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Rare Tropical Storm Spotted in Atlantic Ocean
NASA's Aqua satellite captured an infrared image of Sub-Tropical Storm Arani along the Brazilian coast. Most of the convection and thunderstorms (purple) were limited to the eastern half of the storm.
Credit: NASA JPL/Ed Olsen.

A rare kind of tropical storm recently formed in the Atlantic Ocean off the coast of Brazil and was spotted by a NASA satellite.

The satellite tracked a spot of severe weather that strengthened into Sub-Tropical Storm Arani in the South Atlantic Ocean.

Arani formed near the coast of Brazil and is now moving away from it. Tropical cyclones in the South Atlantic are a rare weather event and Arani is only the third since 2004. (The first known South Atlantic tropical cyclone the collective name for tropical storms, hurricanes and typhoons was seen in 2004 and called "Catarina.")

On March 14, NASA's Aqua satellite captured an infrared image of Sub-Tropical Storm Arani moving away from the Brazilian coast. Most of the convection and thunderstorms associated with the system were limited to the eastern half of the storm at the time. These strong areas of convection (rapidly rising air that condenses and forms the thunderstorms that power a tropical cyclone) appeared on the imagery as a sideways boomerang, and were offshore, paralleling the coast.

The satellite measured the temperatures in those strong areas of convection and found they were minus 63 degrees Fahrenheit (minus 52 degrees Celsius) or colder, indicating some strong thunderstorms with heavy rainfall. That heavy rainfall was occurring offshore.

Despite the storm moving away from the coast, on March 15, the Brazilian Navy issued a special marine warning for the Brazilian coast. Sub-Tropical Storm Arani is forecast to continue moving away from the Brazilian coast over the next couple of days.

Tropical cyclones in the Southern Atlantic Ocean are rare because the waters of the Southern Atlantic are usually too cool for the storms to form.

In 2010, a storm called System 90Q formed in the same region where Sub-Tropical Storm Arani formed this year.

Jack Beven, of the National Hurricane Center in Miami said that Sub-Tropical Storm Arani is not currently as well developed as Tropical Storm Catarina or System 90Q last year.