Observational Study on the Onset of the South China Sea Southwest Monsoon

Based on the long-term marine ship observation data, records of meteorological stations and High-Reflective Cloud(HRC) data by satellite remote sensing , this paper has studied the circulation patterns and variability in elements during onset and the established periods of the South China Sea(SCS) southwest(SW) monsoon. The averaged date of the onset SW monsoon in the SCS occurs in the middle of May climatologically. The corresponding date for the northern part is little earlier (May 12) and those for the southern parts are little later (May 20). The interannual range of the onset dates is about one month. Following the onset of the SW monsoon, the cloud amount and the precipitation increase while the convection activities enhance over the SCS. But there is a strong spatial heterogeneity within the domain. After onset of the SW monsoon the strong convective area moves northwards, while the SCS rain band moves to the center and north. Sea surface temperature(SST) increases rapidly before the onset and the leading time is about one month. The increment of SST supplies heat and vapor for the onset. From April to May the surface heat fluxes display obvious changes, e.g., latent heat exchange and evaporation enhancement. It is one of the reasons why the SW monsoon bursts firstly in the SCS.

ON THE ORIGIN AGE OF THE SOUTHWEST BASIN IN THE SOUTH CHINA SEA

This paper propounds that the origin and evolution of the Southwet Basin (SWB) in the South China Sea (SCSh) are closely related with those of the SCS, reviews various viewpoints on its origin age with a large gap between the oldest age and the youngest age. offers some suggestions on the SWB’s origin age and gives some proposals to ascertain satisfactorily the origin and evolution, and multi-phasal and multiaxial spreading of the SCS and SWB.

Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin,South China Sea

To reveal the tectonic characteristics of the continental margins in the southwest subbasin(SWB)of the South China Sea,a long high-resolution seismic profile was studied using empty basin subsidence.We find that tectonic subsidence features on both margins are uniformly divided into three stages:(1)slow subsidence from Tg to 18.5 Ma(synrift stage);(2)extremely slow subsidence/uplift from18.5 to 16 Ma(spreading stage);and(3)accelerated subsidence from 16 to 0 Ma(post-spreading stage).This feature differs from the classic tectonic subsidence pattern of rifted basins,which exhibits fast subsidence during synrift stage and slow subsidence during the post-rift stage.The tectonic uplift occurred during the spreading stage and the magnitude increased from the continent to the ocean,which is likely related to mantle flow during seafloor spreading.We propose that lower crustal flow played a significant role in the tectonic evolution of the continental margins of the SWB.The lower crust of the SWB margins was warmer and therefore weaker,and more prone to flow beneath the faulting center,which compensated for the upper crustal thinning caused by brittle faulting during the synrift period and thus reduced the tectonic subsidence rate.During the spreading stage,faulting attenuated rapidly,and a necking zone appeared at the continentocean transition formed by lithospheric extension.With upwelling asthenosphere,small-scale secondary mantle convection occurred under the necking zone,which raised the continental margin isotherms and increased the buoyancy.Simultaneously,secondary mantle convection lifted the overriding crust,thus the overall subsidence rate decreased sharply or even reversed to uplift.After seafloor spreading,the effect of mantle convection faded away,and sediment loading drove the lower crust to flow landward.Thermal relaxation,lower crust flow,and vanish of secondary mantle convection together led to rapid subsidence in this stage.

Exploring thermocline and water masses variability in southern South China Sea from the World Ocean Database(WOD)

Study about water characteristics(temperature and salinity) from the World Ocean Database(WOD) was conducted in the area of southern South China Sea(SSCS), covering the area of 0°–10°N, 100°–117°E. From interannual analysis, upper layer(10 m) and deep water temperature(50 m) increased from 1951 until 2014. Monthly averaged show that May recorded the highest upper layer temperature while January recorded the lowest. It was different for the deep water which recorded the highest value in September and lowest in February. Contour plot for upper layer temperature in the study area shows presence of thermal front of cold water at southern part of Vietnam tip especially during peak northeast season(December–January). The appearances of warm water were obviously seen during generating southwest monsoon(May–June). Thermocline study revealed the deepest isothermal layer depth(ILD) during peak northeast and southwest monsoon. Temperature threshold at shallow area reach more than 0.8°C during the transitional period. Water mass study described T-S profile based on particular region. Water mass during the southwest monsoon is typically well mixed compared to other seasons while strong separation according to location is very clear. During transitional period between northeast monsoon to southwest monsoon, the increasing of water temperature can be seen at Continental Shelf Water(CSW) which tend to be higher than 29°C and vice versa condition during transitional period between southwest monsoon to northeast monsoon. Dispersion of T-S profile can be seen during southwest monsoon inside Tropical Surface Water(TSW) where the salinity and temperature become higher than during northeast monsoon.

