Airborne pollution from South Asia is helping to brew monster storms in the Arabian Sea that have claimed thousands of lives and cost billions of dollars, scientists said on Wednesday.
In a paper published in the British journal Nature, researchers pointed the finger at a haze known as the Asian brown cloud, which hangs over parts of the northern Indian Ocean, India and Pakistan.
Several kilometres (two miles) thick, the cloud comprises brownish particles of carbon soot and sulphates spewed by factories, diesel exhaust and poorly-burnt biomass. Previous research has implicated it in disrupting monsoon patterns and in glacier loss in the Himalayas.
Environmental scientists led by Amato Evan of the University of Virginia looked at patterns in cyclones in the Arabian Sea from 1979 to 2010.
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.
As a result, the few cyclones that occurred in the Arabian Sea typically happened before or after the monsoon season -- usually one in May/June and a couple more in August to December -- when the wind shear was far less.
But in the last dozen or so years, the pattern has changed, with the emergence of storms 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.
The team says "brown cloud" particulates have grown six-fold in volume since the 1930s and the pollution is now a disruptive climatic phenomenon in its own right.
Its dark colour absorbs sunlight, making it a source of heat and causing a cooling of the ocean below -- which in turn affects wind circulation and the transport of warmth from the sea to the atmosphere.
Drawn from direct observations and computer models, the conclusion is that the cloud weakens wind shear, essentially lifting the brake on cyclone development.
"This study is a striking example of how human reactions, on a large enough scale -- in this case, massive regional air pollution caused by inefficient fuel combustion -- can result in unintended consequences," said Anjuli Bamzai of the US National Science Foundation.
"These consequences include highly destructive summer cyclones that were rare or non-existent in this monsoon region 30 or so years ago."
Arabian Sea cyclones could be particularly hazardous because they are born in a small area of ocean compared to the tropical western Atlantic and the western Pacific, the study warned.
"Given the relatively small size of the Arabian basin, more than half of all cyclones that form here make landfall, and even weak landfalling Arabian Sea cyclones can cause considerable destruction and loss of life."