引潮力对海洋环流模式的影响

The eight main tidal constituents have been implemented in the global ocean general circulation model with approximate 1° horizontal resolution.Compared with the observation data,the patterns of the tidal amplitudes and phases had been simulated fairly well.The responses of mean circulation,temperature and salinity are further investigated in the global sense.When implementing the tidal forcing,wind-driven circulations are reduced,especially those in coastal regions.It is also found that the upper cell transport of the Atlantic meridional overturning circulation(AMOC) reduces significantly,while its deep cell transport is slightly enhanced from 9×106m3/s to 10×106 m3/s.The changes of circulations are all related to the increase of a bottom friction and a vertical viscosity due to the tidal forcing.The temperature and salinity of the model are also significantly affected by the tidal forcing through the enhanced bottom friction,mixing and the changes in mean circulation.The largest changes occur in the coastal regions,where the water is cooled and freshened.In the open ocean,the changes are divided into three layers:cooled and freshened on the surface and below 3 000 m,and warmed and salted in the middle in the open ocean.In the upper two layers,the changes are mainly caused by the enhanced mixing,as warm and salty water sinks and cold and fresh water rises;whereas in the deep layer,the enhancement of the deep overturning circulation accounts for the cold and fresh changes in the deep ocean.     

Generation mechanisms of tidal residual circulation

Two new types of mechanism for the generation of tidal residual flow are revealed with the use of a hydraulic model experiment. A remarkable anticlockwise tidal residual circulation is formed in a model bay due to the presence of a tidal current, the Coriolis force and a horizontal boundary. A similar circulation is also formed due to the presence of a bottom slope, a horizontal boundary and a tidal current which flows normal to the inclination of the bottom slope. The residual circulation in the Sea of Iyo in the Seto Inland Sea is considered to be due to a combination of the effects of the Coriolis force, a bottom slope, a horizontal boundary and the tidal current. We classified some of the generation mechanisms of tidal residual flow which have been described to date into seven types on the basis of vorticity considerations.     

Tidal residual circulation produced by a tidal vortex

The role of the ‘tidal vortex’ in the mechanism of generation of tidal residual circulation is investigated for a bay with a narrow entrance channel. It is shown that the circulation of residual flow is produced not by the vorticity of the inflowing sidewall-boundary layer, but by a tidal vortex formed by rolling up of the discontinuity surface released from the flow separation point at the entrance. This tidal vortex is affected by the circulation of the inflowing water, that is the inflowing tidal vortex. A returning tidal vortex formed in the bay diminishes the circulation of the tidal vortex of the next generation, while the inflowing tidal vortex formed in the open sea increases it. These cases correspond to tidal vortex life-histories of type-II and type-III, respectively (Kashiwai, 1984a). Tidal vortices of each life-history type have different strength and produce residual circulation of different strength, corresponding to each type. The ratio of kinetic energy of residual flow to that of the tidal current through the bay entrance, that is to say the energy gain of the residual circulation, is proportional to the reciprocal of the Strouhal number, and its rate of increase depends on the life-history type of the tidal vortex. This explains the experimental observation reported by Oonishi (1977) and Yanagi (1978) that the energy ratio of residual flow to tidal flow increases with the Reynolds number not monotonously but goes through a maximum and a minimum at intermediate Reynolds number.     

Tidal residual circulation produced by a tidal vortex

“TIDAL VORTEX” is a term for a kind of starting vortex formed as a pair of vortices at the head of a tidal jet emanating from a narrow entrance into a bay. In this study, its formation and movement have been investigated by means of a hydraulic experiment and an analytical model. A tidal vortex is formed as a result of flow separation at the abrupt widening of a channel entrance followed by rolling up of the discontinuity surface around its free end. The vortex shows three types of life-history (type I, II and III), which are characterized by the Strouhal number and the aspect ratio of the horizontal shape of the entrance channel. In the case of type-I, the tidal vortex proceeds toward the inner region of the bay and there amalgamates with successive vortex cores into a core of tidal residual circulation. In the case of type-II, the tidal vortex core flows out into the entrance channel on the ebb but returns back into the bay on the subsequent flood. And, in the case of type-III, the tidal vortex core which was formed on the bay-side opening of the entrance channel flows out to the open sea and never comes back, whereas the core which was formed on the open-sea side of the entrance flows into the bay and never flows out. The circulation of a tidal vortex core is proportional to the reciprocal of the Strouhal number. The movement of the core near the bay entrance is determined by interaction between the cores and transportation due to the irrotational component of the tidal current. There are three types of tidal residual circulation, corresponding to three life-history types of tidal vortices. In the case of type-I, a strong tidal residual circulation is formed, but in type-II a small and weak circulation is formed. While, in type-III, the circulation having an inverse sense of rotation to that of type-I is formed.     

