We suggest that there are two types of ENSO: an eastern-Pacific (EP) type and a central-Pacific (CP) type. The EP-ENSO is the conventional ENSO that has its SST anomaly center located near the South America Coast and has its generation mechanism tied to thermocline variations. The CP-ENSO has its SST anomaly center located close to the date line and its generation mechanism not related to basin-wide thermocline variations and may be influenced more by atmospheric forcing. The evolution and telelconnection of the CP-ENSO are contrasted with those of the EP-ENSO.

Upper-ocean heat budget analyses are performed to suggest that the generation of the CP-ENSO is related to the trade winds forcing associated with the variation of northern subtropical high. Wind-induced surface heat flux and meridional and vertical advections dominate the onset of the CP-ENSO, which spread SST anomalies from the northeastern subtropical to the central equatorial Pacific. Local air-sea interactions at the equator further intensity the SST anomalies to become CP-ENSO events. The variation of the subtropical high is shown to link to interannual Asian-Australian monsoon variations. Biennial (~2.5 years) timescales are found to appear in CP-ENSO, subtropical high, and monsoon variations.

Experiments are conducted with the NCAR CCSM3, in which the model produces biennial ENSO more close to the central Pacific, to further examine the connection between CP-ENSO and the Indian Ocean and monsoons. It is found that the biennial model ENSO is significantly reduced in a CCSM3 experiment where the Indian Ocean coupling is turned off. It is noticed that the decoupling also weakens the biennial variability in the simulated Australian and Indian monsoons. This modeling result further suggests a close link between the CP-ENSO and the tropical biennial variability.

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