A measure of the difference in sea level pressure between the western (e.g., Darwin, Australia) and central/eastern (e.g., Tahiti) equatorial Pacific, representative of the east-west changes in atmospheric circulation associated with the El Nino/Southern Oscillation phenomenon. Term was introduced by Sir Gilbert Walker (Walker and Bliss, 1932). More information on the SOI can be found here.
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Southern Oscillation Index (“SOI”)