Development Of An Isopycnic-Coordinate Numerical Model And The Simulation Of Internal Tides In The South China Sea

A three-dimensional isopycnic-coordinate numerical model was developed. In this model,the oceanic interior is viewed as a stack of isopycnic layers, and each layer is characterized bya constant density. The model consists of external and internal modes, and barotropic andbaroclinic motions are calculated in the two modes, respectively. An adapted FCT （FluxLimited Transport） method and an “extrapolation” method promoted by previous researcherswere used to deal with numerical difficulties caused by the intersection of model layers withsea surface or bottom topography. The flooding algorithm was also used to overcome theaforementioned numerical difficulties and was compared with previous methods. Theflooding algorithm seems more suitable in dealing with the bottom intersection problem inisopycnic-coordinate models. OBCs （open boundary conditions） suitable for simulatinginternal tides were chosen through comparison of the characteristics of usual OBCs. Themodeling capability of simulating internal tides was verified by comparing the model resultswith an analytical solution. The model was then applied to the simulation of internal tides inthe South China Sea （SCS） with the forcing of M₂and K1tidal constituents, respectively. Theresults show that internal tides in the South China Sea are mainly generated at the LuzonStrait. The generated M₂internal tides propagate away in three different directions （branches）,the branch whose tidal beam is the widest propagates eastwards into the Pacific Ocean, themost energetic branch propagates westwards to the Dongsha Island, and the least energeticbranch propagates southwestwards into the basin of SCS. The generated K1internal tidespropagate in two different directions （branches）, one branch propagates eastwards into thePacific Ocean, and the other branch propagates southwestwards into the basin of SCS. Horizontally non-uniform stratification and background circulation have importanteffects on internal tide generation and propagation. Internal tides in and around the LuzonStrait were studied using a three-dimensional primitive equation model with the forcing ofeight major tidal constituents. Horizontally non-uniform stratification and accompaniedcirculation were included in the simulation, and the effects of horizontally non-uniformstratification and circulation were analyzed. The comparison of model results withobservation at a station west of the Luzon Strait shows that the magnitude of simulatedvelocities are comparable to observation, the simulated phase of K₁and S₂internal tides arenear to observation, and the simulated phase of O₁and M₂have differ a little from observation.At the north part of Luzon Strait, energy flux vectors of semidiurnal bands rotate clockwise,and the rotation of energy fluxes is probably due to the semi-circular distribution ofsupercritical topography at the eastern ridge. The west ridge is the most effective generationsite for semidiurnal internal tides, and the central part of the east ridge is the most effectivegeneration site for diurnal internal tides. Furthermore, internal tide energy can be convertedinto barotropic tide energy in some regions. The distribution of converstion rate differs greatlybetween semidiurnal and diurnal internal tides, the the difference is due to.different phasedistribution of bottom perturbation pressure. The distance between the two ridges at the northpart of the Luzon Strait is near to the wavelength of M₂mode-1internal tide, facilitating thegeneration of semidiurnal internal tides. About69%of semidiurnal tidal energy loss in theLuzon Strait is converted into internal tide field, and about63%of diurnal tidal energy loss isconverted to internal tide field. The M₂internal tide is dominant in semidiurnal internal tides,while energies of K₁and O₁internal tides are comparable, and other internal tidal constituentsare very weak. The internal tide energy propagating westward to the SCS （10.66GW） is morethan the one propagating eastward to the Pacific Ocean （7.80GW）. About30%of diurnal internal tides are dissipated locally, and51%of semidiurnal internal tides are dissipatedlocally. The pase of internal tides is modulated by background circulation. Semidiurnalinternal tides is dominated by the first mode, horizontal non-uniform stratification andbackground circulation do not change their propagation direction. The propagation directionof the first mode diurnal internal tides is hardly affected by horizontally non-uniformstratification and background circulation, but the second and higher modes are seriouslyaffected, their propagation direction change a lot due to background circulation. Horizontalnon-uniform stratification and background circulation have important effects on localdissipation of internal tides and the internal tide energy propagating eastward and westward,and have more effects on diurnal internal tides than semidiurnal internal tides. Semidiurnalinternal tides are mainly affected by horizontally non-uniform stratification, while diurnalinternal tides are mainly affected by background circulation, and the difference is due todifferent frequency of semidiurnal and diurnal internal tides.