Power law correlations in the Southern Oscillation Index fluctuations characterizing El Nino

The southern oscillation index (SOI) is a characteristic of the El Nino phenomenon. SOI monthly averaged data is analyzed for the time interval 1866-2000. The tail of the cumulative distribution of the fluctuations of SOI signal is studied in order to characterize the amplitude scaling of the fluctuations and the occurrence of extreme events. Large fluctuations are more likely to occur than the Gaussian distribution would predict. The time scaling of fluctuations is studied by applying the energy spectrum and the Detrended Fluctuation Analysis (DFA) statistical method. Self-affine properties are found to be pertinent to the SOI signal and therefore suggest power law correlations of fluctuations of the signal. Antipersistent type of correlations exist for a time interval ranging from about 4 months to about 6 years. This leads to favor specific physical models for El Nino description.