The northern Bay of Bengal has been experiencing an intense Marine Heatwave since 28th June 2023. This phenomenon has resulted in India’s usually dry northwest region witnessing extreme rainfall.
Understanding Marine Heatwaves and Their Causes
Marine Heatwaves (MHWs) are sustained periods of exceptionally high Sea Surface Temperature (SST). MHWs have been linked to damaging events such as coral bleaching, the destruction of seagrass, and the loss of kelp forests which significantly affect the fisheries sector. The leading causes of MHWs include ocean currents that accumulate areas of warm water and air-sea heat flux or warming through the ocean surface from the atmosphere. Wind can either amplify or mitigate the warming in a MHW, while climate modes like El Niño can alter the likelihood of such events occurring in certain regions.
Marine Heatwave’s Role in Rainfall over Northwest India
The MHW underway in the Bay of Bengal has elevated sea surface temperatures leading to increased evaporation rates and an increased supply of moisture in the atmosphere. This additional moisture has led to above-average rainfall in northwest India. This MHW has likely influenced the formation and behaviors of depressions in the Bay of Bengal, contributing to an increase in the frequency and intensity of these depressions which play a significant role in monsoon and rainfall patterns. The depressions are now tending to move more towards northwest India, causing a higher concentration of rainfall in the region.
The Broader Impacts of Marine Heatwaves
MHWs have significant effects on ecosystem structures, promoting certain species while suppressing others and leading to mass mortality among marine invertebrates. They can also alter the habitat ranges of certain species, cause economic losses through impacts on fisheries and aquaculture, and drastically impact biodiversity. When occurring alongside other stressors such as ocean acidification, deoxygenation, and overfishing, they not only further damage habitats but also increase the risk of deoxygenation and acidification.
The Bay of Bengal’s Influence on Monsoons
The warm and humid air mass over the Bay of Bengal provides the essential moisture that is carried by monsoon winds towards the Indian subcontinent. During the monsoon season, the seawater’s warmth in the northern part of the Bay of Bengal and the heat up of the Indian landmass create a low-pressure area. This results in a pressure gradient that draws in moisture-laden winds from the Bay of Bengal, leading to monsoon rainfall. The Monsoon winds from the southwest make a U-turn upon reaching the Bay of Bengal and start moving towards the northeast, bringing rainfall across India. Furthermore, the warm sea surface temperatures in the Bay of Bengal contribute to the strengthening of the low-level jet stream or the Somali Jet, enhancing the moisture supply during the monsoon season.
Concerns for the Future
A question posed in the 2015 UPSC Civil Services Examination asked examinees to discuss to what extent they agreed that “the behavior of the Indian monsoon has been changing due to humanizing landscape.” The intense MHW currently being experienced in the Bay of Bengal and its profound impact on rainfall across northwest India make this question even more pertinent today.
