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1.
Dynamics of western boundary currents in the subtropical and subpolar gyres are studied as a source-sink flow of barotropic fluid by means of numerical integration of the time-dependent non-linear vorticity equation. The bottom topography consists of a continental shelf of uniform slope (120 km wide) parallel to the straight western coast and a flat bottom of uniform depth. The steady solution in the case of low Reynolds number (Re≦100) shows the vorticity balance of the western boundary current between theβ-, diffusion-, and bottom relief terms. The cuspidated flow of the western boundary current in the subpolar gyre is observed as a compensating flow for the subtropical western boundary current separating from the western coast. In the case of Re=350, the zonal current separating from the coast meanders with the wave length of the stationary Rossby waves. It is shown that in the present model the separation of the boundary current is controlled by the planetary vorticity (f) of the fluid particle in the boundary flow, with which the same particle flows out the eastern wall at the corresponding latitude. The decrease of the efflux width increases the intensity of the non-linear overshooting of the boundary current separating from the western coast.  相似文献   

2.
Response of the barotropic western boundary current to typhoon passage is investigated by the use of the numerical models described in Part I. Steady states obtained in Part I are chosen as the initial conditions for undisturbed currents. In these models it is assumed that an axially symmetric typhoon (radius = 100 km, maximum wind speed = 27.4 m/sec) moves parallel to the western boundary.For the model with a flat bottom the boundary flow diminishes its strength and broadens its width after the passage of the typhoon offshore. For the model with a continental slope the effect of the typhoon is significantly different depending on the distance of the path of the typhoon from the western boundary. Specifically the north wind on the continental slope causes the formation of cyclonic vortex, which progresses southwards along the slope in the northern hemisphere. Linear theory of continental shelf waves indicates that the phase speed of the propagation of the vortex is 2.4 m/sec. Anomalies of the water level along the coast are also calculated from geostrophic relationships.  相似文献   

3.
Variations of the western boundary currents induced by a periodic change in wind stress are studied in a two-layer model with a continental slope along the western boundary. The variation of the total transport of the western boundary current over the continental slope shows a considerable phase lag with the wind stress and a decrease in amplitude compared with for the flat bottom ocean, though the interior barotropic response is to adjust almost instantaneously to the wind stress. The total transport variation of the western boundary current is well approximated by the upper layer transport variation. That is, almost complete separation of the upper- and lower-layer flows takes place over the slope, and only the upper layer flow contributes to the change in total transport of the western boundary current. Contributions of the interior barotropic and baroclinic responses to the upper layer transport variation depend on the forcing period. With decrease in the forcing period, the barotropic response becomes relatively important for determining the upper layer transport variation although the amplitude of the variation is smaller.  相似文献   

4.
Investigated is a possibility of two-dimensional model in the study of the dynamics of the western boundary current by a numerical experiment. Emphasis is laid on the effect of bottom barrier corresponding to the Izu Ridge.The western boundary current in the model is formed by source and sink of the water prescribed at an artificial eastern wall (600 km offshore). The bottom topographyconsists of a continental slope parallel to the straight western coast, and a ridge protruding from the western coast to 500 km offshore (1,500 m deep and 400 km wide). The grid size of 12 km× 25 km (offshore and longshore directions, respectively) resolves both the western boundary current and the bottom topography.The assumption of homogeneity of the water density makes the western boundary current detour along the isobath of the ridge.A steady state solution is obtained under the assumptions that the horizontal velocity does not change direction vertically (equivalent barotropic), and that the geostrophic relationship holds at the bottom. Homogeneity of the water density is not assumed. The solution shows that most of the volume transport of the western boundary current cross the ridge and the current has cyclonic vorticity near the summit of the ridge. It seems to suggest that the investigation by three-dimensional models is neccesary in order to study the complete dynamics of the western boundary current crossing the ridge.  相似文献   

