Geoscience Reference
In-Depth Information
JULY/AUGUST
DECEMBER
20°N
20°N
ITCZ
ITCZ
20°S
20°S
t
e
40°W
20°W
20°E
40°E
40°W
20°W
20°E
40°E
Figure 18.2. Surface winds and frontal locations (a) during July and August and (b)
during December. (Modified after Nicholson, 1996 , and Gasse et al., 2008 .) ITCZ is
Intertropical Convergence Zone; CAB, Congo Air Boundary; NEM, northerly East
African monsoon. In section: SHC, southern Hadley Cell; NHC, northern Hadley
Cell.
Aridity over Arabia and North Africa is accentuated by the subtropical easterly
jet stream. Past changes in the strength, sinuosity and latitudinal position of the jet
stream would have had important climatic repercussions for both regions (Rognon
and Williams, 1977 ). In southern Africa, the seasonal fluctuations in Antarctic pack
ice have a strong influence on the latitudinal displacement of the winter westerlies,
which would have brought more rain to south-west Africa during times of maximum
ice extent in Antarctica (Stuut et al., 2004 ; Chase and Meadows, 2007 ; Gasse et al.,
2008 ). In peninsular Arabia, the south-west monsoon brings rain to the coastal fringe
and adjacent uplands in summer. In winter, the dominant wind is the Shamal (Arabic
for 'north'), which flows eastwards along the north coast of the Sahara across the
Sinai and southern Negev before curving in a clockwise direction to the south and
south-west ( Chapter 8 , Figure 8.9 ). The Shamal is a dry wind by the time it blows to
the south, mobilising sand and causing dust storms.
18.3 Cenozoic desiccation of the Sahara and adjacent deserts
The Sahara is the largest hot desert in the world. It extends more than 4,800 km, from
the Atlantic coast of Mauritania in the west to the arid Red Sea Hills in the east, and
is continued eastwards across the deserts of Arabia, Iran, Afghanistan and Pakistan
to the Thar Desert of Rajasthan in north-west India, a total distance at the Tropic
of Cancer of about 9,500 km. The northern limits of the Sahara coincide with the
southern margins of the Atlas Mountains in north-west Africa and merge eastwards
into the Sinai and Negev deserts. The southern limit of the Sahara is more diffuse
and has been defined by some French geographers as the northern limit of the spiny
cram-cram grass ( Cenchrus biflorus ), a bane to the traveller on foot but a boon in
times of extreme drought when famine threatens.
 
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