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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3 responses

5 12 2012
jdey123

What a load of baloney. Here’s a link to the global temperatures as recorded by satellite. You can see that some areas of the globe are much warmer than normal and some much colder and that these change and vary over time. Sometimes there are no areas that are colder than normal. The climate appears to me to be chaotic. It is certainly not the case that the earth is at some constant temperature and that single countries are colder or warmer than average because changes in the wind or jetstreams moves cold or warm air over them.

http://www.esrl.noaa.gov/psd/map/images/fnl/sfctmpmer_01a.fnl.anim.html

7 12 2012
Dave Britton

James, You are right that the atmosphere is chaotic and that some areas of the atmosphere will be warmer than normal and some will be colder. Although the atmoshpere is chaotic there are patterns within it and the patterns set up by Sudden Stratospheric Warming is one such example, which can bring colder weather to parts of northwest Europe.

8 12 2012
jdey123

Well I don’t even know why this article was published Dave. It tells us that meteorologists think that a strong SSW might have an effect on surface temperatures but only if it’s sufficiently strong (threshold for this is left undefined). We’re told that the SSW that the MetOffice has allegedly detected is moderate so presumably shouldn’t have any effect on surface temperatures at all.

We then get a typical MetOffice speculation that this minor SSW might have an effect on surface temperatures and if coupled with other (undefined) factors could perpetuate the cold spell which we’ve recently had and has now temporarily ended. For fear that this would be the case, the MetOffice go on to say that the effect on surface temperatures could appear anytime from a few days to a few weeks (or indeed not at all).

Do we actually pay you, Dave, to defend this tosh?

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