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1.
A barotropic model containing large-scale topography and zonal mean flow is established to discuss the effects of large-scale topography on the low-frequency waves. The results show that what affects low-frequency waves mostly is maximal height of topography and topographic slope. The former makes frequency of topographic Rossby waves decrease, the latter makes Rossby waves instable. Moreover, when topographic slope is appropriate, it can also make Rossby waves turn into low-frequency waves. 相似文献
2.
正压模式中大地形作用下的低频波 总被引：1，自引：0，他引：1
利用包含大地形和常数纬向基本气流的正压模式方程组,研究了大地形对低频波激发的作用,结果表明：起作用的地形因子主要是地形的最大高度和地形坡度,地形最大高度的作用主要是使Ｒｏｓｓｂｙ波趋向低频,而地形坡度的作用主要是对Ｒｏｓｓｂｙ波的稳定性决定作用。同时,适当地形坡度也可导致低频波的形成。 相似文献
3.
正压准地转模式中大地形作用下的低频波 总被引：2，自引：0，他引：2
利用包含大地形和常速纬向基本气流的正压准地转位涡度方程,研究了大地形对低频波激发的作用,结果表明,起作用的地形因子主要是地形的最大高度和地形坡度。地形最大高度的作用主要是使Rossby波趋向低频,而地形坡度的作用主要是对Rossby波的稳定性起决定作用,同时适当的地形坡度也可导致低频波的形成。而常速纬向基流在总体上的作用是使波动变得趋向低频,且西风基流更有利于低频（30～60天）波的形成。从纬向波数上看,纬向3波以上的波动更容易出现低频（30～60天）。 相似文献
4.
By using barotropic model equations, this article analyzed the characteristics of Rossby waves, the propagation features of wave energy and the influence of dynamic and thermal effects of the Tibetan Plateau on Rossby waves, and the focus is on discussing the plateau's topographic gradient effects on atmospheric Rossby waves. Then based on the WRF3.2 and the NCEP/NCAR FNL reanalysis data, we devised comparative tests of changing the plateau's topographic gradient and simulated a process of persistent heavy rain that happened in May 2010 in South China. The results are shown as follows. The Tibetan Plateau’s topography is conducive to the formation of atmospheric Rossby waves. while the plateau's terrain, its friction and heating effects can all make the atmospheric Rossby waves develop into the planetary waves; The effects of plateau's north and south slopes on the Rossby wave’ phase velocity is opposite, and when the slope reached a certain value can the quasi-steady normal fluctuations be generated; Simultaneously, due to the plateau's topographic gradient, descending motion appears at the west side of the plateau while ascending motion appears at the east side, and the vertical movement increased with the amplification of topographic gradients. The plateau's topographic gradient also obviously amplified the precipitation in South China, and the rainfall area increased with the amplification of topographic gradients and gradually moved from south to north and from west to east, which is conducive to the occurrence and development of convective activities in the downstream areas of the Tibetan Plateau; Moreover, for the plateau’s dynamic and thermal effects, the Rossby wave’ propagation shows upstream effects of energy dispersion, so the plateau can then affect the weather in downstream areas. Moreover, the wave group velocity increased with the degree of topographic slope. 相似文献
5.
The barotropic, quasi-geostrophic vorticity equation describing large scale, rotating flows over zonal relief supports nonlinear permanent form solutions, namely nonlinear topographic Rossby waves. Through an analytical theory, these solutions have been shown to be neutrally stable to infinitesimal perturbations.Numerical algorithms, which necessarily truncate the infinite number of degrees of freedom of any continuum model to a finite number, are capable of reproducing the numerical equivalent of these form-preserving solutions. Moreover, these numerical solutions are shown to preserve their shape throughout the numerical experiment not only in the limit of small amplitude, but also for high amplitude (Rossby number → O(1)).Through numerical simulation, the stability analysis is carried far beyond the analytical limit of infinitesimal perturbations. The solutions maintain their stability in agreement with the analytical theory, up to perturbations having intensities almost of the same order as the solutions themselves. For higher-amplitude perturbations, the solutions break up and typical turbulent behavior ensues. The passage from wave-like to turbulent behavior, upon surpassing a critical perturbation value, can be observed in the sudden loss of phase locking of the permanent solution Fourier modes. 相似文献
6.
