Characteristics of the transition Layer in the South China Sea in 1998

CTD data on standard levels coolected during July and December in 1998 and the cubic spline interpolating method were used to study the characteristics of the transition layer temperature and salinity.The thermocline undergoes remarkable seasonal variation in the South China Sea (SCS),and especially in the region of the north shelf where the thermocline disappears in december.The thermocline is stronger and thicker in July than in December,There is no obvious seasonal variation in the halocline.Due to the upper Ekman transport caused by monsoon over the SCS,the thermocline slopes upward in July and downward in december from east to west in the northern SCS.The characteristics of the thermocline and halocline are influenced by local eddies in the SCS.The Zhujiang diluted flow influences significantly the SCS shelf‘s halocline.     

MODEL OF UPPER OCEANIC CIRCULATIONS IN THE SOUTH CHINA SEA DURING NORTHEAST MONSOON

MODUrnONTheS0uthChinaSea(SCS)isabophalrnarginalbasinwhereEastAsiamonsoonsprevail.0bviousadjustInentSoftheupperocanoccurduetOthealtematingsurnxneandwintermonsoons.ThemostboohantaspchoflargeanlecurmtSintheSesaretheupperoonnicresponsetothemonsoons(Dale,l956).MostpreviousmrehesfocusedondiagnostiesandmodelingofsuffocecurmtS.Wwti(l96l)plotalsurfacentsbasedonshipdriflsintheNAGAReportNo.2anddescritaltheperiodicallysdri-annualreversingofwindsandrtinthisarea.Xuetal.(l982)calculatalthedy-naAn…     

The modeled atmospheric and oceanic response to the South China Sea SST anomaly

The sensitivity of the global atmospheric and oceanic response to sea surface temperature anomaly (SSTA) throughout the South China Sea (SCS) is investigated using the Fast Ocean-Atmosphere Model (FOAM). Forced by a warming SST, the experiment explicitly demonstrates that the responses of surface air temperature (SAT) and SST exhibit positive anomalous center over SCS and negative anomalous center over the Northern Pacific Ocean (NPO). The atmospheric response to the warm SST anomalies is characterized by a barotropical anomaly in middle-latitude, leading to a weak subtropical high in summer and a weak Aleutian low in winter. Accordingly, Indian monsoon and eastern Asian monsoon strengthen in summer but weaken in winter as a result of wind convergence owing to the warm SST. It is worth noting that the abnormal signals propagate poleward and eastward away in the form of Rossby Waves from the forcing region, which induces high pressure anomaly. Owing to action of the wind-driven circulation, an anomalous anti-cyclonic circulation is induced with a primary southward current in the upper ocean. An obvious cooling appears over the North Pacific, which can be explained by anomalous meridional cold advection and mixing as shown in the analysises of heat budget and other factors that affect SST.     

Vorticity budget study on the seasonal upper circulation in the northern South China Sea from altimetry data and a numerical model

Based on the EOF analyses of Absolute Dynamic Topography satellite data,it is found that,in summer,the northern South China Sea(SCS) is dominated by an anticyclonic gyre whilst by a cyclonic one in winter.A connected single-layer and two-layer model is employed here to investigate the dynamic mechanism of the circulation in the northern SCS.Numerical experiments show that the nonlinear term,the pressure torque and the planetary vorticity advection play important roles in the circulation of the northern SCS,whilst the contribution by seasonal wind stress curl is local and limited.Only a small part of the Kuroshio water intrudes into the SCS,it then induces a positive vorticity band extending southwestward from the west of the Luzon Strait(LS) and a negative vorticity band along the 200 m isobath of the northern basin.The positive vorticity field induced by the local summer wind stress curl is weaker than that induced in winter in the northern SCS.Besides the Kuroshio intrusion and monsoon,the water transports via the Sunda Shelf and the Sibutu Passage are also important to the circulation in the northern SCS,and the induced vorticity field in summer is almost contrary to that in winter.The strength variations of these three key factors(Kuroshio,monsoon and the water transports via the Sunda Shelf and the Sibutu Passage) determine the seasonal variations of the vorticity and eddy fields in the northern SCS.As for the water exchange via the LS,the Kuroshio intrusion brings about a net inflow into the SCS,and the monsoon has a less effect,whilst the water transports via the Sunda Shelf and the Sibutu Passage are the most important influencing factors,thus,the water exchange of the SCS with the Pacific via the LS changes dramatically from an outflow of the SCS in summer to an inflow into the SCS in winter.     

