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 August 07, 2013
Global warming has dried up monsoon over past decades

 While global warming continues to be debated in various circles, latest research has proved that in the past few decades, global warming has "effected a drying up of Indian monsoon" but has anchored rainfall increase over the tropical western Pacific Ocean. This, researchers claim, has increased the fresh water content of the ocean and also increased the sea surface temperature.

Researchers from International Pacific Research Centre (IPRC), University of Hawaii, and Indian Institute of Tropical Meteorology (IITM), lead by H Annamalai, used computer models and performed experiments to study the effect of increase in sea surface temperature on Indian monsoon.

Highlighting the study, Annamalai said, "Various observations have shown that the Indian monsoon has weakened by around 5-6 per cent over the past few decades. Also, there has been an increase in the instances of rainfall over the west Pacific ocean. In fact, India has not observed any strong rainfall activity (e.g. 10 per cent above its climatological seasonal mean rainfall) since the monsoon of 1994," he said.

He said unprecedented concentration of greenhouse gases being injected into the atmosphere has been observed during the same time period.

While the monsoon over the country has been drying up, rainfall activity over the tropical western Pacific ocean has been rising steadily. "Measurement of sea surface salinity (salt amount) of the west Pacific oceanic region has been showing a steady decrease. This is possible because of more fresh water being introduced in the region due to increased in situ rainfall," he said.

Correspondingly, there has been a steady increase in the sea surface temperature over the same region.

Computer model simulations were designed to study the east-west shift in the monsoon rainfall.

Talking about the model, Annamalai said the simulations showed that the shift can be explained only when information about the greenhouse gas concentration was fed in the model simulations. "Carefully designed experiments with computer model confirm that the ocean temperature rise, in particular, over the tropical western Pacific is responsible for the observed east-west shift in monsoon rainfall. Within the current limitations in observations and computer model simulations, our study paints a coherent picture that increase in ocean temperatures, perhaps induced by increase in greenhouse gas concentrations, may be responsible for the weakening tendency in monsoon rainfall," he said.