Types of ENSO Indexes
Air Pressure Indexes
The Southern Oscillation Index (SOI) is the oldest method to indicate ENSO. It is
measured through the atmospheric pressure differences at sea level between Tahiti
and Darwin, Australia. The contrast of pressure at the two ends shows the atmospheric components of ENSO, noticed in the early 1900s by Walker and Bliss
(1932). During El Nino, low pressure is created in Tahiti and high pressure is created
at Darwin, and this represents a negative Southern Oscillation Index. Similarly,
during La Nina pressure is reversed at Tahiti and Darwin and the index changes to
positive. One drawback was that the Tahiti and Darwin stations were established
south of the Equator (Tahiti at 18
S, Darwin at 12
S) but the ENSO effect acts
mostly along the Equator.
This is where the Equatorial Southern Oscillation Index is taken into consideration as it is used to find the sea level pressure difference between two large regions,
that is, Indonesia and the eastern tropical Pacific, which lies centered on the Equator
(5
S to 5
N).
Fig. 10.2 The Indian Ocean dipole and the movements of warmer and cooler air. (Courtesy of
NOAA)
10 Impact of Climate Extremes of El Nina and La Nina in Patterns of Seasonal. . .
233