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 similar to those calculated by the Climate Prediction Center in that they have been derived using normalization factors derived from monthly values.
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. Ropelewski and Jones (1987) describe the data sources and methods used to extend the data. Further comments are provided by Allan and Ansell (2006).
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. The smooth curve denoted by a thick black line is that produced using a decadal filter over thge original monthly values.