Abstract
The existence of multiple types of El Niño-Southern Oscillation (ENSO), termed ENSO diversity, has been well documented, and its mechanism is under active investigation. In this study, an extended recharge-oscillator model for ENSO diversity is derived from first principles based on the Zebiak-Cane framework. The model consists of three independent variables: the eastern Pacific (EP) sea surface temperature anomaly (SSTA), the central Pacific (CP) SSTA, and the basin-averaged equatorial thermocline fluctuation. Formulations of various thermodynamic and dynamical processes, both linear and nonlinear, are individually derived and then combined to yield the model equations. This approach allows model verification against the observation at the process level. The model-simulated ENSO reproduces the observed ENSO amplitude, asymmetry, and phase-locking. Irregular occurrences of multiple ENSO types similar to those identified in the observation are also successfully simulated. This minimalistic conceptual model serves as a promising tool for the process-oriented diagnosis of ENSO and benefits our basic understanding of ENSO diversity. Sensitivity simulations confirm the essential role of nonlinear processes in ENSO asymmetry and diversity.
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