热带气旋影响吕宋海峡输运的研究进展与展望

吕宋海峡是连接南海与西太平洋的唯一深水通道,也是调节南海环流及其热力特征的关键海洋通道。在大尺度西边界流、中尺度涡、热带气旋等众多因子的共同影响下,吕宋海峡输运表现出显著的多时间尺度变率特征,其中热带气旋是影响该海域强烈且频繁的天气过程之一,解析吕宋海峡输运与热带气旋之间的动力联系也是近年来南海海洋研究的热点之一。本文主要从吕宋海峡附近热带气旋活动特征及其对黑潮、吕宋海峡附近环流结构、吕宋海峡输运的影响等方面回顾最新的研究进展。最后,本文认为接下来应当在热带气旋调制吕宋海峡输运的机制,以及对吕宋海峡输运年际变化的贡献等方面加强研究。     

黑潮在吕宋海峡的形变及动力机制

根据１９９０年以来对吕宋海峡和中国南海（ＳＣＳ）北部的ＷＯＣＥ水文资料和其它海洋调查资料的分析，以及对同一海区的卫星遥测海表温度（ＳＳＴ）的资料处理，推断在吕宋海峡常年存在黑潮路径弯曲，西折进入ＳＣＳ并又流出ＳＣＳ的现象，作者将黑潮的这种变形称为“黑潮流套”。黑潮变形进入ＳＣＳ的位置在冬季位于海峡中部、南部附近，范围较大；在夏季略向北移，较集中于海峡中部，范围较小。作者认为，黑潮流套现象可用位涡守恒理论作定性的解释：当黑潮在吕宋海峡失去西边界支持后，其流轴以西贴近西边界的一部分流体，因具有较大的相对正涡度，会脱离黑潮主体在南海东北部形成反时针旋转或顺时针旋转的环流，而黑潮主体会以顺时针旋转的形式在海峡以西的海域出现。整个黑潮以弯曲、扩展的形式在海峡处产生形变，在海峡东侧出现反气旋涡旋的倾向。吕宋海峡黑潮流套及南海北部的诱生环流之流型，会因黑潮本身以及副热带环流整体的变化而变化，也与海峡的宽度有关。总之，吕宋海峡黑潮流套的形成是由当地特殊的地形条件和地转β效应这些内部机制决定的，它的常年存在有其必然性     

西北太平洋反气旋涡的Argos浮标观测结果分析

结合卫星高度计异常资料和2003年10月上旬投放在西北太平洋的25个Argos表层漂流浮标资料,分析观测海域的中尺度涡特征及浮标漂移路径上的温度和流速变化,结果表明:(1)7个浮标受强劲的黑潮流影响直接进入台湾岛以东黑潮表层的主流轴;(2)16个浮标在反气旋涡内旋转,并随中尺度涡向西运动,到达黑潮的东边界,由于中尺度涡旋的消亡,浮标脱离其影响后由黑潮带动向东海运动,浮标的移动轨迹呈螺线型;(3)仅有2个浮标在(123°E、20°N)附近通过吕宋海峡进入南海,且41490号浮标受台湾岛西南外海反气旋涡的影响作了2周旋转后再进入南海。比较分析表明,黑潮在冬季应该存在入侵南海的分支,但浮标能否顺利进入南海受多种随机因素控制,如风生流、潮流和波浪等。另外,西北太平洋向西传播的中尺度涡难以越过强劲的黑潮流屏障继续向西传播通过吕宋海峡进入南海。     

南海东北部叶绿素a浓度对台风“风泵”和黑潮共同作用的响应

南海东北部是寡营养海域,夏季浮游植物叶绿素浓度较低,热带气旋“风泵”效应带来的上层海洋扰动可能引起表层浮游植物的显著增长。以往的研究通常关注热带气旋风应力和海洋中尺度涡对上层海洋浮游植物的影响,本文利用航次CTD、实测叶绿素a浓度、Argo温盐剖面和遥感数据,探讨了台风“风泵”和黑潮共同作用下真光层内浮游植物的变化特征及其成因。结果表明,2015年台风“莲花”过境1周后产生向吕宋海峡西北侧南海海域(A区)入侵的黑潮流套,该入侵的黑潮流套使台风前原有的气旋涡消失,抑制了台风产生的上升流对表层(0～40 m)营养盐供给,使次表层(60～90 m)营养盐富集,进而抑制了表层的叶绿素a增长,促进了次表层叶绿素a的增长;吕宋海峡西侧南海海域(B区)表层的浮游植物叶绿素a浓度增加不仅是源于叶绿素最大层浮游植物的向上输运,更是由于浮游植物的繁殖增长;A区台风引起的流套式的黑潮入侵,促进了B区台风后气旋式流场的形成,产生的持续增强的气旋涡为B区表层叶绿素持续增长提供了充足的营养盐供给。     

