Although the major aspects that give rise to El Niño-Southern Oscillation (ENSO), the dominant mode of climate variability on the interannual time scale, are reasonably well understood, predicting the state of ENSO before the preceding boreal spring has proved to be challenging. Recent investigations suggested that interannual SST variability over the Indian Ocean can influence the following year’s ENSO and its phase transition, thus representing a potential additional source of predictability. This research aims to analyze the coupled interaction between the Indian and Pacific oceans that could provide information beyond the spring predictability barrier. Firstly, an empirical approach is used to study the subsurface temperature covariability across the two oceans to assess the role of the Indian Ocean as a precursor of the ENSO. Furthermore, two possible coupling mechanisms are investigated. Then, we assessed the representation of the CMIP6’s historical simulation of the subsurface covariability between the two oceans and compared the performance with the compressive CLIVAR ENSO metrics. Finally, using a simple conceptual model, we investigate the repercussions of the Indian Ocean over ENSO’s dynamical behavior, represented by the addition of an idealized MJO-forcing parameter over the Western Pacific. Overall, the results highlighted the role of the interbasin subsurface coupling in favoring the ENSO phase transitions and support the view that tropical Indo-Pacific climate variability should be studied as a whole rather than as separated basin modes.