Geoscience Reference
In-Depth Information
N
0
100
km
5
m
2000
1500
ALGERIA
5
1000
Map
Location
500
0
Figure 10.30
Track of a storm and the associated three-hour rainfall (mm) during September 1950
around Tamanrasset in the vicinity of the Ahaggar Mountains, southern Algeria.
Source: Partly after Goudie and Wilkinson (1977).
of South Africa and west of Australia. These are
warm-core systems formed by descending air and
extending through the troposphere. The conti-
nental intensification of the constant eastward
progression of such cells causes pressure maps to
give the impression of the existence of a stable
anticyclone over Australia (
Figure 10.32
). About
40 anticyclones traverse Australia annually, being
somewhat more numerous in spring and summer
than in autumn and winter. Over both oceans, the
frequency of anticyclonic centers is greatest in a
belt around 30
L
HIGH
(200MB)
HIGH
(200MB)
D
°
S in winter and 35-40
°
S in
summer; they rarely occur south of 45°S.
Between successive anticyclones are low
pressure troughs containing inter-anticyclonic
fronts (sometimes termed 'polar') (
Figure 10.33
).
Within these troughs, the subtropical jet stream
meanders equatorward, accelerates (particularly
in winter, when it reaches an average velocity of
60m s
-1
compared with a mean annual value
of 39m s
-1
) and generates upper-air depressions,
which move southeastward along the front
(analogous to the systems in North Africa).
The variation in strength of the continental
Figure 10.31
Interaction between the wester-
lies and the tropical easterlies leading to the
production of Saharan depressions (D), which move
northeastward along a trough axis.
Source: After Nicholson and Flohn (1980). Copyright ©
1980/1982 by D. Reidel Publishing Company. Reprinted by
permission.
5 Australasia
The subtropical anticyclones of the South Atlantic
and Indian Ocean tend to generate high pressure
cells which move eastward, intensifying southeast
