The Southern Oscillation Index (SOI) is one measure of the large-scale fluctuations in
air pressure occurring between the western and eastern tropical Pacific (i.e., the state
of the Southern Oscillation) during El Niño and La Niña episodes. Traditionally, this
index has been calculated based on the differences in air pressure anomaly between Tahiti
and Darwin, Australia. In general, smoothed time series of the SOI correspond very well
with changes in ocean temperatures across the eastern tropical Pacific. The negative phase
of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at
Darwin. Prolonged periods of negative SOI values coincide with abnormally warm ocean
waters across the eastern tropical Pacific typical of El Niño episodes. Prolonged periods
of positive SOI values coincide with abnormally cold ocean waters across the eastern
tropical Pacific typical of La Niña episodes.
The time series of the SOI and sea surface temperatures in the eastern equatorial
Pacific indicates that the ENSO cycle has an average period of about four years, although
in the historical record the period has varied between two and seven years. The 1980's and
1990's featured a very active ENSO cycle, with 5 El Niño episodes (1982/83, 1986/87,
1991-1993, 1994/95, and 1997/98) and 3 La Niña episodes (1984/85, 1988/89, 1995/96)
occurring during the period. This period also featured two of the strongest El Niño
episodes of the century (1982/83 and 1997/98), as well as two consecutive periods of El
Niño conditions during 1991 - 1995 without an intervening cold episode.
Historically, there is considerable variability in the ENSO cycle from one decade to
the next. For example, there are decades in which the cycle was relatively inactive, and
decades in which it was quite pronounced.