The el nino cycle: a natural oscillator of the pacific ocean--atmosphere system.

Research conducted during the past decade has led to an understanding of many of the mechanisms responsible for the oceanic and atmospheric variability associated with the El Niño-Southern Oscillation (ENSO). However, the reason for one of the fundamental characteristics of this phenomena, its quasi-periodicity, has remained unclear. Recently available evidence from a number of sources now suggests that the ENSO "cycle" operates as a natural oscillator based on relatively simple couplings between the tropical atmospheric circulation, the dynamics of the warm upper layer of the tropical ocean, and sea surface temperatures in the eastern equatorial Pacific. This concept and recent field evidence supporting the natural coupled oscillator hypothesis are outlined.