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1.
In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea（SCS）. A wake of baroclinic near-inertial waves（NIWs） commenced after Nesat passed the array. The associated near-inertial currents are surface-intensified and clockwise-polarized. The vertical range of NIWs reached 300 m, where the vertical range is defined as the maximum depth of the horizontal near-inertial velocity 5 cm/s. The current oscillations have a frequency of 0.709 9 cycles per day（cpd）, which is 0.025 f higher than the local inertial frequency. The NIWs have an e-folding time-scale of 10 d based on the evolution of the near-inertial kinetic energy. The depth-leading phase of near-inertial currents indicates downward group velocity and energy flux. The estimated vertical phase velocity and group velocity are 0.27 and 0.08 cm/s respectively, corresponding to a vertical wavelength of 329 m. A spectral analysis reveals that NIWs act as a crucial process to redistribute the energy injected by Typhoon Nesat. A normal mode and an empirical orthogonal function analysis indicate that the second mode has a dominant variance contribution of 81%, and the corresponding horizontal phase velocity and wavelength are 3.50 m/s and 420 km respectively. The remarkable large horizontal phase velocity is relevant to the rotation of the earth, and a quantitative analysis suggests that the phase velocity of the NIWs with a blue-shift of 0.025 f overwhelms that of internal gravity waves by a factor of 4.6.  相似文献   

2.
Nonlinear internal waves(NIWs) are ubiquitous around the Kara Sea, a part of the Arctic Ocean that is north of Siberia. Three hot spot sources for internal waves, one of which is the Kara Strait, have been identified based on Envisat ASAR. The generation and evolution of the NIWs through the interactions of the tide and topography across the strait is studied based on a nonhydrostatic numerical model. The model captures most wave characteristics shown by satellite data. A typical inter-packets distance on the Barents Sea side is about 25 km in summer, with a phase speed about 0.65 m/s. A northward background current may intensify the accumulation of energy during generation, but it has little influence on the other properties of the generated waves. The single internal solitary wave(ISW) structure is a special phenomenon that follows major wave trains, with a distance about 5–8 km. This wave is generated with the leading wave packets during the same tidal period. When a steady current toward the Kara Sea is included, the basic generation process is similar, but the waves toward the Kara Sea weaken and display an internal bore-like structure with smaller amplitude than in the control experiment. In winter, due to the growth of sea ice, stratification across the Kara Strait is mainly determined by the salinity, with an almost uniform temperature close to freezing. A pycnocline deepens near the middle of the water depth(Barents Sea side), and the NIWs process is not as important as the NIWs process in summer. There is no fission process during the simulation.  相似文献   

3.
On the basis of the theoretical research results by the author and the literature published up to date, the analysis and the justification presented in this paper show that the breaking products of oceanic internal waves are not only turbulence, but also the fine-scale near-inertial internal waves (the oceanic reversible finestructure) for inertial waves and the internal solitary waves for internal tides respectively. It was found that the oceanic reversible finestructure may be induced by the effect of the horizontal component f (f = 2Ωcosφ) of the rotation vector on inertial waves. And a new instability of the theoretical shear and strain spectra due to the effect of f occurs at critical vertical wavenumber β c ≈ 0.1 cpm. It happens when the levels of shear and strain of the reversible finestructure are higher than those of inertial waves, which is induced by the effect of f along an "iso-potential-pycnal" of internal wave. If all breaking products of internal waves are taken into account, the average kinetic energy dissipation rate is an order of magnitude larger than the values of turbulence observed by microstructure measurements. The author’s theoretical research results are basically in agreement with those observed in IWEX, DRIFTER and PATCHEX experiments. An important impersonal fact is that on the mean temporal scale of thermohaline circulation these breaking products of internal waves exist simultaneously with turbulence. Because inertial waves are generated by winds at the surface, and internal tides are generated by strong tide-topography interactions, the analysis and justification in this paper support in principle the abyssal recipes Ⅱ:energetics of tidal and wind mixing by Munk & Wunsch in 1998, in despite of the results of microstructure measurements for the turbulent kinetic energy dissipation rate and the diapycnal turbulent eddy diffusivity.  相似文献   

4.
Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.  相似文献   

5.
南海北部陆坡区内孤立波的垂向热量输送   总被引：1，自引：0，他引：1  
An integrated analysis of internal solitary wave(ISW) observations obtained from two moorings over the continental slope in the northern South China Sea(SCS) leads to an assessment of the vertical heat transport of the ISWs. The clusters of ISW packets are phase-locked to the fortnightly cycle of the semidiurnal tide. The ISWs appear during large semidiurnal tides, and there is a period of 5–6 d when no ISWs are observed. The effect of the ISWs on the continental slope heat budget is observed. The ISWs can modify a local temperature field in which the temperature in the upper layer can be changed by O(100) °C after the ISWs passed the mooring. Both ISWinduced diffusion and ISW-induced advection contribute to the temperature variation. The estimates imply an average vertical heat flux of 0.01 to 0.1 MW/m~2 in the ISWs in the upper 500 m of the water column. The vertical heat transport ranges from 0.56 to 2.83 GJ/m~2 with a mean value of 1.63 GJ/m~2. The observations suggest that the vertical heat transport is proportional to the maximum vertical displacement.  相似文献   

