Difference in the prevailing periods of internal tides between Sagami and Suruga Bays: Numerical experiments

Current measurements in the surface layer in Sagami and Suruga Bays showed existence of significant tidal currents which are considered to be mainly due to internal tides (Inaba, 1982; Ohwaki,ea al., 1991). In addition, the prevailing period of the tidal currents is semidiurnal in Sagami Bay, but diurnal in Suruga Bay. To explain this difference in the prevailing, periods, numerical experiments were carried out using a two layer model. The internal tides are generated on the Izu Ridge outside the two bays. The semidiurnal internal tide propagates into Sagami Bay having characteristics of an internal inertia-gravity wave, while it propagates into Suruga Bay having characteristics of either an internal inertia-gravity wave or an internal Kelvin wave. The diurnal internal tide behaves only as an internal Kelvin wave, because the diurnal period is longer than the inertia period. Thus, the diurnal internal tide generated on the Izu Ridge can be propagated into Suruga Bay, while it cannot propagate into the inner region of Sagami Bay, though it is trapped around Oshima Island, which is located at the mouth of Sagami Bay. The difference in the propagation characteristics between the semidiurnal and diurnal internal tides can give a mechanism to explain the difference in the prevailing periods of the internal tides between Sagami and Suruga Bays.     

Multimodal structure of the internal tides on the continental shelf of the northwestern South China Sea

Based on the moored current and temperature observations during the summer of 2005, the vertical structure of the internal tides on the continental shelf of the northwestern South China Sea (SCS) is studied. The vertical structure of the internal tides was found to differ greatly between semidiurnal and diurnal constituents. Generally, the diurnal constituents are dominated by the first-mode motions, which are consistent with the overwhelming first-mode signals in the northeastern SCS. In contrast, the semidiurnal internal tides, unlike the predomination of the first-mode variations in the northeastern area, exhibit a higher modal structure with dominate second-mode signals in the observational region. Moreover, although the diurnal internal tides are much stronger than the semidiurnal component, the shear caused by the latter over various scales was found to be significant compared to that induced by the diurnal tides, probably due to the superposition of the first-mode and higher-mode (smaller scale) semidiurnal variations. Further analysis demonstrates that the shear induced by the diurnal internal tides is larger than that induced by the semidiurnal variations around 45 m depth, where the first-mode current reversal in the vertical happens, while below 45 m depth higher-mode semidiurnal internal tides generally produce larger shear than that by the diurnal component. The northwest-propagating semidiurnal internal tides of higher-mode with small vertical scale, probably do not originate from a distant source like Luzon Strait, but were likely generated near the experiment site.     

Numerical experiments of internal tides in Suruga Bay

Numerical experiments were performed to explain the observed results of the internal tides in Uchiura Bay. The experiments for the generation of the internal tides in Suruga Bay indicate that the internal tides, generated at the slopes in the bay, are not as large an amplitude as those observed in Uchiura Bay. However, when the semidiurnal internal tides incident through the mouth of Suruga Bay are considered, they are amplified. The amplitude at the head of Uchiura Bay is 6–12 times larger than that at the mouth of Suruga Bay under the summer density structure. Under the fall density structure, the amplitude ratio is approximately 4–6. The amplification of the semidiurnal internal tides in Uchiura Bay is considered to be due to resonance of the longitudinal internal seiche of Uchiura Bay. On the other hand, the calculated diurnal internal tides are not as large as those observed. Therefore, the diurnal internal tides are thought to already have these large amplitudes at the mouth of Suruga Bay. Therefore, from the observations and numerical experiments, it is concluded that the internal tides observed in Uchiura Bay are mostly the internal tides originating from the outer region of Suruga Bay, and the semidiurnal tides are the internal seiche which is resonantly amplified.     

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.     

Evidence for predominance of internal tidal currents in Sagami and Suruga bays

Tidal currents observed in a surface layer overlying deep water in Sagami and Suruga Bays frequently have large amplitude in summer and fall. Numerical experiments show that the current amplitude due to the surface tides is below 1.0 cm sec^–1 for the semidiurnal and diurnal constituents in the inner region of the two bays. The observed current amplitudes are larger than the calculated ones due to the surface tides. Therefore, the observed tidal currents are indicated to be due mainly to the internal tides. In addition, the semidiurnal currents dominate the diurnal currents in Sagami Bay, while the opposite occurs in Suruga Bay. These results suggest that the prevailing periods of the internal tides differ between the two bays,i.e., the internal tide has a semidiurnal period in Sagami Bay and a diurnal period in Suruga Bay.     

南海东北部内潮的模态结构和传播

The evolution of energy, energy flux and modal structure of the internal tides(ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a～14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m~2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs.From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.     

