A Further Investigation of the Recharge Oscillator Paradigm for ENSO Using a Simple Coupled Model with the Zonal Mean and Eddy Separated

The recharge oscillator paradigm for ENSO is further investigated by using a simple coupled model, which externally includes the equatorial wave dynamics represented by the Kelvin and gravest symmetric Rossby waves. To investigate the role of eddies in the Pacific basin–wide adjustment to the wind forcing, particularly at the western and eastern boundaries, the zonal mean and eddy parts are treated separately in the current model. It is clearly demonstrated that the basin-wide adjustment of the tropical ocean is accomplished by the net mass transport induced by the meridional transport over the tropical ocean interior and the zonal fluxes at the boundaries. With a reasonable choice of the reflection coefficient, particularly at the western boundary, the meridional transport plays a bigger role than the zonal boundary flux and determines the sign of zonal-mean thermocline depth tendency, in a way that the discharge of equatorial mass in the warm phase and recharge in the cold phase serve as a phase transition mechanism of the coupled system. The meridional mass transport is induced mainly by a geostrophic current associated with the east–west slope of thermocline depth, established quickly by the wind forcing. Also discussed in this paper is the difference between the recharge oscillator and the delayed oscillator in explaining the phase transition mechanism of ENSO.