A quasi-three-dimensional numerical prediction model of salinity structure in Bohai Sea and Huanghai Sea

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A numerical study on the tidal residual flow

Characteristics of the tidal residual flow, the steady current induced in the tidal current system, are studied by a numerical method. The model basin has the same topography as that studied byYanagi (1976) and byOonishi (1977) where only the horizontal motion of the residual flow is concerned. In this study, the effect of the vertical motion is investigated as it is associated with the tidal residual flow. To this end, the bottom friction omitted in the previous study (Oonishi, 1977) is included and a two-leveled model is adopted.The first two experiments exclude the earth's rotation and the buoyancy effect on the flow. The results are as follows. The horizontal flow pattern is essentially the same as that obtained in the previous Oonishi study. The bottom friction results in the reduction of the velocity of the residual flow especially in the bottom level. An important result is that vertical velocity is as strong as the horizontal velocity multiplied by the scale ratio and that it remains even in the time-average. Upwelling appears at the center of the residual circulation. This upwelling explains Yanagi's observation in the hydraulic model that the sediment is swept by the flow and accumulates horizontally in the area at the bottom below the center of the residual circulation. The distribution of a tracer, which is simultaneously calculated in these cases, indicates the important role of vertical motion in the material dispersion in the model.The last experiment includes the earth's rotation and the buoyancy effect presuming a more actual sea. It shows another effect of the vertical motion. The Coriolis term, which operates only under the condition that a horizontal divergence of the flow is present, skews the horizontal residual flow pattern.     

Formation of thermohaline front by cooling of the sea surface and inflow of the fresh water

A numerical study of characteristics of the front formed by cooling of the sea surface and inflow of the fresh water is made within a vertical two-dimensional plane without the rotation of the earth. The convective adjustment technique is employed to parameterize the small scale convective overturning process.A sharply edged shelf causing the marked difference of the heat capacity of the water columns between over the shelf (50 m depth) and in the deeper region (150 m depth), is responsible for the formation of a thermal front over the shelf edge and two cell circulations of the same sense with each other adjoining at the frontal region.Inflow of the fresh water at the upper region of the coast (which is equivalently replaced by the outflow of the salt from the same region) adds a circulation of the opposite sense to the above ones, which transports the water of low salinity and low temperature offshore and forms a sharp front both of temperature and salinity with the offshore water of high salinity and high temperature. This thermohaline front or Oceanic front has a remarkable character that the horizontal density gradient is minimal at the front due to the counteracting contributions of temperature and salinity to density. Response of this front to the sudden ceases of the surface cooling and the fresh water supply, is also studied in order to understand its transient behaviors.     

Reducing horizontal diffusion errors in σ-coordinate coastal ocean models with a second-order Lagrangian-interpolation finite-difference scheme

When a steep bottom slope exists, it is well known that conventional methods for calculating horizontal diffusion in sigma-coordinate coastal ocean models causes spurious transport (e.g. salinity, temperature, and sediments) and currents. In this study, a second-order accurate finite-difference algorithm and program have been developed to reduce the spurious numerical diffusion errors. In the proposed algorithm, the finite differencing is performed in the x–z coordinate system to approximate the horizontal gradient. Each variable in the finite differential formation is calculated in the sigma-coordinate grid cells using a second-order Lagrangian interpolation polynomial. In conjunction with a stepwise bottom boundary condition, numerical experiments show that the proposed finite-difference scheme considerably reduces numerical errors compared to conventional approaches when dealing with horizontal diffusion over steep topography, which often occurs in coastal oceans and navigation channels.     

