Warmer Ocean Blamed for Drought

Dry, dry, dry. A climate model driven largely by growing Indian Ocean warmth can now simulate the Sahel drought of the 1970s and ‘80s.

A warmer Indian Ocean is the culprit behind a devastating drought that hit the Sahel--a 5000-kilometer-long strip of marginally habitable land along the southern edge of the Sahara--in the 1970s, according to new research.

After the drought began, some researchers hypothesized that intensive use of the land might have altered the surface of the Sahel enough to dry it out. The locals, although they have clearly used the land heavily, have since been absolved of fooling with the climate (Science, 31 July 1998).

The indictment of the Indian Ocean comes from two new climate-modeling studies that investigate the effects of the recent warming of tropical oceans. In a paper published online by Science this week, climate dynamicist Alessandra Giannini of the International Research Institute for Climate Prediction in Palisades, New York, and her colleagues report results from the first climate model to simulate accurately the history of Sahel rainfall.

In their model, the Indian Ocean's warming over the decades was the dominant driver of the drying of the Sahel. The model's greenhouse gas amounts were not changed over the 70 years, nor was the vegetation or any other surface property in or around the Sahel. Yet the model faithfully produced the long-term trend in Sahel rainfall. The warming of the tropical Atlantic did play some role in long-term drying, but it was the Indian Ocean--which has warmed more than any other ocean basin--that drove most of the drying in the model.

Now another model adds support to Giannini's conclusions. In a study in press at Geophysical Research Letters, climate dynamicist Mojib Latif of the University of Kiel, Germany, and Jürgen Bader of the University of Cologne report that their model Sahel dried more when tropical seas were warmer during the past half-century than when they were cooler. When they changed sea surface temperatures one ocean basin at a time, it was the Indian Ocean that dominated.

“The Bader and Latif experiments confirm the interpretation of Giannini that the Indian Ocean is indeed driving a Sahel drying,” says climate dynamicist Martin Hoerling of the U.S. National Oceanic and Atmospheric Administration's Climate Diagnostics Center in Boulder, Colorado. No one can yet say whether a warming Indian Ocean will continue to drive drought in the Sahel, but some researchers suggest it could if, as suspected, greenhouse warming proves to be the ultimate cause of rising temperatures there.