Evidence of Upwelling along Peninsular Malaysia during Southwest Monsoon

Upwelling off the east coast of Peninsular Malaysia (PM) was detected from recent cruise data collected during the southwest monsoon. Thermocline lifting was observed at 104?E from a number of parallel transects. To confirm the presence of upwelling, satellite remote sensing data were used, and numerical model experiments were conducted. A cooler sea-surface temperature along the coast was spotted from both in-situ and satellite data while upward movement from the model agreed with field data. The southwesterly wind that blows along PM from June to September is believed to be the important mechanism that contributed to this upwelling through an Ekman dynamics process.

Tectonic subsidence of the Zhu I Sub-basin in the Pearl River Mouth Basin, northern South China Sea

The Pearl River Mouth Basin, which is situated on the northern margin of the South China Sea, has attracted great attention not only because of its tectonic setting but also because of its abundant hydrocarbon resources. We have analyzed the Cenozoic tectonic subsidence history of 4 drilled wells and 43 artificial wells from the Zhu 1 Sub-basin of the Pearl River Mouth Basin by back-stripping, using newly interpreted seismic profiles. We also calculated the average tectonic sub- sidence rates of the four sags in the Zhu 1 Sub-basin. The rifting and post-rifting stages are separated by abrupt changes in the tectonic subsidence curves and average subsidence rates. In the eastem sags of the Zhu 1 Sub- basin, tectonic subsidence started to slow at ca. 30 Ma, compared with ca. 23.8 Ma in the western sags. This probably corresponds to the timing of break-up and suggests that tiffing in the Pearl River Mouth Basin ended earlier in the eastern sags than in the western sags. Anomalously accelerated tectonic subsidence occurred at 17.5-16.4 Ma during the post-tiffing stage, with average subsidence rates as high as 301.9 m/Myr. This distin- guishes the Pearl River Mouth Basin from classical Atlantic passive continental marginal basins, which demonstrate exponentially decaying post-rift tectonic subsidence.

The Late Mesozoic Granodiorites from the Southwest Basin in the South China Sea and Its Tectonic Implication

The southwest basin is a key to study the origin and development of the South China Sea（SCS）.We do not know much about its boundaries,geological history,and the formation of its sea floor because it has a complex and highly re-gional structural background,notable sediment activity,and yet few floor rocks.Here a grano-diorite sample was collected from the southern margin of the southwest basin of the South China Sea.The results indicated that the 40Ar-39Ar ages of biotites in the sample are 110.3±0.5 Ma,suggesting that they were prod-ucts of magmatic intrusion during the Early Cretaceous period.The sample＇s geochemistry showed it had high SiO2,K2O,and Al2O3 but low TiO2 levels.Tectonic discriminant diagrams suggested that the sample might represent extrusion-related magmatism,either in an arc or forearc setting in the SCS area and that the sample mainly belonged to the syncollision type,whose formation was related to orogenies.The sample may be part of the main rock that made up the boundaries of rift system.The process of tension cracking was similar to the development of the Red Sea,in which the rifting and sagging occurred in the continental crust.The southwest basin may not be an original ocean,but a rift developed through finite extension on continental crust basement.The oceanic crust came into being when the width and depth of the rift valley reached a certain scale.The granodiorite sample we collected provides a means of determining the boundary of the southwest basin and the clues that may help researchers expand relevant models.It constitutes an important datum regarding the analysis of the formation and development of the SCS.

