【作者】 荣增瑞；

【导师】 刘玉光； 李明；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2009， 博士

【摘要】 淡水羽(river plume)是淡水从河口入海自然生成的物理现象,研究淡水羽的运动规律和动力机制对于河口附近的泥沙输运与淤积和生态环境的变化趋势具有重要的理论意义和实用价值。淡水从河口入海生成的淡水羽在运动过程中不仅伴随着盐度、温度的水平扩散,因水文环境和海面风的影响,还会产生垂直方向的混合。由于与外海水在盐度、温度及悬浮沉积物浓度差别较大,羽流水会形成性质相近的水团,在水团的交界处形成锋面。影响淡水羽的扩散和混合过程的动力因素具有多样性和多变性的特点。羽流区既受河流径流量变化的影响,如径流的洪、中、枯季的季节变化;又受海洋的动力特征的影响,如潮汐的涨落,大、中、小潮的变化,而且还受风场和陆架环流的影响。这些因素的共同作用使得河口-陆架区的动力过程变得异常复杂,许多羽流区实际上不可能达到一个稳定的状态,因为它们有许多因素在这里相互作用。世界上没有两个河口-陆架羽流系统是相似的,以致人们不知道一些观察到的现象是河口羽流的普遍现象还是个别现象。因此,科学界对羽流动力学的研究一直是方兴未艾。本文从最新发展的区域海洋模式ROMS出发,通过历史资料的诊断分析和模式模拟之间的相互印证,建立了适用于中国近海的高分辨率海洋动力模式。由科学问题出发设计模式实验,主要对认识相对薄弱的潮汐和风影响长江羽流扩散和混合过程的物理机制进行了探讨,揭示了潮流混合影响羽流凸起区和浮力沿岸流的物理机制;还发现潮汐混合会对沿岸流的淡水输送和体积输送产生深远影响,并对此进行了理论探讨;潮汐和风相互作用的涨落潮周期不对称会形成沿岸羽流厚度的半日周期变化,这是对潮汐应变理论的有益补充。潮流一方面可以调制河口向外的淡水输送,另一方面可以产生湍能混合减小羽流水和陆架水之间的密度差异。研究发现:潮流引起的湍能混合变化在大小潮周期上变化显著,小潮向大潮转化时,潮流增大、混合增强;逐渐增强的湍流混合使小潮时形成的海水层化在近岸区域被均匀混合,低盐羽流水团会从河口羽流凸起区(bulge)脱落,该过程周期性发生,这为观测到的长江低盐羽流水的脱落提供了另外一种可能的解释。此外,潮汐会对沿岸羽流动力学产生重要影响:无潮汐混合存在时,浮力沿岸流呈表层局限型,它和底边界无相互作用;考虑潮汐影响后,潮汐混合产生强的垂向动量输送,下溯的沿陆架流可以达到较深的深度,进而在底边界形成Ekman层,底边界Ekman层的离岸输运将驱动盐度锋向外海发展。因此,潮汐作用使得长江口沿岸淡水羽流从近贴岸界的表层局限型向离岸较远的底部输运型转变。更为重要的是,潮汐混合减弱了羽流水和外海水的密度差异,扩展了沿岸流的宽度和深度,潮汐作用改变了沿岸流的淡水输送量以及驻留在羽流凸起区的淡水量。没有潮汐影响时,只有约35%的淡水进入沿岸流向下游输送,这意味着有更多的淡水驻留在羽流凸起区并向上游扩散;有潮汐影响时,约80%的淡水进入沿岸流区,潮汐羽流也趋于稳定。上升流风使得淡水羽向东北方向扩散,由于风生混合的影响,淡水羽盐度在扩散过程中逐渐减弱。研究发现:在没有潮汐强迫时,沿岸淡水羽流以近似定常厚度向外海扩散。有潮汐时,潮汐和风场的相互作用,会产生流场结构的涨落潮不对称。上升流风和潮汐的相互作用在涨潮时会在盐度锋向岸一侧形成辐聚区,使得盐度锋有加深的趋势;落潮时锋区内Ekman平流和潮流同方向,盐度锋厚度变浅。上升流风和潮汐涨落潮周期的不对称使得羽流厚度存在半日周期变化。下降流风时使羽流局限在较窄的范围内并使羽流厚度增加。本文还初步讨论了夏季台湾暖流和季风对长江羽流扩散的影响,模式模拟表明:无季风存在时,长江羽流处于斜压不稳定状态,会形成一系列气旋与反气旋涡;反气旋涡旋可以从河口区和沿岸流区脱落并且向东北方向扩散,间歇性发展的南风可以促进脱落的低盐羽流水的扩散。夏季风的强迫对羽流扩散过程的影响更为明显,夏季风的存在减弱了盐度锋的斜压不稳定特性,羽流水在南风强迫下主要向东北方向扩散进入日本海。以上研究成果主要揭示了潮流、风和陆架环流对长江羽流混合及扩散过程的影响机理,为认识海洋观测结果提供了一定的理论依据。南海海面温度(SST)的变化和ENSO变化密切相关,普遍认为ENSO对南海SST的影响主要是通过大气环流中的“大气桥”。人们早已对南海SST的变化规律展开了广泛而深入的研究。相比之下,对南海海平面变化的研究还比较少,现有的研究也多集中在季节尺度,关于南海海平面年际变化的认识仍非常有限,对于其年际变化的动力学认识更加匮乏。这主要是由于海平面变化不像SST一样更直接受到ENSO的影响,多种动力学过程如海气相互作用、热通量变化、水循环、降水过程等都会影响海平面的时空演变。前人的研究表明SST在ENSO成熟期间和之后会有异常升高,但是SST主要反映的是海表面的热力学过程,关于深层水体在ENSO期间如何变化还需要进一步探讨,以其提高人们对热容海平面和卫星观测实际海平面的认识。本文结合南海海域已有的遥感、常规观测和同化模式资料,包括温度、海平面、海流、降水,建立了各环境要素的相对完备的长时间序列,应用多种统计学方法(例如EOF、小波分析等)分析了各环境要素的低频变异特征和相互关系。并从多个角度对南海年际变化的物理机制进行了探讨,揭示了不同物理过程如热容量、风应力、降水、水循环及环流变化对海平面变化的贡献。研究发现:(1)南海观测海平面和热容海平面均有明显的年际变化特征。南海海平面在El Nino年时异常升高,在La Nina年时异常下降。观测海平面和热容海平面无论在振幅上还是在位相上都有明显的差别,二者之差的年振幅比南海海平面振幅本身的变化还要大,达到63mm,最大值发生在12月份,表明南海和大气、陆地和周边海区之间有明显的水交换。(2)南海SST和海平面对ENSO的响应几乎完全相反,El Nino成熟期过后4个月,海平面达到其最大负异常,但滞后5个月时,SST达到其最大正异常,这个结果和人们的直观认识不同。分析表明SST只表征了上表层的海温特征,而海平面变化和整个水体的热膨胀有关,南海热容海平面主要由南海中下层水体的海温变化控制。(3)海平面在El Nino发展阶段的异常降低主要由于南海和周边海区的水交换和异常的Ekman抽吸造成,异常增强的冷平流和Ekman抽吸一起,使南海热容量降低,南海热容海平面冷却下降。El Nino年时异常减弱的对流活动会减少云量进而减少降水速率。异常的降水速率会对海平面的持续降低产生重要影响,可能会造成El Nino年时海平面异常下降约60mm。更多还原

【Abstract】 River plumes are common features on the continental shelf around the world. They are produced by inflows from a coastal buoyancy source, such as a river or estuary. Plumes of buoyant water emptying into the sea will cause not only the horizontal dispersal of salinity and temperature, but also some vertical mixing process by the effect of the hydrological process and the external forcing. There are remarkable contrasts between plume water and ambient ocean water, such as salinity, temperature and suspending deposit. The plume water has similar property and forms front at the boundary between the plume water and the adjacent ocean water. Many dynamical processes might affect the dispersal and mixing of the river plume, and they are usually characterized by high diversity and variability. The plume region is not only affected by the variation of river runoff, such as the seasonal flood-dry variability of the runoff, but also the ocean dynamical processes, such as the flood-ebb and the spring-neap tidal cycles. Wind forcing and shelf circulation might also play important roles. The interaction of these processes causes the estuary-continental shelf processes to become more complicated. Many plume regions will never reach a steady state, since too many factors are working together on these regions. People don’t know whether the observed phenomena