There is a lot of heated debate going on over the consequences of global warming, due to anthropogenic activities, on climate, plant and animal life on planet earth. Extinction of species and rising sea levels are but two of the possible consequences being discussed in scientific circles. In the light of climate change due to the release of 'greenhouse' gases into the atmosphere and the recent devastating cyclone Nargis which caused unimaginable havoc in Myanmar, one question which comes to one's mind is: What will be the effect of global warming on cyclonic activity, particularly in the Bay of Bengal, in the 21st century?
Dr. M.R. Ramesh Kumar, Scientist & Deputy Director, Physical Oceanography Division, National Institute of Oceanography, Goa, led a study along with colleagues to answer just this question and came up with an interesting answer: cyclones in the Bay of Bengal will be fewer, but those that do form will be of a greater intensity.There is a concern that the enhanced greenhouse effect May be affecting extreme weather events such as tropical cyclone frequency, duration and their intensity. The north Indian Ocean offers a reliable long-term record of tropical cyclone activity though it May not be representative of the rest of the tropical regions. There is substantial scientific evidence that the enhanced greenhouse effect is predominantly of anthropogenic origin. There have been many suggestions based on global circulation and theoretical modelling studies that increases May occur in tropical cyclone frequency.
In order to look into the global warming scenario, Dr. Ramesh Kumar and his team analyzed a long term National Centre for Environmental Prediction/National Centre for Atmospheric Research reanalysis dataset of the tracks of depressions, storms and severe cyclonic storms in the north Indian Ocean for the past 100 years. They further divided the 100 years as two epochs, one from 1901 to 1950 as normal conditions (epoch I) and the period 1951 to 2006 under the influence of global warming conditions (epoch II) to see whether there are any trends in their frequency. The study has not yet been published in a peer-reviewed journal.They restricted their present study to Bay of Bengal only as the number of cyclones which form in the bay is several times higher than that which form in the Arabian Sea.They also looked at the total number of cyclones on a yearly basis rather than individual months as they were more interested in looking at the role of the global warming on their formation.
The study clearly shows that the total number of depressions over the Bay of Bengal is showing an increasing trend from 1901 to 1950. The number, which was 3 per year, rose to about 11 in 1950. However, the epoch II clearly showed a decreasing trend — from 11 to about 3. The analysis was repeated with the number of storms and severe cyclonic storms to check for any significant differences in their formation and trends during the above two epochs. While there was only a marginal decrease in the number of the storms during epoch I, there was a substantial decrease from two storms per year to almost one per year during epoch II.
Decreasing trend
In the case of intense or severe cyclonic storms, a small decreasing trend was seen for epoch I. However, epoch II showed a small increasing trend.For cyclone genesis and sustenance, low level relative vorticity (at 1.5km; 850 hecta Pascals) and mid-tropospheric relative humidity at 500 hPa (hecta Pascals) should be high and the vertical wind shear (difference of the 850 hPa wind and 200 hPa wind) should be low. Dr. Kumar said, "Our study clearly showed that the mid tropospheric humidity showed a substantial decrease from 1951 to 2006. Thus, we feel that this decrease in the mid tropospheric humidity in epoch II May be a major factor for the decrease in the number of depressions and storms in the Bay of Bengal. "Whereas relative vorticity and vertical wind shear between the lower troposphere (i.e. at 850 hecta pascals) and upper troposphere (i.e. at 200 hecta pascals) show a small increase and decrease respectively, which are conducive for cyclogenesis which could be the reason for the slight increase in the number of severe cyclonic storms in the Bay of Bengal."The important question now arises: what is the role of increased sea surface temperatures (SST) on cyclone formation, intensity in the 21st century? Dr. Kumar is of the opinion that even though many believe that rising SSTs May lead to more cyclones and more cyclones of greater intensity, if most of the parameters are not conducive then even in a warm oceanic environment cyclones cannot form.
The Hindu, June 12, 2008
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