A cyclone called Ockhi — why this is raising such an unusual storm

A cyclone called Ockhi — why this is raising such an unusual storm

Context

Cyclones are no strangers to the Indian coast, our east coast in particular witnessing several cyclonic storms each year. Yet, Ockhi, the latest powerful cyclone, is unlike other recent ones

Where do cyclones originate in India?

In India cyclones originate in both the Bay of Bengal and the Arabian Sea sides of the northern Indian Ocean; there is much more frequency on the Bay of Bengal side though, especially of the stronger cyclones — in fact, the Bay of Bengal side witnesses four times more cyclones than the Arabian Sea side on average

Where did Ockhi originate?

Ockhi originated near the south-western coast of Sri Lanka, and travelled very near the southern-most tip of the Indian mainland, along the coasts of Tamil Nadu and Kerala, towards the Lakshadweep islands, where it was at its most powerful

  • It weakened considerably after that and continued further, taking a north-easterly turn towards the Maharashtra and Gujarat coastlines —cyclones in this area are not a common phenomenon

Why does the Bay of Bengal have more cyclones than the Arabian Sea?

Meteorologists say

  • Colder waters of Arabian Sea: The relatively colder waters of the Arabian Sea are not conducive to the formation and intensification of cyclones
  • Typhoons from the Pacific ocean: Additionally, the eastern coast of India receives cyclones that form not just in the Bay of Bengal, mostly around the Andaman Sea near the Andaman and Nicobar Islands, but also those travelling from the Pacific Ocean, where the frequency of ‘typhoons’, as these are called there, is quite high. Most of these cyclones weaken considerably after encountering a big landmass. Therefore, these do not travel to the Arabian Sea side. The western coast of India thus witnesses only those cyclones that originate locally or the ones, like Ockhi, that travel from the Indian Ocean near Sri Lanka

Severity of cyclonic storms

Cyclones are categorized by the maximum wind speed they generate

TypeWind Speed (Kmph)
Very Severe Cyclonic StormB/w 155 and 165
Extremely Sever Cyclonic StormB/w 165 and 220
Super-Cyclones> 220

Example:

  • The most famous instance of a ‘super-cyclone’ was the one that hit the coast of Odisha in October 1999. It was the strongest-ever cyclone recorded in that area, with wind speeds touching 260 km per hour. It was also the most devastating cyclone to have hit India
  • The 2013 Phailin cyclone very nearly got categorized as a super-cyclone. It had maximum wind speeds of around 220 km per hour

Ockhi’s windspeed

Ockhi had wind speeds between 155 and 165 km per hour, touching the upper border for ‘very severe cyclonic storm’

Cyclone forecasts by the IMD in the recent past have been made five to six days in advance, thereby minimizing the damage caused — was the IMD late in issuing a warning for Ockhi?

How early the forecast is depends on how far we are from the place where the cyclone is emerging. Many of the big cyclones in recent years, like Phailin in 2013, Hudhud in 2014 or Vardah in 2016, developed near the Andaman Sea. From there, it took those cyclones about five to six days to hit the Andhra Pradesh or Odisha coasts

How do cyclones form?

Tropical cyclones require certain conditions for their formation.  These are

  • A source of warm, moist air derived from tropical oceans with sea surface temperature normally near to or in excess of 27 °C
  • Winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form
  • Winds which do not vary greatly with height – known as low wind shear. This allows the storm clouds to rise vertically to high levels;
  • Coriolis force / spin induced by the rotation of the Earth. The formation mechanisms vary across the world, but once a cluster of storm clouds starts to rotate, it becomes a tropical depression. If it continues to develop it becomes a tropical storm, and later a cyclone/ super cyclone

Naming of Cyclones

  • Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts and warnings. Since the storms can often last a week or even longer and more than one cyclone can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described.
  • Names were first used in World War II and were subsequently adopted by all regions. In most regions pre-determined alphabetic lists of alternating male and female names are used. However, in the north-west Pacific the majority of names used are not personal names. While there are a few male and female names, majority are names of flowers, animals, birds, trees, foods or descriptive adjectives. By the mid-1960s names were used for all tropical storms except those in the North Indian Ocean . The names currently in use and those to be used in future years are listed.  Various meteorological organisations have responsibility of naming them.
  • The names of cyclones in Indian Seas are not allocated in alphabetical order, but are arranged by the name of the country which contributed the name. It is usual practice for a storm to be named when it reaches tropical storm strength (winds of 34 knots).The list of names to be used for the North Indian Seas is given below:

The names selected for North Indian Ocean cyclones from 2004 onwards

Cyclone names (Source: IMD Website)

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