Typhoon-induced strong surface flows in the Taiwan strait and pacific

Surface Velocity Program drifters drogued at 15 m depth were deployed in the Taiwan Strait (TS) and Luzon Strait in 2005 and 2006. Several drifters in the TS and the Pacific were fortuitously overrun by the typhoon Hai-Tang (July 2005) and Shan-Shan (September 2006), respectively. The drifter and QuikSCAT wind data clearly demonstrate that the surface current over the TS and the Pacific can change dramatically for a period of about two days due to the strong winds of a typhoon during its passage. Our results show that the area of storm-affected surface currents is considerably smaller for a weaker typhoon (category 2 Shan-Shan), about 300∼400 km in radius, than for a stronger typhoon (category 5 Hai-Tang), about 800 km in radius. The maximum observed current speed in the TS was 1.7 ms⁻¹ (or 2.2 ms⁻¹ in net speed change) under the influence of Hai-Tang, and 2 ms⁻¹ in the Pacific under the influence of Shan-Shan. Drifter observations revealed the unusual phenomenon of flow reversal in the surface layer of TS and the Kuroshio induced by the typhoon passage. The effect of a typhoon on surface flows is amplified by the long, narrow geometry of the TS. Surface currents generated by wind forcing along the passage of a traveling typhoon can be explained by the Ekman drift.     

Bashi channel,Luzon strait: A hydraulic model experiment of the tidal current

Bottom current conditions due to tidal currents in the area of the Bashi Channel, Luzon Strait, are examined using a scaled hydraulic model in a uni-directional flow channel. The geometrical model is scaled in both the similitude of Froude's Number and the similitude of the four-thirds power law of eddy viscosity. The currents obtained from the experiment seem to be consistent in order of magnitude with the estimated currents based on observations made by Offshore Environmental System Ltd. In a quasi-steady tidal phase, formation of a strong core flow is found in the Bashi Channel. It can be explained by a simple theoretical model in which the formation of the strong current is attributed to the submarine topography of the Luzon Strait.     

Water exchange and material exchange through a strait due to tidal flow

The process of material transport through a strait due to tidal flow is modeled, and then the differences between various concepts of tidal exchange which have been used hitherto are pointed out using this model. In particular, the exchange of water itself and the exchange of material should be distinguished even in the case where the material of interest is carried by the water,i.e., the material and water move as one body. Further, the physical meaning of “tidal trapping” (Fischeret al., 1979) is discussed by using the model in this paper. The relationship between the exchange ratio for the water itself (r) and the phase lag (δ) of material concentration to the tidal stream in a section of the strait, which is an important factor in tidal trapping, is obtained as follows: $$\delta = \tan ^{ - 1} \left( {\tfrac{1}{r} - 1} \right)$$ Observational results at Lake Hamana (Shizuoka Pref) and at Kabira Cove (Okinawa Pref.) support the validity of the above relationship.     

Three-dimensional numerical experiments on tidal exchange through a narrow strait in a homogeneous and a stratified sea

We have investigated the three-dimensional Lagrangian motion of water particles related with tidal exchange between two basins with a constant depth connected through a narrow strait and the effects of density stratification on the exchange processes by tracking a number of the labeled particles. Tide-induced transient eddies (TITEs), which are similar to those in two-dimensional basin, are generated behind the headlands. Upwelling appears around the center of the eddy and sinking around the boundary. When the basins are filled with homogeneous water, a pair of vortices are produced in the vertical cross section of the strait due to bottom stress, with upwellings along the side walls of the strait and sinking in the center of the strait. These circulations form the horizontally convergent field in the cross-strait direction in the upper layers while the horizontal divergence takes place in the bottom layer. These vertical water-motions produce the three-dimensional distribution of velocity shear and phase lag of the tidal current around the strait, and the Lagrangian drifts of water particles become large. As a result, water exchange through the strait is greatly enhanced: The water exchange rate reaches 94.1% which is much larger than that obtained in the vertically integrated two-dimensional model. When the basins are stratified, the stable stratification suppresses the vertical motion so that a pair of vertical vortices are confined in the lower layers. This leads to a decrease in the exchange rate, down to 88.6%. Our numerical results show that the three-dimensional structure of tidal currents should be taken into account in tidal exchange through a narrow strait.     

