Marine heatwaves impacting Indian monsoon rainfall, study says

Marine heatwaves are on the rise in the Indian Ocean, leaving an impact on the Indian monsoon rainfall, according to a study by researchers of the Pune-based Indian Institute of Tropical Meteorology (IITM).

In the study led by climate scientist Roxy Mathew Koll and published in the journal 'JGR Oceans', the researchers reported a significant increase in marine heatwaves, aided by rapid warming in the Indian Ocean and strong El Niños (weather event associated with unusual warming of surface waters). The study found that marine heatwaves impact the monsoon by reducing the rainfall over the central Indian subcontinent, while enhancing it over the southern peninsula.

Marine heatwaves are periods of extremely high temperatures in the ocean. These events cause habitat destruction due to coral bleaching, seagrass destruction and loss of kelp forests, affecting the fisheries sector adversely. An underwater survey showed that 85 per cent of the corals in Gulf of Mannar near the Tamil Nadu coast got bleached after marine heatwaves 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 study said. “These heatwaves used to be rare in the tropical Indian Ocean, but now they have become an annual affair. The western Indian Ocean region experienced the largest increase in marine heatwaves 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 western Indian Ocean had a total of 66 events while the Bay of Bengal had 94 events,” the researchers said.

Marine heatwaves in the western Indian Ocean and the Bay of Bengal are found to result in drying conditions over the central Indian subcontinent, they said. “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," the report said.

“This is the first time that a study has demonstrated a close link between marine heatwaves and atmospheric circulation and rainfall,” the researchers said. Climate model projections suggest further warming of the Indian Ocean in the future, which will very likely intensify the marine heatwaves and their impact on the monsoon rainfall, Koll said.

“Since the frequency, intensity, and area covered by the marine heatwaves are increasing, we need to enhance our ocean observational arrays to monitor these events accurately, and update our weather models to skilfully predict the challenges presented by a warming world,” he said. IITM is an autonomous institute of the Union Ministry of Earth Sciences.

Watch latest videos by DH here: