Tropical cyclones (TCs) actively form in different ocean basins of the globe and have different characteristics. Typhoons in the western North Pacific are most frequent in genesis, and the annual number of TCs beyond storm intensity in this basin is maximum as compared to other basins of the world. Most typhoons are generated from the inter-tropical convergence zone (ITCZ) or monsoon trough. Super or midget typhoons, typhoons with extra tropical transition and erratic track typhoons can often be found in this ocean basin. Most hurricanes in the north Atlantic develop from an easterly wave moving westward. On the other hand, TC genesis in the ocean around Australia in the southern hemisphere is closely related with the activity of the monsoonal shear line or the south Pacific convergence zone (SPCZ). This essay briefly discusses the characteristics of cyclones in the northern Indian Ocean (NIO).
Impact of cyclones
A TC is a multi-hazard phenomenon, as it causes heavy rain culminating into floods, strong wind leading to structural damage, and storm surges resulting in coastal inundation (Fig. 1). In the BoB, storm surges can be much stronger than in other ocean basins. A higher increment of sea water and storm surges from a landfalling cyclone could submerge some lower islands along the coast and inundate coastal lands and farms, plunging populations into misery and suffering. Another impact is very strong winds from a landfalling cyclone, especially winds blowing towards the shore terrain. These can destroy ships, trees, homes, and built structures. The impact of a TC in the NIO may not be just felt along the coastal region around the BoB, but also in neighbouring countries in peninsular Indochina.
There is also a close link between heavy rainfall in the south-eastern Tibetan Plateau and south-western China and TCs in the BoB. On the other hand, when the TC interacts with a mid-latitude trough, moisture associated with the cyclone can move along the westerly stream into southern and eastern China to bring increased rainfall over an extensive region. TC activity in BoB can also impact movement of a storm in the western North Pacific and marginal sea regions as the TC can influence and modulate the subtropical high ridge.
On the other hand, TCs in the Arabian Sea (AS) are far fewer and less disastrous than those in the BoB. However, a very strong TC can occasionally occur and prove calamitous as well. In June 2007, Cyclone Gonu moved north-westward and made landfall in northeast Oman (Fig. 2). It also cut across the Gulf of Oman and made a second landfall in eastern Iran. Gonu was a well developed cyclone and reached the intensity of a super cyclonic storm, resulting in a heavy toll of life and property in countries in this region.
The extra-tropical transition (ET) process, a common phenomenon with TCs all over the world, is missing for TCs in the NIO. The ET usually arises from the intrusion of weak cold air and the interaction of the TC with a mid-latitude westerly trough. This can expand rainfall distribution and increase the rate of rainfall associated with a TC to cause more damage. Countries in the mid and higher latitudes have paid more attention to the ET phenomenon. The absence of ET process over the NIO is mainly due to the Himalaya and the Iranian Plateau, which blocks the cold air to meet the TC.
TCs rarely occur in three ocean basins, namely the south Atlantic, eastern south Pacific and the near equatorial region encircling the globe. However, the past few years have seen cyclogenesis affect these basins too. Hurricane Katrina occurred in March 2004 in the south Atlantic and made landfall in Brazil. Typhoon Vamei formed in the waters of the southern part of the South China Sea, only 1.5º north of the equator. Typhoon Vamei made landfall in Malaysia, and then cut across Sumatra into the southern part of the BoB. The severe cyclonic storm, Agni, too, formed close to the equator in late November 2004. This was the closest-ever to the equator that a cyclone had formed in history. The formation of Vamei in the South China Sea and Agni in the NIO has challenged all theories of cyclogenesis to date.
Low frequency in genesis
TCs are not frequent in the NIO basin. In fact, only 7 per cent of all global storms occur in this basin. However, TCs in the NIO basin are the most destructive. There are many reasons for this. The trumpet-shaped coastline of the Bay of Bengal (BoB), the special continental shelf with shallow waters of the Bay and the low flat coastal topography makes the region most vulnerable to the impact of a TC. Storm surges from a landfalling TC especially take a heavy toll of life and property. For instance, a TC made landfall at Chittagong east of the Ganges Delta on November 12, 1970, killed over 3,00,000 people. Another TC in April 29, 1991 claimed more than 1,38,000 lives, while Cyclone Sidr that struck on November 15, 2007 killed more than 3,300 persons. Another severe storm, TC Nargis made landfall on May 2, 2008 in the densely populated Ayeyarwady Delta and claimed about 1,38,000 lives. Thus TCs in this region, particularly in the Bay of Bengal (BoB) show lower frequency in genesis but result in the highest fatalities in the world.
Bi-modal distribution of genesis
Cyclogenesis in BoB can be traced to a disturbance in a monsoonal trough in-situ or disturbances moving westward from the South China Sea. Cyclogenesis of TCs in other ocean basins occurs only once a year, such as in August for the western North Pacific and eastern North Pacific, in January for the southern Indian Ocean, and September for the North Atlantic. But the seasonal distribution of storm genesis is different in the BoB and the NIO basin. Here, there are two peak seasons for cyclogenesis—a major peak in November and a secondary one in May.
This is an unusual phenomenon since typhoons and hurricanes frequently occur in ocean basins from a monsoonal trough. Observations prove that monsoonal troughs have a seasonal migration northward from late spring through mid-summer and end up over land or the northern part of BoB over a small area of seawater. This could be presumably the reason behind cyclone formation being suppressed in the mid-summer monsoon season. In addition, the high vertical wind shear (difference of wind in upper and lower tropospheric levels) during the monsoon season and stronger westerlies in the lower tropospheric level are important inhibiting factors.