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1.
Numerical experiments with a multi-level general circulation model have been performed to investigate basic processes of westward propagation of Rossby waves excited by interannual wind stress forcing in an idealized western North Pacific model with ocean ridges. When the wind forcing with an oscillation period of 3 years is imposed around 180°E and 30°N, far from Japan, barotropic waves excited by the wind can hardly cross the ridges, such as the Izu-Ogasawara Ridge. On the other hand, a large part of the first-mode baroclinic waves are transmitted across the ridges, having net mass transport. The propagation speed of the first-mode baroclinic wave is accelerated (decelerated) when an anticyclonic (cyclonic) circulation is formed at the sea surface, due to a deeper (shallower) upper layer, and to southward (slightly northward) drift of the circulation. Thus, when the anticyclonic circulation is formed on the northern side of the cyclonic one, they propagate almost together. The second-mode baroclinic waves converted from the first-mode ones on the ridges arrive south of Japan, although their effects are small. The resulting volume transport variation of the western boundary current (the Kuroshio) reaches about 60% of the Sverdrup transport variability estimated from the wind stress. These characteristics are common for the interannual forcing case with a longer oscillation period. In the intraseasonal and seasonal forcing cases, on the other hand, the transport variation is much smaller than those in the interannual forcing cases. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

2.
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   

3.
In order to fulfill the no-slip condition at the western and eastern boundaries of the ocean basin, introduced ＂effective wind stress＂, which has much larger spatial variations towards the boundaries than in the ocean interior. The effective wind stress can thus be decomposed into spatially slow-varying and fast varying components. Careful scale analysis on the classical Munk winddriven ocean circulation theory, which consists of the interior Sverdrup flow and the western boundary current but of no eastern boundary current, shows that the wind stress curl appearing in the Sverdrup equation must have negligible spatial variations. In the present model the spatially slow-varying component of the wind stress appears in the Sverdrup equation, and the spatially fastvarying component becomes the forcing term of the boundary equations. As a result, in addition to the classical Munk solution the present model has an extra term at the western boundary which （Northern Hemisphere） increases the northward transport as well as the southward return transport, and has a term at the eastern boundary corresponding to the eastern boundary current.  相似文献   

4.
On the assumption that motions of the barotropic mode are horizontally nondivergent, action of the wind stress with longshore variation on a two-layer ocean adjacent to the meridional east coast is studied. Only the equatorward wind stress is considered. Along the east coast, upwelling is induced by the direct effect of the coast and is confined in a narrow strip with the width of the order of the internal radius of deformation. The upwelling propagates poleward with the internal gravity wave speed. Coastal upwelling induced by the wind stress with longshore variation may be interpreted as the generation and propagation of internal Kelvin waves. Associated with the coastal upwelling, the equatorward flow in the upper layer and the poleward flow in the lower layer are formed as an internal mode of motions. When the bottom topography with the continental shelf and slope is taken into account, occurrence of the poleward undercurrent is delayed by a few days because of the generation of continental shelf waves. And, after the forcing is stopped, the shelf waves propagate poleward away from the upwelling region and the poleward undercurrent fully develops. At the margin of the continental shelf, another upwelling region is induced and propagates poleward.  相似文献   

5.
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   

6.
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   

7.
On the general ocean circulation forced by the asymmetric wind stress curl, the role of the eddies which are detached from the western boundary current is studied using an eddy-resolving two-layered quasi-geostrophic numerical model with free-slip boundary condition. An ideal sinusoidal function is used as the wind stress curl, and amplitude is assumed to be larger over the southern basin than over the northern one. In contrast with the antisymmetric wind forcing, in the asymmetric wind stress case, the subtropical western boundary current overshoots to the north from the zero wind stress curl line. As the asymmetricity of the wind forcing becomes larger, the separation point of the time mean field is located further north. The eddies generated in the region of the subtropical recirculation are advected northward by the western boundary current and they are detached from subtropical gyre. The release of these eddies to the north basin leads to weaken the subtropical recirculation system. From the analysis of the potential vorticity budgets, in the asymmetric case, it is shown that detached eddies play an important role in transporting the negative vorticity which is excessively inputted into the southern basin, to the northern basin, in addition to the terms which transport vorticity in the antisymmetric case, i.e., the vorticity transport by the meander of the jet. Under the free-slip boundary, more than a quarter of that excess vorticity is transported by those detached eddies in some cases.  相似文献   