The dynamics of a bathymetrically arrested estuarine front

The mixing and circulation associated with a bathymetrically arrested estuarine front was studied using hydrographic and current data. A quasisteady front, exhibiting strongly convergent surface flows, is formed along the steeply sloping inner margins of the flood tide delta during each semidiurnal tide cycle. This front separates the brackish ambient water within a deep estuarine basin from the incoming oceanic tidal water. The position of the front is dependent on the local water depth and the difference in density between the two water masses. Beneath the surface there is an inclined frontal interface where static stability is very low and vertical mixing intense. A vertically integrated horizontal momentum equation was derived for flow in the upper layer and an estimate made as to the value of the associated entrainment coefficient.     

On the fundamental dynamics of barotropic circulation in shallow seas

It is suggested that the shallow sea circulation should be related to the Lagrangian residual circulation but not to the conventional Eulerjan mean circulation, and further, the first order Lagrangian residual circulation-the mass-transport velocity field is used to define the steady circulation as the lowest order shallow sea circulation.The set of equation governing the shallow sea circulation is reformulated, which shows that there are no the so-called “tidal surface source” and the “tidal stress”. A vorticity equation for the stream function of horizontal transports is derived, which is easily solved by the conventional numerical methods.An intertidal convection-diffusion equation governing the concentration of conservative and passive tracer is derived. Differing from the conventional equation, the equation derived in the present paper reveals the Lagrangian convection and has no need to introduce the artificial hypothesis for the so-called “tidal dispersion”.The theory presented here is base     

Vertical residual flow in Kasado Bay

Some observations were carried out to understand the structure of the vertical residual flow in Kasado Bay. The results of current measurements at three points in the lower layer indicated that a horizontal counterclockwise tidal residual circulation converges in the lower layer. The velocity of upward residual flow was estimated to be about 4.5×10^–3 cm s^–1. The distributions of water temperature, salinity and grain size in the sediment support the existence of this upward motion.     

Broad classification of New Zealand inlets with emphasis on residence times

Thirty‐two New Zealand coastal inlets are tentatively classified into seven groups with probable similar circulation patterns, based on ratios of their physical parameters: volume, tidal compartment, entrance width, surface area, length, and average width. Eighteen of these have a predominant tidal flow, shown from their small ratio (ß < 4) of tidal compartment to total volume, and the cross‐sectional areas of their entrances are controlled by the tidal flow: Moutere, Waimea, Aotea, Whanganui, Avon‐Heathcote, Tauranga, Parengarenga, Porirua‐Pauatahanui, Kawhia, Nelson, Rangaunu, Raglan, Whangarei, Bluff, Otago, Hokianga, Manukau, and Whangaruru. The other fourteen range from long narrow sounds with probable strong vertical circulation (e.g., Pelorus Sound) to large bays with strong mean horizontal circulations (e.g., Tasman and Hawke Bays).

Examples of residence time calculated in inlets with ß > 4 are given for Tasman Bay, a bay with a mean horizontal circulation, which has a residence time of 1–3 months, and Pelorus Sound, an inlet with a vertically complex circulation, which has a residence time of about 20 d.     

Roles of horizontal processes in the formation of density stratification in Hiuchi-Nada

Roles of horizontal processes in the formation of the density stratification in Hiuchi-Nada are investigated by means of a two-dimensional numerical model. In Hiuchi-Nada, vertically mixed and stratified regions are formed due to the regional difference of the tidal currents, and a tidal front is formed between the two regions. The horizontal mixing across the tidal front suppresses the development of the stratification, which is developed too much in the absence of the horizontal mixing. The moderate, realistic stratification cannot be realized in the model without the horizontal mixing. Density currents are formed due to the density distribution associated with the mixed and stratified states. These currents contribute to the horizontal mixing through the shear effect. Horizontal heat transfer from the outside water generates the vertical circulation and causes the stratification. This effect dominantly appears at the early and late stages of the stratified season. The stratification is initiated before the beginning of the surface heating and persists beyond the end, due to the horizontal heat transfer.     

