By analyzing observational data and CMIP3 model simulations, we find the tranistions are controlled by two major factors: (1) the sea level pressure variation in the extratropical atmosphere that are associated with the so-called North Pacific Oscillation (NPO) and (2) the mean thermocline depth at the equatorial Pacific. The NPO works as an "atmospheric linking bridge" to allow the decay of an EP El Nino to excite a CP La Nina. On the other hand, the mean thermocline depth determines whether an EP type of El Nino or La Nina should occur after the decay of a CP El Nino.
Our study indicates that it requires information from the extratropical atmosphere to predict the onset of the CP ENSO and from the equatoral subsurface ocean to predict the decay of the CP ENSO. These results improve our understanding of the generation mechanisms of the EP and CP ENSOs and have importnat implications to the development of method for ENSO prediction.