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.
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.”