南海北部陆架区内波的演变与耗散机制

海洋是多尺度强迫-耗散系统,机械能主要在大尺度输入,在小尺度耗散。在大、中尺度运动的能量向小尺度湍流传递过程中,内波扮演着重要角色。内波的生成和破碎可打破海洋动力平衡,而在陆架区,内波（主要是内孤立波）的浅化演变与耗散则是驱动湍流混合的关键过程。通过长期的理论、观测与数值模拟研究,目前已认识到内波浅化过程中主要发生如下演变：波形调制、极性转变、裂变、破碎与耗散。相较于直接发生破碎,浅化演变过程中的裂变及其引发的剪切不稳定和对流不稳定是内孤立波在陆架区的主要耗散机制,显著调制陆架区的跃层混合。从能量串级的角度讲,内孤立波浅化裂变为动力不稳定的高频内波是潮能串级的重要通道。本文简要回顾南海北部陆架区内波的研究历史,并着重总结内波在陆架区演变与耗散机制的研究进展。     

内孤立波与平顶海山作用的能量耗散实验研究

内波破碎引起的能量耗散和混合是海洋内部的重要物理过程。通过在二维内波水槽进行实验室实验,分析内波与地形的作用,探究内孤立波与平顶海山地形作用时波要素、能量以及湍耗散率的时空变化。本实验利用重力塌陷法在两层流体中制造第一模态内孤立波,通过粒子图像测速技术(particle image velocimetry, PIV)获得内孤立波与地形作用时的流场结构,定量分析整个作用过程。结果表明,地形会改变波形甚至引起破碎,内波与地形作用时,振幅和能量密度会在内孤立波爬坡时迅速增大,在地形前缘产生强烈能量耗散。入射波的能量与塌陷高度呈二次函数关系,透射波能量随地形升高减小,反射波能量随地形升高增大。地形前缘局地湍耗散率极值时间序列在部分实验中呈双峰结构,对应内孤立波界面处剪切加强引起湍流耗散和波后缘翻转破碎。破碎引起的地形前缘区域平均湍耗散率量级在10~(-5)m~2/s~3,局地湍耗散率极值与入射波振幅呈指数关系,所有实验中局地湍耗散率的最大值接近10~(-3) m~2/s~3量级。     

veKdv方程的若干系数在南海北部的地理分布特征及其季节变化分析

应用中国近海及邻近海域海洋再分析资料(简称CORA)研究南海北部第一模态内波场运动学参数的地理分布特征及其季节变化。首先分析了Brunt-Väisälä频率的统计特征;其次,基于弱非线性变系数扩展Kortewed-de Vries (veKdv)方程模型,计算了它的输入系数,即线性长波相速度,平方和立方非线性系数和频散系数,这些参数可用于定性评估内孤立波传播可能的极性,内孤立波的形态,幅度限制以及传播速度等。分析结果表明,南海北部季节性密度跃层从2月开始出现,最大浮力频率约在20 m。它在6—7月达到最强,自8月开始减弱,在10月消退。另一密度跃层出现在8—11月,最大浮力频率约在80 m,冬季大致在120 m。季节性密度跃层在4—9月十分明显,而8—10月双跃层现象显著,冬季仅出现较弱的第二密度跃层。在1—3月和10—12月海盆深水区最大Brunt-Väisälä频率值要大于陆架浅水区;而在5—9月情况则相反。Brunt-Väisälä频率最大值所在深度随季节变化显著,冬季最深,6—7月则最浅。计算的线性内波相速度、频散系数和幅度放大因子的空间特征主要取决于地形变化;平方(立方)非线性系数与地形关系较小,随季节变化明显,它们主要取决于局地海洋环境特征。通过分析veKdv方程的系数特征,解释了为何在夏季南海北部最容易观测到大振幅内孤立波和在吕宋海峡以东海域难以观测到孤立波的原因。     

变系数EKdV模型在模拟南海北部大振幅内孤立波传播和裂变中的应用

利用垂向连续分层变系数EKdV模型,模拟了南海北部海域大振幅内孤立波的传播和裂变过程,并与观测数据进行比较。结果表明:连续分层变系数EKdV模型能够较好地反映振幅小于100m的内孤立波的振幅和波宽,对于更大振幅的强非线性内波,该模型模拟的振幅和波宽均较实测较小;非线性模态函数能够较准确地反映温度振荡的垂直结构,而水平流速的大小和垂直结构则与线性模态较符合。研究结果表明,变系数EKdV模型能够为研究和理解大振幅内孤立波的传播和裂变过程提供较好的理论支持。     

