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  • Author or Editor: Zhiqiang Liu x
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Zhiqiang Liu
and
Jianping Gan

Abstract

A three-dimensional, high-resolution numerical model is used to investigate processes and dynamics of an intensified upwelling that is induced by a coastal promontory over the East China Sea (ECS) shelf. The center of the intensified upwelling around the promontory has been constantly observed, but, so far, it has been dynamically unexplained. Forced by an idealized southeasterly wind stress, the model results well capture the observed upwelling at the lee of the coastal promontory. The intensified upwelling is formed by a strengthened shoreward transport downstream of the promontory as the upwelling jet veers shoreward. The jet is mainly controlled by a cross-shore geostrophic balance and is largely modulated by both centrifugal acceleration associated with nonlinear advection and by bottom stress. The strengthened shoreward transport is mainly attributed to the cross-shore geostrophic current that is induced by a countercurrent (negative) pressure gradient force (PGF) and partly attributed to the bottom Ekman transport. Based on the analyses of the momentum balance and depth-integrated vorticity dynamics, the authors provide a new explanation for the origin of negative PGF. It is found that the countercurrent PGF is generated by negative bottom stress curl and strengthened by negative vorticity advection downstream of the promontory. While the negative bottom stress curl arises from bottom shear vorticity, the source of negative advection downstream of the promontory is the negative shear vorticity on the seaside of the shoreward-bent jet. Nevertheless, cyclonic curvature vorticity at the bottom and positive vorticity advection in the water column at the promontory weakens the negative PGF. Although nonlinear advection strengthens vorticity advection, it weakens bottom stress curl and has little net effect on the countercurrent PGF.

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Jianping Gan
,
Zhiqiang Liu
, and
Chiwing Rex Hui

Abstract

Understanding of the three-dimensional circulation in the South China Sea (SCS) is crucial for determining the transports of water masses, energy, and biogeochemical substances in the regional and adjacent larger oceans. The circulation’s kinematic and dynamic natures, however, are largely unclear. Results from a three-dimensional numerical ocean circulation model and geostrophic currents, derived from hydrographic data, reveal the existence of a unique, three-layer, cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper (<750 m), middle (750–1500 m), and deep (>1500 m) layers in the SCS with differing seasonality. An inflow–outflow–inflow structure in Luzon Strait largely induces the CAC circulation, which leads to vortex stretching in the SCS basin because of a lateral planetary vorticity flux in each of the respective layers. The formation of joint effects of baroclinicity and relief (JEBAR) is an intrinsic dynamic response to the CAC circulation. The JEBAR arises from the CAC flow–topography interaction in the SCS.

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Qi Quan
,
Zhongya Cai
,
Guangzhen Jin
, and
Zhiqiang Liu

Abstract

Topographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

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