Two weeks ago, cyclone Laila tore into Andhra Pradesh. Now there is cyclone Phet in the Arabian Sea.
When the India Meteorological Department (IMD) announced that the monsoon had set in over Kerala on May 31, it also noted the development of a depression in the Arabian Sea. By next day, the storm had intensified into a cyclone and been given a name. On June 2, cyclone Phet grew ominously, becoming a ‘very severe cyclonic storm' and even threatening to turn into a ‘super cyclone,' the most powerful sort of storm.
(The IMD mow forecasts that the storm will cross the Oman coast and weaken. It would then curve back onto the sea and head off towards Pakistan.)
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There have been cyclones before at this time of year in the Arabian Sea, pointed out J. Srinivasan, chairman of the Divecha Centre for Climate Change at the Indian Institute of Science in Bangalore. “But I don't recall that any of them intensified this rapidly.”
One important reason could be that the sea surface temperatures in the Arabian Sea are exceptionally high this year.
Much of the central and northern Arabian Sea is one degree Celsius to 1.5 degrees Celsius warmer than usual, observed P.N. Vinayachandran of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science.
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There was one small patch of ocean where the temperature anomaly was two degrees Celsius. On June 1, the cyclone appeared to be “sitting on that unusually warm patch.” That could be why this cyclone was able to grow as quickly as it did, he added.
A cyclone is basically driven by evaporation of water from the ocean surface. “What a cyclone does is continuously extract energy from the ocean” for a few days, said Dr. Vinayachandran. The warmer the ocean and deeper its warm surface layer of water, more is the energy available to the cyclone to develop and intensify.
It is not clear why the waters of the Arabian Sea became so warm this year, said Dr. Srinivasan. He speculated that it might be linked to changes in global atmospheric circulation that produced an unusually hot summer in India.
Differences in wind speed or direction between the lower and upper atmosphere, known as wind shear, can hamper cyclone formation. Such wind shear would currently be low as the monsoon has not yet set in properly, aiding the development of a cyclone.
Cyclones also require sufficient humidity at a height of about 6 km to develop, observed M.R. Ramesh Kumar of the National Institute of Oceanography at Goa. In the case of cyclone Phet, monsoon winds could have brought in the moisture it needed.
With climate change and the expected warming of the planet, scientific studies indicate cyclones may not increase in frequency and could even decline. But the cyclones that do develop are more likely to intensify and become more dangerous.