南海西部风驱离岸急流次中尺度锋面的动力学分析

本文利用卫星观测资料和500 m分辨率数值模拟结果,结合理论分析,对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明,南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面,在地转和非地转运动的共同作用下,次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示,沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送,有效地在跨锋面方向将表层冷水平流输送至暖水侧,导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV）,表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度,其最大值可达100 m·d^-1。能量评估结果表明,次中尺度湍流的两个主要能量源,即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下,大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此,次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量,可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。     

夏季南海北部粤东陆架锋面的动力特征分析*

本文利用现场观测资料和卫星遥感数据, 并结合ROMS(regional ocean modeling system)数值模拟对南海北部粤东陆架的锋面特征及其影响因素进行探讨。观测结果显示, 夏季南海北部陆架存在活跃的上升流温度锋面, 其水平尺度约为50km, 强度达到0.06℃∙km^-1, 大于同时期卫星遥感观测结果, 垂向影响深度超过20m, 且具有一阶理查森数(Richardson number, Ri)的典型动力学特征。进一步的ROMS 模式诊断分析结果显示, 锋面处水平梯度增强, 且动力学上表现出一阶Ri数, 为锋面不稳定的发生提供了有利条件。高分辨率模拟结果显示, 在夏季西南风的驱动下, 沿锋面地转流方向的风应力引起的跨陆架Ekman输运将锋面处冷水向暖水运移, 导致水平浮力梯度和锋面强度增强并形成负Ertel位涡(Ertel potential vorticity, EPV)。因此, 夏季风场强迫引起的Ekman浮力通量(Ekman buoyancy flux, EBF)可能是南海北部锋面不稳定现象的主要贡献者, 对局地动力环境有重要影响。     

黑潮延伸体海域典型涡旋的次中尺度特征分析

本文基于卫星遥感资料和高分辨率ROMS(Regional Ocean Modeling System)数值模拟结果, 对黑潮延伸体海域典型中尺度涡旋的次中尺度特征进行了探讨。卫星观测和模拟结果显示, 黑潮延伸体涡旋海域伴随着活跃的次中尺度现象。涡旋演变与多尺度能量分析结果表明, 涡旋海域次中尺度动能的强弱与涡旋海域地转流动能有着密切联系, 锋生可能是涡旋边缘次中尺度动能增强的重要机制。次中尺度现象在中尺度涡旋海域具有沿地转流方向的复杂涡丝状结构特征, 意味着涡旋边缘较强的水平浮力梯度和地转流侧向剪切为次中尺度过程形成与发展提供了有利条件。此外, 垂向结构分析表明, 次中尺度过程能引起较大的垂向速度, 最大可达100m·day^-1, 该垂向速度可以影响至混合层下200m深度处, 对海洋内部的垂向物质能量交换、海—气相互作用等有着重要的影响。     

南海北部反气旋涡旋边缘的次中尺度动力过程分析

广泛存在于上层海洋的次中尺度过程能有效地从平衡态的中尺度地转剪切中汲取动能, 并通过非地转斜压不稳定正向串级能量至小尺度的耗散过程, 从而对海洋物质能量输运、中尺度过程变异以及混合层再层化等产生重要影响。文章利用高分辨率(500m)的区域海洋数值模式ROMS(Regional Ocean Modeling System)模拟结果, 并结合理论分析, 对南海北部冬季典型反气旋涡的次中尺度动力过程进行了初步探讨。研究结果表明, 典型中尺度涡边缘存在显著的锋面, 锋面海域强烈的水平浮力梯度能有效地减小Ertel位涡, 有利于诱发次中尺度对称不稳定(symmetric instability); 锋生作用是引起该中尺度涡边缘发生对称不稳定的主要动力机制之一。同时, 次中尺度过程及其不稳定引起的垂向次级环流显著增强了混合层垂向物质能量交换, 最大垂向速度可达95m·d^-1, 影响深度最深至80m。     

南海中西部渔场上升流时空变化特征分析

南海中西部海域是我国外海渔业开发的潜在渔场之一,摸清该海域的物理环境特征可为我国开发中西部渔场提供参考。本文以遥感风场和海温数据为基础,分析了2003~2012年西南季风期起止时间的年变化、上升流中心位置时空变动以及风场对上升流中心位置变动的影响。结果表明:在南海中西部海域,西南季风期通常从5月份开始,到9月份结束,西南季风持续天数平均值为129.4 d,其中,风向角呈51°~60°的天数最多,占总季风天数的21.98%,西南季风期持续时间有增加的现象。上升流中心位置变动范围为11°~15°N,109°~112°E。西南季风期平均风速减小时,年平均上升流中心在经向上向近岸移动,在纬向上向高纬度移动;平均风速增加时,年平均上升流中心在经向上向离岸方向移动,在纬向上向低纬度移动;当西南季风期平均风向角减小时,年平均上升流中心向低纬度移动,平均风向角增大时,年平均上升流中心向高纬度移动。此外,发现厄尔尼诺年此上升流面积显著大于非厄尔尼诺年,而最低温度比2003~2012年平均最低温度低1.3℃。     

