共查询到20条相似文献，搜索用时 15 毫秒
1.
The steady state bifurcation structure of the double-gyre wind-driven ocean circulation is examined in a shallow water model where the upper layer is allowed to outcrop at the sea surface. In addition to the classical jet-up and jet-down multiple equilibria, we find a new regime in which one of the equilibrium solutions has a large outcropping region in the subpolar gyre. Time dependent simulations show that the outcropping solution equilibrates to a stable periodic orbit with a period of 8 months. Co-existing with the periodic solution is a stable steady state solution without outcropping.A numerical scheme that has the unique advantage of being differentiable while still allowing layers to outcrop at the sea surface is used for the analysis. In contrast, standard schemes for solving layered models with outcropping are non-differentiable and have an ill-defined Jacobian making them unsuitable for solution using Newton’s method. As such, our new scheme expands the applicability of numerical bifurcation techniques to an important class of ocean models whose bifurcation structure had hitherto remained unexplored. 相似文献
2.
During October 2003 an intensive oceanographic survey (BIOMEGA) was carried out in the Alboran Sea, coinciding with a migration event of the Western Alboran Sea Gyre (WAG). The observations gathered during that cruise constitute the first field evidence of a migrated stage of the WAG. In this work we present the main differences between the 3D hydrodynamic fields observed during BIOMEGA and those corresponding to a WAG located at its usual position. The migration of the gyre was followed by satellite (altimetry and sea surface temperature) imagery. The causes of the gyre migration are explored in terms of the quasi-geostrophic tendency equation, in particular of the dynamics governing scales larger than the Rossby radius of deformation. It is shown that the steady state gyre must be almost equivalent barotropic and that the key process to break down the stationarity would be a density advection at gyre scale. The mechanisms to explain the migration of the WAG proposed by previous authors are discussed in light of the explanation proposed in this work. 相似文献
3.
长江是亚洲第一大河,巨大的迳流流入东海后,与外海水发生湍流混合,使本海区温盐结构随其发生变化.众所周知,某海区的盐度场结构及其变化是取决于该海区盐量收支的平衡规律.然而,考虑到长江冲淡水区是口股水系交汇的海区,所以影响盐度场的因子众多而复杂.运用统计相关分析,虽然能给出各因子和海水盐度分布的某种程度的相关关系,但对长江冲淡水的动力结构及其混合过程并未揭示.为此,本文想通过对夏季长江冲淡水远岸段的表层平均盐度分布,给出一个可能的数字模式.为进一步探讨长江冲淡水的动力结构及其混合过程提供一些启示,以期对该区的认识乃至预报有所裨益. 相似文献
4.
The Kuroshio Extension System: Its Large-Scale Variability and Role in the Midlatitude Ocean-Atmosphere Interaction 总被引：7，自引：0，他引：7
Bo Qiu 《Journal of Oceanography》2002,58(1):57-75
The Kuroshio Extension and its recirculation gyre form an interconnected dynamic system. The system is located at a crossroads
where the meso-scale and large-scale oceanic variability are highest, and where the ocean-atmosphere interaction is most active
in the Pacific Ocean outside of the tropics. Following a brief review of the mean flow and meso-scale eddy variability, this
study describes in detail the large-scale structural change (an oscillation between an elongated and a contracted state) observed
in the Kuroshio Extension system. Causes for this structural change are explored next, and it is argued that the basin-wide
external wind forcing and the nonlinear dynamics associated with the inertial recirculation gyre are both important factors.
