Global weather phenomena: “Not every collapse leads to a cold snap”

taz: Mr. Köhler, the polar vortex has collapsed. Is it getting cold in Europe now?

Raphael Köhler: The fact that some people were now saying that an ice-cold winter would follow is not particularly trustworthy. However, the probability that weather conditions will occur in the next two to six weeks that will bring colder air masses to Northern and Central Europe is slightly increased. But you can't say more.

Raphael Köhler, born in 1991, has a doctorate in climate physics and is a scientist at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Potsdam.

taz: Nevertheless, the collapse is extraordinary, why?

Köhler: The collapse of the stratospheric polar vortex happens more often. On average, we see about five collapses per decade. But this usually happens in December, January and February. The fact that this is happening at the beginning of November is very unusual.

taz: Maybe you can explain again: What is the polar vortex?

Köhler: The stratospheric polar vortex is a westerly band of wind in the stratosphere. This is roughly the layer of air above a height of 10 kilometers. Normally the vortex builds from September and gets stronger until January and ends in spring. In winter the Arctic hardly gets any sunlight. This causes the air in the stratosphere above the North Pole to cool significantly. Because the air remains warmer further south, a large temperature difference occurs. This ensures that strong westerly winds form around the Arctic. These winds form the polar vortex and hold the cold air largely over the polar region.

taz: How can one imagine the collapse of the polar vortex?

Köhler: As a standard, we measure the wind speed at 60 degrees north at an altitude of around 30 kilometers. At this time of year, the vortex normally has a wind speed of 27 meters per second, which is almost 100 kilometers per hour. We are currently very close to 0 meters per second. If we get into the negative measurement range, that means we would switch from westerly to easterly winds, then we speak of a large, sudden stratospheric warming.

Köhler: We are talking about so-called wave patterns in the troposphere, i.e. the lower layer of the atmosphere, which spread into the stratosphere and can slow down the vortex. These can be influenced by the position of high and low pressure areas. This time too, there was typically a pronounced high-pressure area in the Ural region of Russia and at the same time a pronounced low-pressure area over the Aleutian Islands, an island chain in the North Pacific. These are typical patterns that can lead to a weakening of the vortex.

taz: And other climatic processes can influence these high and low pressure areas?

Köhler: A lot can have an impact on this, for example the melting of the Arctic sea ice. In this way, climate change can generally influence the wave pattern. Exactly what consequences this has is a major research topic. But studies suggest stratospheric polar vortex collapses could easily increase due to climate change.

taz: Despite such collapses, don't we have to fear more extreme winters?

Köhler: We also have to fundamentally differentiate a little between the individual years, where we sometimes have a cold winter or cold two weeks due to the chaos in the weather system, and climate change. In the last 30 years you can clearly see climate change in Germany, winters are on average much warmer. I think many people have simply become a little more sensitive to cold mornings.

taz: Actually, one weather phenomenon in particular is dominating the world right now. A Super El Niño is already showing its consequences: it is increasing the global average temperature and leading to droughts and floods. What influence does it have on the polar vortex?

Köhler: El Niño is keeping us very busy. Not only does it have consequences for global temperature, it also influences global weather systems. The additional heat brought into the atmosphere by El Niño can lead to changes in the global typical high and low pressure areas...

taz: … and they in turn influence the wave patterns! Do we know whether this early disruption of the stratospheric polar vortex is a result of this El Niño?

Köhler: We still have to research that at the moment. Basically, El Niño promotes a weaker stratospheric polar vortex. However, it is assumed that this is particularly the case in January and February. Studies have not yet shown that there is a benefit for November.

taz: However, this could also be the strongest El Niño since measurements began.

Köhler: Yes, that's true. The same applies to the collapse of the vortex in November. The vortex has never been as weak as it is at this time of year since we have good measurement data about it. But it doesn't have to mean that there is a connection, we simply don't know it yet.

taz: How could the polar vortex specifically affect our weather?

Köhler: A weak stratospheric polar vortex can favor weather patterns that bring cooler air masses to Central or Northern Europe. Normally, the jet stream, a westerly wind in the troposphere, brings mild air masses from the Atlantic to Europe in winter. However, if this weakens, this can lead to colder air masses coming to us from the north or northeast, which can bring lower temperatures.

Köhler: There are a few examples in the past where it got colder in our area in the weeks after the vortex collapsed. But not every collapse leads to a cold snap; in about two-thirds of cases we also see changes in the atmospheric circulation in the troposphere in the following weeks. It depends on how long the collapse of the vortex in the stratosphere lasts and how long it influences the troposphere, i.e. the lower layers of the atmosphere that we perceive as weather.

taz: Last year the polar vortex weakened at the end of November. There was no major cold snap.

Köhler: Exactly, it was a little cooler at the beginning of January, which could have just been influenced by the polar vortex. Overall, however, it is a typical example of a weakening vortex that had relatively little impact on our weather. Especially when the polar vortex collapses early in the year, it usually recovers quite quickly, which can mean that we don't have that much of an influence on our weather here. This year too, the vortex will stabilize again into a strong westerly wind band, the only question is when this will happen over the course of the winter.