Scientists link upward trend in pollution to increased intensity of Arabian Sea tropical cyclones

November 28, 2011

NASA MODIS image of Tropical Cyclone Gonu (June 4, 2007).

NASA MODIS image of Tropical Cyclone Gonu (June 4, 2007).

Download here. (Credit: courtesy of NASA)

A 'brown cloud' of pollution over the Indian Ocean resulting from human activities has led to stronger tropical cyclones in the Arabian Sea, according to an international team of scientists.

The findings appear in a paper published this month in the journal Nature titled Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols. The research was conducted by scientists from the University of Virginia, NOAA’s National Climatic Data Center (NCDC) in Asheville, N.C., the Gwangju Institute of Science and Technology, Gwangju, South Korea and the Scripps Institution of Oceanography in La Jolla, Calif.

“We’re showing that pollution from human activity as simple as burning wood or driving a vehicle with a diesel engine can actually change these massive atmospheric phenomena in a significant way,” said study lead author Amato Evan, an assistant professor in the Department of Environmental Sciences at the University of Virginia. “It underscores the importance of getting a handle on emissions in the region.”

While water temperatures in the Arabian Sea are typically warm enough to allow tropical cyclones to form and develop, winds moving at different speeds and directions at different levels in the atmosphere, called wind shear, have limited the strengthening of cyclones in the region. During the past 30 years, however, increased concentrations of airborne particles, or aerosols, in South Asia have altered the pattern of the sun’s heating of the ocean. This has changed the regional wind patterns and weakened the wind shear, making conditions more favorable for intense tropical cyclone development.

The scientists used both observations and models to demonstrate the relationship between decreasing wind shear and the growth of the Atmospheric Brown Cloud, a thick layer of pollution over the North Indian Ocean caused by human emissions of aerosols like black carbon and sulfates. The team then linked the reduced wind shear to an increase in the number of highly intense storms with winds over 120 mph, including five storms since 1998 that have killed more than 3,500 people and caused damages of more than $6.5 billion.

“The research shows that pollution can threaten humans in unexpected ways. In this case, by reducing wind shear in the Arabian Sea and making conditions more favorable for tropical cyclones to intensify,” according to James Kossin, climatologist at NCDC and co-author on the paper.

NOAA researchers are engaged in understanding and assessing changes in climate across many regions of the world since regional climate is influenced by global conditions and patterns. Lessons learned from studying tropical cyclones and their relationship to climate in various regions of the world, including the Indian Ocean, are relevant for improving understanding of hurricanes that directly threaten the United States.

The other co-authors of the study are Chul (Eddy) Chung of the Gwangju Institute of Science and Technology and Veerabhadran Ramanathan of the Scripps Institution of Oceanography.

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