THREE-DIMENSIONAL BAROCLINIC NUMERICAL SIMULATION OF THE SOUTH CHINA SEA CIRCULATION＇S SEASONAL CHARACTERISTICS II. MIDDLE AND DEEP CIRCULATION

A three-dimensional baroclinic shelf sea model‘ s numerical simulation of the South China Sea (SCS) middle and deep layer circulation structure showed that: 1. In the SCS middle and deep layer, a seulhward boundary current exists along the east shore of the Indo-China Peninsula all year long.A cyclonic eddy (gyre) is formed by the current in the above sea areas except in the middle layer in spring, when an anticyclonic eddy exists on the eastern side of the current. In the deep layer, a larges-cale anticyclonic eddy often exists in the sea areas between the Zhongsha Islands and west shore of southern Luzon Island. 2. In the middle layer in snmmer and autumn, and in the deep layer in autumn and winter, there is an anticyclonic eddy (gyre) in the northeastern SCS, while in the middle layer in winter and spring, and in the deep layer in spring and snmmer, there is a cyclonic one. 3. In the middle layer,there is a weak northeastward current in the Nansha Trough in spring and snmmer, while in autumn and winter it evolves inl~ an anticyclonic eddy ( gyre), which then spreads westward l~ the whole western Nansha Islands sea areas.     

NUMERICAL STUDY ON THE INTERANNUAL OSCILLATION OF SEA SURFACE TEMPERATURE IN THE SOUTH CHINA SEA

A two and a half layer oceanic model of wind-driven, thermodynamical general circulation is appliedto study the interannual oscillation of sea surface temperature (SST) in the South China Sea (SCS). Themodel consists of two active layers: the upper mixed layer (UML) and the seasonal thermocline, with themotionless abyss beneath them. The governing equations which include momentum, continuity and sea.temperature for each active layer, can describe the physics of Boussinseq approximation, reduced gravityand equatorial β-plane. The formulas for the heat flux at the surface and at the interface between twoactive layers are designed on the Haney scheme. The entrainment and detrainment at the bottom of theUML induces vertical transport of mass,momentum and heat, and couples of dynamic andthermodynamic effect.Using leap-frog integrating scheme and the Arakawa-C grid the model is forced bya time-dependent wind anomaly stress pattern obtained from category analysis of COADS. The numerical results indicate that t     

Numerical simulation of wave field in the South China Sea using WAVEWATCH III

Wave fi elds of the South China Sea(SCS) from 1976 to 2005 were simulated using WAVEWATCH III by inputting high-resolution reanalysis wind fi eld datasets assimilated from several meteorological data sources. Comparisons of wave heights between WAVEWATCH III and TOPEX/Poseidon altimeter and buoy data show a good agreement. Our results show seasonal variation of wave direction as follows: 1. During the summer monsoon(April–September), waves from south occur from April through September in the southern SCS region, which prevail taking about 40% of the time; 2. During the winter monsoon(December–March), waves from northeast prevail throughout the SCS for 56% of the period; 3. The dominant wave direction in SCS is NE. The seasonal variation of wave height H s in SCS shows that in spring, H s ≥1 m in the central SCS region and is less than 1 m in other areas. In summer, H s is higher than in spring. During September–November, infl uenced by tropical cyclones, H s is mostly higher than 1 m. East of Hainan Island, H s 2 m. In winter, H s reaches its maximum value infl uenced by the north-east monsoon, and heights over 2 m are found over a large part of SCS. Finally, we calculated the extreme wave parameters in SCS and found that the extreme wind speed and wave height for the 100-year return period for SCS peaked at 45 m/s and 19 m, respectively, SE of Hainan Island and decreased from north to south.     

