[Source: NOAA Climate Prediction Center, http://www.cpc.ncep.noaa.gov/
Monthly Southern Oscillation Index (SOI) based on standardized sea level pressure (SLP) difference data measured between Tahiti and Darwin, Austraia from 1882-present. Data consists of SLP anomalies, and standardized data. The standard base period for Darwin and Tahiti is 1951-1980.
Details on the calculation of the SOI can be found at: http://www.cpc.ncep.noaa.gov/data/indices/Readme.index.shtml#SOICALC