Periodic forcing and ENSO suppression in the Cane-Zebiak model

The effect of a periodic forcing on the intensity of El Nino-Southern Oscillation (ENSO) is studied using the Cane-Zebiak model. With a basic seasonal climate close to the present, ENSO can be suppressed by a substantially enhanced seasonal external equatorial wind, which could be induced by monsoon forcing. ENSO suppression is usually more effective for an unstable self-exciting ENSO than for a stable stochastic-exciting ENSO. In addition, ENSO also tends to be suppressed by sufficiently strong periodic forcings of longer periods. The suppression of ENSO seems to be related to the nonlinear mechanism of frequency entrainment. These conclusions are in qualitative agreement with previous studies of conceptual ENSO models, although the Cane-Zebiak model shows a much more complicated dependence of the amplitude of ENSO on periodic forcing.