2011 Atlantic Hurricane Season: an Empirical Assessment

Tropical Storm BRET in a NASA infrared satellite image on 18 July 2011

We are coming up on the 8th week into the 2011 Atlantic Hurricane season, and for those of us who monitor tropical cyclone activity there are some interesting “happenings” that have taken place in the northern tropics so far this year.

A ‘wrong-way’ storm:

Tropical storm BRET, shown on the NASA satellite image from the early morning of 18 July 2011, is an interesting example of a tropical cyclone that departed from the historically traditional sources of cyclogenesis in the larger Atlantic basin.

Typically the main triggers for cyclogenesis in the Atlantic basin involve: (a) tropical waves generated over equatorial Africa moving over the eastern Atlantic, riding the warm waters of ‘hurricane alley‘ until they encounter favorable ocean-atmospheric conditions for tropical cyclone generation; (b) low pressure systems that develop in the Caribbean or the southwestern Gulf of Mexico, which become tropical depressions and eventually tropical cyclones.

The historical record shows tropical cyclones generated in the Atlantic basis generally track from east to west, or from east to northwest or north, and in some cases re-curving toward the north or northeast or even back east under the influences of the Coriolis force and prevailing winds and other atmospheric factors.

The genesis of tropical storm Bret was not, however, propitiated by any of those triggers nor did this storm follow any of the traditional tracks. In fact this tropical cyclone was generated by a front moving from northwest to southeast over the middle of the country, which upon reaching the warm waters of the Gulf Stream and northern Gulf developed thunderstorms and copious rain mainly over a large area just off-shore off Georgia and Florida. This entire system actually tracked southward all the way to the northeastern Bahamas as can be seen in the illustration below:

Track for tropical storm BRET developed by the U.S. Navy Research Laboratory

While not unheard of, as evidenced by some rare examples in the historical record, it is an exception rather than the norm to see a cyclone generated that far north to then travel southward.

Also of interest given the unconventional genesis of Tropical Storm BRET is to see how as this cyclone evolved over time, and it pulled moisture long distance it also began interacting with a weather front moving east by southeast across the eastern USA. As the long “tail” resulting from BRET’s influence got closer to the weather front mentioned before, especially as it emerged over Atlantic waters, the ‘seed’ for another tropical cyclone could be seen. Eventually a new tropical storm, CINDY, was generated [really extra-tropical because of the latitude at which it appeared]. The interaction between weather system that generated CINDY is illustrated by the satellite images below:

GOES satellite image of T.S. BRET and its long 'tail' and an approaching weather front across the southeastern USA on 17 July 2011

On 18 July 2011 the GOES satellite image shows the long 'tail' of T.S. BRET and the USA weather front getting closer over the Atlantic waters







By 21 July 2011 the GOES satellite image shows a weakening BRET and a new full blown cyclone CINDY, which has absorbed most of BRET's long 'tail'








Atmospheric Conditions over Larger Basin:

The series of GOES satellite images spanning from July 13 through the 19th, which follows below, illustrates de atmospheric conditions and development at play during said time interval:

View of regional basin around Florida on 13 July 2011. Main active atmospheric features are highlighted in red

View of basin on 15 July 2011 showing already visible "seed" for cyclone Bret

This how the potential cyclone appeared from space of 16 July 2011


On 17 July 2011 the system goes from a depression to tropical storm strength

On 18 July 2011 tropical storm Bret has 75 kph sustained winds

On 19 July 2011 Bret weakens and it is peaked up by the fron's leading edge moving toward the northeast

The previous series of satellite images also illustrate the atmospheric regime affecting northern South America (Colombia and Venezuela), Panama and Central America, which is a virtual repeat of what we saw in 2010 over the same region. This region has been dominated by frequent and extreme precipitation events and thunderstorms over the past several weeks in a pattern that has already generated four tropical cyclones including the latest one, tropical storm DORA moving generally west by northwest paralleling the Pacific coast line of Central America and Mexico. The satellite view for the aviation industry on 19 July 2011  summarizes the current situation over the larger eastern-Pacific – Atlantic basin:

Satellite view of the larger basin on 19 July 2011 showing tropical storms BRET and DORA, as well as other weather systems active in the region

A Subpar Tropical-Wave Assembly Line:

A well know contributor to cyclogenesis over the Atlantic basin is the so-called tropical wave assembly line that takes weather “pulses” generated over the Indian Ocean and Arabian Sea as they move over equatorial Africa, to generate cells of disturbed weather usually resulting in thunderstorms and rains across the continent, which create a train of tropical waves moving toward the west until they emerge over the warm waters of the Atlantic to the south of the Cape Verde Islands.

The tropical-wave assembly line contributes the storm cells, the disturbed atmosphere, that combine with warm sea surface waters of the Atlantic, another required contributor to the process of cyclogenesis, along the so-called “hurricane alley”. When the combination of tropical waves and warm sea surface waters encounters favorable wind patterns, meaning generally westward steering winds and negligible to no wind-shear aloft, the basic conditions are given that will promote cyclogenesis, provided other external triggers and factors, such as the presence on El Nino or La Nina (ENSO), the current conditions of the Julian-Madden Oscillation over the Pacific, and the location of the Bermuda High, are all in equally favorable combined alignment.

