【作者】 吕新刚；

【导师】 袁业立； 乔方利；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 物理海洋学， 2010， 博士

【摘要】 以国家海洋局第一海洋研究所发展的MASNUM (Laboratory of MArine Science and NUmerical Modeling)海浪-潮流-环流耦合模式为主要工具,结合973环流调查、国家126专项调查、部分908专项调查等数据及卫星遥感资料,通过资料分析、数值模拟和数值试验等手段,研究了黄海、东海沿岸和海南西侧海域上升流的形成机制。着重阐述了上升流的锋生机制和潮运动诱发上升流的机理,丰富了前人“潮致上升流”机制的内涵。提出我国强潮海区夏季上升流的斜压“潮生机制”。研究发现,在斜压海洋环境下,潮混合对于我国沿岸强潮海区的垂向环流(包括上升流)具有深刻影响,甚至是某些海区沿岸上升流的首要诱发因子。该机制与大洋东边界上升流机制不同,其根本原因在于我国陆架海的强潮特性。黄东海作为世界上潮能耗散最强烈的陆架海区之一,其底边界耗散的大量潮能通过向湍动能转化引发强烈的混合,在近岸的陆坡上形成陆架锋(亦称潮汐锋)。锋面的浅水一侧垂向充分混合,在太阳辐射等作用下具有较高的温度;在深水一侧,上层显著层化,深层则以低温高密为特点。所以,陆架锋一旦形成,由于其两侧底层的密度差异甚强,相应的斜压压力梯度会驱动产生锋区次级环流,在锋区的浅水一侧出现明显的上升流分支。该机制尤其适用于潮混合强烈的黄海沿岸、长江口外等海区。受台湾暖流等多种因素影响,浙江沿岸上升流机制更复杂些。利用该机制,本文在海南西侧海区发现了一处夏季上升流区。上述夏季上升流机制与潮混合形成的温度锋相联系,而冬季东海沿岸的盐度锋同样可诱导上升流。温度锋和盐度锋均归结为密度锋,故这两种上升流可统称为锋生上升流。潮运动可通过斜压和正压两种过程诱发沿岸上升流。斜压机制上面已给出。在摩擦等非线性作用下,正压潮流经过复杂地形时的“调整”过程(tidal rectification)产生三维余环流:在黄海和浙江沿岸,表层的水平余流往往表现为强的离岸运动,而底层水平余流则由深水区向岸运动,在沿岸海区诱导出上升运动。在长江口外海,由于缺少海岸的支撑,正压潮致余流通过底部爬坡过程产生上升流。可见,与潮运动相联系的斜压和正压过程均可诱发上升流,潮对于我国沿岸上升流的贡献在过去很可能被低估了。1.长江口外和浙江沿岸的夏季上升流潮、台湾暖流、长江冲淡水是该海区上升流产生的主要驱动力。在长江口外海区,潮混合是诱发上升流的主要因子;与长江冲淡水相联系的斜压过程对上升流也有重要作用,其贡献约占三分之一;台湾暖流对上升流也有影响。在浙江沿岸,潮和台湾暖流均为上升流的重要诱因。底层台湾暖流穿越等深线向岸爬升从而产生上升流;潮运动通过斜压与正压过程产生的上升流均呈带状分布。地形是研究海区上升流形成的重要前提条件。潮汐锋的出现由地形和潮流振幅共同决定,其位置可通过参数log( H U t3)诊断。与大洋东边界流的上升流海区相比,浙江沿岸不存在风生上升流的最佳地形条件。风造成的离岸水体输运有可能通过弱化台湾暖流的爬坡而对上升流造成不利影响。观测得到的浙江沿岸海表低温带一般都不是紧贴海岸的,这与常见的风生埃克曼输运造成的冷水分布特征不同。2.浙江沿岸的冬季上升流冬季,浙江沿岸存在显著的锋面上升流。与夏季不同的是,锋面两侧的密度差异来自盐度而不是温度。冬季随北风南下的长江冲淡水被限制在狭窄的贴岸范围内,在沿岸的低盐水与外海水之间形成显著的盐度锋。盐度锋与夏季潮混合锋诱发上升流的机理是类似的。冬季北风和台湾暖流通过改变盐度锋面的强弱对上升流间接产生影响,这种间接影响与它们各自对上升流的直接作用恰相反。北风是驱动长江冲淡水南下的主要动力,故有利于锋面上升流产生,这与埃克曼输运作用相反;台湾暖流与闽浙沿岸流反向,不利于冲淡水南下,因而对锋面上升流的产生有负面作用,这与海流的爬坡作用相反。3.黄海上升流和夏季表层冷水斑块黄海具有夏季潮汐锋产生的有利条件。在黄海冷水团的周围,模拟得到了水平呈带状分布的锋面上升流区,其位置与诊断得到的潮汐锋面相当吻合。夏季黄海表层存在若干孤立的冷水斑块。两种因素促使冷水斑块的出现:上升流和垂向混合效应。几乎所有黄海冷水斑块都伴随着强至海表的上升流,意味着上升流对表层冷水的形成具有直接贡献;同时,上升流沿底坡将冷水由深层携带至浅层,然后通过混合作用影响到海表。表层冷水斑块的出现具有高度的“地点选择性”,除了苏北浅滩外侧和海州湾以东的冷水以外,其它多出现在半岛顶端的外凸岸线附近。本研究用斜压和正压潮致上升流的观点对此现象做出解释。一方面,在半岛顶端的外凸岸线附近,潮流形成了水平流速的极大值,强混合使得潮汐锋和锋面上升流相应增强;另一方面,正压M2分潮的数值模拟结果表明,多数情况下半岛顶端的垂向余环流是底层向岸、表层离岸的,且在外凸岸线附近形成上升流中心。这可用潮流绕半岛运动的离心力效应来解释(Garrett and Loucks, 1976)。4.海南西(南)侧上升流的发现将在黄东海研究中提出的潮混合诱发上升流的机制应用于北部湾,在海南岛西(南)侧水域发现了一个新的夏季上升流区。该上升流的存在已经得到了卫星遥感和现场调查的部分证实。夏季北部湾盛行偏南季风,相应的埃克曼输运在海南西侧产生的是下降流。研究发现,海南西侧是一个潮流以及潮混合的高值区,在适合的底坡地形上所产生的锋面上升流抵消了风生下降流。在气候平均意义上,海南西侧表现为上升流,并有稳定的海表低温区与之伴随。在静风条件下,上升流约比气候状态增强20%.更多还原

【Abstract】 The coastal upwelling in the Yellow Sea (YS), East China Sea (ECS), and the western Hainan waters is numerically studied by using the MASNUM (MArine Science and NUmerical Modeling) wave-tide-circulation coupled model, in combination with analysis of the field data obtained from the 973 circulation project, national 126 project, and 908 project, and satellite remotely sensed data. On the basis of successful simulation of the general circulation, numerical experiments have been carried out to explore the formation mechanism of the upwelling, with foci on the frontal upwelling mechanism and the role of tidal movements in inducing upwelling. The study provides new understanding for the traditional mechanism of“tidally induced upwelling (TIU)”.A mechanism of TIU in baroclinic mode is proposed in some sea regions with strong tides in China. It is suggested that tidal mixing exerts profound influences on the vertical circulation, as well as coastal upwelling, in some China seas with strong tides, and even serves as the primary inducement for the coastal upwelling. The difference between this mechanism and those responsible for the main eastern boundary upwelling in the world stems from the characteristic of strong tide in China shelf seas. The YS and ECS are among the shelf seas which have the highest tidal energy dissipation