Abstract:

The ocean circulation system in the Western Pacific consists of North Equatoria Current, western boundary currents (WBCs) and other interconnected components, and 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 focused most on the variability of the circulations in specific sections without addressing the intrinsic connectivity and dynamics 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 and underlying physics of Kuroshio western boundary current, in the WBCs. Based on physically sensible definitions of 3D jet stream and currents, it is found that spatial pattern of Kuroshio is fundamentally modulated by mean flow-topography interactions, where continental slope topography regulates the horizontal scales of the western boundary current, and modifies strain and shear characteristics and subsequent along-stream variability while upstream influx in the WBCs act as an external forcing process. Meanwhile, the eddy-mean interactions in Kuroshio alter the characteristics of Kuroshio Current. Specifically, we identified that the along-stream structure of Kuroshio is featured by its velocity, along-/cross-stream transports, meanders and eddies. The strong mean kinetic energy (MKE) with strong along-stream transport and weak cross-stream transport occurs along the steep slope, and the associated strong eddy kinetic energy (EKE) arises from the horizontal shear of the mean current in the shelf break and strait. The MKE is mainly supported by pressure work, while the EKE pattern is supported by barotropic instability, baroclinic instability and wind power input in the stream. The flow is barotropically unstable, and kinetic energy is converted from the mean flow to the eddies and meanders in the Luzon strait, shelf break in East China sea and south of Kyushu Island, with regions of eddy-to-mean conversion in between. By resolving three-dimensional, spatiotemporal variability of western current system, the study provides a new understanding to the dynamics in Kuroshio and its connections with the Western Pacific Current system. 

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