Robert Korty, associate professor in the Department of Atmospheric Sciences, along with his colleague William Boos at Yale University, published their work in the current issue of Nature Geoscience.

The two researchers looked at the precipitation patterns of the Holocene era and compared them to current movements of the Intertropical Convergence Zone, a large region of intense tropical precipitation. Using computer models and other data, researchers found links to precipitation patterns thousands of years ago.

“The framework we developed helps us understand why the heaviest tropical rain belts settle where they do,” says Korty.

“Tropical rain belts are related to what is happening elsewhere in the world through Hadley’s circulation, but they do not directly predict changes elsewhere, as the chain of events is very complex. But it is a step towards this goal.”

The Hadley circulation is a tropical atmospheric circulation that rises near the equator. It is linked to subtropical trade winds, tropical rain belts and affects the position of severe storms, hurricanes and the jet stream. Where it descends in the subtropics, it can create desert-like conditions. The majority of the arid regions of the Earth are located in areas below the descending parts of the Hadley circulation.

“We know that 6,000 years ago what is now the Sahara Desert was a rainy place,” Korty adds.

“It has been a mystery to understand how the tropical rain belt moved so far north of the equator. Our results show that large rain migrations can occur in one part of the globe even if the belt does not move much elsewhere. .

“This framework may also be useful in predicting the details of how bands of tropical rain tend to move during modern El Niño and La Niña events (the cooling or warming of waters in the central Pacific Ocean that have tendency to influence weather conditions around the world). “

The results could lead to better ways of predicting future precipitation patterns in parts of the world, Korty believes.

“One of the implications of this is that we can deduce how the position of precipitation will change in response to individual forces,” he says. “We were able to conclude that the variations in Earth’s orbit that shifted precipitation to northern Africa 6,000 years ago were on their own insufficient to maintain the amount of rain which, according to geological evidence, is fell on what is now the Sahara Desert. Feedbacks between the changes in rain and the vegetation that might exist with it is needed to bring down heavy rains in the Sahara. “

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