Subtropical Storm Cari Forms Near Brazil; South Pacific's Cyclone Pam a Cat 4

A rare subtropical storm, with characteristics of both a tropical and a non-tropical system, has formed in the South Atlantic waters off the coast of Brazil, according to the 8 am EDT March 11, 2015 analysis by the Navy Hydrographic Center in Brazil. The storm has top wind speeds near 45 mph, according to an 3:40 pm EDT Tuesday pass from the Rapidscat instrument on the International Space Station. The surface pressure was near 1000 mb, and a modest amount of heavy thunderstorm activity was firing up near the center of circulation, and a more substantial amount on its southeast side, in a band well removed from the circulation center. Sea surface temperatures are near 27°C, which is about 0.5°C above average, and 1°C above what is typically needed to support a tropical storm. Phase space diagrams from Florida State show the storm has a weak warm core at low levels which should get better defined through Thursday morning, but by Thursday evening the storm will begin losing its tropical characteristics as wind shear increases and Cari moves to the southeast, away from Brazil, over cooler waters. It is unlikely the storm has time to become fully tropical, and Cari is not a threat to make landfall.

Here are the recent IR and VIS satellite loops from GOES-East, courtesy of NOAA's Dan Lindsey.



Figure 1. MODIS satellite image of Subtropical Storm Cari off the coast of Brazil, taken at approximately 9:00 am EDT Wednesday, March 11, 2015. Image credit: NASA.



Figure 2. Surface winds off the coast of Brazil according to an 3:40 pm EDT Tuesday pass from the Rapidscat instrument on the International Space Station. The center of circulation of Subtropical Storm Cari is seen, along with top winds near 45 mph in a band well southeast of the center of circulation.

South Atlantic tropical storm history
Tropical and subtropical storms are so rare in the South Atlantic that until 2011, there was no official naming of depressions or storms done. In 2011, the Brazilian Navy Hydrographic Center instituted a naming system with nine names, of which three have been used so far (Arani in 2011, Bapo in February 2015, and now Cari.) Brazil has had only one landfalling tropical cyclone in its history, Cyclone Catarina of March 2004. Catarina is one of fewer than ten tropical or subtropical storms to form in the South Atlantic, and the only one to reach hurricane strength. Tropical cyclones rarely form in the South Atlantic Ocean, due to strong upper-level wind shear, cool water temperatures, and the lack of an initial disturbance to get things spinning (no African waves or Intertropical Convergence Zone exist in the proper locations in the South Atlantic to help spawn tropical storms). It is uncertain whether climate change may cause an increase in South Atlantic tropical storms in the future. While today's storm formed over waters that were about 0.5°C above average in temperature, Catarina in 2004 formed over waters that were 0.5°C cooler than average. Sea surface temperature is not the main limiting factor inhibiting these storms--wind shear is. How climate change might change wind shear over the South Atlantic has not been well-studied.

Pam hits Category 4 in the South Pacific
In the South Pacific Ocean about 1,800 miles east of Australia, Tropical Cyclone Pam has rapidly intensified to a Category 4 storm with top sustained winds of 135 mph as of 8 am EDT Wednesday. Pam has generated quite a bit of hype over the past few days, thanks to eye-popping model projections by the GFS and European models which show the cyclone intensifying into a Category 5 monster with a central pressure less than 880 mb by late this week. If this forecast verifies, it would make Pam one of the strongest tropical cyclones ever recorded, world-wide. However, these models are not known for making reliable intensity forecasts, and are generally disregarded by NHC for intensity forecasts in the Atlantic and Eastern Pacific. The HWRF model, which is one of our better intensity forecast models, predicted with its 06Z (2 am EDT) Wednesday run that Pam would reach a central pressure of 907 mb by Friday, which would make it a still-formidable Category 5 cyclone. The Tuesday morning official intensity forecast from the Joint Typhoon Warning Center (JTWC) goes along with this idea, making Pam a Category 5 storm by Thursday evening. Pam is an unusually large cyclone over extremely deep warm water, with widespread surface temperatures above 30°C (86°F). Wind shear is currently high, 30 knots, but is expected to fall to the moderate range by Thursday night, which may allow Pam to undergo additional intensification into a Category 5 storm. A westward shift in the predicted track means that Pam will likely pass over or very close to some of the islands of Vanuatu on Thursday and Friday. In particular, a Personal Weather Station at Port Villa appears to be directly in the path of Pam.


Figure 3. Latest satellite image of Pam.

Bob Henson will have a new post Wednesday afternoon, summarizing the U.S. winter of 2014 - 2015.

Jeff Masters