5.
A wind-driven, general circulation for a two-layer ocean with continental shelf-slope along the western boundary is studied numerically. Special attention is focused on the formation process of the western boundary current in the subtropical gyre. The western boundary current develops in the upper layer along the western boundary on the shelf-slope with a bottom trapped poleward flow in the lower layer. The poleward undercurrent is concentrated approximately along the contour lines of the potential vorticity,f/D, wheref is the Coriolis parameter andD the depth of the ocean. The separation of upper- and lower-layer flows on the shelf-slope represents a typical transient response. As the response approaches a steady state, the poleward undercurrent decreases in amplitude, and the motion tends to be confined to the upper layer. The flow pattern becomes similar to that found in a flat bottom ocean. A steady-state response is expected to be isostatic (no motion in the lower layer), even on the shelf-slope, as conservation of potential vorticity would suggest.The remarkable increase in transport of the western boundary current produced by the formation of an anticyclonic vortex on the shelf-slope extending throughout the hemisphere (Holland, 1973) does not occur in the wind-driven general circulation.  相似文献   

6.
Effect of the typhoon passage on the western boundary region of a two-layer ocean with bottom topography is studied. The ocean is initially at rest and is set in motion by a typhoon passing parallel to the west coast. Equations that represent barotropic and baroclinic modes of motions are solved numerically by means of the method of finite differences. Motions of the barotropic mode are assumed to be horizontally non-divergent. In this mode, an elongated vortex is produced by the typhoon and propagates toward the south after passage of the typhoon. Behavior of the vortex may be interpreted as continental shelf waves. It is found that the formation and propagation of continental shelf waves are hardly affected by the density stratification. As for the baroclinic response, the typhoon causes considerable interface displacements along its track. The interface displacements are associated with geostrophic motions and remain for long time, though they are formed on the continental slope. Besides the large scale baroclinic response, internal Kelvin waves are induced along the artificial east wall.  相似文献   

7.
To investigate effects of a continental slope along the western boundary on the abyssal circulation, numerical experiments using multi-level models were carried out. An ocean which extends over the northern and southern hemispheres is forced by cooling inside the ocean at the southwest corner of the basin and uniform heating through the sea surface. When the reference density for the cooling is vertically uniform, effects of the slope emerge clearly for the slope with considerably broad width. The deep western boundary current flowing over the slope feeds no bottom flows in the southern hemisphere, and carries the warmed deep water into the northern hemisphere. This leads to the increased meridional density gradient, which results in the modification of deep flow patterns. When the reference density is vertically distributed, the upper and lower northward flowing western boundary currents form in the deep layer. As the density stratification relaxes the topographic control, the westward intensification of the upper boundary current is achieved over the slope. The intensified flow is accompanied by the countercurrent and they form the horizontal recirculation over the slope. However, the effects are confined around the slope region and the interior flow patterns do not change. The lower boundary current is not significantly affected by the slope and has the large width with no countercurrent. It is found that the actual continental slope does not have significant effects on the gross feature of the thermohaline circulation.  相似文献   

8.
《Ocean Modelling》2000,2(1-2):73-83
The conventional picture of an ocean gyre, based on an ocean with vertical sidewalls, assumes a balance between an input of vorticity by wind stress curl, and a viscous flux of vorticity through the boundary at the same latitude, resulting from a viscous western boundary current which may be significantly modified by nonlinear terms. Potential interactions with topography are also commonly acknowledged as a possible complicating factor. In this idealized picture, the zonal momentum balance is taken to be geostrophic, as numerous model analyses confirm. A theoretical argument is given here which shows that, in an ocean with sloping sidewalls, this geostrophic balance results in bottom pressure torques which balance the wind stress curl at each latitude. This removes the requirement for a viscous western boundary current at each latitude suggesting that the dynamics within a western boundary current may be essentially inviscid. While inviscid western boundary currents have already been found in certain idealized systems, and in one set of diagnostics from an eddy permitting model, the generality of the argument presented here gives a strong reason to believe that these are not special cases. Inviscid western boundary currents are in fact the rule, and the vertical sidewall case is an unrealistic exception.  相似文献   

9.
The generation of small meanders of the Kuroshio off southern Kyûshû is investigated. Basing on the fact that the small meanders tend to follow an increase in velocity of the Kuroshio in the Satsunan region (Sekine andToba, 1981), the influence of this velocity increase on the quasi-stationary path of the Kuroshio is studied numerically. Simplified bottom and coastal topographies are employed in a two layer model ocean. A quasi-stationary numerical solution with a constant inflow is used for the initial condition, and a temporal increase in the inflow with corresponding leakage is employed as the boundary condition to investigate nonlinear effects due to the increase in current velocity. Experiments for four different physical models are carried out to determine the specific roles of the continental slope, the planetary-effect, and density stratification. Temporal increase in the inflow tends to cause offshore shift of the current path. But the topographic effect of the continental slope is strong enough that no significant shift of the current path occurs in the case of the barotropic ocean. However, in the case of a baroclinic ocean, temporal increase in the inflow does cause generation of small meanders, because density stratification diminishes the topographic effect. A larger density stratification provides more favorable condition for the appearance of the small meander, and a cyclonic eddy is formed on the continental side of the small meander path.  相似文献   

