Sahara dust disrupts hurricanes

In a recent NASA study researchers suggest that dust storms over the Sahara Desert in Western Africa could have helped tame what was predicted to be a very active hurricane season in 2006.

Only four tropical storms and five hurricanes formed in the Atlantic basin in 2006, despite predictions from several noted forecasters of a very active season.

NASA�s Terra satellite picture

A plume of dust blew off the northwestern coast of Africa on 21 November, 2006. The Moderate Resolution Imaging Spectroradiometer flying onboard NASAs Terra satellite took this picture the same day. In this image, a faint plume of dust blows primarily out of Western Sahara and over the Atlantic, heading toward the northwest. While the nearby clouds are opaque white, the dust is translucent beige. Photo: NASA

Lead author of the study, William Lau, who is chief of the Laboratory for Atmospheres at NASA’s Goddard Space Flight Centre in Maryland said the research was the first to show that dust has a major effect on hurricane activity.

‘Dust concentrations may play as big a role as other atmospheric conditions, like El Niño,’ he said.

The Saharan dust storms, which are caused because of the larger temperature gradient on the African continent, can be as larges as the continental United States and rise as much as three miles into the atmosphere.

When the dust created by these storms drifts westward over the Atlantic Ocean, the particles block sunlight from reaching the ocean surface, causing a cooling of the waters. Warmer waters cause tropical storm systems to intensify.

Sea surface temperatures in the prime hurricane-breeding areas of the tropical Atlantic and Caribbean were as much as one degree Celsius lower last year as they were in 2005.

Mr. Lau said that following the most significant Saharan dust storms in June and July last year, the ocean waters cooled abruptly in just two weeks, suggesting the dust had an almost immediate effect.

In addition, other scientists on a hurricane research mission in the Cape Verde Islands found that Saharan dust contaminated several fledgling storms that came off the western coast of Africa last year.

The dust storms contain very dry air and also had high wind shear, two factors known to hinder hurricane development.

Dust in the atmosphere not only cooled the ocean but warmed the atmosphere itself by absorbing sunlight. The warmer atmosphere resulted in a shift in the large scale atmospheric circulation that helped to increase sea surface winds, which further cooled the ocean waters, the study showed.

Some scientists, however, discount the effect of the dust storms on last year’s hurricane season and instead see the El Niño that suddenly formed in the Pacific Ocean last summer as the prime reason for the lower than predicted tropical storm activity.

Cayman’s Chief Meteorologist John Tibbetts does not totally discount the effects of African dust on hurricane development, but believes last year’s quiet hurricane season was caused by the El Niño.

‘[El Niño] is the best and most widely accepted reason,’ he said.

Scott Braun, a hurricane specialist from NASA, said the El Niño brought about broad changes to atmospheric conditions that likely had some influence on hurricane formation in the Atlantic.

In particular, Braun noted a large area of high pressure was located over the Eastern Atlantic for most of the 2006 hurricane season. This steered the tropical disturbances away from the warmest waters.

In addition, strong upper level winds helped minimise storm development and intensification.

Braun said there is a pattern in the Atlantic associated with El Niño that suggests it plays a role in seasonal hurricane activity.

‘In fact, the last time the Atlantic produced so few storms was in 1997, when an El Niño pattern was also in place,’ he said.

Research will continue, however, to examine the role dust plays in hurricane development.

Mr. Lua said dust from Africa could offer some predictive value and that it should be monitored to help improve hurricane forecasts.

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