‘Warming Indian Ocean reduces monsoon rains’
The Indian Ocean is warming faster than other deep seas
This temperature rise decreases monsoon rains in the subcontinent
Sri Lankan data shows a seven per cent drop in rainfall over 50 years
Annual average rainfall over Sri Lanka has decreased by seven percent in the 1961—1990 period compared to the 1931—1960 period, according to the department of meteorology’s October 26 bulletin. High variability of annual rainfall is reported from the Batticaloa, Kurunegala, and Rathnapura areas and the bulletin says this may ‘probably be due to global climate change with the increase of greenhouse gases in the atmosphere.’
“Rainfall has decreased, but the intensity of rains has increased,” L. Chandrapala, chief of the department of meteorology tells SciDev.Net. “The next step is making sure that we have appropriate policies in place to face these changes.”
The Sri Lankan findings are consonant with a study on the warming of the Indian Ocean conducted by the Indian Institute of Tropical Meteorology (IITM) and published in the Bulletin of the American Meteorological Society in August. According to the IITM study, rainfall is decreasing from southern Pakistan through central India into Bangladesh. The decrease is highly significant over central India where agriculture is still mostly rain-fed, with a reduction of up to 10—20 per cent in mean rainfall,’ the study found.
“The surface warming in the Indian Ocean, especially that in the western regions, has reached values of up to 1.2 degrees during the past century — much larger than the warming trends in the other tropical oceans,” says Roxy Mathew Koll, IITM scientist and leader of the study. “The land-ocean temperature difference, which is an essential factor driving the monsoon, has also decreased due to the enhanced ocean warming,”
Koll says warmer oceans meant that there was warmer air above them, which could hold more moisture for a longer time. “Warmer oceans have ensured more rains over the oceans, but at the same time resulted in dry air over the Indian subcontinent. Drier, warmer air can hold moisture for a longer time, which results in long dry spells of rainless days intermittent with heavy rainfall events.”
According to Koll there was an increased frequency of drought over the region and by some estimates a third of the last 15 years had been categorised as drought.
Koll says more studies are needed to assess future impact. “The critical role of the warm Indian Ocean deserves special attention for its decisive effect on the food security of a large fraction of the world’s population, and its role in inducing a drought over the Indian subcontinent.”
This piece was produced by SciDev.Net’s South Asia desk.