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.

Southern Oscillation Index (SOI) Last 12 Months
Apr 2013May 2013Jun 2013Jul 2013Aug 2013Sep 2013Oct 2013Nov 2013Dec 2013Jan 2014Feb 2014Mar 2014
+0.2
+0.8
+1.2
+0.8
+0.2
+0.3
+0.7
+0.1
+1.4
+0.1
-0.1
-0.9

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 = ∑ (actual Tahiti SLP - mean Tahiti SLP)² / N

where

N = number of months

and

Standardized Darwin = (Actual Darwin SLP - Mean Darwin SLP)
Standard Deviation Darwin

where

Standard Deviation Darwin = ∑ (actual Darwin SLP - mean Darwin SLP)² / N

where

N = number of months

and

MSD = Monthly Standard Deviation = ∑ (Standardized Tahiti - Standardized Darwin)² / N

where

N = number of summed months