Why aren’t South America and the west coast of Africa hit by hurricanes?

Tropical storms hitting South America have distinct disadvantages compared with the North American situation.

The energy to drive tropical systems is derived from warm ocean waters. For tropical storms to form, the ocean temperature needs to exceed 79.7 Fahrenheit, which is common in the North Atlantic and Caribbean oceans between June and November. Reaching these temperatures is rare in the South Atlantic Ocean, even during their summer. Hurricanes do not form within 5 degrees of the equator and that includes a large geographic region of South America. The small Coriolis forces there inhibit circulation development.

Tropical Cyclone tracks from 1985 – 2005. Credit: NOAA

Tropical storms that develop in the eastern Atlantic will be carried away from the African continent by the winds that steer them. The east coast of Africa does get hit by typhoons that form over the Indian Ocean or Bay of Bengal.

Although the moisture-laden remnants of a tropical storms can result in flooding in California, the West Coast rarely sees landfall of a storm. The only known tropical storm to make an official landfall in California occurred on Sept. 24, 1939. Hurricanes have difficulty forming off the U.S. west coast due to cold water, cold currents and unfavorable winds. Storms that might approach southern California from western Mexico would be steered away by the prevailing upper-level winds.

The waters off the west coast are colder than the threshold for tropical storm formation. The California Current brings cold water north to south along the coast. Also, the prevailing northerly winds in the summer and fall push waters away from the coastlines. This results in upwelling that brings cooler ocean water from below to the surface. This upwelling process also occurs off the western coastline of South America.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Tropical

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