Present-day desert environments - Climatic Change in Deserts - page 17

falling as low as minus 40 ° C, they can experience summer temperatures close to 50 ° C,

which is nearly as hot as the southern Libyan Desert in July and August.
Two other factors may either enhance the aridity resulting from latitude and con-
tinentality or may be the direct and dominant cause of reduced precipitation. These
two factors are the proximity of cold oceanic water immediately offshore and the
rain-shadow effect generated by high mountains. They may operate individually or
together.
The presence of cold upwelling water or a cold ocean current close offshore is an
effective cause of coastal aridity in tropical and even in equatorial latitudes such as
the arid Horn of Africa, flanked by the cold Somali current. The cold Peru/Humboldt
Current flows north parallel to the coast of the Atacama Desert in northern Chile and
the coastal desert of Peru, with the cold California Current as its Northern Hemisphere
counterpart, bringing aridity to Baja California. The cold Benguela Current flows
north parallel to the Namib Desert in southern Africa, and the cold Azores Current
accentuates the aridity of the western Sahara. The cold West Australian Current
likewise flows north parallel to the arid west coast of Australia, but the situation here
is more complex, with the warm Leeuwin Current flowing somewhat erratically from
the Indonesian Warm Pool to the north to counteract the desiccating effect of the cold
West Australian Current.
In fact, the western borders of all the great tropical or Trade Wind deserts in both
hemispheres are washed by cool ocean currents associated with the oceanic circulation
cells or gyres which flow clockwise in the northern hemisphere and anticlockwise in
the southern hemisphere. If cool moist maritime air blows onshore, it often meets a
land surface that is warmer than the adjacent ocean surface, at least in summer and
during the day. The cool maritime air mass becomes warmer on contact with the warm
surface of the land. The relative humidity of this air mass is therefore decreased, and
its ability to absorb additional moisture from surface evaporation is increased. The air
therefore has a desiccating effect on the land. This situation is only reversed if the land
temperatures become significantly cooler than those of the adjacent ocean or if the sea
surface temperatures become periodically warmer, as happens off the coast of Peru
during El Ni no years (see Chapter 23 ). Otherwise, the major sources of moisture in
these often quite narrow coastal deserts are the coastal fogs that blow inland in winter
when the land has cooled down relative to the sea surface temperatures. Coastal fogs
are quite common in deserts where mountain ranges like the Andes or the Rockies, or
uplands of more moderate elevation like the Red Sea Hills, run parallel to and close
to the shore. For example, Erkowit in the Red Sea Hills of the eastern Sudan is a mist
oasis and supports a spectacular flora of tall Euphorbia candelabra trees in the dry
valleys between its rocky granite hills.
The fourth and final general cause of aridity is the rain-shadow effect, which is a
global phenomenon linked to topography and is not restricted to deserts. Wherever
ranges of hills or mountains are located close to the coast, forming a physical barrier