Modelling of the barotropic processes in the Bohai Sea

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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.     

近岸流对波浪传播影响的数值分析

使用近岸波浪模型SWAN计算存在沿岸流和离岸流时的近岸波浪传播。先设离岸流u=0m/s,模拟均匀、非均匀沿岸流的流速和梯度对波高传播的影响;再设沿岸流v=0.5m/s,模拟均匀、非均匀离岸流的流速和梯度对波能高传播的影响。从模拟中得到,近岸波浪传播受沿岸流、离岸流的流速和梯度影响时,波高的变化规律。     

弧形海岸裂流的数值模拟研究

弧形海岸波浪产生的裂流严重危害人类活动,但是目前对其特征缺乏充分认识。本文对Haller物理模型实验和三亚大东海的数值模拟表明FUNWAVE模式具有较好的裂流模拟能力。基于该模式进行了多种弧形海岸条件的裂流数值模拟,给出裂流的一些特征：（1）海岸弯曲度增大,裂流增强;（2）海岸坡度对裂流有比较大的影响,太陡或太平缓的海岸不利于形成裂流;（3）海岸尺寸减小,裂流减弱;（4）波高和波周期增大,裂流增强,但是对于某些海岸而言,0.4m波高可能就存在危害比较大的裂流。     

浅海水域水平扩散系数的推算研究

本文根据拉格朗日流与欧拉流自相关系数的相似关系,用渤中8号平台1987年1月和8月的实测资料,分别推算了水平扩散系数。推算结果表明:研究长时间水平扩散时,把潮流看作湍流,所求的扩散系数实际上是潮流扩散系数,其值在10~6cm~2/s范围内;研究短时间水平扩散时,把潮流看作平流,用富里叶变换的长周期合成值从实测值中扣除,所求的扩散系数消除了长周期潮流部分,可认为是湍流扩散,其值在10~4cm~2/s范围内。     

长江河口北支上口不规则周期潮流的动力机制

数值模拟和动量分析长江河口北支上口枯季大潮期间 1 d内出现"四涨四落"不规则周期涨落潮流现象。长江河口北支上口 1日内两次涨潮流和两次落潮流为常规涨落潮流,受外海半日潮流控制,两次涨潮流和落潮流为非常规涨落潮流。北支上口非常规的涨潮流处于南支落潮的末期,范围小,流速弱,历时约2 h;表层主要是垂向黏滞项和水平扩散项与正压项间的作用,南向的垂向黏滞项起着决定的作用,底层则是斜压项与正压项间的作用;北支上口非常规涨潮流是北风、盐度锋面产生的南向斜压压强梯度力和南支末期落潮流的牵引作用共同造成的;径流抑制非常规涨潮流的产生,持续时间随径流量的增加呈指数递减,当径流量达到22 300 m3/s时,非常规涨潮流现象消失。北支上口非常规落潮流处于南支涨潮流的初期,由于在北支上口南支的涨潮流早于北支涨潮流,导致南支水体进入北支,形成北支上口第二次落潮流,范围较大,流速较强,历时约2.5 h,从表层至底层主要是垂向黏滞项与非线性平流项和正压项之间的作用。本文揭示了北支上口 1 d内出现"四涨四落"不规则周期涨落潮流的动力过程和机制。     

Seasonal variations of the surface currents in the Tsushima Strait

Seasonal variations of the surface currents in the Tsushima Strait were investigated by analyzing the monthly mean surface currents measured with HF radar. Several new features of the surface currents have been found. One notable feature is the large, complicated seasonal variation in the current structure in the eastern channel of the strait. For example, in the southeastern and northwestern regions of the channel, southwestward countercurrents are found in summer while southeastward acrossshore currents are found in autumn and winter. The wind-driven flow (Ekman flow) as well as surface geostrophic currents are responsible for these complicated variations of the surface currents. To quantify each variation of the flow and current, the wind-driven flow was calculated from the monthly wind (more precisely, the friction velocity) using the monthly speed factor and deflection angle estimated in our previous study, and the surface geostrophic currents were then estimated by subtracting the wind-driven flow from the measured surface currents. It was found that the acrossshore currents are the wind-driven flow, and that the surface geostrophic currents flow almost in the along-shore direction, indicating the validity of the decomposition of the surface velocity into the wind-driven flow and the geostrophic currents using the speed factor and deflection angle. A real-vector empirical orthogonal function (EOF) analysis of the surface geostrophic currents shows a pair of eddies in the lee of Tsushima and Iki Islands as the first mode, which indicates that the southwestward countercurrents in the eastern channel are formed primarily by the incoming Tsushima Warm Current.     

