An alternative approach for explaining aperiodicity and chaos in the El-Ni¿o-Southern-Oscillation (ENSO) is proposed, which considers the ocean's equatorial wave dynamics as part of a nonlinear dynamical system driven by random environmental fluctuations. Noise excitation causes the model to jump chaotically between different interacting oscillations generated by the system's implied Pacific ocean-atmosphere cycle and the seasonal cycle. The injection of noise induces chaotic dynamics, triggering an El Ni¿o whenever the model's thermocline depth exceeds a threshold level. A stochastic resonance arises-should noise intensity deviate substantially from optimal, the regular development and coherence of the ENSO cycle is impeded. The model's noise induced chaos provides an attractive explanation for the sporadic yet deterministic character of El Ni¿o events. ¿ 1998 American Geophysical Union