3-D BAROCLINIC NUMERICAL SIMULATION OF THE SOUTH CHINA SEA I. UPPER CIRCULATION

A three-dimensional baroclinic shelf sea model was employed to simulate the seasonal characteristics of the South China Sea (SCS) upper circulation. The results showed that: in summer, an anticyclonic eddy, after its formation between the Bashi Channel and Dongsha Islands in the northeastern SCS, moves southwestward until it disperses slowly. There exists a northward western boundary current along the east shore of the Indo-China Peninsula in the western SCS and an anticyclonic gyre in the southern SCS. But at the end of summer and beginning of autumn, a weak local cyclonic eddy forms in the Nansha Trough, then grows slowly and moves westward till it becomes a cyclonic gyre in the southern SCS in autumn. At the beginning of winter, there exists a cyclonic gyre in the northern and southern SCS, and there is a southward western boundary current along the east shore of the Indo-China Peninsula. But at the end of winter, an anticyclonic eddy grows and moves toward the western boundary after forming in the Nansha Trough. The eddy‘s movement induces a new opposite sign eddy on its eastern side, while the strength of the southward western boundary current gets weakened. This phenomenon continues till spring and causes eddies in the southern SCS.     

Circulation patterns of summer monsoon corresponding to two kinds of indices over the South China Sea

The characteristics of circulation corresponding to two kinds of indices of summer monsoon onset over the South China Sea (SCS) have been discussed using the reanalysis data of the National Centers for Environmental Prediction-National Center for Atmospheric Research. It is found that there are two patterns of deep convection that occur at different locations and influence the summer monsoon onset over the SCS. One is over the Asia continent and the western Pacific corresponding to the southwesterly of summer monsoon prevailing over the northern and central part of the SCS, while the other is near the Philippines that affects the westerly summer monsoon as prevailing over the central and southern southern part of the SCS. Since these two kinds of convection affecting the summer monsoon onset do not always occur together, thus the summer monsoon onset time is different when determined by various indices.     

Reversal process of the South China Sea western boundary current in autumn 2011

Using merged sea level anomaly and absolute geostrophic velocity products from satellite altimetry and Argos drifter data,we analyzed the reversal process of the South China Sea(SCS) western boundary current(SCSwbc) from a summer to winter pattern in 2011 and important oceanic phenomena during this process.Results show that the outbreak time of the northeast monsoon over the southern SCS lagged that over the northern SCS by about 1 month.During the SCS monsoon reversal period,the SCSwbc reversed rapidly into the winter pattern at the Guangdong continental slope in late September.Subsequently,the southward Vietnam coastal boundary current strengthened.However,the northward Natuna Current maintained a summer state until mid-October.Thus,the balance between the southward and northward currents was lost when they met,their junction moved gradually southward.However,a loop current formed southeast of Vietnam because the main stream of the Vietnam Offshore Current(VOC) remained near its original latitude.Meanwhile,the VOC and associated dipole circulation system strengthened.After midOctober,the northward Natuna Current began to weaken,the loop current finally shed,becoming a cool ring.The VOC and its associated dipole sub-basin circulation system also weakened gradually until it disappeared.     

EVIDENCE FOR ABRUPT CLIMATIC CHANGES ONNORTHWESTERN MARGIN OF EAST ASIAN MONSOON REGION DURING LAST DEGLACIATION

Based on investigations of the Zhongwei Nanshan aeolian section situated in the southeastern margin of Tengger Dcsert, carbon-14 and TL (thermoluminescence) dating results and paleoclimatic proxies such as magnetic susceptibility and grain size, we inferred that the northwestern margin of East Asian monsoon region experienced abrupt climatic changes during the last deglaciation. Six oscillation events were identified: Oldest Dryas, Belling, Older Dryas, AllerФd, lntra-AllerФd Cold Period (1ACP) and Younger Dryas (YD). The summer monsoon was weaker during Oldest Dryas and Younger Dryas when the winter monsoon was stronger. However, during the B/A (BФlling/AllerФd) period, the summer monsoon strengthened, reflected by magnetic susceptibility, when the winter monsoon also became strong, which is different from the paleoclimatic pattern established in the East Asian monsoon region. Furthermore,the summer monsoon was nearly in phase with the climate changes inferred from the oxygen isotopic records of Greenland ice cores. It could be speculated that the variations of the sea ice cover in the high latitudes of the North Hemisphere affected the high pressure of Asian continent and the changes of the winter monsoon inland. On the other hand,the sea ice cover variations might have indirectly caused the occurrence of ENSO events that has tightly been related to the summer monsoon in northwest margin of East Asian monsoon region.     

