The role of zonal advection feedback in phase transition and growth of ENSO in the Cane-Zebiak model

The turnabout and growth mechanisms of the ENSO are diagnostically studied by analyzing the SST budget of the Cane-Zebiak model. The SST change rates, which are directly linked to the phase transition and growth of the ENSO, are attributed to two processes: the anomalous vertical advection of subsurface temperature by the mean upwelling (thermocline feedback), and the zonal advection of the climatological mean SST by the anomalous zonal current (zonal advection feedback). The contributions to the phase transition, and growth of the ENSO, can be systematically separated by decomposing the equatorial thermocline depth anomaly, and zonal current anomaly into their zonal mean and zonal contrast fields. It is found that the thermocline feedback, associated with the zonal mean thermocline depth anomaly, and the zonal advection feedback by the equatorial zonal mean zonal current anomaly are responsible for the phase transition of the ENSO. Those associated with zonal contrast fields are responsible for the growth of the ENSO. The two processes in the SST change contribute to the phase transition and growth of the ENSO in an almost equally significant manner. They are closely related, as a result of the geostrophic balance between the meridional gradient of the thermocline depth and the zonal current. These findings suggest that the conceptual understanding of the ENSO in the Cane-Zebiak model, should include both of these two processes.