Geoscience Reference
In-Depth Information
Figure 3.5. The global hydrologic cycle (schematic).
Sarnthein et al.,
1981
; Maher et al.,
2010
). Before and during the Last Glacial Max-
imum, centred on 21
2 ka (Mix et al.,
2001
), Australian and Patagonian desert
dust was also blown as far as Antarctica (Petit et al.,
1981
; Harrison et al.,
2001
;
Revel-Rolland et al.,
2006
; Maher et al.,
2010
). In addition, stronger Trade Winds
linked to a more intense Hadley circulation would have accentuated upwelling of
cold ocean water on the upwind sector of the ocean gyres, strengthening the cold
Benguela Current off Namibia and the cold Humboldt Current off Chile and Peru. In
the Northern Hemisphere, the corresponding cold currents are the Azores Current off
the western Sahara and the California Current off Central and North America, both
of which contribute to enhanced aridity onshore.
Late Cenozoic cooling of the ocean surface was an additional factor responsible
for reducing intertropical precipitation. Galloway (
1965a
) estimated that two-thirds of
present-day global precipitation falls between latitudes 40
±
S and depends
significantly on evaporation from the warm tropical seas. In this context,
Figure
3.5
shows the importance of the oceans in contributing water to the land. The ocean
surface cooling, which was linked to global cooling associated with high-latitude
continental ice build-up and enhanced cold bottom-water circulation, would help
reduce evaporation from the tropical seas, thereby further reducing rainfall across the
tropical deserts.
Tabl e 3 . 4
provides a global summary of some key Cenozoic tectonic
and climatic events, together with their impact on oceanic circulation.
°
N and 40
°
3.3 Cenozoic desiccation was not synchronous
The onset of late Cenozoic aridity and the resultant slow emergence of the deserts
were the results of global tectonic events that led to changes in global atmospheric
circulation linked to changes in the global distribution of land and sea. For example,
the origin of the Sahara as a desert was associated with several independent tectonic