The Record-Breaking Extreme Drought in Yunnan Province, Southwest China during Spring-Early Summer of 2019 and Possible Causes

In spring and early summer of 2019, Yunnan Province experienced the most severe seasonal drought on record,with days of extreme drought area exceeding 10^5 km^2 far more than normal. Consistently, the precipitation in each month from February to June is over 30% less than normal, and about 50% less in the most severe drought period(April–June). The rainy season in Southwest China(SWC) started on the third pentad in June 2019, which is the second latest in history. The rainy season in Yunnan started on 24 June, which is the latest(29 days later than normal). On the contrary, the onset of the South China Sea summer monsoon(SCSSM) is abnormally early. The lag time between the start of the rainy season in SWC and the onset of the SCSSM in 2019 is 7 pentads, which is the largest since 1961, much longer than the climate mean(less than 1 pentad). The present study analyzes the possible reasons why the rainy season came extremely late and the drought disaster persisted and intensified after a much early SCSSM, at both seasonal and subseasonal timescales. The abnormally late onset of the rainy season and the second greatest potential evapotranspiration(PET) since 1981 are the direct reasons for the persistent drought. Statistical results show that the water vapor from southwest of Yunnan in April–June contributes more than that from the east at the seasonal scale. In April–June 2019, however, the southern branch trough(SBT) was abnormally weak, the large and strong anticyclonic wind anomaly prevailed over the Bay of the Bengal(BOB), and the meridional water vapor transport to Yunnan was weak. At the subseasonal scale, the weaker SBT lasted the longest, and the strong convection over the BOB came up late despite of an early onset of the SCSSM, which resulted in reduced low-level moisture convergence in Yunnan and development of drought prior to the SCSSM onset. From the onset of SCSSM to the start of rainy season in SWC, the SBT and meridional water vapor transport from the BOB were still weak, and the water vapor was mainly transported into the coastal area of South and Southeast China rather than Yunnan. After the start of the rainy season in SWC, the SBT was still weak. This led to less moisture transport in the westerlies to the west of Yunnan and the persistent extreme drought. Both the statistical results and case analysis indicate that the stronger Australian high in spring and early summer of 2019 was associated with the abnormally strong anticyclone over the BOB and the always weak SBT. In sum, the anomalous weakness of SBT played a critical role in the extreme drought occurrence and persistence in Yunnan of Southwest China in 2019.

台西南盆地构造特征与演化

通过对南海973航次在该区域的多道地震剖面的解释,并结合重力和磁力资料,发现台西南盆地的主要构造走向为NE-NEE向,沉积上具有独特的双层构造,是一个与南海演化密切相关的新生代伸展型断陷盆地.这个盆地早第三纪开始发育,形成了大量的正断层和由它们控制的一系列半地堑,发育了古新世-渐新世的沉积.渐新世末期盆地受到挤压作用影响,发育了一系列的逆冲断层和反转构造,表明了台西南盆地"先张后挤"的演化特征,从中新世开始进入热力学沉降坳陷发育阶段.

南海台西南盆地自生管状黄铁矿矿物学特征及其对天然气水合物的示踪意义

在南海台西南区GC10岩芯中发现了大量自生管状黄铁矿,它是海洋沉积物缺氧硫酸盐还原过程的主要产物。扫描电镜和高分辨率透射电镜测试结果显示,这些管状黄铁矿主要由草莓状黄铁矿组成,且在管内各圈层分布不均:内层主要是单个的大颗粒草莓状黄铁矿,直径为40μm左右;中间致密层为具外壳结构的草莓状黄铁矿,经后期压实作用后呈五角十二面体紧密堆积;外层是疏松的混有部分杂质的黄铁矿。高分辨率透射电镜观测显示,草莓状黄铁矿中存在面缺陷,在平行于黄铁矿的(001)面存在白铁矿层。草莓状黄铁矿中的黄铁矿与白铁矿是同质异构体,而且,白铁矿的(101)平行于黄铁矿的(001),白铁矿的[010]方向平行于黄铁矿的[100],这是黄铁矿与白铁矿之间最易相互生长的面。草莓状黄铁矿微晶的形态各异,包括有序规则排列的球粒、部分有序排列的五角十二面体、无序排列的球粒、截角立方体、不规则体及亚微晶颗粒等,说明草莓状黄铁矿是在微生物参与下,在由强还原到局部弱氧化的环境中形成的。这种微环境的变化正好是下部上溢的甲烷在管状通道内与硫酸盐发生反应的结果。

南海西南海盆构造演化的热模拟研究

南海西南海盆的张裂和海底扩张是白垩纪末至中始新世南海形成过程中最重要的构造事件.本文采用三维有限单元法对该区的热演化过程进行了模拟计算.通过对变形、温度结构的计算,研究了西南海盆张裂变形、海底扩张持续时间、地幔物质上升、地壳岩墙沿扩张中心的挤入扩张活力、岩浆活动等.计算结果表明:由于其深部动力学条件不足,海盆一次扩张持续时间在10～15Ma之间,其后地幔物质的上升活动逐渐停止,地壳失去扩张动力,使得扩张中心成为残留扩张中心的死亡裂谷,而未构成中脊或中隆带.虽然该处地幔物质上升的潜力不足,但伴随局部的断裂,尤其是盆、缘边界的拆离拉张,仍能产生相当强烈的岩浆喷溢活动,导致此区海盆成型之后的海山崛起.

