The SOI is defined as the normalized pressure difference between Tahiti and Darwin. There are several slight variations in the SOI values calculated at various centres. Here we calculate the SOI based on the method given by Ropelewski and Jones (1987). It uses a second normalization step, and was the Climate Analysis Centre's standard method in 1987. The reader is also referred to Allan et al. (1991) and Können et al. (1998) for details of the early pressure sources and methods used to compile the series from 1866 onwards.
The standard SOI data file has been provided in the same format since 1998. This was supplemented by a version stored with 3 decimal places in 2017. The extra precision is provided for users who are making further calculations where rounding to the appropriate precision is best done at the end of the calculations.
Note that recent values may change a few months later if the data sources we rely upon are updated.
Main reference to cite when using this SOI index:
Related references:
Filename | Description |
---|---|
SOI | |
soi.dat | SOI: 1866 to present, one line per year format(i5,12f7.2,f8.2) year, 12 * monthly value, annual value Missing values represented by -99.99 |
soi_3dp.dat | SOI to 3 decimal places: 1866 to present, one line per year format (i5,12f8.3,f8.3) year, 12 * monthly value, annual value Missing values represented by -99.990 This format changed 21/7/2017 |
Pressure | |
soi_dar.dat | Pressure data for Darwin: 1855 to present, one line per year format (i5,12i6) year, 12 * monthly value Divide by 10 for hPa (millibars) Missing values represented by -990 |
soi_tah.dat | Pressure data for Tahiti: 1855 to present, one line per year format (i5,12i6) year, 12 * monthly value Divide by 10 for hPa (millibars) Missing values represented by -990 |
These datasets are made available under the Open Database License. Any rights in individual contents of the datasets are licensed under the Database Contents License under the conditions of Attribution and Share-Alike.
Updated: December 2023, Ian Harris