Although cyclogenesis taking place in the Caribbean of Gulf sub-basins follows a different causality than described before, the tropical wave trigger in the form of a tropical low or tropical depression or a cell of precipitation and thunderstorms is still an essential link in the chain of events leading to tropical cyclone formation. From this it follows that hurricanes or tropical storms generated over the open waters of the Atlantic require the pulses sent by the tropical-wave assembly line described before.

What has been interesting during the first seven weeks of the 2011 Atlantic hurricane season is how passive the tropical-wave assembly line had been over Equatorial Africa. Whereas at this time of the year we could observe a veritable train of clearly identifiable tropical waves, spaced 150-200 kilometer apart, marching westward over Equatorial Africa toward the eastern Atlantic, during 2011 so far the production of tropical waves has been sparse and lacking the regularity in spacing and intensity required to maintain the assembly-line quality. Few strong tropical waves have made it to the waters of the eastern Atlantic, and in instances when some regularly-spaced waves have finally moved over hurricane alley their intensity and size have lacked the strength to survive the entire crossing over the Atlantic.

Following are satellite views [courtesy of NASA] of the eastern Atlantic and western Equatorial Africa, showing the region where the tropical-wave assembly meets the Atlantic and the approach to hurricane alley south of the Cape Verde Islands. These images are representative of the type of activity we have seen in the Equatorial Africa-Eastern Atlantic-Hurricane Alley continuum over the past four months:

Western Equatorial Africa and the Eastern Atlantic on 27 April 2011

Western Equatorial Africa and the Eastern Atlantic on 19 May 2011

Western Equatorial Africa and the Eastern Atlantic on 17 June 2011

Western equatorial Africa and the Eastern Atlantic on 19 July 2011

From these satellite images it is clear that the pattern of strong evenly spaced storm cells that are typical of the tropical-wave assembly line, during most of the Atlantic hurricane season, have yet to materialize during this 2011 Atlantic season. The net result of this sub-par tropical-wave assembly line is an equally inactive hurricane alley.

Light Traffic on Hurricane Alley:

When compared to previous years, say the last 5 – 7 years, in June and so far in July hurricane alley has had what could be called light traffic both in terms of the number of tropical waves traveling on it and the size and intensity of the same.

Also of interest regarding traffic along hurricane alley observations appear to show whatever traffic there has been, since the start of the 2011 hurricane season, has stayed closer to the equator [to the south of hurricane alley]. As a consequence of this instances of disturbed weather, extreme rain and thunderstorms over northern South America [Colombia and Venezuela] have lingered over the region generating another tropical-wave assembly line, which send pulses toward the Pacific. In turn, this has generated long lasting bouts of disturbed weather, extreme rain events and thunderstorms over Panama and Central America and adjacent Caribbean and eastern east Pacific waters, which have also contributed to cyclogenesis in that region. Hurricane DORA [see GOES satellite image below], which appeared to be strengthening on 20 July 2011, is a good example of such activity.

Color-enhanced infrared GOES satellite image of category 4 Hurricane DORA near the Pacific coast of Mexico

Following are composite satellite full-disk images of Earth’s western hemisphere, which will help illustrate the observations described above:

Full-disk image of Earth's western hemisphere on 19 April 2011

Full-disk image of Earth's western hemisphere on 19 May 2011


Full-disk image of Earth's western hemisphere on 19 June 2011

Full-disk image of Earth's western hemisphere of 19 July 2011

From above images it is clear most of the tropical weather activity has been taking place in the eastern east Pacific and over Central America, Panama and northern South America and adjacent Caribbean waters. While this pattern is not necessarily rare for this time of the year, my observations and comparison to prior years indicate similar conditions of intensity and lingering appeared in 2010 and as a result Central America, Panama, northern South America and even southern Mexico suffered above average precipitation and several flooding events. In 2010 this regional pattern was attributed mainly to the then present La Nina off the coast of Peru, but now in 2011 with a basically neutral ENSO conditions [meaning little to no anomalies exist in sea surface temperature conditions] it may indicate other factors may be at play in the generation of regional weather conditions described before. It is clear continued observations and analysis will be needed to determine what other causes may have been contributing to such weather activity.

What’s Been Happening in the Rest of the World?:

* Earlier during the year, not directly related to the 2011 Atlantic hurricane season, but evidence of the extent of the warm surface waters, we saw the rare occurrence of a tropical cyclone in the Atlantic south of the equator. The satellite image below shows this cyclone near the coast of Brazil to the east of Rio de Janeiro:

Satellite image showing a rare tropical cyclone south of the equator on 16 March 2011

** Out over the West Pacific, in a large triangular region demarcated by the Northern Marianas, the Philippines and Southern Japan, there has been continuous cyclonic activity over the past ninety days. Several full blown typhoons and tropical storms have impacted the Philippines, Taiwan, Japan, mainland China and neighboring territories. Evidence of this activity is Typhoon MA-ON, which at one point reached category 4 strength,  which impacted Japan without making landfall as shown in the satellite image below:

Color enhanced infrared satellite image showing Typhoon MA-ON near Japan on 18 July 2011

*** Also of interest is the monsoon-related weather regime that has prevailed over the northern Indian ocean over the past few weeks, which has generated extreme rain events and flooding in several countries in the basin as well as some cyclonic activity, while at the same time affecting the tropical-wave assembly line over equatorial Africa as has been mentioned herein. The satellite image below is typical of what we have seen over the larger North Indian ocean recently:

Color-enhanced infrared satellite image showing numerous large disturbed weather cells, with embedded rain and thunderstorms, which have prevailed over the northern Indian Ocean basin so far in 2011

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