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. The conceptual ENSO recharge oscillator (RO) model is adopted to examine the ENSO phase-locking behavior in terms of its peak time, strength of phase-locking, and asymmetry between El Niño and La Niña events.
The key factors that control the mechanism of phase-locking of ENSO, in terms of its peak time, strength of phase-locking, and asymmetry of phase-locking can be determined by a simple recharge oscillator paradigm. It is useful to evaluate current coupled general circulation models (CGCMs) in terms of their ability to reproduce the ENSO phase-locking and improve their ENSO simulation and prediction skills of ENSO.
In this paper, the impact on ENSO phase-locking from annual cycle modulation of the growth/decay rate, stochastic forcing, nonlinearity, and linear frequency are examined in the 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 strength of phase-locking is mainly controlled by the linear growth/decay rate, the amplitude of seasonal modulation of growth/decay rate, the amplitude of noise, the SST-dependent factor of multiplicative noise, and the linear frequency. The asymmetry of the sharpness of ENSO phase-locking is induced by the asymmetric effect of state-dependent noise forcing in El Niño and La Niña events.