Decadal changes of ENSO persistence barriers in ocean heat content (OHC) and surface temperature (SST) are examined using observations and ocean data assimilation products for the period 1958-2001. It is found that the SST indices in the eastern and central equatorial Pacific exhibit dramatically different decadal barrier variability. The variability is large for the eastern Pacific SST indices (NINO1+2 and NINO3) whose persistence barriers shifted abruptly in 1976/77 and 1989/90. In contrast, the central Pacific SST indices (NINO3.4 and NINO4) experienced little decadal barrier variability and have had their persistence barriers phase locked to spring in the past four decades. The zonal-mean OHC index averaged over the equatorial acific shows decadal barrier changes similar to those in the eastern Pacific SST indices and leads the NINO3 SST barrier by about one season. The decadal changes in the SST and OHC barriers are related to the regime shifts of the mean thermocline depth at the equatorial Pacific.

These results indicate that ENSO SST anomalies in the equatorial Pacific can be considered as consisting of two different oscillators: a central Pacific oscillator which is strongly phase locked to the seasonal cycle and always onsets in Spring and an eastern Pacific oscillator whose onset timing changes from decade to decade and is related to changes in the mean state of the ocean. Analyses will be presented to further describe this "two-oscillator" view of ENSO. This view will also be used to explain the excessive biennial tendency in the NCAR CCSM3's ENSO simulations.