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1.
Through a suite of three-dimensional, high-resolution numerical modeling experiments, we examine the role of nonhydrostatic effects on O(1 km) submesoscale processes at ocean fronts, with particular focus on the vertical velocity field. Several differences between nonhydrostatic and hydrostatic models are pointed out using a framework that enables precise comparison, but it is difficult to identify categorical differences between the model solutions at the grid resolutions afforded. The instantaneous vertical velocity structure is sensitive to the model choice and, even more so, to grid resolution, but the average vertical flux is similar in both hydrostatic and nonhydrostatic cases.When a frontal region with horizontal density gradients is perturbed by wind, a profusion of submesoscale, O(1 km), secondary circulation features develops in the upper 50 m. Narrow, elongated cells of intense up- and down-welling are found to occur close to the surface, overlying broader regions of weaker up- and down-welling associated with the mesoscale meanders of the baroclinically unstable front. The submesoscale down-welling is considerably stronger than up-welling and is concentrated in 1–2 km width filaments within which velocities can attain magnitudes as high as 200 m day−1. The submesoscale features are found to be robust at horizontal grid resolutions varying between 1 and 0.25 km and exist even in the hydrostatic model. Submesoscale circulation is difficult to observe or resolve in coarser resolution circulation models, but is likely to play a significant role in the exchange of energy and properties between the surface ocean and thermocline. Possible mechanisms for the generation of these features are investigated in a follow-on paper. 相似文献
2.
在南大洋印度洋扇区中部海域,除了地形控制(凯尔盖朗高台),南极绕极流和厄加勒斯回流的汇合流进一步加强了下游的斜压剪切强度,导致涡旋能量显著增强,因此,对该海域涡旋的研究有助于了解该海域的涡旋特征以及地形与涡旋的分布关系。基于2005~2019年卫星遥感数据,对该海域涡旋特征进行统计,并对涡旋产生地分布、跨锋面涡旋的移动状况进行分析,同时结合Argo剖面数据,进一步剖析涡旋内部水文分布特征。结果表明:该海域涡旋生命周期多在20 d以内(64.25%),平均半径多在30~100 km(96.13%);平均半径与平均振幅呈正相关关系(相关系数R=0.55);生命周期越大的涡旋平均传播距离也越大。2014年开始涡旋数量明显增加,主要由短寿命涡旋(<30 d)数量增加所贡献。反之, 21世纪10年代后期年平均涡动能异常呈减小趋势。涡旋产生地随着寿命增长,逐渐从亚南极锋与南极绕极流南部边界之间的锋面区域向亚南极锋以北移动。跨锋面涡旋中,暖涡向高纬,冷涡向低纬移动,大部分具有携带水团移动的能力。由涡旋内部水文特征分析结果可知,不同极性的涡旋能够实现完全不同来源水团的远距离输送,对同一来源水团,气旋涡具有抬升作用,而反气旋涡具有压沉作用。该研究工作有助于提升对南大洋涡旋特征及变动的认识,为进一步的涡旋动力研究提供支撑。 相似文献
3.
An algorithm is proposed for solving three-dimensional ocean hydrodynamics equations without hydrostatic approximation and traditional simplification of Coriolis acceleration. It is based on multicomponent splitting of the modified model with artificial compressibility. The original system of equations is split into two subsystems describing the transport of three velocity components and adjustment of the density and velocity fields. At the adjustment stage, the horizontal velocity components are represented as a sum of the depth means and deviations; the two corresponding subsystems are derived. For barotropic dynamics, the compressibility effect is represented as the boundary condition at the free surface, while for the baroclinic subsystem, it is introduced as ε-regularization of the continuity equation. Then, the baroclinic equations are split into two subsystems describing the hydrostatic and nonhydrostatic dynamics. The nonhydrostatic dynamics is computed at a separate splitting stage. The algorithm is included into the Institute of Numerical Mathematics of the Russian Academy of Sciences model based on “primitive” equations and verified by solving the hydrodynamics problem for the Sea of Marmara. 相似文献
4.