Data analysis results are reviewed and presented, emphasizing that the surface Kuroshio Extension is not simply a well-mixed layer passively responding to heat flux anomalies imposed by the atmosphere. It is argued that large-scale
changes in the Kuroshio Extension system influence the surface ocean heat balance and generate wintertime sea surface temperature
(SST) anomalies through both horizontal geostrophic heat advection and re-emergence to the surface mixed layer of sequestered mode water temperature anomalies.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
The steady response of the ventilated thermocline to an increase in Ekman pumping is investigated, focusing on the effect
of the mixed layer depth distribution on the subsurface density anomaly. We consider only the subtropical gyre, and the mixed
layer is assumed to be deep in the northwest and shallow elsewhere with a narrow transition zone separating the deep and shallow
mixed layer regions. At the intersection of this narrow transition zone and the outcrop line, low potential vorticity fluid
is subducted into and ventilates the thermocline. In such a situation, an enhancement of the Ekman pumping confined to the
northern subtropical gyre leads to pronounced subsurface cold anomalies in the southern subtropics, which is free of anomalous
forcing. These density anomalies are much greater than those that occur when either the mixed layer depth is zonally uniform
or the Ekman pumping is enhanced in the whole subtropical gyre. They are caused by anomalous changes in the trajectory of
the low potential vorticity fluid in response to anomalous Sverdrup flow.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
《Coastal Engineering》1999,36(1):17-36
A time domain method is presented for analyzing simultaneous measurements of pressure and the horizontal components of velocity obtained beneath irregular multidirectional wave fields. This new method differs from the usual linear directional analyses applied to PUV data in two important aspects. First, the essential nonlinearity of the measured waves is not sacrificed to achieve a solution. Therefore, predictions of sea surface elevation and directional kinematics throughout the water column accurately portray the actual nonlinear character of the waves. Second, the analysis method is `local' in that it can be applied to segments of PUV time series much shorter than an individual wave. The viability of the locally nonlinear methodology developed in this paper is proven by demonstrating agreement with higher-order theoretical steady waves. Predictions of sea surface elevation and wave kinematics are also made using actual measurements from PUV instruments at two ocean sites off the west coast of the United States. 相似文献
7.
Some numerical experiments by the barotropic nonlinear two dimensional models are performed to study the water circulations in Lake Biwa, especially to study the large anticlockwise gyre in the north basin. The wind fields used in the experiments have no rotational component. This gyre is induced by the southerly wind and is approximately on the geostrophic balance. The vorticity of this gyre is contributed from the vertical stretching of the vortex tube by the variable bottom topography and the inertia term. But the latter term does not become effective if the bottom profile of the basin is flat. Therefore the horizontal circulation in the barotropic closed basin is the “topographic gyres”. The bottom stress has a little contribution to the vorticity balance of the gyre. 相似文献
8.
B. V. Khar’kov 《Oceanology》2007,47(2):161-171
The nonlinear dynamics of the low-frequency variability of a mid-latitude ocean are studied. The mechanism of the separation of the western boundary current from the western wall, as well as the meridional displacements of the separation point and the separated eastward jet, is analyzed. A regional barotropic quasigeostrophic eddy-resolving numerical model is used for the analysis. The flow in a rectangular domain is simulated by the constant inflow and outflow of fluid through the boundaries. A regime when the nonlinearity prevails over the dissipation and the advection and β terms are of the same order of magnitude is considered, which is characteristic of the actual ocean. When the nonlinearity exceeds the threshold value, a periodic solution is obtained. The solution is determined by the nonslip boundary condition at the western wall. The solution obtained is studied in detail. The meridional displacements of the western boundary current separation point with respect to the western wall and the separated eastward jet can reach a few hundred kilometers. Their intensities and the intensity of the recirculation gyre in the western boundary current are found to oscillate with a period of about five years. 相似文献
9.
渤海环流与输运季节变化的数值模拟 总被引：8，自引：0，他引：8
渤海的风和温度层结有明显的季节变化 ,因而其环流与输运亦有明显的季节信号。以季节平均的海面气象条件和开边界的潮波系统驱动三维斜压水动力模型———HAMSOM ,模拟了渤海冬、夏季的总环流。渤海环流冬强夏弱 ,表层风漂流常被下层逆风流所补偿。深度平均环流 ,即水柱内的输运 ,流型有显著的季节变化 :冬季在渤海中部沿逆时针方向旋转 ,辽东湾顶有一个顺时针流涡 ,阻碍了湾顶水与外海水的交换 ;夏季则为一个大的贴岸的顺时针流环 ,内嵌许多局地涡旋。这些与渤黄东海海洋水文图集中给出的多年观测的环流基本相同 ,同时也被水文要素分布及耐盐浮游动物的出现所佐证。风的季节变化决定了渤海大部分海区、特别是海峡附近环流的季节变化 ,但辽东湾东岸众多的岬角涡旋却不随季节变化 ,因为它们是由潮波系统与岬角岸型变化的非线性相互作用产生的。 相似文献
10.