rate. The huge tidal energy dissipated in the bottom boundary layer transforms into turbulence, resulting in violent verical mixing which leads to tidal mixing front (TMF) straddling on the sloping bottom in coastal waters. The tidal turbulence is usually strong enough on the shallow side of TMF to homogenize the water from the bottom to the surface, and the whole water column can be fully heated by solar radiation, attaining low density. At the other side of the front, the upper ocean is stratified, and the deep water remains cool and dense. Therefore considerable baroclinic pressure gradient force sets up due to density difference across the TMF, and stimulates a cross-frontal, secondary circulation with apparent upwelling branch occurring usually on the shallow side of the front. Such a mechanism is particularly applicable in the YS, the waters off Yangtze River estuary (YRE) and the western coast of Hainan Island, where strong tidal movement can be found. In the coastal waters off Zhejiang, the upwelling mechanism is somewhat complex because of the relatively weak tidal signals and the influences of the Taiwan Warm Current (TWC). In this dissertation, a summertime upwelling is newly found off the western coast of Hainan by using the baroclinic mechanism of TIU.Similarly, the wintertime salinity front off the ECS coast also induces upwelling. The upwelling, induced by either the TMF in summer or the saliity front in winter, can be classified as“frontal upwelling”.This study identifies two upwelling mechanisms associated with tidal movements, the baroclinic and the barotropic mechanism. Besides the baroclinic processes which have been delineated above, upwelling and downwelling can also be induced by a tidal rectification process when barotropic tidal currents flow over complex bottom topography. The residual tidal currents (RTC), even in barotropic aspects, have three-dimensional structure. In the YS and Zhejiang coastal waters, the surface RTC tends to flow offshore while the bottom one moves inversely, thus upwelling is generated near the coast. Without the support from the land, the barotropic upwelling off YRE is produced by the naural uplifting when bottom current flows over bathymetric bumps.1. The summertime upwelling off YRE and Zhejiang coastsTides, TWC, and Yangtze River diluted water (YRDW) are determained as the main inducements of the upwelling off YRE and Zhejiang coasts.In the local region off YRE, tidal mixing plays the the most important role in inducing upwelling, and the baroclinicity resulted from the YRDW also has direct and important effects with a contribution of about 30% to the local upwelling. In Zhejiang coastal waters, both tidal mixing and the encroachment of TWC onto the continent across isobaths are important causations of the upwelling. The TIU resulted from either baroclinic or barotropic process takes in a shape of belt.The existence of proper topography is a premise for the formation of upwelling, either in the case of TIU or the TWC-induced upwelling. A glimpse of the mathematic expression of the SH index, log( H U t3), reminds one the importance of topography in locating the TMF and, subsequently, the occurrence of frontal upwelling.Experimental results make it necessary to reconsider the role of wind in generating the coastal upwelling off Zhejiang coast. Winds may exert a negative influence on the upwelling by the offshore deflection of TWC, impairing its climbing along the sloping bottom. A tentative argument is proposed here that the Zhejiang coastal region is probably not an ideal sea region for wind-driven upwelling like Oregon or Peruvian coastal waters, and the observed low SST belt off Zhejing usually