Structure of tidal mixing at the Hayasui Straits

Based on the observation at the Hayasui Straits, actual conditions of the tidal mixing, of two water masses separated by the strait, are investigated for the time scale shorter than the the tidal period. Analyses show that the tidal mixing of water masses occurs by the disintegration of mutually penetrating small masses of water, which have been produced by the reciprocating tidal motion of the water masses with some topographical effect, especially at the slack water as large tongues. The mutual penetration occurs rather horizontally, and some of these turbulent smaller masses of water keep their own properties for a rather long period of the order of three hours, without mixing even in the violent shear flows. Initial horizontal scale of these turbulent masses is estimated as of the order of three kilometers. These actual conditions reveal themselves as the macroscopic diffusion coefficient.     

钱塘江强潮河段江道缩窄治理研究

对钱塘江强潮河段江道缩窄治理进行研究。首先,回顾治理方案的比选,江道全线缩窄时河轴线线路和河道宽度的确定原则,以及不同河段的指数放宽率。然后,介绍了在涌潮汹涌、滩涂淤坍变化剧烈的强潮河段,实施筑堤所采取的工程措施的步序与要点。最后,分析论证了大规模江道缩窄治理后河床、潮汐、洪水位及涌潮高度等特征值的变化。可供进一步深入理论研究和工程施工参考运用。     

强潮河口海塘基础防冲设计研讨

钱塘江河口是举世闻名的强潮河口,水动力强,有涌潮作用,河床滩地冲淤剧烈,因此给海塘基础防冲工程设计和施工造成较大难度,并成为长期来有待攻克的技术难题。本文就近期海塘建设防冲试验工程采用的板桩防冲墙、小沉井防冲墙、土工布模袋砼和丁坝群等防冲技术措施及其运行情况进行分析讨论。     

Tidal dynamics in the strong tidal current environment of the Uldolmok waterway, southwestern tip off the Korean peninsula

Uldolmok waterway, located between an island off the southwestern tip of Korean peninsula and mainland, is famous for its strong tidal current that has a maximum current of about 6.0m/s. A series of field observations along with numerical modeling have been carried out in order to understand the tidal dynamics in terms of the force balance along the whole waterway and the energy balance in the narrowest part of the waterway. First, analysis of the ADCP current and the tide level variation data reveals that the tidal dynamics along the total waterway (channel) is balanced dominantly between the pressure gradient and linear bottom frictional forces, with the phase lag of sea level difference for the semi-diurnal constituents leading the current phase about by 10°. Secondly, the result of the numerical modeling reveals that the tidal energy flux vector flows toward the narrowest section, indicating that there should be related nonlinear processes. Through the numerical model experiment with multi-components, the convergence of (M₂ + S₂) tidal energy flux of 6.68 × 10⁷ Joule/s in the narrow area of the Uldolmok waterway is explained mainly by the energy consumption of 73% through the nonlinear generation of shallow water components and by the bottom frictional energy dissipation of 27%. This reveals that the remarkably strong nonlinear process dominates in the narrowest section of the Uldolmok waterway, compared with other areas, such as Yellow and East China Seas where the total M2 energy flux through the open boundary is balanced in terms of the bottom dissipation (Kang et al. 2003; Choi 1980).     

