Intense rainfall in the region often leads to flooding and disease outbreaks.
The analysis by a team of U.S. and British researchers shows that unusually heavy rainfalls in East Africa over the past century preceded peak sunspot activity by about one year. Because periods of peak sunspot activity, known as solar maxima, are predictable, so too are the associated heavy rains that precede them, the researchers propose.
"With the help of these findings, we can now say when especially rainy seasons are likely to occur, several years in advance," says paleoclimatologist and study leader Curt Stager of Paul Smith's College in Paul Smiths, New York. Forewarned by such predictions, public health officials could ramp up prevention measures against insect-borne diseases long before epidemics begin, he adds.
The sunspot-rainfall analysis is scheduled to appear on 7 August in the Journal of Geophysical Research - Atmospheres, a publication of the American Geophysical Union.
Increasing sunspot numbers indicate a rise in the sun's energy output. Sunspot abundance peaks on an 11-year cycle. The next solar maximum is expected in 2011-2012. If the newfound pattern holds, rainfall would also peak the year before.
"We expect East Africa to experience a major intensification of rainy season precipitation, along with widespread Rift Valley Fever epidemics, a year or so before the solar maximum of 2011-2012," the team reports. Because mosquitoes and other disease-carrying insects thrive in wet conditions, heavy rains may herald outbreaks of diseases such as Rift Valley Fever.
The new analysis relies on rainfall data going back a century. The scientists also used historical records of water levels at lakes Victoria, Tanganyika, and Naivasha.
The work counters previous research that found no connection between sunspot cycles and rainfall in East Africa. Stager's team concludes that, although the link between sunspots and rainfall was weak between 1927 and 1968, the cyclic pattern held true throughout the 20th century. Previous statistical analysis discounted the link for a variety of reasons, including the influence of El Nino and other climatic disturbances not associated with sunspots.
Scientists have investigated apparent correlations between solar variability and Lake Victoria's water levels since the beginning of the last century, says co-author Alexander Ruzmaikin of NASA's Jet Propulsion Laboratory in Pasadena, California. The new research "shows that these correlations are, in fact, not accidental, effectively resolving a longstanding historical puzzle and improving our knowledge of how solar variability affects Africa's climate," he adds.
Stager, Ruzmaikin and their colleagues offer several reasons why sunspot peaks may affect rainfall. In a simple scenario, increased solar energy associated with sunspots heats both land and sea, forcing moist air to rise and triggering precipitation.
While sunspot peaks augur extraordinarily wet rainy seasons, heavy rains are possible at other times as well, Stager acknowledges. But, most of the rainiest times, he says, are consistently coupled with the predictable rhythms of sunspot peaks. And, to be forewarned is to be forearmed.
"The hope is that people on the ground will use this research to predict heavy rainfall events," Stager says. "Those events lead to erosion, flooding, and disease."
The National Science Foundation funded the study.
Peter Weiss | AGU
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