African Dust and Amazonian Trees

A large dust storm that formed over the Sahara Desert and the adjacent Sahel in late July 2013 has moved west over the tropical Atlantic Ocean where it is expected to suppress storm development in early August. The dust and associated air are expected to dry the Saharan Air Layer (SAL) that forms on top of the moist air over the ocean and make the SAL drier than usual. The fine dust particles absorb solar radiation in the SAL before it reaches the low-lying air mass, heating the air in the boundary layer between the two air masses rather than warming the sea surface that ordinarily occurs this time of  year. A cooling of 1 C could occur in the sea surface temperatures of the broad Atlantic Ocean corridor between the western African coast and the Caribbean Sea in which tropical storms typically develop in August and September. Computer simulation models therefore predict an absence of hurricane formation for the first week of August.

Saharan dust from Mauritania passing over the Atlantic Ocean. NASA photograph. Late July 2013

Saharan dust from Mauritania passing over the Atlantic Ocean. NASA photograph. Late July 2013

The Saharan dust storms that occur regularly in July and August have ecological effects far from their origin. The dust reaches the central Amazon River basin where tropical forests grow on infertile, highly weathered soils. Swap et al. (1992) estimate that during individual rain storms in the basin, about 480,000 tons of dust are deposited basin-wide and that over a year, 13 million tons may be  deposited (another more recent estimate is 40 million tons). Although these estimates are necessarily based on indirect approaches with some error, the magnitude of the material involved suggests that the vegetation growing on the nutrient-limited Amazon soils is enriched by, and perhaps even dependent upon,  essential elements such as phosphorous, nitrogen and potassium that are contained in  the dust. These major nutrients are well-known to limit forest growth in many areas of the world, especially where high temperatures speed decomposition of soil organic matter and high precipitation leaches out nutrients from the forest root zone.

 In a detailed case study using satellite-based approaches, Ben-Ami  et al. (2010) were able to follow specific dust events traceable to the Bode’le’ Depression (BD) of southwestern Chad and deposited about 10 days later in the Amazon region. The Depression is about 1/3 the size of Florida and acts as a large dust source during the winter months when atmospheric conditions favor westward blowing wind storms. It appears to be a supplier of nutrient substances to a large forested area in the Amazon basin. The authors found that compared to the average concentration of crustal elements (iron, manganese, aluminum, silicon and titanium) over the Amazon forest,  about ten times  greater concentration  of crustal elements in their oxide or carbonate forms, was delivered to the Amazon receptor region as a result of the dust material generated in the BD. It was also noted that the dust parcel was augmented by additional dust sources as the BD dust moved westwards towards the Atlantic. Fine aerosol particles produced from biomass burning also were injected into the dust mass as it moved west and probably were the source of much of the potassium that reached the Amazon. Note that the amounts of dust generated in the Saharan-Sahelian region are influenced by local rainfall patterns. Thus the total rainfall over the northern Sahel region (southern Algeria and Libya, northern Niger and Chad) in 2012 was twice the average amount, resulting in increased vegetation growth and correspondingly less bare soil available for dust generation and transport.

Ben-Ami, Y., et al. 2010. Transport of North African dust from the Bode’le’ depression to the Amazon Basin: a case study.  Atmos. Chem. Phys. 10: 7533-7544.

 Masters, Jeff 2013.  Massive African Dust Storm Cooling Atlantic Hurricane Odds for early August.  Dr Jeff Masters’ WunderBlog:  wunderground.com/blog/JeffMasters/comment.html?entry=2478. 

 Swap, R., M. Garstang and S. Greco 1992. Saharan dust in the Amazon Basin. Tellus: 44B: 122-149.

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