Phys.org: An analysis of a decade’s worth of tropical cyclones shows that when hurricanes blow over ocean regions swamped by fresh water, the conditions can unexpectedly intensify the storm.

Although the probability that hurricanes will hit such conditions is small, ranging from 10 to 23 percent, the effect is potentially large: Hurricanes can become 50 percent more intense, researchers report in a study appearing this week in Proceedings of the National Academy of Sciences Early Edition.

These results might help improve predictions of a hurricane’s power in certain regions. Such conditions occur where large river systems pour fresh water into the ocean, such as by the Amazon River system, the Ganges River system, or where tropical storms rain considerably, as in the western Pacific Ocean. “Sixty percent of the world’s population lives in areas affected by tropical cyclones,” said ocean scientist Karthik Balaguru at the Department of Energy’s Pacific Northwest National Laboratory. “Cyclone Nargis killed more than one hundred and thirty eight thousand people in Burma in 2008.

We can predict the paths cyclones take, but we need to predict their intensity better to protect people susceptible to their destructive power.” Most hurricanes passing over the ocean lessen in strength as the ocean water cools off due to mixing by the strong winds under the cyclone: this pumps less heat into them. However, Balaguru, his PNNL colleagues and researchers led by Ping Chang at Texas A&M University and Ocean University of China in Qingdao, China found that when enough fresh water pours into the ocean to form what they call a barrier layer, typically about 50 meters below the surface, the ocean water can’t cool as much and continues to pump heat into the cyclone.

Instead of dying out, the storms grow in intensity by 50 percent on average. A rough estimate for the destruction wreaked by a hurricane is the cube of its intensity. “A 50 percent increase in intensity can result in a much larger amount of destruction and death,” said Balaguru.