Extraction of Intemal Tidal Currents from A Portion of Sea Current Profile

Internal tide is one of the major oceanic phenomena. Determination of internal tide is important for theoretical study and for ocean engineering research. As an inverse problem, extraction of internal tidal currents from sea currents is difficult. In this paper, a method is developed to extract internal tidal currents from a portion of the sea current profile based on the fact that the directions of internal tidal currents above and below the thermocline are inverse. Sea current data collected from the South China Sea is processed with this method. The internal tidal currents and the depth of the thermocline are successfully extracted. The depth of the thermocline determined is in good agreement with that measured in 1959.

Long-Chain Alkenones in the South Yellow Sea Sediments and Their Indicative Significance for Haptophytes Species

Long-chain alkenones(LCAs)have been widely used as important biomarkers in palaeoceanographic studies.However,the commonly used LCAs proxies are mainly based on C_(37) alkenones,and it is still lack of the studies about the distribution and in-dications of LCAs with different chain lengths other than C_(37) alkenones.Here,the composition and distribution of LCAs were ana-lyzed in surface sediments from the southern Yellow Sea(SYS)and a sedimentary core(A02-C)from the central Yellow Sea(YS)mud area.The results showed that C_(37),C_(38) and C_(39) alkenones were the major LCAs in surface sediments of the SYS,and the relative contents of C_(38:2)Et,C_(37:2)Me,C_(37:3)Me,C_(38:2)Me,C_(38:3)Et,C_(38:3)Me,C_(39:2)Et and C_(39:3)Et were 18.3%-59.8%,22.6%-41.2%,7.4%-23.0%,6.6%-15.4%,3.8%-13.3%,3.6%-8.7%,2.8%-6.0% and 0.7%-3.0%,respectively.Then the relationships of U_(38Me)^(K)-U_(38Et)^(K) and U_(37)^(K')-U_(38Et)^(K) indicate that LCAs are mainly derived from Emiliania huxleyi(E.huxleyi).High ratios of total C_(37) alkenones to total C_(38) alkenones(K_(37)/K_(38))(1-1.2)were found in the central SYS,corresponding to the relatively high abundance of E.huxleyi;while low ratios of K_(37)/K_(38)(0.7-0.9)were observed at nearshore area of the SYS where Gephyrocapsa oceanica(G.oceanica)has rela-tively high abundance.The spatial distribution of K_(37)/K_(38) ratio is also consistent with that of coccolithophores nannofossil in the sediments.In addition,K_(37)/K_(38) ratio in core A02-C varied from 0.7 to 1.1 with a gradual decreasing trend during the past 5.5 kyr.This suggests that the relative abundance of E.huxleyi decreased gradually,caused by the changes in the Yellow Sea Warm Current(YSWC)and the East Asian Winter Monsoon(EAWM)during this period.

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.

Progress of studies in China from July 2010 to May 2015 on the influence of the Kuroshio on neighboring Chinese seas and the Ryukyu Current

The influence of the Kuroshio on neighboring Chinese seas and the Ryukyu Current is a very important subject of interest in physical oceanography. To deeply explain the research progress made by Chinese scientists from July2010 to May 2015, the following three aspects are reviewed in this paper. The first concerns the Kuroshio intrusion into the South China Sea（SCS） and its circulation around the Luzon Strait. There are two very important points to be explained： the seasonal and inter-annual variation of the Kuroshio intrusion and the mechanisms of the Kuroshio intrusion and the influence of the Kuroshio on currents in the Luzon Strait and circulation in the northern SCS. The second concerns the variability of the Kuroshio and its interaction with the East China Sea（ECS）. There are following four interesting topics to be explained： an overview of studies on the Kuroshio in the ECS; the Kuroshio intrusion into the ECS, water exchange, and dynamic impacts; the downstream increase of nutrient transport by the Kuroshio; and the application of satellite remote sensing on terrestrial material transport by the Kuroshio intrusion into the ECS. Third, the interaction between the Ryukyu Current and Kuroshio in the ECS are also discussed. Finally, the main results are summarized and areas of further study are simply discussed.

