Rapid warming of Indo-Pacific ocean behind decline in rainfall over north India: Study

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Rapid warming of the Indo-Pacific ocean could be the reason behind erratic rainfall patterns with heavy rains pummeling some parts of the world while others, including north India, experiencing declines, a new study has said. The study, led by Roxy Mathew Koll of the Pune-based Indian Institute of Tropical Meteorology (IITM) warned that warming of the ocean has altered the most dominant mode of weather fluctuation originating in the tropics, known as the Madden Julian Oscillation (MJO), which has reduced rainfall in northern India between the months of November and April.

"The rainfall pattern across the globe is likely changing due to rapid warming of the Indo-Pacific Ocean," it said, adding that the changes in the MJO behaviour have increased the rainfall over northern Australia, west Pacific, the Amazon basin, southwest Africa and southeast Asia (Indonesia, Philippines and Papua New Guinea).

"At the same time these changes have brought a decline in rainfall over central Pacific, along the west and east coast of the United States (e.g. California), north India, east Africa, and the Yangtze basin in China," the study said. The study is part of an Indo-US collaboration, between the Ministry of Earth Sciences (India) and the US National Oceanic and Atmospheric Administration (NOAA), facilitated by the US National Academy of Sciences.

The research was conducted in collaboration with other scientists — Panini Dasgupta (IITM), Michael McPhaden and Chidong Zhang (NOAA), Deahyun Kim (University of Washington) and Tamaki Suematsu (University of Tokyo).

The MJO is characterized by a band of rain clouds moving eastward over the tropics. The MJO regulates tropical cyclones, monsoons, and the El Niño cycle—and occasionally contributes to severe weather events over Asia, Australia, Africa, Europe and America, the study said. "The MJO travels a stretch of 12,000–20,000 kms over the tropical oceans, mainly over the Indo-Pacific warm pool, which has ocean temperatures generally warmer than 28°C. This Indo-Pacific warm pool has been warming rapidly and expanding during the recent decades in response to increasing carbon emissions.

"The warm pool expanded double its size, from an area of 2.2×107 square km during 1900–1980, to an area of 4×107 square km during 1981–2018. The rate of expansion is 4×105 square km, an area equal to the size of California, every year," a statement from IITM Pune said. Though the entire Indo-Pacific has warmed, the warmest waters are over the west Pacific, creating a temperature contrast that drives moisture from the Indian Ocean to the west Pacific Maritime Continent, enhancing the cloud formation there. As a result, the lifecycle of MJO has changed, Koll said.

"Climate model simulations indicate that continued warming of the Indo-Pacific Ocean is highly likely, which may further intensify these changes in global rainfall patterns in the future," said Koll, adding "This means that we need to enhance our ocean observational arrays to monitor these changes accurately...and update our climate models to skillfully predict the challenges presented by a warming world."

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