Spatial scales of mesoscale eddies from GOCI Chlorophyll-a concentration images in the East/Japan Sea 总被引：2，自引：0，他引：2
The spatial scales of mesoscale eddies are of importance to understand physio-biogeochemical processes in the East/Japan Sea. Chlorophyll-a concentration images from the Geostationary Ocean Color Imager (GOCI) revealed numerous eddies during the phytoplankton bloom in spring. These eddies were manually digitized to obtain geolocation information at the peripheries from GOCI images and then least-square fitted to each ellipse. The elliptic elements were the geolocation position of the eddy center, the rotation angle from due east, the eccentricity, the lengths of the semi-major and semi-minor axes, and the mean radius of the ellipse. The spatial scales of eddies had a mean radii ranging from 10 km to 75 km and tended to be smaller in the northern region. The scales revealed a linear trend of about ?7.26 km/°N as a function of the latitude. This tendency depended on the latitudinal variation of the internal Rossby radius of deformation, which originates from the substantial difference in the density structure of the water column. The scales from the sea surface temperature image were larger by 1.30 times compared to those from ocean color image. This implies that physical processes along the periphery of the eddy affect the nutrient dynamics. 相似文献
5.
Mesoscale Eddies Observed by TOLEX-ADCP and TOPEX/POSEIDON Altimeter in the Kuroshio Recirculation Region South of Japan 总被引：1，自引：0，他引：1
Mesoscale eddies in the Kuroshio recirculation region south of Japan have been investigated by using surface current data measured by an Acoustic Doppler Current Profiler (ADCP) installed on a regular ferry shuttling between Tokyo and Chichijima, Bonin Islands, and sea surface height anomaly derived from the TOPEX/POSEIDON altimeter. Many cyclonic and anticyclonic eddies were observed in the region. Spatial and temporal scales of the eddies were determined by lag-correlation analyses in space and time. The eddies are circular in shape with a diameter of 500 km and a temporal scale of 80 days. Typical maximum surface velocity and sea surface height anomaly associated with the eddies are 15–20 cm s–1 and 15 cm, respectively. The frequency of occurrence, temporal and spatial scales, and intensity are all nearly the same for the cyclonic and anticyclonic eddies, which are considered to be successive wave-like disturbances rather than solitary eddies. Phase speed of westward propagation of the eddies is estimated as 6.8 cm s–1, which is faster than a theoretical estimate based on the baroclinic first-mode Rossby wave with or without a mean current. The spatial distribution of sea surface height variations suggests that these eddies may be generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region, though further studies are needed to clarify the generation processes. 相似文献
6.
Mesoscale eddies play vital roles in ocean processes. Although previous studies focused on eddy surface features and individual three-dimensional (3D) eddy cases in the northwestern Pacific Ocean, the analysis of unique eddy 3D regional characteristics is still lacking. A 3D eddy detection scheme is applied to 9 years (2000–2008) of eddy-resolving Regional Ocean Modeling System (ROMS) output to obtain a 3D eddy dataset from the surface to a depth of 1 000 m in the northwestern Pacific Ocean (15°–35°N, 120°–145°E). The 3D characteristics of mesoscale eddies are analyzed in two regions, namely, Box1 (Subtropical Countercurrent, 15°–25°N, 120°–145°E) and Box2 (Southern Kuroshio Extension, 25°–35°N, 120°–145°E). In Box1, the current is characterized by strong vertical shear and weak horizontal shear. In Box2, the current is characterized by the strong Kuroshio, topographic effect, and the westward propagation of Rossby waves. The results indicate the importance of baroclinic instability in Box1, whereas in Box2, both the barotropic and baroclinic instability are important. Moreover, the mesoscale eddies’ properties in Box1 and Box2 are distinct. The eddies in Box1 have larger number and radius but a shorter lifetime. By contrast, Box2 has fewer eddies, which have smaller radius but longer lifetime. Vertically, more eddies are detected at the subsurface than at the surface in both regions; the depth of 650 m is the turning point in Box1. Above this depth, the number of cyclonic eddies (CEs) is larger than that of anticyclonic eddies (AEs). In Box2, the number of CEs is dominant vertically. Eddy kinetic energy (EKE) and mean normalized relative vorticity in Box2 are significantly higher than those in Box1. With increasing depth, the attenuation trend of EKE and relative vorticity of Box1 become greater than those of Box2. Furthermore, the upper ocean (about 300 m in depth) contains 68.6% of the eddies (instantaneous eddy). Only 16.6% of the eddies extend to 1 000 m. In addition, about 87% of the eddies are bowl-shaped eddies in the two regions. Only about 3% are cone-shaped eddies. With increasing depth of the eddies, the proportion of bowl-shaped eddies gradually decreases. Conversely, the cone- and lens-shaped eddies are equal in number at 700–1 000 m, accounting for about 30% each. Studying the 3D characteristics of eddies in two different regions of the northwestern Pacific Ocean is an important stepping stone for discussing the different eddy generation mechanisms. 相似文献
7.
