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.