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South Atlantic tropical cyclone
South Atlantic tropical cyclones are unusual weather events that occur in the southern hemisphere. Strong wind shear (which disrupts cyclone formation) and a lack of weather disturbances favorable for tropical cyclone development make any hurricane-strength cyclones extremely rare. Typically, storms develop during the months from November through to April, however tropical cyclogenesis can occur anytime in the year. Since 2011, the Brazilian Navy Hydrographic Center has started to assign names to tropical and subtropical systems in this basin, when they have sustained wind speeds of at least 65 km/h (40 mph). Below is a list of notable South Atlantic tropical and subtropical cyclones.
==Theories concerning infrequency of occurrence==
Until April 1991 it was thought that tropical cyclones did not develop within the South Atlantic. Very strong vertical wind shear in the troposphere is considered a deterrent. The Intertropical Convergence Zone drops one to two degrees south of the equator, not far enough from the equator for the Coriolis force to aid development. Water temperatures in the tropics of the southern Atlantic are cooler than those in the tropical north Atlantic.
During April 1991, these assertions were proven false when the United States National Hurricane Center reported that a tropical cyclone had developed over the Eastern Atlantic.〔 In subsequent years, a few systems were suspected to have the characteristics needed to be classified as a tropical cyclone including in March 1994 and January 2004. During March 2004, an extratropical cyclone formally transitioned into a tropical cyclone and made landfall on Brazil, after becoming a category 2 hurricane on the Saffir-Simpson hurricane wind scale. While the system was threatening the Brazilian state of Santa Catarina, a newspaper used the headline "Furacão Catarina," which was presumed to mean "furacão (hurricane) threatening (Santa) Catarina (the state)".〔 However, when the international press started monitoring the system, it was assumed that "Furacão Catarina" meant "Cyclone Catarina" and that it had been formally named in the usual way.〔
At the Sixth WMO International Workshop on Tropical Cyclones (IWTC-VI) in 2006 it was questioned if any subtropical or tropical cyclones, had developed within the South Atlantic before Catarina.〔 It was noted that suspect systems had developed in January 1970, March 1994, January 2004, March 2004, May 2004, February 2006 and March 2006.〔 It was also suggested that an effort should be made to locate any possible systems using satellite imagery and synoptic data, however, it was noted that this effort maybe hindered by the lack of any geostationary imagery over the basin before 1966.〔 A study was subsequently performed and published during 2012 which concluded that there had been 63 subtropical cyclones in the Southern Atlantic between 1957 and 2007. During January 2009 a subtropical storm developed over the basin, before a tropical storm developed during March 2010 and was named Anita by Brazilian public and private weather services. During 2011, the Brazilian Navy Hydrographic Center started to assign names to tropical and subtropical cyclones, that develop within its area of responsibility to the west of 20°W when they have sustained wind speeds of at least 65 km/h (40 mph).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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