10.
By using a two-dimensional barotropic model on a-plane, the effect of the bottom topography on the path of the Tsushima Current is investigated. The rectangular model ocean with continental slopes has two openings: one is located at the southern boundary and the other at the eastern boundary. In a steady state, most of the water supplied into the model ocean through the inflow opening, flows along the continental slope with the coast to the right. Continental shelf waves play an important role in the process of adjustment to a steady state. It is suggested that the nearshore branch of the Tsushima Current might be largely topographically controlled.  相似文献   

11.
The Pacific deep western boundary current (DWBC) encounters an unstable continental margin where it flows across the New Zealand convergent plate boundary. Seismic profiles show the DWBC was intercepted by several submarine landslides, the latest (~38-100 ka) being the newly discovered Matakaoa debris flow. Occupying ~650 km3, the flow extends 200 km from Matakaoa re-entrant to Kermadec Ridge to form a 37-68 m high lobe in the current's path. This deposit appears to have (1) reduced the size of gaps in the western boundary, thereby reducing leakage of the DWBC, and (2) temporarily reduced the terrigenous supply into the flow by impeding the passage of turbidity currents from New Zealand.  相似文献   

12.
Measurements of tidal current and wave velocity made at 0.69 and 1.85 m above a rough seafloor exhibit large current gradients (boundary layer) in the water column. The logarithmic boundary layer flow model was fitted to the measurements, and thus roughness (z0) and friction velocity (u*) parameters were derived. The roughness parameter values were generally consistent with the observed upstream physical roughness. The values of both parameters for conditions in the rough turbulence flow regime are generally larger (much larger for ebb) than earlier published values for similar measurements of currents in the absence of significant waves but are comparable to values from recent measurements of currents in the presence of storm waves. The high parameter values here appear to relate more to the magnitude of the current and to the upstream physical bottom roughness than to the magnitude of the seastate. Large boundary layers in the flow at the seabed have a profound effect on the design of offshore structures such as offshore pipelines.  相似文献   

13.
Western boundary currents flow poleward from low latitudes until they ultimately separate from the coast and turn eastward into the ocean interior. The separation is mainly due to either: (i) the variation of the Coriolis parameter with latitude (β) which causes vanishing of the near-wall depth; (ii) vanishing wind stress curl over the ocean interior which forces zero meridional transport; or (iii) opposing currents that flow toward the equator and force the northward flowing currents to turn offshore (Agra and Nof, Deep Sea Research I, 40, 2259–2282). Here, we focus on the third kind of separated currents and show that, due to β, such separated currents migrate along the wall. A nonlinear “reduced gravity” one-and-a-half layer model is used to compute the desired migration speed. Solutions of the primitive equations are constructed analytically assuming that the translation rate is steady. It is found that the migration rate along the wall is given by βRd2 cosα/2 sinγ, where Rd is the Rossby radius, α an angle that measures the inclination of the joint offshore currents relative to the north, and γ is the angle between the axis of the joint offshore currents and the wall. The migration meridional component can be either northward or southward (depending on the inclination of the wall) but the zonal component is always westward. When the separated joint offshore flow is in the east-west direction (i.e. α = π/2 or 3π/2 so that the separated flow is zonal) no migration is taking place. It turns out that the above migration formula is so robust that it is also describes the migration rate in a two-and-a-half layer model where one current is allowed to, at least partially, dive under the other. For most separated currents the computed migration rate is a few centimeters per second.Possible application of this theory to the Confluence zone in the South Atlantic (where significant seasonal movement of the separation latitude has been observed) is discussed.  相似文献   

