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Solar variability helps explain cold winters

Low UV output from the sun leads to easterly winds and cold conditions in Europe and the US

October 2011 - Research from the Met Office has shed new light on a link between decadal solar variability and winter climate in the UK, northern Europe and parts of America.

The study, carried out with Imperial College London and the University of Oxford, shows that low UV output from the sun can contribute to cold winters over parts of the northern hemisphere, such as recently seen in the UK. Years of higher UV have the opposite effect.

Adam Scaife, one of the scientists involved in the research, said that while some studies have observed a link between solar variability and winter climate, our research establishes this as more than just coincidence.

He said: "We've been able to reproduce a consistent climate pattern, confirm how it works, and quantify it using a computer model based on the laws of physics. This isn't the sole driver of winter climate over our region, but it is a significant factor and understanding it is important for seasonal to decadal forecasting."

New data from sensitive satellite equipment shows UV variability over the 11-year solar cycle may be much larger than previously thought and has been key to the research.

By using this information in the Unified Model, researchers were able to reproduce the effects of solar variability apparent in observed climate records.

In years of low UV activity unusually cold air forms over the tropics in the stratosphere, about 50km up. This is balanced by more easterly flow of air over the mid latitudes - a pattern which then 'burrows' its way down to the surface, bringing easterly winds and cold winters to northern Europe.

When solar UV output is higher than usual, the opposite occurs and there are strong westerlies which bring warm air and hence milder winters to Europe.

Sarah Ineson, who performed the experiments, said: "What we're seeing is UV levels affecting the distribution of air masses around the Atlantic basin. This causes a redistribution of heat - so while Europe and the US may be cooler, Canada and the Mediterranean will be warmer, and there is little direct impact on global temperatures."

While UV levels won't tell us what the day-to-day weather will do, they could be important in helping us develop improved forecasts for winter conditions for months or even a few years ahead and this is now being investigated.

Joanna Haigh, Professor of Atmospheric Physics at Imperial College London, said: "Compared with the effect of man-made emissions over the last century, solar variations still have a very minor effect on long-term global climate trends, but this study shows they may have a detectable influence on winter climate.

"Even with the most sophisticated atmospheric models, it is very hard to predict weather patterns on seasonal timescales. This study is adding much detail to our current understanding."

The research has been published in Nature Geoscience and has been carried out as part of the Met Office's programme of independent climate research funded by DECC and Defra

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