Western Regional Climate Center


SOI — Precipitation Relationships

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.

Summer/Autumn Southern Oscillation Index (SOI) versus following October-March Climate
(following methodology of Redmond and Koch, Water Resources Research, 1991).

Important points from these diagrams:
  • Not every El Niño produces the same effect.
  • La Niña has a more consistent signal, in general, than El Niño.
  • The relations are not perfect, other things are happening in the climate system.
  • The 1982/83 El Niño does not fit in with the other points in some locations. Patterns for large El Niños may differ in some ways from typical El Niño patterns.
  • The relationship is lagged. Best associations are found between summer/autumn SOI and the following winter climate, and the following spring and summer streamflow runoff.
Clearwater R. data courtesy David Garen, NRCS-Portland
American R. data courtesy Maurice Roos, DWR-California

A few locations are shown. Others will be added as time permits.

Time Series:

© Western Regional Climate Center