South Atlantic tropical depression dissipates

Satellite images and wind measurements from the Quikscat satellite show that a rare tropical depression in the South Atlantic probably formed for a few hours today, but the storm has since been sheared apart by strong upper-level winds, and is not a threat to re-develop. Although the storm was tropical, had a closed circulation, and winds of up to 35 mph (according to the Quikscat satellite), it only had those characteristics for about three hours today. The National Hurricane Center usually does not designate a system as a tropical depression unless it can hold together for at least six hours. The system formed near 29S 36W, about 600 miles southeast of Rio de Janeiro, Brazil, over waters of about 27 degrees C--well above the 26 C threshold needed for tropical storm formation.


Figure 1.Visible image from 1613 GMT February 24, 2006, taken by the polar-orbiting AQUA satellite, showing a possible tropical depression that formed in the South Atlantic.

Are South Atlantic tropical cyclones a sign of climate change?
Only one hurricane and two tropical depressions have been observed in the South Atlantic since 1970, when accurate tracking methods became available with the advent of weather satellites. There is usually too much wind shear to allow a tropical cyclone to form, and the South Atlantic lacks an active "Intertropical Convergence Zone" (ITCZ)--that stormy band of weather that stretches along the Equator and acts as a source region for many of the disturances that grow into Northern Atlantic hurricanes. With Hurricane Catarina of March 2004, another tropical depression in January 2004, and now yet another "near miss" tropical cyclone in the South Atlantic, I believe is it time that the NHC considered adopting a naming system. Had today's system intensified into a tropical storm, it would not have been given a name, since there is no naming system for the South Atlantic Ocean. It's quite possible that the recent activity in the South Atlantic is due to climate change causing wind shear levels to drop over the South Atlantic. The alternative explanation is that we are seeing an active period that has a long cycle, and last repeated itself before satellites were around. Given the Atlantic Multi-decadal Oscillation (AMO) that affects hurricane activity in the North Atlantic, it is reasonable to think we might see a similar pattern in the South Atlantic. In either case, it's time we had a system in place to start naming these storms to avoid confusion.

My next blog will be on Monday, a discussion of the proposed NWS staff reductions via an early retirement plan. Will we lose our best forecasters at the NHC and other NWS forecast offices?

Jeff Masters