Analysis of Decomposed Surface Currents in a Limited Area

A method of the Helmholtz decomposition for mapped surface currents on grid points is developed. This method is simple, and decomposed currents are not dependent on the distance from the open boundary. This decomposition is applied to surface currents observed by high-frequency (HF) radars. The observation was conducted from April to May 1998 east of Okinawa Island, Japan. The spectral analysis of decomposed currents reveals that the M₂ tidal peak for rotational components is much smaller than that for divergent components, which suggests that the barotropic tidal components are mainly included in divergent currents. The real-vector empirical orthogonal function (EOF) analysis of decomposed currents reveals an embedded eddy, which is related with the convergence of currents     

辽东湾中部西岸浅水海域冬季实测海流分析

2015年12月在辽东湾中部西岸浅水海域进行了8个站、连续半个月的坐底ADCP海流剖面观测,通过对分层潮流和余流分析,得到该海域的海流特征如下:1)实测海流以潮流特征占主导,潮流特征为规则半日潮流,优势分潮为M2;M2椭圆长轴大小为25~50 cm/s、方向多为NE-SW向,具有显著的往复流特征。2)观测期间的平均余流为1~10 cm/s,方向多为SW向,平均余流在水平和垂向上的空间差异明显,日均余流波动剧烈;表层余流方向与局地风向具有很好的同步一致性,且距岸较近站位的表层余流受风影响更大;中、底层余流与风的相关性较差。本文得到的余流方向不支持冬季辽东湾北部的边界顺时针环流的存在。     

赤道中印度洋上层环流结构与季节变化特征分析

本文通过分析RAMA印度洋观测浮标系统锚系ADCP实测资料,对赤道中印度洋上层海流季节变化进行了研究。研究结果表明,0°,80.5°E纬向流垂向剖面呈现上150m层一致的东向流,而经向流在100m以浅呈现表层向北次表层向南的翻转流结构。赤道中印度洋上层纬向流季节信号被半年周期的东向射流Wyrtki Jets(WJs)所控制。WJs发生于季风方向转换的季节,4—5月份较弱,10—11月份较强。赤道中印度洋上层经向流年周期信号显著。北半球夏季与冬季分别出现风应力旋度驱动的Sverdrup南向流与北向流。本文结论为赤道中印度洋上层环流季节变化特征的研究提供了观测角度的支持。     

The statistical structure of synoptic variability ocean currents at the continental slope of the Laptev Sea and features of their generation by anemobaric forces

Based on vector-algebraic analysis of random processes, we study the statistical structure of the synoptic variability of currents measured by an ADCP in the upper mixed layer in the central part of the continental slope of the Laptev Sea in 2006–2007. The results of statistical analysis show that in some cases the synoptic currents in the surface layer of the sea are signs of wind drift currents. This is indicated by the high correlation between the tangential friction of wind and currents, as well as the reversal of the depth of current vectors and the major axes of the ellipses of the mean-square deviation of the Ekman spiral. Due to the large variability of wind flows and stratification of water masses, the penetration depth of these currents is small and varies from 6 to 30 m, with pronounced seasonal variation. In deeper layers, no relationship between the currents and anemobaric forces is traced. It is concluded that the fluctuations of synoptic scale currents in the area of the continental slope of the Laptev Sea represent a superposition of Ekman drift currents and movements associated with free baroclinic Kelvin waves. These currents are the dominant contributor in the upper 30-m layer of the ocean, while waves play a key role in deeper waters.     

2008年8—9月份吕宋海峡西南侧锚定ADCP的斜压海流观测

2008年8月份在吕宋海峡西侧大约4000m深的海域首次布放了两套深水潜标,对上层海洋进行了为期一个月的高频率ADCP海流采样。对数据进行了多重处理,分解得到平均流和逐层潮流,并分析了具有斜压性海流的频率分布。调和分析结果表明,该海域以不规则日周期潮流为主,潮流椭圆随深度旋转。海流的功率谱分析表明,内潮能量在温跃层附近最大,并随深度减小,同时海流有明显的近惯性振荡信号。该观测为了解吕宋海峡深层海洋的潮流、近惯性运动以及海峡水交换提供了第一手的资料。     

Rip currents under obliquely incident wind waves and tidal longshore currents

A field experiment on the nature of rip currents was conducted on the Dutch coast, which differs from previous rip current study sites because it is a wind-sea dominated environment with mostly obliquely incident waves and tidally-driven longshore currents. During the experiment three distinct flow patterns, obtained with GPS tracked drifter instruments, were observed: (1) a locally governed circulation cell, (2) an offshore current that is deflected shore parallel outside the surf zone and (3) a meandering longshore current. The transition from rip currents (flow patterns 1 and 2) to meandering longshore currents (flow pattern 3) occurred gradually within the tidal cycle with longshore currents prevalent at mid to high tide. Rip currents at this site appeared at depressions in the surf zone bar and typically occurred when the water level fell below NAP (equivalent to MSL), even in the presence of obliquely incident waves and tidally driven longshore currents. Hindcast simulations of the drifter experiments were performed with the numerical model XBeach and showed good agreement with field observations. The model was subsequently used to investigate the influence of tidal water level fluctuations, longshore currents and obliquely incident waves on rip currents.Rip currents were initiated when the water level dropped below a specific threshold with the magnitude of the rip current associated with the water level. The strength of the tidal current and its orientation with respect to the incident waves governed the offshore extent and orientation of the rip current. In contrast to other studies that suggest that rip currents solely occur under shore normal (or slightly oblique waves), in this study both observations and numerical model simulations indicate that rip currents can exist under large angles of wave incidence, when the rip channel is sufficiently wide and the wave height is small.     