14.
The western boundary layer (WBL) plays a fundamental role in basin-scale wind-driven ocean circulations. In idealized ocean models with flat bottom topography, this layer is required not only to balance the interior Sverdrup transport to close the gyre circulation, but also to dissipate the vorticity imposed by the wind-stress curl. The width of the WBL in Munk-type models is estimated to be δM(AH/β)1/3, where AH and β are horizontal eddy viscosity and the meridional derivative of the Coriolis parameter respectively. For commonly used values of AH, the boundary-layer width δM ranges from 30 to less than 200 km in the mid-latitude ocean. This scale is often poorly resolved in large-scale climate models.This paper intends to demonstrate some consequences when the western boundary layer is not adequately resolved. It is found that coarse resolution models reach equilibrium states by distorting some important dynamics in order to dissipate wind-imposed vorticity. In three-dimensional models, for instance, very strong spurious upwelling and downwelling can occur along the WBL. In models of two-dimensional flow, however, spurious recirculations may develop near the boundary. These false features can be removed when the boundary layer is better resolved. We propose a method in which a spatially varying AH is used to broaden the WBL without affecting mixing in the interior. The method improves the model results considerably.  相似文献   

15.
A one and a half layer inviscid hydraulic model was introduced to study the dynamics of the flow that brings the bottom cold water southward into the Korea Strait. Two different channel geometries were considered; a rectangular channel and a channel with a sloping western wall, which represents the continental slope near the Korean coast. The lower layer water in the rectangular channel separates from the eastern wall when the depth of the channel,H o, becomes shallower than a critical value donwstream. Hydraulic control of the flow is possible after the flow separation, if the channel becomes shallow enough. Before hydraulic control, the width of the flow decreases asH o decreases, but the effect of the slope of the western wall is negligible. After the control, however, the width increases asH o decreases or the slope becomes weaker. If the slope becomes weak enough or the channel becomes deep enough, which is determined by upstream conditions, the lower layer is observed only over the sloping western wall. This simple model shows that the continental slope between the East Sea (Japan Sea) and the Korea Strait makes the southward flowing North Korean Cold Water bank against the Korean coast in the Korea Strait. The model also shows that the sloping bottom near the Korean coast makes the bottom cold water of the Korea Strait appear only over the continental slope away from the trough of the strait.  相似文献   

16.
Current meter data collected over the last 20 years are presented and used to describe the residual currents on the Celtic and Armorican slope and shelf regions. On the slopes, a poleward current of about 6cm s−1 exists at the 500m depth contour. At mid depths, these currents are directed onslope, whereas near the bottom the flow in markedly downslope, reaching mean speeds of about 15cm s−1 near 6°40′W. The downslope currents are thought to be largely tidally induced and balanced by Stokes transports. The total slope transport near 48°N is about 4Sv. On the upper slopes (<1000m depth) the transport increases poleward. On the outer Celtic shelf, a weak (2 cm s−1) counter-current flowing southeastwards was observed. On the Armorican shelf, the residual flow is again nothwestwards and this coastal flow appears to continue northwards across the mouth of the English Chanel and past the Isles of Scilly with typical mean upper layer speeds of about 2cm s−1. Southwest of Ireland the flow is again northwesterly. Numerical model simulations show that the eastern slope boundary current of the NE Atlantic can be driven by realistic distributions of seawater density. The simulations also show only a small wind driven barotropic response on the Celtic and Armorican shelf region and that a component of the residual shelf flows, like the slope current, may be driven by pressure distributions arising from regional differences in the distribution of seawater density, or from non local wind stress.  相似文献   

17.
Numerical solutions are examined for isolated, intense vortices as influenced by western bounding bottom topography through the use of a rigid-lid, two-layer primitive -plane numerical model. Systematic studies are made of the sense of rotation (cyclonic/anticyclonic), the consequence of varying the gradient of bottom slope, and the different vertical shear in a two layer ocean. In the basin with a bottom slope, the nearly barotropic anticyclonic vortex forms a modon-like vortex for S with fixedRo 2<O(1) (where is the ratio between the variation of the Coriolis parameter across the eddy to the Coriolis parameter in the center, S the topographic effect and,Ro 2 the Rossby number in the lower layer) and its generation is due to a compound effect of the planetary beta, topographic beta, avvection, and mirror image. The formation of the modon-like vortex and the propagation of the original vortex onto the bottom slope depends on the strength of slope gradient and the baroclinicity of the vortex. The nearly barotropic anticyclonic vortex evolves into the stronger upper ocean one with increasing S: the gradient of the bottom slope becomes steeper. Then the original vortex lives longer because the barotropic component of the energy is converted to the baroclinic one and it moves toward southeast in forming a modon-like vortex in the lower layer. The evolution of a vortex in the model results are compared to observational results of a Kuroshio warm core ring (KWCR) obtained from hydrographic data (June, 1985) and from NOAA satellite infrared images (April, 1985 to July, 1985). It is shown that a KWCR (June, 1985) is influenced by the western continental slope/shelf of the East Japan.  相似文献   

