Kuroshio Intrusion Combined with Coastal Currents Affects Phytoplankton in the Northern South China Sea Revealed by Lipid Biomarkers

The northern South China Sea(NSCS)is significantly influenced by the Kuroshio intrusion and the coastal currents.Our knowledge on the roles of both currents on phytoplankton spatial variations is still inadequate.Here,we investigated the concentrations of phytoplankton biomarkers and their proportions in surface suspended particles from 47 sites of the NSCS during summer of 2017 and 2019.Brassicasterol/epi-brassicasterol,dinosterol,and C37 alkenones were used as proxies of biomass for diatoms,dinoflagellates,and haptophytes,respectively,and their sum indicating total phytoplankton biomass.A three end-member mixing model was applied to quantitatively assess the influence extent of the Kuroshio intrusion and the coastal currents.Our results showed that the Kuroshio intrusion and the coastal currents contributed equally to the overall surface water masses in the study area;however,the two currents had distinct effects on the spatial distribution of phytoplankton.For phytoplankton biomass,the eutrophic coastal currents were likely to be the main controlling factors,while the impact of the Kuroshio intrusion was weak and stimulated significant increases in phytoplankton biomass only at certain boundary sites.For phytoplankton community structures,the Kuroshio and its intrusion were the main factors,resulting in an increase in the proportions of dinoflagellates and haptophytes.The proportion of diatoms slightly increased due to the influence of the coastal currents.Our study quantifies the effects of the Kuroshio and the coastal currents on phytoplankton in the NSCS in terms of hydrological parameters,providing an important basis for the understanding of ecological functions and biogeochemical cycles in marginal sea-open ocean boundary regions.

Modeling of random wave transformation with strong wave-induced coastal currents

The propagation and transformation of multi-directional and uni-directional random waves over a coast with complicated bathymetric and geometric features are studied experimentally and numerically. Laboratory investigation indicates that wave energy convergence and divergence cause strong coastal currents to develop and inversely modify the wave fields. A coastal spectral wave model, based on the wave action balance equation with diffraction effect （WABED）, is used to simulate the transformation of random waves over the complicated bathymetry. The diffraction effect in the wave model is derived from a parabolic approximation of wave theory, and the mean energy dissipation rate per unit horizontal area due to wave breaking is parameterized by the bore-based formulation with a breaker index of 0.73. The numerically simulated wave field without considering coastal currents is different from that of experiments, whereas model results considering currents clearly reproduce the intensification of wave height in front of concave shorelines.

Wintertime Guangdong coastal currents successfully captured by cheap GPS drifters

This study introduces a type of self-developed, GPS-based, simple and cheap Surface Current Experiment(SUCE)drifters designated for observing surface coastal currents. By examining trajectories of six drifters deployed in the Daya Bay and the drifter-derived velocities, we conclude that such drifters are generally capable of capturing the characteristics of wintertime surface coastal currents along the Guangdong coast.

Numerical investigation of the control factors driving Zhe-Min Coastal Current

During the northeast monsoon season,Zhe-Min Coastal Current(ZMCC)travels along the Chinese mainland coast and carries fresh,cold,and eutrophic water.ZMCC is significantly important for the hydrodynamic processes and marine ecosystems along its path.Thus,this bottom-trapped plume deserves to be further discussed in terms of the major driving factor,for which different opinions exist.For this purpose,in this study,a high resolution Semi-implicit Cross-scale Hydroscience Integrated System Model(SCHISM)is established and validated.High correlation coefficients exist between along-shelf wind speeds and seasonal variations of both ZMCC volume transport and the freshwater signal.These coefficients imply that the wind is important in regulating ZMCC.However,for similar annual mean ZMCC volume transports,the extreme south boundaries of Zhe-Min Coastal Water(ZMCW)are different among different years.This difference is attracting attention and is explored in this study.According to the low wind/discharge experiment,it was found that although the volume transport of ZMCC is more sensitive to the variation of local wind speeds,the carried freshwater is limited by the Changjiang River discharge,which ultimately determines the south boundary of ZMCW.The momentum analysis at transects I and II shows that,for driving ZMCC,the along-shore wind forcing is as important as the buoyancy forcing.Note that this conclusion is supported by a zero-discharge experiment.It was also found that the buoyancy forcing varies with respect to time and space,which is due to variations of the discharge of Changjiang River.In addition,a particle tracking experiment shows that the substance carried by the Changjiang River diluted water would distribute along the Zhe-Min coastal region during the northeast monsoon season and it may escape due to the wind relaxation.

