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.

Relationship between the Guinea Current and the Coastal Upwelling in Northern of Gulf of Guinea

The variability of ocean circulation and sea surface temperature (SST) in the tropical Atlantic, especially in the Gulf of Guinea (GG), defines this region as exceptionally rich from an oceanographic perspective. The Guinea Current (GC), as the major surface current, plays a significant role in marine productivity and coastal upwelling in the GG. This coastal upwelling is known to influence the climate of the surrounding region, primary productivity and local fisheries. Studies on GC variability and its impact on this coastal upwelling have highlighted that the upwelling downstream of Cape Palmas is influenced by GC detachment, topographic variations and advective processes leading to significant vertical mixing. This study aims to analyze the interannual variability of the GC and its impact on coastal upwelling using the Coastal and Regional Ocean COmmunity model (CROCO). The model’s evaluation is conducted using observational data, specifically Geostrophic and EKman Current Observatory (GEKCO) and Ocean Surface Current Analysis (OSCAR) for currents, and Air-sea Fluxes for the global Tropic ocean-description (TROPFLUX) and Optimum Interpolation-Sea Surface Temperature (OI-SST) for temperature. Thus, the model evaluation indicates that it accurately replicates ocean circulation and SST patterns in the tropical Atlantic and the GG. The joint analysis of upwelling indices (surface and intensity) and the position of the GC core allowed us to conclude that the displacement of the GC core does indeed influence the upwelling indices in the northern part of GG. However, other oceanic and atmospheric mechanisms such as vertical diffusion and horizontal advection as proposed by previous studies may also affect the year-by-year variability of coastal upwelling in the northern GG.

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 mon- soon relaxation in 2006 and corresponding mechanism are investigated based on the field observations. In situ data are ac- quired 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℃） 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 south- ward 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 coast- ally 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.

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.

Coastal Current Systems and the Movement and Expansion of Suspended Sediment from Changjiang River Estuary

This paper, with NOAA/AHHRR data for 2 years, discusses the expanding path and extent of suspended sediment from the Changjiang River, and the relationship between the suspended sediment expanding and coastal current systems by analyzing the thermal infrared imagery with the sediment imagery, which is acquired by correlating the atmosphere corrected AVHRR imagery with in-situ suspended sediment data. The coastal current systems affecting the sediment dispersal mainly include： the Taiwan Warm Current （TWC）, the Huanghai Sea Mixed Water （HSMW）, North Jiangsu near-shore current, and Zhejiang near-shore current etc. In winter, the current systems are stable. Their distribution affects the sediment from north Jiangsu expanding toward the Changjiang estuary in some degree .The front between Zhejiang coastal current and TWC blocks the expanding of sediment toward the sea. In the flood season, apart from the limitation by coastal current systems, the spatial and temporal distribution of suspended sediment is also affected by the runoff, which shows as the jet stream and fresh water. Spring and autumn are the transitional periods of the forming of expanding patterns of flood season and winter respectively. In addition, the re-suspended sediment caused by the wind wave may make the expanding range of near-shore sediment larger.

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.

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.

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.

The difference analysis of physical-mechanical properties of sediments in the central South Yellow Sea and Zhe-Min coastal area in China

The difference analysis of physical-mechanical properties of muddy sediments is made in the central South Yellow Sea and the Zhe-Min（Zhejiang Province to Fujian Province of China） coastal area. The results show that sediments in the two regions are both dominated by mud. There are perfect negative power function correlations between the water content and the density, the compression coefficient and the compression modulus; a good positive power function correlation between the liquid limit and the plastic limit, a perfect positive linear correlation between the water content and the void ratio, and a perfect polynomial function correlation between the miniature vane shear strength and the pocket penetration resistance. In general, compared with sediments in the Zhe-Min coastal area, sediments in the central South Yellow Sea possess high water content, high void ratio,low density, high plasticity, high compressibility, low shear strength. The causes of the differences between physical-mechanical properties of sediments are analyzed from the topographic features, material sources,hydrodynamic conditions, deposition rate, and material composition. Compared with the Zhe-Min coastal area,the central South Yellow Sea is far from the Mainland and low-lying; has poor hydrodynamic condition; the materials diffused to the area are less and dominated by fine clay, have the high content of smectite and organic matters. These factors lead to sediments of the central South Yellow Sea has the higher water content, the higher plasticity, the lower density, and the lower strength than sediments in the Zhe-Min coastal area.

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.