6.
The mode-2 internal solitary waves (ISWs) generated by mode-2 internal tide (IT) are identified by mooring observations in the northern South China Sea (SCS) from 2016 to 2017. Two mode-2 ISWs with a re-appearance period of 24.9 h observed on 29 and 30 July 2016 are characterized by type-b ISWs. They occurred when the isotherms compressed obviously in the vertical direction. Modal decomposition of IT horizontal currents shows that the vertical compression of the isotherms is mainly caused by diurnal mode-2 IT. The analysis of the role of the density stratification reveals that a deeper and thinner pycnocline is favorable for generation of mode-2 ISWs rather than pycnocline intensity. By comparing the mode-2 nonlinear, dispersion coefficients and the Ursell numbers calculated based on the stratification associated with different kinds of ITs with the observation results, it is shown that the diurnal mode-2 IT plays a crucial role in the generation of the mode-2 ISWs. When the diurnal mode-2 IT interacts with the semidiurnal IT and causes a deeper and thinner pycnocline, the mode-2 ISWs are easily excited.  相似文献   

7.
For settlement of the well-known problem of contemporary radar imaging models, i. e. , the problem of a general underestimation of radar signatures of hydrodynamic features over oceanic internal waves and underwater bottom topography in tidal waters at high radar frequency bands （ X-band and C-band）, the impact of the ocean surface mixed layer turbulence and the significance of strat- ified oceanic model on SAR remote sensing of internal solitary waves are proposed. In the north of the South China Sea by utilizing some observed data of background field the nonlinearity coefficient, the dispersion coefficient, the horizontal variability coefficient and the phase speed in the generalized K-dV equation are determined approximately. Through simulations of internal tide transfor- mation the temporal evolution and spatial distribution of the vertical displacement and horizontal velocity of internal wave field are obtained. The simulation results indicate that the maximum amplitudes of internal solitary waves occur at depth 35 m, but the maximum current speeds take place at depth 20 m in this area of the sea （about 20°30＇N, 114°E） in August. It was noticed that considering the effects of flood current and ebb current respectively is appropriate to investigate influence of the background shear flow on coefficients of the K-dV equation. The obtained results provide the possibility for the simulation of SAR signatures of internal solitary waves under considering the impact of ocean surface mixed layer turbulence in the companion paper.  相似文献   

8.
On the basis of the time series observations from a temperature chain and an acoustic Doppler current profiler on the continental shelf of the northern South China Sea, a sequence of internal solitary waves （ISWs） and background waves （BWs, including internal tides and near-inertial waves） on the continental shelf were captured simultaneously after the transit of Typhoon Neast in October 2011. These measurementsprovided a unique opportunity to explore the influence of BWs on the ISWs. The BWs appeared a conversion on the current strength and vertical mode structure during the observational period. The BWs were dominated by weak and mode-one waves before October 2 and then turned to strong and high-mode waves after that time. Meanwhile, the ISWs displayed different wave structures before and after October 2, which was closely related to BWs＇ changes. According to the current profiles of BWs, the high-mode wave structure with strong current could significantly strengthen the vertical shear of ISWs in the near-surface layer and promote the breaking of ISWs, and thus it may play an important role in affecting the background current condition.  相似文献   

9.
A European Space Agency＇ s ENVISAT advanced synthetic aperture radar （ASAR） image covering Zhejiang coastal water in the East China Sea （ECS） was acquired on 1 August 2007. This image shows that there are about 20 coherent internal solitary wave （ISW） packets propagating southwestward toward Zhejiang coast. These ISW packets are separated by about 10 kin, suggesting that these ISWs are tide-generated waves. Each ISW packet contains 5-15 wave crests. The wavelengths of the wave crests within the ISW packets are about 300 m. The lengths of the leading wave crests are about 50 km. The ISW amplitude is estimated from solving KdV equation in an ideal two-layer ocean model. It is found that the ISW amplitudes is about 8 m. Further analysis of the ASAR image and ocean stratification profiles show that the observed ISWs are depression waves. Analyzing the tidal current finds that these waves are locally generated. The wavelength and amplitude of the ECS ISW are much smaller than their counter- parts in the South China Sea （SCS）. The propagation speed of the ECS ISW is also an order of magnitude smaller than that of the SCS ISW. The observed ISWs in the ECS happened during a spring tide period.  相似文献   

10.
季节性层结对孤立内波作用于圆柱载荷的影响   总被引：1，自引：0，他引：1  
On the basis of Morison＇s empirical formula and modal separation method in estimating the force and torque exerted by internal solitary waves （ISWs） on a cylindrical pile, it is found that the loads exerted by the ISWs change largely in different seasons at the same site of the continental shelf in the South China Sea （SCS） even under the condition that the amplitudes of ISWs are the same. Thus, the effect of a seasonal water stratification variation on the force and torque exerted by the ISWs is investigated, and a three-parameter stratification model is employed. It is shown that the loads exerted by the ISWs depend largely on the wa- ter stratification. The stronger the water stratification, the larger the force and the torque; when the depth where the maximum thermocline appears is deepened, the force decreases but the torque increases; when the width of the thermocline is narrowed, the force increases but the torque decreases. The seasonal varia- tion of the force and the torque exerted by the ISWs in four seasons in the SCS is thus explained. Key words： internal solitary waves, force, torque, water stratification, South China Sea  相似文献   

