The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.
TYPE OF INDICATOR
There are three types of indicators used in this report:
1.Describes the CONDITION of important elements of a system;
2.Show the extent of the major PRESSURES exerted on a system;
3.Determine RESPONSES to ... either condition or changes in the condition of a system.
This indicator is one of: CONDITION
DESIGN AND STRATEGY FOR INDICATOR MONITORING PROGRAM
Note: this indicator is mirrored from the Bureau of Meterologys SOI page at http://www.bom.gov.au/climate/current/soihtm1.shtml
Spatial scale: Southern hemisphere
Frequency: Monthly reporting
Measurement technique: There are a few different methods of how to calculate the SOI. The method used by the Australian Bureau of Meteorology is the Troup SOI which is the standardised anomaly of the Mean Sea Level Pressure difference between Tahiti and Darwin. It is calculated as follows:
SOI = 10*(Pdiff - Pdiffav) / SD(Pdiff)
Pdiff = (average Tahiti MSLP for the month) - (average Darwin MSLP for the month),
Pdiffav = long term average of Pdiff for the month in question, and
SD(Pdiff) = long term standard deviation of Pdiff for the month in question.
The multiplication by 10 is a convention. Using this convention, the SOI ranges from about ?35 to about +35, and the value of the SOI can be quoted as a whole number. The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily or weekly values of the SOI do not convey much in the way of useful information about the current state of the climate, and accordingly the Bureau of Meteorology does not issue them. Daily values in particular can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes.
This dataset has obvious implications for climate change research.