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2020-11-30

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Relevant for: Geography | Topic: Important Geophysical Phenomenon - Tropical Cyclones

The story so far: Cyclone Nivar, that barrelled through Tamil Nadu and brought copious rain in its wake, was the third major cyclone to land on India’s coast this year, besides Amphan and Nisarga. The Nivar storm originated in the Bay of Bengal and whipped up windspeeds close to 125-145 kmph, blowing away roofs and felling standing crop. However, relatively fewer lives were lost compared to the havoc wreaked by Amphan in West Bengal in May. What aided relief operations in the anticipation of Nivar was that it largely conformed to forecasts issued by the India Meteorological Department (IMD).

Cyclone Nivar | In pictures

Over the years, India’s ability to track the formation of cyclones has improved significantly. There is a network of 12 doppler weather radars (DWR) along India’s coast if one were to begin counting from Kolkata and trawl up to Mumbai — there are 27 in all in the country. Depending on where a storm is forming, these radars send pulses of radio waves to gauge the size as well as the speed at which water droplets are moving. The earlier generation of radars was unable to track such progress in real time, but with DWRs, now the base standard of weather radars, it is usually possible to detect a potential storm at least four-five days in advance. The IMD also collaborates with similar international networks, such as the Japan Meteorological Agency, the U.S. National Hurricane Center, and the U.S. Central Pacific Hurricane Center, and these bodies constantly send warnings and forecasts about changes in the ocean weather. The near ubiquity of ocean-buoys that track changes in ocean sea surface temperatures as well as dedicated meteorological satellites improve the odds of early detection.

Cyclone Nivar | Improved coordination, action plan helped

Nivar was the second tropical cyclone that formed around India and made landfall this week. Cyclone Gati, which originated in the Arabian Sea and intensified into a ‘very severe cyclonic storm’, made its way towards Africa and made landfall in Somalia on November 22. However around then, another system emerged in the Bay of Bengal, that eventually morphed into a cyclonic storm by November 24. The IMD’s initial forecasts said it would at most be a ‘severe cyclonic storm’, but it then upgraded it to the same league as Gati, i.e, a ‘very severe cyclonic storm’. The IMD follows a five-stage classification for cyclones, with the lowest a ‘cyclonic storm’ generating wind speeds of 62-87 kmph, and the highest a ‘super cyclonic storm’, generating winds over 222 kmph.

April-June and October-December are India’s cyclone seasons. The arriving monsoon, as well as its retreat, stir up the surrounding seas and generate cyclones. Though the Bay of Bengal is three times more likely to generate cyclones, the ones that originate in the Arabian Sea are trickier, as the cyclone, while ostensibly moving away from India’s western coast, can suddenly ‘recurve’ and move back in. There are also fewer radars along India’s west coast than the eastern coast, and all these reasons make the Bay of Bengal cyclones more tractable. In this context, Nivar, because it conformed to a fairly predictable trajectory and was not super cyclonic in intensity, gave State administrations in Puducherry, Tamil Nadu and Andhra Pradesh time to prepare, and was far less damaging than Amphan. However, the cyclone season is not yet over and more systems are likely to form in the coming weeks, according to the IMD.

IMD begins impact-based forecast

Forecasts, on their own, are important, but they cannot override the importance of preparedness by State agencies. The formation of cyclones is preceded by ‘depressions’, and they are often the first warnings. Not all depressions become cyclones, but many coastal States — especially those with a history of being battered — begin organising shelters and evacuation of coastal residents. Sea pockets, where cyclones form, are also places that drive schools of fish and lure fisherfolk. While meteorological agencies give advisories on where fish-catches are likely, they suspend such advisories during storm formation to dissuade fishermen from venturing out. The ubiquity of mobile communication makes it much easier to quickly give warnings. The IMD also issues flood forecast maps, in collaboration with urban bodies that forecast which pockets in a city are likely to be flooded and where crop damage is likely to be maximum.

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