Measurement of the current and spectral analysis on the continental shelf in the East China Sea

Direct measurements of current velocity and water temperature were undertaken at the mooring Sta. M (125°29.38' N,31°49.70'E) on the continental shelf area in the East China Sea in June 1999 by R/V Xiangyanghong 14. The relationship between various time series of oceanic fluctuations is calculated by spectral analysis. The major results are as follows: (1) an average (u,v) of (6.9,-3.0 cm/s) at the 30 m depth is obtained during the 9-day observation, and that at the 45 m depth is (3.7,-1.1 cm/s), I. E., the mean flows are southeastward at both the 30 and 45 m depths;the currents become stronger gradually during the observation period; this may be mainly attributed to the transition of the tidal currents from neap to spring; (2)semidiurnal fluctuation is the most dominant in the current fluctuations, and rotates mainly clockwise; in the next place, there is also diurnal fluctuation;the local inertial period is close to the period of diurnal fluctuation, and an inertial motion is clockwise; thus, local inertial motion combines with diurnal fluctuation, and makes the spectral peaks in clockwise components much higher than those in counterclockwise ones;except for the fluctuations of above main periods, there is also the peak at 3 d period for counterclockwise components in the upper and lower layers; (3) the calculation of cross spectra between two time series of current velocities at the 30 and 45 m depths shows that both the current fluctuations at the 30 and 45 m depths are much alike, I. E.,they are synchro; this shows that the flow field here is rather vertically homogeneous; (4)power spectra of temperature time series at both the 30 and 45 m depths show that the semidiurnal peak is the most predominant, and the second highest peak is the diurnal period; besides spectral peaks at above periods, there are also obvious spectral peaks at 6.8 h and 2 d; (5)plots of temperature time series at 16,30,35,45 and 50 m depths show that the temporal variations of temperature at these depths are synchro, which are like those in the velocity field; temperature records also show a gradual rise in temperature, which are also like those in the velocity field.     

2002年春季吕宋海峡海流观测及其谱分析

基于2002年春季航次在吕宋海峡海域锚碇测流站(20°49'57"N,120°48'12"E)200,500与800m锚碇测流水层观测流,进行的海流特征分析与最大熵方法谱分析,得到以下主要结果.(1)在200m处,观测期间海流平均速度为(47.4cm/s,346°),最大观测海流速度V_max和最大日平均海流速度V_d,max分别为(103.8cm/s,10°)和(71.6cm/s,339°);在500m处,观测期间平均流速为(20.3cm/s,350°),最大观测海流速度V_max和最大日平均海流速度V_d,max分别为(74.1cm/s,17°)和(39.1cm/s,317°).这些都表明黑潮在吕宋海峡锚碇测流站200和500m处向西北方向入侵南海.(2)在800m处,观测期间平均流速为(1.2cm/s,35°),最大观测海流速度V_max和最大日平均海流速度V_d,max分别为(10.8cm/s,76°)和(4.7cm/s,46°).这些都表明,它们的流向皆为东北向.比较在每层实测流的结果,表明在800m层海流状况与200和500m层海流状况是不相同的,流速随深度变深明显减弱,流向向右偏转.(3)在观测期间200,500和800m处,日平均流速在4月皆比3月时要强.(4)在200～800m潮流随深度变深有所变化,除了在500m处f<0情况全日潮峰值高于半日潮峰值以及对于半日潮以逆时针方向为主以外,其余情况在200～800m水层半日潮峰值都要高于全日潮的峰值,并且皆以顺时针方向旋转为主.(5)在200～800m水层都存在15d以上或14d左右的周期振动,例如在逆时针方向分量谱(f>0)在200,500m处存在19d左右的周期振动;在800m处存在14d左右的周期振动(f<0).(6)在200～800m处都存在4～6d周期天气过程的振动和2～3d周期振动.还都存在34.5h左右惯性振动周期,它的振动方向为顺时针方向.(7)通过交叉谱的计算,揭示:1)200与500m层两组流速时间序列对于半日潮周期、全日潮周期、15d以上的周期振动、2～3d的周期振动等都有很好的相关性,且对15d以上的长周期振动几乎是同步的;2)500与800m层两组流速时间序列对于4～6d天气过程的周期振动与2～3d的周期振动等都有很好的相关性,但它们之间有相位差,有滞后或提前现象.     