Sensitivity of CFCs and Anthropogenic CO2 Uptake in a North Pacific GCM to Mixing Parameterization and Surface Forcing

Distributions and characteristics of water mass and chlorofluorocarbons (CFCs) in the North Pacific are investigated by using a General Circulation Model (GCM). The anthropogenic CO₂ uptake by the ocean is estimated with velocity fields derived from the GCM experiments. The sensitivity of the uptake to different diffusion parameterizations and different surface forcing used in the GCM is investigated by conducting the three GCM experiments; the diffusive processes are parameterized by horizontal and vertical eddy diffusion which is used in many previous models (RUN1), parameterized by isopycnal diffusion (RUN2), and isopycnal diffusion and perpetual winter forcing for surface temperature and salinity (RUN3). Realistic features for water masses and CFCs can be simulated by the isopycnal diffusion models. The horizontal and vertical diffusion model fails to simulate the salinity minimum and realistic penetration of CFCs into the ocean. The depth of the salinity minimum layer is better simulated under the winter forcing. The results suggest that both isopycnal parameterization and winter forcing are crucial for the model water masses and CFCs simulations. The oceanic uptake of anthropogenic CO₂ in RUN3 is about 19.8 GtC in 1990, which is larger by about 10% than that in RUN1 with horizontal and vertical diffusive parameterization. RUN3 well simulates the realistic water mass structure of the intermediate layer considered as a candidate of oceanic sink for anthropogenic CO₂. The results suggest that the previous models with horizontal and vertical diffusive parameterization may give the oceanic uptake of anthropogenic CO₂ underestimated. This revised version was published online in August 2006 with corrections to the Cover Date.     

黄海、渤海盐度准三维数值预报模式

黄海、渤海盐度的垂直结构具有典型的自模性，而其水平分布又受平流、水平扩散效应及径流等因素的影响。本文根据黄海、渤海实测资料拟合了盐度垂直剖面的自模函数，并结合描述表、底层盐度及上均匀层厚度这３个特征量水平分布的方程，给出盐度三维结构的准三维模式。在模式中，综合考虑了海面风和热输入的强迫作用以及流场的平流、侧向混合及底层混合的影响因素，同时还考虑了径流、蒸发及降水的作用，较客观地反映了盐度的三维分布及其变化的物理过程。试报结果分析表明，模式的功能较好，结果令人满意。     

长江河口盐度锋

根据1988－1991年河口锋面现场调查、上海市海岸带调查及历次标准断面调查资料对长江盐度场及盐度锋进行了分析，提出了由口门至外海纵向上存在着三级锋面现象：内侧锋面即长江河口锋为长江河口水与长江冲淡水的界面；羽状锋是长江口羽状流水与口外混合水的界面，它是长江口最主要的盐度锋面，也是长江口一个重要的生物地球化学带，对河口沉积过程及水下三角洲发育具有重要的影响。外侧锋面即海洋锋，是长江冲淡水的最外边缘。     

A new hybrid vertical coordinate ocean model and its application in the simulation of the Changjiang diluted water

Based on the analysis of the advantages and disadvantages of some vertical coordinates applied in the calculation of the Changjiang diluted water(CDW),a new hybrid vertical coordinate is designed,which uses σ coordinate for current and σ-z coordinate for salinity.To combine the current and salinity,the Eulerian-Lagrangian method is used for the salinity calculation,and the baroclinic pressure gradient(BPG) is calculated on the salinity sited layers.The new hybrid vertical coordinate is introduced to the widely used model of POM(Princeton Ocean Model) to make a new model of POM-σ-z.The BPG calculations of an ideal case show that POM-σ-z model brings smaller error than POM model does.The simulations of CDW also show that POM-σ-z model is better than POM model on simulating the salinity and its front.     

The effects on water mass formation of surface mixed layer timedependence and entrainment fluxes

An air-sea buoyancy flux out of the ocean between the surface outcroppings of different isopycnals must be balanced by a convergence of advective and diffusive fluxes of buoyancy across those isopycnals (Walin, 1982; Tziperman, 1986; Garrett et al., 1995). For steady conditions, the diapycnal diffusive flux due to vertical mixing in the surface mixed layer is very small, so that the advective buoyancy flux dominates (Speer, 1993; Garrett et al., 1995). The associated advective buoyancy flux can then be used to estimate the volume flux of water out of the base of the surface mixed layer. The resulting thermodynamic algorithm provides a valuable estimate of water mass formation in the ocean.In contrast, for the time-dependent real ocean with horizontal and vertical gradients of the horizontal buoyancy gradient, diurnal and seasonal mixed layer deepening and entrainment in the presence of a buoyancy jump at the base of the mixed layer contributes to the annual volume flux out of the base of the deepest (wintertime) mixed layer. The mismatch between the predictions of the ideal algorithm and measured rates of water mass formation (Speer, 1997) may thus be partly due to mixed layer processes rather than diapycnal mixing in the thermocline.     