南海海气界面潜热通量的分布特征及其对西南季风爆发影响的初步分析

利用1982年1月至2001年12月逐日的Re_NCEP南海海表面潜热通量资料，分析了南海夏季西南季风爆发早年和晚年潜热通量在南海的时空分布特征；并通过相关对比诊断分析了潜热通量对西南季风爆发及强度的影响，初步给出了其动力学机理。结果表明，季风爆发早、晚年的前一年冬季至初春（12～3月），南海南部（5°～13°N，100°～120°E）和北部（13°～22°N，105°～120°E）的潜热通量距平符号相反，呈现反位相，季风爆发早（晚）年，前一年冬季至次年初春，南海北部的潜热通量为正（负）距平，南海南部则为负（正）距平；在季风爆发的早年和晚年，南海潜热通量表现出明显的差异，春、夏、秋季南海潜热通量正距平持续时间短（长），季风强度偏弱（强）。南海北部的潜热通量和南海北部季风强度隔季正相关。当潜热通量为正（负）距平时，同期和滞后1～3个月的海温均为负（正）异常，加大（减小）了春季南海和周围陆地陆暖海冷的海陆温差，有利于西南季风在南海北部的早（晚）爆发，西南风异常偏强（弱）。

利用地震海洋学方法估算南海中尺度涡的地转流速

中尺度涡是重要的海洋学现象,它在很大程度上影响着海洋内部的能量传递过程.由于传统海洋学观测手段的固有局限性,一直以来对中尺度涡观测和研究的程度都比较低.地震海洋学的诞生和发展为海洋学观测提供了一个全新的手段.对南海的历史地震数据重新处理后,我们首次在本研究海域的地震剖面上看到了透镜状结构.它位于南海西南次海盆(～113.6°E,11.4°N),中心深度约为450m,中心厚度约为300m,半径约为55～65km,具有典型的中尺度涡特征,综合解释为反气旋.我们利用地震海洋学方法估算了地转剪切,结合来自于卫星高度数据的海表面地转流速度进一步得到了绝对流速的垂向剖面.结果显示,流速的最大值约为0.7m/s,出现在400～450m处,对应于涡旋的中心深度;西北部分为正,东南部分为负,整体呈现出顺时针的转动方向,说明了它是一个反气旋结构.

2002年南海季风建立及其雨带变化的天气学研究

利用南海海 气通量观测试验资料结合NCEP ,GPCP以及GMS - 5云图资料 ,综合分析了 2 0 0 2年 5～ 6月南海西南季风建立过程及其雨带变化 ,确定 5月 14日西沙及北部海区西南季风爆发 ,5月 15日整个南海季风爆发 ,季风爆发时间属于正常年 ;季风爆发时风向、风速、云量、降水、湿度、辐射及海面温度等要素都发生突变。这种突变是由大气环流的突变造成的。季风爆发前后大气环流变化过程是 :80～ 90°E越赤道气流加强 ,同时印缅低压加深 ,孟加拉湾南北向气压梯度增大 ,而后东亚大陆上气旋发展东移 ,副热带高压东撤 ,孟加拉湾低压槽前的赤道西风突然加强越过中南半岛 ,南海北部首先出现强西南风 ,继而南海季风迅速地全面爆发。孟加拉湾西南风加强到南海季风爆发是一个连续的过程 ,大陆冷空气南下起了重要的作用。南海季风爆发时呈现单雨带型 ,而后由单雨带型转变为双雨带型 ,雨带受副热带高压和季风系统共同影响 ,并且随着副热带高压移动位置变化。