are common features or specific cases, since there are no similar estuary-continental plume systems in the world.In order to understand the plume dynamics, we developed a ROMS model for the Changjiang estuary-East China Sea region. Based on diagnostic analysis of the historic data and the comparisons with model simulations, we have established a high resolution hydrodynamic model for the China Seas. We have conducted many numerical simulations to investigate the little known physical mechanisms of tide and wind effects on the dispersal and mixing processes of the Changjiang River plume. The physical mechanism of tidal mixing affecting the bulge region and the buoyancy coastal current is revealed. We also found that tidal mixing will substantially change the freshwater and volume transport of the buoyancy coastal current and discussed the possible mechanisms. The flood-ebb asymmetry of the interaction between wind and tide generate a semi-diurnal variability of the plume thickness, which provides some new insights to the tidal-straining theory.Tidal currents can affect plume dispersal by modulating estuarine outflow or generating turbulent mixing that reduces the density contrast between the fresh plume water and ambient shelf water. It is found that tidal induced turbulent mixing show large variability over the spring-neap tidal cycle. During the neap tides, the turbulent mixing is relatively weaker and the freshwater plume can spread horizontally and maintains its buoyancy contrast. As the tide transitions from the neap to spring tides, tidal current gets stronger and thus the turbulent mixing. The plume detachment occurs when tidally generated turbulent mixing is strong enough to break down the stratification outside the river mouth, thus causing the disintegration of the river plume into two distinct regions. This might provide another explanation of the observed freshwater patch outside the river mouth. Tide also modifies the structure and dynamics of the buoyancy-driven coastal current on the continental shelf. In the absence of tide, the buoyancy-driven coastal current downstream is a surface-trapped plume attached to the coast. In the presence of tide, however, tidal mixing generates strong vertical momentum flux and leads to the development of an offshore bottom Ekman layer that pushes the plume front away from the coast. Therefore the tide transforms the Changjiang River plume from a surface-trapped plume hugging the coast to a bottom-advected current shifting to an offshore location. More importantly, the tide changes the transport of the buoyancy-driven coastal current and the water accumulation in the bulge region substantially. In the absence of tide, the freshwater transport by the coast current accounts for about 35% of freshwater export from the Changjiang River, suggesting significant freshwater accumulation in the bulge region. In the presence of tide, however, the coastal current carries about 80% of the freshwater export from Changjiang River.In the presence of upwelling favorable wind, the freshwater spreads in the northeastward direction and the density contrast reduces with time due to the wind-induced mixing. It is found that the buoyancy water spread offshore in a quasi-steady uniform thickness in the absence of tide. In the presence of tide, the interaction of wind and tide will produce the current asymmetry over the flood-ebb tidal cycle. The offshore transport is enhanced at ebb but weakened at flood. The flood-ebb asymmetry causes water to accumulate at seaward side of the plume and deepens the front. The plume thickness varies over the flood-ebb tidal cycle, especially under strong wind forcing conditions. Downwelling favorable winds confine the plume to the coast and increase the plume thickness.The effects of Taiwan Warm Current and the summer monsoon wind on the Changjiang River plume have also been investigated. The numerical modeling shows that: in the absence of summer monsoon, the downstream buoyancy current and the upstream Taiwan Warm Current will produce shear-instability at the boundary and forms a chain of anticyclonic and clonic eddies. The