With Puducherry and northern districts in Tamil Nadu facing the brunt of Cyclone Fengal, the regions recorded historic levels of rainfall. Even landlocked districts like Krishnagiri received huge amounts of rain.
By the early hours of December 1, Puducherry had received 48.4cm of rain–the highest cumulative amount for the union territory in thirty years. On December 2, as of 8 am, Uthangarai in Krishnagiri district recorded over 50 cm of rainfall. This is striking because according to the Climate Change Information Portal run by Anna University’s Centre for Climate Change and Disaster Management, Krishnagiri’s average annual rainfall between 1951 to 2020 was only 85 cm.
“Krishnagiri gets the least rainfall during the northeast monsoon, but it had received 50 cm on December 2,” said independent weather blogger Pradeep John. Fengal had weakened into a deep depression by the afternoon of December 1, about 40 kilometres from Villupuram coast, as per the Indian Meteorological Department (IMD). While a cyclonic disturbance would have a surface wind speed of 62 to 88 kmph, a deep depression wind speed would be in the range of 50 kmph to 61 kmph.
“Once the weather system reached Krishangiri, it dumped huge amounts of rain before weakening considerably into a broad low pressure area. This was because it was able to retain its Central Dense Overcast (CDO) until Uthangarai,” Pradeep said. CDO refers to a cirrus cloud shield that forms due to thunderstorms in the walls of a cyclone’s eye and its rain bands. “Even after weakening, the CDO of the cyclone or deep depression remained more or less intact. As a matter of fact, it remained a cyclone for a while even after crossing into land. Wherever the CDO moved, it was able to dump a huge amount of rainfall,” Pradeep said.
Independent weather blogger Srikanth, who runs the site Chennai Rains, explained why interior regions received heavy rainfall as well. “Even after the circulation moved inland, it was able to sustain its structure and its core convective bands. So rainfall events that are usually felt only in coastal areas, were felt in interior districts too,” he told TNM.
So why did Krishnagiri, so far away from any coast, receive nearly 60% of its average annual rainfall in 24 hours due to a cyclone? Similarly, why did several other districts in Tamil Nadu including Villupuram, Cuddalore, Thiruvanamalai and Dharmapuri also receive a battering? Villupuram recorded the highest in the state with 49.8 cm of rain on December 1. On the same day, Cuddalore received the fourth highest amount of rainfall with 17.9 cm and the Thiruvanamalai - Indian Space Research Organisation (ISRO) station recorded 17.5 cm of rain.
Both Pradeep and Srikanth said the cyclonic system’s lack of speed played a significant role. “One reason was obviously the slow movement of Fengal,” Srikanth said.
IMD had stated that the cyclone made landfall near Puducherry between 10:30 pm and 11 pm on November 30. “But it was only on December 2 morning that the system moved near Thiruvanamalai. Effectively we are talking about 36 hours of rain in a 120 kilometre stretch. That’s the biggest issue here. Very slow or no movement means the rainfall remains over the same areas,” Srikanth said.
Further, at the time, there were conflicting predictions from the IMD and weather bloggers regarding the cyclone’s landfall. While IMD insisted that the cyclone had crossed the coast on November 30, weather bloggers said that landfall did not actually take place until the following morning and that the cyclone had been straddling the coast and open seas until then. However, by the afternoon of December 1, the IMD said the cyclone had weakened to a deep depression and that it had remained stationary, close to Puducherry, for 12 hours.
There is another reason for the historic rains, Srikanth said. “What we see now is warmer oceans leading to enhanced rainfall events,” he said. When asked if this relates to climate change, Srikanth said, “I’m not sure, but the northern Indian ocean is the warmest globally.” The Indian Ocean is indeed the warmest ocean basin in the world, and is notably becoming warmer.
Warmer oceans mean the cyclone is able to absorb more moisture, Srikanth said. “This allows it to then spend more time at sea without weakening. As a slow-moving cyclone, it was able to recycle moisture continuously while remaining in the same area and a warmer ocean was like a steroid boost,” he said.
Atmospheric ridges, Pradeep says, is one possible reason for the cyclone’s slow movement. Ridges steer cyclones, deciding their path. India is flanked by Arabian ridges on its western coast and by Pacific ridges on its eastern coast. Most cyclones that form over the Bay of Bengal are steered by the Pacific ridges which explains their west to northwest tracks, Pradeep said. However, Fengal was trapped between the effects of both the Arabian and Pacific ridges. This can happen sometimes, causing the cyclone to slow down, halt or get stuck in a loop. In 2020, the weather blogger recalls, Cyclone Burevi was trapped in the Gulf of Mannar for three days, after being caught between the effects of Arabian and Pacific ridges.