一个概化的潮汐河口羽状流动力学的初步研究

基于区域陆架海水动力-生态耦合模型(COHERENS)中的三维水动力模型,对一个概化的潮汐河口羽状流动力学进行了初步研究。在均一水深和无风的理想条件下,模拟得到了一个潮周期内概化的潮汐河口表层水体:a)羽状流流场和盐度场的平面分布;b)3种不同河流径流量作用下的羽状流流场和盐度场的平面分布;c)4种不同底床粗糙长度下的羽状流流场和盐度场的平面分布。结果分析表明:a)羽状流的长度在潮汐的作用下递增,而其最大宽度近似呈周期性的变化;b)羽状流的长度和最大宽度都随着河流径流量的增大而分别变长和变宽;c)羽状流的长度和最大宽度都随底床粗糙长度的增大而变小;d)无潮情况下羽状流的凸起明显并存在沿岸流,然而在有潮情况下凸起不明显且无沿岸流;e)水平扩散可能限制了沿岸羽状流的发展。     

Fundamental study on the tidal residual circulation—I

The tidal residual circulation in a bay was experimentally investigated with use of a hydraulic model. The model basin is a square bay of 5 m sides with a one-sided mouth of 1 m wide. The depth of the basin is 0.1 m. The tide of a six-minute period was provided by a tide generator of plunger type through the mouth. Tidal currents in the bay always flow in one direction though its strengths change according to the tidal phase, that is, a strong tidal residual circulation occurs in the bay. A similar flow pattern was observed to occur in a field with a horizontal boundary geometrically similar to the present model. The vorticity transfer from tidal current to residual flow is balanced with the vorticity advection of residual flow and the dissipation due to the viscosity.     

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.     

Vertical structure of the tidal current ellipse in a rotating basin

Tidal current ellipses formed by the Coriolis effect are investigated theoretically, taking account of the effect of horizontal boundaries. This study reveals that even in a narrow bay, the tidal current ellipse appears in the inside of the bottom boundary layer, although the ellipticity cannot be recognized in the outside of the boundary layer. In a wide bay, the ellipticity is observed even in the outside and it is larger in the inside. The rotation direction of these ellipses is counterclockwise in the northern hemisphere. And also, the Coriolis force has an effect to deflect the major axis of the ellipse in the inside of the boundary layer. These nature of the tidal current ellipse is well explained by the consideration of the formation mechanism of the ellipse.     

TIDICS—Control of tidal residual circulation and tidal exchange in a channel-basin system

A self-excited oscillation of tidal flow occurs in a tidal channel-basin system with a narrow and high speed inflowing tidal jet. The mechanism of this oscillation was examined through hydraulic experiments. Results show that the oscillation is caused by alternation in the strength of asymmetric residual circulation caused by a biased supply of vorticity by asymmetric tidal vortex pairs, and that the mechanism of alternation also depends on negative feed-back between the residual circulation and the vorticity supply.The unstable state of the flow suggested the possibility of controlling the tidal current and tidal exchange in the channel-basin system and this was confirmed experimentally. Under fixed tidal conditions and with fixed basin dimensions, the tidal residual circulation was controlled to form two strong clockwise and anticlockwise circulations or a single circulation by changing the shape of the bay entrance.     

Models of near-bed dynamics and sediment movement at the Iberian margin

A three dimensional time-dependent baroclinic hydrodynamic model, including sediment transport and incorporating a turbulence energy sub-model, is used in cross sectional form to examine sediment movement at the shelf edge off North West Iberia at 42°40.5’N where measurements were made as part of the OMEX-II programme. These calculations are complemented by a simpler, in essence time-independent model, which is used to examine the sensitivity of the sediment distribution over the slope (from a shelf-break source) to changes in the specified values of horizontal and vertical diffusion coefficients. The philosophy of the paper is to use idealized tidal, wind and wind wave forcing to examine changes in sediment distribution resulting from these processes. Calculations with the time-dependent and steady state models give insight into both the role of events and long-term effects. The steady state model focuses on the off-shelf region, whilst the time-dependent model considers on-shelf events.Tidal calculations showed that for the stratification used here the internal tide in the OMEX region was primarily confined to the shelf edge and ocean. A mean on-shelf sediment transport in the surface layer and off-shelf transport at the bed was found. Across-shelf circulations produced by up-welling/down-welling favourable winds gave rise to on-shelf/off-shelf currents in the bottom boundary layer with an opposite flow in the surface layer. In the case of an up-welling favourable wind, sediment suspension was at a maximum in the near coastal region, with sediment being advected off shore in the surface layer. With a down-welling favourable wind, surface sediment was advected towards the shore, but there was offshore transport at the bed. Near the shelf edge any upwelling flow had the tendency to return this sediment to the surface layer from whence it was transported on-shore. So in essence the sediment was trapped within an on-shelf circulation cell. Wind waves effects increased the total bed stress and hence the sediment concentration and its transport, although its pattern was determined by tidal and wind forcing.The time independent model with increased/decreased lateral diffusivity gave an enhanced/reduced horizontal sediment distribution for a given settling velocity. As the settling velocity increases, the down-slope movement of sediment is increased, with a reduction in the thickness of the near-bed sediment layer, but with little change in its horizontal extent.     

