Context: The supercyclone ‘Amphan’ is likely to make landfall between the Sagar islands of West Bengal and the Hatiya islands of Bangladesh.
Key Points

• Meteorologists and atmospheric science experts state that higher than normal temperatures in the Bay of Bengal (BoB) may be whetting super cyclones and the lockdown, indirectly, may have played a role.
• Another researcher said the elevated ocean temperatures this year could, in part, be explained by the lockdown. Reduced particulate matter emissions during the lockdown meant fewer aerosols, such as black carbon, that are known to reflect sunlight and heat away from the surface.
• Cyclones gain their energy from the heat and moisture generated from warm ocean surfaces.
• This year, the BoB has posted record summer temperatures, a fall-out, as researchers have warned, of global warming from fossil fuel emissions that have been heating up oceans.
• It is said that warming around India is no longer restricted to just the BoB but also the Arabian Sea and the Indian Ocean.
• Tropical cyclones in these seas are a typical feature of the summer months and play a role in aiding the arrival of the monsoon.

Factors responsible for the intensification of cyclones in BoB

• Higher than normal temperatures in the Bay of Bengal (BoB) may be whetting ‘super cyclones’ and the lockdown, indirectly, may have played a role.
• Cyclones gain their energy from the heat and moisture generated from warm ocean surfaces. This year, the BoB has posted record summer temperatures a fall-out, as researchers have warned, of global warming from fossil fuel emissions that has been heating up oceans.
• Lockdown impact: Reduced particulate matter emissions during the lockdown meant fewer aerosols, such as black carbon, that are known to reflect sunlight and heat away from the surface.

General factors responsible for the origin of Cyclones in Bay of Bengal region are:

• Large sea surface with temperature higher than 27° C.
• Presence of the Coriolis force enough to create a cyclonic vortex.
• Small variations in the vertical wind speed.
• A pre-existing weak low-pressure area or low-level-cyclonic circulation.
• Upper divergence above the sea level system.

Arabian Sea is comparatively less prone to cyclonic storms than Bay of Bengal

• Temperature: BOB is hotter than Arabian sea. Hot water temperature is the basic criteria for the development & intensification of cyclones.
• Salinity: Arabian sea has higher salinity than BOB. It’s easier to heat & simultaneously evaporate water having lower salinity.
• Location: The typhoons originating in the Pacific Ocean too influences the cyclones in BOB, not the case in Arabian Sea.
• Movement: According to IMD cyclones originating in Arabian Sea are believed to move northwest. So they actually move away from Indian mainland.
• The Bay receives higher rainfall and constant inflow of fresh water from the Ganga and Brahmaputra rivers. This means that the Bay’s surface water keeps getting refreshed, making it impossible for the warm water to mix with the cooler water below, making it ideal for a depression.

Why the East Coast?

• More cyclones are formed in the Bay of Bengal than the Arabian Sea due to wind patterns (that keep oceans cooler on the western side).
• Even among those formed along the western coast, many move towards Oman instead of hitting Indian shores.
• Storms formed on the eastern coast, however, are more intense and since states on that side have relatively flatter topography compared to the western coast, they can’t deflect the winds.

About Cyclones

• They are the formation of a very low-pressure system with very high-speed winds revolving around it. Factors like wind speed, wind direction, temperature and humidity contribute to the development of cyclones.
• Before cloud formation, water takes up heat from the atmosphere to change into vapour. When water vapour changes back to liquid form as raindrops, this heat is released to the atmosphere.
• The heat released to the atmosphere warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated.
• Hurricanes derive their energy from heated seawater which can be prevented by presence of upper-level-winds that disrupt the storm circulation forcing it to lose its strength.