Rare subtropical storm forms in South Atlantic Ocean

In 2021, an unusual tropical system developed in the southern Atlantic Ocean where tropical systems rarely occur. Only 14 named tropical systems were ever recorded in the southern Atlantic Ocean, a majority of which were 'just' subtropical storms. This means that they have meteorological characteristics of both a tropical storm and a non-tropical storm.

On April 20, 2021, subtropical storm Potira formed off the coast of southeastern Brazil with sustained winds of 65 kilometers per hour outside of the traditional northern Atlantic.

Potira's origin could be traced back to a non-tropical disturbance that moved off the coast of Brazil over the previous weekend before stalling over the ocean just off the coast. As it sat over the water, it gradually started to organize and take on some characteristics of a proper tropical storm, eventually being declared a subtropical storm by Brazil's Navy.

The frequency of this phenomenon has been trending upward in recent years. The most recent was subtropical storm Oquira which developed on December 28, 2020.

Only one system has ever reached hurricane status in the South Atlantic. In 2004 near the end of March, Hurricane Catarina became a Category 2 hurricane before making landfall in Brazil.

Why are tropical systems so rare in the southern Atlantic Ocean? The conditions south of the equator are different than they are north of the equator. One of the biggest factors is windshear. When wind shear is low, it allows disturbances to organize and potentially develop into a tropical storm or hurricane. This disruptive wind is typically higher in the southern Atlantic when compared to the northern Atlantic. Another factor is that the water temperature is typically lower in this region of the southern Atlantic, limiting the potential for tropical development.