Numerical Study And Case Forecast Of Mesoscale Eddies In The Northern South China Sea

Because of the influence of the monsoon,Kuroshio and complex topographical conditions,mesoscale eddies are very active in the South China Sea(SCS).Most of the mesoscale eddies generate,propagate and die in the northern SCS especially near the Luzon Strait,southwest of Taiwan,Dongsha Islands,Paracel Islands and Zhongsha Islands.The horizontal eddy mixing is also very strong in this area.However,in the previous studies,the resolution of their numerical models is mainly 1/10° or 1/12°,which cannot distinguish the distribution of the main islands in the Luzon Strait and simulate the flows in the southern strait accurately.Also,sensitivity tests about the horizontal mixing parameterization schemes and the quantitative assessment of the forecast capability on eddy case is needed in the northern SCS.Therefore,based on the improved 1/30° ocean model system,our study is mainly on data assimilation,horizontal mixing parameterization schemes and the eddy case forecast in the northern SCS.Firstly,we compare the climatological Sea Surface Height(SSH)anomalies,the climatological circulation pattern,and the eddy SST with observations,which validate the model's ability to simulate mesoscale eddies in SCS.However,the simulative mesoscale eddies are difficult to keep pace with the observed results in space-time without data assimilations,so we assimilate the AVISO Along-track Sea Level Anomalies(TSLA)by applying Ensemble Optimal Interpolation(EnOI),which successfully bring the actual mesoscale eddy signal into the model.After two years hindcast with continued assimilation,the model reveales the process of a paired cold and warm eddies which generated in southwest of Taiwan during the winter,2013.Through the analysis of their structure we found:1)The meridional velocity and the zonal velocity of the paired eddies are both not symmetrical and the shear of the horizontal currents is obviously intensive near the adjacent zone of the paired eddies.2)both the vertical axes of the cold eddy and warm eddy are tilt.These characteristics are similar to the observations during the same period,which illustrate that the model can not only simulate the evolution process of mesoscale eddies well,but also have ability to simulate their three-dimensional structure of velocity,temperature and salt with TSLA assimilation.In addition,through the analysis of model results we also found that the warm eddy comes from the shedding of Kuroshio,while the cold eddy is due to the enhanced offshore transport near the Taiwan Strait and the strong cyclonic shear of the currents near the edge of the warm eddy.Secondly,we apply different horizontal mixing parameterization schemes to simulate mesoscale eddy.The results of sensitivity experiments show that:1)The generation,propagation and dying of mesoscale eddies are less different when apply the same horizontal viscosity scheme along constant S-surfaces and along geopotentials,while the Laplacian and Biharmonic horizontal mixing schemes impact on the simulation of mesoscale eddies significantly;2)Biharmonic horizontal mixing scheme is more conducive to the generation of the warm eddy in southwest of Taiwan,which is probably due to the intensive anticyclonic shear of the flow in northern Luzon Strait;3)The warm eddy with Biharmonic horizontal mixing scheme moves faster than the one with Laplacian horizontal mixing scheme,and the trajectory of the warm eddy with Biharmonic horizontal mixing scheme tends to shift northwest shallower waters.The impact of different horizontal mixing schemes on the cold eddy is relatively small.4)Biharmonic horizontal mixing scheme is more suitable for simulating the dying of mesoscale eddy.The warm eddy dies near the Paracel Islands,and the cold eddy dies near Zhongsha islands.The form and the speed of dying are similar to observations.5)Magnitude analysis on the mixing terms in active regions of mesoscale eddy shows that the mixing of Laplacian horizontal mixing scheme is larger than the the mixing of Biharmonic horizontal mixing scheme,which caused the slow attenuation of energy and the extension of dying.Finally,we make a quantitative assessment of the forecast capability about eddy generation and eddy propagation based on the assimilated initial field.The generation test shows that the generation and development of mesoscale eddy can be predicted to a certain extent,but the forecasting warm eddy is stronger than observations.The propagation tests show that the forecast capability of model has some correlation with the intensity of mesoscale eddies in the northern SCS.The forecast capability about strong warm eddy can be 20 days or so,while the forecast capability about weak warm eddy is only a few days.In addition,by comparing different indices,we find that anomaly correlation coefficients are more suitable for examining the forecasting capability of model than the root mean square errors.