David Fogarty, Reuters 28 Aug 09;
SINGAPORE (Reuters) - Small changes in the energy output of the sun can have a major impact on global weather patterns, such as the intensity of the Indian monsoon, that could be predicted years in advance, a team of scientists said.
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 Center 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 vapor. The extra moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, driving more rain.
PREDICTIONS
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: 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."
(Editing by Alex Richardson)
How Sunlight Controls Climate
New computer models begin to suggest how changes in the sun's strength might change weather patterns
David Biello, Scientific American 31 Aug 09;
Small changes in the sun's brightness can have big impacts on our planet's weather and climate. And now scientists have detailed how that process might work, according to a new study published August 28 in Science.
For decades some scientists have noted that certain climate phenomena—warmer seas, increased tropical rainfall, fewer clouds in the subtropics, stronger trade winds—seem to be connected to the sun's roughly 11-year cycle, which causes ebbs and flows in sunspots that result in variations in solar output.
That variation is roughly equal to 0.2 watt per meter squared—far too little to explain, for instance, actual warming sea-surface temperatures. A variety of theories have been proposed to explain the discrepancy: ozone chemistry changes in the stratosphere, increased sunlight in cloudless areas, even cosmic rays. But none of these theories, on its own, explains the phenomenon.
Now, using a computer model that pairs ozone chemistry with the fact that there are fewer clouds in the subtropics when the sun is stronger, climate scientist Gerald Meehl of the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and colleagues have reproduced all the observed cyclical climate phenomena as sunlight waxed and waned in intensity over the course of the last century. "Even though [sunlight variability] is a very small number on a global average, regionally or locally it can be much bigger," Meehl explains. Changes to stratospheric ozone chemistry and cloud cover in the subtropics "kind of add together and reinforce each other to produce a bigger amplitude of this small solar forcing signal," he says.
If the model is correct, the mechanism works like this when the sun is at maximum strength: Ozone in the tropical stratosphere traps slightly more heat under the increased ultraviolet sunlight, warming its surroundings and, in turn, allowing increased ozone production. (Warmer temperatures make it easier for ultraviolet light to break up O2 molecules, thereby allowing the resulting free oxygen ions to hook up with other molecules of their kind to create ozone.) That ozone also warms and the cycle continues, resulting in roughly 2 percent more ozone globally. But this change also begins to affect the circulation of the stratosphere itself, which then alters the circulation in the lowest layer of the atmosphere, known as the troposphere, by reinforcing certain wind patterns that then affect the weather we experience.
Meanwhile, the increased radiance during the solar max also adds slightly more heat to the ocean in areas that are already relatively cloudless because of sinking, cooler air. That produces a little more evaporation, which is carried by the trade winds back into the tropics where it comes down again as increased rainfall, but also helps strengthen the upward convection that causes the subtropical cloudless skies. That, in turn, further increases downward pressure back in the subtropics, resulting in even fewer clouds—again roughly 2 percent less clouds over these parts of the Pacific. "You basically spin up this whole system," Meehl says.
But the model did not exactly reproduce real-world conditions. Whereas sea-surface temperatures in the actual eastern Pacific typically decline by roughly 0.8 degree Celsius under a stronger sun, the model could only replicate about 0.6 degree C of cooling. Nor did the model predict changes where they actually occur on the planet. Other factors are likely at work, Meehl says, and even the best computer model can only begin to approximate the complexity of the actual climate.
Right now, the sun is stuck in a period of extremely low sunspot activity, not unlike the "Maunder Minimum" that may have been responsible for the Little Ice Age that cooled Europe in the late 17th century as well as the fall of imperial dynasties in China. And, for the latter half of the 20th century, the sun's output remained relatively constant as global temperatures rose—ruling out our star itself as the direct source of global warming.
Nevertheless, the research begins to explain the physical mechanisms by which changes in the sun's radiance can have outsized impacts on the planet. And that means that the next uptick in the solar cycle, and thereby the sun's brightness, might bring La Niña conditions—unusually cold surface waters—in the equatorial Pacific. "Whenever it happens," Meehl predicts, "chances are it would behave like a weak La Niña–like pattern."
