Solar variability helps explain cold winters

Study sheds new light on a link between decadal solar variability and winter climate - News

Monday 10 October 2011
Adapted from a press release issued by the UK Met Office

Watch a video of Professor Joanna Haigh explaining solar variability

Research led by 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 and published in the journal Nature Geoscience, 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 UK¹. Years of higher UV have the opposite effect.

Adam Scaife, one of the Met Office 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 equipment² shows UV variability over the 11-year solar cycle³ may be much larger than previously thought and have been key to the research.

By using this information in the Met Office’s climate 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.”

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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.

Co-author Professor Joanna Haigh, from the Department of Physics at Imperial College London, whose research in atmospheric physics laid the groundwork for this study, 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, along with our ongoing research through the NERC Solar Variability and Climate (SOLCLI) consortium, is adding much detail to our current understanding."

The research has been published in Nature Geoscience⁴ and has been carried out as part of the Met Office’s programme of independent climate research funded by DECC and Defra.

Notes:

1. UK mean temperatures for the winters of 2009/10 and 2010/11 were -2.04C and -1.25C respectively below the long-term average of 3.7C. Both years saw extensive snowfall and icy conditions.
2. The data for the research comes from the Spectral Irradiance Monitor (SIM) based on the SORCE (Solar Radiation and Climate Experiment) satellite.
3. UV levels from the sun have been observed to vary over an 11-year cycle. Levels of UV are currently rising as part of this cycle.
4. The full article, Solar forcing of winter climate variability in the northern hemisphere  (S. Ine son, A.A. Scaife, J.R. Knight, J.C. Manners, N.J. Dunstone, L.J. Gray and J.D.Haigh), can be viewed online at Nature Geoscien ce doi:10.1038/ngeo1282