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。     

黑潮流轴在吕宋海峡的变化分析

黑潮在流经吕宋海峡时呈现各种时间尺度的流态变化。本文基于高分辨率的区域海洋环流模式(ROMS)输出数据,分析了黑潮主流轴在吕宋海峡附近的变化特征和可能原因。研究结果表明,黑潮流轴在该区域具有明显的年际、季节和季节内变化,其中季节内变化最为强烈;在年际和季节时间尺度上,黑潮流轴在表层主要受局地风驱动的艾克曼漂流的影响,而在次表层则主要由黑潮本身的惯性决定;在季节内时间尺度上,黑潮流轴的变化主要受制于涡旋与黑潮的相互作用。     

吕宋海峡水交换季节和年际变化特征的数值模拟研究

利用ROMS(Regional Ocean Modeling System)建立了一套覆盖西北太平洋的涡尺度分辨率环流模型,并对吕宋海峡附近的环流进行了模拟研究。结果表明,吕宋海峡120.75°E断面净流量季节变化显著,全年均为西向输运,6月份达到最小,为0.40×106 m3/s,然后逐渐增大,在12月份达到最大,为6.14×106 m3/s,全年平均流量为3.04×106 m3/s。在500 m以浅,秋、冬季都有明显的黑潮流套存在,并伴有黑潮分支入侵南海,而春、夏季黑潮南海分支减弱或消失,黑潮入侵不明显。在500 m以深,冬、春季,吕宋海峡以东有非常明显的南向流存在,流速约10 cm/s,而到了夏、秋季该南向流出现明显的减弱,黑潮与南海的水交换主要通过吕宋海峡以北的吕宋海沟进行。在垂向结构上,120.75°E断面浅层呈多流核结构,并且流核的位置和强弱受黑潮的季节性变化影响显著,深层流的季节变化不大。在年际尺度方面,吕宋海峡年际体积输运量异常与Niño3.4滞后6个月相关系数达到41.6%,吕宋海峡水交换与ENSO现象有较为显著的正相关关系,并存在2~3 a和准8 a周期的年际变化。     

理想台风驱动的涡旋对吕宋海峡黑潮影响的数值研究*

吕宋海峡以东即北太平洋热带地区常年存在着大量的涡旋,这些涡旋在向西运动的过程中遇到吕宋海峡黑潮后是否会穿越黑潮进入南海值得研究。文章用数值模式来模拟吕宋海峡的黑潮以及吕宋海峡以东的众多涡旋,结果表明没有一个涡旋可以穿越吕宋海峡进入南海。在此基础上引入了一个理想台风风场,通过风应力旋度的形式驱动出强劲的气旋式和反气旋式涡旋,这两个涡旋分别添加在源区黑潮附近,也是在源区黑潮流量最小的8月。以往研究表明,黑潮流量小而涡旋强劲的时候涡旋容易穿越吕宋海峡进入南海,但由何种原因产生的涡旋可以穿越吕宋海峡难以确定;而文章的数值计算结果表明,即使在黑潮较弱的夏季8月,由风应力旋度产生的中尺度涡,无论是气旋式还是反气旋式,都受到了吕宋海峡的阻挡而难以穿越。     

入侵南海的黑潮流套及其脱落涡旋

将2003年、2004年和2005年秋、冬季在吕宋海峡投放的卫星跟踪漂流浮标(Argos)资料用于分析黑潮通过吕宋海峡时的流型。结果表明,秋、冬季黑潮表层流存在3种类型:北向型、西向型和流套-涡旋型,后两种入侵南海。统计分析指出吕宋海峡表层流进入流套-涡旋型路径的概率为0.23,黑潮流套的纬向尺度最大达210km,仅发生在恒春海脊西侧的台湾岛西南海域。黑潮流套仅是黑潮流分离的一部分,而非黑潮整体蛇形入侵南海,这与墨西哥湾的蛇形流不一样。在流套内西向流速大于东向流速,这可能是流套西向发展的原因之一,黑潮流套常可演变成脱落涡旋,也可能就地消亡。脱落涡旋以约10cm/s速度西移。     

