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1.
The seasonal cycle of submesoscale flows in the upper ocean is investigated in an idealised model domain analogous to mid-latitude open ocean regions. Submesoscale processes become much stronger as the resolution is increased, though with limited evidence for convergence of the solutions. Frontogenetical processes increase horizontal buoyancy gradients when the mixed layer is shallow in summer, while overturning instabilities weaken the horizontal buoyancy gradients as the mixed layer deepens in winter. The horizontal wavenumber spectral slopes of surface temperature and velocity are steep in summer and then shallow in winter. This is consistent with stronger mixed layer instabilities developing as the mixed layer deepens and energising the submesoscale. The degree of geostrophic balance falls as the resolution is made finer, with evidence for stronger non-linear and high-frequency processes becoming more important as the mixed layer deepens. Ekman buoyancy fluxes can be much stronger than surface cooling and are locally dominant in setting the stratification and the potential vorticity at fronts, particularly in the early winter. Up to 30% of the mixed layer volume in winter has negative potential vorticity and symmetric instability is predicted inside mesoscale eddies as well as in the frontal regions outside of the vortices. 相似文献
2.
Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade, but in the eddy-active South China Sea (SCS) the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood. This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution (~500 m) simulation. The satellite images show that the elongated highly productive stripes with a typical lateral scale of ~25 km and associated filaments are frequently observed at the periphery of mesoscale eddies. The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d) due to submesoscale instabilities. The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m2. This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments. Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity. The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy, but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence. In this context, these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy, as well as enhanced vertical heat transport. 相似文献
3.
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
4.
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
5.
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
6.
Chris Garrett Amit Tandon 《Deep Sea Research Part I: Oceanographic Research Papers》1997,44(12):1991-2006
An air-sea buoyancy flux out of the ocean between the surface outcroppings of different isopycnals must be balanced by a convergence of advective and diffusive fluxes of buoyancy across those isopycnals (Walin, 1982; Tziperman, 1986; Garrett et al., 1995). For steady conditions, the diapycnal diffusive flux due to vertical mixing in the surface mixed layer is very small, so that the advective buoyancy flux dominates (Speer, 1993; Garrett et al., 1995). The associated advective buoyancy flux can then be used to estimate the volume flux of water out of the base of the surface mixed layer. The resulting thermodynamic algorithm provides a valuable estimate of water mass formation in the ocean.In contrast, for the time-dependent real ocean with horizontal and vertical gradients of the horizontal buoyancy gradient, diurnal and seasonal mixed layer deepening and entrainment in the presence of a buoyancy jump at the base of the mixed layer contributes to the annual volume flux out of the base of the deepest (wintertime) mixed layer. The mismatch between the predictions of the ideal algorithm and measured rates of water mass formation (Speer, 1997) may thus be partly due to mixed layer processes rather than diapycnal mixing in the thermocline. 相似文献
7.
近年来的现场观测和理论研究发现, 次中尺度现象广泛存在于上层海洋, 其产生与锋生作用及混合层斜压不稳定存在密切联系。本文利用高分辨率的数值模拟结果并结合动力学及能量诊断分析, 对黑潮延伸体海域次中尺度过程的季节变化进行了探讨。探讨结果表明, 黑潮延伸体海域次中尺度过程具有冬季最强, 春季和秋季次之, 夏季最弱的显著季节变化特征。基于冬、夏季次中尺度能量源的诊断可以看到, 这些季节变化特征主要与上层海洋的斜压不稳定和锋生作用有关。冬季, 黑潮延伸体海域的中尺度能量较弱, 但次中尺度过程在季节尺度上表现最为活跃, 这主要与混合层斜压不稳定的作用有关; 夏季, 黑潮延伸体海域的混合层较浅, 次中尺度过程较弱, 但中尺度涡旋活跃, 中尺度流场变形引起的锋生作用对夏季次中尺度现象的产生具有重要影响。在次中尺度能量的季节变化方面, 冬季次中尺度过程从中尺度过程汲取能量的速率远高于夏季, 这是冬季次中尺度过程比夏季更为活跃的主要原因。本文研究结果有助于加深对黑潮延伸体海域次中尺度过程季节性变化及其动力机制的理解。 相似文献
8.