Tides and short-term variabilities in the Kuroshio west of Yonakuni-jima

INTRODUCFIONTheKuroshioeastofTaiwanIslandflowsnorthwardandenterstheEastChinaalgathroughapaSSagebetweenTaiwanIslandandlriomote-shima.TOstudyfurthertheKuroshiothereandestimateitstransPOrt,itisnecessarytoobservetheKuroshioCurrentdirectly.Therehavebeen*ThisstudywassupportedbytheNationalNaturalaudienceFoundationofChinaundercontractNo.49476278.severaldirectobservationsoftheKuroshionearTaiwanIsland.Forexample,thedirectcurrentmeasurementat(24'24'N1123'35'E)byinabaetal.(1981)showedtheapp…     

Vertical structure and seasonal variation of tidal and residual currents in the northern Huanghai Sea

Analysis of tidal current and sea level has been made based on the observations from the summer of 2006 to the winter of 2007,respectively.The result indicates that a two-layer structure of residual current exists in summer,with its upper layer going northwestward and the lower layer southeastward.In addition,some strong residuals exist in the neighboring depth of the pycnocline during the current period of astronomical tide.In winter,except some individual layersthe residual currents mostly direct to the northwest,from which we can see the fluctuation of abnormal sea-level and the appearance of associated current because of the changes of the wind field.The analysis of tidal ellipse indicates that the direction of the maximum semidiurnal component is clockwise from summer to winter,with an angle of 16-18.Moreover,in summer the semidiurnal component rotates with depth clockwise while the diurnal component counterclockwise.However,the vertical structure is almost homogeneous in winter.     

琼州海峡夏季三塘潮流谱分析和余流特征研究

无论潮流矢量是反时针旋转(f>0)还是顺时针旋转(f<0),全日潮谱峰都高于半日潮谱峰。2个半日潮周期分别为12、12.4h;2个日潮周期分别为23.9和25.8h;在半日潮和全日潮分量中,反时针分量是主要的:顺时针的半日潮能谱只有反时针的84%;顺时针的日潮能谱只有反时针的65%;浅水分潮6.2、8h和超过24h的3、4.2和5.5d等也有明显表现,但是都未通过显著性检验;夏季余流流速在5～10cm/s范围内变化,方向以南偏西为主。造成这种现象的首要的因素是夏季琼州海峡水交换基本态势和三塘附近海底地形,风对流向变化也有重要影响。     

东沙群岛西南海域单站潮流及温盐特征分析

利用东沙群岛西南海域定点站（２０°４１’Ｎ,１１５°４９’Ｅ）３个航次观测的海流、水温资料及同期的温、盐度断面资料,用谱分析方法对该站海流、水温时间序列进行分析。结果表明,该站海流和水温的时间变化都具有明显的全日周期和半日周期波动的特征,潮流为不规则全日潮,主轴顺时针方向旋转。内潮为全日内潮占优,且具有间歇特征。和总流动类似,斜压运动也主要是顺时针方向旋转的。     

Analysis of thermohaline and current distribution characteristics of Zhejiang and Fujian waters in summer

This paper analyzed the distribution of thermohaline and circulation characteristics of Zhejiang and Fujian waters, based on the cross-sectional thermohaline data and on current data (up to 30 d duration) at fixed-point moorings, collected in the summer of 2006. We also performed low-pass filtering and spectrum analysis on the mooring submersible buoy data. Based on that analysis, we discussed the characteristics of low frequency currents and time-variations in these waters. The main conclusions are as follows. (1) There is a low salinity pinnate area near the Hangzhou Bay in summer, and outside the low salinity area, an obvious salinity front is present from surface to bottom near 123°E. There is also a temperature front below the surface at a corresponding position. (2) Bottom water of the Taiwan Warm Current comes from the subsurface of Kuroshio. (3) The direction of low frequency current at fixed anchor stations is N-NE or S, which mainly depends on the interaction of control currents in this waters. (4) Significant spectral peaks at all mooring stations are typically semidiurnal and diurnal tides. Semidiurnal tidal waves are the main ones in these waters, and have more energy closer to the shore. (5) Significant energy spectral peaks of middle period (3 to 8 d) of currents are responses to weather frequency. (6) Significant energy spectral peaks of long periods at the surface or bottom are probably responses to seasonal wind or bottom friction, while, the long period peaks of other depths can reflect cyclical changes of interactions between currents. We conclude that the pulsation period of the Taiwan Warm Current in these waters is 10-17 d.     

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%.     