Huang Weigen 《海洋学报(英文版)》2000,19(3):33-43
I~IOXSatellite infrared measurements are now accepted as an effeCtive way of mapping sea surfacetemperature (SST) distributions on global and regional scales. While regional SST maps are widely uest as background data for oceanographic experiments, they tend to be considered by oceanographers as at best qualitative tools. There is SCOPe for developing the use of satellite SST imagesac extensively for the study of mesoscale d~ical processes, as this paper seeks to demonstrate. As Part O… 相似文献
8.
Nonlinear axisymmetric oscillations of a warm baroclinic eddy are considered within the framework of an reduced-gravity model of the dynamics of a multilayer ocean. A class of exact analytical solutions describing pure inertial oscillations of an eddy formation is found. The thicknesses of layers in the eddy vary according to a quadratic law, and the horizontal projections of the velocity in the layers depend linearly on the radial coordinate. Owing to a complicated structure of the eddy, weak limitations on the vertical distribution of density, and an explicit form of the solution, the latter can be treated as a generalization of the exact analytical solutions of this form that were previously obtained for homogeneous and baroclinic eddies in the ocean. 相似文献
9.
Mesoscale eddies, particularly anticyclonic ones, are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol’ Strait, which has a diameter of ∼100 km, typical surface velocity of 0.2–0.3 m s−1, and less dense core extending to a depth of ∼1200 m. Based on an idea that the generation of mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of ∼70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions. In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances increasing gradually. Typical eddy scale and its westward propagation speed are ∼100 km and ∼0.6 km day−1, respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the β-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of ∼1200 m, can be reproduced. 相似文献
10.
次中尺度过程的水平空间尺度约为0.1~10km, 时间尺度约为1天, 里查森数和罗斯贝数为0(1), 能有效地从中尺度环流中汲取能量向小尺度湍流串级, 并对上层海洋物质的垂向交换有着重要影响。本文基于水平分辨率为~500m的高分辨率ROMS(regional ocean modeling system)数值模拟结果, 采用方差椭圆方法, 评估了黑潮延伸体海域上层海洋次中尺度涡旋的各向异性特征, 并探讨了涡旋各向异性值的大小与次中尺度过程特征参数的相关性。研究结果表明, 黑潮延伸体主轴强流区域的次中尺度涡旋各向异性值明显小于两侧海域, 主轴区域的次中尺度涡旋特征明显强于流轴两侧海域, 各向异性值与次中尺度过程的强弱有着较为显著的负相关关系, 表明次中尺度过程具有较小的各向异性特征(更趋各向同性)。方差椭圆表征了涡与平均流相互作用过程中的能量反馈机制, 较大的各向同性特征意味着动能更趋正向串级。 相似文献
11.
