Abstract:
The University of Washington Joint Institute for the Study of the
Atmosphere and Ocean (JISAO) offers a Southern Oscillation Index (SOI)
dataset. The SOI is calculated from Tahiti and Darwin, Australia
Sea-Level Pressure (SLP) as follows (Chelliah, M., 1990, J. Climate,
3, 158-160 ):
SOI(month,year) = ( T(month,year) - D(month,year) )
... -----------------------------------
S
where: T & D are anomalies of Tahiti and Darwin SLP that have been
normalized by the standard deviation of the monthly values of the
respective index for all months combined for the years 1951 to 1980.
S is the standard deviation of the numerator for all months combined
for the years 1951 to 1980.
This data may be freely downloaded from the JISAO WWW site.
The time series is provided by the NCEP Climate Prediction Center.
[Summary Adapted from the JISAO Homepage] 
