Cross-shelf variation of internal tides west of the Dongsha Plateau in the northern South China Sea

We examine the cross-shelf variation of internal tides(ITs)west of the Dongsha Plateau in the northern South China Sea based on observations from 4 moorings deployed between August 2017 and September 2018.On the slope,the amplitude of diurnal baroclinic current ellipses are 5 times larger than that of barotropic currents.The baroclinic energy quickly dissipates during cross-shelf propagation,and barotropic currents become dominant on the shelf outside of the Zhujiang River Estuary,with the amplitude of semidiurnal barotropic current ellipses being 10 times larger than that of the baroclinic ones.Dynamic modal decomposition indicates the first baroclinic mode is dominant for both diurnal and semidiurnal ITs.The total horizontal kinetic energy(HKE)of the first three baroclinic modes shows spatiotemporal differences among the 4 moorings.On the slope,the HKE for diurnal ITs is stronger in summer and winter,but weaker in spring and autumn;for semidiurnal ITs there is a similar seasonal variation,but the HKE in winter is even stronger than that in summer.On the shallow shelf,both diurnal and semidiurnal ITs maintain a certain intensity in summer but almost disappear in winter.Further analysis shows that only the upper water column is affected by seasonal variation of stratification on the slope,variation of diurnal ITs is thus controlled by the semi-annual cycle of barotropic energy input from the Luzon Strait,while the incoherent baroclinic currents make a major contribution to the temporal variation of semidiurnal ITs.For the shelf region,the water column is well mixed in winter,and the baroclinic energy largely dissipates when ITs propagate to the shelf zone despite of a strong barotropic energy input from the Luzon Strait.

The western South China Sea currents from measurements by Argo profiling floats during October to December 2007

The three dimensional structure of the western boundary current east of the Vietnam coast was determined from measurements by Argo profiling floats which deployed near the east of the Vietnam Coast in October 2007. The trajectories of the Argo floats provided robust evidence that there does exist southward flowing current along the Vietnam coast. The southward current begins at about 15°N, 111°E, flowing along the 1 000 m isobath and extending to 5°N south. The estimated surface and parking depth velocities obtained from the floats suggest that this southward current can extend to 1 000 m depth. The mean surface velocity of the western boundary current is about 49 cm/s, with the maximum speed exceeding 100 cm/s occurring at 11.6°N, 109.5°E in the direction of 245°. The mean parking depth (1 000 m) velocity is 12–16 cm/s with the maximum speed of 36 cm/s occurring at 12.1°N, 109.7°E in the direction of 239°. The water mass analysis suggests that the Kuroshio surface water and NPIW are not apparent in the western boundary current although this time was the favored season for the Kuroshio intrusion into Luzon Strait. The mean geostrophic currents suggest that the Kuroshio intrusion into Luzon Strait during October to December 2007 is very weak, as most of the intruding water was carried eastward at 14°N near the western boundary and little flowed southward along the western boundary, providing an explanation for the absence of Kuroshio surface water and NPIW in the water mass analysis. There is a strong cyclonic circulation in the SSCS during this time, which induces a strong mixing in the western boundary companied by a homogeneous salinity layer between 300–600 m in the salinity vertical distribution. No reversal undercurrent occurred at the intermediate depth along the western boundary east of the Vietnam coast during October to December 2007.

Freshening of the intermediate water of the South China Sea between the 1960s and the 1980s

Variation in intermediate water salinity in the South China Sea (SCS) between the 1960s and 1980s was studied using historical hydrographic data. The results demonstrate that the water was significantly fresher in the 1980s than in the 1960s, indicating that vertical mixing at intermediate water depth was reduced in the 1980s. This was partially because of the change of the SCS meridional overturning circulation (MOC) connecting local intermediate water with deep water. Data assimilation showed a 0.5Sv (1 Sv=10 6m 3/s) reduction in the strength of the MOC, which is about one third of the mean SCS MOC. Because the SCS MOC is linked to the Pacific Ocean, such an interdecadal variation in the intermediate water SCS may reflect anthropogenic climate change in the world ocean.

Evolution and propagation of a mesoscale eddy in the northern South China Sea during winter

In situ observations, satellite data, and the output from an eddy-resolving ocean circulation model were used to study the generations and propagations of an anticyclonic eddy in the northern South China Sea （NSCS） during the winter of 2009-2010. In the NSCS, the anticyclonic eddy firstly appeared to the west of the Luzon Strait, migrated generally along the continental slope and dissipated around the Xisha Archipela- go. The evolution of the warm eddy contains three phases： development, maturation, and decay. The eddy mainly stayed near 119.7°E in December and then gradually moved to 118.7°E until January 15, when its intensity, as indicated by the thermocline temperature and salinity anomalies, increased significantly, re- flecting the growth of the eddy. The eddy reached its peak on January 15 and persisted until February 23. During this period, the eddy propagated westward to 116.4°E. After, the warm eddy weakened significantly and dissipated finally near the Xisha Archipelago.

