What is a Sudden Stratospheric Warming (SSW)?

8 01 2013

You may have heard talk of the UK possibly seeing some colder weather next week 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.

Can we predict these events in advance?

Currently we can reliably predict individual SSWs about a week in advance, and we can detect them early on with satellite and other observations. This means we have some time to see how they develop and may impact our future weather.

The Met Office will continue to monitor the current event and, as ever, will keep everyone up to date on any periods of cold weather through our forecasts and warnings.





Cold weather clue in upper atmosphere

4 12 2012

During a period of colder than average weather across northern Europe in February this year we issued a blog discussing potential causes – including changes in circulation high up in the atmosphere.

We also discussed these disruptions, known as Sudden Stratospheric Warmings (SSWs), in a news release about advances in predicting these events in our long-range guidance for winters.

SSWs happen when the usual westerly winds in the stratosphere, between 10km and 50km up, are disrupted, break down and even reverse.

This signal can then burrow down to the lower atmosphere over the course of a few days to a few weeks, thereby reducing the westerly winds at lower levels.

This illustrates how the reversing of winds in the stratosphere 'burrows' down to surface level.

Figure illustrates how reversal of winds in the stratosphere ‘burrows’ down to surface level.

For the UK in winter, that means a disruption to the westerly flow that usually brings mild air from the Atlantic and there is a potential to allow easterly winds to take hold, bringing in cold air from the continent.

So SSWs can herald cold weather ahead. However, there are variations in their magnitude – sometimes they are fairly minor, just a ‘wobble’ in the flow of the stratospheric winds, but sometimes they are more pronounced, with a complete reversal of the pattern of winds.

The more significant the SSW, the more likely it is to have an impact at the surface and also the greater the potential impact.

Met Office observation systems have picked up a minor SSW in the stratosphere over the past few days, suggesting that this may have an impact on the UK.

Jeff Knight, a Climate Scientist at the Met Office, said: “Satellite and other observation data show that there is a minor SSW going on and this is one factor amongst many others which could perpetuate the colder than average conditions we have seen recently.

“It could take anything from a few days to a few weeks if it is going to have an impact. However, it’s consistent with the current 30-day outlook from the Met Office which favours colder than average conditions – albeit with a fair amount of uncertainty.”

The Met Office will continue to monitor the situation and, as ever, will keep everyone up to date on any periods of cold weather through our forecasts and warnings.








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