The Sahara Dust season in the tropical Atlantic generally starts and finishes about a month ahead of the hurricane season. It produces spells of strange weather – there’s a thin haze that isn’t clouds or fog, sunrise and sunset produce big stretches of brilliant colour, and a thin film of dirt seems to be getting into everything.
The Saharan Air Layer (SAL) is a mass of hot dry air which forms over the Sahara and Sahel desert regions of north-central Africa and usually moves out over the tropical North Atlantic in waves every 3-5 days. The SAL extends between 5,000 and 20,000 feet in the atmosphere and is associated with large amounts of mineral dust and strong winds.
Saharan dust is just what it sounds like – fine particles of dirt from the Sahara and Sahel regions that gets stirred up by storms or human activity and crosses the Atlantic in three to ten days. In winter, it reaches as far south as Brazil, and in summer it gets as far north as Florida.
African dust has been riding the trade winds for centuries, but in the last few years the clouds have been getting bigger and the “dust season” is lasting longer. There are two main reasons. Years of drought have made the deserts bigger, especially in the Sahel region. Also, there has been an increase in human activity –wars, deforestation, tourism and agriculture — in what used to be a mostly empty region. Earth from previously fertile land of the Sahel makes more dust than the sand deserts further north.
When Saharan dust reaches this side of the Atlantic and settles back to earth, it has some positive effects. The nitrogen, phosphorus, iron and magnesium in the dust improve poor soils. In fact, Saharan dust is the lifeline for huge areas of tropical rain forest.
However, there is a lot of downside. Dust of any kind is hard on people with breathing problems. In recent years, public health authorities in some areas have been issuing warnings whenever a heavy wave of dust is expected, advising people to wear masks, stay indoors (with the AC running) and consult their doctors. Medical researchers believe there is a link between increases in Saharan dust and rising levels of serious breathing problems, especially asthma, in places like South Florida, Cuba, Barbados and Trinidad.
The researchers found that Saharan dust isn’t just dust. Samples contained a lot of man-made rubbish — pesticides, aerosols, exhaust fumes and heavy metals – as well as a surprising number and variety of bacteria, viruses and fungus spores. Apparently thick dust clouds shelter these from the sun’s UV rays, which ordinarily would kill them on their trip across the Atlantic.
Some marine scientists suggest that the nasty stuff riding on Saharan dust may also hold a key to some of the unexplained disasters on Caribbean reefs. Peak years for dust match neatly with mass deaths of sea fans, staghorn and elkhorn corals and spiny sea urchins, and with widespread coral disease and bleaching events.
Because it is rich in nutrients – nitrogen, phosphorus and iron – dust promotes the growth of algae (including the dreaded Sargassum Weed) that smother coral reefs and clog beaches. It can also trigger huge plankton blooms. These clouds of tiny marine life block sunlight from the reefs, and some of them, like the deadly Red Tide, are eaten by larger creatures and end up poisoning people.
The problem is twofold. First, the dust itself is changing. It now leaves Africa mixed with an increased load of pollutants from industry, agriculture, tourism and war. Second, in recent years Caribbean countries have been producing ever-growing amounts of their own silt and pollution. Saharan dust, which used to be just a nuisance, can be a serious problem when it is added to the load of locally produced rubbish.
Perhaps the most interesting – and potentially useful – recent discovery about Saharan dust is the love-hate relationship between the dust clouds and tropical storms. More about that next week.