Scientists see that sea surface temperature impacts drought and flooding in the Amazon rainforest

December 14, 1999

Rainfall patterns in the Amazon change when humans alter the land during deforestation and farming, causing some areas to suffer drought while other areas succumb to floods. Now, Rong Fu, an atmospheric scientist at the Georgia Institute of Technology, has found that the ocean surface temperature in the Atlantic and the Pacific surrounding South America has as much of an impact on rainfall as land cover changes do, helping scientists see the human effect on rainfall patterns and helping predict impending drought or floods.

"I was surprised by the strength and influence of Western Pacific's sea surface temperatures on Amazon rainfall," says Fu, "because moisture from the Pacific Ocean has to travel over the Andes Mountains before it reaches the Amazon region." The results of Fu's research will be presented December 15, in San Francisco, Calif. at the American Geophysical Union's Fall Meeting.

Using a computer climate model, Fu compared rainfall patterns over the Amazon to the tropical Atlantic and Pacific Ocean sea surface temperatures. As a result, Fu saw the dramatic affect sea surface temperatures have on rainfall in the Amazon region.

Putting an El Niño event into the computer model, Fu discovered that the rainfall pattern in the eastern equatorial Amazon region of Brazil is very sensitive to sea surface temperature changes. If sea surface temperatures rose, then drought conditions were likely, but if temperatures fell, flooding would result. Surprisingly, just west of Brazil in Columbia and Peru, the precipitation was relatively unaffected by the El Niño, she says.

To find out which ocean had the greatest effect on rainfall changes, Fu removed the Atlantic sea surface temperatures from the model. In the model, a wetter season appeared during spring in Brazil, instead of the normal dry season. Removing the Eastern Pacific Ocean sea surface temperatures had a similar, but weaker effect on rainfall. But it wasn't until Fu removed Western Pacific sea surface temperatures that an unexpected result occurred.

Fu found that the Pacific Ocean had a greater influence on rainfall pattern changes than she expected. Because the evaporation off the Atlantic has a direct route to Brazil, she says, you would expect the Atlantic to have a greater impact, but the Pacific influence is stronger even though evaporation off the Pacific has to travel over mountains to arrive in Brazil.

Fu has clearly seen the affect sea surface temperatures have on Amazon rainfall patterns and plans mimic the results of continued deforestation by changing the lush rainforest cover to semi-arid grasslands in the model. Fu believes that this change will increase the likelihood of drought conditions.

Through her research, Fu hopes to better understand sea surface temperature changes and how these changes affect the Amazon region.
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NASA/Goddard Space Flight Center

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