A Simple Model Study of ENSO Suppression by External Periodic Forcing

Abstract

The study of a forced delayed oscillator ENSO model suggests that the intensity of ENSO can be suppressed significantly by an external periodic forcing due to the nonlinear mechanism of frequency entrainment. This suppression of ENSO is most effective for ENSOs in the regime of unstable self-exciting oscillation and for forcing of frequencies close to that of ENSO. In particular, an annual cycle forcing can suppress ENSO substantially. This ENSO suppression effect by an external annual cycle is in contrast to the effect of the seasonal change of the coupled instability: the latter predominantly generates the seasonal phase locking of ENSO but has little effect on the amplitude of ENSO. Potential implications are also discussed for the evolution of ENSO in the Holocene and the observed monsoon?ENSO relationship.