On Outflow Passages in the South China Sea

Both box and inverse methods are used to study the relative importance of outflow passages of the South China Sea. The physical meaning of the box model is described in detail, and its optimization is based on the most efficient way to remove the warm and fresh water in terms of the heat and salt budgets. The box model results suggest that the Kalimantan Strait is the main outflow passage, because the water near the Kalimantan is the warmest and freshest. The system of the South China Sea advects the warmest and freshest water through the wide Kalimantan Strait out of the South China Sea to achieve a quick heat and salt balance because of the huge amount of heat flux and large precipitation gained from the atmosphere. These results are confirmed by an inverse method, in which detailed dynamics and thermohaline structure are considered. The velocity distribution in the transect of the Luzon Strait revealed by the inverse method is similar to that obtained by earlier studies.

Spring mesoscale high in the western South China Sea

A recurring spring mesoscale eddy in the western South China Sea （SCS） is studied using remote sensing data and historical in situ observations. The feature first appears east of the central Vietnam coast in Febru- ary as a high sea-level anomaly, grows rapidly to a well-developed anticyclonic eddy by March, matures in April, and decays in May. Besides the warm-core feature, it also has an inherent low-salinity property, so it is named ＂spring mesoscale high （SMH）＂. Though with clear interannual variation in terms of intensity and spatial coverage, the SMH always emerges in the region between ll0~E and l14~E and between 12~N and 16~N. The formation of SMH is ascribed to the combined effects of wind forcing and releasing of po- tential energy set up by winter monsoon. In particular, the wind-stress curl plays an important role in its development, maintenance, and dissipation.

Prediction of Scour Depth around Offshore Pipelines in the South China Sea

搜索深度预言近海管道具有到海底的管道工程的设计和构造的大意义。在这份报纸，基于 CFD 软件，流畅的包裹和用户定义功能(UDF ) ，一个 Eulerian 二阶段的模型，包括为水和沉积的联合模型分阶段执行的 Euler-Euler，和为液体的一个狂暴的模型分阶段执行，被采用在管道附近预言 scour 深度。模型被从实验室实验获得的观察数据验证。根据模拟，包括早期的速度的效果，影响 scour 深度的因素等等输送直径和沉积粒子尺寸，被调查。根据各种各样的沉积的早期的速度的公式，同时接近计算在上理论搜索深度在华南海为七个车站的管道被开发，在设计申请的地方，信息是可得到的。

A motion correction on direct estimations of air-sea fluxes from a buoy

A flux system deployed on a moored buoy has been described, which is capable of directly estimating the airsea fluxes after removing the contamination in the signal due to buoy motion. A triple loop fitting method has been demonstrated for determining the three angular offsets between measurement axes of the sonic anemometer and motion pack. The data collected in an experiment in the Northern Huanghai Sea is used to correct the three sonic anemometer measurements of turbulent wind for buoy motion. The effective removal of wave-scale motion from the spectra and cospectra are demonstrated. Estimates of along-wind momentum flux, sensible heat flux and latent heat flux calculated by the eddy correlation method based on data obtained by sonic anemometer 81000V are shown to be in the same trend and scale with those determined by the bulk aerodynamic method after motion correction. The motion correction not only greatly improve the estimation of the momentum flux but also has a great impact on the calculated sensible heat flux.

On the parameterization of drag coefficient over sea surface

Six parameterization schemes of roughness or drag coefficient are evaluated on the basis of the data from six experiments. They present great consistency with measurement when friction velocity u＊〈0.5 m/s （ap- proximately corresponding to 10 m wind speed U10〈 12 m/s） and large deviation from measurement when u＊≥0.5 m/s （approximately U10 ≥ 12 m/s）. In order to improve the deviation, a new parameterization of drag coefficient is derived on the basis of the similarity theory, Charnock relationship and Toba 3/2 power law. Wave steepness and wind-sea Reynolds number are considered in the new parameterization. Then it is test- ed on the basis of the measurements and shows significant improvement when u＊≥0.5 m/s. Its standard errors are much smaller than the ones of the other six parameterizations. However, the new parameteriza- tion still needs more tests especially for high winds.

Air-sea carbon-dioxide flux estimated by eddy covariance method from a buoy observation

Precise measurements of the CO2 gas transfer across the air-sea interface provide a better under- standing of the global carbon cycle. The air-sea CO2 fluxes are obtained by the eddy covariance method and the bulk method from a buoy observation in the northern Huanghai sea. The effects of buoy motion on flux calculated by the eddy covariance method are demonstrated. The research shows that a motion correction can improve the correlation coefficient between the C02 fluxes esti- mated from two different levels. Without the CO2-H20 cross-correlation correction which is termed as PKT correction, the air-sea CO2 fluxes estimated by eddy covariance method using the motion corrected data are nearly an order of magnitude larger than those estimated by the bulk method. After the CO2-H20 cross-correlation correction, some eddy covariance CO2 fluxes indeed become closer to the bulk CO2 flux, whereas some are overcorrected which are in response to small water vapor flux.