内孤立波破碎所致混合的实验研究

为定量分析内孤立波破碎的混合过程,本文在二维内波水槽中进行了两层流体第一模态内孤立波在斜坡上破碎的实验,运用粒子图像测速技术(PIV)测量内孤立波传播、破碎、反射过程的流场,计算涡度、湍动能和湍耗散率。结果表明不同振幅内波在不同角度斜坡上破碎时各个量的分布特征十分相似,各组实验各要素时间序列中均有两个峰值,分别发生于非线性增强和破碎时刻。得到破碎时湍耗散率与内孤立波振幅的关系为:较小振幅内波的湍耗散率与振幅呈2次关系,无因次振幅增大到0.9湍耗散率趋于不变;与斜坡角度的关系为:对于小振幅内波斜坡角度增大,破碎程度降低,耗散率减小;振幅较大时,存在一个角度使破碎程度最大。破碎引起的湍耗散率的量级在10–7到10–4m2/s3之间,比实测海洋中内孤立波传播界面和内潮遇地形破碎的湍耗散大1个量级。     

海洋内波破碎问题的研究

从理论、观测、数值实验和实验室实验四个方面对国内外近20年来关于海洋内波破碎问题的研究成果进行了分析总结.数值实验和实验室实验表明:中高频内波破碎时,初始的不稳定是二维的,当最终有横向对流卷团形成时,能量开始大量耗散,这时不稳定发展成为三维的;从初始的二维不稳定到对流卷团的产生这一过程,到底是一个剪切不稳定过程,还是一个对流不稳定过程,或者是对流不稳定和剪切不稳定共同存在的一个过程,取决于海水的层化、地形、背景剪切流和内波的自身性质.现场曾观测到内孤立波破碎时存在的剪切不稳定过程,数值研究模拟出了内孤立波破碎时存在的对流不稳定过程.现有的海洋内波破碎判据主要是关于中高频海洋内波的.理论分析侧重于确定线性或弱非线性内波的破碎机制和破碎条件.     

缓坡地形上内孤立波的破碎及能量分析

在大型重力式分层流水槽中对内孤立波沿缓坡地形的演化特征进行了实验研究,利用分层染色标识方法和多点组合探头阵列技术对其传播特性做了定性分析和定量测量。实验表明:下凹型内孤立波沿缓坡地形传播过程中的破碎先从波背部发生,继而演化出上凸型内孤立波;内孤立波破碎不仅与入射波波幅相关,而且受到地形坡度的强烈影响;入射波幅参数??0.4是内孤立波不稳定及破碎的实验判据,内孤立波能量损失出现跃升是其发生破碎的重要特征。研究进一步获得了内孤立波沿缓坡地形的三维演化结构、破碎发生条件和能量变化特性,对于复杂海洋环境中非线性内波传播特性认识及其动力学建模具有重要的科学意义。     

单一海脊地形对海洋内波生成与传播影响的分析

在海洋内波多发区,海底地形变化是影响海洋内波生成、传播和演化的重要因素。本文基于不可压缩原始N-S方程,在非静压近似条件下,通过建立适应于非线性海洋内波研究的非静压海洋动力学模型,并将其应用于正压潮驱动下的内孤立波生成和传播的数值模拟研究。根据模拟结果,研究了一类单海脊地形拓扑结构变化对内孤立波生成和传播的影响;分析并讨论了地形拓扑结构参数变化与生成的内孤立波传播至特定位置的抵达时间、强度等特征参数之间的变化关系;提出内孤立波生成之前在海脊一侧形成"L-下陷"结构的观点,并揭示了与该观点相合的能量"积聚"和"释放"机制。     

