What is a Sudden Stratospheric Warming?

In recent years some extreme cold, winter snow events have all been connected to the surface effects of sudden stratospheric warmings, such as those in 2009-10, 2013, and  in 2018.

The term sudden stratospheric warming refers to what is observed in the stratosphere:- 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. This is so high up that we don’t feel the ‘warming’ ourselves. However, usually a few weeks later, we can start to see knock-on effects on the jet stream, which in turn effects our weather lower down (in the troposphere).

However, the stratospheric sudden warming doesn’t happen every year, and it doesn’t always affect our weather when it does.

How does it occur?

  • The stratosphere is the layer of the atmosphere from about 10-50 kilometres up. In the winter hemisphere, the pole is tilted away from the sun and is dark 24 hours. At the equator, the stratosphere receives incoming sunlight.
  • There is, therefore, a large difference in temperature between the high latitude stratosphere and the stratosphere at lower latitudes (a strong temperature gradient). This sets up strong winds blowing in a westerly direction around the cold air over the pole.
  • This arrangement is known as the Stratospheric Polar Vortex (SPV). This forms every winter. On occasions, this vortex can become disturbed. The temperature can rise by up to 50 degrees Celisus in a few days (although it is still cold) and the winds can weaken, or even reverse.
  • If the winds reverse, then a sudden stratospheric warming (SSW) is said to have happened.
  • The SSWs happen around six times a decade in the northern hemisphere, but only one has ever been observed in the southern hemisphere.
  • Every year in winter, strong westerly winds circle around the pole high up in the stratosphere. This is called the stratospheric polar vortex and 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.

How does it affect our weather?

The stratospheric sudden warming can sometimes cause the jet stream to ‘snake’ more, and this tends to create a large area of blocking high pressure. Typically this will form over the North Atlantic and Scandinavia. This means that northern Europe, including the UK is likely to get a long spell of dry, cold weather, whereas southern Europe will tend to be more mild, wet and windy. On the boundary of these areas, cold easterly winds develop and in some cases the drop in temperatures leads to snow, which is what happened in early 2018.