Q+A: Cyclone Ockhi

In India, every year the Indian coast, especially the east coast witness several cyclonic storms. This year, the latest powerful cyclone named Ockhi (means eye in Bangla) has caused extensive damage to the lives of the people living in coastal areas. It brought heavy downpour in southern Tamil Nadu and Kerala and battered the coastal areas of Maharashtra and Gujarat before transforming into a low pressure area over the Gulf of Khambhat. We have discussed the fundamentals of cyclone here, if you would like to review. Some other specific questions are as follows:

How the cyclone Ockhi is different from other cyclones that have hit the Indian coasts?

In India, cyclones are known to occur in both the Bay of Bengal and the Arabian Sea. The Bay of Bengal witnesses four times more cyclones than those experienced by the Arabian Sea. But the case of Ockhi is peculiar. The cyclone originated near the south-western coast of Sri Lanka and followed a very unique trajectory by travelling along the coasts of Tamil Nadu, Kerala and Lakshadweep islands, then it took north-easterly turn travelling along the coasts of Maharashtra and Gujarat. The cyclones in Maharashtra and Gujarat are not a common phenomenon. According to M. Rajeevan Nair, secretary, Earth Sciences, this is only the third time that a cyclone has taken such a unique route since 1891.

Second, Ockhi has originated very near to the equator. It had formed five degrees south. Usually, cyclones do not originate in the equatorial zone due to the earth’s rotation and shape. Cyclones tend to form at least seven degrees north or south of the equator.

Why the Bay of Bengal experiences more cyclones than the Arabian Sea?

The Bay of Bengal experiences more cyclones in a year due to a variety of reasons. The waters of Bay of Bengal are relatively warmer conducive for the formation of cyclones. The shape of the sea is also considered as a factor which encourages cyclones. In Bay of Bengal, every year, at least three cyclones gets developed. These cyclones usually make landfall in the states of Odisha, Andhra Pradesh and Tamil Nadu, or even Bangladesh.

In case of Arabian Sea, the relatively colder waters of this Sea are not considered conducive for the formation and intensification of cyclones. Also, the eastern coast of India does not receive cyclones only from Bay of Bengal but also from those formed in the Andaman Sea near the Andaman and Nicobar Islands. But in case of the western coast, the cyclones originating in the Pacific Ocean gets weakened when it hits a big landmass and hence do not travel to the Arabian Sea. Hence, Arabian Sea witnesses fewer cyclones when compared with the Bay of Bengal.

However, in the recent years, Arabian Sea has become active in terms of cyclone. The study of cyclones in the Arabian Sea since the last 15 years has documented this recent phenomenon. Usually, the Arabian Sea witnesses a big storm in a span of 7 years but nowadays such storms are recorded every second or third year. The reason for this recent phenomenon is not confirmed. There are many theories ranging from climate change to change in temperature of water to aerosols to Arabian Sea’s own cyclicity, as reasons for the frequent occurrence of cyclones in Arabian Sea.

Why the Ockhi cyclone is labelled as a powerful cyclone?

The India Meteorological Department (IMD has described the Ockhi cyclone as a ‘Very severe cyclonic storm’. IMD places cyclones in categories based on the maximum wind speed generated by them. Ockhi cyclone had a wind speed between 155 and 165 km per hour.

IMD’s cyclone classification
  • Super Cyclonic Storm (>222 km/h)
  • Very Severe Cyclonic Storm (118-221 km/h)
  • Severe Cyclonic Storm (88-117km/h)
  • Cyclonic Storm (62-87 km/h)
  • Deep Depression (52-61 km/h)
  • Depression (<51 km/h)

The most famous example of super cyclone is the one experienced in Odisha coast in 1999. The cyclone with the wind speed of 260 km/h is one of the most severe cyclones experienced in India. The Phailin cyclone (2013) had a wind speed of around 220 km per hour.

Discuss about cyclone forecasting in India.

In India, in the recent years, the IMD has been forecasting cyclones five to six days in advance so as to minimise the damages caused by the cyclones. The early forecast of the cyclone depends upon the place where the cyclone is emerging. For example, in case of Phailin (2013), Hudhud (2014) or Vardah (2016), all of them developed in the Anadaman Sea. They took nearly 5-6 days to hit the eastern coast. But in case of cyclones like which have their origins very near to home. IMD cannot issue warnings 5-6 days in advance. The forecasts can be made by IMD only after the detection of an emerging depression which has the properties of a cyclonic storm. The Ockhi cyclone was detected only in the morning of November 29. By the next day many parts of Tamil Nadu and Kerala began to feel the impact of the cyclone. The lead time in the forecast for cyclones such as Ockhi is usually very less.

India employs a very sophisticated cyclone warning system which is one of the best in the world. IMD has a very good observation system with coastal radars supplemented by the US satellites. Recently also, India’s cyclone warning system was updated with the installation of the Global Forecasting System in collaboration with the US. These are mathematical systems that make use of supercomputers placed in Pune and Noida and offer a very high resolution of 12 km. Further, Rs 450 crore has been earmarked for establishing additional machines so that 40 forecasting scenarios could be easily formed making the predictions of IMD more sensitive. The most sophisticated so far developed in the world is capable of predicting the cyclones five to six days in advance.

How are cyclones formed in North Indian Ocean basin are named?

The name of the cyclone Ockhi means ‘eye’ in Bangla language. The cyclones are given names to enable ease of communication and to differentiate one natural phenomenon from the other. Also, at the same time two or more cyclones can occur at the same place at the same time. So naming of cyclones can reduce the confusion among the general public.

Throughout the world, various meteorological organisations are responsible for naming the cyclones. In the North Indian Ocean basin, the cyclones are named by the Indian Meteorological Department (IMD). To name the cyclones originating in the Indian Ocean region, a formula was agreed upon in 2004. As per it, eight countries in the region, namely Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand are contributing a set of names which are assigned to cyclones sequentially. The names of the cyclones which have caused widespread deaths are retired as a mark of respect for the dead. These names are then replaced with new names. So far around 50 names have been replaced since 1972 globally.