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。     

Numerical Study of Influence of Currents on Nonlinear Focusing Waves

In this paper,a numerical model for nonlinear wave propagation in currents is formulated by a set of enhanced fully nonlinear Boussinesq equations with ambient currents.This model is verified by comparison with the published results.Then the influence of currents on nonlinear focusing waves is studied by use of the numerical model.It is found that the effect of currents on the surface elevations at the focal location is negligible.Following currents can augment the maximum crest of focusing wave while decre...     

沿岸流不稳定性的实验研究及理论分析

在坡度为1:40的平面斜坡上进行了单向不规则波的沿岸流不稳定运动实验,观测到了沿岸流的周期性波动(波动周期约为100 s)。利用最大熵方法和三角函数回归法求得这种波动的主频率以及幅值,分析了波动幅值在垂直岸线方向的变化,结果表明该变化与沿岸流变化类似,即在沿岸流最大值附近这种波动强度最大。为了分析波动的机理,利用线性沿岸流不稳定模型对模型实验结果进行了分析,求得了不稳定运动增长模式和波动周期,并与对应实测结果进行了比较,结果表明,计算结果与实测值符合,从而说明实验中观测到的这种周期性波动为沿岸流不稳定引起的剪切波。     

大小门岛海域表层潮流和余流特征分析

利用2012年4月份进行水文调查得到的实测海流数据,依据多周日观测的准调和分析原理,对大小门岛海域表层潮流进行调和分析。基于调和分析结果对大小门岛海域潮流和余流特征进行精细化研究。结果表明:大小门岛海域潮流类型为不正规半日浅海潮流,大小门岛西部海域受浅水分潮影响最大。整个海域潮流运动形式以往复流为主,由于受到多股往复流的干涉作用,大门岛西侧旋转流较强;实测落潮流要强于涨潮流,流速在瓯江口外侧达到最大。余流走向存在区域性差异,小门岛西北、瓯江口内部及黄大峡海域余流方向与涨潮流一致,其它区域与落潮流一致,余流流速大小与最大流速分布基本一致。     

天津港主航道连续观测点潮流和余流特征分析

基于天津港主航道连续观测点31 d的实测海流资料,利用调和分析对主航道潮流和余流特征进行研究,同时结合同步风速资料研究风对表层余流的影响。结果表明:(1)航道附近属于弱流海区,表层平均流速为31.4 cm/s,流速总体上由表至底逐渐减小,流速方向大致集中在NW—SE向。(2)观测海域潮流以正规半日往复潮占主导,优势分潮为M₂,浅水分潮较为显著,涨潮流流速大于落潮流流速。(3)观测期间表层平均余流流速为2.8~13.8 cm/s,随着深度增加余流流速逐渐减小,方向大多为NW向。该站表层余流受风的影响显著,东南风将使余流方向偏向西北。     

黄海潮波系统下的GOCI反演及OSU模式海表流场数据适用性研究

黄海呈现独有的地形条件,且该海域的潮波运动独具特征。本文利用静止海洋水色成像仪（Geostationary Ocean Color Imager, GOCI）遥感反演和俄勒冈州立大学（Oregon State University,OSU）潮流模式分别获取了黄海海域的海表流场,基于该海域独特的潮波系统提出并识别潮波干涉区,进而对GOCI反演的流场做潮流提取,并对两种潮流数据作分区可用性评价,通过实测的漂流浮标数据验证评估。结果表明：利用GOCI反演和OSU潮流模式获取的海表流场具有一定程度的可靠性,GOCI反演的海表流场的流速平均相对大小误差值为0.77,OSU潮流模式获取的海表流场流速平均相对大小误差值为0.49;在靠近潮波干涉区的黄海中部海域,GOCI潮流数据与实测数据在方向上的一致性要优于OSU潮流数据,两者平均角度误差值分别为48.45°和63.10°;在远离潮波干涉区的黄海近岸海域,OSU潮流数据与实测数据在速度大小和方向上的一致性要优于GOCI潮流数据。     

高频地波雷达生成海洋表面矢量流图

武汉大学研制的双站高频地波雷达系统OSMAR2000利用测得的两幅单站径向海流图生成矢量海流图。经典矢量流图生成方法不能直接应用到OSMAR2000系统中。本文提出一种先在极坐标系下用自然三次样条函数将径向流插值到公共网格上然后直接进行矢量合成的矢量海流图生成方法。OSMAR2000在东海的表面矢量流实测结果与作对比验证的传统海流计测量结果十分吻合。对比数据表明，该方法是可行的，且优于先进行径向流线性插值后矢量合成的矢量流图生成方法。这也是国内首次利用高频地波雷达实现海洋表面矢量流的实时监测。