From UPSC perspective, the following things are important :
Prelims level : Tropical Cyclones
Mains level : Rising events of Tropical Cyclone in India
The storm system in the Bay of Bengal, Amphan, developed into a super cyclone and is expected to make landfall along the West Bengal-Bangladesh coast very soon.
Q. In the South Atlantic and South Eastern Pacific regions in tropical latitudes, cyclone does not originate. What is the reason? (CSP 2015)
(a) Sea Surface temperature are low
(b) Inter Tropical Convergence Zone seldom occurs
(c) Coriolis force is too weak
(d) Absence of land in those regions
Super Cyclone Amphan
- Cyclone Amphan is a tropical cyclone formed over the Bay of Bengal that has intensified and likely to turn into a “super cyclonic storm (maximum wind speed is 224 kmph)”.
- It has been named by Thailand.
- Amphan is the equivalent of a Category 5 hurricane on the Saffir-Simpson Hurricane Wind Scale.
- By the time it makes landfall in West Bengal, Amphan is expected to tone down into a category 4 Extremely Severe Cyclonic (ESC) storm with a wind speed of 165-175 kmph and gusting to 195 kmph.
What makes it a nightmare?
- This is the first super cyclone to form in the Bay of Bengal after the 1999 super cyclone that hit Odisha and claimed more than 10,000 lives.
- It is the third super cyclone to occur in the North Indian Ocean region after 1999 which comprises of the Bay of Bengal, the Arabian Sea and the northern part of the Indian Ocean.
- The other two super cyclones were Cyclone Kyarr in 2019 and Cyclone Gonu in 2007.
Recent cyclones in the region
- From 1965 to 2017, the Bay of Bengal and the Arabian Sea collectively registered 46 ‘severe cyclonic storms’.
- More than half of them occurred between October and December.
- Seven of them occurred in May and only two (in 1966 and 1976) were recorded in April, according to data from the IMDs cyclone statistics unit.
- Cyclone Phailin in 2013 and the super cyclone of 1999 — both of which hit coastal Odisha — have been the most powerful cyclones in the Bay of Bengal in the past two decades in terms of wind speed.
- Last year, Fani, which was an ESC made landfall in Odisha and ravaged the State, claiming at least 40 lives.
Back2Basics: Tropical Cyclones
- 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°C 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 an ITCZ in the Bay of Bengal 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 the 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.
What strengthens them?
- A thumb rule for cyclones 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 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.
Grading of Cyclones
- 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 kmph)
- severe cyclonic storms (89 to 117 kmph)
- very severe cyclonic storms (118 to 166 kmph)
- extremely severe cyclonic storms (167 to 221 kmph) and
- super cyclones (222 kmph or higher)