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Junlu Li and Jianping Gan

Abstract

Based on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

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Zhongya Cai and Jianping Gan

Abstract

A process-oriented numerical modeling study was conducted to investigate the formation and underlying forcing of an anticyclonic eddy train observed in the northern South China Sea. Observations showed that long-lived anticyclonic eddies formed an eddy train along an eastward separated jet across the northern South China Sea in summer. The eddy train plays a critical role in regulating ocean circulation in the region. Forced by the southwesterly monsoon and prevailing dipole wind stress curl in the summer, the northward coastal jet separates from the west boundary of the South China Sea basin and overshoots northeastward into the basin. The anticyclonic recirculation of the separated jet forms the first anticyclonic eddy in the eddy train. The jet meanders downstream with a strong negative shear vorticity that forms a second and a third anticyclonic eddy along the jet’s path. These three eddies form the eddy train. These eddies weaken gradually with depth from surface, but they can extend to approximately 500 m deep. The inherent stratification in the region regulates the three-dimensional scale of the anticyclonic eddies and constrains their intensity vertical extension by weakening the geostrophic balance within these eddies. Analyses of the vorticity balance indicate that the eddy train’s negative vorticity originates from the beta effect of northward western boundary current and from the subsequent downstream vorticity advection in the jet. The jet separation is a necessary condition for the formation of the eddy train, and the enhanced stratification, increased summer wind stress, and associated negative wind stress curl are favorable conditions for the formation of the anticyclonic eddies.

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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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Jianping Gan, Ho San Ho, and Linlin Liang

Abstract

This study reveals the dynamics behind the intensified, downslope, cross-isobath transport over a widened shelf (narrowing downwave) in the northeastern South China Sea (NSCS) during a downwelling event. Utilizing a three-dimensional numerical model over an idealized NSCS shelf, this study identified the forcing process and derived an analytical understanding of the invoked dynamics for the intensified downslope cross-isobath transport. This study found that the transport was formed by an amplified geostrophic transport because of an increased positive along-isobath pressure gradient force (PGF), PY*, and by an enhanced bottom Ekman transport due to converging flow over the widened shelf. Based on the depth-integrated vorticity dynamics, PY* was linked with net frictional stress curl in the water column and the curl was primarily associated with the shear vorticity field induced by downwelling jet. The increased positive vorticity seaside of the jet provided positive PY* for the downslope geostrophic transport. The cross-isobath PGF, PX*, which was geostrophically formed and shaped by the spatially asymmetric isobaths of the widened shelf, determined the intensities of the jet and thus the associated shear vorticity that quantified the respective bottom friction transport and PY* for the intensified cross-isobath transport. It was found that the downslope transport over the widened shelf was strengthened in a stratified flow by the linked intensifications of barotropic PX* or the concurrent downwelling jet, shear vorticity field, PY*, and bottom friction because of an increase of volume flux upstream of the widened shelf.

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Wenfeng Lai, Jianping Gan, Ye Liu, Zhiqiang Liu, Jiping Xie, and Jiang Zhu

Abstract

To improve the forecasting performance in dynamically active coastal waters forced by winds, tides, and river discharges in a coupled estuary–shelf model off Hong Kong, a multivariable data assimilation (DA) system using the ensemble optimal interpolation method has been developed and implemented. The system assimilates the conductivity–temperature–depth (CTD) profilers, time series buoy measurement, and remote sensing sea surface temperature (SST) data into a high-resolution estuary–shelf ocean model around Hong Kong. We found that the time window selection associated with the local dynamics and the number of observation samples are two key factors in improving assimilation in the unique estuary–shelf system. DA with a varied assimilation time window that is based on the intratidal variation in the local dynamics can reduce the errors in the estimation of the innovation vector caused by the model–observation mismatch at the analysis time and improve simulation greatly in both the estuary and coastal regions. Statistically, the overall root-mean-square error (RMSE) between the DA forecasts and not-yet-assimilated observations for temperature and salinity has been reduced by 33.0% and 31.9% in the experiment period, respectively. By assimilating higher-resolution remote sensing SST data instead of lower-resolution satellite SST, the RMSE of SST is improved by ~18%. Besides, by assimilating real-time buoy mooring data, the model bias can be continuously corrected both around the buoy location and beyond. The assimilation of the combined buoy, CTD, and SST data can provide an overall improvement of the simulated three-dimensional solution. A dynamics-oriented assimilation scheme is essential for the improvement of model forecasting in the estuary–shelf system under multiple forcings.

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