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1.
Abstract This is a study of the influence of bottom topography of an ocean basin on the wind‐driven, barotropic ocean circulation. A detailed investigation is made of the role of vorticity transfer to the ocean bottom in the presence of varying topography. It is shown that the wind‐driven gyre over the topography of the North Atlantic has a transport in the western part of the basin only half of that obtained in an ocean of constant depth. 相似文献
2.
The transport of the Antarctic Circumpolar Current (ACC) is influenced by a variety of processes and parameters. A proper
implementation of basin geometry, ocean topography and baroclinicity is known to be a fundamental requisite for a realistic
simulation of the circulation and transport. Other, more subtle parameters are those of eddy-induced transports and diapycnal
mixing of thermohaline tracers or buoyancy, either treated by eddy resolution or by a proper parameterization. Quite a number
of realistic numerical simulations of the circulation in the Southern Ocean have recently been published. Many concepts on
relations of the ACC transport to model parameters and forcing function are in discussion, however, without much generality
and little success. We present a series of numerical simulations of circumpolar flow with a simplified numerical model, ranging
from flat-bottom wind-driven flow to baroclinic flow with realistic topography and wind and buoyancy forcing. Analysis of
the balances of momentum, vorticity, and baroclinic potential energy enables us to develop a new transport theory, which combines
the most important mechanisms driving the circulation of the ACC and determining its zonal transport. The theory is based
on the importance of the bottom vertical velocity in generating vorticity and shaping the baroclinic potential energy of the
ACC. It explains the breaking of the -constraint by baroclinicity and brings together in one equation the wind and buoyancy forcing of the current. The theory
emphasizes the role of Ekman pumping and eddy diffusion of buoyancy to determine the transport. It also demonstrates that
eddy viscosity effects are irrelevant in the barotropic vorticity balance and that friction arises via eddy diffusion of density.
In this regime, the classical Stommel model of vorticity balance is revived where the bottom friction coefficient is replaced
by (with the Gent–McWilliams coefficient and the baroclinic Rossby radius ) and a modified wind curl forcing appears. 相似文献
3.
Abstract The adjustment of a nonlinear, quasigeostrophic, stratified ocean to an impulsively applied wind stress is investigated under the assumption that barotropic advection of vortex tube length is the most important nonlinearity. The present study complements the steady state theories which have recently appeared, and extends earlier, dissipationless, linear models. In terms of Sverdrup transport, the equation for baroclinic evolution is a forced advection-diffusion equation. Solutions of this equation subject to a “tilted disk” Ekman divergence are obtained analytically for the case of no diffusion and numerically otherwise. The similarity between the present equation and that of a forced barotropic fluid with bottom topography is shown. Barotropic flow, which is assumed to mature instantly, can reverse the tendency for westward propagation, and thus produce regions of closed geostrophic contours. Inside these regions, dissipation, or equivalently the eddy field, plays a central role. We assume that eddy mixing effects a lateral, down-gradient diffusion of potential vorticity; hence, within the closed geostrophic contours, our model approaches a state of uniform potential vorticity. The solutions also extend the steady-state theories, which require weak diffusion, by demonstrating that homogenization occurs for moderately strong diffusion. The evoiution of potential vorticity and the thermocline are examined, and it is shown that the adjustment time of the model is governed by dissipation, rather than baroclinic wave propagation as in linear theories. If dissipation is weak, spin-up of a nonlinear ocean may take several times that predicted by linear models, which agrees with analyses of eddy-resolving general circulation models. The inclusion of a western boundary current may accelerate this process, although dissipation will still play a central role. 相似文献
4.
5.
