Extraction of Intemal Tidal Currents from A Portion of Sea Current Profile

Internal tide is one of the major oceanic phenomena. Determination of internal tide is important for theoretical study and for ocean engineering research. As an inverse problem, extraction of internal tidal currenls from sea currents is diffi-cult. In this paper, a method is developed to extract internal tidal currents from a portion of the sea current profile based on the fact that the directions of internal tidal currents above and below the thermocline are inverse. Sea current data col-lected from the South China Sea is processed with this method. The internal tidal currents and the depth of the thermocline are successfully extracted. The depth of the thermocline determined is in good agreement with that measured in 1959.     

Extraction of Internal Tidal Currents from A Portion of Sea Current Profile

Internal tide is one of the major oceanic phenomena. Determination of internal tide is important for theoretical studyand for ocean engineering research. As an inverse problem, extraction of internal tidal currents from sea currents is diffi-cult. In this paper, a method is developed to extract internal tidal currents from a portion of the sea current profile basedon the fact that the directions of internal tidal currents above and below the thermocline are inverse. Sea current data col-lected from the South China Sea is processed with this method. The internal tidal currents and the depth of the thermo-cline are successfully extracted. The depth of the thermocline determined is in good agreement with that measured in 1959.     

The high-resolution vertical structure of internal tides and near-inertial waves measured with an ADCP over the continental slope in the Bay of Biscay

ADCP measurements of the velocity structure in the permanent thermocline at two locations over the continental slope in the Bay of Biscay are presented. The vertical variation of the contribution of the inertia-gravity waveband to the kinetic energy, vertical motion, and current shear are analysed. The semi-diurnal tides together with near-inertial waves appear to provide over 70% of the high-frequency kinetic energy (>1/3 cpd). Over the vertical range of the ADCP observations the phase of the harmonic M₂ tide changes up to 155°, while the kinetic energy varies in the vertical by a factor of 3.8, showing the importance of the contribution of internal waves to the observed tidal motion. Both semi-diurnal internal tidal waves and near-inertial waves have a vertically restricted distribution of the variance of the horizontal and vertical velocity, as in internal wave beams. The short-term 14-day averaged amplitude and phase lag of the M₂ tide shows large temporal changes, with a characteristic 40–45 day time scale. These changes are probably related to variations in generation sites and propagation paths of the internal tide, because of changes in the temperature and salinity stratification due to the presence of meso-scale eddies. The relatively large shear in the inertia-gravity wave band, mainly at near-inertial frequencies, supports low-gradient Richardson numbers that are well below 1 for nearly half of the time. This implies that the large shear may support turbulent mixing for a large part of the time.     

东海北部1个潜标连续观测站的潮流垂向特征分析

采用调和分析方法对东海北部海区1个坐底式潜标连续观测的近两个半月的海流数据进行分析,研究了该站点的潮流空间结构特征以及余流信息。调和分析结果表明,该点以半日潮流为主,潮流类型在40 m以上属不规则半日潮流,40 m以下则属规则半日潮流;该海区的潮流以顺时针旋转为主,潮流量值占海流的比率随深度递增,潮流椭圆要素存在明显的垂向变化。本文研究有助于了解该站点潮流的垂直变化特点,并为潮流数值模拟提供校验依据。     

天文大潮对南海北部浮游动物生物量的影响

基于2020年8月至11月在南海北部获取的声学多普勒流速剖面仪观测数据,利用后向散射强度数据估算得到相对体积散射强度并用其表征浮游动物生物量的相对大小,对相对体积散射强度的半月变化与具有半月周期的潮流动能进行相关性分析,进而分析天文大潮对声学估算的浮游动物生物量的影响。结果表明：半月周期的相对体积散射强度与潮流动能之间呈负相关关系,即天文大潮时,潮流动能较强,水体相对体积散射强度较低,浮游动物生物量则较小,天文小潮时则情况相反。初步推测其原因为：天文大潮时,强潮流一方面导致浮游动物生存环境恶化,使其生物量下降,另一方面也改变了浮游动物垂直迁移特性,浮游动物迁移到近海底处使其难以被观测。     

南海东沙岛西南大陆坡内潮特征

2008年4月-10月,在南海东沙岛西南大陆坡底部布放了1套全剖面锚系,同时沿大陆坡底部布放了3套近底锚系,应用谱分析和调和分析方法分析温度和海流连续观测资料,进而研究该海域的内潮特征.结果表明,东沙岛西南大陆坡存在强内潮现象,大陆坡底部温度变化受到内潮波的影响,上层海洋存在强日潮周期的内潮波振动;正压潮和斜压潮均以O...     

