On the vertical structure of tidal currents in a homogeneous sea

Summary. The vertical variation of tidal currents caused by friction at the sea-bed is investigated in both qualitative and quantitative ways with the Coriolis force being taken into account. The simple model with the assumption of constant eddy viscosity is employed to study the effects of friction on the vertical structure of tidal currents. The model explains these effects in vertical change of the maximum velocity, the ellipticity (the ratio of the minor to the major axis) of current ellipse, the time and the direction of the maximum velocity, and the ratio of diurnal current to semidiurnal current.
To reach a quantitative agreement with the data observed by Bowden & Fairbairn in the Irish Sea, the mixing length theory is applied to numerical calculation of the vertical distribution of tidal currents with a finite difference scheme. A fair agreement with the observations is obtained in the calculation. The results show the following features: (1) the vertical profile of amplitude of the main component for the current is almost logarithmic throughout the depth; (2) the corresponding profde for the shearing stress is almost linear with depth; (3) the maximum stress lags the maximum velocity with the delay increasing with elevation; (4) the eddy viscosity coefficient has its maximum around the mid-depth; and (5) the vertically averaged viscosity lags the velocity magnitude.
The dependence of the quadratic resistance coefficient and the coefficient k defined by Bowden's formula = k hū(h — depth, and ū — depth-averaged eddy viscosity and velocity) on the roughness length is given for a steady flow in a non-rotating system. The numerical solution shows that these relations can be approximately applied to the tidal currents in shallow waters.     

大河三角洲河口海岸演化机理模型研究:(II)模型构建与实证

本文以z坐标下的三维斜压海洋动力学数值模式为基本模式原型,在整理渤海基本数据并诊断计算风生环流和热盐环流作为背景环流场基础上,初步建立了渤海海域动力环境数值模式。模式采用了经校正的Bagnold型方程来计算渤海底移质沉积物输运,悬移质计算则是取二维深度平均悬移质输运方程和河床变形方程,计算含沙量分布以及由悬移物引起的冲淤厚度。利用这种方法建立的沉积物输运模式,定量模拟了渤海沿岸和海底的沉积物输运方向和冲淤分布。模拟结果与通过多年实测水深估算获得的渤海海底沉积物的冲淤变化分布相比较,两者之间在基本结论上是比较一致的     

黄河入海泥沙悬移输送机制的控制试验(英文)

Five diagnostic experiments with a 3D baroclinic hydrodynamic and sediment transport model ECOMSED in couple with the third generation wave model SWAN and the Grant-Madsen bottom boundary layer model driven by the monthly sediment load of the Yellow River, were conducted to separately diagnose effects of different hydrodynamic factors on transport of suspended sediment discharged from the Yellow River in the Bohai Sea. Both transport and spatio-temporal distribution of suspended sediment concentration in the Bohai Sea were numerially simulated. It could be concluded that suspended sediment discharged from the Yellow River cannot be delivered in long distance under the condition of tidal current. Almost all of sediments from the Yellow River are deposited outside the delta under the condition of wind-driven current, and only very small of them are transported faraway. On the basis of wind forcing, sediments from the Yellow River are mainly transported north-northwestward, and others which are first delivered to the Laizhou Bay are continuously moved northward. An obvious 3D structure characteristic of sediment transport is produced in the wind-driven and tide-induced residual circulation condition. Transport patterns at all layers are generally consistent with circulation structure, but there is apparent deviation between the depth-averaged sediment flux and the circulation structure. The phase of temporal variation of sediment concentration is consistent with that of the bottom shear stress, both of which are proved to have a ten-day cycle in wave and current condition.     

