Despite the pivotal role of tropical Pacific decadal variability in the global climate system, its key characteristics and underlying mechanisms remain less comprehended. Here we show that observed tropical Pacific decadal variability is most energetic over the equatorial central Pacific and the subtropical northeastern Pacific, contrasting with the interannual El Niño–Southern Oscillation (ENSO), whose largest sea surface temperature (SST) anomalies occur in the eastern equatorial Pacific. Mechanistic analyses reveal that this decadal variability is equatorially originated from ENSO nonlinearity. During strong El Niño episodes, pronounced nonlinear ocean dynamical cooling over the equatorial central Pacific leads the decadal variability by approximately one-quarter cycle (~30 months) and initiates its phase transition. The resulting SST anomalies are subsequently amplified through coupled ocean–atmosphere feedbacks, giving rise to the central-Pacific ENSO/Pacific Meridional Mode-like decadal SST pattern that characterizes its mature phase. In contrast, equatorial ocean dynamical memory plays only a limited role on this decadal timescale, fundamentally distinguishing the underlying mechanism from the classical ENSO recharge–discharge paradigm. Most climate models underestimate this nonlinear rectification process, owing to prevalent mean cold tongue bias that weaken tropical air-sea coupling and degenerate strong El Niño regime. Furthermore, the tropical central Pacific decadal variability induces a similar quasi-decadal SST variability in the tropical South Atlantic primarily through Southern Hemisphere atmospheric teleconnections, highlighting its central role in pan-tropical decadal climate variability.