南海吕宋海峡21°N附近多潜标观测的上层海流

为了了解潮流从西北太平洋经吕宋海峡进入南海内的变化及其垂向结构,本文利用在吕宋海峡附近沿东西方向布放的多套潜标同步获得的高分辨率ADCP长时间连续观测上层海流资料,使用调和分析方法将实测海流分解成3部分:不随时间变化的定常流、周期性潮流和剩余流,并将潮流分解为正压潮流和斜压潮流。通过对实测海流中各组分的分析,得到以下结论:该区域潮流类型在不同深度上有明显变化;M2潮自吕宋海峡传入南海后强度显著减弱75%左右,K1、O1分潮在上层强度减弱约三分之一。从垂向变化来看,在潮流强度上,各站点垂直方向上潮流强度均发生变化。从方向上看,各分潮潮流椭圆东西向特征明显,长轴变化较大,短轴(南北向特征)垂向变化不显著;潮流运动主要沿逆时针方向,垂直方向上潮流明显减弱或增强时会发生转向。斜压潮流主要集中在上表层,100m左右以下随深度逐渐减弱。东西方向斜压潮流能量比正压潮流强,而南北向的流比较稳定,且斜压潮流能量远小于正压潮流。定常流强度在各站点呈现相似的变化趋势,随深度变化减弱。     

海浪混合参数化的渤海、黄海、东海水动力环境数值模拟

在浪-流耦合的概念下,对Princeton Ocean Model(POM)模式进行改进,增加特征波参数下的海浪混合作用,并把潮流和环流同时模拟,得到了渤海、黄海、东海典型的环流和水文特征,特别是夏季黄海的温跃层现象,夏季长江冲淡水扩展路径以及我国东部海域冬季和夏季典型环流等.研究表明,海浪的作用使海洋上层混合得更均匀,潮流的作用使海洋底层混合得更均匀,二者是温跃层形成的重要原因;考虑潮效应模拟流场,由于潮扩散和潮余流的作用,长江冲淡水路径与实际观测更为符合.     

Observations of internal tides in Uchiura Bay

Remarkable tidal currents associated with temperature fluctuations in the subsurface layer have been observed in Uchiura Bay. In order to study the characteristics of these tidal currents, we carried out current measurements in November 1972 and October 1974. It was confirmed from the first set of observations in 1972 that the tidal currents above and below the seasonal thermocline oscillate out of phase with each other and the tidal currents are associated with internal tides.In the second set of observations in 1974 not only current measurements but also serial BT lowerings were made. The phase of the thermocline displacements lagged behind the tidal currents by 81 for the semidiurnal constituent and by 83 for the diurnal constituent, and it is thus concluded that the internal tides in Uchiura Bay behave as standing waves.     

Generation of internal waves by barotropic tidal flow over a steep ridge

A three-dimensional nonhydrostatic numerical model is used to study the generation of internal waves by the barotropic tidal flow over a steep two-dimensional ridge in an ocean with strong upper-ocean stratification. The process is examined by varying topographic width, amplitude of the barotropic tide, and stratification at three ridge heights. The results show that a large amount of energy is converted from the barotropic tide to the baroclinic wave when the slope parameter, defined as the ratio of the maximum ridge slope to the maximum wave slope, is greater than 1. The energy flux of internal waves can be normalized by the vertical integral of the buoyancy frequency over the ridge depths and the kinetic energy of the barotropic tides in the water column. A relationship between the normalized energy flux and the slope parameter is derived. The normalized energy flux reaches a constant value independent of the slope parameter when the slope parameter is greater than 1.5. It is inferred that internal wave generation is most efficient at the presence of strong upper-ocean stratification over a steep, tall ridge. In the Luzon Strait, the strength of the shallow thermocline and the location of the Kuroshio front could affect generation of internal solitary waves in the northern South China Sea.     

基于数值模式的南海北部深层内潮季节变化特征分析

本文基于MITgcm非静力数值模式，采用实际地形、层结和潮流强迫，开展南海北部内潮数值模拟敏感性试验，分析夏冬两个季节南海北部深层内潮的差异。结果显示在南海北部深层，冬季K₁和M₂内潮流速振幅比夏季强10.1%和44.7%。垂向模态分析结果进一步表明，尽管南海北部深层冬季第一模态内潮动能密度比夏季低15.5%，但第二和第三模态内潮则是冬季比夏季高约25.1%和33.2%，导致冬季深层流速的垂向剪切大于夏季，表明冬季较强的高模态内潮可能是冬季南海深层强混合的一个原因。     

Strong tidal currents observed near the bottom in the Suruga Trough,central Japan

