October to December period is among the favourable months for the development of cyclones in the Bay of Bengal and the Arabian Sea. This year, however, October passed without witnessing a cyclonic storm.

Must read: [Burning Issue] Tropical Cyclones and India

https://www.civilsdaily.com/burning-issue-tropical-cyclones-and-india/

When do cyclones form and hit Indian coasts?

• About 80 cyclones are formed around the world annually, out of which five are formed in the Bay of Bengal and the Arabian Sea, together known as the North Indian Ocean.
• India’s east and west coasts are prone to cyclones with the maximum associated hazards—rain, heavy winds and storm surge— faced by coastal districts of West Bengal, Odisha and Andhra Pradesh.
• Cyclones in the North Indian Ocean are bi-modal in nature, that is, they occur during two seasons— April to June (pre-monsoon) and October to December (post-monsoon).
• Of these, May and November remain the most conducive for the development of cyclones.

When have cyclones skipped October, previously?

• Cyclonic disturbances— either in the form of a well-marked low pressure, depression or a deep depression— are common in October.
• Ocean disturbances enter the Bay of Bengal from the South China seaside and head towards the Indian coast.
• IMD officials have attributed it to the weak La Nina conditions along the equatorial Pacific Ocean.
• Cooler than normal sea surface temperatures over this region—termed as La Nina— has been prevailing since August this year.

Madden Julian Oscillation (MJO)

• Because Madden Julian Oscillation (MJO) was positioned in a favourable phase, the low-pressure systems intensified maximum up to a deep depression.
• MJO is kind of an eastward-moving cyclic weather event along the tropics that influences rainfall, winds, sea surface temperatures and cloud cover. They have a 30 to 60-day cycle.
• Most importantly, there was the high wind shear noted between the different atmospheric levels, last month.
• The vertical wind shear— created due to significant wind speed difference observed between higher and lowers atmospheric levels— prevented the low-pressure systems and depression from strengthening into a cyclone.

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