While El Niño continues to influence when the monsoon arrives and how active it is, global warming is increasingly changing the way it rains.
Story continues below this ad“During El Niño years, the number of rainy days is lower.
“Mumbai saw a delayed onset, which is partly explained by El Niño.
But the global warming pattern over West Asia and changes in Arabian Sea winds have also played a role, as expected.
When both systems are active, the core monsoon zone receives heavy rainfall, and that moisture also reaches Mumbai.
Between July 1 and 7, the India Meteorological Department’s (IMD) Colaba observatory recorded 791 mm of rainfall, surpassing its entire July climatological average of 768.5 mm. Meanwhile, the weather bureau’s Santacruz observatory recorded 879 mm of rainfall against its monthly average of 919.9 mm.
The heavy rainfall has come even as the country continues to experience the effects of El Niño, a climate phenomenon in which sea surface temperatures in the central and eastern Pacific Ocean become unusually warm, often delaying the onset of the southwest monsoon. Although the monsoon reached Mumbai nearly two weeks later than usual, it entered an active phase by the end of June, bringing intense rainfall to the city and several parts of coastal Maharashtra.
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El Nino and heavy rain not a contradiction, say scientists
Scientists said there is no contradiction. While El Niño continues to influence when the monsoon arrives and how active it is, global warming is increasingly changing the way it rains. Once weather systems become active, rainfall is becoming more erratic and more intense.
The latest spell also fits a longer-term trend. Between 1981 and 2000, Mumbai recorded an average annual rainfall of 2,325.8 mm. Between 2001 and 2024, the average increased to 2,672.7 mm, an increase of 346.9 mm, or nearly 15 per cent.
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A scooter drives past a boy in Mumbai on a flooded Mumbai road (PTI) A scooter drives past a boy in Mumbai on a flooded Mumbai road (PTI)
Science behind the high-intensity rain
Climate scientists said a warmer atmosphere and rapidly warming oceans are allowing the air to hold more moisture. As a result, rainfall is increasingly occurring in short, high-intensity bursts instead of being spread over many days. Such spells can overwhelm drainage systems, trigger flash floods and expose weaknesses in urban infrastructure.
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“During El Niño years, the number of rainy days is lower. But we know that the character of the monsoon has changed permanently due to global warming. Rainfall now tends to occur in short-duration, high-intensity spells, whether there is an El Niño or not,” said Dr K. J. Ramesh, former Director General of the IMD.
Dr Raghu Murtugudde, Emeritus Professor at the University of Maryland and former professor at IIT Bombay, said several weather systems had come together to produce the exceptionally heavy rainfall.
“Mumbai saw a delayed onset, which is partly explained by El Niño. But the global warming pattern over West Asia and changes in Arabian Sea winds have also played a role, as expected. The Arabian Sea and the Bay of Bengal are both feeding moisture into the monsoon system, and a low-pressure system is developing over the Bay of Bengal. When both systems are active, the core monsoon zone receives heavy rainfall, and that moisture also reaches Mumbai. The Western Ghats force the moisture-laden air to rise, resulting in heavy rain over the city,” he said.
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El Nino directly linked to global warming
He added that El Niño can no longer be viewed in isolation from global warming because the two increasingly interact to shape monsoon behaviour.
Meanwhile, IMD data showed that in the 24 hours ending Tuesday morning, the Santacruz observatory recorded 94 mm of rainfall, while the Colaba observatory recorded 90 mm. This was the first time in the past five days that rainfall at both observatories remained below the three-digit mark.
According to the Brihanmumbai Municipal Corporation (BMC), the western suburbs recorded 78 mm of rainfall during the same period, followed by 77 mm in the eastern suburbs and 46 mm in the island city.
For now, however, the current spell appears to be easing. While the IMD kept Mumbai under a yellow alert on Tuesday, its five-day forecast indicates that the city and neighbouring Thane are likely to receive only light rainfall over the coming week, with no heavy rainfall warning in place.
