Based on the Hybrid Coordinate Ocean Model (HYCOM), this article presents simulations of the three-dimensional dynamic and thermal structure of the North Pacific Ocean (NPO) in the time domain from January 1991 to December 2000. The baroclinic current field and its interannual variability in Northwest Pacific Ocean (NWPO), especially, the Kuroshio Current (KC) and the Japan/East Sea (YES) are analyzed and discussed. The results show that the HYCOM covers a complete dynamic and thermo process with adequate representations of the eddy fields and variability in main spatial and temporal scales. The model is used to simulatethe strong interanual variability of the KC, which may affect the strength of the east bifurcation of the Tsushima Warm Current (TSWC), named the Nearshore Branch (NB) in the YES. When the KC is strong and its axis is close to the Japan Island, the TSWC and its bifurcations in the JES would intensify accordingly. Our results confirm once again the hypothesis of Yang et al. that the KC determines the annual mean NB in the JES via the island integral constraint. This article further extends this hypothesis to study the KC role in the NB on a time dependent scale.