Analysis of Characteristics of Spatio-temporal Evolution of Land Use in Inhabited Islands of Pearl River Estuary

Under the support of the remote sensing and geographical information system(GIS) techniques,we acquire the land use data in 1990 and 2008 regarding 6 inhabited islands,namely Longxue Island,Hengmen Island,Weiyuan Island,Qi'ao Island,Hengqin Island and Gaolan Island in Pearl River Estuary.By using dynamic degree of land use,land use change intensity,relative change rate and other indicators,we conduct quantitative description,and thus quantitatively and qualitatively analyse characteristics of temporal evolution and law of spatial pattern change concerning land use of each island.The study indicates that in the period 1990-2008,the area of construction land and water in 6 islands increased,while the area of agricultural land and unused land increased in some islands and decreased in others.The land use change shows spatial disparity;the holistic land use change degree in Hengmen Island is higher than that of other islands;the dynamic degree of land use,intensity of land use,and relative change rate differ in different islands.

Large eddy simulation of turbulence in ocean surface boundary layer at Zhangzi Island offshore

This study uses a large eddy simulation （LES） model to investigate the turbulence processes in the ocean surface boundary layer at Zhangzi Island offshore. Field measurements at Zhangzi Island （39°N, 122°E） during July 2009 are used to drive the LES model. The LES results capture a clear diurnal cycle in the oceanic turbulence boundary layer. The process of the heat penetration and heat distribution characteristics are analyzed through the heat flux results from the LES and their differences between two diurnal cycles are discussed as well. Energy balance and other dynamics are investigated which show that the tide-induced shear production is the main source of the turbulence energy that balanced dissipation. Momentum flux near the surface shows better agreement with atmospheric data computed by the eddy correlation method than those computed by bulk formula.

Incipient motion of non-uniform coarse grain of bedload considering the impact of two-way exposure

The incipient motion of bedload is due to the interaction between the flow and sediment.It is stochastically correlated with the flow structure,the sediment gradation and the arrangement of grains on the bed surface.The random position of the sediment on bed can be represented by a hiding factor or an exposure degree.Based on the numerical simulation of the disturbed flow in the interstice of grains,the influence of the two-way exposure degree(the vertical exposure degree and the longitudinal exposure degree) on the coarse grain incipient motion was investigated in this work.Results show that the exposure degree varies with the position of the sediment on the bed,which influences the flow structure around the particle and the incipient motion.In this paper,the major research achievements on this phenomenon include:Firstly,a mathematical model is established for the rolling-pattern incipient motion of the coarse grain under a critical state of moment balance.The influence of the partial disturbed flow is considered.Secondly,the two-way relative-exposure-degree probability distribution functions are developed to reflect the influence of the disturbed flow and the random arrangement of sediments.Thirdly,a formula to calculate the incipient velocity is presented based on the above results,which considers the impact of the two-way exposure degree of sediment particles.

Surface pollen distribution patterns in Beibu Gulf and corresponding sediment dynamics environment

The Beibu Gulf is a semi-enclosed gulf in the northwest of the South China Sea.We palynologically analyzed 306 surface sediment samples from the eastern Beibu Gulf to improve bioclimatic interpretation of fossil pollen records there.Surface pollen assemblages could be classified into five pollen regions based on the distributions of total,arboreal,herbaceous and fern pollen concentrations.Four high-concentration and three low-concentration subregions could be distinguished within these regions.The distribution patterns of surface pollen assemblages were consistent with those of grain sizes of surface sediments.Sediments from regions with high pollen concentrations were very fine and fine silts (>7.0,<0.008 mm),whereas those with low pollen concentrations were fine sand (<3.5,>0.088 mm).Sedimentary heterogeneity of surface pollen assemblages was closely related to pollen source,transportation and sedimentation controlled by ocean currents,tides and waves,and oceanic bottom topography.Fern spores exhibited higher percentages along the east margins of the region,while arboreal types like Pinus increased in concentration towards the center.Herbaceous pollen appeared in high percentages around seacoasts near their source areas.Dacrydium and mangrove pollen were distributed near their source regions at low percentages.We discuss the sediment dynamic environments in the eastern Beibu Gulf based on surface pollen distributions.The estuary region is an important access to the sea basin and a depositional site for terrestrial pollen grains.Coastal regions can accumulate pollen due to the back-and-forth movements of tides there.Although ocean currents on a gulf scale tend to spread pollen grains,the interaction of multiple currents could lead to pollen accumulation and deposition.Low surface pollen concentrations in the northeast shallow-water regions of the eastern Beibu Gulf could be attributed to repeated washing and sediment floatation caused by severe wave activities during the summer.Strong tides in Qiongzhou Strait frequently wash the seabed and disadvantage pollen deposition,leading to low surface pollen concentrations there.