A numerical simulation case study of the coastal currents and upwelling in the western Persian Gulf

The present study employed COHERENS model as a three-dimensional hydrodynamic model to address the coastal currents in the western Persian Gulf with a horizontal resolution of 2-minutes of latitude and longitude.The obtained results suggested that Iranian northwestward coastal currents developed from January to April and experienced their maximum intensity from June to August when the rate of the surface inflow current increases through the Strait of Hormuz and the stability of the seasonal thermocline becomes gradually stronger.The simulation results reflected that expansions of the thermocline area began by the arrival of the warm season.Under such circumstances,some stronger coastal currents were generated in summer.The strong coastal surface current in the vicinity of the western part of Iranian coasts flew southeastward,while the reverse current flew with a northwestern direction in the seabed.The obtained results suggested that a consequence of the influence of salinity flux,especially on the Iranian coasts,the speed of currents increases.The findings of this study showed that more kinetic energy was accumulated along Iranian coasts.Moreover,the current vectors on Iranian coasts were greater than those on Arabian coasts,which in turn resulted in the creation of the upwelling currents along western Iranian coasts.

Responses of the Zhe-Min coastal current adjacent to Pingtan Island to the wintertime monsoon relaxation in 2006 and its mechanism

In conjunction with synchronous remotely sensed winds and sea surface temperature(SST),the spatiotemporal features of the Zhe-Min coastal current(ZMCC),especially responses of the ZMCC adjacent to Pingtan Island(PT) to the wintertime monsoon relaxation in 2006 and corresponding mechanism are investigated based on the field observations.In situ data are acquired from Conductivity-Temperature-Depth(CTD) cruise and Bottom-Mounted Moorings(BMM),which are conducted during a comprehensive survey for the Chinese Offshore Investigation and Assessment Project in winter 2006.It is revealed that the ZMCC is well mixed vertically in winter 2006.The ZMCC(<14°C) recedes during the relaxation of the wintertime monsoon and is accompanied by the enhanced northward shift of the warm,saline Taiwan Strait Mixed Water(TSMW,higher than 14°C and is constituted by the Taiwan Strait Warm Water and the Kuroshio Branch Water).And greatly enhanced southward intrusion of the ZMCC can be detected when the wintertime monsoon restores.Correspondingly,the thermal interface bounded by the ZMCC and the TSMW moves in the northwest/southeast direction,leading to periodic warm/cold reversals of the near-seabed temperature adjacent to the PT.By EOF(Empirical Orthogonal Function) analysis of the large-scale wind fields and wavelet power spectrum analysis of the water level,ocean current and the near-seabed temperature,responses of the ZMCC off the PT to wintertime monsoon relaxation are suggested to be attributed mainly to the southward propagating coastally trapped waves triggered by the impeding atmospheric fronts.As a result,ocean current and near-seabed temperature demonstrate significant quasi-5 d and quasi-10 d subtidal oscillations.By contrast,the onshore/offshore water accumulation resulted from Ekman advection driven by the local winds has minor contributions.

Assimilation of surface currents into a regional model over Qingdao coastal waters of China

Surface currents measured by high frequency （HF） radar arrays are assimilated into a regional ocean model over Qingdao coastal waters based on Kalman filter method. A series of numerical experiments are per- formed to evaluate the performance of the data assimilation schemes. In order to optimize the analysis pro- cedure in the traditional ensemble Kalman filter （ENKF）, a different analysis scheme called quasiensemble Kaman filter （QENKF） is proposed. The comparisons between the ENKF and the QENKF suggest that both them can improve the simulated error and the spatial structure. The estimations of the background error covariance （BEC） are also assessed by comparing three different methods： Monte Carlo method; Canadian quick covariance （CQC） method and data uncertainty engine （DUE） method. A significant reduction of the root-mean-square （RMS） errors between model results and the observations shows that the CQC method is able to better reproduce the error statistics for this coastal ocean model and the corresponding external forcing. In addition, the sensibility of the data assimilation system to the ensemble size is also analyzed by means of different scales of the ensemble size used in the experiments. It is found that given the balance of the computational cost and the forecasting accuracy, the ensemble size of 50 will be an appropriate choice in the Qingdao coastal waters.

