Cyclonic storm Nivar, the third tropical cyclone to hit India this year, made landfall approximately 22 km north of Puducherry in Villupuram district around 02.30 am Thursday. With winds in the range of 120-130 kmph, the storm caused heavy to extremely heavy rainfall in Puducherry and neighbouring districts.
During landfall, the storm was classified as a ‘very severe cyclonic storm’ and later it weakened to a ‘severe cyclonic storm’. With dwindling intensity, the wind speed dropped to around 100-110 kmph and by 8.30 am it weakened more and was classified as a severe cyclonic storm.
By Thursday night, the storm will weaken into a deep depression and by Friday morning it will weaken into a low-pressure area.
Even as the storm weakened, the IMD issued a red alert warning indicating heavy to extremely heavy rainfall warning on Thursday for the Rayalaseema region, coastal districts of Andhra Pradesh and parts of Telangana while a yellow alert was issued for Vidarbha, Chhattisgarh and eastern Madhya Pradesh.
Crucially, the eye of the storm- the calm portion that is surrounded by the turbulent, intense thunderclouds was not as well-defined compared to cyclone Nisarga and cyclone Amphan, said Mrutyunjay Mohapatra, Director-General, India Meteorological Department. This is likely to have contributed to a marginally reduced intensity of the storm upon landfall.
Nivar’s intensity was lesser than the 2018 storm Gaja but it was comparable to cyclone storm Vardah, which had made landfall near Chennai on December 12, 2016.
“Not every storm has a well-formed or full-fledged eye. Though we could see the eye clearly in radar images, it was not so well defined compared to other recent storms. When the eye is well-formed, we usually see a greater force of winds,” added Mohapatra.
“Nivar’s intensity similar to what we saw during the landfall of cyclone storm Vardah, though Nivar was a little stronger than Vardha,” he added.
The cyclonic storm’s landfall process lasted for six hours, beginning at 11.30 pm on Wednesday and ending at 05.30 am, till then the entire expanse of the storm had made landfall and it was accompanied by heavy to extremely heavy downpour.
Puducherry, Chennai, Chengalpattu, Cuddalore, Kallalurichi, Karaikal, Nagapattinam, Tiruvallar, Tirivannamalai and Villupuram received rainfall ranging from heavy – very heavy. Up to Thursday morning, the maximum rainfall was recorded in Puducherry, which reported 303.5mm rainfall, followed by neighbouring Villupuram district which reported 140.3 mm rainfall. Chennai recorded 138.5 mm rainfall while Chengalpattu and Cuddalore recorded 124.8 mm and 109.6 mm of rain, respectively.
La Nina and warm ocean favourable for storm genesis
Cyclonic storm Nivar’s genesis and intensification have been linked to La Nina, the global weather phenomenon that cools ocean surface temperatures in the central and eastern equatorial Pacific Ocean, and warming of subsurface and surface temperatures in the Indian Ocean, experts said.
In October, the World Meteorological Organization (WMO) had announced that La Nina had developed and was expected to extend into 2021. WMO had said that it would affect temperatures, rainfall and storm patterns across the world. “This year’s La Niña is expected to be moderate to strong. The last time there was a strong event was in 2010-2011, followed by a moderate event in 2011-2012.”
“In La Nina years, there is a greater chance for formation of cyclonic storms. We have also seen that sea surface temperatures and also subsurface temperatures have been warm. The sea temperature in Bay of Bengal was 29-30 degree Celsius,” said Mohapatra.
Other experts pointed out the occurrence of cyclones in the past during La Nina phases. “During the last 40 years, six cyclones – of the severe cyclone category- hit the Tamil Nadu coast in November. Out of these six, five of them coincided with La Nina like conditions in the Pacific. So that means to some to extent we were expecting a cyclone season in the Bay of Bengal during this time – and it is not a surprise,” said Roxy Mathew Koll, climate scientist at Indian Institute of Tropical Meteorology
A few days later, another low pressure area is expected to develop over the Bay of Bengal, Mohapatra said, and IMD is keeping a close watch on the developments of that system.
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