The KdV equation with topography included in an N-level model is derived. It is shown that if the topography ex-ists. the KdV equation may describe the solitary Rossby waves in the case of basic current without vertical shear, and itis no necessary to introduce the MKdV equation. The results of calculations show that the change of horizontal shearpattern of basic flow may cause an important change of the streamline pattern of the solitary waves with the oddmeridional wavenumber m, and has no effect for the even meridional wavenumber m. The vertical shear increases thesteepness of the barotropic solitary modes, and it has a complicated effect on the baroclinic modes. The influences oftopographic slope on the solitary waves are very great. The southern and northern slopes of topography may cause dif-ferent solitary wave patterns, with the effect of northern slope greater. The effect of Froude number on the solitarywaves is generally to steepen the solitary waves, however, the effect also depends on the meridional wavenumber m andthe modes of solitary wave. 相似文献
7.
Nonlinear permanent form solutions have been found for the barotropic, quasi-geostrophic divergenceless vorticity equation describing large scale, rotating flows over zonal relief. In the linear limit these solutions are topographic Rossby waves. The analytical procedure is an expansion in two small dimensionless parameters, an amplitude parameter (the Rossby number) and the aspect ratio between North-South (cross-relief) and East-West length scales. Permanent form solutions exist when these two parameters, and the related effects of dispersion and nonlinearity, mutually balance. By the same expansion procedure, an analytical linearized stability theory has been formulated which proves the neutral stability of these solutions to infinitesimal, two-dimensional perturbations. 相似文献
8.
设计了一个热带赤道β-平面的两层海洋模式,在准长波近似下,应用最大截断模分析赤道波的基本形态,指出无论是正压模或斜压模Kelvin波、Rossby波及基本流所对应的“地形Rossby波”是最基本的波系,在基本流的一定切变条件下,它们之间可以耦合出一类不稳定波。在浅混合层近似和“快波近似”下,正压模和斜压模是可以分离的,因此可以分别分析它们的色散特征,由于它们的特征量不同,在同样波长(扰动的纬向尺度)下,扰动的增长率也不同,通过分析得出在一定参数下,斜压模扰动增长率为正压模的2倍。近似分析表明,混合层中流场的增长要快于温跃层,但温跃层的温度增长要比混合层明显。 相似文献
9.
An examination of current‐meter data gathered in 1967/68 on the continental shelf and slope off Nova Scotia has shown that meteorological forcing is an important source of energy. The response of currents to wind forcing is concentrated in a frequency band of 2.5 to 7 days. Daily mean currents of up to 25 cm/s appear to be associated with wind‐stress events. The highest correlations between wind and current are for the alongshore components of these variables. Wind‐induced currents may have been responsible for an intrusion of slope water onto the shelf which was observed in hydrographic sections from October and December 1968. Long data series (up to 167 days) formed by patching together shorter records demonstrate the existence of distinct low‐frequency variability at periods greater than 10 days. Some aspects of these motions suggest the presence of topographic Rossby waves on the shelf and slope. However, spatial and temporal coverage of data are not sufficient to define the sources of this variability. 相似文献
10.
《Dynamics of Atmospheres and Oceans》1998,28(2):107-125
Experiments are performed in a 13-m cylindrical tank to study the generation of interfacial internal waves by barotropic sinusoidal waves passing over a slope. At each tidal cycle, there are two waves generated, one propagating onshore and the other propagating offshore. The amplitude of the waves increases with increasing forcing and evolves as nonlinear waves if the shelf width is smaller than the wavelength of the baroclinic tide. Rotation does not modify the generating mechanism but the amplitude of the generated waves decreases with increasing rotation rate; also no internal waves are generated when the forcing period is larger than the inertial period, and at high rotation rate, there are only dispersive waves propagating from the shelf break region. The experiments covered a large range of internal Froude number, Rossby number and temporal Rossby number and compare well with in situ observations. 相似文献
11.
E. A. Zakharchuk A. S. Kudryavtsev V. N. Sukhachev 《Russian Meteorology and Hydrology》2014,39(2):100-108
The features of the spatiotemporal variability of the sea level in the North and Baltic seas during the periods of formation of major Baltic inflows are investigated using the analysis of satellite altimetry data. It is demonstrated that dramatic drops in the sea level between the Baltic and North seas are observed during a few weeks before major inflows. A process of intensive inflows of the North Sea water to the Baltic Sea is accompanied not only by horizontal motions but also by vertical ones manifested in the increase in convergent flows in the North Sea and divergent flows in the Baltic Sea. A pronounced feature of the low-frequency dynamics of water of the North and Baltic seas is its wave structure. In both seas, low-frequency waves with the periods of 14–36 days propagate with the eastern component of the phase velocity along the isobaths and are identified as barotropic topographic Rossby waves. Phase velocities and lengths of low-frequency waves in the Baltic Sea are smaller by several times than those in the North Sea. Using the data of the analysis of meteorological information, a resonance-wave mechanism of generation of major Baltic inflows is studied. 相似文献
12.
