Solar changes help create cold northern winters

Fluctuations in ultraviolet light can set up frigid, snowy conditions

Harsh winters in the United States and northern Europe may partly be the result of changes in ultraviolet radiation coming from the sun.

A new climate simulation study shows how fluctuations in ultraviolet light linked to the sun’s 11-year activity cycle could change winter weather patterns across the Northern Hemisphere. The work appears online October 9 in Nature Geoscience.

“We hope this will open the door to improving ultralong-range predictions,” says co-author Adam Scaife, a climate modeler at the Met Office’s Hadley Centre in Exeter, England.

Scientists have long noted anecdotal links between low solar activity and cold European winters: Part of the Little Ice Age, which gripped the region between about 1550 and 1850, coincided with a record low number of sunspots, which are one measure of solar activity.  But until now, Scaife says, no one had found a physical explanation for how subtle changes in radiation hitting the top of Earth’s atmosphere could translate to changes in weather patterns at the surface.

The answer came from the Solar Radiation and Climate Experiment satellite. From 2004 to 2007, during the low points of the last solar cycle, the satellite measured a surprising drop-off in the amount of ultraviolet radiation coming from the sun, roughly five times greater than previously thought. “I thought, if that’s true, that’s going to do something interesting to the climate system,” Scaife says.

To test what might happen, the scientists put the big ultraviolet decline into the Met Office’s climate model, a massive computer program that can simulate how the ocean and atmosphere respond to such changes. With less ultraviolet radiation, the simulation suggested, parts of the upper atmosphere cooled more than usual and allowed winds to blow more from the east. The anomaly then got bigger and started to burrow down through the atmosphere to altitudes where weather patterns form.  There, the changes affected how storms would normally grow, allowing cold weather to form over northern Europe and the United States.

These changes occurred only in winter, and not during every solar cycle minimum the model analyzed. But over time, the scientists found, more winters saw these cold patterns form during solar minimum than during solar maximum. “It’s changing the odds of what kind of winter you’re going to get by a significant amount,” Scaife says.

At the same time, weather patterns over southern Europe and Canada were milder than normal, essentially canceling out the chill of northern Europe and the United States. The new work thus can’t say much about whether changes in solar radiation affect global temperatures, Scaife says. Other natural factors also affect the severity of winters, including volcanic eruptions and semi-regular weather patterns like El Ni±o.

Kunihiko Kodera, a sun-climate researcher at Nagoya University in Japan, says the new model seems to capture all the steps in the atmosphere, but small details as to how warm or cold it got over particular landmasses may make it difficult to predict localized weather changes.

If the Met Office computer model can accurately reproduce past forecasts, the researchers hope to start incorporating solar variability into long-term weather predictions. Solar activity is currently increasing toward an expected weak maximum in 2013.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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