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。     

1998年南海北部20天定点连续观测的内潮分析（英文）

对1998年6月南海北部20天的海流和温度定点连续观测资料进行分析,得到该海域内潮的特征及其能量分布。分析结果显示内潮的主要成分为O1,K1,M2与S2分量,其中全日内潮（O1与K1）的能量占主要部分。在观测期间,此四个分量的海流失量均为顺时针旋转,其潮流椭圆半长轴的最大值超过14cm/s。海水温度的变化显示出内潮存在准日周期振动,平均垂向振幅达到50m。观测到的内潮携带高能量且其活动存在不连续性,在观测范围内,全日内潮的动能及势能密度的最大值分别达到2kJ/m^2及3.5kJ/m^2,半日内潮的动能及势能密度的最大值分别达到1kJ/m^2及1.5kJ/m^2。     

The variability of internal tides in the Northern South China Sea

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.     

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...     

南海北部陆架海域内潮特征的观测研究

利用2014年南海东沙岛西北部海域70余天的流速剖面高频观测资料,研究分析了该海区正压潮、内潮的时空分布特征。结果表明,观测海区正压潮流以O_1,K_1,M_2,S_2为主;斜压潮流中,除四大分潮之外,MU_2与2Q_1分潮能量也较强;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。全日内潮的锁相部分占全日内潮能量的17.5%,而半日内潮的锁相部分占半日内潮能量的30%;进一步研究发现半日内潮主要由第一模态主导,而全日内潮第二模态占比50%,约为其第一模态能量的两倍;内潮模态能量占比显示出显著的大小潮调制的半月周期。对比不同垂向模态计算方法发现,当流速观测深度有限时,利用全水深温盐资料计算观测范围内流速垂向模态是更为准确的方式。     

Temperature variability caused by internal tides in the coastal waters of east coast of Peninsular Malaysia

The effects of tidal currents (i.e., barotropic and internal tides) are important in the biogeochemistry of a coastal shelf sea. The high-frequency of currents and near-bottom temperatures collected in three consecutive southwest monsoon seasons (May, June, July and August of 2013 until 2015) is presented to reveal the role of the tidal currents to the temperature variability in the coastal shelf sea of the east coast of Peninsular Malaysia (ECPM), south of the South China Sea (SCS). The results of a spectral density and harmonic analysis demonstrate that the near-bottom temperature variability and the tidal currents are influenced by diurnal (O₁ and K₁) and semidiurnal (M₂) tidal currents. The spectral density of residual currents (detided data) at 5, 10 and 16 m depth also shows significant peaks at the diurnal tidal frequency (K₁) and small peaks at the semidiurnal tidal frequency (M₂) indicating the existence of internal tides. The result of the horizontal kinetic energy (HKE) shows a strong intermittent energy of internal tides in the ECPM with the strongest energy is found at 16 m depth during a sporadic cooling event in June and July. A high horizontal cross-shore heat flux (16 m) also indicates strong intrusions of cooler water into the ECPM in June and July. During the short duration of cold pulse water observed in June and July, a cross-wavelet analysis also reveals the strong relationship between the near-bottom temperatures and the internal tidal currents at the diurnal tidal frequency. The intrusion of this cooler water is probably related to the monsoon-induced upwelling in June. It is loosely interpreted that the interaction between the strong barotropic tides and the steep slope in the central basin of the SCS under the stratified condition in southwest monsoon has generated these internal tides. The dissipation of internal tides from the slope area probably has driven the cold-upwelled water into the ECPM coastal shelf sea when the upwelling intensity is the highest in June and July.     

Tides and currents in Fury and Hecla Strait

Under-ice current measurements reveal a homogeneous flow of water along Fury and Hecla Strait; it had a westward direction during the passage, over Igloolik, of low pressure cells in the atmosphere and an eastward direction at other times. It is presumed that barometric pressure differences between the Gulf of Boothia and Foxe Basin control partly the direction of the flow. The diurnal and semidiurnal components of the tidal currents also have a simple structure across the section of measurements. The subtidal and tidal components of the current all have the same order of magnitude, namely some 15 cm/s. The vertical tide progresses eastward; the diurnal component diminshhes abruptly once it enters Foxe Basin while the semidiurnal component increases steadily, its mean amplitude rising from 48 to 68 cm between the two extremities of the channel. Convective and frictional effects are evident in the water level records, indicating marked non-linear effects in the tides and currents, an eastward decrease in the diurnal currents and an increase in the semidiurnal ones.     

Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.     

