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Thursday, December 03, 2020

Explained: Why did cyclones give October a miss?

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.

Written by Anjali Marar , Edited by Explained Desk | Pune | Updated: November 12, 2020 9:46:51 am
cyclones, cyclones in october, indian cyclones, bay of bengal cyclone, arabian sea cyclone, cyclone newsThis May 17, 2020, satellite image released by NASA shows Cyclone Amphan over the Bay of Bengal in India. (NASA Worldview, Earth Observing System Data and Information System (EOSDIS) via AP)

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.

When have cyclones skipped October, previously?

Cyclonic disturbances— either in the form of a well-marked low pressure, depression or a deep depression (weather systems with varying wind intensities ranging from 31 – 61 km/hr formed either over sea or land)— are common in October. Ocean disturbances enter the Bay of Bengal from the South China sea side and head towards the Indian coast. Generally, IMD labels the formation of one cyclone and two cyclonic disturbances in October as normal. This year, however, there was no system which intensified to form a cyclone. Instead, there were three cyclonic disturbances. Two of these cyclonic disturbances in the form of low-pressure systems caused widespread rain in Andhra Pradesh, Telangana, north Karnataka and Maharashtra followed by West Bengal and Bangladesh.

Data on the frequency of cyclone development between 1891-2020 shows no cyclone formation in the month of October on 42 occasions. During the last 130 years, cyclones remained absent for the longest during October of 1950-1954. 📣 Express Explained is now on Telegram

Why were there no cyclone developments this year?

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. 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.

What is the cyclone forecast for November?

Even though November is one of the main months when cyclones develop, at present, IMD officials note that there are no immediate weather systems likely to form in either seas. In addition, since the MJO has just crossed the North Indian Ocean region and has moved eastwards, there will be no supportive factors that could influence the formation of cyclones this month.

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