Geoscience Reference
In-Depth Information
many parts of the North African coast. Depres-
sions are by no means absent in the summer
months, but they are usually weak. The anti-
cyclonic character of the large-scale circulation
encourages subsidence, and air-mass contrasts
are much reduced compared with winter.
Thermal lows form from time to time over Iberia
and Anatolia, although thundery outbreaks are
infrequent due to the low relative humidity.
The most important regional winds in summer
are of continental tropical origin. There are a
variety of local names for these usually hot, dry
and dusty airstreams -
scirocco
(Algeria and the
Levant),
lebeche
(southeast Spain) and
khamsin
(Egypt) - which move northward ahead of
eastward-moving depressions. In the Negev, the
onset of an easterly
khamsin
may cause the relative
humidity to drop to less than 10 percent and
temperatures to rise to as much as 48°C. In
southern Spain, the easterly
solano
brings hot,
humid weather to Andalucia in the summer half-
year, whereas the coastal
levante
- which has a long
fetch over the Mediterranean - is moist and
somewhat cooler (see
Figure 10.28
). Such regional
winds occur when the Azores high extends over
Western Europe with a low pressure system to the
south.
Many stations in the Mediterranean receive
only a few millimeters of rainfall in at least one
summer month, yet the seasonal distribution does
not conform to the pattern of simple winter
maximum over the whole of the Mediterranean
basin.
Figure 10.29
shows that this is found in
the eastern and central Mediterranean, whereas
Spain, southern France, northern Italy and the
northern Balkans have more complicated profiles
with a maximum in autumn or peaks in both
spring and autumn. This double maximum may
be interpreted as a transition between the conti-
nental interior type with summer maximum and
the Mediterranean type with winter maximum. A
similar transition region occurs in the south-
western United States (see
Figure 10.21
), but local
topography in this intermontane zone introduces
irregularities into the regimes.
4 North Africa
The dominance of high pressure conditions in the
Sahara is marked by the low average precipitation
in this region. Over most of the central Sahara, the
mean annual precipitation is less than 25mm,
although the high plateaux of the Ahaggar and
Tibesti receive over 100mm. Parts of western
Algeria have gone at least two years without more
than 0.1mm of rain in any 24-hour period, and
most of southwest Egypt as much as five years.
However, 24-hour storm rainfalls approaching
50mm (more than 75mm over the high plateaux)
may be expected in scattered localities. During a
35-year period, excessive short-period rainfall
intensities occurred in the vicinity of west-facing
slopes in Algeria, such as at Tamanrasset (46mm
in 63 minutes) (
Figure 10.30
), El Golea (8.7mm
in 3 minutes) and Beni Abes (38.5mm in 25
minutes). During the summer, rainfall variability
is introduced into the southern Sahara by the
variable northward penetration of the Monsoon
Trough (see
Figure 11.2B
), which on occasion
allows tongues of moist southwesterly air to
penetrate far north and produce short-lived low
pressure centers. Study of these Saharan depres-
sions has permitted a clearer picture to emerge of
the region. In the upper troposphere at about
200mb (12km), the westerlies overlie the poleward
flanks of the subtropical high pressure belt.
Occasionally, the individual high pressure cells
contract away from one another as meanders
develop in the westerlies between them. These
may extend equatorward to interact with the low-
level tropical easterlies (
Figure 10.31
). This
interaction may lead to the development of lows,
which then move northeast along the meander
trough associated with rain and thunder. By the
time they reach the central Sahara, they are
frequently 'rained out' and give rise to duststorms,
but they can be reactivated further north by the
entrainment of moist Mediterranean air. The
interaction of westerly and easterly circulation is
most likely to occur around the equinoxes or
sometimes in winter if the otherwise dominant
Azores high pressure cell contracts westward. The
