Southern Oscillation Index Based Upon Annual Standardization

Click here to download data files.

The Southern Oscillation Index (SOI) presented below is computed using monthly mean sea level pressure anomalies at Tahiti (T) and Darwin (D). The SOI [T-D] is an optimal index that combines the Southern Oscillation into one series. The SOI noise [T+D] series is a measure of small scale and/or transient phenomena that are not part of the large scale Southern Oscillation. These SOI values are different from those calculated by the Climate Prediction Center because they have been derived using normalization factors based upon annual means. As noted by Trenberth (MWR,1984) this maximizes the signal-to-noise ratio.

The SOI values prior to 1935 should be used with caution. There are questions regarding the consistency and quality of the Tahiti pressure values prior to 1935. Perhaps a better estimate of the SOI prior to 1935 is the normalized Darwin pressure anomaly series.

The smoothed curves below were created using a a filter which effectively removes fluctuations with periods of less than 8 months but includes all others. At 24 months 80% of the variance is retained.

Relevant publications:

Trenberth (1984), "Signal versus Noise in the Southern Oscillation" Monthly Weather Review 112:326-332

Trenberth, K.E. and T.J. Hoar (1996): "The 1990-1995 El Nino-Southern Oscillation Event Longest on Record", Geophysical Research Letters 23:57-60

Ropelewski, C.F., and P.D. Jones (1987): "An Extension of the Tahiti-Darwin Southern Oscillation Index", Monthly Weather Review" 115:2161-2165

SOI Data

The data used are available in both netCDF and ASCII formats. The original
sea level pressure datasets for Darwin and Tahiti from the Climate Prediction
Center are contained on four files. The following datasets have been merged
to facilitate use. (1) Click on the high lighted text and
(2) use "Save As..." from the "File" dropdown menu to save the file.

The data contained on the ASCII files may read via a fortran format statement
read (*,"(i5,(12f6.1))") iyear, (x(m),m=1,12)
Note: a -99.9 indicates a missing value.

Finally, the sea level pressure data spanning the period 1951-1980 were used
to derive the monthly means used to derive the anomalies and standard deviations
used in the calculations.

netCDF format contains all the information contained in the following ASCII files.

darwin.ascii contains the Darwin slp data (-1000 mb).

tahiti.ascii contains the Tahiti slp data (-1000 mb).

darwin.anom.ascii contains the Darwin slp anomalies. The anomalies were based upon the monthly
means for the 1951-1980 period.

tahiti.anom.ascii contains the Tahiti slp anomalies. The anomalies were based upon the monthly
means for the 1951-1980 period.

SOI.signal.annstd.ascii contains the SOI (Standardized Tahiti -- Standardized Darwin) where the
standardizing is done using the approach outlined by Trenberth (1984).
To maximize the signal, the monthly anomalies were standardized using
standard deviations of annual means.

SOI.noise.annstd.ascii contains the noise-SOI (Standardized Tahiti + Standardized Darwin) where the
standardizing is done using the approach outlined by Trenberth (1984).
To maximize the signal, the monthly anomalies were standardized using
standard deviations of annual means.

SOI.darwin.annstdascii contains the Darwin only SOI (- Standardized Darwin) where the
standardizing is done using the approach outlined by Trenberth (1984).
To maximize the signal, the monthly anomalies were standardized using
standard deviations of annual means.

All datasets may also be obtained via traditional ftp:
ftp ftp.cgd.ucar.edu
anonymous
full email address
cd pub/shea/SOI/
binary
prompt
mget *

or

get SOI.nc
get darwin.ascii
get tahiti.ascii
get darwin.anom.ascii
get tahiti.anom.ascii
get SOI.signal.annstd.ascii
get SOI.noise.annstd.ascii
get SOI.darwin.annstd.ascii
quit

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Dennis Shea: shea@ucar.edu /