Transitions between Central-Pacific and Eastern-Pacific Types of ENSO

An increasing number of recent studies has indicated that there are two different types of El Nino-Southern Oscillation (ENSO): an Eastern-Pacific (EP) type that has its sea surface temperature (SST) anomalies centered near the South America coast and a Central-Pacific (CP) type that has its SST anomalies centered around the international datelines. Although these two ENSO types are beleived to have different generation mechanisms, we will show in this talk that there exists certain transition patterns between the EP and CP ENSOs. The demise of one ENSO type can lead to the development of the other type.

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.