Berlin If only you knew how cold winters get in Germany. But it is not so easy with long-term forecasts. How scientists make seasonal climate 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 is whether energy will be scarce in the cold season.
Helmholtz’s vice president for energy, Holger Hanselka, recently predicted that there would probably be no bottlenecks if the winter were mild. Which phenomena in Central Europe influence winter, how seasonal forecasts work and how (un)certain they are.
What affects the course of winter in Central Europe?
A “bouquet of phenomena” determines how winter will be, explains climate scientist Klaus Pankatz from the German Weather Service (DWD). A phenomenon in the stratosphere above the North Pole plays a relatively large role: the polar vortex. This can affect the circulation of the westerly winds, which transport mild, moist air from the Atlantic towards central Europe. If this circulation is impaired, cold air can reach us.
“When the polar vortex is unstable — which is a regular occurrence in winter — it disrupts the stability of the westerly circulation. That increases the likelihood of cold snaps,” says Pankatz. The good news: “Currently, 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 level of snow cover in early winter intensifies the cold Siberian high pressure area. It can spread further west in late winter and bring us cold eastern climates as a result.” In this general weather situation, continental air flows from the east to central Europe.
How are seasonal forecasts made?
For seasonal forecasting, computers use a huge amount of data to simulate different 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 given, and absolute values, such as specific temperatures, are never given. “Seasonal forecasts are climate forecasts, not weather forecasts,” explains DWD climate scientist Andreas Paxian.
What are the differences compared to the weather forecast?
Seasonal forecasts work with the “memory” of long-term processes, Pankatz says. The researchers’ simulations relate to recurring and long-term climatic patterns that extend over large areas. “When you forecast the weather, you look at individual days, such as their maximum and minimum temperatures. But as soon as you get beyond the period of ten days to two weeks, you have to start summarizing, taking an average,” describes the climate scientist.
For the seasonal forecast, it is a three-month average that is compared with that of the reference period. “On the seasonal forecasting scale, it makes no sense to look at individual days.” And that is precisely what is “important for the statement,” emphasizes Pankatz. “If we say: There is a trend towards warmer conditions for the three-month average, then individual days, weeks or even a whole month can be cold during this period.”
What data is assessed?
For seasonal forecasts, the researchers 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, sea, land surface and sea ice is described.
“I need to have observation points all over the globe with a certain spatial resolution at the height of the atmosphere and the depth of the ocean,” describes Paxian. On the one hand, there are measured data and, on the other hand, calculated values for the parts of the earth system for which there is 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 researchers incorporate a huge number of values into their simulation, some of which are based on extrapolations. “Obviously there’s a lot of uncertainty,” Paxian says. For example, if the researchers only know the interval 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, the sea, the land surface and the 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 forecast depends on which period, location and variable are taken into account.”
And what is the current season forecast?
The DWD researchers 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 compared to the respective reference period. DWD itself makes a comparison for the period from 1991 to 2020, other involved models work with reference periods that go further back.
Current status, according to Paxian: “For the three months — November, December, January — we’re seeing a slight trend toward warmer conditions.”
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