根据漂流浮标资料对黑潮15m层流路及流轴特征的分析

利用1979-02—2012-03共33a的水帆位于15m层的Argos漂流浮标资料,绘制黑潮流系15m层的多年年平均和月平均流场,运用特征线方法计算得到黑潮流轴,定义黑潮流动路径的边界为流速大小20~30cm/s的过渡性区域。结果显示:黑潮多年年平均流路大致是一个以(13°30′N,142°00′E)为圆心、2 235km为半径的直角弧段,其在吕宋海峡、台湾东北、九州西南及伊豆海岭附近海区发生气旋式弯曲前先进行反气旋式弯曲调整,弯曲处出现的路径开口主要是支流的并入或分支的流出;黑潮流轴整体性偏向黑潮左边界,其中在吕宋岛东北至台湾以东海域最为显著,在本州岛以南海域次之,而在东海段基本居中;黑潮流路上的流速在总体上由南向北呈增大趋势,但并非沿流路持续性逐渐增加,而是呈现出较平直流段的大流速区和弯曲调整流段的低流速区相互交错的状况,其中四国岛以南至伊豆诸岛以西流段的流速为最大。多年月平均流场显示,2月,5月,8月和11月这4个月份是黑潮流路和流轴发生变化的重要转折期,而1月,4月,7月和10月这4个月份则是各季节的代表月份。其中,冬季月份的黑潮流路和流轴最为曲折,向边缘海发生显著入侵;夏季月份的黑潮流路和流轴最为平直,左侧伴随有北向流动;春、秋两季的过渡性特征则比较明显。     

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.     

基于拉格朗日拟序结构的吕宋海峡表层水交换研究

黑潮通过吕宋海峡入侵南海呈现明显的瞬态特征。以往的研究通常将黑潮在吕宋海峡附近的流态分为几种不同类型。本文基于表层地转流计算得到的有限时间李雅普诺夫指数场（FTLE）,展示了拉格朗日视角下的吕宋海峡上层水交换特征。从FTLE场提取的拉格朗日拟序结构（LCSs）很好地识别了吕宋海峡附近的典型流态和旋涡活动。此外,这些LCSs还揭示了吕宋海峡周围复杂的输运路径和流体域,这些特征得到了卫星跟踪浮标轨迹的验证,且从流速场中是无法直接识别的。FTLE场显示,吕宋海峡附近表层水体的输运形态主要可分为四类。其中,黑潮直接向北流动的“跨越”形态和顺时针旋转的“流套”形态的发生频次明显高于直接进入南海的黑潮分支“渗入”形态和南海水流出至太平洋的“外流”形态。本文还进一步分析了黑潮在吕宋海峡处的涡旋脱落事件,突出强调了LCSs在评估涡旋输运方面的重要性。反气旋涡旋的脱落个例表明,这些涡旋主要源自黑潮“流套”,涡旋脱落之前可有效地俘获黑潮水。LCS所指示的输运通道信息有助于预测最终被反气旋涡所挟卷水体在上游的位置。而在气旋涡的形成过程中,LCS的分布特征表明,大部分气旋涡并未与黑潮水的输运路径相连通。因此,气旋涡对从太平洋到南海的上层水交换的贡献较小。     

Intrusion of the Kuroshio into the South China Sea,in September 2008

Using widespread conductivity–temperature–depth (CTD) data in the Philippine Sea and northern South China Sea near the Luzon Strait together with altimeter data, we identified an intrusion of water from the Kuroshio into the South China Sea (SCS) through the Luzon Strait in September 2008. The Kuroshio water obviously intruded into the SCS from 20 to 21°N, and existed mainly in the upper 300 m. The intrusion water extended as far west as 117°E, then looped around in an anticyclonic eddy and returned to the Philippine Sea further north. The dynamics of the Kuroshio intrusion are discussed using a 1.5-layer nonlinear shallow-water reduced-gravity model. The analysis suggests that the strong cyclonic eddy to the east of the Kuroshio in September 2008 was of benefit to the intrusion event.     

Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan

This study examines the evolution of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are analyzed using a method based on the calculation of the root mean square (rms) difference of the salinity along isopycnals. In combination with analysis of the distribution of the salinity maximum, this method enables water masses in the Kuroshio and the vicinity, to be tracked and distinguished as well as the detection of the areas where water masses are modified. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the right flank of the Kuroshio. In spite of a front dividing the KTW from the South China Sea Tropical Water (SCSTW) on Kuroshio׳s western side, mixing between these two water masses seemingly occurs in the Luzon Strait. These water masses׳ interaction is not evident east of Taiwan. The estimation of eddy diffusivities yields high horizontal diffusivities (K_h~10² m² s⁻¹) all along the Kuroshio path, due to the high current shear along the Kuroshio׳s flanks. The vertical diffusivity approaches 10⁻³ m² s⁻¹, with the highest values in the southern Luzon Strait. Instabilities generated when the Kuroshio encounters the rough topography of this region may enhance both vertical and horizontal diffusivities there.     

Eddy Shedding from the Kuroshio Bend at Luzon Strait

TOPEX/POSEDIENT-ERS satellite altimeter data along with the mean state from the Parallel Ocean Climate Model result have been used to investigate the variation of Kuroshio intrusion and eddy shedding at Luzon Strait during 1992–2001. The Kuroshio penetrates into the South China Sea and forms a bend. The Kuroshio bend varies with time, periodically shedding anticyclonic eddies. Criteria of eddy shedding are identified: 1) When the shedding event occurs, there are usually two centers of high Sea Surface Height (SSH) together with negative geostrophic vorticity in the Kuroshio Bend (KB) area. 2) Between the two centers of high SSH there usually exists positive geostrophic vorticity. These criteria have been used to determine the eddy shedding times and locations. The most frequent eddy shedding intervals are 70, 80 and 90 days. In both the winter and summer monsoon period, the most frequent locations are 119.5°E and 120°E, which means that the seasonal variation of eddy shedding location is unclear.     

Seasonal variation of eddy shedding from the Kuroshio intrusion in the Luzon Strait

Altimeter data and output from the HYbrid Coordinate Ocean Model global assimilation run are used to study the seasonal variation of eddy shedding from the Kuroshio intrusion in the Luzon Strait. The results suggest that most eddy shedding events occur from December through March, and no eddy shedding event occurs in June, September, or October. About a month before eddy shedding, the Kuroshio intrusion extends into the South China Sea and a closed anticyclonic eddy appears inside the Kuroshio loop which then detaches from the Kuroshio intrusion. Anticyclonic eddies detached from December through February move westward at a speed of about 0.1 m s⁻¹ after shedding, whereas eddies detached in other months either stay at the place of origin or move westward at a very slow speed (less than 0.06 m s⁻¹). The HYCOM outputs and QuikSCAT wind data clearly show that the seasonal variation of eddy shedding is influenced by the monsoon winds. A comparison between eddy volume and integrated Ekman transport indicates that, once the integrated Ekman transport exceeds 2 × 10¹² m³ (which roughly corresponds to the volume of an eddy), the Kuroshio intrusion expands and an eddy shedding event occurs within 1 month. We infer that the Ekman drift of the northeasterly monsoon pushes the Kuroshio intrusion into the SCS, creates a net westward transport into the Strait, and leads to an eddy detachment from the Kuroshio.     

A numerical study of the summertime flow around the Luzon Strait

Luzon Strait, a wide channel between Taiwan and Luzon islands, connects the northern South China Sea and the Philippine Sea. The Kuroshio, South China Sea gyre, monsoon and local topography influence circulation in the Luzon Strait area. In addition, the fact that the South China Sea is a fairly isolated basin accounts for why its water property differs markedly from the Kuroshio water east of Luzon. This work applies a numerical model to examine the influence of the difference in the vertical stratification between the South China Sea and Kuroshio waters on the loop current of Kuroshio in the Luzon Strait during summer. According to model results, the loop current’s strength in the strait reduces as the strongly stratified South China Sea water is driven northward by the southwest winds. Numerical results also indicate that Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. The two velocity core structures of Kuroshio can also be observed in eastern Taiwan. Moreover, the water flowing from the South China Sea contributes primarily to the near shore core of Kuroshio.     

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.