The build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific provides necessary conditions for interannual El Niño/Southern Oscillation (ENSO) events to produce transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). A long pre-industrial control run with CESM1 is analyzed to show that there is a greater chance of an ENSO event around the time of an IPO transition when off-equatorial western Pacific ocean heat content reaches either a maximum (for an El Niño to trigger a transition to positive IPO) or minimum (for a La Niña to trigger a transition to negative IPO). If values of off-equatorial western Pacific ocean heat content are below the necessary threshold, an ENSO event can occur without a transition. For example, if off-equatorial western Pacific ocean heat content rises above the threshold for a transition to positive IPO, the convergence associated with westerly anomaly near-equatorial surface winds of an El Niño event can draw that heat content equatorward. This sustains anomalously warm western Pacific SSTs, positive precipitation and convective heating anomalies, a Gill-type response that produces anomalous surface easterlies near 20N and 20S in the western Pacific, and wind stress curl anomalies that continue to feed warm water into the near-equatorial western Pacific to sustain the decadal-timescale transition to positive IPO (with the opposite sense for transition to negative IPO). This timescale interaction, with an ENSO interannual timescale event contributing to a decadal timescale IPO transition, is viewed as a corollary to subseasonal westerly wind burst events contributing to an ENSO interannual timescale transition.