A warmer Bay of Bengal is likely playing a role in increasing faster timescale depressions and reducing slower timescale depressions
The northern Bay of Bengal has been experiencing an intense marine heatwave since June 28, 2023. This has led to India’s usually arid northwest receiving extreme rainfall, experts have told Down To Earth (DTE).
The Bay of Bengal, especially its northern part, is usually warm. This enables it to play an important role in the southwest monsoon’s trajectory.
“During the monsoon, the northern Bay of Bengal tends to be warm and needs to be warm because this is where the monsoon winds blowing from the southwest over the Arabian Sea cross over into the Bay of Bengal and make a U-turn to pump moisture into the Indian subcontinent,” according to Raghu Murtugudde, professor of atmospheric and oceanic science at the University of Maryland, United States.
A warm Bay of Bengal is needed for the monsoon. But we are talking about a warmer-than-normal Bay of Bengal here when we talk about heatwaves. They are likely contributing some extreme rainfall especially over northwest India.
According to the India Meteorological Department (IMD), six of 10 meteorological subdivisions in the southern peninsula and four of 10 subdivisions in central India have reported deficient rainfall during this monsoon season while all the subdivisions in the northwestern region have reported normal to large excess rainfall.
An analysis of the United States National Oceanic and Atmospheric Administration’s (NOAA) data suggests an increase in the intensity of marine heatwaves over the Bay of Bengal and the Andaman Sea ranging from moderate to beyond extreme category marine heatwaves since June 28.
Marine heatwaves, according to NOAA, “are prolonged periods of anomalously high sea surface temperature”.
Since marine heatwaves are also a daily weather phenomenon, with the Bay of Bengal and the Andaman Sea region experiencing some degree of heatwave days throughout June, the intensity increased from June 28, 2023, onwards.
According to Roxy Koll, climate scientist at the Indian Institute of Tropical Meteorology, marine heatwaves in the Bay of Bengal usually means a drier monsoon for central India and enhanced rain over India’s southern peninsula, he said.
According to Genesis and trends in marine heatwaves over the tropical Indian Ocean and their interaction with the Indian summer monsoon, which Koll co-authored, there were 94 heatwave events from 1982 to 2018.
“The western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of about 1.5 events per decade (four-fold rise), followed by the north Bay of Bengal at a rate of 0.5 events per decade (two-to-three-fold rise),” the study said.
But with northwest India getting more rain this time, is 2023 turning out to be an outlier? According to Murtugudde a combination of factors is at play.
“The usual timescales depressions of 10 to 60 days (slower timescale) are reducing while the depressions in the timescales of 3-10 days (faster timescale) are increasing. Their path is now much more towards northwest India than north-central India,” he said.
Northwest India is witnessing above average rainfall because of the current trajectory that the depression has charted, coupled with a faster timescale.
“A warmer Bay of Bengal is likely playing a role in increasing faster timescale depressions and reducing slower timescale depressions”, he said.
In 2023, a warmer-than-usual Bay of Bengal has already experienced Cyclone Mocha, an Extremely Severe Cyclonic Storm, according to IMD.
Apart from meteorological implications, marine heatwaves also adversely affect marine biodiversity. According to the study which Koll had co-authored in 2020, 85 per cent of the corals in the Gulf of Mannar near the Tamil Nadu coast got bleached after the marine heatwave in May 2020, the study said.
The current heatwave in the Bay of Bengal could also negatively impact the mangroves in the Sundarbans, Murtugudde said.
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