Numerical simulation of sediment transport in coastal waves and wave-induced currents

Prediction of coastal sediment transport is of particularly importance for analyzing coast erosion accurately and solving the corresponding coast protection engineering problems.The present study provided a numerical scheme for sediment transport in coastal waves and wave-induced currents.In the scheme,the sand transport model was implemented with wave refraction-diffraction model and near-shore current model.Coastal water wave was simulated by using the parabolic mild-slope equation in which wave refraction,diffraction and breaking effects are considered.Wave-induced current was simulated by using the nonlinear shallow water equations in which wave provides radiation stresses for driving current.Then,sediment transport in waves and wave-induced currents was simulated by using the two-dimensional suspended sediment transport equations for suspended sediment and the bed-load transport equation for bed load.The numerical scheme was validated by experiment results from the Large-scale Sediment Transport Facility at the US Army Corps of Engineer Research and Development Center in Vicksburg.The numerical results showed that the present scheme is an effective tool for modeling coastal sediment transport in waves and near-shore currents.

Numerical calculation of hydrodynamic characteristics of tidal currents for submarine excavation engineering in coastal area

In coastal areas with complicated flow movement, deposition and scour readily occur in submarine excavation projects. In this study, a smallscale model, with a high resolution in the vertical direction, was used to simulate the tidal current around a submarine excavation project. The finite volume method was used to solve Navier-Stokes equations and the Reynolds stress transport equation, and the entire process of the tidal current was simulated with unstructured meshes, generated in the irregular shape area, and structured meshes, generated in other water areas.The meshes near the bottom and free surface were densified with a minimum layer thickness of 0.05 m. The volume of fluid method was used to track the free surface, the volume fraction of cells on the upstream boundary was obtained from the volume fraction of adjacent cells, and that on the downstream boundary was determined by the water level process. The numerical results agree with the observed data, and some conclusions can be drawn: after the foundation trench excavation, the flow velocity decreases quite a bit through the foundation trench, with reverse flow occurring on the lee slope in the foundation trench; the swirling flow impedes inflow, leading to the occurrence of dammed water above the foundation trench; the turbulent motion is stronger during ebbing than in other tidal stages, the range with the maximum value of turbulent viscosity, occurring on the south side of the foundation trench at maximum ebbing, is greater than those in other tidal stages in a tidal cycle, and the maximum value of Reynolds shear stress occurs on the south side of the foundation trench at maximum ebbing in a tidal cycle. The numerical calculation method shows a strong performance in simulation of the hydrodynamic characteristics of tidal currents in the foundation trench, providing a basis for submarine engineering construction in coastal areas.

THEORETICAL STUDY ON COASTAL UPWELLING INDUCED BY ALONGSHORE CURRENT

The perturbation method is used to derive this mathematical model for coastal upwelling induced by alongshore current at the shelf break on a slowly sloping continental shelf. The geostrophic flow velocity with bottom friction as the velocity at the shell" break is presumably the best choice for the homogeneous model.The present analysis will be beneficial to understanding and further study of the upwelling induced by an alongshore current.