海南新海夏季潮流谱分析和余流研究

根据夏季琼州海峡新海附近15 m处地层潮流谱分析结果可得,无论是f>0 或f<0,全日潮谱峰都高于半日潮谱峰。其中,半日潮周期,约为12 h,12.4 h,日潮存在两个,一个在23.9 h,另一个在25.8 h前后;在半日潮和全日潮分量中,反时针分量是主要的、顺时针的日潮能谱只有反时针的84%;顺时针的半日潮能谱只有反时针的63%;浅水分潮8.3 h,6.2 h和超过24 h的4.2 d,3 d等也很明显,但是未通过显著性检验; 大多数余流流速在5~10 cm/s之间,个别情况超过40 cm/s。受反时针运动涡旋影响,余流主要向偏东北、北、西北方向运动。在这个区间内的流向,占总观测数68%以上;风对余流也有重要影响:东北风将使余流方向偏向西北,偏北向风将使余流方向偏南。     

Large amplitude, leaky, island-generated, internal waves around Palau, Micronesia

Three years of temperature data along two transects extending to 90 m depth, at Palau, Micronesia, show twice-a-day thermocline vertical displacements of commonly 50–100 m, and on one occasion 270 m. The internal wave occurred at a number of frequencies. There were a number of spectral peaks at diurnal and semi-diurnal frequencies, as well as intermediate and sub-inertial frequencies, less so at the inertial frequency. At Palau the waves generally did not travel around the island because there was no coherence between internal waves on either side of the island. The internal waves at a site 30 km offshore were out-of-phase with those on the island slopes, suggesting that the waves were generated on the island slope and then radiated away. Palau Island was thus a source of internal wave energy for the surrounding ocean. A numerical model suggests that the tidal and low-frequency currents flowing around the island form internal waves with maximum wave amplitude on the island slope and that these waves radiate away from the island. The model also suggests that the headland at the southern tip of Palau prevents the internal waves to rotate around the island. The large temperature fluctuations (commonly daily fluctuations ≈10 °C, peaking at 20 °C) appear responsible for generating a thermal stress responsible for a biologically depauperate biological community on the island slopes at depths between 60 and 120 m depth.     

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.     

Stochastic Modeling of Short Term Variations of Sea Level in Eastern Canada

Hourly sea level records from three stations in eastern Canada (Québec-Lauzon, Harrington-Harbour and Halifax) are analyzed both in frequency domain from 1970 to 1979 and in time domain during 1973. At the three stations, the deterministic model explains 90 to 96% of the total variability of sea level. The semidiurnal and diurnal tides contribute largely to its variations. The residual series, less than 10% of the initial variations of sea level, contain irregular values including extreme values of seiches and storm surges. Such random variations are analyzed and modeled following the method described by Box and Jenkins (1976). The long period variations (2 to 30 days) can be attributed to meteorological forcing (atmospheric pressure and winds). The short period variations (some hours to one day) can be attributed to longitudinal seiches, semidiurnal and diurnal atmospheric tides, and inertial oscillations. The water discharge from the St. Lawrence River contributes 29% of the monthly residual sea level at Qué     

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.     

南海北部东沙岛附近海洋底层流场特征及时间变化规律

Near-bottom currents play important roles in the formation and dynamics of deep-water sedimentary systems.This study examined the characteristics and temporal variations of near-bottom currents, especially the tidal components, based on two campaigns(2014 and 2016) of in situ observations conducted southeast of the Dongsha Island in the South China Sea. Results demonstrated near-bottom currents are dominated by tidal currents, the variance of which could account for ～70% of the total current variance. Diurnal tidal currents were found stronger than semidiurnal currents for both barotropic and baroclinic components. The diurnal tidal currents were found polarized with predominantly clockwise-rotating constituents, whereas the clockwise and counterclockwise constituents were found comparable for semidiurnal tidal currents. It was established that diurnal tidal currents could induce strong current shear. Baroclinic tidal currents showed pronounced seasonal variation with large magnitude in winter and summer and weak magnitude in spring and autumn in 2014. The coherent components accounted for ～65% and ～50% of the diurnal and semidiurnal tidal current variances,respectively. The proportions of the coherent and incoherent components changed little in different seasons. In addition to tidal currents, it was determined that the passing of mesoscale eddies could induce strong nearbottom currents that have considerable influence on the deep circulation.     

