This article describes the development and application of a threedimensional (3D) multilayer hydrodynamic model of tidal motions and thermohaline circulation. The governing equations were derived from 3D Navier-Stokes equations and were solved using the fractional step method, which combines the finite difference method in the horizontal plane and the finite element method in the vertical plane. The 3D model was first tested against the density current test in order to investigate the performance of the model. Then, the model was applied to the northern part of the Ariake Sea. In the model test, the results of density current were agreed qualitatively with the performance of salt intrusion. In the model application, the hydrodynamic and thermohaline circulation are predicted. The computed amplitudes are agreed within ±3% compared with the observations, and the phase angles agreed well with the observations. The phase angle results were improved when the thermohaline circulation was considered in the computations as compared with the results without the thermohaline consideration. The tidal currents for the Ariake Sea form a round-trip style during the flood and ebb tides. The numerically predicted results show good agreement with field observations of tidal currents, and they improved after considering the thermohaline circulation.
Coastal oceans and semi-enclosed seas are marked by extremely high spatial and temporal variability that challenges the existing predicative capabilities of numerical simulations. During the past few decades, several 3D tidal circulation models have been developed, e.g., those of Heaps (1972), Davies (1983), Oey and Mellor (1985), Stephens (1986), Shen (1987), Casulli and Cheng (1992), Jin and Kranenburg (1993), Zhang and Gin (2000), Ozawa et al. (2003), Yu and Kyozuka (2004), and Oey (2005). Vertical finite difference models with a fixed grid or with a sigma coordinate transformation are the most popular models currently being used.