Tidal and river discharge forcing upon water and sediment circulation at a rock-bound estuary (Guadiana estuary,Portugal)

The relative impacts of tidal (neap, spring) and river discharge (including a flood event) forcing upon water and sediment circulation have been examined at the rock-bound Guadiana estuary. Near-bed and vertical profiles of current, salinity, turbidity, plus surface suspended sediment concentrations (SSC, at some stations only), were collected at the lower and central/upper estuary during tidal and fortnightly cycles. In addition, vertical salinity and turbidity profiles were collected around high and low water along the estuary. Tidal asymmetry produced faster currents on the ebb than on the flood, especially at the mouth. This pattern of seaward current dominance was enhanced with increasing river flow, due to horizontal advection that was confined within the narrow estuarine channel. The freshwater inputs and, at a degree less, the tidal range controlled the vertical mixing and stratification importance. Well-mixed (spring) and partially stratified (neap) conditions alternated during periods of low river flows, with significant intratidal variations induced by tidal straining (especially at the partially stratified estuary). Highly stratified conditions developed with increasing river discharge. Intratidal variability in the pycnocline depth and thickness resulted from current shear during the ebb. A salt wedge with tidal motion was observed at the lower estuary during the flood event. Depending on the intensity of turbulent mixing, the residual water circulation was dominantly controlled either by tidal asymmetry or gravitational circulation. The SSC was governed by cyclical local processes (resuspension, deposition, mixing, advection) driven by the neap-spring fluctuations in tidal current velocities. More, intratidal variability in stratification indicated the significance of tidal pumping at the partially and highly stratified estuary. The estuary turbidity maximum (ETM) was enhanced with increasing current velocities, and displaced downstream during periods of high river discharge. During the flood event, the ETM was expelled out of the estuary, and the SSC along the estuary was controlled by the sediment load from the drainage basin. Under these highly variable river flow conditions, our observations suggest that sand is exported to the nearshore over the long-term (>years).     

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.     

Three-dimensional Layer-integrated Modelling of Estuarine Flows with Flooding and Drying

Details are given of the refinement and application of a thee-dimensional (3-D) layer-integrated numerical model of tidal circulation, with the aim of simulating severe tidal conditions for practical engineering applications. The mode splitting strategy has been used in the model. A set of depth-integrated 2-D equations are first solved to give the pressure gradient, and the layer-integrated 3-D equations are then solved to obtain the vertical distributions of the flow velocities. Attention has been given to maintaining consistency of the physical quantities derived from the 2-D and 3-D equations. A TWO=layer mixing length turbulence model for the vertical shear stress distribution has been included in the model. Emphasis has been focused on applying the model to a real estuary, which is geometrically complicated and has large tidal ranges giving rise to extensive flooding and drying. The model has been applied to three examples, including: wind-driven flow in a rectangular lake, tidal circulation in a model rectangular harbour, and tidal circulation in a large estuary. Favourable results have been obtained for both the simple and complex flow beds.     

Eddy viscosity and tidal cycles in a shallow sea

The very pronounced spring-neap tidal cycle in the South Australian Gulfs leads to an unusually large variation in the magnitude of the tidal currents. Measurements of the currents in these Gulfs show that the non-tidal circulation depends on the strength of the tidal currents and hence on the spring-neap cycle. A simple model is produced in which the vertical eddy viscosity is a function of both wind strength and tidal currents. Results from the model agree with the observations and suggest a means whereby variations in tidal mixing may be accounted for in hydrodynamic modelling of the circulation in shallow seas.     

Similitude of the hydraulic model experiment for tidal mixing

The treatment of diffusion due to the tidal current in the near-shore ocean, and the similitude in the hydraulic model experiment are studied.In broad and shallow tidal bays and in the coastal seawaters near irregular boundaries, horizontal eddy-currents induced geometrically and turbulence caused by their cascading have predominant effect on dispersion of river and waste waters. These turbulent diffusion processes are similarly reproduced in the Froude models of turbulent resume, by adding the similitude for the self-similar structure of the spectral density of turbulence, or the eddy diffusivity. The similitude means to take the scale ratios of the time and the vertical length as the two-thirds power of the scale ratio of the horizontal length.Similitudes are also derived for the system of the gravitational circulation, the stratification and the salt-mass transport in partially and well-mixed estuaries. Generally, the vertical eddy diffusivity must be exaggerated by a half power of the model distortion externally by some methods of agitation. When the tidal bay is broad and very shallow, the Rayleigh number and the Hansen number are small, and the effect of density current and stratification on the flushing is small. Instead the effect of local eddies, geometrically induced tidal residual circulations become predominant. In this special case, there is no need to satisfy the similitude for density difference and vertical shear effects on dispersion.