"It has been something of a mystery to understand how the
tropical rain belt moved so far north of the equator," researcher Robert
Korty said.
Some
6,000 years ago, the Sahara Desert was regularly drowned by tropical rains. It
wasn't a desert at all, but vast grasslands. Today, the Sahara features some of
driest acreage on Earth.
Recently,
a pair of researchers from Texas A&M University and Yale
University set out to explain how such a vast climatic transformation can
happen in such a short amount of time. In order to do so, the researchers built
a model to contrast rain patterns of the Holocene era with those of today.
Their
analysis offers new insights into the nature of the Hadley circulation, the
cycle of warm air rising near the equator and descending in the subtropics. The
Hadley circulation influences everything from the trade winds and tropical rain
belts to jet streams and hurricanes.
"The
framework we developed helps us understand why the heaviest tropical rain belts
set up where they do," Robert Korty, associate professor of atmospheric
sciences at Texas A&M, said in a news release. "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."
Over
time, the rain belt that once provided the Sahara with moderate rainfall has
slowly moved northward toward the Mediterranean.
"It
has been something of a mystery to understand how the tropical rain belt moved
so far north of the equator," Korty said. "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."
Korty
and colleague William Boos of Yale argue the shifting rain belt alone fails to
explain the transformation of the Sahara. Instead, falling precipitation totals
likely created a sort of climatological feedback loop that triggered more
drastic change in the soil and atmosphere.
"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," Korty explained. "Feedbacks between the shifts in
rain and the vegetation that could exist with it are needed to get heavy rains
into the Sahara."
The
pair of scientists hope their findings -- published in the journal Nature
Geoscience -- will improve models designed to predict the impacts of
climate change on regional weather patterns.
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