The circulation of the Southern Ocean is studied in the eddy-resolving model POP (Parallel Ocean Program) by an analysis
of zonally integrated balances. The TEM formalism (Transformed Eulerian Mean) is extended to include topography and continental
boundaries, thus deviations from a zonally integrated state involve transient and standing eddies. The meridional circulation
is presented in terms of the Eulerian, eddy-induced, and residual streamfunctions. It is shown that the splitting of the meridional
circulation into Ekman and geostrophic transports and the component induced by subgrid and Reynolds stresses is identical
to a particular form of the zonally integrated balance of zonal momentum. In this balance, the eddy-induced streamfunctions
represent the interfacial form stresses by transient and standing eddies and the residual streamfunction represents the acceleration
of the zonal current by density fluxes in a zonally integrated frame. The latter acceleration term is directly related to
the surface flux of density and interior fluxes due to the resolved and unresolved eddies. The eddy-induced circulation is
extremely vigorous in POP. In the upper ocean a shallow circulation, reversed in comparison to the Deacon cell and mainly
due to standing eddies, appears to the north of Drake Passage latitudes, and in the Drake Passage belt of latitudes a deep-reaching
cell is induced by transient eddies. In the resulting residual circulation the Deacon cell is largely cancelled and the residual
advection of the zonal mean potential density is balanced by diapycnal eddy and subgrid fluxes which are strong in the upper
few hundred meters but small in the ocean interior. The balance of zonal momentum is consistent with other eddy-resolving
models; a new aspect is the clear identification of density effects in the zonally integrated balance. We show that the wind
stress and the stress induced by the residual circulation drive the eastward current, whereas both eddy species result in
a braking. Finally, we extend the Johnson–Bryden model of zonal transport to incorporate all relevant terms from the zonal
momentum balance. It is shown that wind stress and induction by the residual circulation carry an eastward transport while
bottom form stress and the stress induced by standing eddies yield westward components of transport.
Received: 26 June 2001 / Accepted: 2 November 2001 相似文献
6.
The sea level and the barotropic, frictional circulation response for the New York Bight are used to demonstrate the effects of external sea-level forcing, bathymetry, and variable friction. The governing equation is the steady, integrated vorticity equation and is computed by finite differencing over a curvilinear grid conforming to the 10- and 100-m isobaths and extending for 250 km alongshore. The boundary conditions are based on the hypothesis that the dynamics of the shelf are driven by the external sea-level gradient and the coastal no-flux condition; and consequently the conditions at the lateral boundaries are dependent thereon. Therefore, the external sea-level slope must be independently specified, and the lateral boundary conditions must be dependently generated. The diabathic component of the external sea slope forces the calm wind circulation by its effect on the transport through the upstream boundary; and the parabathic component has also an important modifying effect by forcing a shelf convergent transport. The parabathic sea slope at the coast is independent of its offshore value, being instead a direct product of the coastal boundary condition.The bottom friction is expressed as related to the sea level through a bottom length parameter and a veer angle, both of which are taken to increase shoreward. An additional bottom stress component, related to the surface stress, is determined for bottom depths less than the Ekman depth. Such bottom stress variability produces significant alterations in the nearshore flow field, over the constant bottom stress formulation, by reducing it and causing it to veer downgradient and downwind in the nearshore.The model is forced by different wind directions and the results are discussed. The circulations generally conform to the observed mean flow patterns, but with several smaller-scale features. The strong bathymetric feature of the Hudson Shelf Valley causes a polarized up- and downvalley flow for winds with an eastward or westward component, respectively. Under mean westerly winds, there is a divergence in the shelf valley flow at about the 60-m isobath. The Apex gyre existing off the western tip of Long Island becomes more extensive for winds changing from northeast to southwest. Mean flow reversals (to the northeast) occur off both Long Island and New Jersey for wind directions changing counterclockwise from northwest to southeast and from west to east, respectively. Southeastward transport over the outer New Jersey shelf tends to be enhanced by wind and external sea-level conditions; and the transport over the New Jersey midshelf, particularly in the lee of the shelf valley, tends to be weak and variable also under these mean conditions. 相似文献
7.
