Tropical cyclones are intense low pressure systems with mean sustained winds of 34 knots ( ~63 kmph) and more. These form over warm tropical oceans and are known as hurricanes in the Atlantic Ocean and typhoons in Pacific. Globally on an average 80 cyclones form in a year. Of these five cyclones form over the North Indian Ocean (NIO). Of the five cyclones, four form over the Bay of Bengal and one over the Arabian Sea.
On an average three cyclones intensify into the category of ‘Severe Cyclone’ (as cyclone Nisarga) and 1-2 into ‘Very Severe Cyclone’. Higher frequency of cyclones in the Bay of Bengal as compared to Arabian Sea is because of its warmer sea surface temperatures and movement of remnants of typhoons from the South China Sea to the Andaman Sea.
The frequency of cyclones has a primary peak in November and secondary in the month of May. Though only 7 per cent of annual global cyclones form over NIO, Bay of Bengal (BoB) cyclones cause the highest number of deaths in the world. This accounts for more than 75 per cent of total world wide deaths associated with cyclones in the past 300 years (Dube et al. 2013).
In the last two decades with improved observations, accurate warnings and better disaster preparedness, there has been a significant decrease (less than 100) in the number of deaths (Mohapatra and Sharma 2019). Cumulative impacts of storm surge, strong winds, heavy rainfall and coastal bathymetry cause higher numbers of deaths. This is especially true for the coastal districts of Andhra Pradesh, Odisha, West Bengal and Bangladesh. The 1970 cyclone Bhola (12-13 November) killed about 300,000 people in East Pakistan (now Bangladesh). This is the highest number of deaths associated with any cyclone (Cerveny et al. 2017).
Cyclone Tracks, Landfall and Trend in India
The tracks of tropical cyclones during 1961-2018 are shown in Fig. 1 and frequency of land falling cyclones in coastal states of BoB and Arabain Sea are shown in Fig 2.
Fig 1. Tracks of tropical cyclones over North Indian Ocean (1961-2018)
Fig 2 : Frequency of land falling cyclones (1981-2018)
Most cyclones south of 15o N move west/west-northwest in the Arabian Sea towards the Oman coast. Percentage of cyclones dissipating over the Arabian Sea is higher (63 per cent) as western Arabian Sea is cooler. Maximum land fall occurs over Gujarat coast (18 per cent) followed by the Oman coast.
The cyclones crossing 15o N have a tendency to recurve and move towards Gujarat and the north Maharashtra coast. In the last 128 years (1891-2017) a total of 22 cyclones made landfall over the Gujarat coast. There were 6 cyclones that made a landfall over Maharashtra and Goa coast. Over Kerala coast there were just 2 cyclones making a landfall and none over Karnataka coast (Table 1). The last cyclone over the Maharashtra and Goa coast apart from the current cyclone Nisarga has been way back in 2009.
Table 1: Number of Arabian Sea cyclones making landfall over the west coast of India (1891-2017)
Global warming and cyclones
There is a general impression about the likely increase in the frequency of cyclones due to global warming. Contrary to general perception, Singh et al. (2018) have observed a clear decreasing trend in annual cyclone frequency over the NIO and particularly over BoB during the current warming phase (1947 to 2017). However, cyclone activity over eastern Arabian Sea shows considerable enhancement during the current warming phase. Similar findings for NIO and BoB were also found by Mohapatra et al. (2014) during the satellite era (1961-2010). According to Kunston et al. (2019) there is a significant increase in intense cyclones over the Arabian Sea during the pre-monsoon season. This bears a low confidence as the frequency of intense cyclones are less, the studies are based on recent years only.
Severe Cyclone Nisarga
India Meteorological Department (IMD) in the morning Bulletin on May 30 indicated the formation of a low pressure system. This system has formed over the southeast and adjoining east central Arabian Sea, intensifying in the subsequent 24 hour.
Leading edge of the advancing southwest monsoon, presence of Madden Julian Oscillation (MJO) Index in phase 1 with an amplitude of more than 1. A 30 -32o C sea surface temperature over the Arabian Sea provided favourable conditions for the formation of a low pressure and its subsequent intensification. A low pressure area formed over southeast and adjoining east central Arabian Sea at 0530 hrs of May 31, 2020.
Press Release Issuance
The first press release issued by the IMD in the evening (1730 hrs) of May 31 stated that the cyclone Nisarga was a, “Low pressure area”. They continued by adding that “is very likely to concentrate into a depression over east central and adjoining southeast Arabian Sea in next 24 hours and likely to intensify into a cyclonic storm during the subsequent 24 hours. It is very likely to move nearly northwards and reach near north Maharashtra and Gujarat coast by June 3.”
The cyclone Nisarga system did concentrate into a ‘Depression’ on the morning of June 1 at 0530hrs centred near 13.0o N and 71.4o E, about 370 km southwest of Panjim and 690 km south-southwest of Mumbai.
It further concentrated into a ‘Deep Depression’ at 0530 hrs of June 2. The Deep Depression intensified into the cyclonic storm ‘Nisarga’ at 1130 hrs of June 2. This was centered about 280 km west-northwest of Panjim and 430 km south-southwest of Mumbai. The system was forecasted to intensify into a Severe Cyclonic Storm and cross the coast near Alibag on June 3 afternoon at 1430 hrs. The maximum sustained wind of 110-120 kmph gusting to 120 kmph (Fig.3 ).
Figure 3. Observed and Forecast track along with the cone of uncertainty of Nisarga.
INCOIS also issued a storm surge warning of a maximum of 1.3 m wave height at Alibag (Fig. 4). This is likely to inundate an area of around 1.4 km.
Fig 4: Storm surge warning by INCOIS, Hyderabad