Natural differences in wind speed and direction over different heights in the atmosphere, known as 'wind shear', normally keep cyclones in check — effectively tearing the storms apart before they reach a certain size.
But emissions from sources such as biomass burning and diesel vehicles have interfered with wind patterns, reducing wind shear and enabling cyclones to grow twice as intense, according to a study published in Nature last week (3 November).
Researchers compared cyclones that occurred between 1979 and 1996 with those between 1997 and 2010. They found that the more recent ones were up to three times more intense, with higher wind speeds — and five of the strongest storms during the period occurred after 1998. Meanwhile, wind shear dropped in this latter period.
Aerosol emissions in the region have grown six-fold since the 1930s, creating a three kilometre-thick layer of pollution over the Indian Ocean, known as the South Asian atmospheric brown cloud, which absorbs sunlight, causing the ocean to cool and affecting wind circulation.
"I would say that the effect on cyclones is very strongly linked to the well known effect of these aerosols on the monsoon circulation and rainfall; that is, their propensity to weaken the monsoon circulation and reduce rainfall," Amato Evan, lead author of the study and a researcher at the University of Virginia, United States, told SciDev.Net.
"We are 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. It underscores the importance of getting a handle on emissions in the region," said Evan in a press release.
"If you live in an area where these very strong cyclones can make landfall, this effect [the destruction due to the cyclone] is absolutely significant," Evan told SciDev.Net. "The historical tropical cyclones in the Arabian Sea that were very intense made landfall in India, Pakistan, Oman, and Iran, in each case with tremendous destruction and loss of life."
Ainun Nishat, vice chancellor of BRAC University, Bangladesh, said that the number of disasters caused by cyclones outside the monsoon period in South Asia has increased significantly in recent years.
"The intensity of cyclonic wind in the Bay of Bengal has increased a lot, which we noticed in cyclones such as Sidr and Aila, both of which caused a lot of destruction."
Nature doi:10.1038/nature10552 (2011)