Geoscience Reference
In-Depth Information
The Amazon Basin supports the largest tropical rainforest in existence but has on
occasion been dry enough for sand dunes to develop locally, most probably as source-
bordering dunes (Iriondo and Latrubesse,
1994
; Latrubesse and Ramonell,
1994
;van
der Hammen and Hooghiemstra,
2000
; Teeuw and Rhodes,
2004
). North-east Brazil
is prone to severe droughts during El Ni no years and has been much drier at intervals
during the Quaternary (Auler and Smart,
2001
;Wangetal.,
2004
). The coastal strip
of Peru is very arid, but during El Ni no years it can receive heavy downpours and is
prone to landslides along the Andean foothills.
Vegetation is controlled by temperature, precipitation and evaporation, which are
in turn closely linked to relief. On the windward side of the Andes, precipitation
increases with elevation, and the same is true of the coastal escarpments. However,
temperatures also decrease as elevation increases, with more rapid rates of decrease
occurring in the drier regions (Houston and Hartley,
2003
). As a consequence, there is
a well-marked altitudinal zonation in the vegetation, with mountain forest giving way
to grassland and to alpine scrub (Hooghiemstra and Ran,
1994
, fig. 2). In the lowlands,
tropical rainforest gives way to savanna as rainfall decreases, ceding ultimately to the
low shrubs of Patagonia and the sporadic halophytes of the Atacama.
21.3 Present-day climate and causes of aridity
The southern third of the continent (south of about latitude 30
S) falls within the
domain of the westerlies, which bring winter rainfall to southern Chile and the western
slopes of the southern Andes (Garcıa,
1994
; Sylvestre,
2009
). Patagonia lies in the rain
shadow of the westerlies. Katabatic winds blowing down from the Andes accentuate
the effects of low rainfall, as does the presence of the cold Malvinas/Falklands Current
off the east coast, so this region is sparsely vegetated and exceptionally windy. During
earlier, even drier intervals in the Quaternary, Patagonia was a major exporter of dust
to Antarctica.
The northern half of the continent is under the influence of the south-east Trade
Winds, which bring rain to the east coast of Brazil but blow parallel to the coast on
the west, leading to upwelling of cold ocean water and enhanced aridity. Seasonal
displacement of the Intertropical Convergence Zone (ITCZ) brings summer rainfall
to the region south of the Amazon Basin, which is an area of perennial rainfall
(
Figure 21.2
). The southern boundary of the ITCZ in January at the height of the
austral summer extends to 10
°
S across Amazonia but only to just south of the equator
on the east coast and a few degrees north of the equator on the west coast (Garcıa,
1994
). The Andes therefore lie within the zone of westerly precipitation in the far
south and the zone of easterly precipitation controlled by the seasonal migration of the
ITCZ in the centre and north (
Figure 21.3
). As a consequence, snow accumulation will
reflect winter westerly precipitation in the south and summer easterly precipitation in
the centre and north of the continent. In addition, the Atacama and Peruvian coastal
°
