THESIS
2019
xvi, 172 pages : illustrations (some color) ; 30 cm
Abstract
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...[
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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.
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