Mumbai, 02nd February 2022

Marine heatwaves are increasing in the Indian Ocean and that is having an impact on the Indian monsoon rainfall, reports a study led by Roxy Mathew Koll of the Indian Institute of Tropical Meteorology (IITM), Pune.

In the study published in the journal JGR Oceans, the researchers report a significant increase in marine heatwaves, aided by rapid warming in the Indian Ocean and strong El Niños. They find that these marine heatwaves impact the monsoon by reducing the rainfall over the central Indian subcontinent while enhancing it over the southern peninsula.

What are marine heatwaves?

Marine heatwaves are periods of extremely high temperatures in the ocean (above the 90^th percentile). 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% 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.

Marine heatwaves lead to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely. Image Credits: Chasing Coral, Netflix.

Example of a marine heatwave in the Bay of Bengal during April–May 2020. Marine heatwaves are periods of extremely high temperatures in the ocean (above the 90^th percentile of sea surface temperatures). 85% of the corals in Gulf of Mannar near the Tamil Nadu coast got bleached after the marine heatwave in May.

Increase in marine heatwaves in the Indian Ocean, in response to rapid ocean warming

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.

During 1982–2018, the western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of 1.2–1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.4–0.5 events per decade.