Tropical Cyclone concept
Relevance: Prelims/Mains: G.S paper I: Physical Geography
CONTEXT
As extremely severe cyclone ‘Amphan’ entered the Indian shores in West Bengal and Odisha on Wednesday, lakhs of people have been evacuated from vulnerable areas and shifted to safety. Amphan—the second super cyclone to have had formed over the Bay of Bengal in two decades—made a landfall between Digha, some 180 km south of Kolkata in West Bengal, and Hatiya islands in Bangladesh during the afternoon to evening hours of Wednesday, May 20, with the maximum sustained wind speed of 155-165 kmph gusting to 190 kmph.
ABOUT
- Cyclone Amphan is a tropical cyclone formed over the Bay of Bengal that has intensified and turned 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 the cyclone so serious?
- 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.
- 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)
Why cyclones like Amphan are so deadly?
Five facts about tropical cyclones and storm surges.
– Tropical cyclones –
Cyclones are low-pressure systems that form over warm tropical waters, with gale force winds near the centre. The winds can extend hundreds of kilometres (miles) from the eye of the storm.
Sucking up vast quantities of water, they often produce torrential rains and flooding resulting in major loss of life and property damage.
They are also known as hurricanes or typhoons, depending on where they originate in the world, when they reach sustained winds of at least 119 kilometres per hour (74 miles per hour).
Tropical cyclones (hurricanes) are the most powerful weather events on Earth, according to NASA.
Storm surges –
Cyclones can unleash catastrophic storm surges — tsunami-like flooding — when they make landfall. They can be the deadliest part of a cyclone and are only partially affected by wind speeds.
The term “storm surge” refers to rising seas whipped up by a storm, creating a wall of water several metres higher than the normal tide level.
The large swells move faster than the cyclone and are sometimes spotted up to 1,000 kilometres ahead of a major storm.
The surge can extend for dozens of kilometres inland, overwhelming homes and making roads impassable.
A storm surge is shaped by a number of different factors, including storm intensity, forward speed, the size of a storm and the angle of approach to the coast.
The underlying features of the land at the coast, including bays and estuaries, are also at play.
In previous storms, people failed to flee because they did not grasp the surge’s deadly threat.
That was the case for 2013’s Super Typhoon Haiyan, which left 7,350 dead or missing in the central Philippines, primarily due to the surge.
A storm surge of up to five metres (16 feet) is likely to inundate low-lying areas of West Bengal during Amphan’s landfall, and be up to 3.5 metres in Bangladesh, according to the Indian Meteorological Department.
– Low-lying areas –
India’s eastern coast and neighbouring Bangladesh, a low-lying delta nation, are routinely hit by bad storms between April and December that cause deaths and widespread property damage.
Bangladesh is vulnerable to cyclones due to its location at the triangular-shaped head of the Bay of Bengal, the geography of its coastal area and its high-population density, according to experts.
Hundreds of thousands of people living around the Bay of Bengal have been killed in cyclones in recent decades.
The death tolls have come down in the past few years because of faster evacuations and the building of thousands of coastal shelters.
– Bay of Bengal –
The tropical cyclone season in the Bay of Bengal and neighbouring Arabian Sea has two peaks around May and November, according to the World Meteorological Organisation.
The cyclones can form in the western Pacific Ocean and travel in a northwest direction before arriving in the Bay of Bengal. Some reach the southeastern coast of India but others divert northeast and move up to West Bengal and Odisha states.
The Bay of Bengal has conditions favourable to the development of cyclones, including high sea surface temperatures.
Some of the deadliest storms in history have formed in the Bay of Bengal, including one in 1970 that killed half a million people in what is modern-day Bangladesh.
Some 138,000 died in Bangladesh in 1991 in a tidal wave caused by a cyclone.
In 1999 in India’s Odisha state, 10,000 people were killed by a cyclone.
In 2007, Cyclone Sidr killed at least 4,000 in southern Bangladesh.
Then in 2008 Cyclone Nargis, which devastated Myanmar’s Irrawaddy Delta, killed about 140,000 people.
– Climate change –
Studies suggested a warming climate could bring more destructive cyclones as there would be extra heat in the oceans and atmosphere, although such systems could also become less frequent.
Rising sea levels could boost storm surges from cyclones, making them even more deadly and destructive.
For more such notes, Articles, News & Views Join our Telegram Channel.
Click the link below to see the details about the UPSC –Civils courses offered by Triumph IAS. https://triumphias.com/pages-all-courses.php