涡分辨率模式模拟的南海中尺度涡旋

Mesoscale eddies(MEs) in the South China Sea(SCS) simulated by a quasi-global eddy-resolving ocean general circulation model are evaluated against satellite data during 1993–2007. The modeled ocean data show more activity than shown by the satellite data and reproduces more eddies in the SCS. A total of 345(428) cyclonic eddies(CEs) and 330(371) anti-cyclonic eddies(AEs) generated for satellite(model) data are identified during the study period, showing increase of ~24% and ~12% for the model data, respectively. Compared with eddies in satellite, the simulated eddies tend to have smaller radii, larger amplitudes, a slightly longer lifetime, faster movement and rotation speed, a slightly larger nonlinear properties(U/c) in the model. However, the spatial distribution of generated eddies appears to be inhomogeneous, with more CEs in the northern part of SCS and fewer AEs in the southern part. This is attributed to the exaggerated Kuroshio intrusion in the model because the small islands in the Luzon Strait are still not well resolved although the horizontal resolution reaches(1/10)°. The seasonal variability in the number and the amplitude of eddies generated is also investigated.     

南海上层海洋次中尺度过程空间差异和季节变化特征

南海是西太平洋最大的边缘海, 由于受季风影响显著以及北部海域的黑潮入侵, 其动力环境复杂多变, 次中尺度过程丰富, 且在空间上和时间上存在多变性。文章基于高分辨率数值模式的结果, 通过对次中尺度动力参数的分析, 对比讨论了南海北部、中部、西部和南部海域4个典型子区域上层海洋次中尺度过程的空间差异、季节变化、影响深度、影响因素等问题。研究发现各区域季节性变化特征和机制有所不同: 北部海域受冬季风和黑潮入侵影响, 冬季次中尺度的混合层不稳定较强; 中部海域同样表现为“冬强夏弱”; 西部海域受夏季风影响显著, 夏季次中尺度过程更为活跃; 而南部海域主要受岛屿地形影响较大, 容易产生地形尾涡, 季节性特征不明显。统计分析表明, 次中尺度过程往往表现出强正相对涡度与高应变特征, 在表层更容易出现负位涡, 流体稳定性较差。此外, 文章从能量学角度对次中尺度过程的主要能量来源、控制因素等进行了讨论。     

南海北部陆架区夏季上升流数值研究

采用三维斜压非线性数值模式并结合卫星遥感资料分析,对南海北部陆架区夏季上升流进行了初步研究.研究结果表明,上升流是南海北部陆架区6-9月的一个规律性现象,而不是个别年份的特殊现象;海南岛东部沿岸及雷州半岛以东广州湾东南部一带海域(琼东上升流区)、汕头沿岸直至福建沿岸南日群岛附近海域(粤东上升流区)夏季表层及次表层海水均表现出明显的低温、高盐、高密等陆架上升流特征;上升流中心主要位于海南岛以东清澜湾至七洲列岛之间111°10'E、19°45'N附近,陵水湾至陵水沿岸110°15'E、18°25'N附近,粤东汕头沿岸116°45'E、22°50'N附近及澎湖列岛以西118°E、23°40'N附近.同时通过对模拟结果与QuikSCAT风场的比较分析发现,沿岸上升流与局地风场有着密切的关系,夏季西南风及风应力旋度对琼东沿岸上升流的形成有着非常积极的作用;而粤东沿岸风应力旋度较小,但夏季西南风仍是诱生粤东沿岸上升流的重要因素之一.     

基于HYCOM的南海中尺度涡数值模拟

结合AVISO(Archiving Validation and Interpolation of Satellite Oceanographic Data)高度计资料,利用改进的NERSCHYCOM(Nansen Environmental and Remote Sensing Center-Hybrid Coordinate Ocean Model)大洋环流模式,对南海中尺度涡进行数值模拟研究,主要包括中尺度涡的三维结构、南海EKE(Eddy Kinetic Energy,涡动动能)的垂向变化、黑潮中尺度涡的脱落以及涡旋近岸时的结构变化等。模式再现了2007年2月-3月菲律宾西侧海域的一次暖涡过程,探究了其生命期中各阶段的特征物理量的变化,对其成熟时期的涡旋结构研究表明,中尺度涡的结构呈现不对称性,涡旋两侧的流场空间范围和流场强度均不相同,涡旋的半径和中心位置随深度不断变化,并且由涡旋作用产生的升降流的中心与涡旋自身中心并不重合,二者之间有一定距离。初步探索EKE的垂向分布情况,认为南海年平均EKE在垂向变化上呈现三段式,主要部分分布在300m以浅深度,但同时垂向又能达到海洋深层。分析了一次黑潮中尺度涡脱落的模式模拟个例,推测黑潮中尺度涡脱落原因:黑潮流径西移、外海中尺度涡对黑潮的强迫、地形作用,并且结果表明从黑潮脱落的中尺度涡可以携带大量高温高盐水体进入南海,对南海的温盐性质产生很大的影响。初步探索涡旋近岸时的结构变化,涡旋靠近岸界时,受岸界挤压,流速在一段时间内会增大,继续靠近岸界,由于岸界的摩擦、海底的拖曳,导致能量耗散,流速减小,最终涡旋消亡。     

Spatial-temporal variability of submesoscale currents in the South China Sea

Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.     

