livescience.com Yahoo News 16 Jul 09;
When the sun is at its most active, it can impact Earth's climate in a way that is similar to El Niño and La Niña events, a new study suggests.
The sun experiences a roughly 11-year cycle, during which the activities on its roiling surface intensify and then dissipate. (One noted sign of a highly active period is the number of sunspots dotting the solar surface).
The total energy reaching Earth from the sun varies by only 0.1 percent across the solar cycle.
Scientists have sought for decades to link these ups and downs to natural weather and climate variations and distinguish their subtle effects from the larger pattern of human-caused global warming. But that link has proven difficult to find.
Scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., used computer climate models and more than a century of ocean temperature records to tease out just such a connection.
"We have fleshed out the effects of a new mechanism to understand what happens in the tropical Pacific when there is a maximum of solar activity," said study leader Gerald Meehl. "When the sun's output peaks, it has far-ranging and often subtle impacts on tropical precipitation and on weather systems around much of the world."
Solar La Niña, El Niño
The model results, detailed this month in the Journal of Climate, showed that as the Sun reaches maximum activity, it heats cloud-free parts of the Pacific Ocean enough to increase evaporation, intensify tropical rainfall and the trade winds, and cool the eastern tropical Pacific.
The result of this chain of events is similar to a La Niña event, although the cooling of about 1-2 degrees Fahrenheit is focused further east and is only about half as strong as for a typical La Niña.
Over the following year or two, the La Niña-like pattern triggered by the solar maximum tends to evolve into an El Niño-like pattern, as slow-moving currents replace the cool water over the eastern tropical Pacific with warmer-than-usual water.
Again, the ocean response is only about half as strong as with El Niño.
True La Niña and El Niño events are associated with changes in the temperatures of surface waters of the eastern Pacific Ocean.
After a couple of years of El Niño-like conditions, the event settles down and the system returns to a neutral state.
Weather effects
These solar-induced trends could affect the naturally occurring La Niña and El Niño events, by reinforcing them or counteracting them.
"If the system was heading toward a La Niña anyway," Meehl said, "it would presumably be a larger one."
Meehl and his colleagues found that the solar-driven La Niña tends to cause relatively warm and dry conditions across parts of western North America. More research will be needed to determine the additional impacts of these events on weather across the world.
"Building on our understanding of the solar cycle, we may be able to connect its influences with weather probabilities in a way that can feed into longer-term predictions, a decade at a time," Meehl says.
Sunspots linked to Pacific rain
Sudeep Chand, BBC News 28 Aug 09;
A study has shown how sunspots could affect climate in the Pacific.
Writing in the journal Science, the international team detailed how the 11-year sunspot cycle might influence the amount of rain falling on the ocean.
It is hoped the findings will lead to better models for regional climate predictions.
The authors emphasised the findings "cannot be used to explain recent global warming because of the trend over the past 30 years".
Sunspots are cooler areas on the Sun's surface that are marked by intense magnetic activity.
Although dimmer than their surroundings, their presence is usually accompanied by bright spots, or faculae, which result in a slight general overall brightening of our star when it is most active.
Sunspots and rain
The new study suggests that relatively small variations in sunspot activity might result in changes in climate.
Two mechanisms are involved.
The first is "top-down" where changes in the upper layers of the atmosphere contribute to wetter conditions below.
The second is "bottom-up" where the ocean evaporates and more clouds are produced.
The study used models to show how these two mechanisms might act together to produce rainfall similar to that observed in the tropics.
In addition, the models predict a cooling effect of the surface of the ocean in equatorial regions.
A step forward
Brad Carter, from the University of Southern Queensland, Australia, said: "This paper represents a useful step forward."
Others, however, were not so convinced.
In an interview with the journal Science, Joanna Haigh from Imperial College London, UK, said: "It is not nearly as conclusive as they would have it."
In the same interview, David Rind of the US space agency (Nasa) added that "even if the amplifier exists, its climate leverage is still relatively puny".
In response Dr Meehl, one of the authors of the study, told BBC News: "There could be other mechanisms in the system that also connect solar variability to climate.
"But the new result here is that we have identified these two (mechanisms) that can produce signals of the size we expect to see in the observations.
"And we've reproduced them in a climate model simulation for the first time."