Three-dimensional fully nonlinear waves generated by moving disturbances with steady forward speed without motions are solved using a mixed Eulerian–Lagrangian method in terms of an indirect boundary integral method and a Runge–Kutta time marching approach which integrates the fully nonlinear free surface boundary conditions with respect to time.A moving computational window is used in the computations by truncating the fluid domain (the free surface) into a computational domain. The computational window maintains the computational domain and tracks the free surface profile by a node-shifting scheme applied within it. An implicit implement of far field condition is enforced automatically at the truncation boundary of the computational window.Numerical computations are applied to free surface waves generated by Wigley and Series 60 hulls for the steady problem. The present numerical results are presented and compared with existing linear theory, experimental measurements, and other numerical nonlinear computations. The comparisons show satisfactory agreements for these hydrodynamic problems. 相似文献
11.
G. D. Aburjania 《Izvestiya Atmospheric and Oceanic Physics》2011,47(4):491-502
The generation and further linear and nonlinear dynamics of planetary magnetized Rossby waves (MRWs) in the rotating dissipative ionosphere are studied in the presence of a zonal wind (shear flow). MRWs are caused by interaction with the spatially nonuniform geomagnetic field and are ionospheric manifestations of ordinary tropospheric Rossby waves. A simplified self-consistent set of model equations describing MRW-shear flow interaction is derived on the basis of complete equations of ionospheric magnetohydrodynamics. Based on an analysis of an exact analytical solution to the derived dynamic equations, an effective linear mechanism of MRW amplification in the interaction with nonuniform zonal wind is ascertained. It is shown that operators of linear problems are non-self-adjoint in the case of shear flows, and the corresponding eigenfunctions are nonorthogonal; therefore, the canonically modal approach is of little use when studying such flows; a so-called nonmodal mathematical analysis is required. It is ascertained that MRWs effectively get shear flow energy during the linear stage of evolution and significantly increase (by several orders of magnitude) their energy and amplitude. The necessary and sufficient condition of shear flow instability in an ionospheric medium is derived. Nonlinear self-localization begins with the development of shear instability and an increase in the amplitude, and the process ends with the self-organization of strongly localized isolated large-scale nonlinear vortex structures. Thus, a new degree of freedom and a way for perturbation evolution to occur appear in medium with shear flow. The nonlinear systems can be a pure monopole vortex, a vortex streets, or vortex chains depending of the shape of the sheared flow velocity profile. The accumulation of such vortices in the ionospheric medium can produce a strongly turbulent state. 相似文献
12.
Mesoscale features in the eastward extension of the Kuroshio were investigated using assimilation of TOPEX/POSEIDON (T/P)
data into a three-layer quasi-geostrophic model. The T/P data exhibited an elongated state of the southern recirculation gyre
in 1993–95 and 1997, between whose two periods the gyre had a contracted state in 1995–96. A few stationary eddies were located
in the southern gyre during the contracted state. The baroclinic instability, which was indicated by the phase shift from
the uppermost-to the lowest-layer anomalies toward the downstream side, was evident near the Kuroshio Extension (KE) path.
Since the instability never appeared in the artificial model without bottom topography, the topographic barrier for the eastward
flow in the lowest layer was a necessary condition for the instability. The instability synchronized with the transition in
the western region of the KE axis from the elongated to the contracted states. This evolution was interpreted as if the baroclinic
instability played some part in the KE states and was a trigger for the transition from the elongated to the contracted states. 相似文献
14.
Thomas N. Delmer 《Ocean Engineering》1989,16(2)
The TAAS (Towed Acoustic Array Simulation) computer program, previously used to calculate the large-scale, three dimensional dynamic behavior of series-connected marine structures, is used here to simulate the reaction of a cable-towed weight to small-scale transverse sinusoidal oscillations superimposed upon steady towing. The reaction shows a linear component at the driving frequency and a quadratic, nonlinear component which produces a response at twice the driving frequency as well as systematic distortion. The amplitude and phase of the three components of the response are estimated at points distributed throughout the system. The results compare favorably with those from D.A. Chapman's linear analysis based on mathematical perturbation techniques. Our extension of Chapman's method to estimate the response at twice the driving frequency also compared well. We conclude that full simulation is an effective alternative to linear analysis; it also illuminates nonlinear behavior beyond the reach of traditional linear methods. We find that the nonlinear response of oscillating towed systems shows higher propagation speeds and less damping than the linear response and, consequently, may be significant. 相似文献
15.