18.
A tide circulation model of the Atlantic coast of the Iberian Peninsula has been constructed. This regional numerical model covers the whole continental shelf. The finite element computational grid is made of some 16,300 triangular elements with sizes ranging between 13 km (on the offshore boundary) and 1 km (near the coast), with local refinements on the continental shelf and in the area of Figueira da Foz. This site was selected as experimental site for the study of waves and currents in the frame of the MAST2/WAVEMOD research project.Boundary conditions along the three oceanic limits of this widely open domain are obtained from the North-Atlantic component of a World Ocean tidal numerical model known as FES94 [Le Provost, C., Genco, M.L., Lyard, F., Vincent, P. and Canceil, P. (1994) Spectroscopy of the world ocean tides from a finite element hydrodynamic model, Journal of Geophysical Research, Topex-Poseidon Special Issue]. A new radiation-like boundary condition has been introduced in the modelling system used (Telemac-2D), which solves the Shallow Water Equations (SWE), in order to interface the two models and to allow for the tidal wave to leave the northern limit without reflection. Model calibration has been performed on the dominating M2 constituent. The introduction of the astral static potential generating the tide in the SWE improved this regional model.A long duration run (1 month) has been performed, the model being forced by the eight major tide constituents. Harmonic analysis of results has been performed on 17 tide constituents, due to non-linear interactions of constituents on the continental shelf. Comparisons with the FES94 model on one hand, and with a set of coastal tide gauges on the other hand, are good. A database of tidal harmonics is now available for forecasting sea levels and currents in this area.This work has shown that diurnal shelf trapped tide waves exist in some places along the Portuguese continental shelf, which induce diurnal dominant tidal currents in these places (North of Figueira da Foz).  相似文献   

19.
Recent multibeam bathymetry and acoustic imagery data provide a new understanding of the morphology of the western part of the Gulf of Cadiz. The gulf is under the influence of a strong current, the Mediterranean Outflow Water (MOW). This current is at the origin of the construction of the giant Contourite Depositional System. Canyons and valleys with erosive flanks are observed. Only the Portimao Canyon is presently connected to the continental shelf. Channels occur on the continental shelf but are presently disconnected from the deeper network of channels and valleys. Slumps are localized in steep slope areas. They are caused by oversteepening and overloading, sometimes probably associated with earthquake activity. Slumps transform sharply into turbidity currents, depositing turbidites on the floor of deep valleys. Interaction of the MOW and gravity currents is suggested by the filling of the incisions located on the drifts below the present seafloor, the shifting of valleys and canyons in the direction of the MOW flow inducing an unusual phenomenon of capture of submarine valleys.  相似文献   

20.
Recent observations suggest that the annual mean southward transport of the East Sakhalin Current (ESC) is significantly larger than the annual mean Sverdrup transport. Motivated by this observational result, transport of a western boundary current has been investigated using a simple numerical model with a western slope. This transport is defined as the instantaneous barotropic transport integrated from the western boundary to the offshore point where the barotropic velocity vanishes. The model, forced by seasonally varying wind stress, exhibits an annual mean of the western boundary current transport that is larger than that of the Sverdrup transport, as observed. The southward transport from October to March in the model nearly equals the instantaneous Sverdrup transport, while the southward transport from April to September decreases slowly. Although the Sverdrup transport in July vanishes, the southward transport in summer nearly maintains the annual mean Sverdrup transport, because the barotropic Rossby wave cannot intrude on the western slope. This summer transport causes the larger annual mean. Although there are some uncertainties in the estimation of the Sverdrup transport in the Sea of Okhotsk, the seasonal variation of the southward transport in the model is qualitatively similar to the observations.  相似文献   

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