Agriculture Reference
In-Depth Information
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[270
Figure 20.4 Zimbabwe's annual rainfall departures from the long-term mean from 1901 to
2000. The smooth curve represents the 10-year moving average.
Line
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0.0
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Long
PgEn
1987; Matarira, 1990; Cane et al., 1994). A detailed review of ENSO can
be found in chapter 3. It has been argued that the influence of ENSO events
is strongest during the peak austral summer rainfall months of December-
March because that is when the warm and cold events have reached ma-
turity and when the upper westerlies have retreated significantly poleward
(Mason, 2001). This delayed rainfall response has a potential value in op-
erational long-range forecasting (Cane et al., 1994). Rainfall is reduced
by 20-60% during some warm ENSO events across Zimbabwe for both
the first and second part of the rainfall season. Rainfall deficits tend to be
greatest in the southeast section of the country. Using maize yields from
Zimbabwe's smallholder-farming sector, Cane et al. (1994) showed that
more than 60% of the observed variation in yield could be predicted from
Niño 3 sea-surface temperature anomalies several months in advance.
[270
Se a-Surface Temperatures
Several studies have shown that summer rainfall in southern Africa re-
sponds to anomalous global sea-surface temperature (SST) changes (Cane
et al., 1994; Makarau and Jury, 1997; Rocha and Simmonds, 1997). A
warmer or cooler than normal eastern equatorial Pacific tends to be asso-
ciated with dry or wet conditions, respectively, across the country (Cane et
al., 1994). This response is part of the well-documented ENSO cycle (Ro-
pelewski and Halpert, 1987). It has been shown that anomalously warm
SSTs in the central equatorial Indian Ocean are usually associated with dry
conditions over the central and southern parts of southern Africa. The SST
anomaly pattern of the Indian Ocean could be important in the transmis-
sion of the El Niño signal to southern Africa (Rocha and Simmonds, 1997).
However, it has also been observed that occasionally the Indian Ocean
warm events can occur independent of ENSO events (Mason, 2001).
 
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