James Hurrell of the National Center for Atmospheric Research (NCAR) will present the findings on May 24 in New Orleans at the spring meeting of the American Geophysical Union. The study, conducted with Martin Hoerling (National Oceanic and Atmospheric Administration), was supported by NOAA and the National Science Foundation, NCAR's primary sponsor.
The analysis, which draws on 60 simulations of global climate from five computer models, provides new evidence linking drought in southern Africa to the warming of the Indian Ocean. However, it contradicts earlier studies that also connected the Sahelian drought of northern Africa to the Indian Ocean. Instead, the new results point to a late 20th-century cooling of the North Atlantic Ocean as having been key to Sahelian drought. A subsequent switch to North Atlantic warming, partly consistent with the impact of greenhouse gas increases, is the main factor behind the Sahel's recent swing from drought to moist conditions, the researchers believe.
"Changes in the Indian and Atlantic oceans are causing large regional effects in Africa, and these have substantial impacts on people. Now we can explain these climatic effects," says Hurrell.
Recurrent drought since the 1970s has plagued southern Africa, including Angola, Zambia, and Zimbabwe. Meanwhile, the nearby Indian Ocean has warmed more than 1 degree Celsius (0.6 degree Fahrenheit) since 1950. As showers and thunderstorms develop in the rising air above the warming ocean, says Hurrell, they help lead to sinking air and