New measurements and model results highlight the role of equatorial oceanic wave processes in affecting the evolution of the 1997--98 El Ni¿o in ways not fully anticipated by delayed oscillator theory, a leading paradigm for the El Ni¿o/Southern Oscillation (ENSO) cycle. The onset of the El Ni¿o was linked to eastward propagating equatorial Kelvin waves forced by intraseasonal atmospheric oscillations originating over the Indian Ocean. The demise of the El Ni¿o was related to a complex interplay of wind-forced and western boundary-forced waves that preconditioned the ocean to sudden sea surface temperature cooling initially centered between 125 ¿W and 170 ¿W along the equator. These large-scale ocean wave processes, and the ocean-atmosphere interactions that they mediate, have significant implications for understanding variability associated with El Ni¿o-related climate swings. ¿ 1999 American Geophysical Union