Observational evidence of the Yellow Sea warm current

The Yellow Sea Warm Current (YSWC) is one of the principal currents in the Yellow Sea in winter. Former examinations on current activity in the Yellow Sea have not observed a stable YSWC because of the positioning of current meters. To further understand the YSWC, a research cruise in the southern Yellow Sea was carried out in the winter of 2006/2007. Five moorings with bottom-mounted acoustic Doppler current profilers (ADCP) were deployed on the western side of the central trough of the Yellow Sea. The existence and distributional features of the YSWC were studied by analyzing three ADCP moorings in the path of the YSWC in conjunction with conductivity-temperature-depth (CTD) data over the observed area in the southern Yellow Sea. The results show the following. (1) The upper layer of the YSWC is strongly influenced by winter cold surge; its direction and speed often vary along a south-north axis when strong cold surges arrive from the north. (2) The YSWC near the bottom layer is a stable northwest flowing current with a speed of 4 to 10 cm/s. By combining the analyses of the CTD data, we speculate that the core of the YSWC may lie near the bottom. (3) On a monthly average timescale, the YSWC is stably oriented with northward flow from the sea surface to the sea floor.

The inter-annual variability of the Yellow Sea Warm Current surface axis and its influencing factors

Based on the Pathfinder sea surface temperature(PFSST),the surface axis and its pattern of the Yellow Sea Warm Current(YSWC) are discussed.A structure of double-warm-tongue is found in February and it varies in different years.Two indexes are calculated to represent the westward shift(WSI) and northward extension(NEI) of the warm water in the Yellow Sea(YS).Wavelet analysis illustrates that the WSI and NEI have prominent periods of 3-6 years and 3-4 years,respectively.The Empirical Orthogonal Function(EOF) decomposition is applied to the winter wind stress curl and the Kuroshio Current(KC) transport,which are believed to play important roles in forcing the variability of the YSWC surface axis.Statistics shows that the WSI is significantly related with the second EOF mode of the wind stress curl in February,which may force the YSWC surface axis moving westward and maintaining the double warm tongues because of its opposite curl in the YSWC domain.The first EOF mode of wind stress curl in January is propitious for inducing the warm tongue in the YS to advance more northward.Hence,the wind stress curls both in January and in February could force variations of the YSWC surface axis;however,the effect of the January wind stress curl is relatively weaker than that of the February.The relationship between the NEI and the KC transport is remarkable,and it seems that the stronger KC supplies more power to push the YSWC northward against the southward wind.

Winter heat budget in the Huanghai Sea and the effect from Huanghai Warm Current (Yellow Sea Warm Current)

Four sources of surface heat flux （SHF） and the satellite remote sensing sea surface temperature （SST） data are combined to investigate the heat budget closure of the Huanghai Sea （HS） in winter.It is found that heat loss occurs all over the HS during winter and the area averaged heat content change decreases with a rate of-106 W/m 2.Comparing with the area averaged SHF of-150 W/m 2 from the four SHF data sets,it can be concluded that the SHF plays a dominant role in the HS heat budget during winter.In contrast,the heat advection transported by the Huanghai Warm Current （Yellow Sea Warm Current,HWC） accounted for up to 29% of the HS heat content change.Close correlation,especially in February,between the storm events and the SST increase demonstrates that the HWC behaves strongly as a wind-driven compensation current.

A numerical study of the South China Sea Warm Current during winter monsoon relaxation

Using a Finite-Volume Community Ocean Model, we investigated the dynamic mechanism of the South China Sea Warm Current(SCSWC) in the northern South China Sea(NSCS) during winter monsoon relaxation. The model reproduces the mean surface circulation of the NSCS during winter, while model-simulated subtidal currents generally capture its current pattern. The model shows that the current over the continental shelf is generally southwestward, under a strong winter monsoon condition, but a northeastward counter-wind current usually develops between 50-and 100-m isobaths, when the monsoon relaxes. Model experiments, focusing on the wind relaxation process, show that sea level is elevated in the northwestern South China Sea(SCS), related to the persistent northeasterly monsoon. Following wind relaxation, a high sea level band builds up along the mid-shelf, and a northeastward current develops, having an obvious vertical barotropic structure. Momentum balance analysis indicates that an along-shelf pressure gradient provides the initial driving force for the SCSWC during the first few days following wind relaxation. The SCSWC subsequently reaches a steady quasi-geostrophic balance in the cross-shelf direction, mainly linked to sea level adjustment over the shelf. Lagrangian particle tracking experiments show that both the southwestward coastal current and slope current contribute to the northeastward movement of the SCSWC during winter monsoon relaxation.

NUMERICAL STUDY ON THE FORMATION OF THE SOUTH CHINA SEA WARM CURRENT I. BAROTROPIC CASE

In this work, Princeton Ocean Model (POM) was used to study the formation of the South China Sea Warm Current (SCSWC) in the barotropic case. Monthly averaged wind stress and the inflow/outflow transports in January were used in the numerical simulation which reproduced the SCSWC. The effects of wind stress and inflow/outflow were studied separately. Numerical experiments showed that the Kuroshio intrusion through the Luzon Strait and the slope shelf in the northern SCS are necessary conditions for the formation of the SCSWC. In a flat bottom topography experiment, the wind stress driven northeast current in the northern SCS is a compensatory current.

