A Rare Tropical Storm Forms in the South Atlantic: Iba

March 24, 2019, 4:52 PM EDT

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Above: Tropical Storm Iba as seen on Sunday morning, March 24, 2019 by the MODIS instument on NASA's Terra satellite.

The only ocean basin on Earth that does not regularly see tropical cyclones develop is the South Atlantic, but it had a rare tropical storm form on Sunday. Tropical Storm Iba developed about 600 miles northeast of Rio de Janeiro, Brazil, according to the Brazilian Navy Hydrographic Center (BNHC). Ocean temperatures in this region were quite warm, near 29°C (84°F).

Satellite fixes of the system, initially dubbed “Invest 90Q”, showed a modest increase in organization on Sunday morning. The BNHC rated Iba as a minimal tropical storm with sustained winds of 40 mph and a central pressure of 1008 mb at noon EDT Sunday. Steering currents will take Iba to the southeast, out to sea, and the system is not a threat to any land areas. Iba is expected to slowly strengthen to sustained winds of 45 – 50 mph by Monday morning, but is likely to slowly weaken by mid-week, never attaining hurricane status.

While subtropical storms form about once every year in the South Atlantic, this is the first fully tropical cyclone there since 2010, according to an excellent in-depth article by Jonathan Belles of weather.com. Iba’s name was drawn from the single list of names maintained by the BNHC, and is just the ninth South Atlantic storm ever to get a name.

South Atlantic tropical cyclones: a recent acknowledgment

Until the 21st century, it was widely thought that full-fledged tropical cyclones did not form in the South Atlantic. Although waters can be sufficiently warm, there is often too much wind shear, and tropical waves that can serve as seedlings for tropical cyclones do not stream regularly off the coast of southern Africa as they do from northern Africa.

In 2004, expectations were upended when a non-tropical system off the coast of Brazil gradually transitioned into a tropical cyclone and then turned back westward. The system came to be known as Hurricane Catarina, as it made landfall in the Santa Catarina province of Brazil as a Category 1 equivalent on March 27, 2004. More than 38,000 structures were damaged, and another 1,468 collapsed, with three people killed and 185 others injured.

After Catarina, forecasters and researchers began to pay closer attention to the South Atlantic. Research released in 2012 found that 63 subtropical cyclones had formed between 1957 and 2012, or about one subtropical cyclone every year. The first list of names for subtropical and tropical cyclones in the South Atlantic appeared in 2011.

Because records are so sparse, it would be difficult to detect whether human-induced climate change has played any role in the increased number of tropical cyclones being detected in the South Atlantic. The waters over which Catarina formed were actually slightly cooler than average for the time of year, according to an analysis by Ron McTaggart-Cowen and Lance Bosart (University at Albany, SUNY). This year, sea surface temperatures have been running about 1°C (1.8°F) above average in the region where Iba formed.

We'll have an update on Australia's twin landfalling major cyclones, Veronica and Trevor, in our next post.

Bob Henson contributed to this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995 while working on his Ph.D. in air pollution meteorology at the University of Michigan. He worked for the NOAA Hurricane Hunters from 1986-1990 as a flight meteorologist.


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