Statistical characteristics of interdecadal fluctuations in the Southern Oscillation and the surface temperature of the equatorial Pacific

Abstract

Decadal and interdecadal fluctuations in the Southern Oscillation index, SOI, and in the SSTI (Wright's sea surface temperature index) of the equatorial Pacific are the focus of this research. The SSTI is defined by the average anomaly of sea surface temperature (SST) in regions of the eastern and central equatorial Pacific. Analyses were carried out using the integral wavelet transform. Full understanding of the climate variability that takes place within a century or smaller timescale requires knowing the characteristics of these fluctuations. The following findings are documented in this paper. (1) The interfluctuation span in decadal and bidecadal components of SOI and SSTI are normally distributed; standard deviation values cast doubts on the potential of statistical models for long-term — 10 to 20 years ahead — forecasting. (2) There is a large correlation between positive (negative) extreme of SOI fluctuations and associated negative (positive) extreme of SSTI fluctuations at decadal, bidecadal, and interdecadal timescales. (3) In mid-1930s, a change in the relative phase between decadal fluctuations of SOI and SSTI occurred. (4) Early 1960s pulsations at decadal and total interdecadal timescales of SOI and SSTI started to amplify. (5) Summer bidecadal components of SOI and SSTI started to amplify at about mid-1930s, whereas winter bidecadal components do not show amplification. (6) Signals in the band of wavelet timescale smaller than 10 year — encompassing ENSO signals — do not show indication of climate variability throughout the period of study (1896–1985). (7) The controlling process on the variability of the decadal components of winter SOI and winter SSTI during that period is a joint amplitude modulation (for brevity, denominated amplitude modulation of decadal component of Southern Oscillation, AMDecSO). This amplitude modulation attains maximum amplitude in 1913–1917. Thereafter, amplitude decreases until about 1959. In early-1960s, starts the growing branch of this amplitude modulation of decadal fluctuations. The state of AMDecSO and contributions from the bidecadal component of SOI and SSTI account for the evolution observed in this century in the values of these two indexes. The modulation of the decadal component of SSTI is clearly seen in summer and winter averages.