Environmental Engineering Reference
In-Depth Information
Figure 6.18
(left) The index cycle of flow in the upper
westerlies. The amplitude of the waves increases from (a) to
(d) before type (a) becomes re-established.
EL NIÑO
-
SOUTHERN OSCILLATION
new developments
In addition to the major north-south exchanges represented by the Hadley cell, we find a
major east-west exchange in the tropical Pacific Ocean which has been called the Walker
circulation. The normal situation is a strong flow from the subtropical high-pressure cells,
emphasized by subsidence over the cool ocean currents near South America (Figure 1).
Over Australia, the archipelago of Indonesia and the warm Pacific Ocean area, air tends
to be rising and precipitation is abundant. Periodically this circulation is transformed by
the cool Humboldt current off Peru being disrupted and replaced by much warmer waters.
As a result, the normally dry areas are wetter as subsidence stops and the wet areas are
drier as subsidence zones shift to dominate these locations. This state of affairs is known
as an El Niño or an ENSO (El Niño-Southern Oscillation) event. Climatologists have
derived an index termed the Southern Oscillation Index (SOI) based on pressure values at
Tahiti in the Pacific and Darwin in northern Australia to represent the state of the
circulation (Figure 2).
