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The Sahara - world's largest hot desert - was covered in grassland that received plenty of rainfall as little as 6,000 years ago, a new study has found. Researchers at Texas A&M University in the US and colleagues found that shifts in the world's weather patterns abruptly transformed the vegetated region into some of the driest land on the Earth. They looked into precipitation patterns of the Holocene era and compared them with present-day movements of the intertropical convergence zone, a large region of intense tropical rainfall. Using computer models and other data, the researchers found links to rainfall patterns thousands of years ago. The finding could lead to better rainfall predictions worldwide. "The framework we developed helps us understand why the heaviest tropical rain belts set up where they do," said Robert Korty, associate professor in the Department of Atmospheric Sciences. "Tropical rain belts are tied to what happens elsewhere in the world through the Hadley circulation, but it won't predict changes elsewhere directly, as the chain of events is very complex. But it is a step toward that goal," said Korty. The Hadley circulation is a tropical atmospheric circulation that rises near the equator.
It is linked to the 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 Earth's arid regions are located in areas beneath 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 said. "It has been something of a mystery to understand how the tropical rain belt moved so far north of the equator. Our findings show that that large migrations in rainfall can occur in one part of the globe even while the belt doesn't move much elsewhere. This framework may also be useful in predicting the details of how tropical rain bands tend to shift during modern-day El Nino and La Nina events - the cooling or warming of waters in the central Pacific Ocean which tend to influence weather patterns around the world. The findings could lead to better ways to predict future rainfall patterns in parts of the world, Korty said. "One of the implications of this is that we can deduce how the position of the rainfall will change in response to individual forces," he said. "We were able to conclude that the variations in Earth's orbit that shifted rainfall north in Africa 6,000 years ago were by themselves insufficient to sustain the amount of rain that geologic evidence shows fell over what is now the Sahara Desert. "Feedbacks between the shifts in rain and the vegetation that could exist with it are needed to get heavy rains into the Sahara," said Korty. The findings were published in the journal Nature Geoscience.
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