Geophys. Res. Lett. 35, L15710 (2008)

Credit: NASA

The frequency and intensity of summer tropical cyclones forming in the north Indian Ocean could increase in the coming century, according to scientists. Tropical storms such as the recent Cyclone Nargis, which made landfall in May and caused thousands of fatalities, are common in spring and late fall. But the onset of the summer monsoon system has usually prevented summertime tropical storm activity.

V. Brahmananda Rao at the Center for Weather Forecast and Climate Studies, São Paulo, Brazil, and colleagues assessed the causes of increased summer tropical-storm activity over the north Indian Ocean, which saw its first-ever category 5 hurricane in June 2007. They found a strong statistical link between increasing numbers of storms and the weakening of the Tropical Easterly Jet associated with the summer monsoon. The decline of the jet resulted in decreased wind shear in the atmosphere above the ocean, creating a prime breeding ground for violent storms, say the scientists.

Greater summer storm activity poses a hazard to millions of people living in low-lying deltas and coastal plains surrounding the Indian Ocean, especially those already inundated with monsoonal rains and floods.