Pollution may be strengthening Asian cyclones

Pollution may be strengthening Asian cyclones

Sooty brown cloud cools water and lowers wind speeds, study finds

By Janet Raloff
Web edition : Wednesday, November 2nd, 2011

A large and growing brown cloud of persistent air pollution hovering over northern India and surrounding regions has doubled — and occasionally tripled — the intensity of late spring cyclones in the Arabian Sea during the past three decades.

Within the past decade, several notable early-season tropical cyclones have ripped through the region. Gonu, the strongest, smashed through the Middle East in 2007, killing dozens and causing more than $4 billion worth of damage. “This supercyclonic storm was Katrina-like in size and intensity,” says climate scientist Amato Evan of the University of Virginia in Charlottesville.

These big storms, which invariably make landfall, represent a new environmental impact that can wreak havoc on people from northern India through the Middle East, Evan and his colleagues propose in the Nov. 3 Nature.

Warming sea-surface temperatures can boost the intensity of hurricanes, known in the Indian Ocean as cyclones. And for many decades, Evan says, water in the Arabian Sea has been “really toasty.” But winds in the upper and lower atmosphere there tend to blow briskly in opposing directions, in a phenomenon known as vertical wind shear. “This is the most hostile environment one could imagine for a hurricane,” Evan explains. “It literally tears a storm apart.” That appears to explain why major cyclones here were rare — until recently.

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