Seesaw Climate: Drought Here Means Flood There

Weather buffs know that highs and lows swing in tandem across a continent, but a new study shows that climate extremes on one part of the planet are routinely reflected by opposite conditions a world away.

The result is a newfound climatic yin-yang that can have unfortunate and tragic consequences for the regions involved.

When the Congo Basin floods, South America's Amazon basin experiences a drought. And conversely, when tropical storms lash in the Amazon to the point of flooding, it's dangerously dry in the Congo Basin.

The see-saw climate oscillation was discovered by a team of MIT environmental engineers who studied regional satellite observations of tropical rainfall and river flow.

"We were just curious to find out if any relationship existed," said Elfatih Eltahir, one of the co-authors of the study published in a recent issue of the journal Geophysical Research Letters.

Intense tropical storms over these two basins, along with tropical rainfall over the Pacific Ocean, are largely responsible for the Earth's climate on a large scale. Most of Earth's tropical rain falls in these three regions.

The Amazon and Congo rivers carry the largest and second largest annual discharges of water, about 6,300 cubic kilometers and 1,250 cubic kilometers respectively. (For comparison, 1,250 cubic kilometers is equivalent to 11 times the volume of rock and dirt packed into the mile-high San Salvador volcano in El Salvador). Heating of the atmosphere above the South American and African continents results in 12,000 cubic kilometers of rain over the Amazon and 5,600 cubic kilometers of rain over the Congo, annually.

As a result, rainfall and atmospheric movement over these two basins have a huge influence over global climate.

The see-saw pattern can be explained by an atmospheric cycle that repeats itself. In flooded areas, hot moist air rises, which then condenses into vapor that helps form rain clouds. That rising and falling motion associated with the flood forces clear skies and inhibits rain in surrounding regions, in this case over the corresponding basin across the Atlantic Ocean.

The flood-drought see-saw was found to be most significant during 1945-1955, the 1960s and the 1970s in the Southern Hemisphere summers.

The weather pattern is a weak but independent cousin of the now well-known El Nino weather pattern that brings floods and droughts to the Ganges, Congo, Amazon, and Murray (Australia) basins, as well as parts of North America, depending upon the movement of an enormous "bathtub" of warm water in the Pacific Ocean.

The discovery of the Amazon Congo see-saw could be useful as a forecasting tool, Eltahir told LiveScience. "Like any new understanding of natural variability, these findings have implications regarding climate variability and climate forecasting and prediction in the tropics," he said.