How Mumbai steered clear of cyclones

Despite cyclones frequently forming in the Arabian Sea, Mumbai has been shielded from being directly hit by these weather systems thanks to what climate scientists

Despite cyclones frequently forming in the Arabian Sea, Mumbai has been shielded from being directly hit by these weather systems thanks to what climate scientists describe as steering winds and a subtropical ridge.

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Steering winds are way above the earth’s surface and control the movement of weather systems. In case of a cyclone, these upper-level winds steer the cyclone in the direction that they are flowing.

“Cyclones in the Arabian Sea are mostly formed in the central Arabian Sea. The reason Mumbai has been shielded is because they [cyclones] do not form close to the coast. Even if they do form close to the coast, they move towards Oman, Yemen or Somalia because of upper-level steering winds that move from east to west,” said Sridhar Balasubramanian, associate professor, department of mechanical engineering and associated faculty, IDP in Climate Studies, Indian Institute of Technology Bombay.

Looking at cyclone Nisarga making landfall south of Mumbai and last year’s severe cyclonic storm Vayu, professor Raghu Murtugudde, from University of Maryland’s department of atmospheric and oceanic science, said the possibility of a change in the steering wind pattern could be a factor.

“Something is changing about these winds, given last year cyclone Vayu and this year Nisarga have come straight to the coast,” said Murtugudde, who is currently in Mumbai. “My guess is that [ocean] warming has weakened the steering wind in a way that the direction of cyclones will probably begin to change. Overall, this dynamic change is in response to global warming, which is similar to observations of a slight weakening and changes in monsoon circulation.”

Mumbai has also evaded being hit by cyclones in the past because of a high-pressure zone called the subtropical ridge. “Normally, during the pre-monsoon season, the subtropical ridge acts as a barrier and doesn’t allow the cyclone to come towards us,” said Balasubramanian. “The cyclone moves along the line of the ridge, which is usually over Mumbai-Gujarat and therefore the system ends up towards the Gulf.”

Taking into account the probability of climate change-driven increase in pre-monsoon cyclones hitting the coastline, Murtugudde said that going forward, policy makers must incorporate cyclone-level storm surge and inundation in the city’s disaster management plan. “There is a need to modify the plan that goes beyond flooding to factor in the consequences of the ocean being pulled into the city. Policy makers need to keep in mind that a one-in-100-year event can begin to occur once in 20 years,” he said.