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What is the Southern Oscillation Index (SOI)?


The Southern Oscillation Index or SOI is a standardised index of the barometric pressures over Darwin, Australia and Tahiti. Climate scientists use the SOI to assess the strength of the El Niño Southern Oscillation phenomenon (or ENSO), which in Queensland accounts for nearly 25 per cent of our year-to-year rainfall variability. For example, it is often wetter during a La Niña classified year (when the sustained SOI is very positive (higher than +7)) and drier during an El Niño classified year (when the sustained SOI is very negative (lower than -7)). The index scale ranges from about +35 to -35 using the Troup method of calculation used on Long Paddock.

 

What is included in the Southern Oscillation Index (SOI)?


The Southern Oscillation Index (SOI) is often used as an input for seasonal climate forecasting in Australia and other El Niño Southern Oscillation (ENSO)-sensitive areas around the world. We provide daily SOI data from 1991 and monthly SOI data from 1876 up to present day. We’ve also used the SOI to develop seasonal climate outlooks including rainfall probabilities and pasture growth outlooks for graziers and other land managers to freely access from this web site. These SOI datasets are generated automatically from third-party sources of Mean Sea Level Pressure (MSLP) that we access and so, occasionally there can be unscheduled interruptions to our service that are outside our control.

SOI data are generally updated by 7pm Australian Eastern Standard Time (AEST) and include:

  • Summary page
  • Table of previous 30 days
  • Data files available in CSV format
  • Graph of historical values

 

Troup's SOI Calculation (used on this site)


10x [PA (Tahiti) - PA (Darwin)] / Std Dev Diff

SOI Calculation formula: 10x [PA (Tahiti)  - PA (Darwin)] / Std Dev Diff

SOI Calculation formula: 10x [PA (Tahiti) - PA (Darwin)] / Std Dev Diff

where: PA() = the Pressure Anomaly = monthly mean minus long-term mean (derived from 1887-1989 base period)

St.Dev.Diff. = Standard Deviation of the Difference (derived from 1887-1989 base period)

A Troup SOI of -10 means the SOI is 1 standard deviation on the negative side of the long-term mean for that month. Troup's monthly SOI from 1876 onwards is derived from normalised Tahiti minus Darwin mean sea level pressure (mslp)

NCC SOI Calculation


The National Climate Centre (NCC) have a revised SOI calculation although still based on the Troup formula.

The formula for calculating the NCC 1933-92 base period SOI is:10x [PA (Tahiti) - PA (Darwin)] / Std Dev Diff

where: PA() = the Pressure Anomaly
= monthly mean minus long-term mean (derived from 1933-1992 base period)

St.Dev.Diff. = Standard deviation of the difference (derived from 1933-1992 base period)

Reference


A.J. Troup (1965) The Southern Oscillation. Quarterly Journal of Royal Meteorological Society. 91, 490-506.

Note: These calculations differ slightly from those issued in some other centres where the value may not be multiplied by 10.

Last updated: 7 March 2018