Note that recent values may change a few months later if the data sources we rely upon are updated.
Latest updates (see below for earlier updates):
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.
Main reference to cite when using this SOI index:
|File format (SOI)|
for year = 1866 to endyear format(i5,12f7.2,f8.2) year, 12 * monthly value, annual value
|Missing values represented by -99.99|
|File format (SOI with 3 decimal places)|
for year = 1866 to endyear format(i5,12f8.3,f8.3) year, 12 * monthly value, annual value
NOTE THIS FORMAT WAS CHANGED ON 21 JULY 2017|
Missing values represented by -99.990
|File format (Station pressure)|
for year = 1855 to endyear format(i5,12i6) year, 12 * monthly value
Missing values represented by -990
Divide by 10 for hPa (millibars)
|soi_3dp.dat||14 kilobytes||SOI (stored with 3 decimal places)|
|soi_dar.dat||12 kilobytes||Pressure data for Darwin|
|soi_tah.dat||12 kilobytes||Pressure data for Tahiti|