2011 Atlantic Hurricane Season: Was that it?

Although the historical record shows there have been Atlantic Basin tropical cyclones in every month of the year the “official” Atlantic Hurricane Season runs from June 1 through November 30.

GOES satellite image showing a good portion of the tropical Atlantic basin on 4 December 2011

So, today being Saturday 3 December 2011, we could say that “officially” at least the 2011 Atlantic Hurricane Season is over. The question is: does mother Nature agree?  Looking at various satellite images from around the world, but well aware of the caveat that nature is quite capable of surprising us and that change is a constant of nature, there is an absence of the triggers and contributors in the regions that usually lead to cyclogenesis in the Atlantic basin. On that basis I would say the 2011 Atlantic hurricane season does appear to have come to an end.

Color-enhanced infrared satellite image of a partial North Atlantic basin on 3 December 2011 showing the large regions of high pressure and dry air that prevail over much of the basin

There are no tropical waves or cells of disturbed stormy weather over the region of Equatorial Africa designated as the tropical-wave assembly line. Nor do we see any seeds for potential tropical cyclone development along hurricane alley, the Atlantic Ocean corridor reaching from the western coast of Africa near the Cape Verde Islands to the Lesser Antilles and the eastern Caribbean.

GOES satellite image showing large sectors of high atmospheric pressure and dry air and the absence of tropical waves or any other cells that might qualify as seeds for tropical cyclone development

A very large bubble of high atmospheric pressure and dry air has prevailed over the Gulf of Mexico and most of the Caribbean sub-basins, as well as along most of the USA Atlantic seaboard and adjacent Atlantic coastal waters., which has prevented any sort of tropical weather from developing or penetrating the region.

Composite of satellite images creating a global view of the northern tropics from the eastern Pacific to the Indian Ocean on 3 December 2011

The composite global mosaic of satellite images shows a large swath of the northern tropics from the eastern Pacific to the northern Indian Ocean that on 3 December 2011 was basically clear of any disturbed weather, or tropical waves. Likewise a full-disk composite satellite image of Earth’s Western Hemisphere shows a disrupted belt of tropical activity from the coast of western Africa, north of the equator, pretty much all the way to Hawaii except for some stormy weather over the Andes and other mountains in northern South America.

Composite of satellite image give a full-disk view of Earth's western hemisphere on 3 December 2011; notice the absence of the belt of tropical activity that regularly girdles the planet near the equator













In fact the only potential for tropical cyclonic activity worldwide is in the southern Indian Ocean where two tropical waves, one south of the Indian subcontinent and the other to the northeast of Madagascar, are showing some organization and plenty of stormy weather over warm waters that may lead to future tropical cyclone generation. Other than these specific cells all remains quite on the tropical front.


Color-enhanced infrared satellite image of the Indian Ocean on 3 December 2011 showing the only two cells of disturbed weather with some potential for cyclonic activity on a worldwide basis

What kind of a season was it in 2011 in the Atlantic basin? With a total of 18 named storms the season qualifies as quite an active season, certainly a continuation of the multidecadal period of increased activity that started in 1995. However only 6 storms reached hurricane strength and of these, only 3 became major hurricanes [ category 3 or higher in the Saffir-Simpson scale] and in this sense it was a below-average season. But if we assess the death toll, injury and damage caused by land-falling storms or hurricanes that otherwise affected land, the 2011 Atlantic season clearly ranks way above average based on a death toll that included at least eleven in New Jersey and six in Vermont, more than $7.0 billion in damages  and the worst flooding seen in the USA Northeast in more than 80 years. The negative consequences of the 2011 Atlantic hurricane season were made even worse when tropical storm Lee followed in hurricane Irene’s footsteps contributing additional intense rain from Virginia to Vermont where already saturated soils could not keep up with the pace of precipitation exacerbating flood conditions throughout the region.

The above reinforces the point that it only takes one impact by a tropical cyclone to cause plenty of damage and human suffering. Also, in terms of damage, it does not have to be “the big one” meaning a monstrous category 5 with extreme winds pounding a region; a lot of damage can be caused by what could be classified as a minor hurricane, but one that carries a lot of water in it as was the case with Irene.

If we assess the 2011 Atlantic hurricane season in the context of tropical cyclone activity on a worldwide basis so far in 2011, it is interesting to note that we are seeing a repeat of  what happened in 2010 when the total number of named storms worldwide was close to 17% below average. So far in 2011 there have been a total of 65 named tropical cyclones  worldwide, including the 18 that generated over the Atlantic. With  only a few days left before the end of the year and little potential for cyclogenesis in the near future, based on current atmospheric conditions worldwide, it appears we will end the year with another below-average global season.

While two consecutive years of below-average tropical cyclone activity on a global basis do not signify a trend one can not, but ask if this might not be a signal from Nature telling us that the threshold for cyclogenesis has shifted to a higher level, in response to global warming? In my opinion the multidecadal cycle of increased cyclogenesis in the Atlantic is masking this possible shift in the threshold for tropical cyclone generation in this basin, while elsewhere in the world we continue to see fewer storms. Certainly food for thought and an important area for research exists relative to tropical cyclone activity on a global basis, and how this is being affected by global warming or others aspects of global climate change.

Relative to this topic it is critical to keep in mind that what really matters is our own relative vulnerability to tropical cyclones in the community where each of us lives, because it is the impact at the local level that can bring death, injury, damage and human suffering, regardless of whats happens during a season in a given basin or of what may be happening on a global basis in response to global warming. All it takes is one impact. We must pay attention, be prepared, and above all we must practice mitigation!

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