渤、黄、东海三维潮波运动数值模拟

采用POM三维海流数值模式模拟渤、黄、东海潮波运动。所得潮汐调和常数与77个测站的观测值的平均绝对误差：m1振幅1．92cm,迟角4．39°;M2振幅4．70cm,迟角5．08°。模拟结果显示M2分潮流共存在8个圆流点,m1岁潮流共存在9个圆流点。其中,M2分潮流在舟山群岛附近与m1分潮流在黄海北部和台湾海峡谷存在一个圆流点,这三个圆流点是本文数值模拟所首次显示的,需进一步的观测验证它们实际存在与否。模式还得出湍流涡动系数分布。     

盐度对太平洋牡蛎面盘幼虫垂直分布的影响

于1986年夏、秋两季在福建省惠安、罗源采集太平洋牡蛎,以人工授精法获取受精卵,在空调室内研究盐度变化对太平洋牡蛎面盘幼虫趋性行为的影响。结果表明,盐度的提高能促使牡蛎D形面盘幼虫从正趋地性转变为负趋地性,同时幼虫的正趋光性也得到加强。在D形面盘幼虫期,能引起趋光性加强的盐度增加阈值为2.9×10~(-12);在8日龄壳顶面盘幼虫,提高至5.0×10~(-12)以上。当D形面盘幼虫和壳顶面盘幼虫分别从25×10~(-12)移置于30×10~(-12)时,趋光水平能得到明显提高的时间阈值分别为4h和6h。据以上结果,作者认为该种幼虫在河口区有无阻留的主动性取决于该区环境的变化,同时也与幼虫不同发育时期有关。盐跃层的存在对牡蛎幼虫垂直分布的影响表明,只要盐跃层为2.5×10~(-12),即能抑制幼虫的向上游泳;一旦盐跃层增加,其阻抑作用仍维持接近低盐跃层水平。这说明盐跃层的存在对牡蛎幼虫的垂直迁移具有重要作用。     

Wind-induced shear dispersion and genesis of the shelf-break front

Through a simple analytical model, we examine the shear dispersion associated with oscillatory winds in an unstratified coastal ocean. As noted previously in the tidal regime, the vertical-integrated (total) horizontal diffusivity has a maximum where the water depth equals the diffusive depth – defined as the reach of the vertical diffusion during one forcing cycle. Due principally to the long synoptic timescale that characterizes the wind forcing, this depth lies over the outer shelf. When combined with effective mixing of the slope water by meso-scale eddies, the total diffusivity exhibits a minimum around the shelf break, thus facilitating frontogenesis. Due again to the long forcing period, the bottom Ekman flow is well developed at the diffusive depth, which would accentuate the gradient enhancement of the front over the inshore water, which however is bounded above by doubling.Calculations from a primitive-equation numerical model are carried out for both unstratified and stratified oceans. From an initially uniform property gradient, a front is seen to emerge around the shelf break after an oscillating wind is switched on, in a visual demonstration of the proposed frontogenesis. The unstratified solution closely agrees with the analytical solution, and although the front is not particularly sharp, it is comparable to that observed. The stratified solution renders a more realistic simulation of the observed front, but it retains the basic features, suggesting the dominance of the proposed mechanism even in the presence of the cross-frontal circulation.     