Response of the equatorial western Pacific thermohaline structure to wind variation

During the Global Weather Experiment oceanographic measurements were recorded during winter and summer in the western Pacific region 5°S−5°N, 160°E−175°E. The variations of the upper ocean temperature and salinity fields were produced by the large seasonal and spatial wind fluctuations. The vertical temperature structure of the thermocline at the equator, the meridional slope of the thermocline south of the equator, and the northward penetration of high salinity water were related to the direction and intensity of the zonal wind-stress. (NOAA Pacific Marine Environmental Laboratory) Contribution No. 1307 from the Institute of Ocean., Academia Sinica. Received Sept. 3, 1985     

Comparison of remote sensing data with in-situ wind observation during the development of the South China Sea monsoon

Wind measurements derived from QuikSCAT data were compared with those measured by anemometer on Yongxing Island in the South China Sea (SCS) for the period from April 2008 to November 2009. The comparison confirms that QuikSCAT estimates of wind speed and direction are generally accurate, except for the extremes of high wind speeds (>13.8m/s) and very low wind speeds (<1.5m/s) where direction is poorly predicted. In-situ observations show that the summer monsoon in the northern SCS starts between May 6 and June 1. From March 13, 2010 to August 31, 2010, comparisons of sea surface temperature (SST) and rainfall from AMSR-E with data from a buoy located at Xisha Islands, as well as wind measurements derived from ASCAT and observations from an automatic weather station show that QuikSCAT, ASCAT and AMSR-E data are good enough for research. It is feasible to optimize the usage of remote-sensing data if validated with in-situ measurements. Remarkable changes were observed in wind, barometric pressure, humidity, outgoing longwave radiation (OLR), air temperature, rainfall and SST during the monsoon onset. The eastward shift of western Pacific subtropical high and the southward movement of continental cold front preceded the monsoon onset in SCS. The starting dates of SCS summer monsoon indicated that the southwest monsoon starts in the Indochinese Peninsula and forms an eastward zonal belt, and then the belt bifurcates in the SCS, with one part moving northeastward into the tropical western North Pacific, and another southward into western Kalimantan. This largely determined the pattern of the SCS summer monsoon. Wavelet analysis of zonal wind and OLR at Xisha showed that intra-seasonal variability played an important role in the summer. This work improves the accuracy of the amplitude of intra-seasonal and synoptic variation obtained from remote-sensed data.     

海洋涡旋移动特征的区域划分方法及应用分析

为了合理有效地分析和挖掘海洋涡旋移动数据中的规律和模式,本文以基于空间交互性流聚类的区域化方法为基础,提出了一种海洋涡旋移动特征的网格区域化方法。该方法以网格为统计单元,对涡旋移动数据进行组织,通过图论模型构建海洋涡旋的移动网络图,然后采用基于平均邻接的层次聚类和基于模块度的划分2个步骤,实现涡旋移动特征的区域划分。基于该算法,对1992-2011年中国南海海洋涡旋移动数据进行算法实验,结果表明,南海海洋涡旋按照其移动频繁性特征可分为越南东南部（R1）、越南东部-巴拉望岛（R2）、南海北部（R3）3个区域。其中,R1区域包含了南海西南部深海盆地区的涡旋活跃条带;R2区域体现了南海中部涡旋向西移动的活动规律;R3区域则包含了南海北部东北-西南走向条带。3个区域内冷涡和暖涡具有明显的季节性变化特征：R1和R3区域冷暖涡变化相似,暖涡在夏秋季移动最多,冬季最少,而冷涡则相反,夏秋季移动最少,随后逐渐增加,并在春季达到峰值;R2区域暖涡在春季移动最多,而冷涡在夏冬移动最多,春秋移动相对较弱。     

