The Deadly Bay – ‘Bay of Bengal’; The Motherhouse of Tropical Cyclones

“The Bay of Bengal is hit frequently by cyclones. The months of November and May, in particular, are dangerous in this regard.” – Dr. A. P. J. Abdul Kalam

-By Dr. Arup Giri

The Deadly Nature of Bay of Bengal

The Bay of Bengal is the ‘Mother House’ of 90% deadly tropical cyclones in the world. This bay located within 5° to 22° (N) Latitude and 80° to 90° (E) Longitude which covers about 2,173,000 km² of South Asia. The average depth of this bay is 2600 meters (m), whereas the maximum extent is about 4694 m.

Bay of Bengal responsible for 50% cyclone related deaths, whereas it only domiciliates 0.6% of the global ocean. Still, it has the highest percentage for the origination of tropical cyclones.

Generally, tropical cyclone originates in tropical waters which located within the 5-30 latitude north or south of the equator. Within this region, the sea surface temperature belongs within the 27°C. The terminology of cyclones varies on the location of the Globe. It is known as ‘hurricanes‘ in the North Atlantic, and the central and North Pacific region. It is known as ‘typhoons‘ in the northwest Pacific, whereas it is known as ‘tropical cyclones‘ in the South Pacific and the Indian Ocean.

From year after year, the tropical cyclones originated in the Bay of Bengal (BOB); most of them converted to severe storms, which caused a lot of causalities and destruction of human civilization of different countries located around the Bay of Bengal.

It has been found that, mainly, the East coast of India, Bangladesh, was being affected by these tropical cyclones.

All the cyclones with their typical characteristics that are wind speed, depression pressure, year of origin, and their effects in the affected country has been listed in the following table.

Table 1: Description of cyclones originated at the Bay of Bengal from 1970 to 2020

To know more about all the cyclones (Click the following Box)

Historically it has been found that 19^th and 20^th century has the rising century for the deadly cyclones. Most of them affected the east coast of the Indian subcontinent. The following images are showing the origination and the track of cyclonic storms, severe cyclonic storms, very severe cyclonic storms, extremely severe cyclonic storms, and super cyclonic storms from 1972 to 2017.

Cyclonic storms	Severe cyclonic storms	Very Severe cyclonic storms
Extremely cyclonic storms		Super cyclonic storms

Classification of Cyclonic Disturbances

According to the Indian Meteorological Division (IMD), depending upon the intensity, there are different types of cyclones which are originated at the Bay of Bengal.

Table 2: Classification of cyclones (Data collected from Mishra et al., 2019).

Cyclones Affected Tamil Nadu in Last 20 years

Many cyclonic storms affected Tamil Nadu for last 20 years. Followings are the list of those cyclones.

The Structure of Tropical Cyclones

The total body of tropical cyclone consist of the ‘Eye,’ the ‘Eyewall,’ ‘Rainbands,’ and ‘Tropical Cyclone Size.’

The Eye

The central calm and clear area of the connective clouds is known as the ‘Eye’ of tropical cyclones. It formed might be due to the combination of “the conservation of angular momentum” and centrifugal force. These cumulative effects can create a clear and calm region in the center of a cyclone.

The Eyewall

The Eyewall is the ring around the eye, which mainly gets the clouds which are characterized by the thunderstorm, rain, and control of the wind speed. The storm’s intensity depends upon the nature of the cloud in the eyewall, which controlled by the eye.

Rainbands

Rainbands are the curved tail of the eyewall, which spun around all the structures contacting thunderstorms called rainbands. In the rainbands, some gaps may be found which have no rain and wind.

Tropical Cyclone Size

Tropical Cyclone Size is mainly, the intensity of the cyclone known as the tropical cyclone size.

Bay of Bengal Vs. The Arabian Sea for Cyclone Bed

IMD data showed that several cyclones originated at the Bay of Bengal are higher than the number of cyclones originated at the Arabian Sea. The graph depicted that in all the decade, the percentage of the tropical cyclone was higher.

The study indicated that dry air from the Middle East Desert shocked all the vapors, which reduced the probability of cyclogenesis at the Arabian Sea. In contrast, the warmer BOB is the typical bed for tropical cyclone formation.

The “Cyclogenesis” at BOB and Their Movement

It has been found that about 77% of tropical cyclones were originated at BOB in November month, followed by 63% in December, 61% in May. Cyclones of BOB mainly heated on Bangladesh in May, Oct and Nov month and Myanmar in May month. West Bengal, Odisha, and Andhra Pradesh coastal region of India heated in June, August, and September month. Whereas the south coast of India struck mainly in October, November, and December month.

