Data Assimilation Of Tides In The South China Sea Using Adjoint Method Considering The Internal Tide Dissipation

The South China Sea (SCS) is located in the south of China mainland. It is animportant area to connect the Pacific Ocean and Indian Ocean. The steep bottomtopography and numerical islands form a complex tidal system in the SCS. The tidalenergy dissipation in the SCS is very strong. In the deep water area, the bottomfriction dissipation can not represent the dissipation of tidal energy sufficiently. Thetidal energy dissipation also occurs through the scattering of surface tides into internaltides, which is called internal tide dissipation.In this paper, the tidal and adjoint model are built in the SCS. A parameterizationof internal tide dissipation is added to the traditional2-D tidal equations, consideringthe internal tide dissipation on the influence of the tidal wave system in the SCS.Based on previous studies, the parameterization of internal tide dissipation isimproved, and the expression of the bottom roughness parameter is given.In order to obtain better open boundary conditions, the global tide modelsTPXO7.2, GOT00.2, NAO.99b and DTU10are compared in the SCS. Accoding tothe results of comparison, DTU10is chosen to compute the open boundary conditionsin this study.The harmonic constants at63tide gauge stations and24TOPEX/Poseidonsatellite altimeter crossover points are used as observations. The bottom frictioncoefficient and the internal tide dissipation coefficient are optimized by using adjontmethod to minimize the distance between the simulated results and observations. Inorder to get the best optimizing scheme, seven experiments are designed. The rootmean square deviations between the results for M2tide of seven experiments andobservations are29.78cm,24.08cm,12.64cm,12.56cm,10.19cm,10.63cm and10.15cm respectively. It can be seen that the results of Experiment7is best. After the optimizing scheme is confirmed, numerical models for M2tide with thehorizonal resolution of1/8degree and1/4degree are built. By comparing the results,the computational time of the model with the horizontal resolution of1/4degree isone five of that of the model with the horizontal resolution of1/8degree. But the erroris slightly bigger. In order to improve the computional efficiency, the model with thehorizontal resolution of1/4degree is choosen to simulate the principal tidalcomponents M2, S2, K1and O1simultaneously.By using the scheme of Experiment7, the adjoint numerical simulation of tidalcomponents M2, S2, K1and O1in the SCS are achieved. The root mean squaredeviations between the simulations of the four tidal components and obsernations are12.35cm,6.47cm,9.79cm and8.21cm respectively. The simulated results coincidewith the observed. The cotidal charts for M2, S2, K1and O1are given, and they reflectthe characteristics of tides in the SCS.The tidal energy flux of the four principal tidal constituents is analysed in thispaper based on the numerical results. The tidal energy propagates to the SCS throughthe Luzon Strait from the Pacific Ocean, and then propagates southwestward. Themagnitude of tidal energy flux for S2tide is less than the other three tides. Thestrongest tidal energy is in the Luzon Stait for both semidiurnal and diurnal tidalconstituents.According to the numerical results, the tidal energy dissipation in the SCS isstudyed. In the SCS, the tidal energy is dissipated not only by the bottom friction butalso by scattering of the surface tide into the internal tide. The bottom frictiondissipation occures in shallow water area, the internal tide dissipation mainlyconcentrates in the Luzon Strait.