强潮河口三维无结构网格盐度数学模型

采用平面无结构网格和垂向直接分层,建立了强潮河口盐度三维斜压力数学模型。无结构网格使模型能够适应河口复杂的边界,并可以根据需要进行局部加密。Casulli的半隐式法离散模型水位梯度和垂项紊动扩散项,克服了水位梯度和垂项紊动扩散对模型稳定性的影响,Semi-Lagrangian法用于模型对流项的离散,使模型具有"无条件"稳定性。采用水池风生流和盐度异重流算例检验了模型,并应用模型模拟了强潮河口(瓯江口)的潮流盐度运动,计算和实测结果进行比较,表明该模型是准确、可靠的。     

长江口北支强潮河道悬沙运动及输移机制

随着崇明北侧岸滩的自然淤涨和人工圈围,北支河道显著束窄,"喇叭口"顶点位置下移。在新的地形及流域来水来沙变异背景下,作为长江河口的分支强潮汊道,其悬沙运动与输移特点值得探讨。根据2010年4月小潮至大潮连续8 d的半个半月潮水沙观测,结合多年不同河段水沙观测数据得到的含沙量过程曲线显示:整个河道潮流强、含沙量高,含沙量过程曲线呈 "单峰-双峰-单峰"的变化特点;河道悬沙的输移以平流输移和"潮泵输移"为主,以"喇叭口"顶点为界,上游段河道平流输移占主导地位,"潮泵输移"次之;下游段"潮泵输移"占主导,平流输移次之。净输沙总量呈:上段河道向海,下段河道向陆,在"喇叭口"顶点附近存在一个泥沙汇聚的最大浑浊带区域。     

Prediction models for tidal level including strong meteorologic effects using a neural network

Accurate prediction of tidal level including strong meteorologic effects is very important for human activities in oceanic and coastal areas. The contribution of non-astronomical components to tidal level may be as significant as that of astronomical components under the weather, such as typhoon and storm surge. The traditional harmonic analysis method and other models based on the analysis of astronomical components do not work well in these situations. This paper describes the Back-Propagation Neural Network (BPNN) approach, and proposes a method of iterative multi-step prediction and the concept of periodical analysis. The prediction among stations shows that the BPNN model can predict the tidal level with great precision regardless of different tide types in different regions. Based on the non-stationary characteristic of hourly tidal record including strong meteorologic effects, three Back-Propagation Neural Network models were developed in order to improve the accuracy of prediction and supplement of tidal records: (1) Difference Neural Network model (DNN) for the supplementing of tidal record; (2) Minus-Mean-Value Neural Network model (MMVNN) for the corresponding prediction between tidal gauge stations; (3) Weather-Data-based Neural Networks model (WDNN) for set up and set down.The results show that the above models perform well in the prediction of tidal level or supplement of tidal record including strong meteorologic effects.     

Sea ice evolution and internal wave generation due to a tidal jet in a frozen sea

The purpose of this study is to investigate the influence of tidal currents on sea ice in Spitsbergen fjords which may cause rapid decrease of the ice thickness due to erosion and melting of the ice. The effect was studied in-situ near the narrow channel connecting the Van Mijen Fjord and Lake Vallunden. The strong jet-like tidal currents in the strait driven by semidiurnal tide continue into the lake preventing ice freezing along a narrow strip during high tide and relatively warm weather. Understanding the formation of open water regions or regions with thin ice is important for the safe transportation on ice. We estimate conditions and representative time over which strong tidal current influences ice thickness along a narrow strip in solid ice. Changes of tidal phase and decrease in air temperature influence freezing of the strip in one-two days. While the tidal flow leaves the strait it overflows a shallow bar and generates internal lee waves propagating downslope and mixing the water. Tidal forcing of internal waves was measured using pressure gauges and by scanning of the ice surface during flood and ebb phases. Internal waves were measured using three types of CTD instruments and an ADCP current meter. The generation of wave packets occurs every tidal cycle when the current flows into the lake, but no generation occurs during the ebb phase of the tide because the currents over the bar slope are low. Parameters of internal waves are estimated. Model simulations confirm generation of internal wave train by the tidal current descending downslope.     

