Patterns of upper layer circulation variability in the South China Sea from satellite altimetry using the self-organizing map

Patterns of the South China Sea (SCS) circulation variability are extracted from merged satellite altimetry data from October 1992 through August 2004 by using the self-organizing map (SOM). The annual cycle, seasonal and inter-annual variations of the SCS surface circulation are identified through the evolution of the characteristic circulation patterns. The annual cycle of the SCS gener- al circulation patterns is described as a change between two opposite basin-scale SW-NE oriented gyres embedded with eddies: low sea surface height anomaly (SSHA) (cyclonic) in winter and high SSHA (anticyclonic) in summer half year. The transition starts from July--August (January--February) with a high (low) SSHA tongue east of Vietnam around 12°～14° N, which de- velopa into a big anticyclonic (cyclonic) gyre while moving eastward to the deep basin. During the transitions, a dipole structure, cyclonic (anticyclonic) in the north and anticyclonic (cyclonic) in the south, may be formed southeast off Vietnam with a strong zonal jet around 10°～12° N. The seasonal variation is modulated by the interannual variations. Besides the strong 1997/1998 e- vent in response to the peak Pacific El Nino in 1997, the overall SCS sea level is found to have a significant rise during 1999～ 2001, however, in summer 2004 the overall SCS sea level is lower and the basin-wide anticyclonic gyre becomes weaker than the other years.     

南海大尺度动力场年循环和年际变化

应用COADS风应力、Levitus温度资料，描述南海上层海洋动力场的年循环及其与热力场之间的关系和南海大尺度动力场的年际变化。针对冬、夏2个季节，分析Sverdrup环流场与上层海温之间的关系。研究发现，上层海温变化与上层海洋环流基本结构非常相似，即上层海温变化在一定程度上反映了南海Sverdrup平衡，而且随着浓度的增加，平均海温场与流函数场之间的对应关系更好。本文还着重分析了El Nino期间和La Nina期间的南海异常流函数场。研究发现，异常流函数场在El Nino期间的夏季主要是强化南海自身的环流结构，即强化南部反气旋式涡流（gvre)和强化北部气旋式涡流；冬季则削弱整个南海的气旋式流场。LaNina期间对夏季环流态的影响主要集中在南海北部，即削弱北部气旋式涡流，而对于南海南部的影响甚微；冬季则强化整个南海的气旋式流场。     

中国南海表层叶绿素a季节内变化特征及成因

对中国南海表层叶绿素a季节内变化的研究有助于深入认识其海洋特征,满足渔情预报等实际应用需求。利用卫星观测资料分析南海表层叶绿素a不同季节的季节内变化特征,结果表明南海表层叶绿素a季节内振荡强度冬季最高。冬季和春季的季节内振荡最强区域都位于吕宋岛西北侧海区,夏季和秋季振荡较强的区域偏向菲律宾群岛一侧。分析表明研究海区表层温度和表层叶绿素a 存在负相关,冬强夏弱,北强南弱。大部分海区海面高度和叶绿素a 相关性不显著,但南海东南边缘海区海面高度和叶绿素a在季节内存在正相关。冬季海盆尺度逆时针旋转的环流结构应是这些现象产生的原因。除南海东南边缘海区、海南岛东南海区和吕宋岛西侧海区之外,风应力大小和热通量均与叶绿素a 在季节内呈正相关。这显示非局地风场和海流等因素、海洋动力调整过程可能在吕宋海峡以西和南海东南边缘的表层叶绿素a 季节内变化中起到重要作用。     

Diagnostic calculation of the circulation in the South China Sea during summer 1998

On the basis of hydrographic data obtained from 12 June to 6 July, 1998, the three-dimensional structure of circulation in the South China Sea (SCS) is computed using a three-dimensional diagnostic model. The combination of sea surface height anomaly from altimeter data and numerical results provides a consistent circulation pattern for the SCS, and the main circulation features can be summarized as follows: In the northern SCS there are a cyclonic eddy C1 near Dongsha Islands and an anti-cyclonic eddy W1 west of Luzon Island. In the central SCS a strong anti-cyclonic eddy W3 and a cyclonic eddy C3 compose a quasi-dipole southeast of Vietnam. A coastal northward jet is present at the western boundary near the Vietnam coast above 300 m level. This northward coastal jet flows northward and turns eastward at about 14°N, and then flows southeastward into the area between eddies W3 and C3. In the southern SCS the current is weaker. The most important dynamic mechanism underlying the circulation in the SCS is the joint effect of the baroclinicity and relief (JEBAR), and the second dynamical mechanism is the interaction between the wind stress and relief (IBWSR). Comparison of the characters of circulation in the SCS during summer 2000 with that during summer 1998 reveals no obvious variability of the main characteristics.     

