A sudden stratospheric warming (SSW) is an event where the polar vortex of westerly winds in the winter hemisphere slows down or even reverses direction over the course of a few days.
Following a sudden stratospheric warming, the high altitude winds reverse to flow eastward instead of their usual westward. The eastward winds progress down through the atmosphere and weaken the jet stream, often giving easterly winds near the surface and resulting in dramatic reductions in temperature in Europe including the UK.
You may have heard talk of the UK possibly seeing some colder weather in the next few weeks and that ‘things going on’ in the upper atmosphere may be playing a part.
The ‘thing’ happening in the atmosphere is known as Sudden Stratospheric Warming (SSW). When it does happen, it attracts a lot of interest in the UK because it is sometimes linked to the onset of cold weather in winter.
Here we shed a little bit more light on the phenomenon.
What is an SSW?
The term SSW refers to what we observe – rapid warming (up to about 50 °C in just a couple of days) in the stratosphere, between 10 km and 50 km up.
You may have heard of the jet stream which helps to steer Atlantic weather systems towards the UK.
Well there are other jet streams high up in our atmosphere in both the northern and southern hemisphere which circumnavigate the Earth from west to east. One of these, the Polar Night Jet, circles the Arctic.
Sometimes the usual westerly flow can be disrupted by natural weather patterns or disturbances in the lower part of the atmosphere, such as a large area of high pressure in the northern hemisphere.
This causes the Polar Jet to wobble and these wobbles, or waves, break just like waves on the beach. When they break they can be strong enough to weaken or even reverse the westerly winds and swing them to easterlies.
As this happens, air in the stratosphere starts to collapse in to the polar cap and compress. As it compresses it warms, hence the stratospheric warming.
How does it move down through the atmosphere?
As it turns out, waves can only move around the Earth’s atmosphere in westerly winds.
Fluctuations in our weather send waves up through the atmosphere to the easterly winds in the stratosphere, where they travel no further, and instead break and reinforce the easterly winds, bringing the easterlies lower.
This pattern continues until the easterlies have moved down to the troposphere – the lowest part of the atmosphere where our weather is.
It can take anything from a few days to a few weeks for this process to take place.
What impact does this have on the UK?
We normally expect our weather to come in from the west – with a flow of relatively mild air coming in off the Atlantic.
When an SSW brings easterly winds this tends to alter our weather patterns slightly, weakening areas of low pressure and moving our jet stream further south.
This leads to high pressure over the North Atlantic, ‘blocking’ that flow of mild Atlantic air and dragging in cold air from the continent to the east. Exactly how cold it might be depends on the details of where the air comes from.
SSWs don’t always result in this outcome – but a cold snap follows more often than not, so the SSW greatly increases the risk of wintry weather.