ON generation source sites of internal in the Luzon strait

This effort aims to determine the generation source sites in the Luzon Strait for energetic, long-crest, transbasin internal waves (IW) observed in the northern South China Sea (NSCS). The roles of islands distributed on eastern side of the strait, Kuroshio, submarine ridges, shoaling thermocline, and strait configuration played in the IW generation are examined using the cruise data analysis, satellite data interpretation, and dynamical analysis. The islands and channels on eastern side of the strait are excluded from a list of possible IW source sites owing to their unmatched horizontal dimensions to the scale of IW crest line length, and the relative low Reynolds number. The Kuroshio has a potential to be a radiator for the long-crest IW disturbances, meanwhile, the Kuroshio west (east) wing absorbs the eastward (westward) propagating IW disturbance. Namely, the Kuroshio blockades the outside west-east propagating IW disturbances. The 3-D configuration of the Luzon Strait is characterized by a sudden, more than one order widening of the cross-section areas at the outlets on both sides, providing a favorable condition for IW type initial disturbance formation. In the Luzon Strait, the thermocline is featured by a westward shoaling all the year around, providing the dynamical conditions for the amplitude growth (declination) to the westward (eastward) propagating IW type disturbance. Thus,the west slope of western submarine ridge at the western outlet of the Luzon Strait is a high possibility source sites for energetic,long-crest, transbasin Iws in the NSCS. The interpretation results of satellite SAR images during a 13 a period from 1995 to 2007 provide the convincing evidence for the conclusions.     

Year-to-year variation of cold waters around the korea strait

Year-to-year variation of bottom cold waters around the Korea Strait was investigated based on bottom temperatures measured by submarine telephone cable between Pusan, Korea and Hamada, Japan from 1982 to 1992. The characteristics of bottom temperatures could be divided into three different groups: the Korean side, the middle, and the Japanese side. Temperature drops in summer appeared in all the three regions implying the intrusion of cold waters into the Korea Strait. Significant decreases in the Korean side were observed in 1983, 1986, 1990, 1991, and 1992 when bottom temperatures were high in the middle. In contrast, bottom temperatures significantly decreased in the middle in 1985, 1988, and 1989 when the temperature drops in the Korean side were relatively small. This tendency for a negative relationship was also shown in the second mode of an EOF analysis. In the years when bottom temperatures significantly decreased in the Korean side, the cold water along the east coast of Korea expanded offshore and its temperature was low. On the contrary, cold water in the southern region of the Ulleung Basin developed in the years when bottom temperatures decreased considerably in the middle.     

A three-dimensional model of the breeze-induced circulation of waters in the Kerch strait

We generalize the problem of evaluation of currents caused by the action of breeze winds to the three-dimensional case. In the “solid-lid” approximation, the problem is reduced to the numerical solution of a two-dimensional equation for the integral current function (with complex-valued coefficients) with subsequent evaluation of the components of current velocity according to analytic formulas. The breeze is specified as acting in a narrow coastal strip in the form of a zonal wind. The three-dimensional structure and the variations of currents near the west boundary of the Kerch Strait as functions of time are studied in detail.     

Calculation of energy of the barotropic-baroclinic interaction in the strait of Bab el Mandeb

Equations for the kinetic, potential, and total energy in the curvilinear boundary-fitted coordinates are considered in the strait area and adjacent regions of the Red Sea and Gulf of Aden. The budgets of different forms of energy are presented, making it possible to assess the correctness of the formulation of the three-dimensional boundary-value problem, the accuracy of the calculation of the barotropic-baroclinic interaction in an energy norm, and the relationship between the components of energy balance. The energy fluxes in the boundary sections of the strait are estimated for the interaction of thermohaline currents with main tidal harmonics and with the total tide, as well as for the amplitude of fluctuations in boundary energy fluxes in a synodic month. The results illustrating the behavior of some dynamical characteristics and components of energy balance at vertical and horizontal sections of the strait are given.     

