Why India should be worried about monsoon drying up

After a sluggish start, the monsoon may have picked up over several parts of the Indian landmass, but it may not be a cause of cheer if we go by emerging behaviour of this annual weather phenomenon. Below-average rainfall, such as the one predicted for this year, may actually be part of a long-term trend of gradual drying up of the monsoon.

One of the main reasons for the weakening monsoon is warming of the Indian Ocean, which in turn is a consequence of pollution caused by greenhouse gases and an increase in the number and magnitude of El Nino events, according to a new study released on Tuesday.

The likely impact of climate change on the monsoon has been a hotly debated subject among scientists. Some studies have suggested that climate change should actually boost rainfall in tropics due to increase in moisture availability because of change in surface temperatures. Another reason for this optimism is evidence that land surfaces in the northern hemisphere are warming much faster than the oceans, which should mean stronger monsoon circulation.

“We have found something different. The land-sea surface temperature difference over South Asia has been decreasing due to rapid warming in the Indian Ocean and a relatively subdued warming – and even cooling in some parts - over the subcontinent. This enhanced warming of the Indian Ocean reduces land-sea temperature difference, dampens monsoon circulation and reduces rainfall over parts of South Asia,” explained Dr Roxy Mathew Koll, who led the study at Indian Institute of Tropical Meteorology, Pune. The study results have been published in journal Nature Communications.

For a normal monsoon, land surface temperature should be warmer and sea surface temperature relatively cooler. Weather data of past hundred years shows a different trend in the Indian sub-continent – the warming of land surfaces is rather subdued while the ocean surface is getting warmer. A warming Indian Ocean results in surplus rains over the ocean at the cost of the monsoon rains over land. The decrease in rainfall is highly significant over central India where agriculture is still mostly rain-fed - ten to 20 per cent drop in mean rainfall.

The surface warming in the Indian Ocean, particularly in the western regions, has reached values of up to 1.2 degree C during the past century – which is much higher than warming of the other tropical oceans.

The decrease in the land-sea temperature difference is also visible in the upper atmosphere, as the warming from the ocean surface is transferred to atmosphere above. In addition to ocean warming, Koll pointed out, suppressed warming over the Indian landmass is creating imbalance in land and sea surface temperatures. The reason for less warmer land surfaces could be increased aerosols or reasons which are still uncertain. Aerosols can reduce incoming solar radiation over the Indo-Gangetic plain, thereby cooling the surface.“Climate models suggest that Indian Ocean will continue to warm under the influence of increasing greenhouse gases. Will the monsoon decrease further? This remains uncertain. The critical role of the warm Indian Ocean deserves special attention for its decisive effect on food security of a large section of the world’s population, and its role in inducing a drought over the Indian subcontinent,” Koll suggested.

The study was a part of India-French collaboration under the National Monsoon Mission, and research team included IITM scientists (Ritika Kapoor, Ashok Karumuri and BN Goswami), Raghu Murtugudde from University of Maryland and French scientist Pascal Terry.