Diel vertical migration of mesozooplankton in the northern Yellow Sea

The Yellow Sea Cold Water Mass(YSCWM),one of the most vital hydrological features of the Yellow Sea,causes a seasonal thermocline from spring to autumn.The diel vertical migration(DVM) of zooplankton is crucial to structural pelagic communities and food webs,and its patterns can be affected by thermocline depth and strength.Hence,we investigated zooplankton community succession and seasonal changes in zooplankton DVM at a fixed station in the YSCWM.Annual zooplankton community succession was affected by the forming and fading of the YSCWM.A total of 37 mesozooplankton taxa were recorded.The highest and lowest species numbers in autumn and spring were detected.The highest and lowest total densities were observed in autumn(14 464.1 inds./m3) and winter(3 115.4 inds./m3),respectively.The DVM of the dominant species showed obvious seasonal variations.When the YSCWM was weak in spring and autumn,most species(e.g.Paracalanus parvus,Oithona similis,and Acartia bifilosa) stayed above the thermocline and vertically migrated into the upper layer.Calanus sinicus and Aidanosagitta crassa crossed the thermocline and vertically migrated.No species migrated through the stratification in summer,and all of the species were limited above(P.parvus and A.crassa) or below(C.sinicus and Centropages abdominalis)the thermocline.The YSCWM disappeared in winter,and zooplankton species were found throughout the water column.Thus,the existence of thermocline influenced the migration patterns of zooplankton.Cluster analyses showed that the existence of YSCWM resulted in significant differences between zooplankton communities above and below the thermocline.     

Surface inflow into the South China Sea through the Luzon Strait in winter

We studied the driving force of the Kuroshio intrusion into the South China Sea (SCS) during the winter monsoon, using satellite-tracked drifters entering the Luzon Strait (LS) through the Balintany and Babuyan Channels from the Philippine Sea. Most drifters passing through the Babuyan Channel in winter entered the interior SCS without a significant change in velocity. However, half of the drifters passing through the Balintany Channel entered the SCS at ~30 cm/s, which was faster than when they entered the LS. The other half continued moving northwestward into the Kuroshio and returned to the North Pacific. Quantitative analyses, using surface climatological wind and sea surface height anomaly (SSHa) data explained both the difference in velocity of drifters between the two channels and their acceleration through the Balintany Channel. The results suggest that the positive meridional gradient of sea surface height in the Luzon Strait, caused by the pileup of seawater driven by the Northeast monsoon, as well as Ekman flow, contribute to the Kuroshio intrusion into the SCS through the Babuyan and Balintany Channels. The former may be the main driving force.     

The regional air-sea coupled oscillation in the South China Sea

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅＳｏｕｔｈＣｈｉｎａＳｅａ (ＳＣＳ)ｉｓａｕｎｉｑｕｅｓｅｍｉ ｅｎｃｌｏｓｅｄｄｅｅｐｏｃｅａｎｂａｓｉｎｌｏｃａｔｅｄｎｅａｒｔｈｅｗｅｓｔ ｅｒｎｐｅｒｉｐｈｅｒｙｏｆｔｈｅＰａｃｉｆｉｃＯｃｅａｎ .Ｓｐｒｅａｄｉｎｇｆｒｏｍｔｈｅｅｑｕａｔｏｒｔｏ 2 0°Ｎａｎｄｓｐａｎｎｉｎｇｚｏｎａｌｌｙａｂｏｕｔ1 5°Ｎ ,ｔｈｅＳＣＳｌｉｅｓｂｅｔｗｅｅｎｔｈｅＳｏｕｔｈＣｈｉｎａｃｏａｓｔａｎｄｔｈｅｍａｒｉｔｉｍｅｃｏｎｔｉｎｅｎｔ.ＴｈｅＳＣＳｂｏｔｔｏｍｔｏｐｏｇｒ…     

Modeling seasonal variations of subsurface chlorophyll maximum in South China Sea

