‘Atmospheric Rivers caused 70% of India's floods between 1985 and 2020’

K Praveen Kumar | PTI

Thiruvananthapuram: The devastating floods that occurred in the country between 1985 and 2020 during the summer monsoon season were directly associated with Atmospheric Rivers, a phenomenon of a stream of water vapour moving in the sky like a river flowing on the land, says a new study. It says severe weather events like the 2013 Uttarakhand floods and the 2018 floods in Kerala that claimed several lives were all due to severe Atmospheric Rivers (ARs).

The study, jointly conducted by climate scientists from the Indian Institute of Technology (IIT) Gandhinagar, the National Institute of Technology, Srinagar, and the University of Washington, says a warming climate is increasing the moistureholding capacity of Atmospheric Rivers, which leads to concerns about more devastating floods in the future. "We examined landfalling ARs during the summer monsoon, which has implications for water resources and deleterious flood hazards in India since they can deliver enormous amounts of precipitation within a few hours or days," the study quoted.

The research team used the highresolution atmospheric fields from the European Reanalysis Version, along with observed precipitation from the India Meteorological Department and a historical flood database from the Dartmouth Flood Observatory of the University of Colorado, USA, to study the impact of ARs formed during the summer monsoon season on flooding in India, a first-of-its-kind comprehensive study in this regard in India. The study was published in the latest issue of Communications Earth and Environment journal.

"We found that India experienced 596 major AR events from 1951 - 2020. More than 95% of these ARs occurred during the summer monsoon season, i.e., between June and September," Prof. Shanti Shwarup Mahto, lead author from IIT, Gandhinagar, told PTI. He said out of one-third of the top AR events, 54% occurred in the recent three decades, i.e., between 1991 and 2020, indicating a direct link between severe ARs and rising global temperatures.

"The frequency and severity of ARs show an increasing trend in India in recent decades. Daily and sub-daily precipitation extremes have increased in recent decades and are likely to rise as the climate continues to warm," the report said.

Seven out of the 10 floods with the highest mortality during 1985-2020 were associated with ARs. These floods caused the loss of more than 9,000 human lives and several billion dollars worth of damages and displacements. "Overall, 70% of India's major flood events in the summer monsoon season were directly associated with ARs during the 1985–2020 period," it says.

The report says the 2013 Uttarakhand floods, which claimed 6,000 lives, the 2007 floods in South East Asia, including India, with 2,000 human lives lost, the 1988 floods in Punjab, the 2018 floods in Kerala that claimed 400 lives, the 2006 floods in Gujarat, the 1993 floods in Assam, and the 2004 floods that caused extensive damage in Eastern India and Bangladesh were all due to severe ARs. "The ARs are narrower and carry more moisture. The precipitation levels of these ARs are more in Western Ghats and Eastern Ghats regions, causing extreme rain events," Mohto said.

The report also considered extreme daily precipitation to have occurred when the rainfall for a particular day exceeded 1 mm of rainfall. The study has indicated that South Asian monsoon system is projected to transport more moisture under a warming climate, and that may lead to an increased frequency of ARs making landfall in India.

"Warmer sea surface temperatures over the south-central Indian Ocean play a crucial role in the development of ARs. Evaporation from the Indian Ocean has significantly increased in recent decades due to an increase in Vapour Pressure Deficit (VPD).

VPD is the measurement of pressure required to convert liquid to vapour." "The frequency of ARs and floods caused by them has increased recently as the climate has warmed," the report said. With the Indian Ocean warming up faster, there can be a considerable increase in evaporation, which could lead to more severe ARs, the study indicated.

"More warming, more evaporation, and the capacity of the atmosphere to hold more moisture increase. These are all proven facts. This is the first time we have looked at the connection between ARs and flooding in India. More needs to be studied regarding the impact of global warming on ARs," Dr. Vimal Mishra, co-author and professor at IIT, Gandhinagar, told PTI.