Influence of Mesoscale Eddies on Variations of the Kuroshio Path South of Japan

The influences of mesoscale eddies on variations of the Kuroshio path south of Japan have been investigated using time series of the Kuroshio axis location and altimeter-derived sea surface height maps for a period of seven years from 1993 to 1999, when the Kuroshio followed its non-large meander path. It was found that both the cyclonic and anticyclonic eddies may interact with the Kuroshio and trigger short-term meanders of the Kuroshio path, although not all eddies that approached or collided with the Kuroshio formed meanders. An anticyclonic eddy that revolves clockwise in a region south of Shikoku and Cape Shionomisaki with a period of about 5–6 months was found to propagate westward along about 30°N and collide with the Kuroshio in the east of Kyushu or south of Shikoku. This collision sometimes triggers meanders which propagate over the whole region south of Japan. The eddy was advected downstream, generating a meander on the downstream side to the east of Cape Shionomisaki. After the eddy passed Cape Shionomisaki, it detached from the Kuroshio and started to move westward again. Sometimes the eddy merges with other anticyclonic eddies traveling from the east. Coalescence of cyclonic eddies, which are also generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region south of Japan, into the Kuroshio in the east of Kyushu, also triggers meanders which mainly propagate only in a region west of Cape Shionomisaki. This revised version was published online in July 2006 with corrections to the Cover Date.     

东海黑潮周边中尺度涡的分布、运动规律以及生成机制

为了探究东海黑潮周边涡旋分布、形成机理及运动规律,基于法国国家空间研究中心(CNES)卫星海洋学存档数据中心(AVISO)的中尺度涡旋数据集展开了研究。首先,统计了近27年东海黑潮周边的涡旋分布,发现在黑潮弯曲海域产生了650个涡旋,在黑潮中段海域产生了271个涡旋,其中直径100~150 km之间的涡旋数量最多,涡旋振幅主要集中在2~6 cm。其次,分析了东海黑潮的运动路径和涡运动过程,结果表明,黑潮气旋式弯曲海域内侧易产生气旋涡,且移动路径较长,如台湾东北海域黑潮流轴气旋式弯曲处产生的涡旋,其平均位移达到了87.6 km;当反气旋式弯曲海域内侧产生反气旋涡时,涡旋往往做徘徊运动。黑潮中段海域的涡旋呈现出气旋涡在黑潮主轴西侧、反气旋涡在黑潮主轴东侧的极性对称分布特征,两类涡都沿黑潮主轴向东北方向移动。最后,结合再分析的流场、海面高度数据,讨论了涡旋运动规律和生成机制。黑潮弯曲处涡旋的生成与黑潮流体边界层分离有关,奄美大岛南部到冲绳岛西侧的黑潮逆流对黑潮中段海域涡的极性对称分布起到了关键作用,涡旋在运动过程中通常经历生长、成熟和衰变三个阶段。     

Path Transition of the Kuroshio Due to Mesoscale Eddies: A Two-Layer, Wind-Driven Experiment

We have executed numerical experiments using a two-layer, wind-driven ocean model with simplified coastal geometry and bottom topography to investigate the possibility of the Kuroshio path transition due to mesoscale eddies. A straight path easily changes into a meandering path due to the eddy action. For this transition, an anticyclonic eddy is preferable to a cyclonic one when imposed in the beginning region of the Kuroshio (east of Taiwan). When imposed southeast of Kyushu, on the other hand, a cyclonic eddy is more effective than an anticyclonic one. The reverse transition, from a meandering to a straight path, did not occur at all in this experiment. This revised version was published online in August 2006 with corrections to the Cover Date.     

琉球群岛附近海域中尺度涡的初步研究

采用1977年1月至2006年12月高分辨率全球大洋环流模型OFES输出结果对琉球群岛附近海域的中尺度涡进行了研究分析。结果表明:(1)尺度较大的涡旋的分布密集区主要有台湾以东海域、琉球海沟上层海域和四国以南海域。(2)琉球海流流经海域的反气旋涡旋占优势,有利于琉球海流的发展。(3)琉球海流受中尺度涡的影响十分显著,纬度越低,其受中尺度涡的影响越明显,而黑潮相对比较稳定,受中尺度涡的影响并不显著。(4)四国以南海域暖涡从黑潮脱落之后向西南移动,该涡旋的移动对琉球海流和黑潮产生特别显著的影响。文章的最后还讨论了中尺度涡与黑潮弯曲以及琉球海流可能存在的联系。     

