Climatic Research Unit : Data : Pressure/Circulation Indices

Southern Oscillation Index (SOI)

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:

Related references:

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
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)

Filename SizeDescription
soi.dat 14 kilobytesSOI
soi_3dp.dat 14 kilobytesSOI (stored with 3 decimal places)
soi_dar.dat 12 kilobytesPressure data for Darwin
soi_tah.dat 12 kilobytesPressure data for Tahiti

Change log

Last updated: January 2019, Tim Osborn

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.
Please use the attribution Climatic Research Unit, University of East Anglia