Geoscience Reference
In-Depth Information
persisted throughout August, causing severe floods in much of central Sudan, includ-
ing Khartoum. Extreme rainfall events in central Sudan are historically associated
with a warm equatorial Indian Ocean, a strong summer monsoon over both Africa
and India, a northward shift of the Intertropical Convergence Zone earlier and further
north than usual, and the presence of deep, well-developed westerly air masses accom-
panied by a strong Tropical Easterly Jet that allowmore moisture transport into Africa
from the South Atlantic via the Congo Basin, which leads to very heavy precipitation
in the Ethiopian uplands and the central Sudan (Camberlin,
1997
; Camberlin et al.,
2001
;Moetal.,
2001
; Osman and Shamseldin,
2002
; Vizy and Cook,
2003
). Satellite
imagery revealed that the intense 1999 wet season rains in central Sudan filled the
depressions between the extensive dunes immediately east of the lower White Nile
and recreated the geography of early Holocene times (Williams and Nottage,
2006
).
This was a time when bands of Mesolithic hunter-gatherers used to camp seasonally
on top of the dunes overlooking the flooded ponds between them in order to fish,
harpoon hippos and collect
Pila wernei
shells for food (Adamson et al.,
1974
; Clark,
1989
).
23.12 Conclusion
Changes in sea surface temperatures in the equatorial Pacific Ocean and adjacent
Indian Ocean associated with the Southern Oscillation are a major source of inter-
annual variations in the strength of the summer monsoon. The Southern Oscillation
reflects the surface atmospheric pressure differences between the eastern and western
limbs of the equatorial Pacific and gives rise to the Walker Circulation. The strength of
the Southern Oscillation is measured as the pressure difference between Darwin and
Tahiti, expressed as the Southern Oscillation Index (SOI). In years when the SOI is
strongly negative (i.e., there is anomalously high pressure at Darwin and anomalously
low pressure at Tahiti), there tends to be below-average precipitation or droughts in
north-east Brazil, eastern Australia, eastern China, peninsular India, north and central
Thailand, Java, parts of southern Africa and at the Ethiopian headwaters of the Blue
Nile. Conversely, when the SOI is strongly positive, these same regions experience
above-average precipitation and often severe floods.
These patterns have changed only slightly over the past hundred and more years, but
there are signs of a change in the previously well-established, statistically determined
correlations (or teleconnections). For example, since the 1970s, summer rainfall in
India has become less linked to El Ni no-Southern Oscillation (ENSO) events and
more linked to phase changes in the Indian Ocean Dipole. There is also evidence that
from 1977 to 2006, when tropical sea surface temperatures were the highest on record
and mean surface atmospheric pressure at Darwin was the highest recorded, there was
a weakening of the Walker Circulation, and June-December values of the SOI were
the lowest on record. A corollary to these changes in previous patterns is that global