中国近海及其附近海域若干涡旋研究综述Ⅱ.东海和琉球群岛以东海域

综述东海和琉球群岛以东海域若干气旋型和反气旋型涡旋的研究.对东海陆架、200m以浅海域,主要讨论了东海西南部反气旋涡、济州岛西南气旋式涡和长江口东北气旋式冷涡.东海两侧和陆坡附近出现了各种不同尺度的涡旋,其动力原因之一是与东海黑潮弯曲现象有很大关系,其次也与地形、琉球群岛存在等有关.东海黑潮有两种类型弯曲:黑潮锋弯曲和黑潮路径弯曲.黑潮第一种弯曲出现了锋面涡旋,评述了锋面涡旋的存在时间尺度与空间尺度和结构等;也指出了黑潮第二种弯曲,即路径弯曲时在其两侧出现了中尺度气旋式和反气旋涡,讨论了它们的变化的特性.特别讨论了冲绳北段黑潮弯曲路径和中尺度涡的相互作用,着重指出,当气旋式涡在冲绳海槽北段成长,并充分地发展,其周期约在1~3个月时,它的空间尺度成长到约为200km(此尺度相当于冲绳海槽的纬向尺度)时,黑潮路径从北段转移到南段.也分析了东海黑潮流量和其附近中尺度涡的相互作用.最后指出在琉球群岛以东、以南海域,经常出现各种不同的中尺度反气旋式和气旋式涡,讨论了它们在时间与空间尺度上变化的特征.     

Anticyclonic Eddy Revealing Low Sea Surface Temperature in the Sea South of Japan: Case Study of the Eddy Observed in 1999–2000

Various kinds of datasets, such as satellite-derived sea surface temperature (SST), sea surface height, surface velocity produced by combining surface drifter and satellite altimeter data, and hydrographic data, led to the discovery of an anticyclonic eddy with lower SST than those of surrounding waters in the Kuroshio recirculation region south of Shikoku, as if the eddy were cyclonic. This anticyclonic eddy was formed east of Kyushu in late August to early September 1999 from the merger of two anticyclonic eddies which had migrated in the recirculation region to the sea south of Japan from the east. After the merger, the anticyclonic eddy strengthened abruptly and began to exhibit the low SST. In October, this eddy coalesced with the Kuroshio and moved swiftly eastward, accompanied by an amplitude growth of the Kuroshio meander. In mid November, off the Kii Peninsula, the eddy detached from the meandering Kuroshio. It then moved southwestward and again slowly propagated westward along the 30°N line. During this period, at least from late October 1999 to January 2000, SSTs over the anticyclonic eddy were found to be continuously lower than those of surrounding waters. This case tells us that we have to pay careful attention to the interpretation of mesoscale SST distributions. This revised version was published online in July 2006 with corrections to the Cover Date.     

台湾以东中尺度涡对黑潮入侵东海路径的影响

利用AVISO数据集的卫星高度计资料,分析了中国台湾以东中尺度涡的时空特征,通过具体的中尺度涡实例探讨了其对台湾以东黑潮路径的影响。研究表明气旋式中尺度涡在春夏季节的数目要少于反气旋式中尺度涡,在秋冬季节气旋式涡旋个数则多于反气旋涡;并且台东以东区域涡旋传播存在多种路径,涡旋的存在对台湾东北部黑潮入侵东海的路径具有重大影响,特别是2004年夏季台湾以东区域存在多个涡旋,相应的吕宋海峡黑潮主轴向东偏移明显,台湾东北黑潮入侵东海的路径发生了显著变化。     

进入中国南海的黑潮脱落中尺度涡的特征——基于OFES模式数据

自黑潮脱落并由吕宋海峡进入中国南海的中尺度涡(简称脱落涡旋)对黑潮与南海的水体交换、热量及物质输送等过程均有十分重要的作用。基于1993—2013年OFES(OGCM for the Earth Simulator)模式数据产品,分析研究了脱落涡旋的统计特征及其温盐流三维结构,并与卫星观测结果进行对比分析。OFES模式的海表面高度数据和卫星高度计数据的统计结果都表明气旋式脱落涡旋(脱落冷涡)绝大部分在黑潮西侧边缘生成,反气旋式脱落涡旋(脱落暖涡)则大部分在黑潮控制区(包括黑潮流套区)生成,脱落暖涡的数量远多于脱落冷涡的。OFES模式数据得到的脱落涡旋个数和出现频率较卫星观测结果要明显偏低。此外,由OFES模式数据得到的脱落涡旋三维结构表明,黑潮控制区和黑潮西侧边缘生成的脱落冷涡的流场垂向影响深度差异较大,而脱落暖涡的流场垂向影响深度一般达水深1000 m以深,脱落涡旋的位势温度的垂向影响深度与该涡的流场垂向影响深度相当,其盐度的垂向影响深度则较浅;脱落涡旋的温盐结构受黑潮的影响较大。     

