Geoscience Reference
In-Depth Information
Figure 20.13
Long-term mean annual TOMS aerosol index over southern Africa.
the east. As the area to the west of the basin is blocked
by high mountains, convergence forces the winds to be-
come northwesterly at the western part of the basin. The
Kunlun Mountains to the south block the flow, causing
an accumulation of dust over the southern part of the
basin, where the occurrence of high dust loadings is an
order of magnitude higher than the northern parts of the
Tarim (Xuan and Sokolik, 2002; Kurosaki and Mikami,
2005).
China has been the focus of several modelling studies
(Zhao
et al.
, 2003; Laurent
et al.
, 2006; Darmenova
et al.
,
2009). The importance of dust as a health issue in urban
areas has no doubt prompted efforts to simulate both the
emission and transport of dust (Gong
et al.
, 2003; Shao
et al.
, 2003). Like the SEQ, specification of soil area pa-
rameters such as particle size, roughness and soil moisture
remain a key limitation to better constrained modelling.
sphere source regions are southern Africa and Australia.
In contrast to China and the SEQ, these source regions are
more accessible and as a result have been the feature of
recent geomorphological investigation.
TOMS AI long-term mean data (Figure 20.13) indicate
two relatively small but clearly developed dust sources
over southern Africa. The more intense of these is centred
over the Etosha Pan in northern Namibia and has an AI
value of more than 1.1. The other is over the Mgkadik-
gadi Depression in northern Botswana and has AI values
greater than 0.8.
Etosha, which covers an area of about 6000 km
2
,isa
salt lake that occupies the sump of a much larger basin and
often floods in the summer months but is often dry enough
in winter for deflation. The Mkgadikgadi is another ma-
jor structural feature, which includes the Sua (Sowa) Pan.
Deflation is influenced by the extent and frequency of lake
inundation, sediment inflows and surface wind speed vari-
ability (Bryant
et al.
, 2007). Deflation events occur with
the passage of transient eddies, in the form of west-to-east
migrating anticyclones that enhance the prevailing east-
erly (trade) winds across southern Africa for periods of
20.2.4
Southern Africa and Australia
