A satellite image of Gonu from June 2007, the largest recorded cyclone ever to have formed in the Arabian Sea. New research suggests pollution from South Asia could be contributing to more intense cyclones in the region.
Credit: NASA/ Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center.
PARIS: Clouds of aerosol pollution from South Asia are helping to brew monster cyclones in the Arabian Sea that have claimed thousands of lives and cost billions of dollars, new research suggests.
In a paper published in the journal Nature today, scientists pointed to a haze known as the Asian brown cloud, which looms over parts of the northern Indian Ocean, India and Pakistan.
Roughly 3 km thick, the cloud comprises brownish particles of carbon soot and sulphates spewed by factories, diesel exhaust and poorly burnt biomass. Previous research has implicated the cloud in disrupting monsoon patterns and in glacier loss in the Himalayas.
Environmental scientists led by Amato Evan of the University of Virginia in the U.S. looked at patterns in cyclones in the Arabian Sea from 1979 to 2010.
"In addition to the multitude of known human health impacts associated with aerosols that comprise the [Asian brown cloud], we suggest that the increasing intensity of landfalling tropical cyclones is a consequence of regional emissions of pollution aerosols," the authors wrote.
And because these tropospheric aerosols have a relatively short residence time, they suggest that decreasing emissions should have a "nearly immediate effect on the propensity of pre-monsoon tropical cyclones to reach their maximum potential intensity."
Altered vertical wind shear
They found the region historically only averaged two or three cyclones a year and these typically were weak - even though the sea was clearly hot enough to fuel very powerful storms.
The reason for the weakness and infrequency, they discovered, lies in a phenomenon called vertical wind shear, which occurs in July and August during the hot months of the monsoon season.
Vertical wind shear occurs when strong winds flow in the upper and lower atmosphere in opposite directions. In the lower levels, it blows from the southwest, and in the upper atmosphere, from the east. The shear rips the top off a would-be cyclone, preventing it from developing the circular winds that are its muscular hallmark.
Changing storm pattern
As a result, the few cyclones that occurred in the Arabian Sea typically happened before or after the monsoon season - usually one in May or June and a couple more in August to December - when the wind shear was far less.
The pattern has changed in the last dozen or so years with the emergence of storms coming in the weeks immediately before the monsoon season. They include a cyclone that killed nearly 3,000 people in Gujarat, India, in June 1998.
In June 2007, cyclone Gonu, a category five storm, killed 49 people in Oman and Iran, causing more than four billion dollars in damage. It was the first documented storm ever to enter the Gulf of Oman. And in June 2010, 26 people were killed in Pakistan and Oman by a category-four cyclone, Phet, inflicting losses of nearly two billion dollars.
