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Friday, July 03, 2009

African Dust Can Affect Caribbean Corals

African dust has been implicated in various negative effects on coral reefs in the Caribbean.  The dust brings in iron, which may be driving an increase in algae growth, and it may also be bringing pesticide dust, other airborne pollution, or even new coral diseases.

This dust is not new; it has always been around, forming an important source of fertile soil in the Caribbean.  But with increasing desertification in Africa, as well as increasing use of pesticides and herbicides in agriculture in that area of the world, it certainly bears attention now.

A number of operational products have been generated in the past few years to monitor African dust and minerals transported across the Atlantic and beyond. These aerosols are transported within a unique and detectable air mass, termed the Saharan Air Layer. The Saharan Air Layer, or SAL, is a relatively deep layer of very warm, dry air originating over the Sahara, and typically extends from about 5,000 feet often to as high as 18,000-20,000 feet.

The SAL circulates around the periphery of the cyclonic circulations over the African continent that develop into tropical waves, and then moves out ahead of the tropical waves as they exit the west coast.  Jason Dunion and the folks at NOAA HRD and Chris Veldon of CIMSS (U of Wisc-Madison) among other colleagues, have developed a satellite product, using channel differencing, to detect these very dry air masses moving across the Atlantic (view data by clicking here).

This product depicts areas of dry air, not the dust/minerals itself. Dust content suspended within the SAL can further be defined using NOAA aerosol thickness products found here.

An animation of the SAL imagery, as well as the static optical thickness product, can reveal the SAL and dust circulating cyclonically around the vortices or broad areas of low pressure associated with tropical waves exiting African and transiting the Atlantic. It is the mid-level steering currents across the Atlantic that influence the trajectory of the SAL and dust, and this can very inter-seasonally, as well as annually. 2006 and 2007 were dominated by a strong and broad high pressure ridge across the Atlantic, and a strong mid-level African Easterly Jet that transported much SAL and much dust across the basin.

These strong mid-level winds also act to induce increased wind shear in the vertical, which has a negating effect on tropical cyclone development, and also influence the strength of the low level and potentially surface wind flow across the Atlantic.  Prolonged periods of strong trade winds across the Atlantic produce upwellings and thus cooler surface water temperatures. The dust has been shown to reflect incoming solar radiation, which also has a slight limiting effect on surface warming. All of these factors combine cumulatively to make tropical cyclone formation more difficult to occur; in general, the stronger the jet, the more SAL and associated dust is transported.

The SAL and dust typically do not produce a continuous plume or stream, but generally are seen in more broken broad zones exiting Africa with and ahead of each tropical wave, every 2.5 to 6 days.  Hundreds of millions of tons of African dust are transported annually from the Sahara and Sahel to the Caribbean and southeastern U.S.  Various peaks in the dust record, at Barbados and elsewhere in the western Atlantic, coincide with benchmark events on reefs throughout the Caribbean.

The mechanisms by which dust may affect reefs include deposition of nutrients or pollutants that may:

1) Interfere with a coral’s immune system, making it more susceptible to disease pathogens
2) Interfere with some stage in reproduction (gamete production, fertilization, settling, larval survival)
3) Induce pathogenicity in a microorganism in the reef environment
4) Trigger a rapid increase in the number of pathogenic microorganisms
5) Fuel macro-algae or phytoplankton growth, as has been shown for Red Tides in the Gulf of Mexico
6) Directly deposit pathogenic microorganisms (e.g., Aspergillus sydowii, the fungus that causes sea fan disease throughout the western Atlantic)

Additional information about this phenomenon can be found by clicking here. (Source:  VOMIL, NOAA-NWS, USGS)

Posted by Susan Davis on July 03, 2009 at 9:49am AST
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