南海内波研究前沿与热点

内波为发生在层结海洋内部的亚中尺度波动,是物理海洋学研究,特别是海洋混合及能量级串研究,不可或的缺环节。孤立内波的突发性巨大冲击能量可对水下航行和工程设施构成灾难性威胁,实现实时监测与预报海洋内波具有重大现实意义。南海是全球海洋中超强内波多发海区之一。长期现场观测表明,吕宋海峡以西海域内孤立波振幅高达150～200 m,且终年发生。因此,南海是目前海洋内孤立波观测与研究热点海域。本文以2015年至2021年间发表的论文为依据,评述南海内波研究新进展,认为7 a来研究成果取得质的提升。第一,实现了由卫星为主要手段2D观测到以卫星与潜标同步3D观测为主要手段的提升。由此催生出振幅240 m超强内孤立波、中尺度涡对内波的调制作用、重现周期23 h 内孤立波、浅海内孤立波裂变现象、深海盆内波及动能级串等创新成果。第二,研究区开始呈现向中部深海盆扩展趋势。迄今为止,南海内波观测与研究集中在吕宋海峡以西和北部陆架,现已出现向中部深海盆扩展趋势。第三,海洋探测高新技术应用于南海内波观测与研究,取得了突破性成果。由卫星高度计沿轨海面高度场二维平面波分解技术得出的南海M₂内潮辐射图,解决了多年争论不休的南海北部内波生成机制和生成源地问题。人工智能技术成功应用于建立南海邻近的边缘海内波传播预报模式。模式预报的一个潮周期之后内波波峰线位置与后续卫星图像上显示的位置之间的平均相关系数达95%,平均距离均方根差为3 km。快速深潜剖面浮标技术应用于南海北部深海盆,得出0～3 500 m 全水深内波波段（周期为0.1～1.8 d）波动引起的水温起伏幅度垂直分布。高分辨水下声成像技术,包括人工地震技术和回声探测仪,成功应用于南海北部陆架内波观测与研究。其中回声探测仪图像空间分辨率达10 cm,清楚显示出内孤立波波包精细结构,可精确测得水平尺度仅为2 m的孤立波特征半宽度。可以预期,大量科学研究成果的积累,特别是采用人工智能技术建立内波传播预报模式的成功实例,必将为开发南海内波精准预报模式奠定基础。     

跃层厚度对内孤立波能量耗散的影响

内孤立波破碎混合是陆架地形上海洋混合过程重要的能量汇,为了探究内孤立波在连续跃层密度分层中能量变化及能量耗散规律,本文使用OpenFOAM建立数值水槽,基于双曲正切曲线设置连续跃层密度分层开展了一系列工况的模拟,针对跃层厚度对内孤立波能量传递及其耗散进行了详细分析。结果表明跃层厚度与跃层处流速剪切存在着负相关关系,随着跃层厚度的增加能量耗散先减小后增大。跃层厚度较小时,跃层处流速剪切强,理查德森数小,易产生开尔文-亥姆霍兹(Kelvin–Helmholtz)不稳定现象;随着跃层厚度的增大,流速剪切减小,理查德森数增大,开尔文-亥姆霍兹不稳定消失;跃层厚度达到临界值后,层结稳定性减小,理查德森数减小,流场的翻转混合过程加强,能量耗散也明显增强。     

Analysis of kinematic parameters of Internal Solitary Waves in the Northern South China Sea

The monthly climatology of observed temperature and salinity from the U.S. Navy Generalized Digital Environment Model (GDEM-Version 3.0) is used to derive the geographical and seasonal distribution of kinematic parameters of nonlinear internal waves in the Northern South China Sea (NSCS). Coefficients of the Generalized Extended Korteweg-de Vries Equation (GEKdV) with a background current are investigated (phase speed, dispersion, quadratic and cubic nonlinearity parameters, normalizing factor). These parameters are used to evaluate the possible polarities, shapes of internal solitary waves, their limiting amplitudes and propagation speed. We show that the long wave phase speed and dispersion parameters mainly depend on topography characteristics and have no obvious seasonal variation. The nonlinear parameters and normalizing factor are sensitive to variations in the density stratification and topography. Background current also exerts the distinct effects on the kinematic parameters; especially the nonlinear parameter can change by an order of magnitude. The nonlinear parameters take on larger values in the summer (July), and linear internal waves are prone to become steeper and develop into large-amplitude internal solitary waves under such circumstances. This explains why nonlinear internal solitary waves occur more frequently in summer. From the kinematic viewpoint, the dispersion parameter takes on larger values in the Pacific Ocean (PO) due to deeper water depth when compared with that in the NSCS. The stronger dispersion effect in the PO hinders the formation of large amplitude internal solitary waves, explaining why nonlinear internal solitary waves are rarely found to the east of the Luzon Strait. Large near-bottom velocities dominate the shallow area and tend to increase in the warm season. The largest values are induced by internal solitary waves, indicating that internal waves are the major drivers of sediment re-suspension and erosion processes.     