Reasons Behind the Deadly Nature of ‘Bay of Bengal’

Years after year, the deadly nature of BOB is devastating the Indian subcontinent, Bangladesh, Sri Lanka, Myanmar. In the Globe, there are many other coastlines located like the Gulf Coast of Louisiana. Those are not experiencing such calamities as coastlines of BOB. But, why BOB is the motherhouse of all these deadly cyclonic storms?

According to Gray (1979), high sea surface temperature, lower vertical wind shear, higher vorticity, an increase of Coriolis force, higher relative humidity are the main component to form tropical cyclones.

All these properties are present in BOB. For these reasons, Bob Henson, meteorologist, said that the Bay of Bengal is a “Textbook example of this type of geography.” Director of IMD, M Mohapatra, says, “It is a very warm sea.”

So how all these properties are attributing to the formation of tropical cyclones in BOB?

• The shape of the Bay: The funnel shape of BOB is one factor. This funnel-shaped caused a massive surge of cyclones.
• Depth of the Bay: Shallow depth of the BOB caused the extra surge and clipped the coastal region with severe rain, flood, and strong winds.
• Sea Surface Temperature: The most crucial factor is the increase in sea surface temperature (SST). The average SST of BOB near 28°C due to sluggish wind and massive rainfall caused higher humidity. These are the ideal for the genesis of cyclones. According to Girishkumar and Ravichandran (2012), El Nino caused an increase of 0.25°C SST. The sluggish wind has further increased the SST. On the other way, El Nino caused a reduction of convective activity, which also reduced the reaching of solar radiation. These created a further increase in SST. 

• Relative humidity: According to Gray (1968), at least 40% relative humidity is necessary for the typical cyclogenesis. Due to global warming, El-Nino, and higher rain at BOB makes the motherhouse of tropical cyclones.  
• Low-level winds (850 hPa): Low levels wind is another typical parameter for the formation and strengthening of tropical cyclones. A low magnitude of winds helps in the creation of a dense cloud.
• Vertical wind shear: Lower vertical wind shear (VWS) favors the cyclogenesis. 

‘Amphan‘ – The Deadly Cyclone in 2020

‘Amphan‘ the super cyclone brought back the decade-old memories of Odisha, a state of India. In 1999, Odisha was washed out after the super cyclone as nearly 10 000 peoples were killed and damaged nearly Rs 20,000 crores.

With an average wind speed of 185km/h (115mph), Amphan gusted up to on West Bengal and Odisha, states of India and Bangladesh. It took 128 life in India and caused the damage of thousand crores.

Conclusion

The Bay of Bengal is one of the world’s most deadly Bay for cyclones formation due to the presence of almost all the favorable conditions.  Year after year, the most destructive cyclones have originated and destroyed the east coast of India, Bangladesh, Myanmar, Sri Lanka. Mainly for the tropical cyclones, people live perpetually at risk from this destructive weather phenomenon in the coastal states along the Bay. The northern part of India experienced only stormy winds (50-60 km/h) and little rainfall. But, in coastal regions of India, what will be in the future as the climate is getting worsen day by day? We all know how human beings are the most destructive factor for the increase in global temperature to increase pollution at sea. So, the time is now for taking steps for a healthy future.

Source

• Bhardwaj, P., & Singh, O. (2020). Climatological characteristics of Bay of Bengal tropical cyclones: 1972–2017. Theoretical and Applied Climatology, 139(1), 615-629. (PDF)
• Gray, W.M. (1979). Hurricanes: Their formation, structure and likely role in the general circulation. Meteorology over the Tropical Oceans, D. B. Shaw, Ed., Royal Meteorological Society, 155–218. (PDF)
• Mishra SP, Sethi KC, Mishra DP, Siddique M. 2019. Pre Monsoon Cyclogenesis Over Bay of Bengal. International Journal of Recent Technology and Engineering, 8, 4895-4908. (PDF)

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About the Author

The author is Dr. Arup Giri, Associate Professor of Baba Mastnath University, Rohtak, Haryana, India. To know more about him, you may follow the links.

1. ResearchGate: https://www.researchgate.net/profile/Arup-Giri
2. LinkedIn: https://www.linkedin.com/in/dr-arup-giri-b5657391/
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4. Google Scholar: https://scholar.google.com/citations?user=hMvFttkAAAAJ&hl=en

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