SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded inNino3
NOAA/National Weather Service NOAA Center for Weather and Climate Prediction, Climate Prediction Center, Climate Glossary, University Research Court,College Park, Maryland, USA 2004. http://www.cpc.ncep.noaa.gov/products/outreach/glo ssary.shtml, (Wthr - Amer) {862}