South American deserts - Climatic Change in Deserts - page 410

Figure 21.2. Mean annual precipitation, South America. (From The Times Atlas of
Americas , 2010.)
deserts lie within the zone of the eastern rain shadow of the Andes in the centre and
north ( Figure 21.2 ). The precise limits of these rain-shadow zones have probably
fluctuated during the Quaternary according to how far north the westerly air masses
reached in winter and how far south the ITCZ reached in summer. Such changes need
not have been in phase.
In addition to topography and seasonal shifts in atmospheric circulation, the ocean
currents close to the coast have a significant influence on regional climate. In particular,
the cold Humboldt/Peru Current accentuates aridity along the west coast and the cold
Malvinas/Falklands Current accentuates aridity off the east coast of Patagonia. In
contrast, the warm South Equatorial Current and its southern counterpart, the warm
Brazil Current, favour the passage of warm, moist air masses onto the north-east and
south-east tropical margins of the continent. In the far north-west, a warm current
known locally as the El Ni no Current flows from the north, and in years when it
reaches the arid coast of northern Peru, it is associated with heavy local rainfall (see
Chapter 23 ).
The Atacama Desert owes its aridity to four main factors. In common with all the
other hot tropical deserts of the Southern Hemisphere, it lies beneath the descending
southern arm of the Hadley Cell, which brings warm, dry air to that latitude (see
Chapter 1 ). As noted in the previous section, it lies within the western rain shadow