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.

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.