El Nio 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 component 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 Pacific is associated with high pressure and dry conditions. Generally, when pressure rises over the eastern Pacific Ocean, it tends to drop in the western Pacific and vice versa (Maunder, 1992). A large-scale pressure seesaw between the western and eastern Pacific Oceans is called the Southern Oscillation, 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 Peruvian sailors (Glantz, 1996), the term "La Niña" was recently coined by the

Walker Circulation

Downward Branch

Figure 3.1 A schematic view of the normal atmospheric and oceanic features in the Pacific Ocean indicating the Walker circulation (from Glantz, 1996).

Deep

Convection

Deep

Convection

Westerlies

Downward Branch

Figure 3.1 A schematic view of the normal atmospheric and oceanic features in the Pacific Ocean indicating the Walker circulation (from Glantz, 1996).

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 circulation. 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.

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