Diagnostic experiments for transport mechanisms of suspended sediment discharged from the Yellow River in the Bohai Sea

Five diagnostic experiments with a 3D baroclinic hydrodynamic and sediment transport model ECOMSED in couple with the third generation wave model SWAN and the Grant–Madsen bottom boundary layer model driven by the monthly sediment load of the Yellow River, were conducted to separately diagnose effects of different hydrodynamic factors on transport of suspended sediment discharged from the Yellow River in the Bohai Sea. Both transport and spatio-temporal distribution of suspended sediment concentration in the Bohai Sea were numerially simulated. It could be concluded that suspended sediment discharged from the Yellow River cannot be delivered in long distance under the condition of tidal current. Almost all of sediments from the Yellow River are deposited outside the delta under the condition of wind-driven current, and only very small of them are transported faraway. On the basis of wind forcing, sediments from the Yellow River are mainly transported north-northwestward, and others which are first delivered to the Laizhou Bay are continuously moved northward. An obvious 3D structure characteristic of sediment transport is produced in the wind-driven and tide-induced residual circulation condition. Transport patterns at all layers are generally consistent with circulation structure, but there is apparent deviation between the depth-averaged sediment flux and the circulation structure. The phase of temporal variation of sediment concentration is consistent with that of the bottom shear stress, both of which are proved to have a ten-day cycle in wave and current condition.     

黄河入海泥沙悬移输送机制的控制试验

以三维斜压动力—悬沙模型ECOMSED联合第三代海浪模型SWAN及Grant-Madsen浪流耦合底边界层模型,对黄河入海泥沙悬移扩散的时空变化过程进行了数值模拟。通过5项控制试验,初步揭示了不同动力因子对黄河入海泥沙悬移输送的贡献和影响。研究结果表明,单纯潮流作用下黄河入海悬沙不能形成长距离输送。风海流作用下,绝大部分悬沙直接沉降在黄河三角洲的附近,只有非常小的部分发生长距离输送。风场作用下,黄河入海悬沙主要向北－西北方向输送,部分输往莱州湾的悬沙继续向北输送。环流作用下,悬沙输送具有明显的三维结构,分层结构与环流结构基本一致,而深度平均的悬沙输送通量与环流结构有较大的差别。浪流联合作用下的悬沙浓度变化与底剪切应力都呈现为10天左右的变化周期,表明其变化主要是由风应力所控制。     

南黄海辐射沙脊群里磕脚11DT02孔沉积相分析及环境演化

11DT02孔位于南黄海辐射沙脊群东北部里磕脚沙脊南坡,通过沉积相分析、粒度统计,结合年代地层数据,重建晚更新世以来里磕脚沙脊的长周期环境演化,分析沙脊成因、厘定沙脊发育年代。研究表明:里磕脚沙脊从下至上由潮滩相、硬黏土相、潮滩相和潮流沙脊相组成。潮流沙脊和潮滩由粉砂、细砂和少量黏土组成,两者均成正偏态分布。潮流沙脊和潮滩在概率累积曲线上分别表现为三段式和四段式分布,以跳跃次总体为主,并呈双跳跃特点。研究区晚更新世经历海平面下降发育潮滩环境,到完全暴露形成标志性的“硬黏土”层;全新世初期经历海侵,研究区复又开始潮滩沉积,随后被海水完全淹没,潮流作用渐强,改造下伏潮滩和“硬黏土”,逐渐堆积成高差最大达30 m的脊槽地形。     

白令海峡及其邻近海域潮汐潮能数值模拟

基于非结构三角形网格的FVCOM(Finite-Volume Coastal Ocean Model)海洋数值模式,对白令海峡及其邻近海域的潮汐、潮能进行数值模拟研究。模拟结果同验潮站和实测海流资料符合良好,较好地反映了白令海峡及其邻近海域的潮汐、潮流分布特征和运动状况。根据计算结果绘制了主要分潮的同潮图和潮流椭圆图,对该海域潮汐潮流特征进行了系统分析。结果表明,白令海陆架区、白令海峡和楚科奇海主要以M2分潮为主,而在诺顿湾海域以K1分潮为主,M2分潮潮流在白令海陆架东南部及阿纳德尔湾较强,K1分潮潮流在诺顿湾潮流达到最大值。在此基础上,对其潮汐能的传播与耗散进行分析,结果发现研究海域潮能通量较小,主要分潮在研究海域潮能耗散总量约为751 MW,M2潮能耗散占该总量的52%,K1潮能耗散占38%,潮能进入白令海陆架后,M2分潮主要在圣劳伦斯岛以南陆架区耗散,K1分潮主要在诺顿湾海区耗散。     