吕宋海峡內潮的季节变化特征及其对背景流的响应研究

基于2008年秋季至2009年夏季共9个月的锚定潜标流速资料,分析了吕宋海峡西南內潮的时空特征.谱分析结果显示,该观测点全日內潮和半日內潮较为显著,尤其体现在顺时针旋转部分.除春季第二模态占优外,全日內潮主在其余三个季节均以第一模态为主,而半日內潮呈现变化的多模态结构.此外,全日內潮的动能具有明显的季节差异,冬季能量最强,夏季紧随其后,而在春、秋两季能量最小.通过分析发现,非相干运动对此季节性特征起主要作用,它反映了內潮与背景场的相互作用.然而,半日內潮却没有显著的季节性差异,而且能量较全日內潮更小,尤其在冬季,只有全日內潮动能的三分之一.同时,半日內潮的不规则变化也是与多变的背景场相关的.半日內潮的非相干部分占到了半日內潮总能量的37%左右,而全日內潮更小一些,只有22.2%.     

内潮耗散与自吸-负荷潮对南海潮波影响的数值研究

利用非结构三角形网格的FVCOM海洋数值模式,在其传统二维潮波方程中加入参数化的内潮耗散项和自吸-负荷潮项,计算了南海及其周边海域的M_2、S_2、K_1和O_1分潮的分布。与实测值的比较表明,引入这两项对模拟准确度的提高有明显效果。根据模式结果本文计算分析了研究海域的潮能输入和耗散。能量输入计算表明,能通量是潮能输入的最主要构成部分,通过吕宋海峡断面进入南海的M_2和K_1分潮能通量分别为38和29GW;半日周期的自吸-负荷潮能量输入以负值居多,而全日周期的自吸-负荷潮能量输入以正值居多,因而自吸-负荷潮减弱了南海的半日潮,并加强了南海的全日潮。引潮力的作用也减弱了半日潮而加强了全日潮,但其作用要小于自吸-负荷潮。潮能耗散的分析显示底摩擦耗散在沿岸浅水区域起主导作用,内潮耗散则主要发生在深水区域。内潮耗散的最大值出现在吕宋海峡,且位于南海之外的海峡东部的耗散量大于位于南海之内的海峡西部的耗散量。对M_2和K_1分潮吕宋海峡的内潮耗散总值分别达到16和23GW。     

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.     

南海西北部半日内潮参量次谐频不稳定生成的近惯性波研究

为了研究非临界纬度上参量次谐频不稳定(parametric subharmonic instability,PSI)过程生成的近惯性波(near-inertial wave,NIW),本文基于国家重点研发项目的 准实时传输深海潜标资料,对内波速度谱、近惯性流速和动能、D2-f(半日频减惯性频)流速和动能、半日内潮流速和...     

M2 tidal currents near the continental shelf margin in the East China Sea

Time series of velocity and water temperature were measured at three stations on the continental shelf, on the shelf margin and on the slope off the northwest Tokunoshima in December 1980 to study influences of the slope on tides.Tidal currents with semidiurnal periods were dominant at the stations on the shelf and shelf margin. However, semidiurnal components in temperature fluctuations were dominant at the stations on the shelf margin and the slope. We estimated horizontal currents due to semidiurnal internal tides from the vertical distribution of water density and temperature, assuming that the temperature fluctuations were caused by the vertical displacement of water particles due to semidiurnal internal tides. The tidal ellipses at the station on the shelf and the phase relation of the tidal currents between the two stations on the shelf and shelf margin indicated that the M₂ surface tide on the shelf was a Sverdrup wave propagating to the northwest.Semidiurnal tidal currents on the slope were also caused by tides of surface and internal modes. Furthermore, the axis of the tidal ellipse was not perpendicular to the co-tidal line estimated by Ogura (1934) but rather parallel to the isobaths on the slope, which shows a striking effect of the bottom topography on the tidal currents.     

Transition of Tidal Waves from the East to South China Seas over the Taiwan Strait: Influence of the Abrupt Step in the Topography

Observations of tidal waves between the East and South China Seas (ECS and SCS) over the Taiwan Strait (TS) suggest that the diurnal tides simply appear as one southward-propagating wave from the ECS to the SCS through the TS. The semidiurnal tides, however, behave differently in that they appear as a southward-propagating Kelvin wave in the western TS and a nearly standing wave in the eastern TS, and then diminish rapidly over the shallow shoal in the southern TS. A smaller-domain model, with sea-level boundary conditions derived from a larger-domain tidal model, was first used to simulate tides in the TS to an overall percentage of accuracy of about 90%. Subsequent numerical experiments and theoretical analysis revealed that the southward-propagating semidiurnal tides to be impeded and then reflected as they arrive at an abrupt, deepened step in the topography of the southern TS. This reflection enhances the amplitudes of the incident semidiurnal tides and contributes to the formation of a nearly standing wave in the eastern TS. The southward-propagating diurnal tides in the TS are connected by the diurnal tides in the northern SCS when the amplitudes of the two tide systems are comparable and their phases nearly equal at the step. This revised version was published online in July 2006 with corrections to the Cover Date.