The SOI, or Southern Oscillation Index, is the normalized difference in sea surface air pressure between Tahiti, French Polynesia and Darwin, Australia. In the early 20th century, Sir Gilbert Walker documented a pattern of reversing air pressure between stations in the eastern tropical Pacific and the western tropical Pacific/Indian Ocean. He coined this relationship the "Southern Oscillation". Later, this "seesawing" atmospheric pattern was connected to El Nino and La Nina events. In order to quantify these events, an index was created using one station to represent the eastern Pacific region and one station to represent the western Pacific/Indian Ocean region. Tahiti and Darwin were chosen as the stations because of their long data records. In general terms, the equation for the Southern Oscillation Index is the atmospheric pressure at Tahiti minus the atmospheric pressure at Darwin.

During El Niño, higher than average air pressure covers Indonesia and the western tropical Pacific and below-average air pressure covers the eastern tropical Pacific (Fig. 6). Because winds flow from regions of high to low pressure, the trade wind activity is weakened or in a strong El Nino even reversed. During El Niño events the SOI has a large negative value (generally below -1) due to lower-than-average air pressure at Tahiti and higher-than-average pressure at Darwin. During La Niña, the situation is reversed. Below-average air pressure covers Indonesia and the western tropical Pacific and above-average air pressure covers the eastern tropical Pacific (Fig. 6). The trade winds become stronger than normal. During La Niña events the SOI has a large positive value (generally above +1) due to higher-than-average air pressure at Tahiti and lower-than-average pressure at Darwin.

Figure 7 gives the three month running mean of the SOI for a 25 year period. Note the large negative SOI values (during the 1997-1998 and 1982-1983 El Niños and the large positive deviation during 1988-1989 La Niña.