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Ocean warming, Bay of Bengal cyclones

K.S. RAJGOPAL

While global warming has resulted in increasing sea-surface temperatures and fears that there will be an increase in frequency and intensity of tropical cyclones, it has been found that atmospheric factors also play an important role. Studies have shown that apart from ocean warming, atmospheric parameters like decreasing vertical wind shear have given rise to the increasing frequency and intensity of tropical Atlantic Ocean cyclones.

As far as the Bay of Bengal is concerned, a recent study by National Institute of Oceanography, Goa, scientists led by Dr. M.R. Ramesh Kumar, Senior Scientist, Physical Oceanography Division, and published in the Indian Journal of Geo-Marine Sciences, clearly indicates that warm sea surface temperatures alone are not sufficient for the initiation of convective systems over the Bay of Bengal.

According to Dr. Ramesh Kumar, other environmental parameters, such as the low-level relative vorticity (the rotational flow of winds), the mid-tropospheric relative humidity (the troposphere extends from sea-level to an altitude of 15 Km) and the vertical wind shear (the resultant of mutually opposing winds — one, at 1.5km above sea level and the other, at 12 km above sea level), also play an equally important role in their genesis and intensification. While high values of mid-tropospheric relative humidity and low-level relative vorticity are conducive to cyclone formation, a high value of vertical wind shear has the opposite effect.

In order to focus on the effects of global warming the study period was divided into two parts — 1951-1978 (epoch I) and 1979-2007 (epoch II) and the frequency and intensity of cyclones which formed in the Bay of Bengal were examined statistically. Bay of Bengal was chosen because of the higher frequency of convective systems forming in that region.

In the study, it was found that mid-tropospheric relative humidity and relative vorticity were decreasing throughout epoch II and were therefore not conducive for cyclone formation. It was found that there is no direct relationship between the intensity of storms or severe cyclonic storms with the sea surface temperatures over the Bay of Bengal.

The frequency of storms and severe storms in the two epochs were considered. It was seen that the number of storms and severe storms have decreased largely in the second epoch over the Bay of Bengal. Though the sea surface temperatures were higher in the second epoch, the number of storms decreased in the second epoch leading Dr. Ramesh Kumar to say to this Correspondent: “Rising sea-surface temperatures alone cannot give rise to cyclones over the Bay of Bengal. If that were so, we would be having a cyclone every other day, given the present rate of global warming. The atmosphere plays an equally important role.”

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