African Dust May Have Suppressed Hurricanes

By David L. Brown

Last season was expected to be a big year for hurricanes in the Atlantic Ocean, but following the extremely stormy 2005 season the 2006 hurricane season came in like a lamb. Climatologists were puzzled. Climate change deniers were elated. Coastal residents were relieved.

So if global warming is real, and if a warmer ocean means greater numbers of hurricanes, and stronger storms, what happened last year to make the experts who had predicted an active season eat humble pie? The answer may be another facet of the global warming picture, drought in Africa that caused large clouds of dust to be blown across the central Atlantic where the big storms are born. Here is a satellite picture that gives an indication of the severity of the dust storms that blew out of Africa last year:


Two climatologists writing in the February 27 issue of Eos magazine have pointed a finger at the unusual dust storm activity as a major cause of last year’s quiet hurricane season. Reporting on the paper, Science magazine explains that “The dust seems to have suppressed storm activity over the southwestern North Atlantic and Caribbean by blocking some energizing sunlight….”

The article continued:

As the 2006 season approached, conditions looked propitious for another blustery hurricane season. In particular, there was no sign of El Niño, whose Pacific warming can reach out to the Atlantic and alter atmospheric circulation to suppress hurricanes there. But, unremarked by forecasters, an unusually heavy surge of dust began blowing off North Africa and into the western Atlantic at the 1 June beginning of the official hurricane season. Two weeks later, the surface waters of the western Atlantic began to cool compared with temperatures in the previous season.

Climatologists William Lau of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and Kyu-Myong Kim of the University of Maryland, Baltimore County, in Baltimore argue that the arrival of the thick dust and the subsequent cooling were no coincidence. The dust blocked some sunlight and cooled the surface, they say. That cooling went on to trigger a shift toward less favorable conditions for the formation and intensification of storms in the western Atlantic, they argue. As a result, no storm tracks crossed where nine had passed the previous season.

But that wasn’t the whole story, either, because an unexpected reappearance of El Nino in August added another factor to the complicated mix of effects that can affect hurricane activity. “We’re not denying El Niño had an impact,” says Lau, but “maybe we have neglected an equally important factor, if not a more important factor.”

Climatologists have a far-from-complete understanding of all the factors that affect weather patterns, and considering the evolving interaction of effects and feedbacks resulting from global warming, we may never be able to predict with complete certainty what Nature has in store for us. As other portions of a warming Earth turn dryer and winds stir clouds of dust into the air, many other surprises may lie ahead.

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