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.     

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.     

Circulation pattern and current variations with respect to tidal frequency in the sea near the head of Suruga Bay

Current measurements were conducted 10 m below the sea surface near the head of Suruga Bay intermittently from 1970 to 1978. The circulation pattern is usually counterclockwise; northward along the east coast (off Heda and at the mouth of Uchiura Inlet), westward along the north coast (off Fuji), and southwestward along the west coast (off Shimizu). The amplitudes of the four major tidal constituents of current variation, M₂, S₂, K₁ and O₁, are much larger than those expected from sea level variations along the coast. The amplitudes of the diurnal constituents of current variation are much larger than those of the semidiurnal constituents, while the amplitudes of the semidiurnal constituents of sea level variation are much larger than those of the diurnal constituents. The observed amplitude of the predominant diurnal constituents exhibit large seasonal changes and tend to increase with the development of the stratification of the upper part of the water in Suruga Bay. These facts strongly suggest that the observed current variations are mainly associated with internal tides in Suruga Bay.     

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.     

Internal tides in Uchiura Bay

Long-term temperature measurements of the subsurface layer near Uchiura-Bay head were made in summer 1978 and 1979 to further investigate the characteristics of the internal tides in more detail. The temperature measurements indicate that the internal tides were present at all times during the observational period. The semidiurnal internal tides are usually dominant over the diurnal, but at times the diurnal internal tides have comparable amplitudes with the semidiurnal. When the semidiurnal internal tides are predominant, the fundamental period of the longitudinal internal seiche in Uchiura Bay estimated from the observed density structure is close to the semidiurnal period. However, the reason for the occasional amplification of the diurnal internal tides is not clear.     

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.     

福建海区的潮汐和潮流

本文根据实测资料和潮汐学基本理论,分析了福建海区潮波结构和特征、潮汐和潮流主要特征值的分布规律。结果表明,泉州湾以北海区潮汐很强,为半日潮;港湾区因受地形影响潮流很强,性质为半日潮;外海区潮流很弱,性质多为混合潮。福建南部海区潮汐很弱,性质多数是混合潮;潮流很强,性质为半日潮。     

Current variation in the sea near the mouth of Suruga Bay

In order to investigate the circulation pattern and the characteristics of tidal currents in the sea near the mouth of Suruga Bay, current meausrements were conducted at two stations at near-surface levels during the warm months of the year (late May to early October). The mean currents at the eastern and western stations are inversely correlated with each other: when inflow occurs at the eastern station outflow occurs at the western one andvice versa. The circulation pattern, therefore, can be determined from the mean current at either station,e.g., inflow or outflow at the eastern station corresponds to counterclockwise and clockwise circulation, respectively. The predominant tidal constituents of the measured currents are diurnal while those of the sea level along the bay are semidiurnal. The amplitudes of the diurnal constituents of the current show clear long-term variations besides the semimonthly variation corresponding to the phase of the moon.Variation in the path of the Kuroshio off the bay mouth seems to influence not only the circulation pattern but also the tidal currents in the sea near the bay mouth. When the Kuroshio axis is to the north of Zenisu, a shoal off the eastern side of the bay, the circulation pattern is counterclockwise and the amplitude of the tidal current is small. On the other hand, when the Kuroshio is to the south ofZenisu, the circulation pattern is weakly clockwise or stagnant and the amplitude of the tidal current is large.     

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.     

A combined numerical tidal model for the Hangzhou Bay and Qiantang River

-In order to avoid prescribing open boundary condition on the upstream side of the Hangzhou Bay, in numerical simulation of the tides and residual currents of the Bay, a 1-D model for the Qiantang River is connected to the 2-D model for the Hangzhou Bay. The harmonic constants of diurnal constituent [ (K1 O1)/2],semidiurnal constituent (M2) and shallow water constituent (M4) are obtained. The results produced by the combined model are in better agreement with the observed ones than those produced solely by the original 2-D model. The combined model gives much more reliable results for tide-induced residual water level and current.     

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...     

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。     

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。     

Variability of internal tides in the coastal water area of Oahu Island (Hawaii)

The variability of the parameters of semidiurnal internal tides in Mamala Bay (Oahu Island, Hawaii) was investigated using the experimental data obtained with ADCP bottom current profilers and ther-mistor strings. It was stated that the size, shape, and orientation of the orbits, as well as the sign of the rotation of the orbital currents, are rarely similar to those characteristic of progressive internal waves on a rotating Earth. It is supposed that such unusual features of the orbital currents are related to the interference of the waves that arrive from the straits rimming Oahu Island and to the waves that, under favorable conditions, are episodically generated at the shelf edge of Mamala Bay. Due to the fact that the floor inclinations of the shelf in the bay are supercritical for semidiurnal internal tides, the local generation of internal tides is poorly efficient.     

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.     

大亚湾和大鹏湾两个相邻海湾潮波浅水变形的异同分析

利用实测资料分析重构了大亚湾和大鹏湾潮汐水位“双峰”现象,确定了浅水分潮的异常增长是潮位“双峰”现象的主要成因,其中四分之一日分潮和六分之一日分潮起着至关重要的作用。通过SCHISM模型构建大亚湾和大鹏湾附近海域高分辨率水动力模型,模拟结果表明近岸海域,在大亚湾以东,潮汐类型为不规则全日潮,以西为不规则半日潮,在两个海湾内均为不规则半日潮;研究海域的潮流均表现为不规则半日潮流。四分之一日分潮和六分之一日分潮在大亚湾和大鹏湾的不同变形过程是造成两个相邻海湾水文差异的直接原因。通过构建不同底摩擦强度、消除水底地形以及改变海湾水深的数值实验研究表明,分潮传播方向与水深变浅方向是否一致,是导致两个海湾潮波浅水变形不同的根本原因。     

Pelagic chaetognaths in Sagami Bay and Suruga Bay,Central Japan

Pelagic chaetognaths in Sagami Bay and Suruga Bay, Central Japan, were studied. Their community structure was very similar in both of these bays. Four genera and 26 species were identified. In Sagami Bay the habitat segregation by depths was clearly obtained. The population and species number of epipelagic (0–200 m) chaetognaths were larger in summer than in winter. The populations of meso- (200–500 m) and bathypelagic (500–1,000 m) species were smaller than that of epipelagic ones and fairly stable seasonally.     

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.     

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.