The warming of the Indian Ocean is weakening the monsoon circulation, according to a study led by Roxy Mathew Koll, scientist at the Indian Institute of Tropical Meteorology, Pune, and published in Nature Communications.
Increased warming in the ocean promotes large-scale upward motion of warm, moist air over the equatorial ocean. This leads to surplus rain over the ocean at the cost of land, drying up the subcontinent in the process.
Monsoon Drivers
Land-sea temperature difference and sea surface temperatures are the main monsoon drivers, says Koll. Under the global warming scenario, the monsoon drivers should get stronger, which should result in an increased rainfall.
Previous studies suggested that the land in the northern hemisphere is warming much faster than the oceans.
Also, the warm ocean entails increased moisture availability from increase in evaporation and moisture holding capacity of air. Increased land-sea temperature contrast and moisture availability should hence ideally increase the monsoon rainfall.
Thermal Contrast
However, that is not the case for the Indian monsoon. Why is this so?
The study finds that, contradictory to what earlier studies have noted, the land-sea thermal contrast over the South Asian domain has reduced in the past decades.
The primary reason is warming of the Indian Ocean, especially in the western basin, to much beyond trends in the other tropical oceans.
In contrast, the land has been warming to a lesser degree, possibly due to increased aerosols or reasons not adequately explained.
Using the data from 1901-2012, Koll and co-authors found that the rainfall is decreasing over central South Asia – from south of Pakistan through central India to Bangladesh.
Cooling Land
With an observed reduction in mean rainfall between 10- to 20 per cent, this is highly significant for central India where agriculture is still mostly rain-fed. The Indian landmass is showing some cooling but the Indian Ocean has warmed up much rapidly than other regions, Koll says.
“This has resulted in the weakened land-sea temperature difference, a major monsoon driver, and reduced monsoon circulation/rainfall.”
The study is part of Indo-French collaboration under the National Monsoon Mission setup by the Ministry of Earth Sciences.
Intern Ritika Kapoor and collaborators Pascal Terray, Raghu Murtugudde, Ashok Karumuri and B.N. Goswami contributed to the analysis and research.