南海西部风驱离岸急流次中尺度锋面的动力学分析

本文利用卫星观测资料和500 m分辨率数值模拟结果，结合理论分析，对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明，南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面，在地转和非地转运动的共同作用下，次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示，沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送，有效地在跨锋面方向将表层冷水平流输送至暖水侧，导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV），表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度，其最大值可达100 m·d^-1。能量评估结果表明，次中尺度湍流的两个主要能量源，即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下，大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此，次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量，可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。     

Reducing horizontal diffusion errors in σ-coordinate coastal ocean models with a second-order Lagrangian-interpolation finite-difference scheme

When a steep bottom slope exists, it is well known that conventional methods for calculating horizontal diffusion in sigma-coordinate coastal ocean models causes spurious transport (e.g. salinity, temperature, and sediments) and currents. In this study, a second-order accurate finite-difference algorithm and program have been developed to reduce the spurious numerical diffusion errors. In the proposed algorithm, the finite differencing is performed in the x-z coordinate system to approximate the horizontal gradient. Each variable in the finite differential formation is calculated in the sigma-coordinate grid cells using a second-order Lagrangian interpolation polynomial. In conjunction with a stepwise bottom boundary condition, numerical experiments show that the proposed finite-difference scheme considerably reduces numerical errors compared to conventional approaches when dealing with horizontal diffusion over steep topography, which often occurs in coastal oceans and navigation channels.     

Numerical analysis of the mesoscale features of circulation in the Black Sea coastal zone

The results of simulating the hydrophysical fields of the Black Sea with a resolution of 1.64 × 1.64 km for January–September 2006 with the use of real atmospheric forcing are analyzed. Both vertical turbulent momentum exchange and vertical turbulent heat and salt diffusions are parameterized using the Mellor-Yamada level 2.5 scheme. The results of this numerical experiment are compared with similar data obtained with a horizontal resolution of 5 km. The features of the meso- and submesoscale dynamics of waters in individual sea regions are given. Possible physical mechanisms of forming meso- and submesoscale vortices are studied on the basis of energy analysis. It is shown that, in the absence of significant wind forcing, the main contribution to kinetic energy is made by the buoyancy force and wind-field inhomogeneities result in significant variations in both total vertical viscosity and total vertical diffusion.     

夏季南海北部粤东陆架锋面的动力特征分析*

本文利用现场观测资料和卫星遥感数据, 并结合ROMS(regional ocean modeling system)数值模拟对南海北部粤东陆架的锋面特征及其影响因素进行探讨。观测结果显示, 夏季南海北部陆架存在活跃的上升流温度锋面, 其水平尺度约为50km, 强度达到0.06℃∙km^-1, 大于同时期卫星遥感观测结果, 垂向影响深度超过20m, 且具有一阶理查森数(Richardson number, Ri)的典型动力学特征。进一步的ROMS 模式诊断分析结果显示, 锋面处水平梯度增强, 且动力学上表现出一阶Ri数, 为锋面不稳定的发生提供了有利条件。高分辨率模拟结果显示, 在夏季西南风的驱动下, 沿锋面地转流方向的风应力引起的跨陆架Ekman输运将锋面处冷水向暖水运移, 导致水平浮力梯度和锋面强度增强并形成负Ertel位涡(Ertel potential vorticity, EPV)。因此, 夏季风场强迫引起的Ekman浮力通量(Ekman buoyancy flux, EBF)可能是南海北部锋面不稳定现象的主要贡献者, 对局地动力环境有重要影响。     

Assimilation of climatic data in a model of circulation of waters in the Black Sea with regard for the space and time behavior of the variances and cross-covariance functions of forecast errors

We propose an improvement of the algorithm of joint assimilation of the data on climatic temperature, salinity, and altimetric sea level in a model of circulation. Unlike the previous works, the variances of the forecast errors of temperature and salinity and the cross-covariance functions of of the forecast errors of salinity-level and temperature-level depend on the dynamics of waters. It is shown that the structure of the fields of cross-covariance functions in the upper mixed layer is formed by the vertical turbulent diffusion of the variances of forecast errors of temperature and salinity. At greater depths, these statistical characteristics are mainly determined by the vertical advection. We compared the results of calculations with and without taking into account the dynamics of the statistical characteristics. The analysis of the influence of the dynamics of these characteristics makes it possible to reconstruct the mutually adapted climatic fields of temperature, salinity, and horizontal and vertical current velocities in the Black Sea with the assimilation of data in the numerical model in each time step. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 18–31, July–August, 2008.     

