The month of October passed this year (2020) without witnessing a cyclonic storm whereas October to December period is among the favourable months for the development of cyclones in the Bay of Bengal and the Arabian Sea. Some 80 cyclones occur around the world annually, out of which five are formed in the North Indian Ocean, i.e., the Bay of Bengal and the Arabian Sea. Two of India’s coasts-east and west coasts-are prone to maximum hazardous cyclones, bringing rain, heavy winds, and storm surge, faced by West Bengal, Odisha, and Andhra Pradesh. These cyclones are bi-modal in nature, occurring during two seasons: April to June (pre-monsoon) and October to December (post-monsoon).
As per Indian Meteorological Department (IMD), the absence this year is due to the weak La Nina conditions (cooler than normal sea surface temperatures over the region) along the equatorial Pacific Ocean. La Nina has been prevailing since August this year because Madden Julian Oscillation (a kind of an eastward-moving cyclic weather event along the tropics that influences rainfall, winds, sea surface temperatures, and cloud cover, having a 30 to 60-day cycle) was positioned in the low-pressure systems, intensified maximum up to a deep depression.
Most significantly, there was the high wind shear (due to significant wind speed difference observed between higher and lower atmospheric levels), noted between the different atmospheric levels, which prevented the low-pressure systems and depression from strengthening into a cyclone. IMD officials do not see any immediate weather systems likely to form in November. Besides, the MJO has just crossed the North Indian Ocean region and has moved eastwards. So, there will be no supportive factors which could influence the formation of cyclones this month.
Data on the frequency of cyclone development shows that there have been no cyclonic formations in the month of October on 42 occasions between 1891-2020 as well. There have been cyclonic disturbances either in the form of a well-marked low pressure, depression or a deep depression. Generally, in October, wind intensities ranging from 31 to 61 km/hr formed either over sea or land, entering from the South China sea side and heading towards the Indian coast. In general, IMD labels the formation of one cyclone and two cyclonic disturbances in October as normal. 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, causing widespread rain in Andhra Pradesh, Telangana, north Karnataka, and Maharashtra followed by West Bengal, and Bangladesh.
———–Sidelight——–
La Nina, also known as El Viejo, anti-El Nino, or simply ‘a cold event’ means the Little Girl in Spanish. This episode represents periods of below-average sea surface temperatures across the east-central Equatorial Pacific. Its impacts tend to be opposite to those of El Nino. In the tropics, ocean temperature variations in La Nina also tend to be opposite to those of El Nino. Winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest during a La Nina year.
El Nino means the Little Boy, or Christ Child in Spanish, and was originally recognised by fishermen off the coast of South America in the 1600s, when the water in the Pacific Ocean appeared unusually warm around December. It refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific. There are warmer-than-average temperatures over western and central Canada, and over the western and northern United States. Similarly, wetter-than-average conditions are likely over portions of the U.S. Gulf Coast and Florida, while drier-than-average conditions can be expected in the Ohio Valley and the Pacific Northwest. It influences weather patterns, ocean conditions, and marine fisheries across large portions around the world for a long period.
Courtesy: Indian Express, oceanservice.noaa.gov