A Numerical Study on the Density Driven Circulation in the Yellow Sea Cold Water Mass

The circulation of Yellow Sea Cold Water Mass （YSCWM） in the Southern Yellow Sea is investigated using a diagnostic 2D MITgcm model. The resolution of the computational grid is 900 m in the horizontal and 2 m in the vertical where an initial tem- perature distribution corresponding to a typical measured Yellow Sea Cold Water Mass was applied. The existence of YSCWM that causes fluid density difference, is shown to produce counter-rotating cyclonic horizontal eddies in the surface layer： the inner one is anti-cyclonic （clockwise） and relatively weaker （8-10cms-1） while the outer one is cyclonic （anti-clockwise） and much stronger （15-20cms-~）. This result is consistent with the surface pattern observed by Pang et al. （2004）, who has shown that a mesoscale anti-cyclonic eddy （clockwise） exists in the upper layer of central southern Yellow Sea, and a basin-scale cyclonic （anticlockwise） gyre lies outside of the anti-cyclonic eddy, based on the trajectories and drifting velocities of 23 drifters. Below the thermocline, there is an anti-cyclonic （clockwise） circulation. This complex current eddy system is considered to be capable of trapping suspended sediments and depositing them near the front between YSCWM and the coastal waters off the Subei coast, providing an explanation on the sediment depth and size distribution of mud patches in the Southern Yellow Sea. Moreover, sensitive test scenarios indicate that variations of bottom friction do not substantially change the main features of the circulation structure, but will reduce the bottom current velocity, increase the surface current velocity and weaken the upwelling around the frontal area.

A THEORETICAL SOLUTION FOR THE THERMOHALINE CIRCULATION IN THE SOUTHERN YELLOW SEA

Application of the thermocline equations in the thermocline areas and the boundary layer and the asymptotic matching techniques in each boundary in order to satisfy the surface and bottom conditions yielded a theoretical 2- D solution of the vertical thermohaline circulation of the Southern Yellow Sea in summer when the quasi-statically varying seasonal thermocline (density layer) is the background density structure , the deviations from which cause the secondary vertical circulation . The results show that the thermocline can be considered as an internal boundary or a barrier to the vertical heat advection so that in the central areas of the Southern Yellow Sea or the center of the Yellow Sea Cold Water Mass(YCWM)】 the downwelling in the upper layer and upwelling in the lower or bottom layer form a double cell vertical circulation . The solution is similar to Hu’s conceptual model ( 1986) in the central areas of the YCWM and is consistent with observed temperature . salinity and dissolved oxygen

The baroclinic circulation structure of Yellow Sea Cold Water Mass

A theoretic solution of one-dimensional heat transfer equation and a numerical simula-tion of 3D baroclinic circulation by MOM2 are investigated to understand the roles of bottom boundary mixing and the Topographic Heat Accumulation Effect (THAE) in the Yellow Sea Cold Water Mass (YSCWM) circulation. Our results show: (i) The time scale of heat transfer changes from days to weeks and from shallow to deep water column. Strong bottom boundary mixing makes the thermocline domed. (ii) The circulation of YSCWM has a two-layer structure. The upper layer is cyclonic, while the lower layer is anticyclonic, and the lower layer is thinner (about 10—20 m) and weaker than the upper layer. The depth-integrated (net) circulation is cyclonic. (iii) The strength of the bottom boundary mixing influences the temperature structures greatly but has less effect on the velocity structure.

A two-time-level split-explicit ocean circulation model (MASNUM) and its validation

A two-time-level, three-dimensional numerical ocean circulation model（named MASNUM） was established with a two-level, single-step Eulerian forward-backward time-differencing scheme. A mathematical model of large-scale oceanic motions was based on the terrain-following coordinated, Boussinesq, Reynolds-averaged primitive equations of ocean dynamics. A simple but very practical Eulerian forward-backward method was adopted to replace the most preferred leapfrog scheme as the time-differencing method for both barotropic and baroclinic modes. The forward-backward method is of second-order of accuracy, computationally efficient by requiring only one function evaluation per time step, and free of the computational mode inherent in the three-level schemes. This method is superior to the leapfrog scheme in that the maximum time step of stability is twice as large as that of the leapfrog scheme in staggered meshes thus the computational efficiency could be doubled. A spatial smoothing method was introduced to control the nonlinear instability in the numerical integration. An ideal numerical experiment simulating the propagation of the equatorial Rossby soliton was performed to test the amplitude and phase error of this new model. The performance of this circulation model was further verified with a regional（northwest Pacific） and a quasi-global（global ocean simulation with the Arctic Ocean excluded） simulation experiments. These two numerical experiments show fairly good agreement with the observations. The maximum time step of stability in these two experiments were also investigated and compared between this model and that model which adopts the leapfrog scheme.

