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.

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.

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.

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.

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.

Effects of topography on the sub-tidal circulation in the southwestern Huanghai Sea(Yellow Sea)in summer

A nested circulation model system based on the Princeton ocean model （POM） is set up to simulate the currentmeter data from a bottom-mounted Acoustic Doppler Profiler （ADP） deployed at the 30 m depth in the Lunan（South Shandong Province, China） Trough south of the Shandong Peninsula in the summer of 2008, and to study the dynamics of the circulation in the southwestern Huanghai Sea （Yellow Sea）. The model has reproduced well the observed subtidal current at the mooring site. The results of the model simulation suggest that the bottom topography has strong steering effects on the regional circulation in summer. The model simulation shows that the Subei （North Jiangsu Province, China）coastal current flows north- ward in summer, in contrast to the southeastward current in the center of the Lunan Trough measured by the moored currentmeter. The analyses of the model results suggest that the southeastward current at the mooring site in the Lunan Trough is forced by the westward wind-driven current along the Lunan coast, which meets the northward Subei coastal current at the head of the Haizhou Bay to flow along an offshore path in the southeastward direction in the Lunan Trough. Analysis suggests that the Subei coastal current, the Lunan coastal current, and the circulation in the Lunan Trough are independent current systems con- trolled by different dynamics. Therefore, the current measurements in the Lunan Trough cannot be used to represent the Subei coastal current in general.

Interaction of an anticyclonic eddy with sea ice in the western Arctic Ocean:an eddy-resolving model study

The dramatic decline of summer sea ice extent and thickness has been witnessed in the western Arctic Ocean in recent decades, which has motivated scientists to search for possible factors driving the sea ice variability. An eddy-resolving, ice-ocean coupled model covering the entire Arctic Ocean is implemented, with focus on the western Arctic Ocean. Special attention is paid to the summer Maskan coastal current （ACC）, which has a high temperature （up to 5℃ or more） in the upper layer due to the solar radiation over the open water at the lower latitude. Downstream of the ACC after Barrow Point, a surface-intensified anticyclonic eddy is frequently generated and propagate towards the Canada Basin during the summer season when sea ice has retreated away from the coast. Such an eddy has a warm core, and its source is high-temperature ACC water. A typical warm-core eddy is traced. It is trapped just below summer sea ice melt water and has a thickness about 60 m. Temperature in the eddy core reaches 2-3℃, and most water inside the eddy has a temperature over 1℃. With a definition of the eddy boundary, an eddy heat is calculated, which can melt 1 600 km2 of 1 m thick sea ice under extreme conditions.

The circulation in the southern Huanghai Sea and northern East China Sea in June 1999

On the basis of hydrographic data and current measurement (the mooring system, vessel-mounted ADCP and toward ADCP) data obtained in June 1999, the circulations in the southern Huanghai Sea (HS) and northern East China Sea (ECS) are computed by using the modified inverse method. The Kuroshio flows northeastward through eastern part of the investigated region and has the main core at Section PN, a northward flow at the easternmost part of Section PN, a weaker anti-cyclonic eddy between these two northward flows, and a weak cyclonic eddy at the western part of Section PN. The above current structure is one type of the current structures at Section PN in ECS. The net northward volume transport (VT) of the Kuroshio and the offshore branch of Taiwan Warm Current (TWCOB) through Section PN is about 26.2 x 10(6) m(3)/s in June 1999. The VT of the inshore branch of Taiwan Warm Current (TWCIB) through the investigated region is about 0.4 x 10(6) m(3)/s. The Taiwan Warm Current (TWC) has much effect on the currents over the continental shelf. The Huanghai Sea Coastal Current (HSCC) flows southeastward and enters into the northwestern part of investigated region, and flows to turn cyclonically, and then it flows northeastward, due to the influences of the Taiwan Warm Current and topography. There is a cyclonic eddy south of Cheju Island where the Huanghai Sea Coastal Current flows to turn cyclonically. It has the feature of high dense and cold water. The uniform and cold water is occurred in the layer from about 30 m level to the bottom between Stations C306 and C311 at the northernmost Section C3. It is a southern part of the Huanghai Sea Cold Water Mass (HSCWM).

