Monitoring Agricultural Drought Using El Niño and Southern Oscillation Data - Monitoring and Predicting Agricultural Drought: A Global Study - page 30

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[30],

Fig ure 3.1 A schematic view of the normal atmospheric and oceanic features in the Pacific
Oc ean indicating the Walker circulation (from Glantz, 1996).

Line
——
-0.0
——
Long
PgEn

scientific community (Philander, 1989). When the SOI is negative, El Niño
or ENSO warm phase occurs. During an El Niño event, the expanding warm
waters tend to relax the easterly trade winds and weaken the Walker circula-
tion. As a result, the Southern Oscillation is reversed and pressure rises in the
west but falls in the east. When the SOI sustains high positive values, La Niña
or ENSO cold phase occurs, which represents other extremes of the ENSO
cycle. The intensification of the Walker circulation, in contrast, causes the
eastern Pacific to cool down. These changes often bring widespread rain and
flooding to western Pacific areas such as Australia and Indonesia.

[30],

EN SO and Droughts
The ENSO events have worldwide consequences. Many researchers have
studied the relationships between ENSO events and weather anomalies
around the globe to determine whether links between ENSO events and
droughts exist. ENSO events perturb the atmosphere around the tropics
to varying degrees, generating anomalous climatic patterns at the regional
and local levels. As a result, possible connections between an ENSO event
and, for example, drought in northeast Brazil, Australia, and Africa can be
identified (figure 3.2).
Not all anomalies, even in ENSO years, are due to ENSO phenomenon.
In fact, statistical evidence shows that ENSO accounts for only a small
fraction of the interannual variance in rainfall. In many ENSO-affected
places such as eastern and southern Africa, the ENSO accounts at most
for about 50% of the variance in rainfall (Ogallo, 1994), but many of the
more extreme anomalies, such as severe droughts, flooding and hurricanes,
have strong teleconnections to ENSO events. (A “teleconnection” refers to
a link between two events [e.g., ENSO, drought, flood] that are separated
by large distances.)