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Marine heat waves over Indian Ocean are reducing monsoon rain over central India

Published on Feb 01, 2022 10:57 PM IST

The marine heatwaves in the western Indian Ocean and the Bay of Bengal are leading to drying conditions over the central Indian subcontinent

Climate model projections suggest further warming of the Indian Ocean , which will very likely intensify the marine heat waves (MHW) and their impact on the monsoon rainfall. (AFP File Photo)
ByJayashree Nandi

NEW DELHI: The rise in marine heat waves (periods of extremely high temperatures in the ocean) over the Indian Ocean is leading to reduction in monsoon rainfall over Central India but an enhancement of monsoon rain over the South Peninsular region, according to a new paper.

The research paper by the Centre for Climate Change Research at the Indian Institute of Tropical Meteorology was published in the Journal of Geographical Research—Oceans on Tuesday.

The western Indian Ocean region experienced the largest increase in marine heatwaves (MHW) at a rate of about 1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.5 events per decade during 1982–2018, the paper said adding that the western Indian Ocean had 66 MHW events while the Bay of Bengal had 94 MHW events during the 36-year period.

Of the 66 events over the western Indian Ocean, 21 were during monsoon months (June to September) while 34 of the 94 MHW events over Bay of Bengal.

“The marine heatwaves in the western Indian Ocean and the Bay of Bengal are found to result in drying conditions over the central Indian subcontinent. At the same time, there is a significant increase in the rainfall over south peninsular India in response to the heatwaves in the north Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the heatwaves. This is the first time that a study has demonstrated a close link between marine heatwaves and atmospheric circulation and rainfall,” a statement released by authors of the research paper said on Tuesday.

Climate model projections suggest further warming of the Indian Ocean in the future, which will very likely intensify the MHWs and their impact on the monsoon rainfall. Since the frequency, intensity, and area of MHWs are increasing, it is essential that we closely monitor and forecast these events in advance in order to mitigate their impacts, the paper has concluded.

Marine heatwaves are periods of extremely high temperatures in the ocean--above the 90th percentile. These events cause habitat destruction due to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector, the authors said.

An underwater survey showed that 85% of the corals in Gulf of Mannar near the Tamil Nadu coast got bleached after the marine heatwave in May 2020. Though recent studies have reported their occurrence and impacts in the global oceans, they are least understood in the tropical Indian Ocean, the authors said in a statement.

These marine heatwaves occur due to background ocean warming in the Indian Ocean and also in response to El Niño events in the Pacific Ocean. Locally, a peak in solar radiation and a dip in evaporative cooling due to weak winds lead to the formation of these marine heatwaves. In the western Indian Ocean, the weak winds also reduce the heat transported by ocean currents from the near-equatorial regions toward the north, intensifying the marine heatwave.

In the southeast Indian Ocean, coral bleaching events due to the MHWs were reported, mostly associated with the El Niño and Madden-Julian Oscillations (MJO) events and exacerbated due to climate crisis. Studies have also reported high sea surface temperatures in the western Indian Ocean (Seychelles region), Arabian Sea, and the Bay of Bengal region (Andaman Sea) and also point out the coral bleaching events due to intense warming in these regions.

The IITM team used international datasets for sea surface temperatures and monsoon rainfall data from IMD for the analysis.

“Rapid Ocean warming due to climate change is the key factor leading to the rise in marine heatwaves. As global emissions continue unabated, we might see permanent marine heatwave conditions in the Indian Ocean that will have severe impacts on the fisheries and extreme weather events over South Asia. We need urgent measures to monitor and forecast these events and their impacts,” said Roxy Mathew Koll, climate scientist at IIITM and co-author of the paper.

“This is a very important study. Marine heat waves are a new research topic. I am happy to see the IITM group has come out with this study. Heat waves over land regions are studied very well but marine heat waves are not well explored. Marine heat waves will also have a major impact on marine ecosystems and biodiversity which we should study separately,” said M Rajeevan, former secretary, ministry of earth sciences.

The Intergovernmental Panel on Climate Change (IPCC), in their ‘The Physical Science Basis’ report released last August, said that the surface Indian Ocean has warmed faster than the global average. Marine heatwaves have become more frequent over the 20th century globally and are also projected to increase around the globe over the 21st century.

Ocean warming is likely to further decrease ocean oxygen concentrations, and this deoxygenation is projected to persist for thousands of years. Global mean sea level will continue to rise over the 21st century in response to continued warming of the climate system, the report said.

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