作为中尺度过程与小尺度过程中的过渡,次中尺度过程[空间尺度为O(1～10)km,时间尺度为O(1)天]是海洋动力过程中重要的一环。海洋次中尺度过程具有明显的非地转特征,从而促进垂向热量和物质的输运,因此在海洋上层热量与物质垂直交换中肩负着极为重要的作用。黑潮作为全球最强的西边界流之一,是海洋能量的重要聚集区。针对黑潮流区大尺度环流和中尺度涡旋等动力过程的研究,受到海洋和气象学者的广泛关注,但对黑潮流区次中尺度过程的相关研究相对较少。本文基于高分辨率ROMS数值模式(空间分辨率为1公里),针对黑潮流区(25.5°～29.4°N, 124.4°～131°E)次中尺度过程的空间分布特征及其诱导的热量输运特征进行了研究。模拟结果表明,黑潮流区存在着十分活跃的次中尺度过程,尤其是在黑潮流区及岛屿周边等地形变化剧烈的海区。相对涡度和垂直流速的分布特征表明,次中尺度相对涡度和垂向流速上表现出了明显的不对称性,正相对涡度强于负相对涡度,向下垂向流速强于向上垂向流速,而这主要是由惯性不稳定所导致。通过计算次中尺度引起的热量输运,结果表明次中尺度的水平热量通量为东北方向,从较低纬度朝较高纬度输运,这意味着次中尺度可以促进不同纬度的热量交换;而垂向热量通量则表现出向上输运的特征,即由深层往表层输运,这意味着次中尺度过程可以导致热量在垂直方向上的再分配,从而使得海洋趋于再分层。 相似文献
12.
采用海洋再分析结果,研究了海洋涡旋和锋面波动对台湾以东黑潮锋的影响,结果表明,Rossby波第一斜压模态形成的冷涡(暖涡),减弱(增强)台湾以东黑潮温度锋强度,减小(加大)锋的宽度.在再分析结果中,捕获到1991年1-2月台湾以东的一次黑潮锋面波动.锋面波动的波槽(波脊)到达时,该温度锋强度减弱(增强),宽度和厚度减小... 相似文献
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14.
Dax Matthews Brian PowellRalph Milliff 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(10):979-987
We utilize a variety of available observations with a semivariogram technique to quantify the oceanic variability around the Hawaiian Islands. The Hawaiian Islands have a significant impact on the North Pacific circulation, and quantifying the characteristics of the variability is important for understanding the eddy energy, as well as required for statistical techniques to work with the data, such as optimal interpolation, data assimilation, etc. Both satellite sea surface height and temperature data are used to determine horizontal scales of variability, while Argo profiles, ship-borne profiles, and autonomous Seagliders provide estimates of the vertical scales. In the lee of the islands, satellite data reveal an increase in horizontal variability attributed to enhanced eddy activity that persists for over 1000 km westward; however, only within 400 km of the immediate lee the horizontal length scales are greatly reduced. Further west, length scales increase significantly indicating a change in the generation mechanism for eddy variability and where eddies merge and coalesce. The meridional length scale gradient is found to be larger than previous results and more representative of the gradient of the first baroclinic mode of the internal Rossby radius. Vertical length scales are shown to increase in the lee, with vertical temperature variability doubled from the windward side. 相似文献
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16.
The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model).In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved. 相似文献
17.
A new method of assimilating sea surface height (SSH) data into ocean models is introduced and tested. Many features observable by satellite altimetry are approximated by the first baroclinic mode over much of the ocean, especially in the lower (but non-equatorial) and mid latitude regions. Based on this dynamical trait, a reduced-dynamics adjoint technique is developed and implemented with a three-dimensional model using vertical normal mode decomposition. To reduce the complexity of the variational data assimilation problem, the adjoint equations are based on a one-active-layer reduced-gravity model, which approximates the first baroclinic mode, as opposed to the full three-dimensional model equations. The reduced dimensionality of the adjoint model leads to lower computational cost than a traditional variational data assimilation algorithm. The technique is applicable to regions of the ocean where the SSH variability is dominated by the first baroclinic mode. The adjustment of the first baroclinic mode model fields dynamically transfers the SSH information to the deep ocean layers. The technique is developed in a modular fashion that can be readily implemented with many three-dimensional ocean models. For this study, the method is tested with the Navy Coastal Ocean Model (NCOM) configured to simulate the Gulf of Mexico. 相似文献
18.