16.
An updated Lagrangian finite element formulation of a three-dimensional annular section beam element is presented for large displacement and large rotation dynamic analyses of flexible riser structures. In this formulation a new linearization method is used to avoid inaccuracies normally associated with other linearization schemes. The effects of buoyancy force as well as steady state current loading are considered in the finite element solution for riser structures response. The formulation has been implemented in a nonlinear finite element code and the results are compared with those obtained from other schemes reported in the literature. 相似文献
17.
海流作用下海底管道局部冲刷数值分析 总被引：1，自引：0，他引：1
在任意拉格朗日-欧拉参考坐标系下,采用基于雷诺平均的Navier-Stokes方程组(RANS)、流线迎风有限元方法、泥沙输运模型以及底床变形方程,对海流引起的海底管道局部冲刷进行了数值模拟。着重讨论了均匀来流流速和海底管道直径对局部冲刷发展过程及平衡冲刷深度的影响作用。数值结果表明,在冲刷的初始阶段,冲刷深度随时间迅速增加,之后缓慢逼近极限平衡深度;在管径一定的情况下,管道附近的局部平衡冲刷深度与流速大致呈线性关系;当流速超过某一临界区域后,最大平衡冲刷深度出现的位置并不在管道正下方,而是随流速的增加向管道下游方向移动;另外,管道直径也会对平衡冲刷深度产生比较明显的影响,在相同流速下,平衡冲刷深度大致随管径呈线性增大。在本文的计算范围内,海底管道的相对局部平衡冲刷深度基本随雷诺数线性增加,但流速对冲刷深度的影响作用要比管径的影响作用更为明显。 相似文献
18.
Using an eddy-resolving two layer primitive-equation model forced by symmetric wind stress, we investigate the asymmetry of the recirculation of a double gyre (subpolar gyre and subtropical gyre). In the case where the layer thickness change is large, cyclonic recirculation becomes unstable and splits into meso-scale vortices more easily than anti-cyclonic recirculation in their developing stage. The subpolar gyre is, therefore, filled with more vortices than the subtropical gyre. Moreover, the effect of the upper layer potential vorticity on the lower layer potential vorticity in the subpolar gyre is stronger than that in the subtropical gyre. The characteristic of turbulence in the subpolar gyre is different from that found in the subtropical gyre and, therefore, the asymmetry of the recirculation of the double gyre is maintained by this difference. The asymmetry can not be produced in a quasi-geostrophic model because it ignores the nonlinearity associated with layer thickness change. Moreover, we investigate the effects of layer thickness and lateral viscosity on the asymmetry of the recirculation of the double gyre. In the case of realistic physical parameters, the asymmetry of the recirculation of the double gyre is noticeable from the view of the activities of the eddies. In the case with the shallowest upper layer, the position of separation of the western boundary current moves further southward. 相似文献
19.
A P - vector method is optimized using the variational data assimilation technique(VDAT). The absolute geostrophic velocity fields in the vicinity of the Luzon Strait (LS) are calculated, the spatial structures and seasonal variations of the absolute geostrophic velocity field are investigated. Our results show that the Kuroshio enters the South China Sea (SCS) in the south and middle of the Luzon Strait and flows out in the north, so the Kuroshio makes a slight clockwise curve in the Luzon Strait, and the curve is strong in winter and weak in summer. During the winter, a westward current appears in the surface, and locates at the west of the Luzon Strait. It is the north part of a cyclonic gyre which exits in the northeast of the SCS; an anti-cyclonic gyre occurs on the intermediate level, and it exits in the northeast of the SCS, and an eastward current exits in the southeast of the anti-cyclonic gyre. 相似文献
20.
A new analysis of nonlinear rolling carried out by the multiscale perturbation method is herewith presented. The behaviour of a ship in a regular beam sea is considered and approximate analytical solutions in three nonlinear resonance regions are obtained. These concern the transient and the steady state roll oscillations. The latter fits in well with a previous one obtained through the averaging method and with the results of the numerical simulation.The obtained results appear to be particularly convenient due to their major mathematical simplicity. Moreover, they allow a simple estimation of the maximum roll amplitudes predictable for a given excitation intensity. 相似文献