苏禄—苏拉威西海内孤立波动力参数时空变化特征

基于弱非线性理论及再分析同化数据, 计算了苏禄—苏拉威西海冬季及夏季内孤立波动力参数, 包括内孤立波线性速度、一阶和二阶非线性参数及线性色散参数, 并研究了这些参数的时空变化特征。我们发现, 虽然苏拉威西海域受到更加显著的西北太平洋水入侵, 但苏禄海内孤立波动力参数的时空变化特征却比苏拉威西海更为显著。夏季苏禄海内孤立波线性速度总体上比冬季约大0.1m·s^-1; 与此相反, 夏季苏拉威西海内孤立波线性速度却比冬季约小0.05m·s^-1。无论是一阶或二阶非线性动力参数, 其在苏拉威西海的时空变化均比较微弱, 但在苏禄海则较为显著。苏禄海夏季一阶非线性动力参数比冬季高出约3×10^-3s^-1, 但是夏季二阶非线性动力参数却比1月份低约3×10^-5m^-1·s^-1。此外, 相比冬季, 夏季苏禄海和苏拉威西海的色散动力参数均有所减弱, 但其在苏禄海减弱的幅度更大。综上, 苏禄—苏拉威西海环流引起的水体层化最大浮力频率所在深度的时空变化是造成上述内孤立波动力参数时空变化的根本原因。     

南海北部陆架区内孤立波向岸传播过程研究

南海北部是全球海洋中内孤立波最强和最为活跃的海域。然而,内孤立波在传入陆架区后,其形态发生显著变化,其传播演变过程表现出高度的复杂性。本研究综合卫星图像和数值模式手段研究了内孤立波在向岸传播过程中的空间变化特征。可见光卫星图像研究结果显示,南海北部陆架区存在三种形态的内孤立波,分别为第一模态下凹型内孤立波、第一模态上凸型内孤立波和第二模态内孤立波。受水深和层结变化的控制,它们的分布区域显著不同。基于MITgcm的数值模拟研究表明,上凸型内孤立波由第一模态下凹内孤立波经过极性转换过程发展而来,而第二模态内孤立波由第一模态下凹内孤立波与急剧变浅地形相互作用而产生。     

Numerical investigation of internal solitary waves from the Luzon Strait: Generation process,mechanism and three-dimensional effects

A fully nonlinear, non-hydrostatic model, MITgcm, is used to investigate internal solitary waves (ISWs) from the Luzon Strait (LS). As the ISWs in the South China Sea (SCS) have drawn more and more attention in recent years, they are studied in various ways, i.e., via remote sensing images, in situ measurements, and numerical simulations. The inspiration of this paper derived from the potential flaws of different numerical models that were employed to examine ISWs. In this study, we performed three-dimensional (3D) experiments with realistic topography and stratification, as well as with fully non-hydrostatic terms in the model, which was rather important for investigating the ISWs.Modeling results showed that baroclinic tides in the LS were essentially three-dimensional (3D), and that wave structures around two ridges in the strait were complicated with interesting internal oceanic phenomena. Several zonal cross-sections were chosen to illustrate vertical structures of zonal velocity field, and to show their meridional variances together with surface horizontal velocity gradients in order to highlight the advantages of 3D modeling with fully nonlinear, non-hydrostatic terms. Following Vlasenko et al. (2005), analysis of two parameters (Froude number and slope parameter that is defined as the ratio of inclination of topography to slope of radiated rays) that govern generation regime indicated that internal waves produced in the LS were subject to a mixed lee wave regime rather than baroclinic tide regime or unsteady lee wave regime.The propagation of ISWs beyond the generation area showed that manifestation of 3D effects was not very obvious, which, through further analysis, was mainly attributed to homogeneity of topography, inaccuracy of barotropic forcing, and Kuroshio intrusion in the LS. To better understand the necessity of 3D modeling, we chose several zonal cross-sections and performed various sensitivity experiments to show discrepancies between 2D and 3D cases.     

Monthly variation of some parameters about internal solitary waves in the South China sea

