Extratropical air–sea interactions have become increasingly involved in promoting the transition to El Niño–Southern Oscillation (ENSO) with climate change. In our work, we explore how sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP) affect ENSO development at two different timescales. First, we break down the effects of future warming on the 1-year lead relationship between a cold WNP SSTA and El Niño development the following winter and how this relationship may shape ENSO in a warming world. With enhanced warming, cold WNP SSTAs in the boreal winter further strengthen summer westerly anomalies in the western equatorial Pacific, which promote the intensification of surface convergence and anomalous Ekman and geostrophic advection, and positive SSTAs in the central equatorial Pacific in the seasons prior to the El Niño. Downwelling equatorial Kelvin waves induced by the westerly wind stress facilitate entrainment of subsurface water into the mixed layer during the transition period to trigger stronger thermocline feedback in the central–eastern equatorial Pacific. As a result, the amplitude and frequency of El Niño and its tropical precursors are projected to increase with warming under the WNP influence. ENSO diversity modulated by this relationship depends on the relative strength of advective and thermocline feedbacks, as well as the background state at the time of the event. Next, we discern the mechanisms behind the formation and preservation of the warm SSTA in the western North Pacific and its unusually prolonged propagation to the tropics that culminates in the development of an El Niño 2–3 years later, possibly extending ENSO prediction efforts up to 3 years in advance. Although the second part of this study focuses on the propagation specific to the warm WNP SSTA, we intermittently reapply some dynamics to the cold WNP SSTA–La Niña relationship to verify plausibility of our hypothesis and highlight its application to the oscillatory nature of ENSO. The direction and speed of currents from the subtropics to tropics work in conjunction with the topography of the ocean and Earth’s rotational effects, and may be used to explain the formation, propagation, and persistence of subtropical SSTAs that have the potential to promote ENSO long after they form.