm fields, on the other hand, are much more accurate.” Basically, the accuracy of forecasts over days is 70 percent and more. However, this level of accuracy cannot be achieved for long-term forecasts.
And what is the current seasonal forecast?
The experimental calculations of the American weather service NOAA currently show that December 2022 and January 2023 are only slightly too warm, while February 2023 is said to be significantly too warm, explains Alexander. But there are also indicators for cold phases, “especially since the trends based on the European weather model also predict a rather cold December 2022”. According to Alexander, the precipitation currently “looks like it will be slightly too dry to average overall”.
The scientists from the DWD are currently evaluating data from various climate models for Germany. To do this, they calculate how many of the models have a proportion of warm, normal or cold results in relation to the respective reference period. The DWD itself compares for the period 1991 to 2020, other included models work with reference periods going back longer. Current status, according to Paxian: “For the three months – November, December, January – we see a slight trend for warmer conditions.”
