In a rare move yesterday, the Brazilian Navy's weather division declared a swirling storm off the southeastern coast of Brazil (near São Paulo) a subtropical storm named Bapo. Tropical and subtropical cyclones in the South Atlantic Ocean are relatively rare, with only a hundred or so recorded in the past sixty years.

The organization decided to name the system after determining that the storm acquired subtropical characteristics. We're pretty familiar with tropical cyclones in the United States—they're highly organized low pressure systems that form over warm water and feature a warm core (warm air from bottom to top) and tightly-packed thunderstorms around the system's center. Subtropical storms, on the other hand, feature warm air at the surface with cooler air aloft, and the system's thunderstorm activity is often displaced a good distance from the center. They're not completely tropical, but they're close enough to be considered subtropical. We see them every once and a while in the Atlantic (especially early and late in the season), and sometimes they can transition into a fully tropical system, as we saw with Bermuda's Hurricane Fay last October.

Here's a look at a surface analysis from the Brazilian Navy Hydrography Center issued today at 7:00 AM EST, showing Subtropical Storm Bapo (or "Tempestade Subtropical Bapo" on the map) sitting off the southeastern coast of Brazil. The "B" denotes the center of low pressure (or "baixa pressão" in Portuguese), just as English-speaking meteorologists use an "L."

The storm is moving south-southeast away from land, and shouldn't affect anyone but shipping interests.

Thanks in large part to wind shear and cooler waters, tropical cyclones in the South Atlantic are relatively rare. Out of the hundred or so tropical or subtropical cyclones seen in the South Atlantic since the 1950s, the strongest on record was Cyclone Catarina in March 2004.

The cyclone, pictured above, produced 100 MPH winds at its peak, making Catarina the equivalent to a category two hurricane in the United States. The storm made landfall in the Brazilian state of Santa Catarina (hence its name), producing major damage and killing several people.

The Southern Hemisphere is currently in the middle of summer. Hurricane season in the North Atlantic doesn't begin until June 1, but we can see tropical activity sooner in some cases. Back in 2003, Subtropical Storm Ana developed near Bermuda in the middle of April, becoming one of only a handful of storms to develop in the North Atlantic before May.

[Images: NASA, BNHC, NASA]

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