Explained: Why Cyclone Fani is an unusual storm

Amitabh Sinha
Clouds loom ahead of Cyclone Fani in Visakhapatnam on May 1, 2019. (Reuters Photo)

A powerful cyclonic storm named Fani (pronounced Foni) is headed towards the Odisha coast, with its landfall forecast near Puri Friday. Expected to generate storms with wind speeds as high as 200 km per hour, it has the potential to cause widespread damage in Odisha and neighbouring states.

The last time such a powerful cyclonic storm had emerged in the Bay of Bengal at this time of the year, in 2008, it had killed more than 1.25 lakh people in Myanmar. But that was mainly because of the lack of a sophisticated warning system and enough logistical preparedness to evacuate people.

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Fani, on the other hand, has been continuously monitored ever since it developed southeast of Sri Lanka about a week ago, warnings have been issued after every few hours to fishermen and people living in coastal regions, and a massive emergency preparedness has been mounted. In the last few years, India has impressively managed disasters caused by cyclones, most remarkably during Cyclone Phailin of 2013, which was even stronger than the approaching Fani.

Cyclone Fani, the outlier

The eastern coast of India is no stranger to cyclones. On an average, five to six significant cyclonic storms emerge in the Bay of Bengal region every year. The months of April and May just before the start of the monsoon, and then October to December immediately after the end of the monsoon, are the prime seasons for tropical cyclones.

Yet, Fani is a little outlier, mainly on account of its strength, and the route it has taken. Cyclones emerging in April-May usually are much weaker than those during October-December. There have been only 14 instances of a "severe cyclone" forming in the Bay of Bengal region in April since 1891, and only one of them, which formed in 1956, touched the Indian mainland. The others all swerved northeast to hit Bangladesh, Myanmar or other countries in the southeast Asian region. Since 1990, there have been only four such cyclones in April.

Also read | Puri's Jagannath temple replaces flag to sound alert: Fani on way, brace for impact

Explained: Why Cyclone Fani is an unusual storm

Fani is not just a severe cyclone but an "extremely severe cyclone". Tropical cyclones in the Bay of Bengal are graded according to maximum wind speeds at their centre. At the lower end are depressions that generate wind speeds of 30 to 60 km per hour, followed by cyclonic storms (61 to 88 kph), severe cyclonic storms (89 to 117 kph) and very severe cyclonic storms (118 to 166 kph). At the top are extremely severe cyclonic storms (167 to 221 kph) and super cyclones (222 kph or higher).

Fani is, thus, unusual, and that is mainly because of the place it originated, very close to the Equator, and the long route it has taken to reach the landmass.

Strengthening over seas

Cyclones are formed over slightly warm ocean waters. The temperature of the top layer of the sea, up to a depth of about 60 metres, need to be at least 28 degrees Celsius to support the formation of a cyclone. This explains why the April-May and October-December periods are conducive for cyclones.

Then, the low level of air above the waters needs to have an 'anticlockwise' rotation (in the northern hemisphere; clockwise in the southern hemisphere). During these periods, there is a zone in the Bay of Bengal region (called the inter-tropical convergence zone that shifts with seasons) whose southern boundary experiences winds from west to east, while the northern boundary has winds flowing east to west. This induces the anticlockwise rotation of air.

Once formed, cyclones in this area usually move northwest. As it travels over the sea, the cyclone gathers more moist air from the warm sea, and adds to its heft.

A thumb rule for cyclones (or hurricanes and typhoons as they are called in the United States and Japan) is that the more time they spend over the seas, the stronger they become. Hurricanes around the US, which originate in the vast open Pacific Ocean, are usually much more stronger than the tropical cyclones in the Bay of Bengal, a relatively narrow and enclosed region. The cyclones originating here, after hitting the landmass, decay rapidly due to friction and absence of moisture.

Fishermen in Odisha on Monday. (PTI Photo)

Cyclone Fani in Odisha: In situ origins

A big difference between the strengths of cyclones in April-May and October-December is that the former originate in situ in the Bay of Bengal itself, barely a few hundred kilometres from the landmass. On the other hand, cyclones in October-December are usually remnants of cyclonic systems that emerge in the Pacific Ocean, but manage to come to the Bay of Bengal, considerably weakened after crossing the southeast Asian landmass near the South China Sea. These systems already have some energy, and gather momentum as they traverse over the Bay of Bengal.

"April-May is not the season for typhoons in the west Pacific Ocean. Most of the typhoons in west Pacific in northern hemisphere form between June and November. That is why almost all the cyclones in the Bay of Bengal in April-May period are in situ systems," said P V Joseph, a former director of the India Meteorological Department.

How Cyclone Fani grew muscle

The in situ cyclonic systems in the Bay of Bengal usually originate around latitude 10 , in line with Chennai or Thiruvananthapuram. Fani, on the other hand, originated quite close to the Equator, around latitude 2 , well below the Sri Lankan landmass. The forecast landfall on the Odisha coast is at a latitude of almost 20 . It has traversed a long way on the sea, thus gaining strength that is unusual for cyclones originating in the Bay of Bengal in this season.

It was initially headed northwestwards, towards the Tamil Nadu coast, but changed course midway, and swerved northeast away from the coastline to reach Odisha. That has given it even more time on the sea.

"If it had remained on its original course, and made a landfall over the Tamil Nadu coastline, Fani would only have been a normal cyclone, not the extremely severe cyclone it has now become. The recurve it has taken gave it more time over the sea and has ensured that it has gathered unusual strength," said meteorologist U C Mohanty of IIT Bhubaneswar.