"Our work firmly documents the existence of the relationship between the timing of sunspot peaks and heavy rainfall in East Africa, something that has long been controversial," Stager told environmentalresearchweb. "Although we have yet to determine exactly why solar cycles trigger the rain anomalies, we believe that this pattern is too strong and widespread to ignore. If it holds true in the future, then it should be useful for predicting solar-induced rainfall extremes (and associated disease outbreaks) in this region up to several years in advance."
Sunspots are dark regions on the solar surface that are often accompanied by bursts of activity such as solar flares. They are produced by magnetic activity inside the Sun and higher numbers of sunspots indicate an increase in the Sun's energy output. Sunspots vary in number with a period of 11 years and the next cycle is expected in 2011–2012.
The researchers reckon that increases in solar energy output might warm tropical sea and land surfaces. This would increase the moisture in the air and strengthen convective storm activity during the tropical rainy seasons. "Another idea is that the rhythm of the sunspot cycle might also affect the timing of El Niño events, which occur roughly twice as often as sunspot peaks [and] often cause heavy rains in East Africa as well," explained Stager.
Anticipation is everything
The US–UK team hopes its result will encourage other scientists to work out exactly how the sunspot-rainfall connection works. "In addition, we hope that it will allow people in the affected region to anticipate upcoming major rainfall events, by simply monitoring the sunspot cycle," added Stager.
The researchers insist that their method cannot be used to predict droughts and that heavy rains could also fall during low sunspot-number times because of other factors. "However, the occurrence of heavy rainfall roughly one year before sunspot peaks has been consistent for more than a century," insisted Stager. "We hope that people who have the know-how and the resources to deal with diseases like Rift Valley fever, malaria and cholera (as well as possible flood damage to infrastructures) can benefit from the advance warning."
Stager and co-workers obtained their results by analysing rainfall data from the Lake Victoria watershed in the 20th century. They also used historical records of water levels at lakes Tanganyika and Naivasha. Finally, they looked at computerized weather maps of Africa from the last half of the century.
The researchers will now examine tropical river discharge records in Africa and South America for similar patterns. "And, we will also watch the weather in East Africa closely as the next sunspot peak approaches, to see if the long-standing pattern still holds true," said Stager.
The work was published in the Journal of Geophysical Research and was funded by the US National Science Foundation.