State of stress in the oceanic lithosphere in response to loading

Summary. Flexure studies of the oceanic lithosphere constrained by bathymetry and gravity data suggest that the lithosphere behaves elastically over geological time-scales. For loads to be supported, however, large bending stresses (approaching 10 kb in some cases) are required at the top and bottom of the elastic plate.
These stress-differences can be significantly reduced by introducing more complex rheologies: we propose a model of layered lithosphere, consisting of a purely elastic upper layer, a transition zone with viscosity varying with depth and a perfectly plastic lower layer. The transition layer is grossly centred at the bottom of the elastic plate. Such a model results in a noticeable reduction of stress differences; reaching 60 per cent for flow laws representing creep mechanisms in olivine. When applied to a number of seamount loads, this model leads to maximum stress-differences which do not exceed 1–2kb.
The approach used in this study allows us to follow stress relaxation over time. Taking account of the thermal cooling of the lithosphere, we show that the elastic thickness of the lithosphere is stabilized after a given time, while the time required for stabilization is found to be of the order 5—6 per cent of the age of the lithosphere at the date of loading.     

On the excitation of resonant double Kelvin waves in the Barents Sea Opening

In the northern Barents Sea Opening (BSO) the K₁ tidal energy is predominant in the diurnal tidal frequency band, suggesting the generation of a topographic wave with the K₁ tidal frequency. Tidal energy of the K₁ component becomes strong where bottom topography undulates in the BSO and the scale of the undulation is close to the wavelength of the K₁ wave. An analytical model is developed to investigate the energy enhancement mechanism of the tidally induced topographic wave due to a resonance between tidal current, a topographic wave and periodic topography. The wave excited by the resonance is identified as a resonant double Kelvin wave (DKW) and the significant K₁ energy in the BSO could be due to the excitation of the resonant DKW.     

Glacial rebound of the British Isles—II. A high-resolution, high-precision model

Observations of ice movements across the British Isles and of sea-level changes around the shorelines during Late Devensian time (after about 25 000 yr BP) have been used to establish a high spatial and temporal resolution model for the rebound of Great Britain and associated sea-level change. The sea-level observations include sites within the margins of the former ice sheet as well as observations outside the glaciated regions such that it has been possible to separate unknown earth model parameters from some ice-sheet model parameters in the inversion of the glacio-hydro-isostatic equations. The mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere, the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km, the transition zone down to 670 km depth, and the lower mantle. No evidence is found to support a strong layering in viscosity above 670 km other than the high-viscosity lithospheric layer. Models with a low-viscosity zone in the upper mantle or models with a marked higher viscosity in the transition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary. In contrast, a marked increase in viscosity is required across this latter boundary. The optimum effective parameters for the mantle beneath Great Britain are: a lithospheric thickness of about 65 km, a mantle viscosity above 670 km of about (4-5) 10²⁰ Pa s, and a viscosity below 670 km greater than 4 × 10²¹ Pa s.     

近30 年来长江口岸滩沉积物与地貌演变特征

通过对长江口潮间带宽度、表层和浅地层柱状样沉积物、典型水下岸坡剖面的现场测量,以及对潮间带宽度及水下岸坡剖面的多幅历史海图分析,研究近30 年长江口岸滩演变特征及岸滩演变的主要原因。结果表明:(1) 岸滩沉积物整体上呈“北细南粗”的变化特征,横沙东滩北岸、长江和东海大桥下等主要大型工程附近潮间带柱状样粒序向上显著变粗;(2) 长江口部分潮间带宽度减少90%以上;水下岸坡由1980 年的“S”形向凹形、斜坡形等侵蚀型形态转变,岸坡明显变陡,侵蚀型岸滩特征凸显;南汇嘴至芦潮港岸段水下岸坡闭合水深1978-2004 年为-8 m,近10 年来滩底发生侵蚀,刷深约1 m;(3) 流域来沙量锐减导致长江口侵蚀型水下岸滩格局的形成;河口大面积围垦、青草沙水库和长江与东海大桥等大型工程建设是造成潮间带大面积损失和局部沉积物粗化的主要原因;沙体变迁是狭长型河槽局部岸滩演变的主要影响因素。     