Laboratory experiments concerning the nature of density fronts in a two-layer fluid in the vicinity of a continuous ridge were conducted. The experiments were carried out in a circular rotating test cell containing an annular ridge of uniform cross-section. The density fronts were established by releasing a lighter fluid contained in a bottomless cylinder in the interior of the region defined by the topography into a heavier fluid occupying the rest of the test cell. The system was also equipped with an oscillating plunger located along the test cell axis to produce simulated tidal currents impinging in the normal direction on the ridge; experiments without and with tidal forcing were conducted. The governing parameters for the physical system considered are the Rossby, temporal Rossby, Burger and Ekman numbers and geometrical parameters. It is found that for both the non-forced and tidally forced experiments the fronts were stabilized by the ridge. The fronts in the simulated tidal currents experiments were found to advect radially outward more rapidly at early times than their non-forced counterparts; at large times, the temporal evolution of the front for these forced experiments approached that of the non-forced experiments. In the region interior to the annular ridge, the motion field is highly baroclinic, while outside this region, the flow response at the forcing frequency is barotropic. Scaling arguments regarding frontal position, viscous decay and barotropic oscillatory flow responses are advanced and supported by experimental observations. 相似文献
13.
We have determined free Rossby waves in the North Pacific Current by numerical methods. We have found only two stable solutions — the barotropic and first-order baroclinic Rossby shear modes. The influence of the current on the dispersion features of these waves is small for the barotropic shear mode, but is significant for the baroclinic shear mode. An explicit comparison of the dispersion relations for the baroclinic wave in case of vanishing and non-vanishing current is given. We have found at most one unstable solution per wave number. The unstable wave with largest growth rate has an e-folding time of 1.1 year. We have calculated vertical profiles of the stream function and the temperature for the various shear modes at various wave numbers. The temperature shear modes have been calculated for later usage in a Rossby wave model to be fitted to observed temperature data from the North Pacific Current area. 相似文献
14.
Howard J. Freeland 《大气与海洋》2013,51(3):288-302
Abstract The effect of an abrupt headland on a barotropic oceanic boundary current with variable bottom topography is investigated. The objective is to explore with a very simple model some of the observed features of flow past Brooks Peninsula, an obstacle to boundary currents on the west coast of Vancouver Island. It is shown that the seasonal variation in the background current field causes a large change in the response to the headland. The difference is both quantitative and qualitative and results from the ability of southward alongshore flows to support topographic Rossby lee waves. As a result of the presence of the lee waves a strong offshore flow occurs just downstream of the Peninsula and this ejects water from the continental shelf into deep water producing features reminiscent of the so‐called “squirts” and “jets”. 相似文献
15.
In this study we investigate which conditions are needed to assure uniqueness of solutions of the barotropic QG model, and, how these conditions are related to the conservation laws of mass, vorticity and energy. Uniqueness and conservation laws are analyzed for a simply connected domain (a closed basin) and for a doubly connected domain (a periodic or re-entrant channel).For the multiply connected domain we find, besides the model proposed by McWilliams in 1977, another consistent model whose solutions satisfy the conservation laws. The additional conditions in our model are: (a) the sum of the circulations around all closed solid walls is time-independent; (b) the value of the QG streamfunction (Φ) is the same at all closed walls. McWilliams' model and ours are not equivalent.As a simple application we study the free Rossby normal modes in a channel. For a non-zonal channel on a β-plane there are solutions (modes) that are independent of the coordinate along the channel. These are used to compare the modes and frequencies obtained from three different, but well posed, models. Solutions that are independent of the along-channel coordinate do not exist for the planetary geostrophic and for the shallow water equations on a β-plane. 相似文献
16.
It is shown how symmetric dipolar vortices can be formed by the action of an impulsive jet in a homogeneous single layer of fluid in a rotating tank. These dipoles are allowed to interact with a constant topographic slope, which can model a β-plane or a continental shelf. A dipole's trajectory bends toward the right when climbing a slope and to the left when descending, as predicted by numerical simulations and analytical arguments. The maximum penetration of the dipoles over a slope, the adjustment to the slope, and formation of trailing lobes are compared with both numerical simulations and a two-point vortex model. The results suggest that Rossby wave radiation plays an important role in the interaction process. 相似文献
17.
