British scientists say they've established a link between the sun's 11-year cycle and cold winters in Northern Europe and parts of America.
The study, by the Met Office, Imperial College London and the University of Oxford, finds that levels of ultraviolet light can influence the severity of winter weather in parts of the northern hemisphere.
"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," says Adam Scaife, one of the Met Office scientists involved in the research.
"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 the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) satellite indicates that UV levels may vary much more than thought over the 11-year solar cycle.
Adding this data into the Met Office’s climate model produced results that mimicked observed climate records.
It seems that in years of low UV activity, unusually cold air forms over the tropics in the stratosphere, about 50km up. This is balanced by a greater easterly flow of air over the mid latitudes. The pattern spreads down to the surface, bringing easterly winds and cold winters to northern Europe.
When solar UV output is higher than usual, the opposite occurs, with strong westerlies bringing warm air.
What the data doesn't do, though, is explain global warming, as there seems to be little impact on temperatures globally.
"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," says professor Joanna Haigh from Imperial College London.