The baroclinic response of a stratified coastal embayment (Lunenburg Bay of Nova Scotia) to the observed wind forcing is examined using two numerical models. A linear baroclinic model based on the normal mode approach shows skill at reproducing the observed isotherm movements and sub-surface currents during a time of strong stratification in the bay. The linear model also shows that the isotherm movement in Lunenburg Bay is influenced by the wind forcing and propagation of baroclinic Kelvin waves from neighbouring Mahone Bay. The effects of nonlinearity and topography are investigated using a three-dimensional nonlinear coastal circulation model. The nonlinear model results demonstrate that the nonlinear advection terms generate a gyre circulation at the entrance of Lunenburg Bay, and the slope bottom topography at the mouth of the bay strengthens the sub-surface time-mean inflow on the southern side of the bay. A comparison of model-calculated currents in different numerical experiments clearly shows that baroclinicity plays a dominant role in the dynamics of wind-driven circulation in Lunenburg Bay. 相似文献
8.
Two-dimensional (cross-shelf and depth) circulation by downwelling wind in the presence of a prograding front (with isopycnals that slope in the same direction as the topographic slope) over a continental shelf is studied using high-resolution numerical experiments. The physical process of interest is the cross-shelf circulation produced by northeasterly monsoon winds acting on the Kuroshio front over the East China Sea outer shelf and shelfbreak where upwelling is often observed. However, a general problem is posed and solved by idealized numerical and analytical models. It is shown that upwelling is produced shoreward of the front. The upwelling is maintained by (1) a surface bulge of negative vorticity at the head of the front; (2) bottom offshore convergence beneath the front; and (3) in the case of a surface front that is thin relative to water depth, also by upwelling due to the vorticity sheet under the front. The near-coast downwelling produces intense mixing due to both upright and slant-wise convection in regions of positive potential vorticity. The analytical model shows that the size and on-shore propagating speed of the bulge are determined by the wind and its shape is governed by a nonlinear advection–dispersion equation which yields unchanging wave-form solutions. Successive bulges can detach from the front under a steady wind. Vertical circulation cells develop under the propagating bulges despite a stable stratification. These cells can have important consequences to vertical exchanges of tracers and water masses. 相似文献
9.
W. Lawrence Gates 《Pure and Applied Geophysics》1972,99(1):169-200
Summary The time-dependent primitive equations for a shallow homogeneous ocean with a free surface are solved for a bounded basin on the sphere, driven by a steady zonal wind stress and subject to lateral viscous dissipation. These are the vertically integrated equations for a free-surface model, and are integrated to 60 days from an initial state of rest by an explicit centered-difference method with zero-slip lateral boundary conditions. In a series of comparative numerical solutions it is shown that at least a 2-deg resolution is needed to resolve the western boundary currents adequately and to avoid undue distortion of the transient (Rossby waves. The -plane formulation is shown to be an adequate approximation for the mean circulation in the lower and middle latitudes, but noticeably intensifies the transports poleward of about 50 deg and both slows and distorts the transients in the central basin. The influence of the (southern) zonal boundary on the transport solutions is confined to the southernmost gyre, except in the region of the western boundary currents where its influence spreads to the northern edge of the basin by 30 days. The total boundary current transport is shown to be approximately proportional to the zonal width of the basin and independent of the basin's (uniform) depth, while the elevation of the free water surface is inversely proportional to the basin depth, in accordance with linear theory. The introduction of bottom friction has a marked damping effect on the transient Rossby waves, and also reduces the maximum boundary-current transport. The solutions throughout are approximately geostrophic and are only slightly nonlinear.The root-mean-square (rms) transport variability during the period 30 to 60 days is concentrated in the southwest portion of the basin through the reflection of the transient Rossby waves from the western shore and has a maximum corresponding to an rms current variability of about 3 cm sec–1. The transport variabilities are about 10 percent of the mean zonal transport and more than 100 percent of the mean meridional transport over a considerable region of the western basin (outside the western boundary current regime). Some 99 percent of the total kinetic energy is associated with the zonal mean and standing zonal waves, which are also responsible for the bulk of the meridional transport of zonal angular momentum. Although the transient Rossby waves systematically produce a momentum flux convergence at the latitude of the maximum eastward current, much in the manner of their atmospheric counterparts, this is only a relatively small contribution to the zonal oceanic momentum balance; the bulk of the mean zonal stress is here balanced by a nearly stationary net pressure torque exerted against the meridional boundaries by the wind-raised water. In an ocean without such boundaries the role of the transient circulations may be somewhat more important. 相似文献
10.