南海西北部上层海洋对台风“杜苏芮”的响应分析

基于锚碇观测资料，本文分析了南海西北部陆坡区上层海洋对台风“杜苏芮”的动力学和热力学响应特征。在动力学响应方面，台风“杜苏芮”期间上层流速显著增强，混合层纬向流速可达1.20 m/s；“杜苏芮”经过后上层海水运动以近惯性振荡为主(流向顺时针旋转周期在36～40 h之间)。近惯性能量在垂向分布上存在两个高值中心，分别位于混合层和温跃层深度上。近惯性能量耗散过程的e折时间尺度约为3.7 d，我们认为能量的向下传播在局地近惯性能量衰减过程中起主要作用。对能量谱的分析表明，“杜苏芮”作用期间近惯性频率能量相对于其作用前增大了约29.4倍，而全日和半日频率(K₁和M₂)能量有所减弱。此外，能量谱显示近惯性频率存在明显的“蓝移”现象，即对于纬向和经向流速分量在400 m以浅平均的近惯性振荡频率分别为1.167 f₀和1.170 f₀(f₀为局地惯性频率)。蓝移与近惯性内波的向下传播及正的相对涡度的输入有关。在热力学响应方面，上层海洋在台风的搅拌作用下，40～250 m深度均出现较小增温，最大增温幅度接近1°C；此外70 m以浅盐度的降低可能与台风过境时的降水相关，而Ekman抽吸引起的上升流则可能对70～100 m深度盐度的升高具有重要作用。     

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.     

Spectrum of mesoscale sea level oscillations in the northern Black Sea: Tides,seiches, and inertial oscillations

Long-term data from 23 tide gauges were used to analyze the spectrum of mesoscale sea level variability of the Black Sea. The tides have sharp spectral peaks, and they are detected at diurnal and semidiurnal frequencies for all stations. A local wide spectral peak associated with inertial oscillations is located between the diurnal and semidiurnal tidal peaks. This peak is well known in the spectra of the current velocity variations of the Black Sea, but in the sea level spectrum it has been identified for the first time. At frequencies of >3 cpd, sea level spectra of the Black Sea have (1) wide maxima in the continuous spectrum, which correspond to the main eigenmodes of the sea with periods of 5.6, 4.8, 4.1, and 3.1 h, and (2) sharp peaks of radiational harmonics S₃, S₄, S₅, and S₆. The periods of seiches calculated in this study are close to the periods of eigenmodes of the Black Sea, obtained by the numerical modeling of other authors. The main factors influencing the formation of radiational tides in the Black Sea are presumably breezes and runoff from large rivers. The significant predominance of a harmonic with frequency of 5 cpd (S₅) over other radiational harmonics is caused by the influence of an eigenmode, with a frequency of about 5 cpd. The proximity of the periods of these oscillations leads to resonant amplification and to a corresponding increase in amplitude of the radiational harmonic S₅.     

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...     

Observation of the abyssal current in the West Mariana Basin

In order to investigate long-term variations of deep sea currents and temperature in the western North Pacific, a direct current measurement was made at 12.5°N, 137°E from July 1985 to July 1986. The current meter was moored at a depth of about 4,000 m (bottom depth 4,604 m) in the West Mariana Basin, very close to the deep water passage to the North Pacific Basin. Throughout the observed period, the current flows southward with an average speed of 0.8 cm sec^–1. There are significant variations of both currents and temperature within the period of 60 days. For the shorter time scales, in addition to the tidal oscillations with one day and half day periods, there is a notable spectral peak of the current with a clockwise rotation at a period of 2.2 days, which is slightly shorter than the local inertial period of 2.3 days.The observed southward current seems to indicate that the deep sea water in the West Mariana Basin flows out through the sill which is deeper than 4,000 m and is located about 200 km southeast of the mooring point. A simple analysis of the linear plane wave indicates that the medium time-scale variation with a period of 60 days is associated with the barotropic Rossby wave whose wavelength is 390 km and whose trough direction is 30° clockwise from the north.     

Tidal oscillations in the Caspian Sea

Long-term hourly data from 12 tide gauge stations were used to examine the character of tidal oscillations in the Caspian Sea. Diurnal and semidiurnal tidal peaks are well-defined in sea level spectra in the Middle and South Caspian basins. High-resolution spectral analysis revealed that the diurnal sea level oscillations in the Middle Caspian Basin have a gravitational origin, while those in the South Caspian Basin are mainly caused by radiational effects: the amplitude of diurnal radiational harmonic S₁ is much higher than those of gravitational harmonics О₁, P₁, and K₁. In the North Caspian Basin, there are no gravitational tides and only weak radiational tides are observed. A semidiurnal type of tide is predominant in the Middle and South Caspian basins. Harmonic analysis of the tides for individual annual series with subsequent vector averaging over the entire observational period was applied to estimate the mean amplitudes and phases of major tidal constituents. The amplitude of the M₂ harmonic reaches 5.4 cm in the South Caspian Basin (at Aladga). A maximum tidal range of 21 cm was found at the Aladga station in the southeastern part of the Caspian Sea, whereas the tidal range in the western part of the South Caspian Basin varies from 5 to 10 cm.