Summer monsoon rainfall over central India has reduced by 10% on average, while there has been three-fold increase in extreme rainfall events between 1950 and 2015. There will be further increase in extreme rainfall events over most parts of the Indian subcontinent by the end of the century.

There has been an average 10% decline in summer monsoon (June to September) rainfall over central India between 1950 and 2015 as a result of weakening of the summer monsoon winds. However, the frequency and intensity of extreme rainfall (more than 150 mm per day for two-three days) events during the same period over central India (from Gujarat in the west to Odisha and Assam in the east) has been on the rise.

There has been a three-fold increase in widespread extreme events over central India during 1950-2015. In the 1950s, there were two extreme rainfall events per year, while in recent years the number of events has increased to six per year. There have been 268 flooding events in India during 1950-2015. These results were published in the journal Nature Communications.

Models suggest further warming of the Arabian Sea and adjoining areas and further increase in extreme events over most parts of the Indian subcontinent by the end of the century.

What causes extreme rainfall

“The weakening of the monsoon winds has resulted in less supply of moisture to the Indian subcontinent. The warm ocean temperatures in the northern Arabian Sea result in large fluctuations in the monsoon winds leading to occasional surges of increased moisture transport. These sudden surges of the monsoon winds brings in plenty of moisture and that is what is causing extreme rainfall events across the central Indian belt,” says Dr. Roxy Mathew Koll from the Centre for Climate Change Research at Pune’s Indian Institute of Tropical Meteorology (IITM) and the first author of the paper.

The Arabian Sea contributes 36% of the total moisture to central India, while the Bay of Bengal contributes 26% and the Indian Ocean contributes 9%. Land evotranspiration contributes a surprising 29%!

While the central Indian Ocean has warmed up, the Indian peninsular region has not warmed up compared to other regions in the tropics leading to reduced land-sea temperature difference. “Probably the cooling caused by aerosol and the reduced land-sea temperature difference in recent years is what is causing the weakening of the monsoon winds and decline in monsoon rainfall. There are large-scale daily fluctuations in the monsoon wind over central India,” Dr. Koll says.

At the same time, the northern Arabian Sea is becoming increasingly warm leading to more moist air over the Arabian Sea. In addition, the northern Arabian Sea gets warmer (1-2 degrees C) 2-3 weeks prior to extreme events. As a result, there is 20-40% more evaporation and increased moisture levels over the Arabian Sea before an extreme event. This gets transported over central India resulting in extreme rainfall events.

Arabian Sea — main contributor of moisture

While earlier studies have indicated the Bay of Bengal and central Indian Ocean as the source of moisture for extreme events over central India, the latest study by Dr. Roxy and others indicates otherwise. The weak low-pressure system over the Bay of Bengal does not become strong moisture-carrying cyclonic storms. Increased warming of the central India Ocean has increased the moisture level over the Ocean but has not resulted in increased rainfall over central India.

“The Arabian Sea supplies more moisture to the extreme rainfall events than the Bay of Bengal and the central Indian Ocean combined,” says Prof. Subimal Ghosh from the Department of Civil Engineering, Indian Institute of Technology (IIT) Bombay and one of the authors of the paper. The study found that the Arabian Sea contributes 36% of the total moisture to central India, while the Bay of Bengal contributes 26% and the Indian Ocean contributes 9%. Prof. Ghosh had used the Dynamic Recycling model to find the source of moisture as well as its residence time.

Role of land evotranspiration overlooked

“Interestingly, land evotranspiration contributes 29% moisture, which is much more than even the Bay of Bengal. Moisture from land evotranspiration is often neglected in monsoon studies,” says Prof. Ghosh. “Even for summer monsoon rainfall in central India, the average moisture contribution from land evotranspiration is 25%.”

Published in The Hindu on October 3, 2017