Climate Variability: Southern Oscillation Index
In addition to the Ocean Niño index, atmospheric measurements can indicate the status of the El Niño - Southern Oscillation cycle. As El Niño conditions develop, sea surface temperatures in the eastern Pacific Ocean increase and air pressure over the region decreases. Simultaneously, higher air pressure develops over the cooler sea surface in the western Pacific, near Indonesia. Thus, the difference in air pressure between these two regions provides another way to monitor the ENSO cycle.
The Southern Oscillation Index or SOI represents the difference in average air pressure measured at Tahiti and Darwin, Australia. More specifically, the SOI is calculated as the difference in monthly averages of standardized mean sea level pressure at each station.
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Additionally, a third index is used to characterize the phase of ENSO: the Equatorial Southern Oscillation Index or EQSOI represents the difference in air pressure measured over the eastern and western Pacific. The EQSOI is calculated as the difference in standardized mean sea level pressure over a swath of the eastern equatorial Pacific Ocean (5°N-5°S, 80°W-130°W, labeled EPAC above) and another swath that spans Indonesia (5°N-5°S, 90°E-140°E, labeled INDO).
During El Niño conditions, when air pressure is above average over the western Pacific and below average over the eastern Pacific, values for the SOI and EQSOI are negative. Conversely, La Niña conditions result in positive values for the two atmospheric indices.