The understanding of the western boundary currents (WBCs) and the entire current system in the western Pacific Ocean (WPO) is still very limited, particularly regarding to the processes and underlying formation dynamics. We develop a numerical ocean model to conduct a process-oriented study and investigate the three-dimensional current system with the associated forcing mechanism over the WPO. We explore the linkage and dynamics among different components in the system. In the upper ocean, the current system includes WBCs of Kuroshio (KC) and Mindanao Current (MC) and their upstream North Equatorial Current (NEC). They have same seasonality with strong and weak magnitudes during spring/summer and autumn, respectively. The strengths of the WBCs are also modulated by the meridiona...[ Read more ]

The understanding of the western boundary currents (WBCs) and the entire current system in the western Pacific Ocean (WPO) is still very limited, particularly regarding to the processes and underlying formation dynamics. We develop a numerical ocean model to conduct a process-oriented study and investigate the three-dimensional current system with the associated forcing mechanism over the WPO. We explore the linkage and dynamics among different components in the system. In the upper ocean, the current system includes WBCs of Kuroshio (KC) and Mindanao Current (MC) and their upstream North Equatorial Current (NEC). They have same seasonality with strong and weak magnitudes during spring/summer and autumn, respectively. The strengths of the WBCs are also modulated by the meridional migration of the NEC bifurcation (NB). Corresponding with a southerly (northerly) NB in the first (second) half of a year, the domain- and layer-integrated relative vorticity (Vr) composed by shear and curvature vorticity is relatively small (large). Change of Vr associated with the corresponding NB migration measures the circulation and relative strength of the KC and MC. Analysis based on vorticity dynamics reveals that the planetary vorticity (PV) influx from the NEC is the major vorticity source in the upper current system, while compensated by the Vr outflux in WBCs. The intrinsic dynamics modulates the Vr and NB location. The local wind stress curl imparts large positive Vr into the circulation during autumn and is offset by the bottom pressure torque induced by the latitude-dependent tilting of the isopycnal bottom of the WBCs. Below the upper ocean, the current system consists of North Equatorial Undercurrent (NEUC), Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC). The NEUC flows eastward with a longitude-independent seasonality. Spatially, we found that the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S). The NEUC_N is fed by the LUC with ~5 Sv in summer, while the NEUC_S is fueled by the MUC with ~6 Sv in autumn. The NEUC is dominated by the PV carried in the LUC and MUC. These external influxes interact with the internal dynamics of the nonlinearity, pressure torques and stress curls, and jointly govern the NEUC. The LUC is linked with the westward undercurrent in the higher latitude, which bifurcates at ~25°N flowing southward along the western boundary. The seasonality of the MUC exhibits two peaks that are contributed by the westward subsurface jets in the tropical and equatorial regions. This study fills the gap of understanding on processes and dynamics in the current system of tropical WPO, which, in turn, provides a better reasoning for the associated ocean circulation in the marginal China Seas.