《Ocean Modelling》2011,40(3-4):311-331
Recognizing the potential role played by submesoscale processes in both the energy cascade in the ocean and biogeochemical transport, we conduct a series of large eddy simulations of isolated mixed layer instabilities. The primary objective is to generate freely evolving velocity and density fields representative of submesoscale flows and then use these to examine potential observational sampling strategies. Mixed layer instabilities are explored in two parameter regimes: a strongly-stratified regime which results in a system with surface-intensified eddies and high vertical shear, and a weakly-stratified regime exhibiting weaker, smaller scale eddies that penetrate across the entire domain depth as Taylor columns. Analysis of a variety of mixing measures derived from both particle and tracer based sampling strategies indicates the differing importance of vertical processes in the two flow regimes. 相似文献
9.
A coupled air–sea general circulation model is used to simulate the global circulation. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled‐model approach avoids strong artificial water mass transformation associated with relaxation to prescribed mixed layer conditions. Instead, transformation results from the more physical non‐local, nonlinear interdependence of sea‐surface temperature, air–sea fluxes, and circulation in the model's atmosphere and ocean. The development of a stronger mid‐depth circulation cell and associated upwelling when eddy fluxes are present, is examined. The strength of overturning is diagnosed in density coordinates using the transformation framework. 相似文献
10.
本文利用现场观测资料和卫星遥感数据, 并结合ROMS(regional ocean modeling system)数值模拟对南海北部粤东陆架的锋面特征及其影响因素进行探讨。观测结果显示, 夏季南海北部陆架存在活跃的上升流温度锋面, 其水平尺度约为50km, 强度达到0.06℃∙km-1, 大于同时期卫星遥感观测结果, 垂向影响深度超过20m, 且具有一阶理查森数(Richardson number, Ri)的典型动力学特征。进一步的ROMS 模式诊断分析结果显示, 锋面处水平梯度增强, 且动力学上表现出一阶Ri数, 为锋面不稳定的发生提供了有利条件。高分辨率模拟结果显示, 在夏季西南风的驱动下, 沿锋面地转流方向的风应力引起的跨陆架Ekman输运将锋面处冷水向暖水运移, 导致水平浮力梯度和锋面强度增强并形成负Ertel位涡(Ertel potential vorticity, EPV)。因此, 夏季风场强迫引起的Ekman浮力通量(Ekman buoyancy flux, EBF)可能是南海北部锋面不稳定现象的主要贡献者, 对局地动力环境有重要影响。 相似文献
11.
The unbalanced submesoscale motions and their seasonality in the northern Bay of Bengal(BoB) are investigated using outputs of the high resolution regional oceanic modeling system. Submesoscale motions in the forms of filaments and eddies are present in the upper mixed layer during the whole annual cycle. Submesoscale motions show an obvious seasonality, in which they are active during the winter and spring but weak during the summer and fall. Their seasonality is associated with the mixed layer... 相似文献
12.
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
13.
本文基于卫星遥感资料和高分辨率ROMS(Regional Ocean Modeling System)数值模拟结果, 对黑潮延伸体海域典型中尺度涡旋的次中尺度特征进行了探讨。卫星观测和模拟结果显示, 黑潮延伸体涡旋海域伴随着活跃的次中尺度现象。涡旋演变与多尺度能量分析结果表明, 涡旋海域次中尺度动能的强弱与涡旋海域地转流动能有着密切联系, 锋生可能是涡旋边缘次中尺度动能增强的重要机制。次中尺度现象在中尺度涡旋海域具有沿地转流方向的复杂涡丝状结构特征, 意味着涡旋边缘较强的水平浮力梯度和地转流侧向剪切为次中尺度过程形成与发展提供了有利条件。此外, 垂向结构分析表明, 次中尺度过程能引起较大的垂向速度, 最大可达100m·day-1, 该垂向速度可以影响至混合层下200m深度处, 对海洋内部的垂向物质能量交换、海—气相互作用等有着重要的影响。 相似文献
14.