济州岛南部海区的双跃层现象

本文分析了济州岛南部区域温度双跃层现象的类型及其成因。特别阐明了双跃层的波状现象,指出:强流锋区的侧向效应,黑潮次—中层混合水的爬升,黄海冷水团边缘密度环流引起的侧向流动,以及上、下层流速、流向不一致引起的剪切作用,是导致双跃层波状现象的主要原因。发现上、下跃层之间存在着“跃层间环流”,这是一个有趣的海洋学现象,它将导致双扩散的发生和有利于双跃层的维持。

北黄海冷水团环流结构探讨──潮混合锋对环流结构的影响

简述北黄海冷水团环流结构研究现状，指出已有研究成果中的主要问题。然后用一个诊断模型给出了冷水团环流结构，得到冷水团环向流动主要存在于海洋上层接近冷水团边界处；径向运动也主要存在于断面两端，上层为离岸流，下层为向岸流；冷水团中心的上升流极为微弱，且仅存在于海洋上层；温跃层下的冷水团中心区域的流动极为微弱，几乎为“死水”一般，上述环流结构对冷水团中心部分的温、盐度长期保持不变及跃层底部溶解氧最大值的形成和保持极为有利。最后用卫星及实测海流资料佐证了所述环流结构。

夏季南黄海风漂流的不同计算方法的对比分析

分别用Ekman有限深海风漂流计算公式,普林斯顿海洋模式(POM)数值计算了南黄海风漂流。通过对这两种方法的计算结果的时比分析得出:在水深较小时,模式计算的风漂流场与公式计算结果较为接近;而在水深较大时公式计算所得风漂流场与实际流场相差较大,模式计算的风漂流场则基本再现了夏季南黄海的环流结构;风速越大,两种计算方法所得的流场之间的差别越大。同时从数值试验的角度还得到夏季由于风速普遍偏小,风场对环流结构的影响都相对较小,热盐效应是控制夏季环流的主导因素。但当风速增大到一定程度时,环流结构会发生根本改变。

台风过境对黄海冷水团及其环流结构的影响

台风能够对黄海的水文结构及人民群众的生产生活产生重要的影响,严重威胁了人们的生命财产安全。利用ROMS(regional ocean modeling system)模式,分析了台风“灿鸿”在过境黄海期间对黄海温度及环流结构的影响过程。结果表明,台风期间强烈的风致混合能够使温跃层的深度增大,强度减弱,同时,使得近岸的底层温度迅速升高,推动底层的温度锋面向黄海内区移动。台风过境也会对黄海冷水团环流产生重要的影响,台风过境前,混合层中的北向流会迅速加深增强,同时伴随着混合层及温跃层的下移,从而使得黄海冷水团环流的流核下移至跃层以下。当台风过境时,黄海上空的气旋式风场会加剧黄海上层的气旋式环流,导致黄海冷水团环流的流幅及流量迅速增加。当台风登陆后,黄海上层的温度及黄海冷水团环流的结构开始逐渐恢复。

青岛冷水团的生成与演变研究

本文基于现场观测资料并结合FVCOM三维海洋模式的模拟结果,研究了2010年青岛冷水团生消过程和演变机制。结果表明,山东半岛东南海域的中层冷水是青岛冷水团的雏形,于4月中旬演变为青岛冷水团,位于青岛东南外海40m以下的盐度锋面中;刻画了青岛冷水团的消亡过程:5月青岛冷水团的北部底层水并入南黄海底层冷水中,构成南黄海的西部冷中心;而南部水团面积大幅减小,温盐特征大幅上升;6月上旬,青岛冷水团完全被南黄海底层冷水吞并,青岛冷水团完全消亡;揭示了青岛-石岛近海反气旋涡、黄海冷水团锋面密度环流对青岛冷水团的作用,前者是青岛冷水团存在的动力机制,后者加剧了底层海域的水平热量交换,促使了青岛冷水团的消亡。

A harmonic analyzed parameterization of tide-induced mixing for ocean models

The tide-induced mixing plays an important role in the regulation of ocean circulation.Numerical simulation of continental shelf circulation is found to exhibit an unreasonable vertical thermohaline structure without consideration of tide effects.In this study,we establish a harmonic analyzed parameterization of tide-induced（HAT） mixing,by which means to derive time-depended function of mixing coefficient based on harmonic analysis of the vertical mixing coefficient.By employing HAT mixing parameterization scheme,a series of numerical experiments are conducted for the Yellow Sea.Numerical results show that an ocean circulation model with the HAT mixing involved is capable of reproducing the reasonable thermohaline structure of the Yellow Sea Cold Water Mass,similar to structures produced by explicit tidal forcing on the open boundary.The advantage of the HAT method is its faster computation time,compared with models that directly resolve explicit tidal motion.The HAT parameterization for the tide-induced mixing has potential to improve both the accuracy and efficiency of ocean circulation and climate models.

An ocean circulation model based on Eulerian forward-backward difference scheme and three-dimensional, primitive equations and its application in regional simulations

A two-time-level, three-dimensional numerical ocean circulation model is established with a two-level, single-step Eulerian time-difference scheme. The mathematical model of the large-scale oceanic motions is based on the terrain-following coo-rdinated, Boussinesq, Reynolds-averaged primitive equations of ocean dynamics. A simple but very practical Eulerian forward-back-ward method is adopted to replace the most preferred leapfrog scheme as the time-difference method for both barotropic and barocli-nic modes. The forward-backward method is of the second order of accuracy, requires only once of the function evaluation per time step, and is free of the computational mode inherent in the three-level schemes. It is superior in many respects to the original leapfrog and Asselin-filtered leapfrog schemes in the practical use. The performance of the newly-built circulation model is tested by simula-ting a barotropic （tides in marginal seas of China） and a baroclinic phenomenon （seasonal evolution of the Yellow Sea Cold Water Mass）, respectively. The three-year time histories of four prognostic variables obtained by the POM model and the two-time-level model are compared in a regional simulation experiment for the northwest Pacific to further show the reliability of the two-level scheme circulation model.