Three-dimensional characteristic and variability of the current system in the western Pacific

The ocean circulation system in the Western Pacific consists of western boundary currents (WBCs, Kuroshio Current, Mindanao Current, Ryuku Current) and connected with North Equatorial Current (NEC). The system is one of the most complicated current systems, vitally regulating the exchange of mass, energy, and heat transport between the open ocean and the adjacent marginal seas. Previous studies in Western Pacific circulations most focused on the variability of the circulations in specific sections without addressing the intrinsic connectivity and dynamics of currents in the system. Using the high-resolution, validated three-dimensional and time-dependent China Sea Multi-scale Ocean Modeling System (CMOMS, https://odmp.hkust.edu.hk/cmoms/), we quantitatively characterize the variability of the western boundary currents and related circulations in Western Pacific, and investigate their underlying physical processes. Based on physically sensible definitions of the jet stream and currents in the system, we identified characteristic width, depth, and along-/cross-stream transports and their unique spatiotemporal variability in the 3D current system. The momentum and vorticity analyses show the couplings between extrinsic inflow and intrinsic dynamic response of the Kuroshio Current in connections among currents in the system and between the marginal seas and open oceans. Synchronized structures in downstream variations of core velocity, cross-stream transport, eddy kinetic energy and path variability is pronounced along the Kuroshio Current. We found that the spatial patterns of Kuroshio are fundamentally modulated by mean flow-topography interactions, where shelf slope and shelf-current separation distance regulate the horizontal scales of the western boundary current, and thereby modify strain and shear characteristics and subsequent along-stream variability. The effect of topography on the synchronized spatial patterns is studied by energy budgets along the Kuroshio Current. Upstream influx and local flow-topography interaction acts as an external and internal forcing process to modulate the barotropic and baroclinic instability in Kuroshio variability, respectively. Associated time-averaged eddy fluxes are fundamentally reshape the mean current. By resolving three-dimensional, spatiotemporal variability of western current system, the study provides a new understanding to the dynamic connections in the Western Pacific Current system.