Days after India Meteorological Department (IMD) downgraded its southwest monsoon forecast for 2015, a study shows that rapid warming of the Indian Ocean is responsible for reduced rainfall over parts of South Asia during the past century.
The study, led by Indian Institute of Tropical Meteorology scientist Roxy Mathew Koll, used data from 1901 till 2012 and found a decreasing trend in summer monsoon rainfall over the central Indian subcontinent. While rainfall decreased over the region from south Pakistan up to Bangladesh, central India saw a significant reduction of up to 10 to 20 per cent in mean rainfall.
The findings of this study contradict previous studies that had shown a warmer ocean and increased land-sea temperature difference would lead to a stronger Indian monsoon.
“The changes in the Indian Ocean and correspondingly in the monsoon became prominent since the 1950s,” says Koll. “The trends have been steady since though there are decadal variabilities also.”
Koll and his team found that land-sea temperature difference, a key monsoon driver, has actually reduced over the South Asian region because the ocean has warmed much faster. During the past century, the ocean surface temperatures of the Indian Ocean have risen by up to 1.2°C, much larger than the warming trends in other tropical oceans. At the same time, the Indian subcontinent land mass has witnessed “subdued” warming due to reasons which have not yet been established.
Koll says these findings are typical of the Indian Ocean. “The land-sea temperature difference is increasing everywhere in the northern hemisphere, except in the Indian Ocean-South Asian domain,” he adds.
The study explains that ocean warming also affects monsoon circulation. A warmer ocean sees large-scale upward motion of moist air. This is compensated by subsidence (downward movement) of dry air over the subcontinent, resulting in surplus rains over the Indian Ocean at the cost of the monsoon rains over land.
The study was published in Nature Communications journal on Tuesday. Results of the study have wider implications for food security in the Indian subcontinent as agriculture is still largely rain-fed.
Climate models show that the Indian Ocean will continue to warm and Koll warns the threat of anthropogenic warming is manifesting itself closer home.
“We need to be as watchful of the changes in the Indian Ocean as we are about other oceans and land-atmosphere systems. This is a global issue linked to greenhouse gas emissions and needs to be tackled at all possible levels,” he says.
If the southwest monsoon is deficient yet again this year, Indian farmers are headed for their fifth consecutive crop damage and an unprecedented agrarian crisis.