21 July 2009

Solar cycle links to global climate

Maximum solar activity and its aftermath have the kind of impact on Earth that La Nina and El Nino have on the tropical Pacific Ocean, researchers said.

That understanding may pave the way toward better predictions of temperature and precipitation patterns at certain times during the Sun’s cycle, which lasts approximately 11 years, said researchers at the National Center for Atmospheric Research (NCAR).

The total energy reaching Earth from the Sun varies by only 0.1% 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.

Building on previous work, NCAR researchers used computer models of global climate and more than a century of ocean temperature data to answer longstanding questions about the connection between solar activity and global climate. Changes in greenhouse gases were also included in the model, but the main focus of the study is to examine the role of solar variability in climate change.

“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 NCAR scientist Gerald Meehl, the lead author of a paper on the subject. “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.”

The new paper, along with an earlier one by Meehl and colleagues, shows 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 Nina event, although the cooling of about 1-2 degrees Fahrenheit focused further east and is only about half as strong as for a typical La Nina.

Over the following year or two, the La Nina-like pattern triggered by the solar maximum tends to evolve into an El Nino-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 Nino.

True La Nina and El Nino events associate with changes in the temperatures of surface waters of the eastern Pacific Ocean. They can affect weather patterns worldwide.

While they do not analyze the weather impacts of the solar-driven events, the researchers found the solar-driven La Nina tends to cause relatively warm and dry conditions across parts of western North America. They will have to do more research 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 said.

Long-term solar variations affect certain weather patterns, including droughts and regional temperatures, scientists said. But establishing a physical connection between the decadal solar cycle and global climate patterns has proven elusive. One reason is only in recent years have computer models been able to realistically simulate the processes underlying tropical Pacific warming and cooling associated with El Nino and La Nina. With those models now in hand, scientists can reproduce the last century’s solar behavior and see how it affects the Pacific.

To ferret out these sometimes subtle connections between the Sun and Earth, Meehl analyzed sea surface temperatures from 1890 to 2006. They then used two computer models based at NCAR to simulate the response of the oceans to changes in solar output.

They found, as the Sun’s output reaches a peak, the small amount of extra sunshine over several years causes a slight increase in local atmospheric heating, especially across parts of the tropical and subtropical Pacific where Sun-blocking clouds are normally scarce. That small amount of extra heat leads to more evaporation, producing extra water vapor. In turn, trade winds carry the moisture to the normally rainy areas of the western tropical Pacific, fueling heavier rains.

As this climatic loop intensifies, the trade winds strengthen. That keeps the eastern Pacific even cooler and drier than usual, producing La Nina-like conditions.

For related information, go to www.isa.org/environment.