We propose that the broad spectrum of interannual variability in the tropics with a peak around four years results from an interaction between the linear low frequency oscillatory mode of the coupled system and the nonlinear higher frequency modes of the system. We illustrate this using a conceptual model where the higher frequency modes, represented by a nonlinear low-order model, are coupled to a low frequency linear oscillator. ¿ American Geophysical Union 1993