8.
The generation process of internal waves by strong tidal flow over a continental shelf slope is reproduced using a multi-level numerical model. On the basis of the numerical results, the crucial role of the tidal advection effect in the generation process of internal waves is demonstrated. The close relation between the resulting internal waveform and the strength of the tidal advection effect is also examined. The barotropic forcing on the internal wave actually works within a relatively small horizontal scale over the top of the continental shelf slope. When the maximum internal Froude number at the shelf break (Frm) is less than about 0.6, the amplitude of the resulting internal wave is almost proportional to Frm. When Frm is more than about 0.6, however, the amplitude of the resulting internal wave becomes larger than predicted by linear theory. In particular, when Frm is more than unity, the time period during which the shoreward propagating internal wave stays in the barotropic forcing region becomes much longer. Consequently, the internal wave is significantly amplified with the horizontal scale approaching that of the barotropic forcing, which concentrates in a relatively small region over the top of the continental shelf slope. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
Horizontal and meridional volume transports on timescales from intra-seasonal to interannual in the North Pacific subarctic region were investigated using a reanalysis dataset for 1993–2001 that was constructed from an assimilation of the TOPEX altimeter and in situ data into an eddy-permitting North Pacific ocean general circulation model. The barotropic flow is excited along east of the Emperor Seamounts by the western intensification dynamics. The volume transport of this flow compensates for that across the interior region east of the Seamounts below the summit depth of the Seamounts. The Oyashio, which is also considered as a compensation flow for the transport in the whole interior region, includes baroclinic as well as barotropic components. Baroclinic transports in the whole interior region exceed those in the western boundary region in the upper (200–1000 m) and lower (2000–5000 m) layers, and the total transport is northward (southward) in the upper (lower) layer. These excesses of the baroclinic transport are balanced by a vertical transport of the meridional overturn. The meridional overturn has a complementary relation to the basin-scale baroclinic circulation in the North Pacific subactic region. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
结合南沙群岛海域出现的内潮现象 ,利用一个两层数值模式 ,通过数值试验来探讨潮汐性质、水体的层结程度、跃层所在的深度、风应力等因子对陆架坡折处内潮特性的影响 ,结果表明 :内潮的波动周期只与海区的潮汐性质有关 ;风应力、非线性效应以及正压潮源地的深度等都可改变内潮的波动振幅 ;内潮的波动振幅随海区的水体层结程度及温跃层所在的深度不同而改变  相似文献   

11.
To investigate an mechanism of the seasonal variation of transport through the Tokara Strait, two numerical experiments with real geometry and wind forcing were carried out. The models are linear barotropic models which are a North Pacific Ocean model and a limited-area model with a fine grid. The seasonal variation of volume transport with a maximum in the summer and a minimum in the autumn could be well reproduced by both models. The results demonstrate the wind stress component normal to a gradient vector of bottom topography is crucial for determining the seasonal variation. The similar seasonal variation widely covers the East China Sea and has a large amplitude near the Tokara Strait. Finally, it can be concluded that winds north of 35°N have little influence on the seasonal response of our model at the Tokara Strait.  相似文献   

12.
Semidiurnal tides, and especially the lunar tide M2, are dominant dynamics in the Bay of Biscay. Strong tidal currents are associated with the presence of a significant continental slope. By combining Newton's gravitation laws and Euler's equations, Laplace's equations contain the astronomical forcing responsible for the observed semidiurnal tides. In shallow waters, this direct forcing is often neglected. We study here its influence on the tidal dynamics over the continental slope through the development of a simple model describing the barotropic semidiurnal dynamics on a transect perpendicular to the slope. This new model results from the combination of two different models, i.e. the one developed by Rosenfeld and Beardsley (1987), which takes into account the tide-generating force, and that of Battisti and Clarke (1982), which neglects it. A first model is developed by neglecting the direct astronomical forcing in equations: it consists in solving a second-order homogeneous propagation equation for the barotropic semidiurnal tide and needs only coastal conditions as well as the knowledge of the along-slope wave number of the solution. For a mean slope typical of the South Brittany area, this non-forced model provides results in accordance with those of Battisti and Clarke and Le Cann (1990): in particular, in the upper part of the slope, it shows a polarization inversion of tidal ellipses characteristic of the tidal dynamics observed in this area. Then, the direct astronomical forcing is kept in equations. The simple model developed without this forcing is fitted in order to solve the resulting forced propagation equation for the barotropic tide. The solution of this second model is the sum of a forced wave responding to the direct astronomical forcing and of a free wave generated at the coastal boundary. Under the same boundary conditions, the results obtained with the influence of the tide-generating force are then compared with those obtained without it. This comparison allows one to apprehend the importance of the direct astronomical forcing on tidal dynamics across the slope: in particular, the main difference appears in deep waters where this forcing induces a phase-lag between the plain and the shelf for the sea-surface slope.  相似文献   

