Dynamics Of Typical Mesoscale Eddies In The Northern South China Sea And Its Adjacent Region

Mesoscale eddies are a common form of water movement,which undertake more than 90% of the material and energy transport in the ocean.A mesoscale eddy carries a large amount of high kinetic energy water with abnormal temperature during its propagation.Therefore,changes of the salt,temperature and velocity field in the local ocean caused by a mesoscale eddy can not be neglected.The South China Sea(SCS)has a vast area and complex topography,providing ample space for the generation,movement,evolution and extinction of mesoscale eddies.In the northern SCS,a mesoscale warm eddy propagates westwards along the continental shelf of the SCS,which can have an influence on the ecological environment of the local marine system.The purpose of this paper is to analyze the structural characteristics of mesoscale eddies and their evolution in the northern South China Sea from the perspective of dynamics.Based on the structure of mesoscale eddies captured by hydrological observations in the SCS,an idealized mesoscale eddy is established by using the MIT general circulation model(MITgcm).The influence of the planetary ?-effect on the vertical structure of mesoscale eddies and its mechanism is investigated using theoretical analysis and numerical model experiments.Results of the both approaches show that the planetary ?-effect and nonlinear advection effect are the most common factors causing the vertical structure of the eddy tilts westwards from the sea surfact to the bottom.The generation of vertical velocity gradient is a direct embodiment of the vertical structure tilt of a mesoscale eddy.Based on the formation mechanism of mesoscale eddies in the northern South China Sea,the eddy-current interaction on the eastern side of the Luzon Strait(LS)is analyzed.Satellite sea surface altimeter data suggests trajectories of these mesoscale eddies over the Kuroshio zone can be categorized into three different patterns,and only 3.5% of the mesoscale eddies propagating westward in the North Pacific can enter the South China Sea through the Luzon Strait.Results of numerical model experiments indicate that a mesoscale eddy can be merged into the loop anticyclone when the current is weak as filamentous structure,which can strengthen exchanges of the material and energy between the South China Sea and the North Pacific.The strong Kuroshio and seamounts in the LS effectively prohibit mesoscale eddies entering the SCS.Based on propagation pathes of mesoscale eddies in the northern South China Sea,effects of isolated terrain(islands,seamounts,etc.)on the movement path and structure evolution of eddies are explored using a numerical model.The dependence of eddy behaviors on the horizontal scale and submergence depth of an obstacle can be summarized using two dimensionless parameters,namely R and S(where R and S are two dimensionless parameters of the island size and submergence depth;R is the ratio of the island radius to the eddy radius,and S is the ratio of the seamount submergence depth to the eddy vertical length).Results of numerical experiments show that the qualitative range of eddy-splitting is 1/4<R<2,S<1/5.In the ocean,topography is not only one of the factors leading to the disappearance of mesoscale eddies,but also the source of mesoscale/sub-mesoscale eddies.The numerical simulation shows that the critical Reynolds number for vortex street formation behind submerged 3-D obstacles is twice as large as that for 2-D cylinders.The effects of the Coriolis parameter,inflow conditions,bottom friction and seamount shape on the leewake are explored in a systematic way using a numerical model.Results from this research lay the foundation for accurately simulating and predicting lee-wakes of an island or a seamount under different hydrodynamic conditions.