The concept of tidal exchange and the tidal exchange ratio

The principal character of the tidal exchange process is neither diffusion nor advection, but a third category of transport, “Massenaustausch”, which appears in the space/time averaged transport. The exchange process can be divided into four fluxes: the flux of standing eddies, the flux of tidal exchange, the flux of tidal eddies and the flux of local eddies. The results of observations at the entrance channel of Kumihama-Bay show a typical example of transport dominated by tidal exchange. The tidal exchange ratio defined by Parkeret al. (1972) applies to the process of exchange between the outflowing watermass and the surrounding watermass outside of the bay mouth, but this should also be considered as being coupled with the ratio for the process of exchange between the inflowing watermass and the surrounding watermass inside of the bay mouth. These two exchange ratios can be combined into a single exchange ratio which describes the exchange process between the outer watermass and the bay water.     

Deglaciation and meltwater events in hudson strait and the Eastern Canadian Arctic

Stable isotope measurements on molluses from raised marine deposits, and on foraminifera in a piston core, define a series of light isotope events. A major event is¹⁴C dated at ca. 10.3 ka and is correlated with a marked isotopic excursion in HU77-021-156. This event is followed by a dramatic rise in diatom and, slightly later, in benthic foraminiferal abundances. Molluses record another light event around 8 ka. Major fluctuations in both ¹⁸O and ¹³C around 10.3 ka suggests that the Hudson Strait ice stream may have responded to the Younger Dryas climatic oscillation.     

乐清湾的潮位、潮流和余流特征

2008年7月至2009年4月在乐清湾进行了代表春、夏、秋、冬4个季节的航次调查,设置了Y4、Y14、Y15和A共4个连续观测站位,共得到12组实测的海流流速和10组CTD数据。采用潮汐调和分析法分析了距江厦潮汐能试验电站3 km处的潮汐站位连续19个月的潮位资料,结合调查数据特性和乐清湾潮汐特点,引入M2与S2、O1与K1、M4与MS4、2MS6与M6分潮之间的差比关系,对连续观测站位的潮位和潮流进行准调和分析。潮位的统计和准调和分析结果显示:Y4、Y14、Y15站位和潮汐站位8个分潮振幅和的航次调查平均值为3.75,4.02,3.94和4.03 m,(HO1+HK1)/HM2的航次调查平均值为0.32,0.28,0.32和0.24。Y4、Y14、Y15和潮汐站位的M4、MS4、M6和2MS6浅水分潮振幅的航次调查平均值分别为0.20,0.31,0.35和0.25 m,M6和2MS6浅水分潮振幅的航次调查平均值分别为0.03,0.15,0.17和0.15 m。不同航次调查4个连续观测站位涨潮最大流速的平均值为81.5 cm/s,落潮最大流速的平均值为103.1 cm/s。Y4、Y14和Y15站位潮流的M2和S2分潮振幅百分比分别为86%,65%和68%,浅水分潮振幅百分比分别为11%,29%和25%。M4、MS4分潮振幅之和分别是M6、2MS6分潮振幅之和的2.1,1.2和1.7倍。由潮位和潮流的分析结果可知:从乐清湾湾口至湾顶,潮汐逐渐增强,半日潮比率逐渐增大,半日潮型的特性更为明显;浅水分潮强度逐渐增加,其中M6和2MS6分潮强度增强更为明显。位于湾口的Y4站位在秋季(2008年10月)航次调查时的日平均余流流向为西南偏南方向,冬季(2009年1月)和春季(2009年4月)航次调查时的余流流向为东南偏南方向。Y4站位余流受灵霓大坝影响,大坝建成后湾口余流改变方向,向南流出乐清湾。位于湾顶的Y14站位,余流流速变化不大,但方向变化明显,夏季(2008年7月)为西南方向,秋季为西南偏南方向,冬季为西南偏西方向,春季又为西南偏南方向。Y15站位余流流速较小,但方向变化明显。A站位两个航次调查时的余流流向均为东北方向。