The unprecedented rains in Kolkata- the highest since 1986, and the sixth highest in a single day in the last 137 years - are being classified as a ‘rare’ occurrence by scientists, which is the result of incessant warming of the Bay of Bengal. The immediate impact of this could be a delayed withdrawal of the monsoon from cities such as Kolkata and Mumbai.
The warming over the ocean, on account of increased temperature, leads to higher evaporation. In such a situation, a low-pressure area attracts more moisture from the oceans, making weather activity more intense.
It’s a situation, though rare, that will be more common in the coming times, said Mahesh Palawat, vice president, Meteorology and Climate Change, Skymet Weather.
Over 250 mm of rainfall flooded Kolkata in less than 24 hours. Though intense, it falls short of the Indian Meteorological Department’s classification of a cloudburst (100 mm of rain in a single hour). This unusual rain is just one example of the extreme costs that climate change-induced weather will bring.
VIDEO | Kolkata: "Cyclonic circulation forms over the Bay of Bengal, fishermen advised to avoid sea; rain and gusty winds likely in South and North Bengal," says IMD Regional Director Habibur Rahaman Biswa.
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A powerful low-pressure system that developed over Odisha on September 22, migrated towards Gangetic West Bengal, north Odisha, and the northwest Bay of Bengal. This system created a ‘conveyor belt’ of moisture from the Bay of Bengal directly into Kolkata, powering relentless convective cloud formation and static cloud build-up.
The Bay of Bengal has undergone significant warming in recent years, leading to more rapid evaporation and enhanced moisture supply to weather systems. This warming is not isolated: climate scientists point to rising global ocean temperatures as the key driver behind heightened atmospheric moisture and the increasing intensity of rainfall events.
With multiple typhoons forming in the warming Pacific Ocean, there is now a greater competition for atmospheric moisture, further influencing monsoon circulations across South Asia. Dr. Raghu Murtugudde, a prominent climate scientist, emphasised that these oceanic changes are disrupting established monsoon withdrawal patterns, leading to extended monsoon showers and delayed transitions to winter.
Since the weather systems brewing in the Pacific Ocean are strong, their moisture demand is also high. These systems try to pull moisture from the North Indian Ocean, affecting the monsoon circulations. Under this external influence, there has been the formation of weather systems in the Bay of Bengal as well. The low-pressure system that affected Kolkata was the result of this. With more systems expected in the Bay of Bengal, the withdrawal process of the monsoon will get stalled for a bit. Places like Mumbai and Kolkata are likely to witness extended monsoon showers, explained Murtugudde.
Research published in Nature estimates that global oceans have warmed by over 1.5°C since the start of the industrial era, with the Indian Ocean proving to be a significant hotspot. The region has recorded the fastest surface warming since the 1950s. This has led to marine heatwaves and changing weather dynamics. Marine heatwaves are periods of anomalously high ocean temperatures, and these are predicted to increase tenfold, possibly transforming the tropical Indian Ocean into a near-permanent heatwave zone. Such dramatic changes mean cyclones can rapidly intensify and rainfall patterns can become more unpredictable.
Climate models further suggest that sea surface temperatures in the Indian Ocean will exceed 28°C year-round by the late 21st century under high-emission scenarios. These temperatures foster deep convection and cyclogenesis, increasing the likelihood of severe rainfall events and extremely intense cyclones. Historically, the Indian Ocean Dipole- a major climate phenomenon impacting monsoon and cyclone formation- is expected to see an increase in extreme events by as much as 66%, changing the frequency and intensity with direct implications for South Asian weather.
The rapid intensification and unusual persistence of these weather systems amplify existing lags in city infrastructure, emergency response readiness, and climate adaptation planning. Marine heatwaves are also slated to devastate marine habitats, with implications for local fisheries and food security due to coral bleaching and seagrass destruction.
Experts warn that as the Indian Ocean continues to heat up, South Asia will see even more erratic monsoon cycles, heavier rainfall spells, and accelerated urban flooding risks. Robust investment in climate resilience planning and urban infrastructure adaptation is urgently needed to confront the stark realities of a warming world.
