Tidal Dynamics in the South China Sea and Estuarine & Adjacent Shelf Circulation in the Pearl River Estuary: Modeling Studies

Numerical studies of the tides in the South China Sea (SCS) and circulations in the Pearl River Estuary (PRE) and adjacent shelf are carried out by developing a numerical modeling system that includes a tidal circulation model in the SCS basin, a conceptual and a realistic three-dimensional coupled estuary-shelf circulation models forced by wind stress, tides, and river discharge around the PRE. The high resolution tidal model utilizes a physically sensible domain and assimilates Topex/Poseidon (T/P) data through an efficient generalized inverse scheme. The coupled estuary-shelf modeling is a physically novel study that investigates the interactive roles of the wind, tide and buoyancy forcing and dynamic processes of buoyant plume, estuarine circulation and shelf circulations in the PRE region.;Tidal simulation in the SCS well simulates the observed feature and provides tidal forcing for the subsequent coupled estuarine-shelf circulation models around the PRE, and investigates the dynamics and energy of tides in a semi-enclosed sea. The study, for the first time, reveals the amplified K1 tide in the SCS basin as a result of the Helmholtz resonance. Analysis of tidal energy shows that the energy dissipates mostly in the LS and strong dissipation of M2 tide also occurs in the Taiwan Strait (TS). The work rate of the tidal generating force in the SCS basin is negative for M2 and positive for K1. The different responses of the M2 and K 1 tides in the SCS are largely controlled by the intruding directions of the tides from the Pacific, the tidal frequency, the wavelengths, the local geometry and the bottom topography.;A conceptual coupled estuary-shelf model that utilizes representative, but idealized forcing and topography is developed to better identify the fundamental but extremely complex physical processes in a simple but physically well-defined system. Results show that a buoyancy-driven anti-cyclonic eddy is developed inside the idealized PRE under both the gravitational and Coriolis effects, but dissolved by additional tide or wind forcing. The shape and spreading speed of the Pearl River plume are greatly modified by the wind and tidal forcing, in which, the thickness of the plume is increased and the propagation speed is retarded by strong tidal mixing in the estuary, while the plume is narrowed and accelerated by the wind-driven coastal currents over the shelf . The upwelling/downwelling coastal jet intrudes into the estuary with different current patterns and these shelf processes largely influence the estuarine circulation.;Forced by observed time-dependent wind, buoyancy, and tidal forcing, the direct simulation in the coupled estuarine-shelf model in the PRE adopts high spatial resolution that better resolves the realistic topography and coastal geometry. With the implementations of suitable numerical schemes and physically sensible open boundary conditions, circulation and related dynamics resulting from the interactions between the topography and multi-forcing processes are explored. It is found that the patterns of the coastal upwelling currents are largely controlled by the topography and greatly modified by the tide and buoyancy forcing over the shelf, and the coastal bays and estuaries with wide entrance are favorable for the intrusion of the upwelling coastal jets. Tidal residual currents and tide-induced redistribution of the sea surface pressure gradient field strengthen/weaken the coastal upwelling jet and bottom onshore currents on the eastern/western shelf. The buoyant plume enhances the eastward and southward motion of the surface upwelling currents as a result of thinned surface Ekman layer and formation of pressure gradient between the plume and ambient sea water. Inside the PRE, the circulation has little response to the wind forcing in the upper part, but is largely influenced by the intrusion of the time-dependent coastal circulation. Analysis of the plume and current features in spring-neap cycle reveals that the variable competition of the tide and buoyancy forcing determines the circulation around the entrance of the PRE.