Southern Oscillation Index (SOI)
The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia. The 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. 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 (positive) SOI values coincide with abnormally warm (cold) ocean waters across the eastern tropical Pacific typical of El Niño (La Niña) episodes. The methodology used to calculate SOI is available below. More information can be found at the Climate Prediction Center SOI page.
| Jul 2012 | Aug 2012 | Sep 2012 | Oct 2012 | Nov 2012 | Dec 2012 | Jan 2013 | Feb 2013 | Mar 2013 | Apr 2013 | May 2013 | Jun 2013 |
|---|---|---|---|---|---|---|---|---|---|---|---|
Calculation of SOI
Note the anomalies are departures from the 1951-1980 base period.
| SOI | = | (Standardized Tahiti - Standardized Darwin) |
|---|---|---|
| MSD |
where
| Standardized Tahiti | = | (Actual Tahiti SLP - Mean Tahiti SLP) |
|---|---|---|
| Standard Deviation Tahiti |
where
Standard Deviation Tahiti =
where
N = number of months
and
| Standardized Darwin | = | (Actual Darwin SLP - Mean Darwin SLP) |
|---|---|---|
| Standard Deviation Darwin |
where
Standard Deviation Darwin =
where
N = number of months
and
MSD = Monthly Standard Deviation =
where
N = number of summed months