11.
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   

12.
Submesoscale processes in marginal seas usually have complex generating mechanisms, highly dependent on the local background flow and forcing. This numerical study investigates the spatial and seasonal differences of submesoscale activities in the upper ocean of the South China Sea (SCS) and the different dynamical regimes for sub-regions. The spatial and seasonal variations of vertical vorticity, horizontal convergence, lateral buoyancy gradient, and strain rate are analyzed to compare the submesoscale phenomenon within four sub-regions, the northern region near the Luzon Strait (R1), the middle ocean basin (R2), the western SCS (R3), and the southern SCS (R4). The results suggest that the SCS submesoscale processes are highly heterogeneous in space, with different seasonalities in each sub-region. The submesoscale activities in the northern sub-regions (R1, R2) are active in winter but weak in summer, while there appears an almost seasonal anti-phase in the western region (R3) compared to R1 and R2. Interestingly, no clear seasonality of submesoscale features is shown in the southern region (R4). Further analysis of Ertel potential vorticity reveals different generating mechanisms of submesoscale processes in different sub-regions. Correlation analyses also show the vertical extent of vertical velocity and the role of monsoon in generating submesoscale activities in the upper ocean of sub-regions. All these results suggest that the sub-regions have different regimes for submesoscale processes, e.g., Kuroshio intrusion (R1), monsoon modulation (R2), frontal effects (R3), topography wakes (R4).  相似文献   

13.
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   

14.
利用卫星遥感资料和区域海洋数值模式ROMS(regional ocean modeling system)高分辨率数值模拟结果, 对南海西部夏季上升流锋面的次中尺度特征及其非地转过程进行了探讨。高分辨率卫星遥感观测和数值模拟结果显示, 南海西部夏季锋面海域存在活跃的次中尺度现象, 其水平尺度约为1~10km, 且具有O(1)罗斯贝数(Rossby number, Ro)的典型次中尺度动力学特征。进一步的诊断分析表明, 在夏季西南风的驱动下, 沿锋面射流方向的风应力(down-front wind stress)引起的跨锋面埃克曼输运有利于将海水由锋面冷水侧向暖水侧输运, 减小了锋面海域的垂向层结和Ertel位涡, 加剧了锋面的不稳定, 并形成跨锋面的垂向次级环流。高分辨率模拟结果显示, 锋面海域最大垂向流速可达100m?d -1, 显著增强了上层海洋的垂向物质交换。因此, 活跃在锋面海域的次中尺度过程可能是增强南海西部上升流海域垂向物质交换的重要贡献者。  相似文献   

15.
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   

16.
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   

17.
南海东北部亚中尺度过程时空分布特征   总被引：6，自引：3，他引：3  
基于高分辨率模型2009-2012年的模拟结果，本文对南海东北部亚中尺度过程的时空分布特征进行了研究。模拟结果表明，南海东北部上层广泛存在着相对涡度接近于局地行星涡度的亚中尺度过程。统计结果发现，亚中尺度过程的相对涡度的分布具有着明显的非对称性，即正涡度明显强于负涡度。这意味着相比于负涡度，具有正涡度的亚中尺度过程要更为活跃，而这主要是由离心不稳定导致。同时，亚中尺度过程在时间分布上表现出明显的冬强夏弱的季节变化特征。通过对该海区亚中尺度过程可能生成机制的分析发现，该季节变化与流场拉伸和混合层的厚度有着密切关系，冬季更强的流场拉伸和更深的混合层有利于通过锋生过程和混合层不稳定为亚中尺度过程生成提供更多的能量。  相似文献   

18.
Amala Mahadevan   《Ocean Modelling》2006,14(3-4):222-240
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   

19.
南海是西太平洋最大的边缘海, 由于受季风影响显著以及北部海域的黑潮入侵, 其动力环境复杂多变, 次中尺度过程丰富, 且在空间上和时间上存在多变性。文章基于高分辨率数值模式的结果, 通过对次中尺度动力参数的分析, 对比讨论了南海北部、中部、西部和南部海域4个典型子区域上层海洋次中尺度过程的空间差异、季节变化、影响深度、影响因素等问题。研究发现各区域季节性变化特征和机制有所不同: 北部海域受冬季风和黑潮入侵影响, 冬季次中尺度的混合层不稳定较强; 中部海域同样表现为“冬强夏弱”; 西部海域受夏季风影响显著, 夏季次中尺度过程更为活跃; 而南部海域主要受岛屿地形影响较大, 容易产生地形尾涡, 季节性特征不明显。统计分析表明, 次中尺度过程往往表现出强正相对涡度与高应变特征, 在表层更容易出现负位涡, 流体稳定性较差。此外, 文章从能量学角度对次中尺度过程的主要能量来源、控制因素等进行了讨论。  相似文献   

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