AN EQUATORIAL OCEAN RECHARGE PARADIGM FOR ENSO .2. A STRIPPED-DOWN COUPLED MODEL

The conceptual recharge oscillator model intuitively established in Pa rt I is derived from a dynamical framework of a Cane-Zebiak type model for tropical ocean-atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SS T dynamics are employed to obtain a stripped-down coupled model that c aptures the main physics of the Cane-Zebiak type model. It is shown th at the conceptual recharge oscillator model can be obtained from the s tripped-down coupled model with a two-box approximation in the zonal d irection or a low-frequency approximation to filter our high-frequency modes. Linear solutions of the stripped-down model are analytically s olved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupl ed model describes a coupled-wave mode and a mixed SST-ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These two coupled oscillatory modes undergo a further merger. In the neighborhood of this merger, the leading mode o f the system becomes a generalized mixed mode. It is suggested that th e slow ENSO regime can be best characterized by this generalized mixed mode whose essential physics are described by the conceptual recharge oscillator model proposed in Part I.