Three-imensional Numerical Simulation Of Current And Sea Temperature In The South China Sea And The Research Of Its Vertical Coordinates

For the needs of the marine environment forecasting support in the South China Sea(SCS)this paper carried out a three-dimensional numerical simulation of current and sea temperature based on the FVCOM.For the tide numerical simulation and prediction in the SCS,the application research of global ocean tide model data in the SCS-Indian Ocean was carried out and the numerical model of the SCS tide was established by using the FVCOM.In order to reduce the error of the baroclinic gradient force in the SCS baroclinic circulation simulation,the seawater motion equation was reconstructed with the vertical layer number as the independent variable.Based on the equation,the FVCOM was improved.Four new generalized terrain following coordinates were proposed,including uniform sigma stratification and equal z-plane stratification block mixed coordinate(B1 coordinate),uniform sigma stratification and equal z-plane stratification fully mixed coordinate(B2 coordinate),exponential function stratification and equal z-planes stratification fully mixed coordinate(B3 coordinate),hyperbolic function stratification and equal z-plane stratification fully mixed coordinate(B4coordinate).The characteristics of the thermocline in the SCS were analyzed by using WOA18 and Argo data.The numerical simulations of sea temperature and thermocline were carried out with the FVCOM used B4 coordinate,considering the tide and baroclinic circulation.The conclusions are as follows:The tidal wave characteristics of the global ocean tide models DTU10,TPXO8,GOT00.2,and NAO.99 b were basically the same in the SCS-Indian Ocean.The tide forecast results based on the harmonic constants of the four global ocean tide models showed that the GOT00.2 had the best forecasting effect in the Indian Ocean and the TPXO8 had the best forecasting effect in the SCS.In general,TPXO8 was more suitable for providing the open boundary condition for the SCS tide numerical model.The simulation results of the SCS tide numerical model based on the FVCOM were good.The Mean square error between simulation results and tide tables of Yongxing Island and Yongshu Reef were 10.0 cm and 14.1 cm respectively.In the FVCOM,the calculation error of baroclinic gradient force for generalized terrain following coordinates was significantly reduced by using the seawater motion equation with the vertical layer number as the independent variable.Compared with calculation results of uniform sigma stratification coordinate,exponential function stratification coordinate,hyperbolic function stratification coordinate,the B1?B4coordinates proposed in this paper significantly reduced the calculation error of baroclinic gradient force,and B2 and B4 coordinates were more suitable for the numerical simulation of SCS baroclinic circulation.The thermocline types in the SCS were mainly composed of single thermocline and multiple thermocline.The inversion thermocline mainly appeared in Guangdong coast,Fujian coast and Beibu Gulf during the late Autumn to early Spring.In Winter,the thermocline was the deepest,the southeast of SCS was shallow,the northwest of SCS was deep.The thermocline was thin,and the thermocline strength was mainly in the range of 0.06?0.1°C/m.In Spring,the thermocline becomed shallow,the spatial distribution was relatively uniform,the thickness was increased,and the thermocline was appeared at the continental shelf.The thermocline strength of the deeper sea was mainly in the range of 0.07?0.1°C/m,that of the continental shelf was mainly in the range of 0.11?0.15°C/m.In Summer,the thermocline was shallower,it was divided into two parts by the southwest-northeast line.In the north of line,the thermocline was deeper and the thermocline was shallower in the south of line.The thermocline was thickest,the thermocline strength of the deeper sea was mainly in the range of0.08?0.11 °C/m,and that of the continental shelf was mainly in the range of0.12?0.2°C/m.In Autumn,the thermocline got deeper and the thickness got thin.The thermocline at continental shelf began to be disappeared.The thermocline strength of the deeper sea was mainly in the range of 0.08?0.1°C/m,that of the continental shelf was mainly in the range of 0.11?0.15°C/m.Based on the Argo data and the WOA18 reanalysis data,the upper boundary depth,the lower boundary depth and the thickness of the thermocline were basically consistent,while there were also differences between them.The main reason may be that the WOA18 reanalysis data reflected the climatic characteristics of thermocline,ignoring the interannual and intra-seasonal changes,while the Argo data reflected the direct or indirect effects of atmospheric fluctuations,solar radiation,and wind on the thermocline characteristics in a short time.The thermocline characteristics obtained from the simulation results of the SCS sea temperature were basically consistent with those of the WOA18 reanalysis data.