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Homes are submerged after Category 5 hurricane Katrina's passage.

Hurricane Katrina, which flooded New Orleans in 2005, may have gained strength over river mouths.

Photograph by Tyrone Turner, National Geographic Stock

Charles Choi

For National Geographic News

Published August 20, 2012

Hurricanes can get supercharged when they hit river mouths, researchers now find.

Hurricanes keep alive by converting the warmth of tropical waters into motion. The strong winds they kick up in turn cause surface water heated by the sun to mix with deeper, cooler waters, and that drop in warmth causes hurricanes to weaken.

(See photos of hurricanes.)

Rivers and rainfall alter this pattern by adding freshwater, which is less dense than saltwater. Therefore, this freshwater sits on top of cold seawater for much the same reason oil sits on top of water. The resulting "barrier layer" of freshwater keeps winds from mixing the warm surface layer with cooler, deeper water, giving hurricanes more heat to intensify with.

Scientists analyzed 587 tropical storms and cyclones between 1998 and 2007 in the western tropical Atlantic, the western Pacific, and the northern Indian Oceans. They found that tropical storms over thick barrier layers cooled off 36 percent less than storms over areas lacking barrier layers and drew in 7 percent more heat from the ocean.

 

picture: thunderstorm

 

"The Amazon plume spreads out over an area of nearly 1 million square kilometers (386,000 square miles), so we're talking about a pretty big area where hurricanes might intensify," said ocean scientist Karthik Balaguru at Pacific Northwest National Laboratory in Richland, Washington.

Although the chances that hurricanes will hit regions swamped by freshwater is small at only 10 to 23 percent, the effect can be startlingly large—hurricanes can become up to 50 percent more intense in regions where freshwater pours into the ocean, such as from river systems like the Ganges, or where tropical storms rain considerably, as in the western Pacific Ocean.

(See "Ocean Color Can Deflect Hurricanes, Study Says")

"Our study finds that when hurricanes pass over oceans covered by freshwater from rivers, such as the Amazon and Mississippi River, and rain, they can really intensify rapidly," Balaguru said.

"Knowing more about this effect might give people in places such as New Orleans more time to evacuate or take other vital measures," said Balaguru. "We'd also like to monitor the upper ocean salinity structure in these regions to keep track of how hurricanes might behave."

The scientists detailed their findings online August 13 in the Proceedings of the National Academy of Sciences.

Watch video of how Hurricane Katrina formed, video of the storm's destruction, and see why New Orleans was doomed.

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