In this paper, by non-dimensional analysis, it is found that finite-depth theory is more appropriate to the study of internal solitary waves (ISWs) in the South China Sea (SCS) than shallow-water theory. The 1-degree grid data of monthly mean temperature and salinity data at standard levels in the SCS are used to solve the linearized vertical eigenvalue problem. The nonlinear parameter and the wave phase speed are computed, then the nonlinear phase speed and the characteristic half-width of ISWs are calculated respectively by two different theories to investigate the difference between these two parameters in the SCS. The nonlinearity is the strongest near the continental slope of the SCS or islands where the bottom topography changes sharply, it is stronger in summer than that in winter; it increases (decreases) as pycnocline depth deepens (shallows), stratification strengthens (weakens) and pycnocline thickness thins (thickens). The nonlinear wave phase speed and the characteristic half-width are the largest in deep sea area, they then reduce peripherally in shallower water. The nonlinear wave phase speed in the SCS changes slightly with time, but the characteristic half-width changes somewhat larger with time. In most of the SCS basin, the nonlinear wave phase speed derived from shallow-water theory is very close to that derived from finite-depth theory, but the characteristic half-width derived from shallow-water theory is about 0.2–0.6 times larger than that derived from finite-depth theory. The ISW induced horizontal current velocity derived from shallow-water theory is larger than that derived from finite-depth theory. Some observed and numerical modeled ISW characteristic half-widths are compared with those derived from shallow-water and finite-depth theories, respectively. It is shown that, the characteristic half-widths derived from finite-depth theory agree better with observational and numerical modeled results than those derived from shallow-water theory in most cases, finite-depth theory is more applicable to the estimation of ISW characteristic half-widths in the northern SCS. It is also suggested that, to derive the precise ISW parameters in further study, the physical non-dimensional ratios which are related with ISW characteristic half-width, amplitude, thermocline and water depths should be calculated, so that an appropriate theory can be chosen for estimation.     

Internal solitary waves in the East China Sea

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.     

Remote sensing survey and research on internal solitary waves in the South China Sea-Western Pacific-East Indian Ocean (SCS-WPAC-EIND)

Internal solitary waves (ISWs) are common mesoscale dynamic processes in the ocean that are spread throughout the world’s oceans. The South China Sea (SCS), Western Pacific (WPAC) and Indian Ocean (EIND) (SCS-WPAC-EIND) are areas where ISWs frequently occur. In particular, in the northern part of the South China Sea, Sulu Sea, Celebes Sea, Andaman Sea, Lombok Strait and northeastern part of Taiwan Island, ISWs exist almost year-round. Remote sensing is an important technique to carry out investigations and research on ISWs on a large scale. In particular, optical sensors represented by the Moderate Resolution Imaging Spectroradiometer (MODIS) can observe ISWs for a long time and on a large scale, while SAR sensors such as Sentinel-1 A/B can compensate for the deficiencies in optical sensors and comprehensively observe ISWs. Based on many years of remote sensing surveys of ISWs, this paper uses MODIS and Sentinel-1 satellite remote sensing images of more than 70 000 scenes from 2010 to 2020 to carry out survey studies of ISWs in the SCS-WPAC-EIND. The survey systematically gives the temporal and spatial distribution characteristics of ISWs in the SCS-WPAC-EIND and focuses on the analysis of the ISW characteristics in main areas in the SCS-WPAC-EIND, thereby providing basic data for further research on ISWs.     

海洋内孤立波预警监测识别技术及其在流花16-2油田群开发中的应用

我国南海内孤立波发生频繁,由于其振幅大、流速强、能量集中等特点,已成为深海油气开发海上安装工程必须考虑的重大风险之一。依托现场守护船,提出一种内波流结构单体式监测、识别技术,采用KdV方程实现内波流预警,并成功应用于流花16-2油田群开发项目海管管线终端(PLET)安装、浮式生产储卸油装置(FPSO)锚系及水下管缆回接、空气潜水等高风险作业的内波流预警安全保障中,同时基于此次内波流预警监测结果对该海域的内波生成源和生成机制进行了探讨。应用期间采用该技术共监测、识别并发布内波预警450次。应用结果表明,约93%的内波预警时刻误差在10 min以内,平均误差为±3.90 min。统计分析结果表明,该海域的内孤立波主要来源于巴士海峡而非局地的潮地相互作用,大致可以分为两类:一类是直接产生于巴士海峡,西传至该海域;另一类是局地产生,主要是由巴士海峡产生的内波西传至陆坡浅化分裂生成或是由巴士海峡产生的内潮西传至陆坡由于非线性变陡机制产生。该内波流监测技术在流花16-2油田群开发项目的成功应用,可为南海其他类似项目提供直接的借鉴。     

Seasonal and spatial variations of kinematic parameters of internal solitary waves in the Andaman Sea

The horizontally variable density stratification and background currents are taken into the variable-coefficent extended Korteweg-de Vries(evKdV) theory to obtain the geographical and seasonal distribution of kinematic parameters of internal solitary waves in the Andaman Sea(AS). The kinematic parameters include phase speed,dispersion parameter, quadratic and cubic nonlinear parameters. It shows that the phase speed and dispersion parameter are mainly determined by the topographic feature and ha...