Only three tropical cyclones are known to have formed in the Southern Atlantic Ocean, the most recent of which was Tropical Storm Anita in 2010.

On Sunday, Brazilian meteorologists began tracking a new tropical storm, Iba, that has formed off the country's southeast coast. According to the Brazilian Navy Hydrographic Center, the storm formed maximum winds of 40mph and a central pressure of 1008 millibars. Present estimates suggest it has intensified to 50mph, but it is not expected to directly threaten land.

Although Iba is not all that far from the Brazilian coast, atmospheric steering currents will pick up the storm and carry it to the south and then southwest by Tuesday or Wednesday. Once over the Southern Atlantic Ocean, cooler waters and unfavorable winds should act to dissipate the storm. Over the next day or so, it should come nearest to Brazilian towns Caravelas and Vitoria.

A rarity

Such cyclones are a rarity. Only one hurricane, in fact, is known to have formed in this area. In 2004, Hurricane Catarina reached 100mph before making landfall in southern Brazil, about halfway between Rio de Janeiro and the country's border with Uruguay. This is further south than where Iba has developed.

Scientists were not even sure that the Southern Atlantic Ocean could support tropical systems until the early 1990s. There are several reasons for this. One is that wind shear—varying wind speeds and directions at different altitudes—is typically higher in the Southern Atlantic Ocean in the tropics near Brazil and disrupts organized storm activity.

Another factor is that the "Intertropical Convergence Zone," or ITCZ, typically lies close to the equator. The ITCZ is caused by overhead heating from the Sun, so it migrates from south during the Southern Hemisphere summer in January into the Northern Hemisphere in July. This heating causes air at the surface to rise, pulling in warm, moist air from the tropics (hence, convergence). This rising air creates a fertile atmosphere for showers and thunderstorms to form.

During the Northern Atlantic summer, the storms within the ITCZ often spawn tropical systems and sometimes hurricanes. However, the Southern Hemisphere ITCZ is less pronounced, and it typically remains closer to the equator, where cyclones cannot form because the Coriolis effect (which gives cyclones their spin) is just too weak.

So lacking a potent ITCZ away from the equator and favorable wind shear conditions, tropical cyclones typically don't form in the Southern Hemisphere tropics in the Atlantic. Iba is an exception, but then again it's been an exceptional year for the Southern Hemisphere. Already, the 2018-2019 South-West Indian Cyclone season has set a record for number of intense storms, with nine to date.

There is no reason to think this activity will translate into a particularly active North Atlantic hurricane season, which begins June 1. The University of Colorado's Tropical Meteorology Project is expected to release its first forecast for the 2019 Atlantic season on April 4.