‘Sudden stratospheric warming (SSM)’

Published: 18th Jan, 2021

A “sudden stratospheric warming” event took place in early January 2021, according to weather forecasting models.

Context

A “sudden stratospheric warming” event took place in early January 2021, according to weather forecasting models.

About

What is sudden stratospheric warming (SSM)?

The term sudden stratospheric warming refers to what is observed in the stratosphere.
It is a rapid warming (up to about 50 °C in just a couple of days), between 10 km and 50 km above the earth’s surface.
The stratosphere is the layer of the atmosphere from around 10 km to 50 km above the Earth’s surface.
However, usually a few weeks later, knock-on effects on the jet stream can be seen, which in turn effects weather lower down (in the troposphere).
However, the stratospheric sudden warming doesn’t happen every year, and it doesn’t always affect weather when it does.
It was first discovered in 1952.

How does it occur?

Every year in winter, strong westerly winds circle around the pole high up in the stratosphere.
This is called the stratospheric polar vortexand it circulates around cold air high over the Arctic.
In some years, the winds in the polar vortex temporarily weaken, or even reverse to flow from east to west.
The cold air then descends very rapidly in the polar vortex and this causes the temperature in the stratosphere to rise very rapidly, as much as 50°C over only a few days; hence the term sudden stratospheric warming.
As the cold air from high up in the stratosphere disperses, it can affect the shape of the jet stream as the cold air sinks from the stratosphere into the troposphere.
It is this change in the jet stream that causes our weather to change.

Any role of climate change?

Sudden stratospheric warming events are a natural atmospheric fluctuation, not caused by climate change.
So even with climate change, these events will still occur, which means that we need to be adaptable to an even more extreme range of temperatures.

What is Polar Vertex?

Polar Vertex can refer to one of two different, but related, weather patterns.
The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles. It ALWAYS exists near the poles, but weakens in summer and strengthens in winter.
The term "vortex" refers to the counter-clockwise flow of air that helps keep the colder air near the Poles.

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