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Upper-level Lows

NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite captured this image of the rare Atlantic hurricane on March 26, 2004. Courtesy of NASA's EOS

On March 28, 2004, a rare south Atlantic hurricane made landfall near the town of Torres just south of the resort town of Laguna in the southern Brazilian state of Santa Catarina, about 500 miles south of Rio de Janeiro (track map of "Hurricane Catarina"). NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite captured this high-resolution visible image(resize your window) of the rare Atlantic hurricane on March 26, 2004.

Before meteorologists had reliable GOES imagery (in the mid 1970's), tropical cyclones could have formed over the south Atlantic Ocean and nobody would have ever known. But, if the last three decades are any indication, there weren't many. Indeed, the tropical cyclone that lashed Brazil was the only hurricane ever observed during the 30 years of geostationary satellite imagery. There have been other tropical cyclones observed in the south Atlantic during the modern satellite era - April, 1991, and perhaps January 19, 2004 (a short-lived tropical depression), but never a Category 1 hurricane.

There are two primary reasons why tropical cyclones are rare in the south Atlantic basin. First, vertical wind shear between 850 mb and 200 mb is typically greater than 10 meters per second (check out the long-term average of vertical wind shear between 850 mb and 200 mb). To make matters worse, westerly shear dominates over latitudes where tropical cyclones would be most likely to form. Second, easterly waves from Africa do not form south of the equator (the MLAEJ is a northern hemispheric singularity. Most people maintain that sea-surface temperatures are too low, but that's a red herring, in my opinion.

So what triggered the rare south Atlantic hurricane? An upper-level low became cut-off from the main 500-mb flow over the middle latitudes. When an upper-level trough in the mid-latitude westerlies moves equatorward, a low will sometimes "close off" and then detach from the main current of westerlies. The low is then said to be cut-off from the main westerlies (it lies equatorward of this relatively fast current of air). Detached from fast steering winds, a cut-off low is typically quasi-stationary. As a metaphor, I like to think of cut-off lows as pools of standing water left behind after a flooded stream has receded back within its banks.

A 500-mb trough at 00Z on March 24, 2004. The trough moved equatorward from the main current of mid-latitude westerlies and subsequently cut-off, helping to trigger the first documented hurricane in the south Atlantic Ocean. (large image)

At any rate, check out this loop of 500-mb images (requires Quicktime plug-in) from 12Z March 14, 2004, to 00Z on March 29, 2004 (small version of 500-mb loop). It shows a trough in the westerlies along the east coast of South America moving equatorward and then cutting off. Remember, this is the southern hemisphere, folks, so don't mistake the 500-mb trough for a 500-mb ridge.

With a mid-latitude, upper-level low intruding over the subtropics, the hurricane had a baroclinic beginning (such a hybrid life cycle is not rare, and we'll investigate these storms in the next section). You can see the footprint of the developing low-pressure system and its clockwise but cyclonic circulation on the 925-mb height and wind-vector analyses at 12Z on March 24. The corresponding analysis of 925-mb isotherms shows the baroclinic underpinnings that characterized the early stages of this system.

Just so you don't get the wrong impression, I emphasize that a mid-latitude, upper-level low cutting off over the south Atlantic Ocean is not a rare event. I submit to you, however, that this particular low affected the region for quite some time - perhaps as long as two weeks, upping the probability that something unusual might happen. To understand the tenacity of this 500-mb low, check out the mean two-week 500-mb pattern below (there's a clear signal from the trough along the east coast of South America). Also note the stationary high anchored off the coast of Africa and ridging westward almost to South America. Essentially, the high blocked the eastward advance of the 500-mb trough (and the subsequently cut-off low).
The mean 500-mb heights from March 14-28, 2004, shows a stationary high anchored off the west coast of Africa.

With the path of the cut-off low essentially blocked, moisture from South America had sufficient time to wrap into the stalled system's embryonic circulation. Take a look at the water-vapor image from 12Z on March 26. It shows a conveyor belt of moisture streaming southeastward from the Brazilian tropics, and cyclonically wrapping into the circulation of the storm. You can see the footprint of this moist stream of air on the 600 mb and 700 mb charts of specific humidity. Check out this short primer on specific humidity, which is an absolute measure of the amount of water vapor in the air. With the exception of this stream of moisture from the Brazilian tropics air in the middle troposphere was exceptionally dry over and west of the developing cyclone.

At 12Z on March 24 (the upper-left image below, which is a composite infrared and radar image from the TRMM satellite), a comma-shaped cloud pattern indicates the still extratropical nature of the system. By 06Z on March 27 (lower left), however, the cyclone had ostensibly made the transition to a tropical cyclone (more on this transition in the next section). Indeed, infrared imagery showed the presence of an eye while radar detected strong thunderstorms on the southern flank of the storm's center (apparently, these storms were part of the eyewall).

A sequence of composite infrared and radar images from the Tropical Rainfall Measuring Mission (TRMM). The times and dates are: 12Z March 24 (upper left), 12Z March 26 (upper right), 06Z March 27 (lower left) and 11Z March 27 (lower right).

Forming in the vicinity of latitude 28 degrees South, this unnamed hurricane likely went through a stage of development where it had both tropical and extratropical characteristics. In the Atlantic basin, these hybrid storms are called subtropical cyclones. Let's investigate.