A preliminary study on the response of marine primary production to monsoon variations in the South China Sea Basic characteristics

Seasonal, interannual and interdecadal variations of monsoon over the South China Sea （SCS） directly influence the ocean circulation and the mass transport process, etc. , especially the changes of horizontal circulation pattern and upwelling area. These changes directly influence the nutrient transport and the photosynthesis of phytoplankton, which induce the change of the marine ecosystem in the SCS, including the change of marine primary production in this sea area. On the basis of climatic data for long-time series and primary production estimated by remote sensing, the multi-time scale variations of monsoon, seasonal and interannual variations of primary production, and the response of primary production to monsoon variations were analyzed. Furthermore, the spatio-temporal variations of primary production in different sea areas of the SCS and their relations to the monsoon variations were given. The results showed that the strong southwesterly prevailed over the SCS in summer whereas the vigorous northeasterly in winter. The seasonal primary production in the entire sea area of the SCS also produced a strong peak in winter and a suhpeak in summer. And the seasonal primary production distributions displayed different characteristics in every typical sea area. The variations of the annual and summer averaged primary production in the entire sea area of the SCS showed almost the same rising trend as the intensity of the summer monsoon. Especially for 1998, the summer monsoon reached almost the minimum in the past 54 a when the primary production was also found much lower than any other year （ 1999--2005 ）. The responses of annual primary production to monsoon variation were displayed to different extent in different sea areas of the SCS ; especially it was better in the deep sea basin. Such research activities could be very important for revealing the response of marine ecosystem to the monsoon variations in the SCS.     

南海叶绿素浓度季节变化及空间分布特征研究

以南海海域1997年10月至2002年9月SeaWiFS卫星遥感叶绿素浓度的资料为基础,分析了多年平均的南海叶绿素浓度的时空分布,初步分析结果表明,冬季南海大部分海域叶绿素浓度普遍较高,春季大部分海域较低;南海各个海区的叶绿素月平均最低浓度基本出现在春季的4月或5月,而最高浓度出现的月份却有不同的特征,在中央海盆区出现在12月,在广东沿岸海区出现在7月,在越南东南部近岸海域在8月和12月有两个最高值;在吕宋海峡的西部区域,尽管叶绿素浓度的最高值也出现在12月,但是叶绿素浓度的最低值却出现在夏季的7月.在空间上近岸区域的叶绿素浓度明显高于中央海盆区,西部海域普遍高于东部海域.南海叶绿素浓度的这一时空分布特征与流场(如上升流等)、海面温度场和风场等的变化有关,也与陆源物质的输入等关系密切.     

Features of the horizontal variability of the sea surface temperature in the Western Black Sea from high resolution modeling

A parallel version of the NEMO complex ocean circulation model has been implemented for the Black Sea basin; the results of circulation numerical modeling with a high spatial resolution are presented. Analysis of the spatial variability is performed for the reconstructed hydrophysical fields in 2005–2008. The resulting simulated spatial variability characteristics of the sea surface temperature are compared with available satellite observational data.     

