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East Africa’s Climate Under El Niño Examined

Posted on | August 12, 2011 | 1 Comment

Lake Challa

Lake Challa with Mt. Kilimanjaro in background. Photo courtesy of Stephan Opitz.

Floods and droughts in East Africa are often unleashed by far-away events in the tropical Pacific–the warm (El Niño) or cool (La Niña) phases of the El Niño-Southern Oscillation. A catastrophic drought is currently wreaking havoc in wide regions of Kenya, Ethiopia, Djibouti and Somalia, affecting food security and putting millions of people in urgent need of assistance. Scientists have attributed the severe drying to La Niña conditions that prevailed from June 2010 to May 2011 in the Pacific.

The waxing and waning of rainfall in eastern tropical Africa in unison with ENSO is nothing unusual and existed already 20,000 years ago, according to a study published in the Aug. 5 issue of Science by Mānoa’s Axel Timmermann and a group of scientists from Germany, Switzerland, the U.S., the Netherlands and Belgium.

Clues to this imprint come from Lake Challa, a crater lake in the foothills of Mt. Kilimanjaro. The thickness and color of the layers allowed the team to reconstruct the history of East African rainfall back in time to the Last Ice Age.

“Even though rainfall did not vary much during that period, the sediment layers still reflect the beat of El Niño and La Niña cycles,” says co-author Timmermann, a professor at Mānoa School of Ocean and Earth Science and Technology’s International Pacific Research Center. “Compared with this coldest time, the last 3,000 years have been wetter, but more variable with severe century-long droughts sprinkled throughout.”

These observations fit the growing consensus that the current global-warming trend will affect the large-scale rainfall patterns. They are also consistent with computer models simulating the 21st century climate in eastern equatorial Africa in response to increasing greenhouse gas concentrations. As climate warms, the atmosphere holds more moisture, and thus the east African short rainy period in October to November is likely to get wetter but also show more variability with occasional severe droughts such as the present one.

“Will these projected changes affect East Africa’s unique biodiversity in its national parks, such as the Serengeti?” asks Timmermann. “We do not yet know, but there are fascinating links to explore further.”

Read the news release.