Context

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.

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.

Reason behind ‘no cyclone’ developments this year

• Weak La Nina: The weak La Nina conditions along the equatorial Pacific Ocean can be the reason behind the no cyclone development.
  • Cooler than normal sea surface temperatures over this region—termed as La Nina— has been prevailing since August this year.
• 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 lower atmospheric levels— prevented the low-pressure systems and depression from strengthening into a cyclone.