Sea surface height anomaly and geostrophic circulation variations in the South China Sea from TOPEX/POSEIDON altimetry

The sea surface height anomaly (SSHA) and geostrophic circulation in the South ChinaSea (SCS) are studied using TOPEX/POSE1DON (T/P) altimetry data. The SSHA, which is obtained after tidal correction based on the tidal results from T/P data, is predominated by seasonal alternating monsoons. The results reveal that the SSHA in the central part of the SCS is positive in spring and summer, but negative in autumn and winter. It is also found that the SSHA in the SCS can be approached with the sum of tidal constituents SA and SSA. The geostrophic circulations in the SCS are calculated according to sea surface dynamic topography, which is the sum of SSHA and mean sea surface height. It is suggested that the circulation in the upper layer of the SCS is generally cyclonic and notably western intensified during autumn and winter, while the western intensification is weak during spring and summer. It is also indicated that the Kuroshio intrudes into the northeastern SCS throuth the Luzon Strait in winter. But ther     

Numerical investigation of the South China Sea deep circulation

Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea（SCS）, the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer（July）, a northeastsouthwest through-flow pattern across the deep basin without a western boundary current in winter（January）,and a transitional pattern in spring and autumn. The sensitivity ...     

吕宋海峡Ekman输运和南海SST的相关性分析

在利用1950—2009年NCEP(National Center for Environmental Prediction)资料分析风场数据的基础上,计算吕宋海峡的Ekman输运,研究表明其存在显著的季节变化,除了夏季外,其它季节均为由太平洋向南海输运。分析吕宋海峡Ekman输运和南海海盆表征上层热力状况的海表面温度SST(Sea Surface Temperature)之间的关系发现:在年内时间尺度上,两者不存在显著的同期相关,Ekman输运对SST的影响开始于一个月之后,从北部向南扩展,第二个月最为明显,并扩展至整个海盆,第三个月开始衰减,第四个月影响消失,且相关性为正;在年际尺度上,吕宋海峡Ek-man输运的异常同南海SSTA(Sea Surface Temperature Abnormal)的第二模态存在显著的相关联系,并且吕宋海峡Ekman输运和南海SSTA的相关关系在北部为正,南部为负。吕宋海峡Ekman输运调制南海大尺度环流,通过暖、冷平流的作用影响南海SST的变化。     

南海及邻近海域异常海温影响局域大气环流的初步试验

南海及邻近海域表层海温（SST）异常的气候意义可以通过IAP－AGCM的数值模拟得到反映。数值试验表明，东亚大气环流对这种SST异常的响应具有时变性（或称季节性），在空间上维持一定的经向结构和纬向结构。确切地说，南海冷水年份，即南海和孟加拉湾负距平、西太平洋正距平的年份，2，3月份南海有东北风异常；夏季菲律宾附近维持一个反气旋式的差值环流，它的低频活动造成了这个地区九水场的低频振荡；与环流较一致的是夏季西太平洋副热带高压活动有所减弱、东移，造成了水汽经向输送的异常分布。     

南海海面温度的年际模态及其与季风强迫的关系

通过对COADS和OISST海洋气象资料的分析，以南海的海面温度异常(SSTA)为指示因子，在研究年际尺度上的南海与大尺度ENSO关系的基础上，进一步探讨了季风强迫下Ekman抽吸对SSTA年际变化的贡献。研究发现，南海SSTA滞后5个月时与：Nifio 3的SSTA相关系数最大，且其年际变化依方差贡献大小，分别有42．7月、25．6月及36．6个月的周期，这3个年际模态基本上能够描述海面温度的年际变异，为南海在大尺度ENSO背景下存在区域响应提供了重要的证据；应用Krause—Turner混合层温度方程对南海异常暖事件的研究表明，除了经向风应力异常，Ekman抽吸也是影响SSTA年际变异的主要因素，在某些异常年份甚至是主导因素，为南海SSTA年际变化的研究提供了重要的补充。     

南海表层水温年际变化的大尺度特征

通过对COADS海洋气象资料的分析,得出南海表层水温(SST)年际变化的若干大尺度特征.结果表明:南海SST年际具有一定的准周期性,其显著周期为24～30个月;南海SST年际变化与年循环之间有着一种锁相关系,关键位相在于北半球冬春季节;南海典型冷暖年份合成SST距平场的时空结构十分相似;在年际时间尺度上,南海SST和南方涛动指数有反相关系,与经向风海面热收支之间有同位相关系;南海暖池面积指数的年际变化与南海SST年际变化一致.     