南海北部深水区东西构造差异性及其动力学机制

This paper overviews research progress in observation, theoretical analysis and numerical modeling of submesoscale dynamic processes in the South China Sea(SCS) particularly during recent five years. The submesoscale processes are defined according to both spatial and dynamic scales, and divided into four subcategories as submesoscale waves, submesoscale vortexes, submesoscale shelf processes, and submesoscale turbulence. The major new findings are as follows.(1) Systematic mooring observations provide new insights into the solitary waves(ISWs) and the typhoon-forced near-inertial waves(NIWs), of which a new type of ISWs with period of 23 h was observed in the northern SCS(NSCS), and the influences of background vorticity, summer monsoon onset, and deep meridional overturning circulation on the NIWs, as well as nonlinear wave-wave interaction between the NIWs and internal tides, are better understood. On the other hand, satellite altimeter sea surface height data are used to reveal the internal tide radiation patterns and provide solid evidence for that the ISWs in the northeastern SCS originate from the Luzon Strait.(2) Submesoscale offshore jets and associated vortex trains off the Vietnam coast in the western boundary of the SCS were observed from satellite chlorophyll concentration images. Spiral trains with the horizontal scale of 15–30 km and the spacing of 50–80 km were identified.(3) 3-D vertical circulation in the upwelling region east of Hainan Island was theoretically analyzed. The results show that distribution patterns of all the dynamic terms are featured by wave-like structures with horizontal wavelength scale of 20–40 km.(4) Numerical models have been used for the research of submesoscale turbulence. Submesoscale vertical pump of an anticyclonic eddy and the spatiotemporal features of submesoscale processes in the northeastern SCS are well modeled.     

南海东北部亚中尺度过程时空分布特征

基于高分辨率模型2009-2012年的模拟结果,本文对南海东北部亚中尺度过程的时空分布特征进行了研究。模拟结果表明,南海东北部上层广泛存在着相对涡度接近于局地行星涡度的亚中尺度过程。统计结果发现,亚中尺度过程的相对涡度的分布具有着明显的非对称性,即正涡度明显强于负涡度。这意味着相比于负涡度,具有正涡度的亚中尺度过程要更为活跃,而这主要是由离心不稳定导致。同时,亚中尺度过程在时间分布上表现出明显的冬强夏弱的季节变化特征。通过对该海区亚中尺度过程可能生成机制的分析发现,该季节变化与流场拉伸和混合层的厚度有着密切关系,冬季更强的流场拉伸和更深的混合层有利于通过锋生过程和混合层不稳定为亚中尺度过程生成提供更多的能量。     

南海上升流研究概述

本文对近40a来南海上升流研究结果进行了概述，重点介绍了南海北部陆架区上升流的时空分布特征，及其消长和形成机制等研究成果．这些研究成果揭示上升沈是整个南海北部陆架区夏季的普遍现象，具有南海海盆的空间尺度．引起南海北部陆架区夏季上升流存在的动力因素是盛行的西南季风．该上升流在空间和时间上的分布是不均一的，海南岛东北和闽、粤边界海域是上升流中心区；台湾浅滩周围的上升流呈多元结构，各上升流区海水的理化性质存在看明显差异；在粤东，上升流的影响可达沿海港湾内部，并支配看这些港湾的夏季水文条件．南海除了在其北部陆架区存在看夏季上升流外，夏季在越南东部沿岸和冬季在吕宋岛沿岸均存在看上升流。     

High-resolution simulation of upper-ocean submesoscale variability in the South China Sea: Spatial and seasonal dynamical regimes

Submesoscale processes in marginal seas usually have complex generating mechanisms, highly dependent on the local background flow and forcing. This numerical study investigates the spatial and seasonal differences of submesoscale activities in the upper ocean of the South China Sea (SCS) and the different dynamical regimes for sub-regions. The spatial and seasonal variations of vertical vorticity, horizontal convergence, lateral buoyancy gradient, and strain rate are analyzed to compare the submesoscale phenomenon within four sub-regions, the northern region near the Luzon Strait (R1), the middle ocean basin (R2), the western SCS (R3), and the southern SCS (R4). The results suggest that the SCS submesoscale processes are highly heterogeneous in space, with different seasonalities in each sub-region. The submesoscale activities in the northern sub-regions (R1, R2) are active in winter but weak in summer, while there appears an almost seasonal anti-phase in the western region (R3) compared to R1 and R2. Interestingly, no clear seasonality of submesoscale features is shown in the southern region (R4). Further analysis of Ertel potential vorticity reveals different generating mechanisms of submesoscale processes in different sub-regions. Correlation analyses also show the vertical extent of vertical velocity and the role of monsoon in generating submesoscale activities in the upper ocean of sub-regions. All these results suggest that the sub-regions have different regimes for submesoscale processes, e.g., Kuroshio intrusion (R1), monsoon modulation (R2), frontal effects (R3), topography wakes (R4).     

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.