Polar vortex may weaken by November 5 in a possible record low

The polar vortex over the Arctic could weaken noticeably in early November. According to the forecast of the European Centre for Medium-Range Weather Forecasts (ECMWF), around November 5 the average speed of the westerly wind in the stratosphere will fall to roughly 12 metres per second. That is almost half of what is usual for this time of year.

One of the main indicators of the vortex's strength is precisely the average speed of the stratospheric westerly wind. What is unusual here is the timing: in the Northern Hemisphere the vortex starts to form in autumn, as the Arctic cools quickly, and in early November it normally grows steadily stronger.

A record dating back to 1959 could be broken

A comparison of data going back to 1959 shows that in the first ten days of November the speed fell below 12 metres per second in only three years. It was lowest on November 9, 1960, at 10.49 metres per second. A historic record this year therefore cannot be ruled out.

Why the vortex is weakening

The cause is a strong disruption of the usual flows in the lower layers of the atmosphere. Meteorologist Judah Cohen points to a powerful blocking anticyclone between Scandinavia and the Urals. It may amplify large atmospheric waves that sometimes rise into the stratosphere and break up the strong westerly flow around the Arctic. The vortex then becomes deformed, shifts away from the pole or, in extreme cases, splits into two separate cores.

The latest models do show such deformation: instead of a compact vortex, early November brings an elongated structure with two areas of stronger circulation.

What it means for the weather

The effects on weather at ground level usually appear with a delay of several weeks. A weakened vortex can alter the jet streams and support blocking anticyclones at high latitudes, which in turn make it easier for cold Arctic air to push into the mid-latitudes.

But not every weakening of the vortex leads to a cold snap: while one part of the Northern Hemisphere freezes, another may stay noticeably warmer. It is too early to say whether this will affect November weather in Europe. All that is clear is that the vortex may behave far more unusually in early November than is normal for autumn.

What the polar vortex is

It is a vast area of low pressure and very cold air at an altitude of roughly 10 to 50 kilometres above the Arctic. It forms in autumn as the stratosphere cools, is usually strongest in winter, and breaks down in spring when sunlight returns. A strong, compact vortex keeps the coldest air at high latitudes. One of the most notable disruptions is a sudden stratospheric warming, in which the temperature in the stratosphere rises sharply and the vortex's circulation weakens markedly or even reverses.

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Around Prague, based on reporting by novinky.cz