Westward intensification in marginal seas 总被引：1，自引：0，他引：1
An idealized model was used to examine why the strong western boundary current (WBC) is observed in the South China Sea (SCS) but not in the Gulf of Mexico (GOM) and Japan/East Sea (JES). Results suggested that the stronger WBC in the SCS is mainly attributed to the direct contribution of the inflow and the strong monsoon. Although the Gulf Stream transports a large amount of water into the GOM, the passage in the southeast corner guides the inflow out of the gulf and inhibits the inflow from intensifying the WBC. Meanwhile, the wind stress in the GOM is weakest among the three marginal seas. The meridional ocean ridge and the particular layout of the continental slope of JES prevent the whole basin from participating in the westward intensification. Besides, the throughflow has adverse effects on the formulation of WBC in JES. The variation of Coriolis parameter with latitude leads to the westward intensification in marginal seas. However, a strong WBC cannot be observed in the absence of reasonable collocation of wind, inflow, and topography. 相似文献
11.
12.
Jacques Verron 《地球物理与天体物理流体动力学》2013,107(1-4):257-276
Abstract In this paper we examine the behaviour of oceanic unsteady flow impinging on isolated topography by means of numerical simulation. The ocean model is quasigeostrophic and forced by an oscillatory mean flow. The fluid domain is of the channel type and open-boundary numerical conditions are used to represent downstream and upstream flow. In certain cases, vortex shedding, either cyclonic or anticyclonic, is observed in the lee of obstacles. Such shedding can be explained as the consequence of both an enhanced process of vorticity dissipation over the topography which locally affects the balance of potential vorticity on the advective timescale, and a periodic dominance of advective effects which sweep the fluid particles trapped on the seamount. For refined resolution and smallest viscosity the model will predict flows in which the shed eddies are coherent structures with closed streamlines. The model suggests a mechanism by which topographically generated eddies may be swept away from a seamount in the ocean. 相似文献
13.
Andrew F. Bennett 《地球物理与天体物理流体动力学》2013,107(1):113-142
AbstractThe flow in a mechanically driven thin barotropic rotating fluid system is analysed. The linear theory of Baker and Robinson (1969) is modified and extended into the non-linear regime.An internal parameter, the “local Rossby number”, is indicative of the onset of nonlinear effects. If this parameter is 0(1) then inertial effects are as important as Coriolis accelerations in the interior of the transport-turning western boundary layer and both of its Ekman layers. The inertial effects in the Ekman layers, ignored in previous explorations of non-linear wind driven oceanic circulation, are retained here and calculated using an approximation of the Oseen type. The circulation problem is reduced to a system of scalar equations in only two independent variables; the system is valid for non-small local Rossby number provided only that the approximate total vorticity is positive.To complete the solution for small Rossby number a boundary condition for the inertially induced transport is needed. It is found by examining the dynamics controlling this additional transport from the western boundary layer as the transport recirculates through the rest of the ocean basin. The strong constraint of total recirculation within the western boundary layer (zero net inertial transport) is derived.The calculated primary inertial effects are in agreement with the observations of the laboratory model of Baker and Robinson (1969).The analysis indicates the extent to which three-dimensional non-linear circulation can be reduced to a two dimensional problem. 相似文献
14.
