NEW research by US scientists shows that maximum solar activity and its aftermath have impacts on earth that resemble La Niña and El Niño events in the tropical Pacific Ocean.
Establishing a key link between the solar cycle and global climate in this research answers some of the longstanding questions about the impact of the solar cycle on global climate.
The research, announced July 16, may pave the way toward predictions of temperature and precipitation patterns at certain times during the approximately 11-year solar cycle.
The research is being led by scientists at the National Science Foundation (NSF)-funded National Centre for Atmospheric Research (NCAR) in Boulder, US.
"These results are striking in that they point to a scientifically feasible series of events that link the 11-year solar cycle with ENSO, the tropical Pacific phenomenon that so strongly influences climate variability around the world," said Jay Fein, program director in NSF's Division of Atmospheric Sciences.
"The next step is to confirm or dispute these intriguing model results with observational data analyses and targeted new observations."
The total energy reaching earth from the sun varies by only 0.1pc 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, the NCAR researchers have used computer models of global climate and more than a century of ocean temperature to answer these longstanding questions.
The research, published this month in a paper in the Journal of Climate, was funded by NSF, NCAR's sponsor, and by the US 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," said NCAR scientist Gerald Meehl, the paper's 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 an earlier one by Meehl and colleagues, shows 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°F 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.
The paper does not analyse the weather impacts of the solar-driven events. However, Meehl and his co-author, Julie Arblaster of NCAR and the Australian Bureau of Meteorology, found that the solar-driven La Niña tends to cause relatively warm and dry conditions across parts of western North America.
To tease out the sometimes subtle connections between the sun and Earth, Meehl and his colleagues analysed 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 that, 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, 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 Niña-like conditions.
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