Observations over the past 20 years indicate that equatorial Pacific Ocean heat content variations associated with El Ni¿o-Southern Oscillation (ENSO) cycles lead sea surface temperature (SST) anomalies in the equatorial cold tongue by about 7 months. However, an asymmetry exists in the relationship between SST and heat content: positive SST anomalies related to El Ni¿o are stronger than negative SST anomalies related to La Ni¿a for the same magnitude (but opposite sign) heat content anomaly. In this study, we analyse a multi-century simulation using the CSIRO Mark 3 coupled climate model to show that a similar asymmetry exists in some decades but not in others. This non-stationarity appears to be a consequence of modulations by a mode of multidecadal variability which affects the temperature of upwelled water and the efficiency with which upwelling generates SST anomalies.