南海西南次海盆广角地震探测

2010年12月—2011年3月在南海西南次海盆开展了海底地震仪(Ocean Bottom Seismometer,OBS)探测,采用Sedis IV型、I-4C型和MicrOBS3种不同型号的OBS,以4×24.5L的大容量气枪为震源,获得了覆盖西南次海盆残留扩张脊的3D人工地震数据。从处理的地震数据可知,此次试验是一次比较成功的地震实验,OBS地震记录清晰、震相丰富,所使用的气枪有足够的能量输出,显示了其良好的工作能力。选取剖面1中的7台OBS进行了2D剖面处理。初步建模结果表明,南海西南次海盆地壳结构为普通洋壳,海山顶部沉积层很薄,莫霍面埋深较浅。

南海北部神狐西南海域天然气水合物成矿远景

通过对南海北部神狐西南海域的构造、沉积及气源环境研究,探讨了神狐西南海域天然气水合物的成矿条件,认为该区具有天然气水合物良好的成矿环境;通过穿越神狐西南海域陆坡深水区的典型高分辨率地震剖面资料的解释,初步判识出天然气水合物的可能分布区,预测了天然气水合物的成藏远景。

南海西南部新生代盆地类型及演化历史

南海西南部地区发育了多种类型的新生代沉积盆地。各种类型的盆地具有各自的结构特征和演化历史。走滑一周缘前陆盆地──曾母盆地发育了渐新一中中新统层系、晚中新统层系和上新一第四系层系等三套地层。曾母盆地经历了三个发育阶段。拉张剪切盆地──万安盆地发育了下第三系一下渐新统层系、上渐新航一中中新统层系和上中新统一第四系层系三套地层。万安盆地经历了四个发育阶段。

利用重力梯度反演南海西南海盆深部构造

重力梯度异常反映的是重力异常的变化，其分辨率比重力异常高。重力梯度空间参量图能给出构造倾角和倾面的信息，结合重力梯度剖面和梯度空间参量图可以构建出地下构造的几何模型，进而对一些复杂构造进行解释。本文利用重力梯度异常对南海西南海盆进行了解释，得到大致以西南海盆北东向扩张轴为对称轴的穹隆状构造面。该构造面在西南海盆下6～15km处形成一个密度界面，此界面可能是西南海盆北西-南东向海底扩张期间地幔上隆所引起的。

南海西南次海盆北缘海底地震仪测线深部地壳结构研究

基于2009年和2011年采集的海底地震仪数据,辅以多道地震数据,对南海西南次海盆北缘的地壳结构进行了探索.利用二维射线追踪的反演方法建立测线上的模型.利用声学基底面的反射波,下地壳顶界面的反射波和莫霍面的反射波走时的反演勾绘了地层中的不连续界面,利用自声学基底面下的折射波和来自上地幔的首波来反演整条测线的P波速度结构.结果发现在下地壳顶界面和莫霍面都出现了较大的起伏,据此推断西南次海盆的张裂模式,并得到西南次海盆与非火山型的大陆边缘伊比利亚-纽芬兰的张裂具有一定程度上的相似性的结论.说明了大陆张裂的两种端元力学模型不能很好的解释西南次海盆的拉张模式,而应用弹性梁的力学模型可以解释这种拉张模式.虽然在西南次海盆北部陆坡区的下地壳中没有发现大规模的岩浆活动,但是得出在莫霍界面上方可能有小规模熔融物质存在的结论.

影响南海夏季风爆发年际变化的关键海区及机制初探

利用1958—2011年NCEP/NCAR再分析资料和ERSST资料,采用Lanczos时间滤波器、相关分析、回归分析、合成分析和交叉检验等方法,研究了影响南海夏季风爆发年际变化的关键海区海温异常的来源与可能机制。结果表明,前冬(12—2月)热带西南印度洋和热带西北太平洋是影响南海夏季风爆发年际变化的关键海区。冬季热带西南印度洋(热带西北太平洋)的异常增暖是由前一年夏季El Ni?o早爆发(强印度季风异常驱动的行星尺度东-西向环流)触发、热带印度洋(西北太平洋)局地海气正反馈过程引起并维持到春季。冬季热带西北太平洋反气旋性环流(气旋性环流)及印度洋(热带西北太平洋)的暖海区局地海气相互作用使得印度洋(热带西北太平洋)海温异常维持到春末。春季,逐渐加强北移到10°N附近的低层大气对北印度洋(热带西北太平洋)暖海温异常响应的东风急流(异常西风)及南海-热带西北太平洋维持的反气旋性环流(气旋性环流)异常,使得南海夏季风晚(早)爆发。