Variations in ENSO phase locking

Abstract

A hybrid coupled model (HCM) and a simple coupled model (two variables) for the tropical Pacific ocean-atmosphere system are employed to examine the variations of ENSO phase-locking behavior. The HCM consists of an ocean general circulation model coupled to an empirical atmospheric model. While it is 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 observations suggest that the phase-locking behavior is more complicated. A scattered 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 included. A similar variation in phase-locking behavior is found in observations, suggesting that the observed scatter of onset and termination phases is a fundamental 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 season) and the tendency to phase-lock to a preferred season due to the nonlinear interaction between the ENSO cycle and the annual cycle. The phase-locking behavior is associated with the frequency-locking behavior but has additional aspects. For instance, the season of maximum warming can change with parameters, even when the locked ENSO frequency is unchanged. Analysis of observations motivated by the model variation in onset phases suggests defining a category for 'early-onset' El Ninos. Such events appear to be more irregular than other El Ninos.