岱山—洋山跨海通道水沙环境影响数值模拟研究

杭州湾是世界闻名的强潮河口,具有不规则的岸线边界、独特的地形地貌特征和复杂的水动力泥沙环境。岱山—洋山跨海通道位于杭州湾湾口水域,将岱山与洋山近35km连通,实现了上海—洋山—岱山—舟山—宁波跨海通道的全线贯通,其建设具有非常重要的意义。基于考虑波浪作用的潮流泥沙数值模型,讨论了岱山—洋山跨海通道几种工况对杭州湾大范围海域水沙环境的影响。研究结果表明:跨海通道实施后潮位变化在大桥轴线附近较为敏感,远离则基本没有影响,高低潮位变化幅度大多在2 cm以内。杭州湾涨落潮量减小小于0.6%。水流流态和水下地形仅在大桥附近水域存在明显变化。从对杭州湾水沙环境影响角度考虑,岱山—洋山跨海通道几种工况均可行,建议关注桥墩和人工岛附近的局部冲刷问题。

海岸植被防护机制的物理模型试验研究进展

基于自然的海岸修复与防护越来越受到人们关注,从海岸植被对水动力及泥沙运动影响机制的物理模型试验研究方面,介绍了试验研究中植被的描述方法,包括植被的分类、材料选择、特性表征等;综述了植被影响下的流场结构、消浪特征、形态阻力相关的研究进展;论述了悬沙浓度分布、泥沙再悬浮、沉积和冲刷模式等泥沙输运方面的研究成果;结合大量模型试验研究,提出了未来的研究趋势。

粤东大亚湾水沙环境及岸滩演变分析

根据现场实测水文泥沙及地形资料,对大亚湾海域的自然条件、水沙环境特征及岸滩演变特征进行分析,研究结果表明:大亚湾潮差较小,波浪和潮流动力弱,水体含沙量较低,泥沙来源有限,主要来自近岸附近滩面就地搬运的少量泥沙。湾内底质多为黏土质粉砂,呈现淤积质海岸的沉积特点。因此大亚湾海域水清沙少,自然状态下海床长期保持稳定。近期受港口航道建设、围填海等人类活动的影响,部分海床地形出现较大幅度的冲淤变化。湾内港池、航道疏浚开挖后航槽稳定,回淤强度较小。正确认识该海域的水动力条件、泥沙运移及海床冲淤变化,不仅是开展海岸工程开发建设的关键技术问题,也有利于近岸海洋环境保护及资源开发利用。

Analysis of characteristics and mechanism of current system on the west coast of Guangdong of China in summer

On the basis of data of drifting bottles＇ tracks and the current measured in anchored stations, as well as temperature and salinity observed in cruise investigations and coastal stations, ADCP current data and AVHRR surface sea temperature （SST） data on the western coast of Guangdong, synthetic results of analysis showed that the coastal currents in the west of the mouth of the Zhujiang River were mainly westward in summer, which constituted the north branch of cyclonic gyre in the east of the Qiongzhou Straits. Part of its water flowed westward into the Beibu Gulf through the Qiongzhou Straits. The coastal current pattern was not identical with the traditional current system which flowed westward in the Qiongzhou Straits in winter and eastward in summer. The summertime＇s coastal current was always westward, maybe temporarily turning northeast only when the southwest wind was strong. The important characteristics of coastal current on the western coast of Guangdong, in the Qiongzhou Straits and in the north of the Beibu Gulf were analyzed and their mechanisms also were explained.

The spatio-temporal variation of wintertime subtidal currents in the western Taiwan Strait

A new data set of observations by six cruises of ship-mounted acoustic doppler current profiler（SADCP） and three 40 d long bottom-mounted ADCPs（BADCPs） is employed to reveal the spatiotemporal variability of tidal and subtidal currents in the western Taiwan Strait（TWS） during winter season. The results confirm the existence of intense cotidal lines for M2 tidal current, which is located north of 25°N. In this case, no existence of an amphidromic point can be identified. It is also revealed that the counter-wind current（CWC） can extend through the whole western TWS and even occupy the entire water column during winter monsoon relaxation. However,this CWC is observed to be thoroughly overwhelmed by the downwind China coastal current（CCC） during the two big monsoon bloom events in the winter of 2007, and the CCC consequently extends southward throughout the western TWS instead. Most importantly, the variation of the spatial extent for the CWC and the CCC in the western TWS is found to be well explained by the first two modes of the vector empirical orthogonal function（VEOF） analysis, that is, it is mainly controlled by a wind-driven quasi barotropic current as the first mode and slightly modulated by a relatively weak background current with a first-order baroclinic structure as the second mode.