Measurement of the current and spectral analysis on the continental shelf in the East China Sea

Direct measurements of current velocity and water temperature were undertaken at the mooring Sta. M (125°29.38' N, 31°49.70' E) on the continental shelf area in the East China Sea in June 1999 by R/V Xiangyanghong 14. The relationship between various time series of oceanic fluctuations is calculated by spectral analysis. The major results are as follows: (1) an average (u,v) of (6.9,-3.0 cm/s) at the 30 m depth is obtained during the 9-day observation, and that at the 45 m depth is (3.7,-1.1 cm/s), i. e., the mean flows are southeastward at both the 30 and 45 m depths;the currents become stronger gradually during the observation period; this may be mainly attributed to the transition of the tidal currents from neap to spring; (2)semidiurnal fluctuation is the most dominant in the current fluctuations, and rotates mainly clockwise; in the next place, there is also diurnal fluctuation;the local inertial period is close to the period of diurnal fluctuation, and an inertial motion is clockwise; thus, local inertial motion combines with diurnal fluctuation, and makes the spectral peaks in clockwise components much higher than those in counterclockwise ones; except for the fluctuations of above main periods, there is also the peak at 3 d period for counterclockwise compo-nents in the upper and lower layers; (3) the calculation of cross spectra between two time series of current velocities at the 30 and 45 m depths shows that both the current fluctuations at the 30 and 45 m depths are much alike, i. e., they are synchro; this shows that the flow field here is rather vertically homogeneous; (4)power spectra of tempera-ture time series at both the 30 and 45 m depths show that the semidiurnal peak is the most predominant, and the second highest peak is the diurnal period; besides spectral peaks at above periods, there are also obvious spectral peaks at 6.8 h and 2 d; (5)plots of temperature time series at 16 , 30 , 35 , 45 and 50 m depths show that the temporal variations of temperature at these depths are synchro, which are like those in the velocity field; temperature records also show a gradual rise in temperature, which are also like those in the velocity field.

NUMERICAL STUDY ON THE FORMATION OF THE SOUTH CHINA SEA WARM CURRENT II. BAROCLINIC CASE

In this part, Levitus’ climatological temperature and salinity are incorporated in the numerical model developed in Part I. Diagnostic and prognostic experiment on the thermohaline circulation were conducted. The smooth Levitus’ data do not include any information on the South China Sea Warm Current (SCSWC), so it is not in the model produced diagnostic thermohaline circulation. Although the SCSWC does not appear in the wind driven circulation in the barotropic case, it appears in the prognostic wind driven circulation in the baroclinic case. This implies that the differing circulation patterns between barotropic case and baroclinic case are due to the stratification. The prognostic thermohaline circulation with wind stress and inflow/outflow transports at open boundaries are also discussed. Coupling of density and dynamic forces makes the circulation pattern more complicated. Even though the stratification is not always a direct cause of the formation of the SCSWC, it is at least an indirect cause.

Application of ESPRIT in Broad Beam HF Ground Wave Radar Sea Surface Current Mapping

HF surface wave radar system OSMAR2000 is a broad-beam sea-state detecting radar. ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) algorithm is proposed to apply in DOA (direction of arrival) determination of sea echoes. The algorithm of ESPRIT is briefly introduced first. Then discussions are made on the technique for application in the OSMAR2000 framework. Numerical simulation results are presented to demonstrate the feasibility of radial current mapping based on this method. The algorithm manifests significant performance and computational advantages compared with that of MUSIC. Data acquired by OSMAR2000 are processed to give radial current map and the synthesized vector currents are compared with the in-situ measurement with traditional means. The results show the validity of ESPRIT application in DOA determination for broad-beam radar. Key words HF ground wave radar - sea surface current - ESPRIT - MUSIC CLC number TN 911.72 - TN 958.95 Foundation item: Supported by the National Natural Science Foundation of China (60201003) and the National High Technology Development 863 Program of China (863-818-01-02)Biography: Liu Dan-hong (1976-), female, Master candidate, research direction: radar signal processing.