Oxygen isotopic composition and its application to the study of tracing oceanographical process in Bering Sea and Chukchi Sea

In this paper, the 18 O distribution of surface water from the central sea areas of the Bering Sea and the Chukchi Sea was studied. The δ 18 O value of surface water from the Bering Sea is averagely -0.5‰; the δ 18 O contents of the Chukchi Sea are distributionally lower in northeast and higher in southwest; the δ 18 O value at the margin of Canadian Basin is -2.8‰, and averagely -0.8‰ in the southern area of the Chukchi Sea. The δ 18 O vertical distribution in some deep water stations from the Chukchi Sea and the Bering Sea is also studied. In the southern margin of Canadian Basin, the δ 18 O value is -2‰ -3‰ for surface layer and rises to 0 at 100 m depth layer. In the Bering Sea, the δ 18 O is about -0.5‰ for surface layer and increases to 0 at the depth of 300 m. The NO tracer can reflect obviously three water masses vertically distributed in the central Bering Sea: the upper Bering water mass, the middle Bering water mass and the deep Pacific water mass. The distributive ranges of NO and temperature for the various water masses are T<7℃, NO>780 μmol/dm 3 and T≥7℃, NO>650 μmol/dm 3 for upper Bering water mass, T<4℃, 550相似文献   

Numerical models of mantle convection with secular cooling

A 2-D time-dependent finite-difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects. Two basic cooling modes are considered: in the first, both upper and lower boundaries are cooled at the same rate, while maintaining the same temperature difference across the layer; in the second, the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0°C. The first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from, and thereby cools, the Earth's core. The mathematical analogue between the effects of cooling and internal heating is verified for finite-amplitude convection. It is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer. For upper-mantle models the transient stage lasts for about 30 per cent of the age of the Earth, while for the whole mantle it lasts for longer than the age of the Earth. Consequently, in our models the bulk cooling of the mantle lags behind the cooling of the core-mantle boundary. Models with temperature-dependent viscosity are found to cool in the same manner as models with depth-dependent viscosity; the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth. If the Earth's mantle cools in a similar fashion, secular cooling of the planet may be insensitive to lateral variations of viscosity.     

The tidal dynamics of the Irish and Celtic Seas

Summary. Current meter data collected over periods of more than 14 day from the Irish and Celtic Seas are harmonically analysed and presented in maps of tidal stream information. Making use of the analysed current data, and by constructing time series of frictional and inertial stresses which are also harmonically analysed, harmonic constituents of the surface tidal slopes at current meter stations are obtained. Using these with data collected from offshore tide gauges, and in conjunction with coastal tide data, cotidal maps are drawn with some confidence for M ₂, S ₂, O ₁ and K ₁, the M ₂ chart resolving the discrepancy which exists between the different charts of the Celtic Sea already produced. Cotidal maps for M ₃ and M ₄ are also presented.
The mean over a tidal cycle of the energy flux for M ₂, S ₂ and O ₁ is also presented in the form of the total energy flux in these constituents which crosses different sectional lines. A flux of 44 × 10⁶ kW is observed to enter the Celtic Sea from the Atlantic and this is compared with previous estimates. An energy budget is also performed for M ₂, including all the effects of astronomical forcing and Earth tides to enable comparison to be made between the true energy inflow and the estimated frictional dissipation. Finally, comparison is made between the mean of the instantaneous energy flux and the sum of the energy fluxes associated with the major harmonics.     