Current measurements carried out at the depth of 4 m above the sea bottom near the northern edge of the Suruga Trough in the early fall of 1985 indicated the existence of strong semidiurnal tidal currents, which were considered to be associated with internal tides. In order to examine the spatial structure of the bottom intensified tidal flow, more detailed current observations were carried out at three or four depths at two stations along the main axis of the Suruga Trough during about 70 days from August to October 1988. We obtained the following results: (1) the variations of the current velocity caused by the semidiurnal and diurnal internal tides are evident in all of the records, and the orientation of the major axis of each tidal ellipse nearly coincides with that of the main axis of the trough; (2) the semidiurnal internal tide is dominant over the diurnal internal tide at 4 m above the sea bottom at both stations; (3) at the northern station the semidiurnal internal tide is dominant over the diurnal internal tide, whereas they are nearly equal at the southern station except at 4 m above the sea bottom; (4) the biharmonic internal tides with 1/3 day and 1/4 day periods, are found near the sea bottom and the major axis of the tidal ellipse is perpendicular to the orientation of the main axis of the Suruga Trough.     

THE VERTICAL CHARACTERISTICS OF INTERNAL WAVES IN SHALLOW REGION OF THE EAST CHINA SEA ANALYSED FROM CTD DATA

The vertical characters of temperature, salinity and density fluctuations in shallow region of the East China Sea are analysed by using 12 CTD short records with 1-hour intervals. The average S,t and p profiles can be divided into three distinctive layers: the thermocline, and the layers above and below it. The fluctuations of t, S and p are basically caused by internal waves, but there are somewhat intrusions or entrainments at the thermocline boundaries. The vertical displacement of the thermocline centre is up to more than 6.35m and the thermocline thickness changes from 3.5 to 8.0 m. Their varature with time are intermittent and rather irregular. The vertical wavenumber spectra of temperature fluctuations have been estimated by using the maximum entropy method, indicating that energy is mainly contained in the low-wavenumber range (<0.13cpm) with two sharp peaks probably corresponding to the first and second modes of internal waves, whose dependence is estimated to be less than β-2. There exist dep     

Measurements of the Turbulent Energy Dissipation Rate around the Shelf Break in the East China Sea

Using the micro-structure profiler, TurboMAP, large values for the turbulent energy dissipation rate ε were found just above the bottom of the shelf and around the thermocline near the continental shelf break in the East China Sea. The values found above the bottom are produced by the bottom stress due to tidal currents, resulting in a distinct bottom mixed layer where the vertical eddy diffusivity Kz is also large. Distinct maxima in the values of ε detected around the thermocline are located at the depth of the fine-scale shear maxima detected with the moored ADCP. The vertical profiles of ε were compared with those of the current velocity, and it was found that the maxima in ε appear to correspond to those of the shear with fine scale. The magnitude of the observed ε coincided approximately with the ε calculated from the fine-scale shear and the buoyancy frequency according to the parameterization proposed by Gregg (1989), if the large-scale mean shear caused by the Kuroshio is subtracted. However, it is not clear whether the parameterization for the internal wave fields in the open ocean is applicable to the estimation of ε in the shelf break. Whereas the most predominant value of ε was found just above the bottom and around the thermocline, the maxima of ε could be found in the internal area. They could have been caused by the propagation of the vertically high wave number internal tides along the characteristic ray.     

Cross thermocline mixing effect due to tide in the shallow sea

Using the data specially observed on the Bohai Sea oil platform by the authors for studying thermocline for two weeks, the thermocline depth, the temperature and the salinity of the layers below and above the thermocline were numerically modelled. The model is based on the Stigebrandt model and some important supplements are made in this paper in accordance with the local condition of the sea. The modelling results fit well in the observations. It is proved that the entrainmem due to both the tide and the wind can give rise to rather intense mixing cross the thermocline. This process makes the temperature rising below the thermocline,and causes the nutrient increasing above it,which has important significance for the marine physical environment and the marine organism.     

2002年夏季广东陆架边缘上升流海域的内潮现象及其局地反馈初探

基于2002年夏季开展的"中国近海环流形成变异机理、数值预测方法及对环境的影响"观测项目获得的往复走航温盐流资料,结合同期的卫星观测(风、SST、海表动力高度)数据,初步探讨了粤东陆架边缘上升流区的内潮现象及其局地反馈特征.研究结果表明:粤东陆架边缘海域存在显著的、间歇性的相对低温海水的沿陆坡涌升现象,其中A航段的低温(18～23℃)海水涌升发生在约50～150m深度,B航段的低温(<18℃)海水涌升则主要位于150 m以深.由于观测期间海表风变化很小,而海洋涡旋在几周至几个月内相对稳定,并且研究海域涡旋对流场的影响似乎局限于75 m以浅的上层海洋,因而A、B航段的差异显然不能用海表风或海表动力高度变化来解释.进一步的分析表明粤东陆架边缘上升流区存在显著的内潮现象,尽管资料所限使得我们无法准确判定该内潮性质,往复走航海流剖面的确揭示了1阶和5阶内潮模的存在.内潮的不同模态极大地改变了上层海洋的热力和环流结构;低阶内潮模导致沿最大温度水平梯度处水温的剧烈垂向起伏(>30 m)以及跃层两侧的海流反向现象;高阶内潮模导致垂直陆坡方向水平流速的多次反向,强烈的流剪切可能与增强的混合联系在一起.导致低阶内潮模(A航段)向高阶内潮模(B航段)转变的原因可能是由于内潮特征线倾角与地形坡度比较接近而激发的"临界反射"效应.分析结果还表明,不同内潮模态导致的环流结构变异叠加在背景环流场之上,会显著影响粤东陆架边缘上升流的空间结构及强度.     

