Mumbai monsoon: Why the city is seeing record rain amid El Niño

Mumbai has come to a standstill, and the credit goes to the non-stop rainfall the city is witnessing. Between June 30 and July 6, the financial capital received as much rain as major metros typically do in an entire year.

India Meteorological Department's (IMD) Colaba observatory recorded 882. 6mm of rain and Santacruz 988. 8mm during the seven-day period. According to this, the city surpassed Delhi's average annual rainfall, nearly matching Bengaluru's yearly total.

This sheer volume of rain has submerged roads and trees, affecting drainage systems and railways. But the heavy rainfall has come despite the country continuing 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.

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Hence, the question arises: why is the city seeing record rain amid an El Niño year?

We take a look.

Mumbai's rain paradox in an El Niño year

This year, the monsoon reached Mumbai nearly two weeks later than usual, but its intensity is unmatched by that of any other metro city. Scientists say El Niño and heavy rain are not a contradiction.

While El Niño continues to affect the onset and strength of the monsoon, global warming is increasingly changing how rainfall occurs. Once the weather system is active, rainfall intensity increases, making it more erratic.

Therefore, instead of steady rainfall spread over several weeks, cities are increasingly experiencing fewer rainy days but far more intense cloudbursts. In simple words: the monsoon hasn't disappeared; it has gone far more extreme.

Mumbai's average annual rainfall rose from 2,325.8 mm between 1981 and 2000 to 2,672.7 mm during 2001–2024, an increase of 346.9 mm, or nearly 15 per cent, according to reports.

What is El Niño?

El Niño is a naturally occurring climate phenomenon in which the waters of the tropical Pacific Ocean become warmer than usual. It has traditionally been associated with weaker monsoons and below-normal rainfall in India.

Initially this year, this pattern held, with the southwest monsoon reaching Mumbai nearly two weeks later than usual. However, once it arrived, it brought extraordinary rain instead of the gradual, slow showers.

What's the science behind high-intensity rain

According to Climate scientists, a warmer atmosphere and rapidly warming oceans are allowing the air to hold more moisture.

Hence, rainfall is increasingly occurring in short periods, with intense downpours rather than being spread over several days. And this trend is visible in Mumbai's rain records as well.

Dr KJ Ramesh, former Director General of the IMD, said, "During El Niño years, the number of rainy days is lower. But we know the monsoon's character 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," the Indian Express reported.

Dr Raghu Murtugudde, Emeritus Professor at the University of Maryland and former professor at IIT Bombay, asserted that a combination of several weather systems led to the exceptionally heavy rainfall. "Mumbai saw a delayed onset, which is partly explained by El Niño," he added.

Is El Niño directly linked to global warming?

According to Murtugudde, as anticipated, the global warming pattern over West Asia and changes in Arabian Sea winds are playing a vital role.

"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," he explained, the Express quoted.

Professor Murtugudde added that the Western Ghats force moisture-laden winds to rise, triggering heavy rainfall over the city.

Professor pressed the point that El Niño is, and can no longer be, viewed in isolation from global warming. Citing the reason, he said both El Niño and global warming interact to shape monsoon behaviour.

However, Mumbai has received a much-needed breather after days of relentless rain. For the first time in five days, daily heavy rainfall fell below the 100 mm mark, with 94 mm recorded at Santacruz and 90 mm at Colaba, according to IMD data.

While the weather department kept Mumbai under a yellow alert on Tuesday (July 7), the forecast over the five days indicates that the city and neighbouring Thane are likely to receive only light rainfall.

With inputs from agencies