Variations in ENSO phase locking

A hybrid coupled model (HCM) and a simple coupled model (two variables) for the tropical Pacific ocean-atmosphere system are employed to examine the v ariations of ENSO phase-locking behavior. The HCM consists of an ocean gene ral circulation model coupled to an empirical atmospheric model. While it i s often stated that the warm peak phase of El Nino in observations tends to occur in a preferred season, both model simulations and analysis of observ ations suggest that the phase-locking behavior is more complicated. A scatt ered phase-locking behavior in ENSO onset and termination phases is seen in results of both models, even when model climatology is not changing during the integration and weather noise is not included. The mechanisms for this scatter appear to be robust when atmospheric stochastic forcing is include d. A similar variation in phase-locking behavior is found in observations, suggesting that the observed scatter of onset and termination phases is a f undamental ENSO property. The scattered phase-locking behavior is explained as a result of a competition between the inherent ENSO frequency (giving a cycle in which warm peaks would not necessarily occur in a preferred seaso n) and the tendency to phase-lock to a preferred season due to the nonlinea r interaction between the ENSO cycle and the annual cycle. The phase-lockin g behavior is associated with the frequency-locking behavior but has additi onal aspects. For instance, the season of maximum warming can change with p arameters. even when the locked ENSO frequency is unchanged. Analysis of ob servations motivated by the model variation in onset phases suggests defini ng a category for "early-onset" El Ninos. Such events appear to be more irr egular than other El Ninos.