Warming ocean alters monsoon

THIRUVANANTHAPURAM: What does global warming portend for the monsoon that sweeps over India during the four-month period from June to September sustaining life in the subcontinent?  Discouraging signals have already started becoming visible, according to the former Director of India Meteorological Department (IMD) P.V. Joseph.  "A new finding is that the sea surface temperature of the equatorial Central Indian Ocean [south of Sri Lanka] has had a phenomenal increase of about 1.5 degrees Celsius during the last 50 years…This might weaken the monsoon heat engine [the vertical Monsoon Hadley Cell that drives the monsoon circulation over the subcontinent]," Dr. Joseph said at a seminar on global warming, organised by the State Forest Department, here on Tuesday.  He said that greater heating of the sea surface in Central Indian Ocean would create an area of increasing rainfall in the ocean near the equator.  Latent heat released through the condensation of moisture would heat up the atmosphere over that part of the ocean tampering with the seasonal pressure gradient that pulled the moisture-laden winds towards the subcontinent, Dr. Joseph said.  He said the number of depressions [that form over the Bay of Bengal] during the monsoon season had come down in the recent decades. Monsoon rainfall over south Kerala, the gateway of the Indian monsoon, too was on the decrease.  The strength of the low level monsoon winds over peninsular India had fallen by nearly 20 per cent over the last 50 years.  Dr. Joseph said the data for the last 100 years showed that, while rainfall in certain meteorological divisions such as Kerala was on the decreasing trend, in certain other regions such as Maharashtra it was on the increase.  Occurrences of very heavy one-day rainfall over the subcontinent and severe tropical cyclones in the seas of the region too were on the increase, he said.  Analysing meteorological data for the last 100 years, he said: "The observed change in climate has been in two ways: Changes noticeable across decades (a few decades of increasing rainfall followed by a few decades of decreasing rainfall, in a sort of multi-decadal oscillation) and long term trends indicating either increase or decrease in rainfall [in specific meteorological divisions]," Dr. Joseph said.

 The Hindu, 21st January 2009