Published: 2007-08-06 10:20:52
Updated: 2007-08-06 10:20:52
Posted August 6, 2007
MIKE MOSS SAYS: Michael, The most proximate reasons for the lack of activity in the South Atlantic are sea surface temperatures that tend to run a shade cooler than ideal for tropical cyclone formation even in the southern summer, climatologically high values of vertical wind shear across that basin throughout the year, and a lack of pre-existing centers of rotation (vorticity) in that area. This last point is due both to the absence of tropical waves similar to those generated over west Africa in the northern hemisphere and to the fact the the Intertropical Convergence Zone (ITZC), which shifts toward the north in our summer and toward the south in the southern summer, usually stays at or north of the equator in the Atlantic Ocean (this zone of convergence, thunderstorms, horizontal wind shear and periodic low level rotation, and its cousin the Monsoon Trough in the Indian and Australian basins, are often formation regions in those basins and the central and eastern Pacific). A broad convergence zone like this need to reach about 5 degrees of latitude away from the equator in order for the Coriolis Force to have sufficient intensity to organize a full-fledged tropical cyclone, and the Atlantic ITCZ almost never shifts that far south, while the Monsoon trough in the Indian and Australian basins, and the western South Pacific, regularly does so.
The eastern South Pacific is just about as void of activity as the South Atlantic, due largely to the ITCZ also having a tendency to stay near or north of the equator, and to climatalogically cool sea surface temperatures. Of course, one can then ask why are these factors aligned in such a way, which would make for a very long answer - the short version is they are all the result of the shapes, sizes, and distribution of continents and ocean basins, together with the character of the landform surfaces, and the Earth's tilted rotation axis and not quite circular orbit around the sun.
Even though the South Atlantic is a very unfavorable location for tropical cyclones, at least three possible such systems have been observed there since satellite monitoring began. The first was off Angola just west of Africa in 1991, another in January 2004 off Brazil, and most famously what has come to be named Cyclone Catarina, which reached apparent category 2 intensity before making landfall in southern Brazil on March 28, 2004.