An unexpected cyclone in the Arabian Sea crossed Gujarat’s coast which left meteorologists puzzled.
- The western Arabian Sea is usually inhospitable to cyclone formation due to cooler sea temperatures and dry air from the Arabian Peninsula.
About Cyclones
- Cyclones are big, swirling masses of air rotating around a centre of low pressure.
- This is a powerful natural phenomenon characterized by strong winds and heavy rainfall.
- It is a low-pressure area in the atmosphere in which winds spiral upward
- Condition for Formation for tropical cyclone: Heat, Air, and Forces
- Temperature: Warm sea surface temperature above 27 degrees Celsius.
- Supply of Warm Air: Large and continuous supply of warm and moist air that can release enormous latent heat.
- Coriolis force: Strong Coriolis force that can prevent the filling of low pressure at the centre (absence of Coriolis force near the equator prohibits the formation of tropical cyclones between 0°-5°°latitude).
- Unstable Condition: Catalyst for Cyclone Development
- Through the troposphere that creates local disturbances around which a cyclone develops.
- Absence of strong vertical wind wedge, which disturbs the vertical transport of latent heat.
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About Cyclone Asna
- This is the first cyclonic storm in the Arabian Sea in August since 1976.
- Between 1891 and 2023: Only three cyclonic storms formed in the Arabian Sea during August (in 1976, 1964, and 1944), according to the IMD.
- Naming: The name Asna has been given by Pakistan.
- Path : The deep depression over the Kachchh coast and adjoining areas of Pakistan and the Northeast Arabian Sea
- It moved westward and intensified into cyclonic storm ‘Asna’ and lay centred at 1130 hours over the same region, 190 km west-northwest of Bhuj (Gujarat).
- Usual Path: Tropical cyclones generally do not cross the equator due to the Coriolis effect, which is responsible for the rotation of cyclones.
- The Coriolis effect is weakest at the equator and increases as you move towards the poles.
- This weak effect near the equator makes it difficult for cyclones to cross from one hemisphere to the other.
- Deep Depression vs Cyclone: A deep depression is a low-pressure system with wind speeds ranging from 52 kmph to 61 kmph, while a cyclone has wind speeds between 63 kmph and 87 kmph.
- Geographically Rare: It is considered geographically rare because it originated over land in Rajasthan, an area far from the typical oceanic origins of cyclones.
- The system then moved into the Arabian Sea, which generally sees fewer cyclones compared to the Bay of Bengal, particularly during the month of August.
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Why More Cyclones Occur In The Bay of Bengal Than In The Arabian Sea?
There are more cyclones in the Bay of Bengal than in the Arabian Sea for several reasons:
- The Bay of Bengal is a shallow embayment: The Bay of Bengal is more shallow than the Arabian sea so there is more heat transfer from the water to the air.
- This increases the potential for moisture and instability, which are necessary ingredients for cyclone formation.
- Surrounding Landmass:The Bay of Bengal is surrounded by land on three sides, which generates additional moisture and instability.
- The low-lying coastal regions often get inundated when the storm makes landfall.
- The Bay of Bengal has a much larger area than other water bodies in the region.
- This makes it easy for a storm to dissipate and West Bengal is one of the most active areas for tropical cyclone formation in the world.
- Landlocked Sea of Arabian Sea: The sea is partially landlocked as it is bordered by Oman, Iran, Pakistan, India, and the Arabian Peninsula.
- The partial landlocked system of the sea makes it more saline in nature.
- The sea is also cooler than the Bay of Bengal, which makes it less prone to cyclones.
- Bigger River Discharge in Bay of Bengal: Large rivers like the Ganges, Brahmaputra, and Irrawaddy discharge warm fresh water into the Bay of Bengal, which keeps the surface layer warm and less saline.
- This warm water helps in the formation and intensification of cyclones.
- Less Convective Activity: Arabian Sea has much less convective activity during the pre-monsoon and is relatively less prone to cyclogenesis.
- The northeast monsoon and dry continental air ends up cooling the Arabian Sea during the post-monsoon.
- The number of cyclones in the Arabian Sea is thus about half of that over the Bay of Bengal in both seasons.
- Deeper and Narrower: The Arabian Sea is deeper, so the waves are not as high as in the Bay of Bengal.
- The Arabian Sea is much narrower than the Bay of Bengal, which means that cyclones have a harder time forming.
- The Bay of Bengal wide expanse of shallow water heats up more quickly than deep water, providing the necessary conditions for storms to form.
- Minimum Temperature Requirement: A sea surface temperature of 27 degrees Celsius and above is needed for a low-pressure system to intensify into a cyclone.
- Currently, the sea surface temperature in the Bay of Bengal is 28-30 degrees Celsius.
- It is around 27-28 degrees Celsius in the Arabian Sea
- It’s colder (below 26 degrees Celsius) in the west-central Arabian Sea and very warm (above 32 degrees Celsius) in the Gulf of Aden.
- The Tropical Cyclone Heat Potential is high in the central Bay of Bengal but low in the northern and central Arabian Sea.
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Reasons are attributed to increasing cyclones in Arabian Sea
- Rising Sea Surface Temperatures (SST): One of the most significant factors is the warming of the Arabian Sea due to global climate change.
- Warmer sea surface temperatures provide more energy and moisture, which are crucial for the formation and intensification of cyclones.
- Reduced Vertical Wind Shear
- Changing Wind Patterns: Historically, the Arabian Sea experienced strong vertical wind shear, which often prevented cyclones from forming or intensifying.
- However, recent years have seen a reduction in vertical wind shear, particularly during the pre- and post-monsoon seasons, allowing cyclones to develop more easily.
- Weakened Monsoon Winds: The southwest monsoon winds over the Arabian Sea have shown signs of weakening, reducing the disruption of cyclonic systems.
Vertical wind shear
- Vertical wind shear refers to the change in wind speed and/or direction with height in the atmosphere.
- Low Vertical Wind Shear: Ideal for tropical cyclone development, as it allows the storm to maintain a vertically aligned structure, concentrating its energy and facilitating intensification.
- High Vertical Wind Shear: Can tear apart the structure of a developing cyclone, preventing it from organizing properly and potentially leading to its dissipation.
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- Positive IOD Events: The Indian Ocean Dipole, a climate phenomenon characterized by the difference in sea surface temperatures between the western and eastern Indian Ocean, has been more frequently positive in recent years.
- A positive IOD warms the western Indian Ocean, including the Arabian Sea, making it more conducive to cyclone formation.
- El Niño and La Niña: These global climatic events have a significant impact on the Indian Ocean region, including the Arabian Sea.
- El Niño tends to reduce cyclone activity in the Bay of Bengal while increasing it in the Arabian Sea, particularly during the post-monsoon season.
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September 2, 2024