We analyze model simulations of a wind-forced upper ocean front to understand the generation of near-surface submesoscale, O(1 km), structures with intense vertical motion. The largest vertical velocities are in the downward direction; their maxima are situated at approximately 25 m depth and magnitudes exceed 1 mm/s or 100 m/day. They are correlated with high rates of lateral strain, large relative vorticity and the loss of geostrophic balance. We examine several mechanisms for the formation of submesoscale structure and vertical velocity in the upper ocean. These include: (i) frontogenesis, (ii) frictional effects at fronts, (iii) mixed layer instabilities, (iv) ageostrophic anticyclonic instability, and (v) nonlinear Ekman effects. We assess the role of these mechanisms in generating vertical motion within the nonlinear, three-dimensionally evolving flow field of the nonhydrostatic model. We find that the strong submesoscale down-welling in the model is explained by nonlinear Ekman pumping and is also consistent with the potential vorticity arguments that analogize down-front winds to buoyancy-forcing. Conditions also support the formation of ageostrophic anticyclonic instabilities, but the contribution of these is difficult to assess because the decomposition of the flow into balanced and unbalanced components via semigeostrophic analysis breaks down at O(1) Rossby numbers. Mixed layer instabilities do not dominate the structure, but shear and frontogenesis contribute to the relative vorticity and strain fields that generate ageostrophy. 相似文献
15.
The results of simulating the hydrophysical fields of the Black Sea with a resolution of 1.64 × 1.64 km for January–September 2006 with the use of real atmospheric forcing are analyzed. Both vertical turbulent momentum exchange and vertical turbulent heat and salt diffusions are parameterized using the Mellor-Yamada level 2.5 scheme. The results of this numerical experiment are compared with similar data obtained with a horizontal resolution of 5 km. The features of the meso- and submesoscale dynamics of waters in individual sea regions are given. Possible physical mechanisms of forming meso- and submesoscale vortices are studied on the basis of energy analysis. It is shown that, in the absence of significant wind forcing, the main contribution to kinetic energy is made by the buoyancy force and wind-field inhomogeneities result in significant variations in both total vertical viscosity and total vertical diffusion. 相似文献
16.
MPAS-Ocean is used to simulate an idealized, density-driven overflow using the dynamics of overflow mixing and entrainment (DOME) setup. Numerical simulations are carried out using three of the vertical coordinate types available in MPAS-Ocean, including z-star with partial bottom cells, z-star with full cells, and sigma coordinates. The results are first benchmarked against other models, including the MITgcm’s z-coordinate model and HIM’s isopycnal coordinate model, which are used to set the base case used for this work. A full parameter study is presented that looks at how sensitive overflow simulations are to vertical grid type, resolution, and viscosity. Horizontal resolutions with 50 km grid cells are under-resolved and produce poor results, regardless of other parameter settings. Vertical grids ranging in thickness from 15 m to 120 m were tested. A horizontal resolution of 10 km and a vertical resolution of 60 m are sufficient to resolve the mesoscale dynamics of the DOME configuration, which mimics real-world overflow parameters. Mixing and final buoyancy are least sensitive to horizontal viscosity, but strongly sensitive to vertical viscosity. This suggests that vertical viscosity could be adjusted in overflow water formation regions to influence mixing and product water characteristics. Lastly, the study shows that sigma coordinates produce much less mixing than z-type coordinates, resulting in heavier plumes that go further down slope. Sigma coordinates are less sensitive to changes in resolution but as sensitive to vertical viscosity compared to z-coordinates. 相似文献
17.
