Long Paddock

SOI data files

These files are updated on a regular basis (near daily and monthly)

Please note: The Southern Oscillation Index (SOI) used by this site is calculated using a climatic base period of 1887 to 1989. The National Climate Centre (NCC) has made a number of changes to the way the SOI is calculated, in particular changing the climatic base period to 1933-1992. Long Paddock will continue to provide the original SOI data (1887 to 1989 base period) as the SOI phase maps on Long Paddock are based on this data.

The file called 'MonthlySOIPhase1887-1989Base' contains a column of numbers under the heading 'Phase'. The five Phases of the SOI are as follows:

  1. consistently negative
  2. consistently positive
  3. rapidly falling
  4. rapidly rising
  5. consistently near zero

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 (1887-1989 base period)

St.Dev.Diff. = Standard Deviation of the Difference (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 (1933-1992 base period)

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

Reference

A.J. Troup (1965) The Southern Oscillation. Quarterly Journel 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: 29 June 2010

Southern Oscillation Index (SOI)