Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical Convergence Zone (ITCZ), or sometimes the Intertropical Discontinuity (ITD).
Nearly all of these systems form between 10 and 30 degrees of the equator and 87% form within 20 degrees of it. Because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones rarely form or move within about 10 degrees of the equator, where the Coriolis effect is weakest. It is, however, possible for tropical cyclones to form within this boundary if there is another source of initial rotation. These conditions are extremely rare, and such storms are believed to form at most once per century. An example of this is Typhoon Vamei which formed at only 1.5 degrees north of the equator in 2001.
There have traditionally been seven basins in which tropical cyclones are formed. These include the North Atlantic Ocean, the eastern and western parts of the Pacific Ocean (considered separately because tropical cyclones rarely form in the central Pacific), the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. The North Atlantic is the most studied of the basins, while the Western Pacific is the most active and the North Indian the least active. On average there are around 80 tropical cyclones forming worldwide each year.
In addition to the institutions in the table above, the Joint Typhoon Warning Centre (JTWC) issues informal advisories in all basins except the North Atlantic and Northeastern Pacific. The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues informal advisories, as well as names, for tropical cyclones that approach the Philippines in the Northwestern Pacific. The Canadian Hurricane Centre (CHC) issues advisories on hurricanes and their remnants that affect Canada.
Northern Atlantic Ocean
The most-studied of all tropical basins, it includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Tropical cyclone formation here varies widely from year to year, ranging from over twenty to one per year with an average of around ten. The United States Atlantic coast, Mexico, Central America, the Caribbean Islands and Bermuda are frequently affected by storms in this basin. Venezuela, the south-east of Canada and Atlantic "Macaronesian" islands are also occasionally affected. Many of the more intense Atlantic storms are Cape Verde-type hurricanes, which form off the west coast of Africa near the Cape Verde islands.
Northeastern Pacific Ocean
This is the second most active basin in the world, and the densest (a large number of storms for a small area of ocean). Storms that form here can affect western Mexico, Hawaii, northern Central America, and on extremely rare occasions, California and Arizona.
Northwestern Pacific Ocean
Tropical storm activity in this region frequently affects China, Japan, Hong Kong, the Philippines, and Taiwan, but also many other countries in South-East Asia, such as Vietnam, South Korea and Indonesia, plus numerous Oceanian islands. This is by far the most active basin, accounting for one-third of all tropical cyclone activity in the world. The coast of China sees the most landfalling tropical cyclones worldwide.
Northern Indian Ocean
This basin is divided into two areas, the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating (5 to 6 times more activity). This basin's season has an interesting double peak; one in April and May before the onset of the monsoon, and another in October and November just after. Tropical cyclones which form in this basin have historically cost the most lives - most notably, the 1970 Bhola cyclone killed 200,000. Nations affected by this basin include India, Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan. Rarely, a tropical cyclone formed in this basin will affect the Arabian Peninsula.
Southwestern Pacific Ocean
Tropical activity in this region largely affects Australia and Oceania.
Southeastern Indian Ocean
Tropical activity in this region affects Australia and Indonesia.
Southwestern Indian Ocean
This basin is the least understood, due to a lack of historical data. Cyclones forming here impact Madagascar, Mozambique, Mauritius, and Kenya.
The following areas spawn tropical cyclones only very rarely.
South Atlantic
A combination of wind shear and a lack of tropical disturbances from the Intertropical Convergence Zone (ITCZ) makes it very difficult for the South Atlantic to support tropical activity. However, three tropical cyclones have been observed here - a weak tropical storm in 1991 off the coast of Africa, Cyclone Catarina (sometimes also referred to as Aldonįa), which made landfall in Brazil in 2004 at Category 1 strength, and a smaller storm in January 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometer winds.
Southeastern Pacific
Tropical cyclone formation is rare in this region; when they do form, it is frequently linked to El Niņo episodes. Most of the storms that enter this region formed farther west in the Southwest Pacific. They affect the islands of Polynesia in exceptional instances. During the 1982/83 El Niņo event, French Polynesia was affected by six tropical cyclones in five months.
Mediterranean Sea
Storms which appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Examples of these "Mediterranean tropical cyclones" formed in September 1947, September 1969, January 1982, September 1983, and January 1995. However, there is debate on whether these storms were tropical in nature.
Temperate Subtropics
Areas further than thirty degrees from the equator are not normally conducive to tropical cyclone formation or strengthening, and areas more than forty degrees from the equator are very hostile to such development. The primary limiting factor is water temperatures, although higher shear at increasing latitudes is also a factor. These areas are sometimes frequented by cyclones moving poleward from tropical latitudes.
Low Latitudes
Areas within approximately ten degrees latitude of the equator do not experience a significant Coriolis force, a vital ingredient in tropical cyclone formation. However, in December 2001, Typhoon Vamei formed in the Southern South China Sea and made landfall in Malaysia. It formed from a thunderstorm formation in Borneo that moved into the South China Sea.
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