Sea surface temperature evolution in the Yellow Sea Warm Current pathway and its teleconnection with high and low latitude forcing during the mid-late Holocene

Sea surface temperature(SST)in the Yellow Sea Warm Current(YSWC)pathway is sensitive to the East Asian Winter Monsoon(EAWM)and YSWC.However,the role of the YSWC in the evolution of regional SST remains unclear.Here,we present new U 37 k′based SST and grain size sequences spanning the last 6092 years in the sediment core Z1,which was retrieved from the central Yellow Sea muddy area.Overall,U 37 k′-SST gradually increased since 6.1 ka BP,with a series of centennial-scale fl uctuations.Its variation was mainly caused by EAWM when YSWC was weak between 6.1 and~3.9 ka BP,as shown by the end-member content of grain size.However,after YSWC was fully developed,i.e.,since~3.9 ka BP,it exerted critical eff ects on SST evolution in its pathway.The 1010-and 538-year cycles of the SST sequence indicated a basic control of solar activity on the oceanic conditions in the Yellow Sea.It is suggested that the variation of total solar irradiance was amplifi ed by thermohaline circulation and then transmitted to the Yellow Sea through the EAWM.Meanwhile,the tropical Pacifi c signal of El Niño was transmitted to the YSWC through the Kuroshio Current.The dual properties of warm water transported by YSWC to compensate the EAWM and driving by Kuroshio Current closely linked the variation of SST in the YSWC pathway to the Northern Hemisphere high latitude climate and the tropical Pacifi c.These fi ndings highlight the signifi cance of YSWC on regional SST evolution and its teleconnection to high and low latitude forcing,which grains a better understanding of the long-term evolution of SST in the middle latitude Yellow Sea.

The study of the Kuroshio in the East China Sea and the currents east of the Ryukyu Islands

In this study, the inverse method is used to compute the Kuroshio in the East China Sea and southeast of Kyushu and the currents east of the Ryukyu Islands, on the basis of hydrographic data obtained during September-October, 1987 by R/V Chofu Maru. The results show that: (1)A part of the Taiwan Warm Current has a tendency to converge to the shelf break; (2) the Kuroshio flows across the section C3 (PN) with a reduced current width, and the velocity of the Kuroshio at the section C3 increases and its maximum current speed is about 158 cm/s, and its volume transport here is about 26×106m3/s; (3) the Kuroshio has two current cores at the sections C3 (PN) and B2 (at the Tokara Strait); (4) the currents east of the Ryukyu Islands are found to flow northward over the Ryukyu Trench during September-October, 1987. The velocities of the currents are not strong throughout the depths. At the section C2 east of the Ryukyu Islands, the maximum current speed is at the 699 m levei and its magnitude is 25 cm/s, and its volume transport is about 21×06 m3/s; (5) the volume transports of the Kuroshio through the sections B2 (at the Tokara Strait) and C6 (southeast of Kyushu) are 23. 33, 67. 31×106 m3/s, respectively; (6) there are two meso-scale anticyclonic warm eddies between 135° E and the area east of the Ryukyu Islands, and their characters and hydrographic structure are discussed.

Vertical structure and seasonal variation of tidal and residual currents in the northern Huanghai Sea

Analysis of tidal current and sea level has been made based on the observations from the summer of 2006 to the winter of 2007,respectively.The result indicates that a two-layer structure of residual current exists in summer,with its upper layer going northwestward and the lower layer southeastward.In addition,some strong residuals exist in the neighboring depth of the pycnocline during the current period of astronomical tide.In winter,except some individual layersthe residual currents mostly direct to the northwest,from which we can see the fluctuation of abnormal sea-level and the appearance of associated current because of the changes of the wind field.The analysis of tidal ellipse indicates that the direction of the maximum semidiurnal component is clockwise from summer to winter,with an angle of 16-18.Moreover,in summer the semidiurnal component rotates with depth clockwise while the diurnal component counterclockwise.However,the vertical structure is almost homogeneous in winter.

On the Huanghai (Yellow) Sea circulation:a review by current measurements

The Huanghai (Yellow) Sea circulations suggested in the past were constructed mostly based on hydrographic data or simple numerical models, and were not rigorously compered with direct current measurements. Ourpresent understanding of the Huanghai Sea circulation is still very poor and superficial. To evaluate our present knowledge about the circulation and to sketch a more realistic circulation pattern, the previous studies on the circulation arebriefly reviewed and directly measured current data collected in the past are revisited. The Huanghai Sea Warm Current and the northeastward extension in summer of the Changjiang diluted water are paid more attention to, which arethe major currents of the basic circulation, and then attempted to sketch the Huanghai Sea circulation by synthesizinghydrography and current data. The Huanghai Sea circulation is not considered to be persistent since the thin water column, less than 100 m, responds quickly and sensitively to external driving forces. Directly measured current data showthat saline water originating from the Kuroshio water turns clockwise around Cheju-do throughout the year and eventually joins the Tsushima Warm Current at the western channel of the Korea Strait. This clockwise current is called theCheju Warm Current. The current data do not support the persistent existence of the Huanghai Sea Warm Current,which has ho believed to supply heat and salt to the Huanghai Sea throughout the year. However, a northward flowis generated intermittently during the weakening phase of cold air outbreaks in winter. The northeast extension of theChangjiang diluted water in summer is evidenced by satellite-tracked surface drifters.

