Do hurricanes form in the South Atlantic?

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  • Map displaying tracks of all known Atlantic tropical cyclones from 1842 to 2012. Note that only Catarina's track is plotted in the South Atlantic. Credit: www.csc.noaa.gov/hurricanes.

    Satellite image of Cyclone Catarina prior to landfall over Brazil in March 2004. Image: NASA

    Tropical storm Anita off the Brazillian coast in March 2010. Image: NASA.

  • Do hurricanes form in the South Atlantic?
    26.09.2013 16:49

    Tropical cyclones are a regular occurrence across the North Atlantic through the summer and autumn months. In fact, twelve named systems and six hurricanes develop across the basin in an average season, some threatening communities across the Caribbean, Central America and the east coast of the United States. But why do we hear so little about tropical cyclones in the South Atlantic?

    A number of environmental conditions are required for tropical cyclogenesis to occur. Two of these factors, namely pre-existing disturbances and weak vertical wind shear, are lacking in the South Atlantic, making tropical cyclone formation here a rare occurrence.

    Tropical cyclones develop from low-level disturbances with sufficient rotation. In other ocean basins, these disturbances can take the form of thunderstorms in the intertropical convergence zone (ITCZ). This is the region where the trade winds come together to produce areas of cloud and showers, which migrate north and south of the equator with the position of the overhead sun. However, the northward and southward extent of the ITCZ varies around the world, and over the Atlantic it tends to remain north of the equator throughout the year. North Atlantic tropical cyclogenesis also benefits from the African Easterly Jet. This low-level jet stream, which results from the north-south temperature contrast between the Sahara desert and Gulf of Guinea, is responsible for producing easterly waves. These troughs of low pressure move west across the Atlantic and the thunderstorms they bear can develop into tropical cyclones. In fact, easterly waves account for the majority of tropical cyclone development in the North Atlantic. The African Easterly Jet owes its existence to the shape of the African continent in the Northern Hemisphere, and therefore has no counterpart in the South Atlantic.

    Tropical cyclone development in the South Atlantic is also constrained by high vertical wind shear, that is the change in wind speed and direction with height. When wind shear exceeds 10m/s between the lower and upper troposphere, the organised convection required for intensification of disturbances can be disrupted through shearing and the removal of heat and moisture from the core.

    Despite this hostile environment, tropical cyclones have occasionally been observed in the South Atlantic since the availability of satellite imagery in the 1970s. The most notable of these was Cyclone Catarina in March 2004, the only hurricane ever known to have developed in the basin. Catarina, named unofficially by Brazillian meteorologists, formed when a mid-latitude upper level low became slow moving off the coast of Brazil and acquired heat and moisture from the tropics. The low developed into a subtropical cyclone, possessing characteristics of both tropical and extratropical systems, and transitioned into a fully-tropical warm core system under favourable vertical wind shear conditions. Catarina made landfall near the Brazilian town of Torres as a Category 1 hurricane with winds of around 85mph, killing at least three people and damaging 36,000 homes.

    Catarina was one of only nine tropical or subtropical storms officially recorded in the South Atlantic since 1974. Five of these have occurred since Catarina’s landfall in 2004, and several have led to damage and fatalities in southern Brazil. In 2011, Subtropical storm Arani became the first system to receive an official name by the Brazillian Navy Hydrographic Centre.

    By: Billy Payne