Solar Effects on Rainfall at Lake Victoria, East Africa

J. Curt Stager [], Paul Smith's College, New York.

            Evidence from Lakes Naivasha and Victoria suggests that prolonged sunspot minima caused lake levels to rise there during the Little Ice Age (LIA). This is a highly unusual pattern for tropical Africa; during most of the last 100,000 years, cooling phases usually caused tropical drying, as cooler sea surfaces reduced evaporation and weakened monsoon circulation. Could solar cooling have caused symmetrical equatorward shrinking of the tropical rain belts, thus increasing rainfall in equatorial Africa while causing drought at higher tropical latitudes? More surprisingly, these apparent sun-rainfall relationships apparently reversed sign during the Dalton sunspot minimum (ca. 1800 AD), and recent positive sunspot-lake level correlations suggest that this pattern persisted in the Victoria basin through the end of the 20th century. Could this reflect the end of an unusual, short-lived form of sun-climate interactions linked to the LIA that occurred sporadically throughout the Holocene, perhaps including the 8200 BP event? And how widespread was this pattern of hydrological change in the tropics? Historical records of Lake Victoria levels, when compared to the ca. 11-year sunspot cycle, show that rainfall peaks (lake level maxima) occurred during every sunspot peak of the 20th century. These associations were more consistent than the widely recognized links between ENSO and equatorial rainfall. A possible mechanism: sunspot cycle peaks are thought to cause higher SST's in the equatorial Indian Ocean, which can increase East African rainfall. But why would this have differed under LIA conditions?