Preliminary Application Of The Isopycnic-coordinate Numerical Model For Simulation Of Internal Tides With Adjoint Method In Parameter Estimation And The Northeastern South China Sea

Internal tides, which are ubiquitous in the oceans, are generated by the interaction of barotropic tidal flows with sloping topography in a density-stratified ocean. They are an important form of seawater motion. On the one side, the internal tides play an important role in the dissipation of barotropic tides and in mixing the deep ocean, and consequently contribute to large-scale ocean circulation. On the other hand, internal tides with large amplitude and generating solitary waves will disturb the work of oceanic engineering and be dangerous to marine oil platform and submarine oil pipe. At the same time, internal tides generation and propagation laws are of great research value for marine fishery production and energy development, etc. The adjoint assimilation method is an advanced data assimilation method and has the advantage of directly assimilating various observations distributed in time and space into the numerical model. By assimilating the interior observations, the adjoint method can effectively optimize the control variables in the model, and then makes the numerical model organically combined with observation data and further improves the simulation precision. This paper presents the application of adjoint assimilation method in the numerical simulation of internal tides in the northeastern South China Sea.In this paper, the numerical model for simulation of internal tides with adjoint assimilation method is introduced briefly. This model consists of two sub-models:the forward model (i.e. the isopycnic-coordinate internal tide model) and the adjoint model. Mode splitting technique is employed in both forward and adjoint models. This model is expected to make an improvement in the precision of internal tidal simulation through an organic combination of the numerical model and the observation data. Based on this model, some intensive studies on the inversion of control variables (open boundary conditions (OBCs) and bottom friction coefficients (BFCs)) are also conducted in this paper. Then, ideal experiments are conducted to investigate the stability of the isopycnic-coordinate internal tide model. At last, on the basis of optimized control variables, adjoint assimilation method is applied to the numerical simulation of internal tides, discussing the influence that seasonal stratification exert on the internal tides in the northeastern South China Sea.In this paper, the isopycnic-coordinate internal tidal model with adjoint assimilation method is developed into a three-layer version in order to investigate the estimation of spatially varying OBCs. The attempt of three-layer version is necessary. Next, the estimation of spatially varying BFCs is performed based on this model. The results of numerical experiments indicate that:the model has a good ability in estimating the spatially varying OBCs and BFCs; the use of independent points schemes (IPSs) can indeed effectively improve the estimation precision of control variables, and proper IPSs can further improve the inversion accuracy; the inversion accuracies vary depending on the prescribed distributions of BFCs; the assimilation precision as well as the cost function can be reliable and is able to reflect both of the inversion and simulation results in the whole field.Ideal experiments with different Gaussian terrains are performed to preliminary investigate the stability of the simulation of internal tides based on the isopycnic-coordinate internal tide model. The results show that, the model for simulation of internal tides is stable with low terrains, and we can analyze internal tides during100-300period in spite of the stability with high terrains deserving further research. As a preliminary application of the isopycnic-coordinate internal tide model to the real case, some actual numerical experiments are conducted to simulate the M2internal tides around the Luzon Strait in the northeastern South China Sea, by assimilating the T/P (TOPEX/Poseidon) altimeter data. The numerical experimental results indicate that:By optimizing the control variables, the adjoint method makes the numerical model organically combined with observation data; The instantaneous zonal baroclinic velocity are much stronger in Luzon Strait than other areas and the internal tides generated in Luzon Strait propagate to the Pacific Ocean, the Dongsha Islands and the South China Sea; The amplitudes of zonal baroclinic velocity in Summer are larger than that in Winter.