Mesoscale Eddies Observed by TOLEX-ADCP and TOPEX/POSEIDON Altimeter in the Kuroshio Recirculation Region South of Japan

Mesoscale eddies in the Kuroshio recirculation region south of Japan have been investigated by using surface current data measured by an Acoustic Doppler Current Profiler (ADCP) installed on a regular ferry shuttling between Tokyo and Chichijima, Bonin Islands, and sea surface height anomaly derived from the TOPEX/POSEIDON altimeter. Many cyclonic and anticyclonic eddies were observed in the region. Spatial and temporal scales of the eddies were determined by lag-correlation analyses in space and time. The eddies are circular in shape with a diameter of 500 km and a temporal scale of 80 days. Typical maximum surface velocity and sea surface height anomaly associated with the eddies are 15–20 cm s^–1 and 15 cm, respectively. The frequency of occurrence, temporal and spatial scales, and intensity are all nearly the same for the cyclonic and anticyclonic eddies, which are considered to be successive wave-like disturbances rather than solitary eddies. Phase speed of westward propagation of the eddies is estimated as 6.8 cm s^–1, which is faster than a theoretical estimate based on the baroclinic first-mode Rossby wave with or without a mean current. The spatial distribution of sea surface height variations suggests that these eddies may be generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region, though further studies are needed to clarify the generation processes.     

Primary Study of the Mechanism of Eddy Shedding from the Kuroshio Bend in Luzon Strait

The mechanism of the anticyclonic eddy's shedding from the Kuroshio bend in Luzon Strait has been studied using a nonlinear 2 1/2 layer model, in a domain including the North Pacific and South China Sea. The model is forced by steady zonal wind in the North Pacific. Energy analysis is adopted to detect the mechanism of the eddy shedding. Twelve experiments with unique changes of wind forcing speed (to obtain different Kuroshio transports at Luzon Strait) were performed to examine the relationship between the Kuroshio transport (KT) and the eddy shedding events. In the reference experiment with KT of 22.7 Sv (forced with zonal wind idealized from the annual mean wind stress from the COADS data set), the interval of eddy shedding is 70 days and the shed eddy centers at (20°N, 117.5°E). When the Kuroshio bend extends westward, the southern cyclonic perturbation grows so rapidly as to form a cyclonic eddy (18.5°N, 120.5°E) because of the frontal instability in the south of the Kuroshio bend. In the evolution of the cyclonic eddy, it cleaves the Kuroshio bend and triggers the separation of the anticyclonic eddy. In statistical terms, anticyclonic eddy shedding occurs only when KT fluctuates within a moderate range, between 21 Sv and 28 Sv. When the KT is larger than 28 Sv, a stronger frontal instability south of the Kuroshio bend tends to generate a cyclonic eddy of size similar to the width of the Luzon Strait. The bigger cyclonic eddy prevents the Kuroshio bend from extending into the SCS and does not lead to eddy shedding. On the other hand, when the KT decreases to less than 21 Sv, the frontal instability south of the Kuroshio bend is so weak that the size of corresponding cyclonic eddy is smaller than half the width of the Luzon Strait. The cyclonic eddy, lacking power, fails to cleave the Kuroshio bend and cause separation of an anticyclonic eddy; as a result, no eddy shedding occurred then, either.     

Numerical study on the summer circulation of the upper South China Sea and its establishment

A coupled single-layer/two-layer model is employed to study the South China Sea (SCS) upper circulation and its response before and after the onset of summer monsoon. It is found that, in summer, due to the β effect and the first baroclinic mode of the wind-driven current, a northward western boundary jet current is formed along the Indo-China Peninsula coast, and it leaves the coast at about 13° N and diffuses towards northeast; next to the Indo-China Peninsula, a large anticyclonic     