From January 9 to 17, 1981, detailed observations of the horizontal and vertical structure beneath one of the quasi-permanent semi-stationary mesoscale offshore eddy signatures in the California Current System (CCS) discussed by Bernstein, Breaker and Whritner (1977), Burkov and Pavlova (1980), and Simpson (1982) were made. The vertical sections of temperature and density show the presence of three-layer system. A subsurface warm-core eddy, whose diameter is about 150 km at the 7°C isotherm, is the dominant feature. A warm surface layer, which extends to a depth of 75 m, lies over the eddy. Between the warm surface layer and the subsurface warm-core eddy, there is a cold-core region which extends to a depth of about 200 m. There is a high degree of symmetry about the vertical axis of rotation. Vertical sections of salinity and dissolved oxygen are entirely different from sections of temperature and density. Diagrams of water mass characteristics confirm that the core of the eddy, found between 250–600 m, consists of inshore water from the California Undercurrent (CU). Below about 700 m, local waters from the Deep Poleward Flow (DPF) have been incorporated into the eddy. The observed distributions of properties (T, S, δθ, O2) are inconsistent with a single, local generation process for the eddy system. Radial distributions of angular velocity, normalized gradient velocity and relative vorticity support the use of a Gaussian radial height field as an initial condition in eddy models. Possible reasons why CCS eddies may differ dynamically from Gulf Stream rings are given in the text. At the time the observations were made, the system as a whole was in near geostrophic balance. Local geostrophic balance, however, cannot explain the observed distribution of properties and structure. The observed symmetry in the structure of the eddy system, chemical evidence (Simpson, 1984), biological distributions (Haury, 1984) and satellite images of the CC (Koblinsky, Simpson and Dickey, 1984) suggest that lateral entrainment of warm (oceanic) and cold (coastal) water into the upper two layers of the three-layer system by the subsurface eddy is a likely generation mechanism for the cold-core region. The coastal origin of the frontal structure along the northeastern quadrant and the oceanic origin of the frontal structure along the southwestern quadrant of the eddy system further support lateral entrainment as a generation mechanism for the cold core. This entrainment makes the CCS eddy system different from cold-core rings in the Gulf Stream and rather similar to some warm-core eddies found in the East Australian Current. The presence of CU water in the core of this eddy raises the question of how CU water was transported from the continental slope. Eddy generation mechanisms, other than baroclinic instability of the CC, may be required to explain the distribution, persistence, and core composition of offshore mesoscale eddies in the CCS. There is evidence that barotropic, in addition to baroclinic, processes may be important. 相似文献
19.
V.M. Benítez-Barrios J.L. Pelegrí A. Hernández-Guerra K.M.M. Lwiza D. Gomis P. Vélez-Belchí S. Hernández-León 《Progress in Oceanography》2011,91(4):516-533
High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05 m s−1 over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1 Sv, with maximum velocities of 0.18 m s−1 at surface decreasing to 0.05 m s−1 at 300 db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12 m day−1, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7 m day−1). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08 Sv), integrated linear Ekman pumping (−0.05 Sv) or diapycnal transfer (about 0.1–0.2 Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures. 相似文献
20.
Nesting in large-scale ocean modeling is used for local refinement to resolve eddy dynamics that would not be accessible otherwise. Unstructured meshes offer this functionality too by adjusting their resolution according to some goal function. However, by locally refining the mesh one does not necessarily achieve the goal resolution, because the eddy dynamics, in particular the ability of eddies to release the available potential energy, also depend on the dynamics on the upstream coarse mesh. It is shown through a suite of experiments with a zonally re-entrant channel that baroclinic turbulence can be out from equilibrium in wide (compared to a typical eddy size) zones downstream into the refined area. This effect depends on whether or not the coarse part is eddy resolving, being much stronger if it is not. Biharmonic viscosity scaled with the cube of grid spacing is generally sufficient to control the smoothness of solutions on the variable mesh. However, noise in the vertical velocity field may be present at locations where the mesh is varied if momentum advection is implemented in the vector invariant form. Smoothness of vertical velocity is recovered if the flux form of momentum advection is used, suggesting that the noise originates from a variant of the Hollingsworth instability. 相似文献