Simulation of the Circulation and Space Structure of Thermohaline Fields in the Sevastopol Bay with Regard for the Actual External Data (Winter, 1997)

We discuss the results of the numerical experiment aimed at the simulation of the behavior of currents and transformations of the temperature and salt modes in the Sevastopol Bay in January–February 1997. In the numerical analysis, we use actual data on the velocity and direction of the wind, sea surface temperature, and the discharge of River Chernaya. It is shown that the circulation and structure of hydrological fields are mainly connected with the direction of the wind, its intensity, and variability in the course of time. Since the analyzed water area is shallow, the currents inside the bay undergo rapid transformations (less than for an hour after changes in the wind). At the same time, the transformations of the thermohaline fields are slower. Due to the inflow of fresh waters of River Chernaya and salt waters from the open sea through the strait, the structure of thermohaline fields formed in the bay is nonuniform (both in the vertical and horizontal directions). The distribution of salinity plays the main role in the formation of the vertical stratification, which is natural for the winter season. Due to the process of freshening of water, a quite high vertical salinity gradient is formed in the upper layer of the sea. As a result, the process of cooling does not lead to the appearance of convection and inversions of temperature are formed in the case where warmer waters are located in the bottom layers. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 60–76, March–April, 2005.     

Possible Density Change of the Origin of North Pacific Intermediate Water Due to Double Diffusion in the Mixed Water Region

In this study we test Talley's hypothesis that Oyashio winter mixed-layer water (26.5–26.6σ _θ) increases its density to produce the North Pacific Intermediate Water (NPIW) salinity minimum (26.7– 26.8σ_θ) in the Mixed Water Region, assuming a combination of cabbeling and double diffusion. The possible density change of Oyashio winter mixed-layer water is discussed using an instantaneous ratio of the change of temperature and salinity along any particular intrusion (R _l ). We estimate the range of R _l ^DD required to convert Oyashio winter mixed-layer water to the NPIW salinity minimum due to double diffusion, and then assume double-diffusive intrusions as this conversion mechanism. A double-diffusive intrusion model is used to estimate R _l ^DD in a situation where salt fingering dominates vertical mixing, as well as to determine whether Oyashio winter mixed-layer water can become the NPIW salinity minimum. Possible density changes are estimated from the model R _l ^DD by assuming the amount of density change due to cabbeling. From these results, we conclude that Oyashio winter mixed-layer water contributes to a freshening of the lighter layer of the NPIW salinity minimum (around 26.70σ_θ) in the MWR.     

Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan

This study examines the evolution of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are analyzed using a method based on the calculation of the root mean square (rms) difference of the salinity along isopycnals. In combination with analysis of the distribution of the salinity maximum, this method enables water masses in the Kuroshio and the vicinity, to be tracked and distinguished as well as the detection of the areas where water masses are modified. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the right flank of the Kuroshio. In spite of a front dividing the KTW from the South China Sea Tropical Water (SCSTW) on Kuroshio׳s western side, mixing between these two water masses seemingly occurs in the Luzon Strait. These water masses׳ interaction is not evident east of Taiwan. The estimation of eddy diffusivities yields high horizontal diffusivities (K_h~10² m² s⁻¹) all along the Kuroshio path, due to the high current shear along the Kuroshio׳s flanks. The vertical diffusivity approaches 10⁻³ m² s⁻¹, with the highest values in the southern Luzon Strait. Instabilities generated when the Kuroshio encounters the rough topography of this region may enhance both vertical and horizontal diffusivities there.