Characteristics of zooplankton community in North Yellow Sea unveiled an indicator species for the Yellow Sea Warm Current in winter: Euchaeta plana

Zooplankton distributions are largely influenced by both biotic and abiotic factors in the surrounding environment.Some zooplankton species can be used as bio-indicators for particular currents and water masses to help discover the dynamics of water current in the ocean.In this study,we investigated the distribution of zooplankton in North Yellow Sea(NYS)during winter when the Yellow Sea Warm Current(YSWC)influenced that area.Zooplankton communities in the NYS were dominated by temperate and warm-temperate species,such as Calanus sinicus,Paracalanus parvus,Acartia bifilosa,and Sagitta crassa.Two warm-water species,Eucheata plana and Sagitta enflata were also present.Cluster analysis grouped NYS zooplankton into three communities,the Shandong Coastal Community(SCC)in the Shandong neritic area,the Yellow Sea Central Community(YSCC)in the central waters of the NYS,and the Liaoning Coastal Community(LCC)in the Liaoning neritic area.Abundances varied significantly among these communities,with an average of 102.2 ind./m^3 in SCC,179.8 ind./m^3 in YSCC,and 1244.2 ind./m^3 in LCC.Depth and the bottom(3 m from the sea floor)temperature were likely the primary abiotic factors influencing zooplankton distributions.The appearance of E.plana,an indicator species for the YSWC path,indicated a significant northwestward trend for the YSWC in 2007.

A review on the South China Sea western boundary current

The advances in understanding the South China Sea （SCS） western boundary current （SCSwbc） have been reviewed since the works of Dale （1956） and Wyrtki （1961） in the middle of the 20th century. The features of the pattern of SCSwbc and the oceanic phenomena associated with it are focused on. The current is driven mainly by monsoon over the SCS and partially by winds over the tropical Pacific governed by the island rule. The SCSwbc exhibits strong seasonal variation in its direction and patterns. In winter, the current is strong and flows southwestward along the South China shelf and slope from the east of Dongsha Islands to the northern central Vietnamese coast, then turns to the south along the central and southern Vietnamese coast, and finally partially exits the SCS through the Karimata Strait. In summer and early fall, the SCSwbc can be divided into three segments based on their characteristics. The southern segment is stable, flowing northward from the Karimata Strait up to about 11 N, where it separates from the coast forming an eastward offshore current. The separation of the current from Vietnamese coast induces some striking features, such as upwelling and cold sea-surface temperature. The middle segment off the central Vietnamese coast may have a bimodal behavior： northward coastal current and meandering current in early summer （June–July）, and cyclonic gyre in later summer and early fall （August–September）. The northern segment is featured by the summer SCS Warm Current on the South China shelf and a southwestward subsurface current along the continental slope.

Vertical Structure of the Tidal Currents on the Continental Shelf of the East China Sea

The available data on tidal currents spanning periods greater than six months for the continental shelf of the East China Sea (26°30.052′N, 122°35.998′E) were analyzed using several methods. Tidal Current Harmonic Analysis results demonstrated that semi-diurnal tides dominated the current movement. The tidal currents of the principal diurnal and semidiurnal rotated clockwise with depth, with the deflection of the major semi-axes to the right in the upper layer and to the left in the lower layer. The vertical structures of two principal semi-diurnal constituents-M2 and S2-were similar, which indicates that the tidal currents are mainly barotropic in this area. The main features of the variation of the four principal tidal constituents with depth demonstrate that the currents in this region are influenced by the upper and lower boundary layers. Therefore, the tidal constituents of the shallow water are similar. Different vertical modes were calculated based on the Empirical Orthogonal Function (EOF) analysis of the Eastern and Northern components of the tidal currents, with a variance contribution for the zero-order model of at least 90%. The variance contribution of the baroclinic model is minimal, which further reveals a strong barotropic character for the tidal currents of this region.