夏季北黄海南部定点高分辨率实测海流分析

对夏季北黄海南部一定点高分辨率连续ADCP(Acoustic Doppler Current Profiler)海流实测资料,使用调和分析方法分解成3部分:不随时间变化的定常余流,周期性潮流和剩余流,再将潮流分解为正压潮流和斜压潮流。通过对实测海流中各组分的分析,结合同时期卫星反演海面风场资料,温度、盐度断面调查资料,得到以下结论:夏季该站点上层定常余流的主导动力控制因素是风应力,上层表现出明显的Ekman风海流特征,中、下层流速方向与表层流向基本成反向,体现出"上进下出"的垂向空间结构,定常流速最大位于近表层,可以达到5cm/s以上;各层的潮流类型均为正规半日潮流,主要半日潮潮流椭圆长轴的方向基本上呈东南-西北方向,其椭率在近底层达到最大值,中、上层较小;从能量角度分析该站点各海流组分,潮流与剩余流所占能量较大,平均起来看,潮流能量占测量海流能量的77%,而定常余流仅占0.6%,该点的斜压潮流较弱,平均斜压潮流能量仅占正压潮流能量的5%。     

青岛外海潮致第二模内孤立波的观测研究

根据在青岛附近海域开展的现场观测数据介绍了一种可能的新的第二模内孤立波的产生机制。温度链观测数据显示在2 h内孤立波特征由显著的第一模内孤立波变为了显著的第二模内孤立波。所观测到的第二模内孤立波的波面起伏和它的垂直结构同KdV(Korteweg-de Vries)方程的理论结果吻合良好,对应涨潮引起的内波非线性系数、频散系数以及Ursell数的变化和第二模内孤立波的产生相匹配。这些结果表明,所观测到的第二模内孤立波可能是由于涨潮导致的局地层结快速变化引起的。进一步分析表明,局地层结快速变化可能由潮汐推动黄海冷水团边缘锋面移动引起。     

南海西北陆坡区内潮与近惯性内波观测研究

通过对2006年南海西北部海域近3个月的全水深流速观测资料的分析,研究了该海区正压潮、内潮及近惯性内波的时空分布特征。结果表明,全日内潮明显强于半日内潮,且最大潮流均出现在海洋上层;内潮的主轴方向基本沿东南-西北方向,近似与局地等深线垂直;内潮能量显示出明显的时间长度约为半月的大小潮调制周期;全日内潮的coherent部分占全日内潮能量的70%,而半日内潮的coherent部分占半日内潮能量的53%;进一步研究发现半日内潮主要由第一模态主导,而全日内潮第三模态能量占总能量的比例仅次于第一模态且量值上与之相当;强风过程可激发出强的近惯性运动,暖涡使得近惯性内波能量更有效地向海洋深层传播,冷涡则不利于近惯性内波能量向下传播。     

Oceanic response to Typhoon Nari (2007) in the East China Sea

The oceanic response to a typhoon in the East China Sea (ECS) was examined using thermal and current structures obtained from ocean surface drifters and a bottom-moored current profiler installed on the right side of the typhoon’s track. Typhoon Nari (2007) had strong winds as it passed the central region of the ECS. The thermal structure in the ECS responded to Typhoon Nari (2007) very quickly: the seasonal thermocline abruptly collapsed and the sea surface temperature dropped immediately by about 4°C after the typhoon passed. The strong vertical mixing and surface cooling caused by the typhoon resulted in a change in the thermal structure. Strong near-inertial oscillation occurred immediately after the typhoon passed and lasted for at least 4–5 days, during which a strong vertical current existed in the lower layer. Characteristics of the near-inertial internal oscillation were observed in the middle layer. The clockwise component of the inertial frequency was enhanced in the surface layer and at 63 m depth after the typhoon passed, with these layers almost perfectly out of phase. The vertical shear current was intensified by the interaction of the wind-driven current in the upper layer and the background semi-diurnal tidal current during the arrival of the typhoon, and also by the near-inertial internal oscillation after the typhoon passage. The strong near-inertial internal oscillation persisted without significant interfacial structure after the mixing of the thermocline, which could enhance the vertical mixing over several days.