The significance of water column nitrification in the southeastern Bering Sea

Nitrate is considered the nutrient that limits new primary production in the southeastern Bering Sea shelf.Nitrate regenerated through biological nitrification has the potential to significantly support primary production as well.Here we use measurements of the specific rate of water column nitrification in a 1-D ecosystem model to quantify the resupply of nitrate from nitrification in the middle shelf of the southeastern Bering Sea.Model sensitivity studies reveal nitrification rate is an important control on the dominant phytoplankton functional type,and the amount of nitrate in summer bottom waters and in the winter water column.Evaluation of nitrification using the model supports the hypothesis that increases in late-summer nitrate concentrations observed in the southeastern Bering Sea bottom waters are due to nitrification.Model results for nitrate replenishment exceed previously estimated rates of 20-30%based on observations.The results of this study indicate that nitrification,potentially the source of up to～38%of the springtime water column nitrate,could support～24%of the annual primary production.     

Deglacial land emergence and lateral upper-mantle heterogeneity in the Svalbard Archipelago—II. Extended results for high-resolution load models

In Paper I (Breuer & Wolf 1995), a preliminary interpretation of the postglacial land emergence observed at a restricted set of six locations in the Svalbard Archipelago was given. The study was based on a simple model of the Barents Sea ice sheet and suggested increases in lithosphere thickness and asthenosphere viscosity with increasing distance from the continental margin.
In the present paper, the newly developed high-resolution load model. BARENTS-2, and land-uplift observations from an extended set of 25 locations are used to study further the possibility of resolving lateral heterogeneity in the upper mantle below the northern Barents Sea. A comparison of the calculated and observed uplift values shows that the lithosphere thickness is not well resolved by the observations, although values above 110 km are most common for this parameter. In contrast to this, there are indications of a lateral variation of asthenosphere viscosity. Whereas values in the range 10¹⁸-10²⁰Pas are inferred for locations close to the continental margin, 10²⁰-10²¹ Pa s are suggested further away from the margin.
A study of the sensitivity of the values found for lithosphere thickness and asthenosphere viscosity to modifications of load model BARENTS-2 shows that such modifications can be largely accommodated by appropriate changes in lithosphere thickness, whereas the suggested lateral variation of asthenosphere viscosity is essentially unaffected. An estimate of the influence of the Fennoscandian. ice sheet leads to the conclusion that its neglect results in an underestimation of the thickness of the Barents Sea ice sheet by about 10 per cent.     

Three-dimensional model for tides and surges with vertical eddy viscosity prescribed in two layers—I. Mathematical formulation

Summary. The mathematical basis of a spectral method for the numerical solution of the three-dimensional hydrodynamic equations for tides and surges is described. Vertical eddy viscosity is prescribed in two layers, upper and lower. In each layer, horizontal components of current are expanded through the vertical in terms of a set of eigenfunctions. Coefficients of these expansions are evaluated in the horizontal and through time using a two-dimensional numerical time-stepping procedure. Thence the three-dimensional current structure is determined.     

Post-glacial rebound and transient lower mantle rheology

Summary. Although post-glacial rebound data have been conventionally interpreted as being governed by the steady state component of the mantle viscosity spectrum, the radial profile of this parameter, which is then inferred by fitting a model to observations, is characterized by the fact that it exhibits rather slight variation with depth. This disagrees with expectations based upon microphysical models of the solid state creep process. It also disagrees with very recent inferences of the viscosity stratification based on isostatic geoid anomalies expected on the basis of the internal lateral heterogeneity of mantle density obtained from seismic tomographic analyses. The new calculations of the signatures of post-glacial rebound reported here show that these two types of information are easily reconciled if the previously inferred value of the lower mantle viscosity is interpreted as a transient value, as originally suggested by Weertman on the basis of qualitative considerations. In these new models considered here the steady state creep resistance of the lower mantle is not constrained at all by post-glacial rebound observations. It can be fixed only by an appeal to other geophysical data. Whether such models are actually required by the data should become clear in the very near future.     

Effect of viscosity structure on fault potential and stress orientations in eastern Canada

Previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern Canada have focused on the mode of failure and the observed timing of the pulse of earthquake/faulting activity following deglaciation. In this study, the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times. It is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 10²¹ Pa s mantle.
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (10²² Pa s) in the lower mantle is rejected by the stress orientation and rotation observations. A low-viscosity (6 times 10²⁰Pa s) upper mantle with 1.6 times 10²¹ Pa s in the upper part of the lower mantle and 3 times 10²¹ Pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations.