By Adam Sobel
As I write, the India Meteorological Department has just declared the weather disturbance over the southeast Arabian Sea — just off India’s west coast — to be a Cyclonic Storm (equivalent to ‘Tropical Storm’ on the Saffir-Simpson scale used in the U.S.) and given it the name “Nisarga.” It is currently forecast to make landfall near Mumbai on Wednesday, June 3, with the city bearing the brunt of the storm’s winds, rain and storm surge.
Cyclones in the Arabian Sea are relatively rare. In the historical average, there are only one or two per year. And of those, only a small fraction hit India. India’s west coast does not experience cyclones with the frequency or intensity that its east coast, exposed to the more storm-prone Bay of Bengal, does – as a prominent example, Cyclone Amphan did tremendous damage to the West Bengal coastline last week, and hit the city of Kolkata head-on. And of those which do strike the west coast, the most powerful ones have historically gone north of Mumbai, to the state of Gujarat.
So a major cyclone landfall in Mumbai is a rare event indeed. If current predictions bear out, this one will not be truly major either, on the scale of what is possible. The current predictions of the IMD are for winds of around 110 kilometers per hour (~70 mph), rainfall of perhaps as much as 200 millimeters (~8 inches) in the heaviest spots, and a storm surge of 1-2 meters (~3-6 feet). (Fortunately, the storm surge is likely to peak closer to low tide than high, which would reduce the coastal flooding.) These numbers are not nearly as bad as what the worst theoretically possible storm could produce. But still, in a city of 12 million people — 20 million in the greater metropolitan area — and one exposed to the sea, with many vulnerable, low-lying areas, and little experience of cyclones, these predictions easily justify the high state of alert currently in place in Mumbai and its surrounding areas.
Several colleagues and I have been studying the risk of a major cyclone landfall in Mumbai for several years. This work was originally motivated by the great writer (and anthropologist) Amitav Ghosh, who asked me, after my book about Hurricane Sandy came out, whether something similar to that event could happen in Mumbai, and whether the probability of one was increasing due to global warming. That led to a series of wide-ranging conversations that continues today. The first few of those conversations are described in Amitav’s book, The Great Derangement. They also inspired me and my colleagues to do some research into the question, supported by the Columbia President’s Global Innovation Fund and in collaboration with the Columbia Global Center Mumbai as well as colleagues in India.
Our research, published here, indicates that a truly major cyclone is possible in Mumbai. By “truly major” I mean one with winds in excess of 100 knots (Category 3, a major hurricane on the U.S. Saffir-Simpson scale) and a greater storm surge than the 1-2m currently predicted with Nisarga. The storm surge would probably be the greatest threat, possibly causing floods equal or maybe even worse than what extreme monsoon rains (not associated with a cyclone) produced in 2005. It looks as though Nisarga will not approach these magnitudes, thankfully, notwithstanding the threat it still poses.
Our research on Mumbai hasn’t yet addressed the role of global warming. Other studies have shown, though, that climate models predict increasing cyclone activity over the Arabian sea as a consequence of warming, and one recent study by colleagues at Princeton even attributes the spate of recent active years to human influence. And it is certain that sea level rise increases the risk, since any storm surge starts on top of a higher baseline. Mumbai in particular was originally just a series of small, mostly flat islands, connected over the last few centuries by landfill, and is one of the world’s most vulnerable cities as the sea rises.
In the course of doing our work, I have been fortunate to visit Mumbai several times. It’s a truly great city, and one where a New Yorker can feel at home. My thoughts are with everyone there. Hopefully this will not be the disaster we might have feared, and will instead lead to greater resilience and preparedness for the future.
A historical note: In several articles in the Indian print and online media over the last day or so, I have been quoted as saying that Mumbai has not, at least since 1891 (when modern data began), experienced a “serious” or “significant” cyclone. I regret those quotes, as they are arguably misleading in the present context. They were based on the research cited above, looking for high-intensity storms coming close to Mumbai, and are correct as such, but I should have been more precise rather than using qualifiers “serious” and “significant,” since a storm like Nisarga, though not reaching the high intensities we were thinking about, can still have major impacts. There have, in fact been a few cyclones in the modern record which have come close to Mumbai with intensities comparable to what is predicted for Nisarga. The IMD historical data, analyzed by my colleague Chia-Ying Lee, shows storms with intensities of 65 knots (just above the threshold for Category 1 hurricane intensity on the U.S. scale, Very Severe Cyclonic Storm on the Indian scale) within 100 km of Mumbai in 1903, 1940, and 1948. Nonetheless, if the forecasts for Nisarga verify, it will still be the strongest storm to affect the city directly in at least 70 years. (Cyclone Phyan, in 2009, affected Mumbai more recently, but made landfall further south and at a weaker intensity than Nisarga is currently forecast to do.) A more detailed analysis should wait until after the storm.