Synoptic scale variability of the Southern Ocean wind field in the high-frequency range of barotropic Rossby waves results
in transport variations of the Antarctic Circumpolar Current (ACC), which are highly coherent with the bottom pressure field
all around the Antarctic continent. The coherence pattern, in contrast to the steady state ACC, is steered by the geostrophic
f/h contours passing through Drake Passage and circling closely around the continent. At lower frequencies, with interannual
and decadal periods, the correlation with the bottom pressure continues, but baroclinic processes gain importance. For periods
exceeding a few years, variations of the ACC transport are in geostrophic balance with the pressure field associated with
the baroclinic potential energy stored in the stratification, whereas bottom pressure plays a minor role. The low-frequency
variability of the ACC transport is correlated with the baroclinic state variable in the entire Southern Ocean, mediated by
baroclinic topographic–planetary Rossby waves that are not bound to f/h contours. To clarify the processes of wave dynamics and pattern correlation, we apply a circulation model with simplified
physics (the barotropic–baroclinic-interaction model BARBI) and use two types of wind forcing: the National Centers for Environmental
Prediction (NCEP) wind field with integrations spanning three decades and an artificial wind field constructed from the first
three empirical orthogonal functions of NCEP combined with a temporal variability according to an autoregressive process.
Experiments with this Southern Annular Mode type forcing have been performed for 1,800 years. We analyze the spin-up, trends,
and variability of the model runs. Particular emphasis is placed on coherence and correlation patterns between the ACC transport,
the wind forcing, the bottom pressure field and the pressure associated with the baroclinic potential energy. A stochastic
dynamical model is developed that describes the dominant barotropic and baroclinic processes and represents the spectral properties
for a wide range of frequencies, from monthly periods to hundreds of years. 相似文献
15.
John A. Johnson 《Continental Shelf Research》1982,1(2):143-157
Stratification is incorporated into an unsteady model of shelf currents by splitting the dynamic response of the flow into two parts, each with its own time scale. The barotropic part of the response is independent of depth and varies rapidly on a short time scale, whereas the baroclinic part depends on depth and changes slowly with time on a long time scale.The three-dimensional model has a continental shelf sloping down from an eastern boundary to the deep ocean. The equations for the barotropic component of the pressure field contain forcing by the wind stress and feedback from the baroclinic field. An integral of the heat equation over the long time scale determines the slow changes in the temperature field and hence in the baroclinic component of the velocity distribution.The temperature field is specified at the start of the numerical calculation. Its subsequent development is controlled by the numerical procedure. It is found that significant changes in the temperature field require a long period of upwelling favourable winds, whereas the longshore currents react more quickly to changes in the wind stress. 相似文献
16.
Array measurements of the bottom boundary layer and the internal wave field on the continental slope
AbstractAn array of current meters was placed on the continental slope and rise for two months in the autumn of 1970. The bottom boundary layer was penetrated on the slope. On the smallest array scale, of the order of 1 kilometer, the array functioned as a directional internal wave antenna. Moving shoreward, the current spectra show strong suppression of the inertial peak and strong enhancement of the semidiurnal tide. The measured wave number spectra show that the tidal energy is almost completely baroclinic, and probably being generated in the region where the slope becomes “critical” for the tidal period. If this area is typical of worldwide conditions, a substantial fraction of the dissipation of surface tides takes place on the continental slopes by conversion to baroclinic waves. The bottom boundary layer has been modeled by an extension of the work of Ellison (1956) to a sloping boundary in a fluid of positive stability. An equivalent constant eddy coefficient has the value 3 cm2/sec as determined from the measurements. 相似文献
17.
Based on eddy-permitting ocean circulation model outputs, the mesoscale variability is studied in the Sea of Okhotsk. We confirmed that the simulated circulation reproduces the main features of the general circulation in the Sea of Okhotsk. In particular, it reproduced a complex structure of the East-Sakhalin current and the pronounced seasonal variability of this current. We established that the maximum of mean kinetic energy was associated with the East-Sakhalin Current. In order to uncover causes and mechanisms of the mesoscale variability, we studied the budget of eddy kinetic energy (EKE) in the Sea of Okhotsk. Spatial distribution of the EKE showed that intensive mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. We revealed a pronounced seasonal variability of EKE with its maximum intensity in winter and its minimum intensity in summer. Analysis of EKE sources and rates of energy conversion revealed a leading role of time-varying (turbulent) wind stress in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk in winter and spring. We established that a contribution of baroclinic instability predominates over that of barotropic instability in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the simulated circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. In April, the mesoscale anticyclonic eddies are observed along the western boundary of the Sea of Okhotsk. The role of the sea ice cover in the intensification of the mesoscale variability in the Sea of Okhotsk was discussed. 相似文献
18.