Seasonal and spatial distributions of phytoplankton biomass associated with monsoon and oceanic environments in the South China Sea

Seasonal variations of phytoplankton/chlorophyll-a （Chl-a） distribution, sea surface wind, sea height anomaly, sea surface temperature and other oceanic environments for long periods are analyzed in the South China Sea （SCS）, especially in the two typical regions off the east coast of Vietnam and off the northwest coast of Luzon, using remote sensing data and other oceanographic data. The results show that seasonal and spatial distributions of phytoplankton biomass in the SCS are primarily influenced by the monsoon winds and oceanic environments. Off the east coast of Vietnam, Chl-a concentration is a peak in August, a jet shape extending into the interior SCS, which is associated with strong southwesterly monsoon winds, the coastal upwetling induced by offshore Ekman transport and the strong offshore current in the western SCS. In December, high Chl-a concentration appears in the upwelling region off the northwest coast of Luzon and spreads southwestward. Strong mixing by the strong northeasterly monsoon winds, the cyclonic circulation, southwestward coastal currents and river discharge have impacts on distribution of phytoplankton, so that the high phytoplankton biomass extends from the coastal areas over the northern SCS to the entire SCS in winter. These research activities could be important for revealing spatial and temporal patterns of phytoplankton and their interactions with physical environments in the SCS.     

Intercomparison of three South China Sea circulation models

1IntroductionTheSouthChinaSeaisthelargesttropicalmarginaldeepsealocatingbetweenthewesternPacificOceanandtheeasternIndianOcean.AsapartofAsia-Australiamaritimecontinent,monsoonisaprimaryfactorforcingtheSouthChinaSeaCurrent(SCSC)variation.Drivenbynortheasterlymonsooninwinterandsouth-westerlymonsooninsummer,respectively,theSCSCbehavesacyclonicgyreandananticy-clonicgyre,correspondingly(Wyrtki,1961;Xuetal.,1982).Owingtotheshortageandexpen-sivenessofdirectobservationsintheSCS,fur-therunder…     

Mesoscale variability in the South China Sea from the TOPEX/Poseidon altimetry data

Using wavelet transform we studied the mesoscale variability in the South China Sea (SCS) by analyzing 5-yr (October 1992 to August 1997) TOPEX/Poseidon (T/P) altimetry data. Our analysis suggests that mesoscale variability inside the SCS is weaker than that outside the SCS in the Kuroshio. It is found that despite the large temporal variation in the mesoscale variability, there are two narrow bands of significant mesoscale variability north of 10°N throughout much of the 5-yr period. The stronger one lies along the western boundary, while the weaker one is oriented in a southwest-to-northeast direction across the central SCS. In the rest of the SCS, the mesoscale variability is much weaker. In light of the numerical simulation by Metzger and Hurlburt (1996, Journal of Geophysical Research, 101, 12,331–12,352) and an XBT section along 15°N, the broad characteristic structure of the mesoscale variability indicates that the large-scale mean circulation in the SCS is primarily a cyclonic gyre north of about 10°N. In addition to the mesoscale variability, analysis of both the T/P and the XBT data indicates that there also exists significant intra-annual variability within similar geographic locations. The intra-annual variability is found to be primarily a subsurface feature with a very weak surface signature.     

基于多源融合卫星高度计观测数据的南海海浪有效波高的季节变化研究

The seasonal variability of the significant wave height(SWH) in the South China Sea(SCS) is investigated using the most up-to-date gridded daily altimeter data for the period of September 2009 to August 2015. The results indicate that the SWH shows a uniform seasonal variation in the whole SCS, with its maxima occurring in December/January and minima in May. Throughout the year, the SWH in the SCS is the largest around Luzon Strait(LS) and then gradually decreases southward across the basin. The surface wind speed has a similar seasonal variation, but with different spatial distributions in most months of the year. Further analysis indicates that the observed SWH variations are dominated by swell. The wind sea height, however, is much smaller. It is the the largest in two regions southwest of Taiwan Island and southeast of Vietnam Coast during the northeasterly monsoon, while the largest in the central/southern SCS during the southwesterly monsoon. The extreme wave condition also experiences a significant seasonal variation. In most regions of the northern and central SCS, the maxima of the 99 th percentile SWH that are larger than the SWH theoretically calculated with the wind speed for the fully developed seas mainly appear in August–November, closely related to strong tropical cyclone activities.Compared with previous studies, it is also implied that the wave climate in the Pacific Ocean plays an important role in the wave climate variations in the SCS.     