东海黑潮活跃区表层流场的半年内时间尺度变化研究

利用高时空分辨率的资料,对东海黑潮表层海流的半年内时间尺度变化特征进行了分析研究,得到主要以下结论:(1)东海黑潮表层海流在台湾东北海区和吐噶喇海峡附近海区存在着最为显著的变化;(2)那里的表层海流都存在半年内时间尺度的变化,其谱峰主要在50~70d及90~140d两个频段内,两个准周期变化的基本特征都是异常气旋涡和反气旋涡的准周期转换;(3)异常气旋涡和反气旋涡的活动都与东海黑潮在两个海区的流轴变化相联系,气旋涡与黑潮流轴在该海区的向东南退缩相伴,而反气旋涡与黑潮流轴在该海区的向西北推进相伴;(4)初步分析表明,在台湾东北海域导致50~70d变化的异常涡旋主要源于黑潮自身存在的中尺度过程,而90~140d的变化则主要受从台湾以东传来的中尺度涡影响。类似地,吐噶喇海峡附近的黑潮海流同样在50~70d及90~140d两个频段内存在显著的准周期变化,回归分析表明,导致50~70d变化主要源于上游黑潮海流的中尺度过程,而90~140d的变化则主要受琉球群岛以东传来的中尺度涡影响。     

台湾以东黑潮的低频变化及机制研究

本文基于AVISO（Archiving, Validation and Interpretation of Satellite Oceanographic data）1993-2015年间的海表面绝对动力高度数据,研究了台湾以东黑潮的低频变化特征,并探讨了影响其变化的机制。结果表明,台湾以东多年平均的黑潮流幅值约为136 km,表层流量值约为7.75×104 m2/s,对应的标准差分别为28 km和2.14×104 m2/s。台湾以东黑潮不仅具有显著的季节变化特征,还具有显著的年际变化特征。功率谱分析结果表明,台湾以东黑潮表层流量具有1 a和2.8 a的显著周期。空间上,台湾东南部黑潮的年际变化幅度比东北部强烈。相关性及合成分析结果表明,台湾以东黑潮的年际变化与PTO（Philippines-Taiwan Oscillation）之间存在显著的相关性。PTO年际震荡所导致的副热带逆流区反气旋式涡旋与气旋式涡旋的相对强度是影响台湾以东黑潮年际变化的主要动力因素。     

The Kuroshio east of Taiwan and the currents east of the Ryūkūy-guntōduring October of 1995

On the basis of hydrographic data obtained during two October cruises of 1995, a modified inverse method is used to compute the Kuroshio east of Taiwan and the currents east of the Ryukyu-gunto.The net northward volume transport(VT) of the Kuroshio through Section TK₂-K₂ southeast of Taiwan is about 57.8×10⁶ m³/s.There are four current cores of the Kuroshio at Section TK₂-K₂.Its main core is near the south of Taiwan, and its maximum speed is about 257 cm/s at the surface.After the Kuroshio flows through Section TK₂-K₂, there are three branches of the Kuroshio.The main branch of the Kuroshio flows northward into Section TKa east of Su''ao.The second branch of the Kuroshio flows northward through Section TKa and then enters the East China Sea through the region between Yonakunijima and Iriomote-shima.The net northward VT of the Kuroshio through Section TK4 is about 21.6×10⁶ m³/s.The eastern branch of the Kuroshio flows northeastward through the region between a stronger cyclonic eddy and a recirculating anticyclonic gyre, and then flows continuously northeastward to the region east of the Ryūkyū-guntō and becomes a part of the origin of the western boundary current east of the Ryūkyū-guntō.Another part of the origin of the western boundary current east of the Ryūkyū-guntō comes from a recirculating anticyclonic gyre.From the above, in the regions east of Taiwan end east of the Ryūkyū-guntō the pattern of circulation during October of 1995 differs from the pattern of circulation during early summer of 1985.There are several eddies of different scales in this computational region.For example, there is a meso-scale stronger cyclonic eddy whose center is located at about 23°N, 124°20''E.     

中国近海及其附近海域若干涡旋研究综述 Ⅰ.南海和台湾以东海域

综述了南海和台湾以东海域若干气旋型和反气旋型涡旋研究.在南海存在着许多活跃的中尺度涡,我们分别对南海中、南部海域和南海北部海域中尺度涡作了评述.在南海北部海域,目前最感兴趣的问题为:南海水与西菲律宾海通过吕宋海峡的交换的物理过程,以及黑潮是否以反气旋流套形式进入南海.这些问题目前尚不清楚,尤其是这些问题的机理.这些问题必须通过今后深入和细致的、长时间的海流和水文观测,以及长时间卫星遥感观测资料的论证才能逐渐认识清楚.台湾以东海域,黑潮两侧经常出现中尺度涡,而且变化较大而复杂.文中着重讨论兰屿冷涡和台湾东北的气旋式冷涡.     