基于高分辨率CFSR(climate forecast system reanalysis)风场资料、气候态海洋混合层厚度资料和卫星高度计海面高度异常资料,本文估计了大气风场向全球海洋混合层的近惯性能通量和近惯性能量输入功率,并探究了混合层厚度、风场时间分辨率、经验衰减系数和中尺度涡旋涡度对近惯性能通量和能量输入功率的影响。浮标实测风场和流速表明,本文所用的风场和阻尼平板模型可用于估计风场向全球海洋的近惯性能通量。本文计算得到的大气向全球海洋输入近惯性能量的功率为0.56TW(1TW=10~(12)W),其中北半球贡献0.22TW,南半球贡献0.34TW。在时间上,风场的近惯性能通量呈现各个半球冬季最强、夏季最弱的特征,这和西风带风场的季节变化有关。在空间上,近惯性能通量的高值海域为南、北半球西风带海洋,尤其是南大洋。混合层厚度和风场空间不均匀性使得西风带近惯性能通量呈现纬向变化,即海盆西部强于海盆东部。风场时间分辨率对近惯性能通量的估计至关重要,低时间分辨率风场对近惯性能通量的低估达到13%—30%。阻尼平板模型中的经验衰减系数对近惯性能通量估计的影响不超过5%。中尺度涡旋涡度仅改变近惯性能通量的空间分布,而对全球近惯性能量输入功率的影响可以忽略。 相似文献
18.
南海西部风驱离岸急流次中尺度锋面的动力学分析 总被引：1，自引：0，他引：1
本文利用卫星观测资料和500 m分辨率数值模拟结果，结合理论分析，对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明，南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面，在地转和非地转运动的共同作用下，次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示，沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送，有效地在跨锋面方向将表层冷水平流输送至暖水侧，导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV），表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度，其最大值可达100 m·d-1。能量评估结果表明，次中尺度湍流的两个主要能量源，即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下，大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此，次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量，可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。 相似文献
19.
WRF模式空间分辨率对南海台风模拟的敏感性分析 总被引：3，自引：0，他引：3
为准确模拟南中国海台风路径和强度，对建立的WRF模式开展水平和垂向网格分辨率的敏感性分析。水平网格分辨率分别采用5 km，10 km，20 km，30 km以及15 km和5 km的嵌套方案等5种情况，垂向网格分辨率采用35层、28层和20层等3种情况，顶部最大压强采用1000 Pa，2000 Pa，3500 Pa和5000 Pa等4种情况，针对台风"启德"的路径和强度进行模拟分析。对不同条件组合下的台风路径、风速和海面压强进行对比，发现水平和垂向网格分辨率对台风路径的影响比较有限，对台风强度的影响相对显著。基于高精度空间分辨率模拟得到的台风强度与实测值吻合更好，采用嵌套网格技术对计算结果精度的提高效果显著。采用35层的垂向分辨率得到的模拟结果明显优于基于20层和28层得到的模拟结果。经过对比分析，确定适用于本研究的最优参数设置：水平空间分辨率采用15 km和5 km的双向嵌套网格方案；垂向分层为35层，顶部最大压强为2000 Pa。 相似文献
20.
Two numerical studies (Endoh, 1977;Harashima
et al., 1978) have been proposed on a front formed by a coupling effect of cooling of the sea surface and inflow of the fresh water in a vertical two-dimensional plane without the rotation of the earth. It is, however, not easy to interpret their numerical results. A simple interpretation will be proposed by an analytical study in this paper.It is found that local convection due to the density inversion, which is expressed by the convective adjustment of the vertical diffusion coefficient in the actual numerical calculations, plays an important role on the front formation.The characteristics of the front is also clarified in the case of steady state. Namely, simple functional dependences are obtained of the position and the width of the front, the horizontal and the vertical velocities and the distribution of the buoyancy and the salinity in the neighborhood of the front on the horizontal coordinate, the cooling rate, the eddy coefficients of diffusion and viscosity, the water depth and the vertically averaged horizontal fluxes of buoyancy and salinity. 相似文献