TIME-FREQUENCY VARIABILITY OF ENSO AND STOCHASTIC SIMULATIONS

The time-frequency spectral structure of El Nino-Southern Oscillation (ENSO) time series holds much information about the physical dynamics of the ENSO system. The authors have analyzed changes of the spectrum with time of three ENSO indices: the conventional Southern Oscillation index (SOI)I Nino3 sea surface temperatures, and a tropical Pacific r ain index, over the period 1871-1995. Three methods of time-frequency analysis-windowed Fourier transform, wavelet analysis, and windowed Pr ony's method-were used, and the results are in good agreement; The tim e-frequency spectra of all the series show strong multidecadal variati ons over the past century. In particular, there was reduced activity o f ENSO in the 2-3-yr periodicity range during the period 1920-60, comp ared with both the earlier and later periods. The dominant frequencies in the spectra do not appear to be constrained to certain frequency b ands, and there is no evidence that the ENSO system has fixed modes of oscillation. The qualitative behavior of the real SOI time series has been compared with that of time series simulated by an autoregressive stochastic process of order 3 and time series created by phase-random izing the spectral components of the SOI. The decadal variability of t he amplitude and time-frequency spectra was found to be very similar b etween the observed and simulated SOIs. This suggests that the decadal variability of ENSO can be well simulated by a stochastic model and t hat stochastic forcing may be an important component of ENSO dynamics.