A Tropical Connection
The Amazon and the Congo River Basins have a see-saw relationship
By Courtney Humphries

The amazon and Congo River Basins together cover more than 11 million square kilometers, and the intense rainfall they receive helps shape the global climate. Now, researchers at MIT have found that these two giant watersheds appear to be engaged in a climatic tug of war. A study led by Elfatih Eltahir, SM '93, ScD '93, a professor of civil and environmental engineering, shows that when the Congo Basin is dry, the Amazon Basin tends to be wet, and vice versa.

Eltahir, who reported the discovery in the December 2004 Geophysical Research Letters, has termed this back-and-forth relationship a "see-saw oscillation." His team uncovered the connection by examining both short-term and long-term weather data for the two regions. A bet-ter understanding of the relationship between their climate systems could lead to more-accurate predictions of periods of drought and flood.

The oscillation first drew notice in 2003, when Teresa K. Yamana '04, then an undergraduate, analyzed recent satellite data from NASA's Tropical Rainfall Measuring Mission, which monitors weather conditions over the tropics. She noticed that the driest months in one region were consistently the wettest in the other.

But the satellite data had been collected only since 1997. "That's clearly not long enough to investigate this phenomenon," Eltahir says. Since the rainfall over these vast and in large part inaccessible regions has not been measured, Eltahir's team turned to river flow data collected between 1905 and 1985. "Fluctuations in river flow from year to year reflect the fluctuations in rainfall over a large area," he says. "We think of it as a big rain gauge."

When another undergraduate, Brian Loux '04, analyzed the river flow data, he found that the relationship between the watersheds was not as simple as Yamana's analyses originally suggested. The see-saw effect was most noticeable during the Southern Hemisphere's summer, between the months of January and March. And it appeared stronger in some decades than in others. "It has its own variability, like other oscillations in the atmosphere," Eltahir says. He compares the phenomenon to El Nino, the unusually warm water current in the Pacific Ocean that shows up irregularly to shake up global weather patterns.

Loux, now a graduate student in civil and environmental engineering, says the most exciting implication of the work is "the idea that climate is somehow largely connected across the Atlantic Ocean." So how do regions half a world apart manage to play tug of war with water? Eltahir says that the mechanism of the effect is still unknown. But he believes it may resemble the one that lets El Nino exert far-reaching effects on tropical river flow. When air moves up in one region, he explains, it forces air down in the other. The upward-moving currents that bring rainfall to the Amazon might force the air over the Congo to sink, bringing dry weather.

To test this hypothesis, Eltahir and his colleagues will attempt to re-create the see-saw effect in a mathematical climate model. Then they can begin to test how other factors change the natural climatic balance between the two regions.