Molecular phylogeography and population genetic structure of the planktonic copepod Calanus sinicus Brodsky in the coastal waters of China

Planktonic copepod Calanus sinicus is the dominant meso-zooplankton in the Northwest Pacific Ocean. To better understand its population dynamics and phylogeographic patterns, 243 C. sinicus individuals were collected from seven locations across the shelf waters of China and its population genetics was studied by mitochondrial DNA cytochrome oxidase I（mtCOI） sequences analyses. Thirty-nine different sequences, or haplotypes, were detected with moderate haplotype diversity（h=0.749） and low nucleotide diversity（π=0.003） for all populations. The evolutionary divergence between geographic populations varied from 0.24% to 0.37%, indicative of very limited genetic differentiation. Visualized minimum spanning network（MSN） and phylogenetic analysis of all the detected haplotypes did not reveal any clear phylogeographic pattern. Furthermore, AMOVA data showed no significant spatial population differentiation existed among the individuals collected across China shelf waters. Pairwise FST values showed that population collected from northwest of the East China Sea（ECS） displayed a low difference to other populations. Mismatch distribution analyses and neutrality tests indicated that C. sinicus might undergo a demographic/population expansion. No significant population genetic structuring was detected, indicating an extensive gene flow among the C. sinicus populations. Our results provide molecular evidence for the hypothesis that C. sinicus in the northwestern South China Sea in winter is transported from the East China Sea and the Yellow Sea by the China Coastal Current during the northeast monsoon period.

Observed residual currents off the Changjiang (Yangtze) River mouth in wintertime of 1959 and 1982

Data taken in the two large-scale ocean investigations in China in winter 1959 and 1982 are used to analyze the residual current off the Changjiang (Yangtze) River mouth in this paper. The current in wintertime off the river mouth consist of the Changjiang runoff, wind-driven current, coastal current, density-driven current and Taiwan Warm Current (TWC). The TWC occurs in wintertime off the mouth. The surface TWC reaches only to the east side of Dinghai, then tums southeastward. The bottom TWC can flow to the area off the Changjiang mouth along west slop of the submerged river valley (SRV) and to the area off the Subei coast. The simulated currents by 3D model are basically consistent with the observed currents, although the model was run with climatological forces and the observations was done in episodic time manner.

Numerical study of current fields near the Changjiang Estuary and impact of Quick-EnKF assimilation

A 30-d current numerical simulation is running for the Yangshan Port,the Changjiang Estuary,the Hangzhou Bay and their adjacent seas using a finite volume coastal ocean model （FVCOM）,with Changjiang River runoff and wind effect being considered.At the open boundary,this model is driven by the water level obtained from prediction including eight main partial tides.After the harmonic analysis,the cotidal chart and the iso-amplitude line as well as the current ellipse distribution map are displayed to illustrate the propagation property of a tidal wave.Horizontal velocity of both the U and V components coincides with the actual measurement,which shows that the model result is credible to describe the hydrodynamic pattern in this sea area.On this basis,real-time current data from high-frequency radar is assimilated with the implementation of quick ensemble Kalman filter,which takes the variation tendency of the state vector to compute the analysis field,instead of integrating the field for N （the number of ensemble） times as it used to in the standard EnKF,aiming at raising the efficiency of computation,reducing the error of prediction and at the same time,improving the forecast effect.

A coastal acoustic tomography experiment in the Tokyo Bay

Eight sets of coastal acoustic tomography (CAT) systems were deployed during November 29 to December 10, 2002 at the coasts on both sides of Tokyo Bay to measure tidal current structures at 15-min interval.Sound transmission across the Tokyo Bay (between Yokohama and Chiba)was successfully traced,even under severe interference from ship generated wakes and bubbles.Tidal current fields changing from northward to southward flow are well reconstructed by the inverse analysis of travel-time difference data for a period with the best sound transmission condition. It is suggested that the CAT is the most powerful tool to continuously map tidal current fields in the coastal seas with heavy shipping traffic and fisheries activity.