This article presents a suite of long-term numerical simulations that investigate the dynamical mechanisms controlling the circulation in the South Brazil Bight (SBB). The overarching goal of these simulations is to quantify the relative contributions of local wind forcing and the Brazil Current (BC) to the upwelling of nutrient-rich slope water onto the shelf. The model results indicate that the water mass structure of the SBB is controlled by the synergy between wind-driven, inner-shelf upwelling and geostrophic, shelf-break upwelling. The later extends yearlong but the former peaks during the austral summer and decreases towards the winter. The interaction between the poleward flow of the BC and the bottom topography greatly influences the shelf circulation, particularly in the bottom boundary layer. Changes of the SBB coastline direction and shelf width modulate the along-shore pressure gradient and the magnitude of the shelf-break upwelling and downwelling. Thus, although the summer upwelling winds extend over large part of the SBB surface temperatures are warmer in the south because of the cooling effect of the shelf-break upwelling in the northern region. At difference with previous studies of shelf-break dynamics the shelf-break upwelling in our model is not controlled by the uplifting associated with the presence of instabilities of the boundary current or nonlinear accelerations under a variable shelf width. The proposed mechanism is relatively simple. As the boundary current flows along the continental slope, changes in the coastline orientation and along-shore bottom topography modify the along-shore pressure gradient which through geostrophy leads to inshore bottom flow and hence shelf-break upwelling. Such a mechanism can provide insight into upwellings on other western boundary current regions where similar topographic variations exist. 相似文献
19.
Rationalized by the observational circulation pattern in the upper ocean of the North Pacific, meridional friction term is
first incorporated in a barotropic theoretical model of the wind-driven circulation. The governing potential vorticity equation
thence has β term and wind stress curl term (the two of the Sverdrup balance), zonal friction term and meridional friction
term. The analytical solution satisfactorily captures many important features of the wind-driven circulation in the North
Pacific: Kuroshio, Oyashio, Kuroshio extension, North Equatorial Current, and especially the eastern boundary currents in
the North Pacific, i.e. California current and Alaska current. 相似文献
20.
A cyclonic gyre controls the advection of source waters into the formation areas of bottom water in the southern and western parts of the Weddell Sea and the subsequent transport of modified water masses to the north. Determination of the structure of the Weddell Gyre and of the associated transports was one of the objectives of the “Weddell Gyre Study” which began in September 1989 and ended in January 1993. The collected data set comprises records of moored current meters and profiles of temperature and salinity distributed along a transect between the northern tip of the Antarctic Peninsula and Kapp Norvegia. The circulation pattern on the transect is dominated by stable boundary currents of several hundred kilometers width at the eastern and western sides of the basin. They are of comparable size on both sides and provide nearly 90% of the volume transport of the gyre which amounts to 29.5 Sv. In the interior, a weak anticyclonic cell of 800 km diameter transports less than 4 Sv. Apart from the continental slopes, the near-bottom currents flow at some locations in an opposite direction to those in the water column above, indicating a significant baroclinic component of the current field. The intensity of the boundary currents is subject to seasonal fluctuations, whereas in the interior, time scales from days to weeks dominate. The large-scale circulation pattern is persistent during the years 1989 to 1991. The heat transport into the southern Weddell Sea is estimated to be 3.48×1013 W. This implies an equivalent heat loss through the sea surface of 19 W m−2, as an average value for the area south of the transect. The derived salt transport is not significantly different from zero; consequently, the salt gain by sea ice formation has to compensate almost entirely the fresh water gain from the melting ice shelves and from precipitation. Estimation of water mass formation rates from the thermohaline differences of the inflow and outflow through the transect indicates that 6.0 Sv of Warm Deep Water are transformed into 2.6 Sv of Weddell Sea Bottom Water, into 1.2 Sv of Weddell Sea Deep Water, and into 2.2 Sv of surface water. 相似文献