Using the SODA-POP ocean assimilation data, the decadal evolution in the tropical Pacific Ocean spanning from 1958 to 1997 is analyzed. During the pre-1980, the ocean temperature anomalies in the meridional cross section were positive in the subsurface southwestern Pacific and negative in the surface layer of the tropical Pacific. This more or less stationary pre-1980's pattern reversed after early 1980s. Rather than gradual changing, an abrupt phase transition occurred between 1977 and 1981. During the transition period, the trade wind became weaker, and the warm subsurface water in the western Pacific moved toward the surface layer of the eastern equatorial Pacific as well as toward the subsurface in the northwestern Pacific. The transition mechanism resembles the recharge-oscillator hypothesis on the ENSO such that the warm water is accumulated in the western Pacific before the ENSO starts and the weakening of the trade wind may be related to the triggering mechanism.