Potential suitable sites for the calibration of Scientific Echo Sounder in the marginal seas around Antarctica

The in situ calibration of Scientific Echo Sounders(SESs)in the Southern Ocean is crucial for accurate assessments of Antarctic krill and fishery biomasses.Because of the occurrence of strong winds,waves,and sea ice coverage in most seasons,SES calibration is usually difficult to perform in the Southern Ocean.Accordingly,it is essential to identify potential sites suitable for SES calibration in the marginal seas around Antarctica to successfully calibrate SESs in the Southern Ocean.Using synthetic analyses of the wind,surface current,and sea ice concentration in the targeted seas,we found that the polynya in the southeast Prydz Bay,close to the Chinese Zhongshan Station,is an ideal location to calibrate SESs based on its weak wind and surface current and its ice-free coverage during Antarctic cruises.Calibrations of the SESs onboard the research vessels of Xuelong and Xuelong 2 during the 36th and 37th Chinese National Antarctic Research Expeditions indicate that this location is a potential suitable site for conducting SES calibration with the vessel in a drifting mode.

Coastal upwelling off the southwest coast of India:observations and simulations

Spatial hydrographic data collected from August 2007 through May 2008 and the Princeton Ocean Model(POM)are utilized to explain and document the upwelling in the south eastern Arabian Sea(SEAS).The decrease in the magnitude of winds towards the coast favors local anticyclonic vorticity,resulting in the formation of cyclonic eddy and reversal of coastal currents.The Ekman transport due to alongshore winds,Ekman pumping due to wind stress curl,cyclonic eddy and southward West Indian Coastal Currents play different roles in the SEAS upwelling.In the offshore,wind stress curl leads to the formation of meso-scale eddies,resulting in Ekman pumping at the center and consequent upwelling.The rapid depth variation on the western side of Quilon Mount intensifies upwelling in the offshore.The upwelling Kelvin waves generated at the equator terminates in the Bay of Bengal and do not contribute to the SEAS upwelling.The possible role of local and remote winds,especially around Sri Lanka on the upwelling dynamics of SEAS is described utilizing POM.The Kelvin waves generated off the coast of Sri Lanka during the active phase of the summer monsoon also contribute to the upwelling dynamics off the west coast of India.

Origin of the springtime South China Sea Warm Current in the southwestern Taiwan Strait: Evidence from seawater oxygen isotope

Oxygen isotope(δ^18O)of seawater is an excellent proxy for tracing the origins of water masses and their mixing processes.Combining with hydrographic observation,hybrid coordinate ocean model(HYCOM)analysis data,and seawater oxygen isotope,we investigated the source of the South China Sea Warm Current(SCSWC)in the southwestern Taiwan Strait and its underlying mechanism.Results show that the Kuroshio subsurface water(KSSW)can intrude the continental slope in the southwestern Taiwan Strait,and thereby climb up the continental slope coupled with upwelling.Theδ^18O-salinity relationship further indicates that in spring,the SCSWC in the southwestern Taiwan Strait originates from the upslope deflection of the slope current formed by the KSSW intrusion into the South China Sea,rather than from the west segment of the SCSWC formed to the east of Hainan Island.In addition,the southward flowing Zhe-Min Coastal Current(ZMCC)can reach as far as the Taiwan Bank(TB)and deflects offshore over the western TB at approximately 23.5°N,to some extent affecting the SCSWC.Moreover,this study reveals that seawaterδ^18O is exquisitely sensitive to the determination of the origin and transport of water masses as compared with traditional potential temperature-salinity plot(θ-S)and HYCOM analysis data.In addition,their coupling can more reliably interpret the mixing processes of shelf water masses.