Tropical cyclones (TCs) are intense low pressure systems where the wind speed in the surface circulation system exceeds 33 nautical miles per hour (knots). The low-pressure systems over the northern Indian Ocean (NIO) are classified (Table 1) on the basis of associated maximum sustained surface wind (MSW) at the surface level. Systems with the intensity of depressions and above are considered cyclonic disturbances. TCs occurring over the north Atlantic Ocean are known as hurricanes and those occurring over the Pacific are known as typhoons. A TC that has a MSW of 64 knots corresponds to a very severe cyclonic storm (VSCS) over the NIO. TCs occurring near the Australian continent are the willy-willies. Tropical cyclones usually do not occur over the south Atlantic Ocean or SE Pacific (east of 140oW) mainly because of the colder sea surface temperatures (SSTs) and other unfavourable ocean and atmospheric conditions.
About five to six TCs occur in a year over the NIO during the pre-monsoon (March-April-May) and post-monsoon (October-November-December) seasons (Fig. 1). This accounts for about seven per cent of all global TCs. The ratio of formation over the Bay of Bengal (BoB) vis-à-vis the Arabian Sea (AS) is 4:1 (Fig. 2). Globally, the frequency is maximum over the northwest Pacific followed by the north Atlantic Ocean. In the NIO, the frequency of TCs is bimodal, with primary maxima in the month of November followed by secondary maxima in the month of May.
TCs generally move in a north-westerly direction. However, they may sometimes re-curve depending upon atmospheric conditions. The BoB TCs mainly strike the Odisha-West Bengal coast in October, Andhra coast in November and the Tamil Nadu coast in December. Over 50 per cent of the TCs in the BoB strike different parts of the eastern coast of India, 30 per cent strike the coasts of Bangladesh, Myanmar and Sri Lanka and about 20 per cent dissipate over the sea (Fig. 2). The percentage of TCs dissipating over the AS is higher (60 per cent) as the western AS is cooler. Maximum landfall occurs over the Gujarat coast (18 per cent of the total cyclones in the AS) in India, followed by coastal Oman.
The life period of a TC over the NIO is 5-6 days. The VSCS with an intensity of 64 knots or more lasts for 2-3 days as against a global average of 6 days. The vertical and horizontal structures of a tropical cyclone are described in Fig. 3 and 4. There are several stages that comprise the life-cycle of an average TC. These stages are not discrete entities, but represent a continuous process. The period from the formation of an initial disturbance or low-pressure area, to its intensification into a depression, a deep depression, and thereon to a cyclonic storm and its ultimate weakening make up the life cycle of a TC. This can be divided into four stages—formative, immature, mature and decaying.
Formative stage: Since the development of a TC is a continuous process, features associated with the earliest stages may overlap. The formative stage marks the period when an initial low-pressure disturbance intensifies into a depression, a deep depression and finally into a TC. The pressure falls gradually along with an increase in surface wind speed. Clouds and rain associated with the storm occur in a disorganised pattern at this stage. The development of marked circular cloud masses also occurs at this stage and lasts a few days.
Immature stage: In this stage, two things occur —rapid fall of pressure in the central region of the TC, and strengthening of winds in the surface circulation. At the end of this stage, the lowest pressure and the strongest winds associated with the storm are reached. The winds, clouds, and precipitation pattern become more organised, and form spiral bands directed inward. This stage lasts for half a day to 2-3 days. The duration is dependent upon the ocean basin. For BoB and the AS, the stage lasts up to a day while for the Atlantic Ocean, it takes 2-3 days.
Mature stage: In this stage, the system reaches a steady state. The central pressure no longer drops and wind strength does not increase. However, the circulation expands in area and the size of the system expands horizontally in all directions to reach its maximum size. Strong winds extend up to 200 miles from the centre. The symmetry in circulation associated with the cyclone is lost and the maximum wind and maximum pressure gradient are concentrated in the right forward sector of the TC in the northern hemisphere. This stage lasts a few days to a week depending on the basin in which it is formed.
Decaying stage: In this stage, the TC weakens into a depression, and gradually or rapidly subsides depending upon the ocean basin and atmospheric conditions. Dissipation occurs when:
- Landfall results in supply of moisture being cutoff and surface friction increases.
- TC enters into an area of relatively cold waters which is below 26°C.
- TC remains for too long in the same area of the ocean and upper 100 feet of warmer water mixes with the colder water due to upwelling.
- Entrance of colder and dry air in lower levels of the TC that causes weaker circulation.
- TC experiences a high vertical wind shear and the convective heat engine moves away from the centre, preventing further development.
- Formation of an outer eye wall, typically around 50-100 miles from the centre of the storm, chokes off convection within the inner eye wall. Such weakening is generally temporary unless it meets the other conditions mentioned above. If the outer eye walls merges with the inner eye wall, the TC may strengthen.
Sometimes, if the TC meets mid-latitude westerlies, it can turn into an extra-tropical cyclone. When passing over the sub-tropical ridge while moving north or north-eastwards, cold air is likely to favour its transformation into an extra-tropical cyclone.
About half the depressions formed develop into TCs, and only less than a quarter intensify into severe TCs. However, the rate of intensification differs with the seasons (Fig. 3). While it takes about 2-4 days for a low-pressure area to develop into a depression, intensification into severe or very severe cyclones can occur in 24-48 hours.
Climatological information is crucial for effectively planning hazard management during severe cyclonic conditions. The understanding can also be utilised to predict the characteristics of a TC based on analogue techniques.