3 days ago
Why doesn't the South Atlantic get many tropical storms?
If you look at a map showing the tracks of every tropical depression, subtropical storm, tropical storm and hurricane (called typhoons in the Western Pacific basin and cyclones in the Southern Hemisphere), you will notice something peculiar. There is a complete absence of storm tracks west of South America and very few tracks east of the continent. Why is that?
The waters are too cold
The lack of activity off the west coast of South America is primarily because of cold waters, AccuWeather Lead Hurricane Expert Alex DaSilva says. The Peru Current in the southeastern Pacific Ocean brings cold water unusually far northward along the west coast, too cold for tropical storms and hurricanes. Water temperatures need to be in at least 79 F (26 C) for tropical storms to develop and strengthen.
Wind shear is stronger in these regions
Another factor is higher wind shear, which tears apart most tropical storms before they can strengthen. Wind shear is particularly high in the Southern Hemisphere, due to a larger temperature gradient.
"The waters off Brazil, in the Southern Atlantic, are not as cold, thanks to warmer water coming down from the north, but the wind shear is still too strong to support many tropical storms," DaSilva explained.
The 'wave train' is missing in the South Atlantic
Another missing piece of the puzzle in the South Atlantic is the African wave train, which pushes clusters of thunderstorms off the continent and over the North Atlantic during hurricane season. These storms can then go on to become a tropical depression, storm or hurricane.
"Approximately 80 percent of major North Atlantic hurricanes develop in this area. This conveyor belt of tropical seedlings is absent from the southern Atlantic," DaSilva said.
Only one hurricane is known to have traversed this basin
Only one hurricane in recorded history has formed in the South Atlantic. In 2004, an unnamed storm, locally called Catarina, formed off the coast of Brazil. The storm made landfall near northeastern Rio Grande do Sul with 100-mph winds, killing three people and causing $300 million (2004 USD) in damage.
According to the U.S. National Hurricane Center, only two additional unnamed tropical storms ever roamed the southern Atlantic basin, in 2010 and 2011.
More South Atlantic storms likely missed by official records
Other storms shown on the map above are not recognized in NOAA's database. Many of these are subtropical storms, which possess some tropical characteristics but are not fully developed tropical systems. The map also includes tropical storms named by the Brazilian Navy Hydrographic Center's Marine Meteorological Service (HCMM).
The HCMM has been naming subtropical and tropical storms off the Brazilian coast since 2011, but its threshold for naming storms may not be as rigorous as in the rest of the world, so it may name more storms than other agencies, such as NOAA. On the other hand, because the records in the other basins stretch back much longer, 75 to 150 years, there are likely many storms in the South Atlantic that were never tracked.
Out of roughly two dozen storms named by HCMM in the last 15 years, fewer than 10 of the named storms tracked since 2011 have made landfall on the South American coast.
You may also notice that no storm has ever crossed the equator. This is because, at 0 degrees latitude, the Coriolis force is essentially zero, making it impossible for a tropical system to cross over from one hemisphere to another.