has a distance away from the land, rather than hugging closely to the coast like most cases in typical wind-driven upwelling area.2. The wintertime upwelling off Zhejiang coastsFrontal upwelling off Zhejiang coast is very clearly shown in numerical modeling in winter. Different from the summer case, it is the horizontal gradiet of salinity, rather than temperature, that leads to the formation of the front. In winter, the YRDW is confined within a narrow band off the coasts of Zhejiang and Fujian when the northerly monsoon is dominant in winter. The salinity front between the low salinity coastal water and open sea is quite strong, and a remarkable baroclinic pressure gradient sets up. Accordingly, upwelling will be driven by the similar mechanism to that associated with TMF in summer.Wind and TWC affect upwelling through altering the intensity of salinity front. The notherly wind plays positive role in enhancing upwelling for its driving the YRDW southward, while the net effects of TWC on upwelling are negative since its direction is opposite to that of the low salinity coastal current.3. The upwelling in Yellow Sea and the surface cold patchesThe numerical model produces systematic upwelling belts around the Yellow Sea Cold Water Mass (YSCWM). The highly coincidence between the upwelling belts and TMF in location shows the intrinsic physics of upwelling. As manifested by numerical experiments, the upwelling is mainly induced by tidal motions.Surface cold patches (SCPs) are often observed scattering around YSCWM, especially off the convex coastlines of peninsula tips. The formation mechanism of the SCPs is ascribed to both upwelling and vertical mixing. Almost all the SCPs are accompanied by upwelling strong enough to reach the sea surface, implying the direct contribution of upwelling to the generation of the SCPs. The cross-frontal ascending movement of deep water cools down the bottom of the water column in the frontal zone, and pure vertical mixing may continue to homogenize water column.Another interesting feature of the SCPs is the site-selectivity, meaning their occurrences off convex coastlines. The tidally induced upwelling, both in baroclinic and barotropic aspect, shed some lights on the physics of such site-selectivity. On the one hand, tidal currents tend to be particularly strong off the peninsula tips, resulting in high vertical mixing and, of course, strong TMF and the resultant frontal upwelling. On the other hand, the“centrifugal upwelling mechanism”suggested by Garrett and Loucks (1976) seems to be valid in the Yellow Sea. Numerical modeling indicates that in most cases, the RTC induced by barotropic M2 constituent flows onshore in the bottom layer and offshore in the upper layer, leading to upwelling concentrations off the convex coastlines.4. The detection of the upwelling off Hainan western (southwestern) coast A summertime upwelling region is newly detected off the western and southwestern coast of Hainan Island by applying the baroclinic TIU mechanism in the Gulf of Tonkin, and the existence of this upwelling has been partially verified by both remotely sensed SST climatology and the latest cruise observation.As a matter of fact, the prevailing wind in the Gulf of Tonkin is downwelling favorable in summer. At the same time, study also shows that the coastal waters off west and southwest Hainan are characterized by the maxima of both tidal current and tidal energy dissipation. The TMF-induced upwelling dominates the wind-driven downwelling in a climatological sense, accompanied by a climatologically stable low SST area. Without wind-driven downwelling, numerical modeling brings about approximately 20% increase of the upwelling.更多还原