13.
A numerical experiment is made using a barotropic model for the western boundary currents. The time-dependent, non-linear vorticity equation is integrated with and without the variable of bottom topography. The inertial and frictional boundary flow is resolved with a fine grid size of 10 km. Connection of the western boundary currents with the general circulation is facilitated by giving the fixed Sverdrup transport at the eastern boundary of the model (400 km offshore).For the flat bottom topography, steady flow forRe=35 shows dynamical balance essentially of a frictional model. The transient response leading to the formation of the western boundary currents in the model seems to support theLighthill's theory (1969). ForRe=350, unsteady features revealed byBryan (1963) is re-established. A phenomenon of barotropic instability is also observed with sufficient resolution. For the model with a continental slope the steady flow is also obtained forRe=35. The boundary currents flow over the continental slope, deviating offshore as they flow northward.  相似文献   

14.
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   

15.
利用一个全球海洋环流模式在3组风应力资料的强迫下模拟分析了副热带太平洋向热带太平洋密跃层水量输送的年际变化特征及其和风应力的关系,并设计数值试验,研究了密跃层水量输送的变化机制.结果表明,副热带太平洋向赤道太平洋的密跃层水量输送具有显著的年际变化.在年际时间尺度上,南北太平洋西边界密跃层水量输送都起着补偿内部路径输送的...  相似文献   

16.
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   

17.
本文利用南海海洋再分析产品REDOS(Reanalysis Dataset of the South China Sea)和风场资料CCMP(Cross-Calibrated,Multi-Platform),通过能量诊断探讨了越南沿岸南海西边界流(南海贯穿流主体部分)区域夏季(6—9月)涡流相互作用的年际变化特征以及平均流对中尺度过程的贡献。结果显示,在季风和西边界强流、南海贯穿流的共同影响下,越南沿岸东向急流和双涡结构的能量分布和收支有显著的年际差异。尽管涡动能(EKE,Eddy Kinetic Energy)和涡动有效势能(EPE,Eddy available Potential Energy)的量级基本一致,但二者在水平和垂向空间分布上存在明显差异,这与夏季风影响下的南海西部边界流,越南离岸流的上层海洋密度梯度、流速大小和剪切导致的斜压、正压不稳定性等因素相关。同时随着深度的增加,密度梯度变化相对水平速度剪切对海洋涡流过程的影响逐渐凸显。EKE能量收支分析表明,压强与风应力主要做正功,是维持EKE稳定的主要能量来源,而EKE平流项既可以促进涡旋的增长,也会造成涡旋的消耗,对EKE的年际变率影响比较显著。正压不稳定导致的能量转换主要影响南海西部边界流区域,并存在显著年际变化,并且在风和平均流的影响下,沿贯穿流方向存在显著空间分布差异。越南离岸流正异常年,整体呈现平均流向涡旋传递能量;负异常年,出现EKE反哺平均动能的情况。  相似文献   

18.
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   

19.
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   

20.
The effects of intra-seasonal wind forcing on the mean field of the tropical Pacific Ocean has been studied using an ocean general circulation model (GCM). Idealized intra-seasonal zonal wind forcing with zero mean, which propagates eastward, induces net eastward jets at the equator that shift the warm water pool to the east. The mean temperature of the upper 200 m of the ocean increases off the equator and decreases at the equator. The change is independent of the propagation speed of the intra-seasonal wind forcing. The magnitude of the change depends on the amplitude and the period of the forcing, and the ocean structure, while the spatial pattern is independent of these parameters. A simple shallow water model is used to explain these changes. It is found that the term responsible for the enhanced eastward Equatorial jet is the Reynolds stress term, which arises from a phase shift of the zonal current due to friction. The resultant convergence of eastward momentum on the equator and geostrophic adjustment of the interface to the change of zonal current brings about the thermal redistribution of the upper ocean seen in the GCM.  相似文献   

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