The sun swings through an 11-year cycle measured in the number of sun spots on the surface that emit bursts of energy.
The difference in energy is only about 0.1 percent between a solar maximum and minimum and determining just how that small variation affects the world’s climate has been one of the great challenges facing meteorologists.
Using a century of weather observations and complex computer models, the international team of scientists led by the National Center for Atmospheric Research (NCAR) in the United States showed that even a small increase in the sun’s energy can intensify wind and rainfall patterns.
“Small changes in the sun’s output over the 11-year solar cycle have long been known to have impacts on the global climate system,” said Julie Arblaster, from the Centre for Australian Weather and Climate Research, a co-author of the study published in the latest issue of the journal Science.
“Here we reconcile for the first time the mechanisms by which these small variations get amplified, resulting in cooler sea surface temperatures in the tropical Pacific and enhancing off-equatorial rainfall.”
The researchers found that during periods of strong solar activity the air in the upper atmosphere, in a layer called the stratosphere, heats up. This occurs over the tropics, where sunlight is typically most intense.
The extra warming alters wind patterns in the upper atmosphere, which in turn increases tropical rainfall.
Increased sunlight at solar maximum also causes a slight warming of ocean surface waters across the subtropical Pacific, where clouds are normally scarce, says the study.
This extra heat leads to more evaporation, producing additional water vapour. The extra moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, driving more rain.
In the tropical eastern Pacific, sea surface temperatures cool a little, creating conditions similar to a La Nina event. La Nina is the opposite phenomenon to El Nino, producing wetter weather in the western Pacific and drier weather in parts of South America.
The Indian monsoon and many other regional climate patterns are largely driven by rising and sinking air in the tropics and subtropics. Solar-cycle predictions could help meteorologists estimate how those circulation patterns, changes in sea surface temperatures and regional weather patterns might vary.
“The sun, the stratosphere, and the oceans are connected in ways that can influence events such as winter rainfall in North America,” says NCAR scientist Gerald Meehl, lead author of the study.
“Understanding the role of the solar cycle can provide added insight as scientists work toward predicting regional weather patterns for the next couple of decades.”
The sun is presently in a calm period after reaching a solar minimum at the end of last year, according to the Space Weather Prediction Center at the National Oceanic and Atmospheric Administration in the United States.
The next solar peak is expected in May 2013. (For more details, see: http://www.swpc.noaa.gov/SolarCycle/)
“This paper represents a useful step forward in understanding how solar activity may lead to modest but detectable climatic effects,” said Brad Carter, senior lecturer in physics at the University of Southern Queensland, Australia.
“It is a good reminder that solar activity is not an explanation of global warming over recent decades.”