Uncertainty Reduction of Estimated Geostrophic Volume Transports with Altimeter Observations East of Taiwan

The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference level within the depth range between 800 and 2000 dbar. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of deep-sea eddies on the northern KwaZulu-Natal shelf,South Africa

This paper describes the westward movement of a cyclonic eddy across the Mozambique Channel and the subsequent south-westward propagation of the eddy along the east coast of South Africa and its interaction with the shelf. A hydrographic survey on 13 September 2006 off Nine-mile Reef (NMR), Sodwana Bay, showed a well-developed Agulhas Current along the continental shelf inshore of a cyclonic eddy flanked by two anti-cyclonic eddies, further offshore. A satellite-tracked drifter and complementary altimetry data confirmed the dimensions of the eddy and tracked its movement towards the coast. Shelf-edge upwelling was measured at NMR by an underwater temperature recorder (UTR) when the cyclonic eddy first came into contact with the shelf and again when the cyclonic eddy interacted with the leading edge of the anti-cyclonic eddy moving onto the shelf. Further shelf–eddy interactions off Aliwal Shoal, south of Durban, and consequent upwelling were similarly caused by the same cyclonic eddy as it progressed south-westward along the east coast. Analysis of UTR data between 2004 and 2006 indicated that between two and five cyclonic eddies impact the shelf off NMR per year.     

台湾岛以东涡旋及东海黑潮的变化特征

台湾岛以东海域是黑潮变异最为强烈的区域之一,副热带逆流(STCC)区产生的大量涡旋西传至此,对台湾以东的黑潮流径和流量产生较大影响。但是受观测资料限制,目前对这些涡旋的垂向结构尚不清楚。利用ARGO剖面浮标资料并结合卫星高度计资料,对台湾岛以东的涡旋垂向结构及其运动特征进行了详细的分析。通过分析表明,这些涡旋垂向尺度可以深达2 000 m,水平尺度约200 km,其表层切向速度可以达60 cm/s,与黑潮流速相当,而在1 000 m层上的平均切向流速还可达8 cm/s左右。根据高度计资料分析,这些涡旋是源自STCC区的Rossby波西传结果,其Rossby变形半径为43 km,其相速度约4.2 cm/s。另外,根据浮标观测表明,1 000 m层上东海黑潮按照流速可明显分成3个区段。     

南海北部陆坡中尺度涡的演化及传播规律分析

The continental slope in the northern South China Sea(SCS) is rich in mesoscale eddies which play an important role in transport and retention of nutrients and biota. In this study, we investigate the statistical properties of eddy distributions and propagation in a period of 24 years between 1993 and 2016 by using the altimeter data. A total of 147 eddies are found in the continental slope region(CSR), including 70 cyclonic eddies(CEs) and 77 anticyclonic eddies(ACEs). For those eddies that appear in the CSR, the surrounding areas of Dongsha Islands(DS) and southwest of Taiwan(SWT) are considered as the primary sources, where eddies generated contribute more than 60% of the total. According to the spatial distribution of eddy relative vorticity, eddies are weakening as propagating westward. Although both CEs and ACEs roughly propagate along the slope isobaths, there are discrepancies between CEs and ACEs. The ACEs move slightly faster in the zonal direction, while the CEs tend to cross the isobaths with large bottom depth change. The ACEs generally move further into the basin areas after leaving the CSR while CEs remain around the CSR. The eddy propagation on the continental slope is likely to be associated with mean flow at a certain degree because the eddy trajectories have notable seasonal signals that are consistent with the seasonal cycle of geostrophic current. The results indicate that the eddy translation speed is statistically consistent with geostrophic velocity in both magnitude and direction.     

东海黑潮及琉球群岛以东海流研究进展

东海是西太平洋的一个边缘海,它西部有广阔的大陆架,东面以九州岛、琉球群岛(由奄美诸岛、冲绳诸岛、先岛诸岛组成)和台湾岛连线为界,具有较深的海槽。黑潮是一支高温、高盐、高流速的西边界流,它起源于菲律宾以东海域、流经台湾东岸进入东海,沿东海大陆架外缘向东北向流动，穿越吐噶喇海峡返回太平洋。通常的把自台湾东北端起至吐噶喇海峡一段称为东海黑潮。此外,在琉球群岛以东海域也常年存在一支稳定的西边界流,称为琉球海流。 对于东海黑潮和琉球海流的硏究,主要起步于20世纪80年代,通过进行中日联合调查研究、中日副热带环流调查研究等,取得了许多重要成果,苏纪兰(1994)、袁耀初(2000，Yuan,2006)、管秉贤(2002)等人都做过评述。本文基于以上研究评述,主要叙述200年以来对东海黑潮及琉球海流的硏究新进展。