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be much longer or shorter. El Niño often begins early in the year and peaks
in the following boreal winter. Although most El Niño events have many
features in common, no two events are exactly the same. The presence of
El Niño events during historical periods can be detected using climatic data
interpreted from the tree ring analysis, sediment or ice cores, coral reef sam-
ples, and even historical accounts from early settlers. Many researchers are
currently working to determine whether global warming would intensify
or otherwise affect El Niño.
El Niño and the Southern Oscillation
Sea surface variations in the equatorial Pacific have a profound influence
on the global atmospheric circulation and are an integral part of the ENSO
phenomenon. The western equatorial Pacific is characterized by a region
of heavy precipitation and an intense and thermally driven circulation with
low-level easterly trade winds and upper-level westerly winds to the east
of the date line. This circulation is called the Walker circulation after Sir
Gilbert Walker, who discovered and named a number of global climate
phenomena, including the Southern Oscillation, while trying to predict the
Indian summer monsoon (Walker, 1924).
The easterly trade winds in the tropics are part of the low-level com-
ponent of the Walker circulation. Typically, the trade winds bring warm
and moist air toward the Indonesian region (figure 3.1), where the moist
air moves over the normally very warm seas and rises to high levels of the
atmosphere. The air at high altitudes traveling eastward meets the westerly
currents before sinking over the eastern Pacific Ocean. The rising air in the
western Pacific is associated with a region of low air pressure, towering
cumulonimbus clouds, and rain, whereas the sinking air in the eastern Pa-
cific is associated with high pressure and dry conditions. Generally, when
pressure rises over the eastern Pacific Ocean, it tends to drop in the west-
ern Pacific and vice versa (Maunder, 1992). A large-scale pressure seesaw
between the western and eastern Pacific Oceans is called the Southern Os-
cillation, which causes variations in rainfall and winds. When the Southern
Oscillation is combined with variations in the sea temperatures, it is often
called the El Niño and Southern Oscillation or ENSO.
The Southern Oscillation can be measured by the difference between
eastern sea-level pressure (ESLP) at Tahiti (on a French island in the Central
Pacific) and western sea-level pressure (WSLP) at Darwin (in Australia).
This pressure difference is called the Southern Oscillation Index (SOI) or
Tahiti-Darwin Index (TDI). Although the SOI is the most widely used
indicator of the Southern Oscillation, there are other indices using east—
west pairs involving the locations such as Jakarta in Indonesia or Santiago
de Chile in South America.
Two important events, El Niño and La Niña, are closely linked to the
SOI. While the term “El Niño” was extracted from the folklore of the Peru-
vian sailors (Glantz, 1996), the term “La Niña” was recently coined by the
[29],
Line
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-0.1
——
Norm
PgEn
[29],
 
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