Fundamental Behavior of ENSO phase-locking

El Niño-Southern Oscillation (ENSO) events tend to peak at the end of the calendar year, a phenomenon called ENSO phase-locking. This phase-locking is a fundamental ENSO property that is determined by its basic dynamics. In this study, the conceptual ENSO recharge oscillator (RO) model is used to examine the ENSO phase-locking behavior in terms of peak time, the sharpness of phase-locking and the asymmetry of phase-locking between El Niño and La Niña events. The RO model framework basically reproduces the main phase-locking characteristics found in observations and the results show that the phase-locking of ENSO is mainly generated by the seasonal variation associated with the periodic modulation of ENSO's growth/decay rate. However, the asymmetric property of phase-locking isn't readily quantified by the seasonal variance. Base on this result, it is suggested that the peak time histogram measure of ENSO is a more precise method to characterize the phase-locking feature.

Further, the impact onto ENSO phase-locking from annual cycle modulation of the growth/decay rate, stochastic forcing, nonlinearity, and linear frequency are examined in RO model. The preferred month of ENSO peak time depends critically on the phase and strength of the seasonal modulation of the ENSO growth/decay rate. Furthermore, the sharpness of phase-locking is mainly controlled by the linear growth rate, the strength of seasonal modulation of growth rate, the amplitude of noise, the SST-dependent multiplicative noise, and the linear frequency. The asymmetry of the sharpness of ENSO phase-locking is mainly induced by the asymmetric effect of noise in El Niño and La Niña events.