When the sun's weather is 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 Earth's natural weather and climate variations, and to 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.
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.