The current delayed oscillator theory attributes the phase transition of ENSO cycle to a negative feedback provided by equatorial Rossby wave reflection at the western ocean boundary. While the theory is relevant to interpretation of the interannual oscillations simulated in some coupled ocean-atmospheric models, it is not known to what extent it works in nature.
Observational evidences presented here reveal a critical role of the equatorial central Pacific (ECP)-western North Pacific (WNP) atmospheric teleconnection in the transition of ENSO cycles. During mature phases of major El Ni o episodes, a surface high pressure anomaly rapidlyestablishes over the WNP as a remote response to enhanced atmospheric heating in the ECP. Astrong anticyclonic anomaly forms and persists over the Philippine Sea with persistent easterly anomalies prevailing north of New Guinea. The latter force upwelling equatorial Kelvin waveswhich propagate eastward, switching the warming trend to cooling and leading to a cold state in the eastern Pacific. The turnabout from a cooling to warming involves similar negative feedback processes but with reversed anomalies.