anticyclonic eddies will detach from the bulge region and spread in the northeastward direction. The episodic northward winds will spread the freshwater patches further offshore. The summer monsoon will change the dispersion trajectory substantially. In the presence of summer monsoon, the baroclinic instability of the plume front is suppressed; the plume water spreads northeastward and enters into the Japan Sea.The research result outlined above mainly revealed effects of tide, wind and shelf circulation on the dispersal and mixing processes of the Chanjiang River plume through some process-oriented numerical modeling, which provide some theoretical evidence to understand the observations. Sea surface temperature (SST) in the South China Sea (SCS) is closely related to El Ni?o and Southern Oscillation (ENSO). Possible mechanisms have also been widely discussed. The influence of ENSO on the SCS SST is considered to be through the atmospheric bridge of atmospheric circulations. Compared to SST, investigations of sea level in the SCS have mainly been focused on seasonal scale. Until now, our understanding of interannual sea level variability in the SCS is still very poor. In particular, the mechanisms that are responsible for the SCS interannual sea level variations are not clear. SST anomalies are found to be increased during and after the mature phase of El Ni?o. However, since SST data reflects mainly the sea surface thermal phenomena, deep layer water temperature must be investigated in order to enhance our understanding obtained from interpretation of altimeter observations. The main purpose of this study is to examine the interannual variability of the SCS sea level and its relationship with ENSO.We have gathered the available data of the South China Sea, including Sea level observed by altimeter, seawater temperature, ocean circulation data, thermosteric sea level, and tide gauge records and established relatively long time series for each component. Based on multiple statistical methods, including EOF and wavelet coherence, we investigate the interannual variability of these environmental components in the SCS and their relationship with ENSO. We have also investigated the possible physical mechanisms from the perspective of volume transports between the SCS and adjacent oceans, air-sea interaction, and water mass cycle. The main and new results are listed as flows:(1) Both the interannual variations of the observed sea level and the thermosteric sea level are closely related to ENSO. The SCS sea level anomalies are negative during El Ni?o years and positive during La Ni?a years. Both the amplitude and the phase show difference between the observed and thermosteric sea level anomalies. The annual variation of the altimeter observed sea level corrected for thermosteric effect has amplitude of 63mm with a maximum in December. This variation should result from ocean mass exchange between the atmosphere and the continent via precipitation, evaporation and runoff. Since the SCS is not a closed basin, the water mass exchange between the SCS and the adjacent oceans may also play a role.(2) An‘enigma’that the SST and sea level in the SCS have inverse response to ENSO is revealed. While SST reaches its maximum with a lag of five months behind the mature phase of El Ni?o, sea level reaches its minimum with a lag of four months. Such an‘enigma’is revealed by calculating the heat expansion of seawater in the 0～700m layer. The SST is mainly determined by seawater temperature in the surface layer. The sea level is related to the heat expansion of seawater in all the layers. The thermosteric sea level anomalies are dominantly controlled by seawater temperature anomalies of the intermediate layers.(3) The volume transports between the SCS and the adjacent oceans and the anomalous Ekman pumping contribute a lot for the sea level fall in the developing stage of El Ni?o, while the mass exchange, which is dominated by precipitation, plays a more significant role in the following continuous negative sea level anomalies, which will account for about 60mm sea level fall during El Ni?o years.更多还原