[Coral-List] African dust over the Atlantic basin

[scott.stripling]

Apologies to Jim for file size violation. This is a retran.

Gene and those interested in dust:

Thought I would chime in on this since the African dust is a frequent 
visitor to us in Puerto Rico :-)

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 airmass, termed the Saharan Air Layer. A 
Google search will yield
much research on this topic. 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 5K 
ft often to as high as 18-20K ft.
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. See 
http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal.html
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 
http://www.osdpd.noaa.gov/PSB/EPS/Aerosol/aerday.cf.gif
An animation of the CIMSS 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 interseasonally, as well as annually. As you state Gene, 
the 2006 and 2007 years
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 upwelling and cooling of the 
SST's. The dust has
been shown to reflect incoming solar radiation, which also has a slight 
limiting effect on surface SST
warming. So all of these factors combine cumulatively to make tropical 
cyclone formation more difficult
to occur. As a tropical meteorologist, the key ,to me, appears to be the 
strength of this mid level
jet. In general, the stronger the jet, the more SAL and associated dust 
transported.

Hope this offers some insight Gene. 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.

Saludos,

Scott Stripling
NOAA-NWS
San Juan, Puerto Rico