Distribution characteristics of wave energy in the Zhe-Min coastal area

A 10-year(2003–2012)hindcast was conducted to study the wave field in the Zhe-Min coastal area(Key Area OE-W2)located off Zhejiang and Fujian provinces of China.Forced by the wind field from a weather research and forecasting model(WRF),high-resolution wave modelling using the SWAN was carried out in the study area.The simulated wave fields show a good agreement with observations.Using the simulation results,we conducted statistical analysis of wave power density in terms of spatial distribution and temporal variation.The effective duration of wave energy in the sea area was discussed,and the stability of wave energy was evaluated using the coefficient of variation of wave power density.Results indicate that the wave energy resource in the study area was about 4.11×10^(6) kW.The distribution of wave energy tends to increase from the north(off Zhejiang coast)to the south(off Fujian coast),and from near-shore area to the open sea.The sea areas with wave power density greater than 2 kW/m are mostly distributed seaward of the 10-m isobath,and the contours of the wave power density are almost parallel to the shoreline.The sea areas around the islands that are far from the mainland are rich in wave energy,usually more than 6 kW/m,and therefore are of obvious advantages in planning wave energy development and utilization.The effective duration of wave energy in the offshore area shows an increasing trend from north(off Zhejiang coast)to south(off Fujian coast),with values of∼3500 h in the north and∼4450 h in the south.The coefficient of variation of wave energy in this region is mostly in the range of 1.5–3.0,and gradually decreases from the north to the south,suggesting that the wave energy in the south is more stable than that in the north.

考虑海岸植被影响的水沙动力数学模型研究进展

介绍了植被模型的不同考虑方式,分别从水流阻力、床面阻力、植被消长等方面展开讨论。基于工程尺度和实验室尺度水沙动力数学模型的研究,综述了植被影响下的流速、紊动能、拟序结构,不同波浪形态的传播特征以及悬沙浓度、地形演变等相关的研究进展。结合大量数学模型研究,提出了未来的研究趋势。

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。

人类活动和沿岸流影响下的粤东近海浮游动物群落特征

粤东附近海域具有丰富的渔业资源和复杂的水文环境。在气候变化背景下,为系统提升该海域海洋观测水平和生态动力过程的认知,于2022年5月(春季)、7月(夏季)、10月(秋季)和2023年1月(冬季)开展了四个季节的水文动力、生物、化学等生态环境要素的综合考察。基于镜检浮游动物的鉴定结果,分析该海域浮游动物群落特征,探讨其时空分布的影响因素。根据环境因子聚类,调查海域可分为近岸(主要受人类活动影响)和远岸(主要为海流影响)区域,不同季节和区域浮游动物群落结构存在显著差异。春、夏季浮游动物动物种类、生物量和丰度均高于秋、冬季;远岸区浮游动物种数、生物量和丰度显著高于近岸,并且浮游动物群落结构的季节差异在远岸区更加明显。桡足类和毛颚类为调查海域的主要浮游动物类群。浮游幼虫和鱼卵的丰度在春季显著高于其他季节,并且峰值集中在南澳岛西南韩江口附近海域,说明该区域为粤东重要的产卵场。受粤东沿岸上升流影响,夏季桡足类的种类和丰度增加;而冬季,浮游动物主要由闽浙沿岸流所携带的暖温带种所组成,中华哲水蚤(Calanus sinicus)在S8站位高达1000 ind.·m^(-3)。另外,近岸养殖区内的浮游动物出现种类少、生物量和丰度较低,个别调查站网采样品基本无浮游动物检出。在人类活动和气候变化的双重影响下,粤东近海浮游动物需要进行长期系统监测分析其对海洋环境变化的响应。

Coastal Erosion Feature and Mechanism at Feiyantan in the Yellow River Delta

Feiyantan was the discharge area of Diaokou River distributary of the Yellow River during the period of 1964 to 1976. The coastal erosion feature and morphological evolution at the Feiyantan coast are studied in the light of the topography and section depth, and the corresponding dynamics of wave and current. Results indicate that the protruding topography left after the Diaokou River distributary was abandoned is the main cause of strong coastal erosion. Further research suggests that waves start up the sediment and the tidal current transports it, and the waves and tidal current are combined to be the dominant dynamic mechanism of coastal erosion, in which the tidal residual current takes and transports the sediment outward, thus causing the sediment to wane in the coast.

A Regional Ocean Reanalysis System for Coastal Waters of China and Adjacent Seas

A regional ocean reanalysis system for the coastal waters of China and adjacent seas has been developed by the National Marine Data and Information Service（NMDIS）.It produces a dataset package called CORA （China ocean reanalysis）.The regional ocean model used is based on the Princeton Ocean Model with a generalized coordinate system（POMgcs）.The model is parallelized by NMDIS with the addition of the wave breaking and tidal mixing processes into model parameterizations.Data assimilation is a sequential three-dimensional variational（3D-Var） scheme implemented within a multigrid framework.Observations include satellite remote sensing sea surface temperature（SST）,altimetry sea level anomaly（SLA）,and temperature/salinity profiles.The reanalysis fields of sea surface height,temperature,salinity,and currents begin with January 1986 and are currently updated every year. Error statistics and error distributions of temperature,salinity and currents are presented as a primary evaluation of the reanalysis fields using sea level data from tidal gauges,temperature profiles,as well as the trajectories of Argo floats.Some case studies offer the opportunity to verify the evolution of certain local circulations.These evaluations show that the reanalysis data produced provide a good representation of the ocean processes and phenomena in the coastal waters of China and adjacent seas.