UEA ENV Climatic Research Unit Climate Data Pressure & Circulation




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Southern Oscillation Index (SOI) data

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:

Ropelewski, C.F. and Jones, P.D. (1987) An extension of the Tahiti-Darwin Southern Oscillation Index. Monthly Weather Review 115, 2161-2165. doi: 10.1175/1520-0493(1987)115%3C2161:AEOTTS%3E2.0.CO;2

Related references:

Allan, R.J., Nicholls, N., Jones, P.D. and Butterworth, I.J. (1991) A further extension of the Tahiti-Darwin SOI, early SOI results and Darwin pressure. J. Climate 4, 743-749. doi: 10.1175/1520-0442(1991)004%3C0743:AFEOTT%3E2.0.CO;2

Können, G.P., Jones, P.D., Kaltofen, M.H. and Allan, R.J. (1998) Pre-1866 extensions of the Southern Oscillation Index using early Indonesian and Tahitian meteorological readings. J. Climate 11, 2325-2339. doi: 10.1175/1520-0442(1998)011%3C2325:PEOTSO%3E2.0.CO;2

FilenameDescription
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

License

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

Updated: December 2023, Ian Harris