"In our models, the Indian Ocean shows very clear and dramatic warming into the future, which means more and more drought for southern Africa," says Hurrell. "It is consistent with what we would expect from an increase in greenhouse gases."
Hurrell and Hoerling compared model results from 1950-99 to several control runs that omitted the Indian Ocean warming. None of those runs showed the magnitude of drying that actually occurred in southern Africa. When the models did include the Indian Ocean warming, southern Africa consistently dried out. The models also project that by 2049, monsoons across southern Africa could be 10% to 20% drier than the 1950-99 average.
A different process appears to shape rainfall in the Sahel. When sea-surface temperatures are warmer in the South Atlantic than in the North, it pulls the Sahelian monsoon cycle south as well, depriving the region of its usual rains.
"This was the situation during much of the latter half of the 20th century." says Hurrell. "We believe the North Atlantic Ocean cooling was natural and masked an expected greenhouse-gas warming effect."
Since 1990, the sea-surface temperature pattern has reversed, warming more rapidly in the North Atlantic than in the South. The models examined by Hurrell and Hoerling show this trend intensifying in future decades. They project that the Sahel monsoon will be some 20% to 30% wetter by 2049 compared to the 1950-99 average.
The warming of Indian Ocean waters is well beyond the range expected from natural processes. This strengthens the case that greenhouse gases are involved, says Hurrell. In the Atlantic, natural variability affects ocean temperatures more strongly, making it more difficult to attribute changes there to greenhouse-gas effects.
Paleoclimate records show that even greater climate swings have occurred in Africa's monsoons, most likely related to past v