A barotropic channel model in β-plane is used to study the effect of topographic forcing on the formation and maintenance of blocking. The approximate analytical solution of potential vorticity equation can show the main property of the whole process of blocking. It is indicated that the topographic forcing is one of the main factors caus-ing the blocking process. The results suggest that the nonlinear interaction plays a very important role in the stable “Ω” situation of blocking. The atmospheric circulation with periodic and low-frequency oscillation, perhaps, is partly caused by topographic forcing. 相似文献
18.
Abstract The eddy flux of a conservative scalar in a time‐dependent rotary velocity field may have a component that is normal to the scalar gradient. This component is the “skew flux”, which consists of the scalar transport by the Stokes velocity and a part that is always non‐divergent (and hence does not affect scalar evolution). Since tidal velocity fields usually have rotary features, tidal‐band eddy scalar fluxes may include a skew component that can be useful in indicating the occurrence of non‐linear current interactions. The skew temperature flux associated with the semidiurnal tide in a continental shelf region is demonstrated using simple models, and moored current and temperature observations from Georges Bank. The observed fluxes on the Bank are largely directed along isobaths, with apparent contributions from the topographic rectification of the barotropic tidal current over the Bank's side and from the rotary tidal ellipses in a frontal region. Simple models indicate that the weaker cross‐isobath fluxes can arise through the influence of frictionally induced vertical structure on topographic tidal rectification, a baroclinic tidal current interaction, or the interaction of baroclinic and barotropic tidal currents. In some cases, the simple models show qualitative agreement with the observed fluxes and currents but, in general, more realistic models and better estimates of the background mean temperature field are required to obtain quantitative estimates of the relative importance of these interactions and other processes. Nevertheless, the observations and models suggest that non‐linear interactions involving both barotropic and baroclinic tidal currents are occurring on Georges Bank. 相似文献
19.
本文进一步对旋转正压大气中的包络Rossby孤立波进行了研究，结果发现当Rossby波的波数m（m为纬向波数）满足1≤m≤2时，旋转正压大气中才存在包络Rossby孤立波，并且只有m＝2的包络Rossby孤立波才具有阻塞高压的结构。我们还对这种m＝２的包络Rossby孤立波随纬度变化的持续性进行了计算，得到了许多结果。 相似文献
20.
The boundary currents over the Western Australian continental shelf and slope consist of the poleward flowing Leeuwin Current (LC) and the equatorward flowing Leeuwin Undercurrent (LUC). Key properties of the LC are its poleward strengthening, deepening to the south, and shelfbreak intensification. The alongshore flow reverses direction below about 300 m, forming the LUC at greater depths. To investigate the processes that cause these features, we obtain solutions to an idealized, regional ocean model of the South Indian Ocean. Solutions are forced by relaxing surface density to a prescribed, meridionally varying density profile ρ*(y) with a timescale of δt. In addition, vertical diffusion is intensified near the ocean surface. This diffusion establishes the minimum thickness over which density is well-mixed. We define this thickness as the “upper layer”. Solutions are obtained with and without a continental shelf and slope off Western Australia and for a range of values of δt and mixing parameters. Within this upper layer, there is a meridional density gradient that balances a near-surface, eastward geostrophic flow. The eastward current downwells near the eastern boundary, leading to westward flow at depth. The upper layer's meridional structure and zonal currents crucially depend on coastal processes, including the presence of topography near the eastern boundary. Kelvin waves inhibit the upper layer from deepening at the coast. Rossby waves propagate the coastal density structure offshore, hence modifying the interior currents. A comparison of the solutions with or without a continental shelf and slope demonstrate that topographic trapping of Rossby waves is a necessary process for maintaining realistic eastern boundary current speeds. Significant poleward speeds occur only onshore of where the upper layer intersects the slope, that is, at a grounding line. Its poleward transport increases when surface-enhanced vertical mixing is applied over a greater depth. When the timescale δt is sufficiently short, the poleward current is nearly barotropic. The current's spatial structure over the shelf is controlled by horizontal mixing, having the structure of a Munk layer. Increasing vertical diffusion deepens the upper layer thickness and strengthens the alongshore current speed. Bottom drag leads to an offshore flow along the bottom, reducing the net onshore transport and weakening the current's poleward acceleration. When δt is long, poleward advection of buoyancy forms a density front near the shelf break, intensifying poleward speeds near the surface. With bottom drag, a bottom Ekman flow advects density offshore, shifting the jet core offshore of the shelf break. The resulting cross-shelf density gradient reverses the meridional current's direction at depth, leading to an equatorward undercurrent. 相似文献