Page last updated at 23:33 GMT, Wednesday, 14 April 2010 00:33 UK

Low solar activity link to cold UK winters

By Mark Kinver
Science and environment reporter, BBC News

Satellite image showing the British Isles covered in snow (Image: NASA)
A period of low solar activity could lead to more cold winters in the UK

The UK and continental Europe could be gripped by more frequent cold winters in the future as a result of low solar activity, say researchers.

They identified a link between fewer sunspots and atmospheric conditions that "block" warm, westerly winds reaching Europe during winter months.

But they added that the phenomenon only affected a limited region and would not alter the overall global warming trend.

The findings appear in the journal Environmental Research Letters.

"By recent standards, we have just had what could be called a very cold winter and I wanted to see if this was just another coincidence or statistically robust," said lead author Mike Lockwood, professor of space environment physics at the University of Reading, UK.

To examine whether there was a link, Professor Lockwood and his co-authors compared past levels of solar activity with the Central England Temperature (CET) record, which is the world's longest continuous instrumental record of such data.

The researchers used the 351-year CET record because it provided data that went back to the beginning of the Maunder Minimum, a prolonged period of very low activity on the Sun that lasted about half a century.

Europe is particularly susceptible because it lies underneath the jet stream
Professor Mike Lockwood

The Maunder Minimum occurred in the latter half of the 17th Century - a period when Europe experienced a series of harsh winters, which has been dubbed by some as the Little Ice Age. Following this, there was a gradual increase in solar activity that lasted 300 years.

Professor Lockwood explained that studies of activity on the Sun, which provides data stretching back over 9,000 years, showed that it tended to "ramp up quite slowly over about a 300-year period, then drop quite quickly over about a 100-year period".

He said the present decline started in 1985 and was currently about "half way back to a Maunder Minimum condition".

This allowed the team to compare recent years with what happened in the late 1600s.

"We found that you could accommodate both the Maunder Minimum and the last few years into the same framework," he told BBC News.

Big chill

Professor Lockwood said that there were a number of possibilities that could explain the link, but the team favoured the idea of a meteorological phenomenon known as "blocking".

This affects the dynamics of jet streams, which are very strong winds about 7-12km above the Earth's surface that can have a major influence on weather systems. There is one jet stream present in each hemisphere.

The Sun (Image: AP)
Solar activity has been in decline since 1985, says Professor Lockwood

"Europe is particularly susceptible because, firstly, it lies underneath the (northern hemisphere's) jet stream," he explained.

A "blocking" occurs when the jet stream forms an "s" shape over the north-eastern Atlantic, causing the wind to fold back over itself.

"If you haven't got blocking, then the jet stream brings the mild, wet westerly winds to give us the weather we are famous for."

But, he added, if the jet stream is "blocked", and pushed further northwards, then cold, dry winds from the east flow over Europe, resulting in a sharp fall in temperatures.

"This... 'blocking' does seem to be one of the things that can be modulated by solar activity," he said.

Recent studies suggest that when solar activity is low, "blocking" events move eastwards from above north-eastern North America towards Europe, and become more stable.

A prolonged "blocking" during the most recent winter was responsible for the long spell of freezing conditions that gripped Europe.

Written observations from the period of the Maunder Minimum referred to the wind coming from the east during particularly cold winters, which strengthened the team's "blocking" hypothesis.

The way in which solar activity affects the behaviour of blocking episodes is linked to the amount of ultraviolet (UV) emissions being produced by the Sun.

Solar UV heats the stratosphere (20-50km above the surface), particularly the equatorial stratosphere. This results in a temperature gradient, which leads to the formation of high level winds.

"The change in solar activity undoubtedly changes the stratospheric winds," said Professor Lockwood.

Studies have shown that the state of the stratosphere can make a considerable difference to what happens in the troposphere, which is where the jet stream occurs, Professor Lockwood explained.

"There has been some quite simple modelling that indicated that heating the equatorial stratosphere with more UV would actually move the jet streams a little bit, by just a few degrees.

"That, of course, has the potential to change the behaviour of the jet streams - and that is the sort of thing that we think we are seeing."

'Blocking central'

Professor Lockwood was keen to stress that "blocking" only affected a limited geographical region, and would not have a widespread impact on the global climate system.

To illustrate the point, he said that while the CET record showed that this winter was the UK's 14th coldest in 160 years, global figures listed it as the fifth warmest.

He said that one of his colleagues at the University of Reading referred to Europe as "blocking central".

Picture of a Thames "forest fayre" in 1716 (Getty Images)
"Frost fayres" were held on the Thames during the Maunder Minimum

"The reason is largely because the jet stream has to come to us over the Atlantic Ocean and it is slowed down when it hits the land in Europe.

"You don't quite have the same combination of circumstances anywhere else in the world that gives you such strong blocking."

While the current decline in solar activity is expected to continue in the coming decades, he cautioned that more frequent "blocking" episodes would not result in Europe being plunged into sub-zero temperatures every winter.

"If we look at the last period of very low solar activity at the end of the 17th Century, we find the coldest winter on record in 1684, but the very next year - when solar activity was still low - saw third warmest winter in the entire 350-year (CET) record."

A number of other meteorological factors also influenced the weather systems over Europe, so a number of parameters had to be met before a "blocking" occurred, he observed.

Responding to the team's findings, Dr Peter Stott, head of climate monitoring and attribution at the UK Met Office, said: "This paper provides some additional evidence that what happens in the stratosphere could be important for climate at the surface."

But he added: "The findings are suggestive of a possible effect but more research is needed to pin down the mechanisms and determine how significant such effects could be for determining the probability of cold winters in the UK.

"At the Met Office, we are already working on research into incorporating better representation of the stratosphere into our seasonal and decadal forecasting models."

Professor Lockwood said he now planned to examine the influence of low solar activity on European weather during the summer months.

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