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Solar Cycle and El Nino: New research points to stronger connection by Steve LaNore, Chief meteorologist at KDAF-TV Dallas (1998-2000), Chief at KXII-TV (Sherman) since 2006
Saturday, July 18th 2009, 10:26 AM UTC
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New research led by the National Center for Atmospheric Research (NCAR) in Boulder, Colorado shows that maximum solar activity and solar fluctuations through the complete solar cycle have impacts on Earth that mimic La Nina and El Nino events in the Pacific Ocean. This research may set the stage for more accurate predictions of weather patterns at various times during the Solar Cycle, which lasts approximately 11 years.

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The total solar output reaching Earth has a very small range which only changes by 0.1 percent from max to min within the solar cycle. Researchers have searched for decades to link these variations to natural weather and climate variations.

NCAR researchers used computer models of global climate and more than a century of ocean temperature data to answer questions about the correlation between solar activity and global climate. While change in greenhouse gases was also included in the model, but the main point of the study is to examine the role of solar variability in climate change.

The research was funded by the National Science Foundation and by the Department of Energy.

"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," says NCAR scientist Gerald Meehl, the lead author. "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 a previous one by Meehl and colleagues, shows that as the Sun reaches its maximum, it heats cloud-free parts of the Pacific Ocean, increasing evaporation. This in turn intensifies tropical rainfall and the trade wind pattern, and cools the eastern tropical Pacific. This sequence of events is similar to a La Nina event, although the cooling 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 peak tends to morph into an El Nino-like pattern. The slow-moving currents replace the cool water over the eastern tropical Pacific with warmer water. The ocean change is once again is only about half as strong as with a “regular” El Nino.

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

The new paper does not analyze the weather impacts of the solar-driven events. But Meehl and his co-author, Julie Arblaster of both NCAR and the Australian Bureau of Meteorology, found that the solar-driven La Nina 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.

Establishing a physical connection between the 11-year solar cycle and global climate patterns has proven challenging. Only in recent years have computer models been able to simulate with some accuracy the mechanisms underlying tropical Pacific warming and cooling associated with El Nino and La Nina.

Meehl and his colleagues analyzed sea surface temperatures from 1890 to 2006. They then used two computer models based at NCAR to project the response of the oceans to changes in solar output.

Their findings show that as the Sun's output reaches a peak, the small amount of extra sunshine (over several years) causes a small but measurable increase in local atmospheric heating, especially where sun-blocking clouds are in short supply. That small amount of extra heat leads to more evaporation, producing extra water vapor. In turn, the moisture is carried by trade winds 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.

Although this Pacific pattern is produced by the solar maximum, the authors found that its switch to an El Nino-like state is likely triggered by the same kind of processes that normally lead from La Nina to El Nino. The transition starts when changes in the strength of the trade winds produce slow-moving off-equatorial pulses known as Rossby waves in the upper ocean, which take about a year to travel back west across the Pacific.

The energy then reflects from the western boundary of the tropical Pacific and ricochets eastward along the equator, deepening the upper layer of water and warming the ocean surface. As a result, the Pacific experiences an El Nino-like event about two years after solar maximum. The event settles down after about a year, and the system returns to a neutral state.

"El Nino and La Nina seem to have their own separate mechanisms," says Meehl, "but the solar maximum can come along and tilt the probabilities toward a weak La Nina. If the system were heading toward a La Nina anyway," he adds, "it would presumably be a larger one."
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