atmospheric pressure. The force exerted on the surface of the Earth by the weight of the atmosphere above it. Barometers are used to measure this force. Southern Oscillation Index (SOI). A measure for monitoring the Southern Oscillation. The index is compiled by measuring the atmospheric pressure differences between Tahiti and Darwin (monthly or seasonally) and comparing the result with the mean for that time of year. The index scale ranges between about +30 and -30. A strongly negative (more negative than -10) SOI for several months indicates an El Niņo event; a strongly positive (greater than +10) SOI for several months indicates a La Niņa. For more information about the SOI and how it is calculated see Southern oscillation index (Bureau of Meteorology, Australia).
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Posted November 1997.
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