Elements of a Stochastic-Dynamical Theory of the Long-Term Variability of the El Niño/Southern Oscillation

In this paper, we present the elements of a theory of the long-term variability of the El Niño/Southern Oscillation (ENSO). Three basic processes, i.e., unstable air-sea interaction, the seasonal variation and stochasfic forcings from high-frequency (relative to ENSO) transients are identified to be the crucial factors leading to the long-term behavior of the tropical ocean-atmosphere system. We hypothesize that the occurrence of the ENSO is the result of an inherent instability in the tropical ocean-atmosphere triggered by stochastic forcings. Such a process is strongly modulated by the seasonal variation. For interannual time scale of the ENSO, the tropical ocean-atmosphere system is formally described in terms of a stochasticaly forced climate system. In a simple prototype model, we demonstrate how the abovemendoned processes can interact to produce many of the salient features of the long-term variability of the Southern Oscillation, including frequency of occurrence of ENSO, autocorrelation, spectral characteristics nd phase-locking properties The proposed theory provides an interesting framework for further investigation into the long-term variability of the tropical ocean-atmosphere system, as well as more general coupled dynamical climate systems.