In the South China Sea(SCS), the subsurface chlorophyll maximum(SCM) is frequently observed while the mechanisms of SCM occurrence have not been well understood. In this study, a 1-D physical-biochemical coupled model was used to study the seasonal variations of vertical profiles of chlorophyll-a(Chl-a) in the SCS. Three parameters(i.e., SCM layer(SCML) depth, thickness, and intensity) were defined to characterize the vertical distribution of Chl-a in SCML and were obtained by fitting the vertical profile of Chl-a in the subsurface layer using a Gaussian function. The seasonal variations of SCMs are reproduced reasonably well compared to the observations. The annual averages of SCML depth, thickness, and intensity are 75 ± 10 m, 31 ± 6.7 m, and 0.37 ± 0.11 mg m-3, respectively. A thick, close to surface SCML together with a higher intensity occurs during the northeastern monsoon. Both the SCML thickness and intensity are sensitive to the changes of surface wind speed in winter and summer, but the surface wind speed exerts a minor influence on the SCML depth; for example, double strengthening of the southwestern monsoon in summer can lead to the thickening of SCML by 46%, the intensity decreasing by 30%, and the shoaling by 6%. This is because part of nutrients are pumped from the upper nutricline to the surface mixed layer by strong vertical mixing. Increasing initial nutrient concentrations by two times will increase the intensity of SCML by over 80% in winter and spring. The sensitivity analysis indicates that light attenuation is critical to the three parameters of SCM. Decreasing background light attenuation by 20% extends the euphotic zone, makes SCML deeper(~20%) and thicker(12% – 41%), and increases the intensity by over 16%. Overall, the depth of SCML is mainly controlled by light attenuation, and the SCML thickness and intensity are closely associated with wind and initial nitrate concentration in the SCS.     

Water masses in the South China Sea and water exchange between the Pacific and the South China Sea

Water masses in the South China Sea (SCS) were identified and analyzed with the data collected in the summer and winter of 1998. The distributions of temperature and salinity near the Bashi Channel (the Luzon Strait) were analyzed by using the data obtained in July and December of 1997. Based on the results from the data collected in the winter of 1998, waters in the open sea areas of the SCS were divided into six water masses: the Surface Water Mass of the SCS (S), the Subsurface Water Mass of the SCS (U), the Subsurface-Intermediate Water Mass of the SCS (UI), the Intermediate Water Mass of the SCS (I), the Deep Water Mass of the SCS (D) and the Bottom Water Mass of the SCS(B). For the summer of 1998, the Kuroshio Surface Water Mass (KS) and the Kuroshio Subsurface Water Mass (KU) were also identified in the SCS. But no Kuroshio water was found to pass the 119.5°E meridian and enter the SCS in the time of winter observations. The Sulu Sea Water (SSW) intruded into the SCS through the Mindoro Channel between 50–75 m in the summer of 1998. However, the data obtained in the summer and winter of 1997 indicated that water from the Pacific had entered the SCS through the northern part of the Luzon Strait in these seasons, but water from the SCS had entered the Pacific through the southern part of the Strait. These phenomena might correlate with the 1998 El-Niño event.     

Seasonal variation of the barrier layer in the PN section

In this paper, we use the conductivity-temperature-depth (CTD) observation data and a three-dimensional ocean model in a seasonally-varying forcing field to study the barrier layer (BL) in the PN section in the East China Sea (ECS). The BL can be found along the PN section with obviously seasonal variability. In winter, spring and autumn, the BL occurs around the slope where the cold shelf water meets with the warm Kuroshio water. In summer, the BL can also be found in the shelf area near salinity front of the Changjiang (Yangtze) River Dilution Water (YRDW). Seasonal variations of BL in the PN section are caused by local hydrological characteristics and seasonal variations of atmospheric forcing. Strong vertical convection caused by sea surface cooling thickens the BL in winter and spring in the slope area. Due to the large discharge of Changjiang River in summer, the BL occurs extensively in the shelf region where the fresh YRDW and the salty bottom water meet and form a strong halocline above the seasonal thermocline. The formation mechanism of BL in the PN section can be explained by the vertical shear of different water masses, which is called the advection mechanism. The interannual variation of BL in summer is greatly affected by the YRDW. In the larger YRDW year (such as 1998), a shallow but much thicker BL existed on the shelf area. Supported by National Basic Research Program of China (973 Program, No. 2005CB422303 and 2007CB411804), the Key Project of the International Science and Technology Cooperation Program of China (No. 2006DFB21250), the “111 Project” of the Ministry of Education (No. B07036), the Program for New Century Excellent Talents in University, China (No. NECT-07-0781)