Circulation in the South China Sea is in a state of forced oscillation: Results from a simple reduced gravity model with a closed boundary

The South China Sea (SCS) is a narrow semi-enclosed basin, ranging from 4°–6°N to 21°–22°N meridionally. It is forced by a strong annual cycle of monsoon-related wind stress. The Coriolis parameter f increases at least three times from the southern basin to the northern basin. As a result, the basin-cross time for the first baroclinic Rossby wave in the southern part of the basin is about 10-times faster than that in the northern part, which plays the most vitally important role in setting the circulation. At the northernmost edge of SCS, the first baroclinic Rossby wave takes slightly less than 1 year to move across the basin, however, it takes only 1–2 months in the southernmost part. Therefore, circulation properties for a station in the model ocean are not solely determined by the forcing at that time instance only; instead, they depend on the information over the past months. The combination of a strong annual cycle of wind forcing and large difference of basin-cross time for the first baroclinic Rossby wave leads to a strong seasonal cycle of the circulation in the SCS, hence, the circulation is dominated by the forced oscillations, rather than the quasi-steady state discussed in many textbooks.The circulation in the SCS is explored in detail by using a simple reduced gravity model forced by seasonally varying zonal wind stress. In particular, for a given time snap the western boundary current in the SCS cannot play the role of balancing mass transport across each latitude nor balancing mechanical energy and vorticity in the whole basin. In a departure from the steady wind-driven circulation discussed in many existing textbooks, the circulation in the SCS is characterized by the imbalance of mechanical energy and vorticity for the whole basin at any part of the seasonal cycle. In particular, the western boundary current in the SCS cannot balance the mass, mechanical energy, and vorticity in the seasonal cycle of the basin. Consequently, the circulation near the western boundary cannot be interpreted in terms of the wind stress and thermohaline forcing at the same time. Instead, circulation properties near the western boundary should be interpreted in terms of the contributions due to the delayed wind stress and the eastern boundary layer thickness. In fact, there is a clear annual cycle of net imbalance of mechanical energy and vorticity source/sink. Results from such a simple model may have important implications for our understanding of the complicated phenomena in the SCS, either from in-situ observations or numerical simulations.     

Formation of an anticyclonic eddy and the mechanism involved: a case study using cruise data from the northern South China Sea

Data from satellite altimetry and in situ observations together with the Hybrid Coordinate Ocean Model (HYCOM) reanalysis data were used to investigate the mechanism and formation of an anticyclonic eddy in the northeastern South China Sea (SCS). Analysis of water mass using cruise data indicated that the water captured in the eddy diff ers from those in the SCS, the Kuroshio intrusion, and the eddy-forming region. Data from sea surface height (SSH) and sea level anomaly (SLA) indicate that the eddy formed due both to the Kuroshio intrusion and the local circulation in the SCS. The Kuroshio intrusion is present at the start of the eddy growth (March 5-9) before Kuroshio leaps the Luzon Strait. The eddy then becomes larger and stronger in the absence of the Kuroshio intrusion. From the eddy budget of the HYCOM reanalysis data, the formation of the eddy goes in three steps. By the third step, the eddy had become aff ected by variations of local SCS circulation, which is more strongly than in the fi rst step in which it is aff ected more by the Kuroshio intrusion. The variability of the temperature and salinity inside the eddy provide a support to this conclusion. The water in the SCS intruded into the eddy from the southeast, which decrease the salinity gradually in the southern part of the eddy during the growth period.     

南海深水海盆环流和温跃层深度的季节变化

受南海季风和复杂地形的影响,南海环流场具有复杂的空间结构和明显的季节变化,同时此海域又是中尺度涡多发海域,这些特征必然对南海温跃层深度的水平分布及季节变化有显著影响。首先,基于GDEM(General-ized Digital Environmental Model)的温、盐资料和利用P矢量方法计算并分析了南海的表层环流和多涡结构的空间分布特征和季节变化规律。在此基础上,分析了南海温跃层深度的空间分布特征和季节变化规律。结果表明,南海环流和多涡结构对南海温跃层具有显著的影响。