【作者】 周锋；

【导师】 苏纪兰； 黄大吉；

【作者基本信息】 浙江大学 ， 流体力学， 2011， 博士

【摘要】 中国北部近海主要包括渤、黄海以及东海的北部,上述海域均位于中国东部.,是彼此相连且陆架宽阔的中纬度海域。有多种因素影响本海域的季节性层化现象,如海气热通量、潮、水平环流、径流、风场、波浪等。传统研究认为海气热通量和潮主导了本海域的季节性层化,两者的平衡处即为层化的边界,也即潮锋。潮锋的位置能较好地被该理论预测,但是,在局部海域仍有一定的不足。在这些局部海域,其他因素对层化的作用也很重要,譬如径流和水平环流。同时,季节性层化是影响本研究海域生态过程的重要物理过程之一本文基于大量的海上现场观测、遥感资料、并结合区域海洋数值模拟系统(ROMS-Regional Ocean Modeling Systems)及不同的湍流封闭模型,对28°N以北的中国近海的层化特征及影响因素作了较为系统的研究；并通过层化初期的春季水华与夏季河口层化区域的底层缺氧等两个典型案例详细阐述了平流对层化区生态过程的重要影响。主要的工作和获得的认识有：研究并揭示了渤海的层化除了受增温与潮混合控制之外,还受北黄海和辽东半岛西侧深层冷水的平流过程的影响。渤海温跃层下的底层冷水是显著且长期稳定的水文现象,底层冷水分别位于辽东湾内和渤海南部,前者的温度较后者偏低1-3℃左右,从而形成南、北不对称的底层冷水结构。热通量和热容量的区域差异是早期形成双中心冷水不对称结构的主要原因。北黄海和辽东半岛西侧深层冷水的平流作用对北部冷中心的贡献大于对南部冷中心的影响,是维持不对称的双中心冷水结构的主要原因。研究并揭示除了海表增温之外,冬季黄海高温高盐水入侵黄海可增强夏季黄海的层化；潮流的水平扩散使更多的长江冲淡水向江苏北部沿岸和黄海西部扩展,显著地增强了南黄海西部的层化。模拟再现了南黄海中部海槽和渤海海峡东部洼地附近的潮流及其混合偏弱的特征,由此成为黄海深层冷水较为稳定及其上方跃层较强的主要因素之一。模拟表明黄海冷水团周围以气旋式环流为主,是底层冷水相对稳定的另一个因素。发现了南黄海海域春季次表层水华位于跃层附近,揭示了表、底层的平流过程对水华变化规律的影响。南黄海的海槽西侧斜坡和海槽中央的春季层化发展的过程有差异。冬季,在海槽西侧的斜坡海域,底层受黄海高温高盐水北上平流的影响,而表层受低温低盐的沿岸水南下平流的影响,即使春季海表增温之后,层化也相对较弱,因此水华强度较弱且持续时间较短。在海槽中部,平流以弱潮流的周期性运动为主,潮混合较弱,冬季主要是低温高盐水,因此,春季海表增温之后的层化发展较快、较强,因此水华的浓度更高、持续时间更久。分析了长江冲淡水、台湾暖流的平流效应以及海表增温共同导致长江口毗邻海域强层化的特征,揭示了长江冲淡水的走向是影响邻近海域缺氧区空间变化的物理原因。稳定的水体结构使该海域的底层溶解氧消耗后不能得到有效补充,从而成为底层水体缺氧的前提。风场、台湾暖流和径流等因素的年际差异导致了研究海域冲淡水扩展范围的年际变化,由此不仅影响了营养盐和悬浮有机质的分布,还影响盐度层化的分布,成为缺氧区年际变化的物理机制。通过ROMS对典型年份1999年和2006年的比较研究证实了上述机制的作用。本文还通过实际海域的算例,比较了四种湍流封闭模型在不同海域的表现。四种湍流封闭模式反映的层化结构具有不同特点,需要根据实际情况选择合适的湍流封闭模式。更多还原

【Abstract】 The Bohai, Yellow and East China Seas are temperate marginal seas with broad shelves and significant seasonality at mid-latitude in the east of China. The evolution of stratification is complicated and is influenced more or less by factors like the air-sea heat flux, tides, advection, run-offs, wind fields and waves. The historical studies have demonstrated the the balance between the air-sea heat flux and tides play a dominant role to the pattern of the stratification. But it is not satisfied in some local regions like the Bohai Sea and the area adjacent to the Changjiang River Estuary where advection and run-offs are crucial, too. The seasonal stratification is also a critical factor to the ecological processes in the coastal waters.Based upon analysis of massive in situ observations in addition to the use of remote sensing data, a serial of numerical tests with primitive equation model-Regional Ocean Modeling Systems (ROMS) and several turbulence models, the thesis is dedicated to understanding of the evolution and characteristics of the stratification in the China seas north of 28°N, and clarification of the contributions from multi-factors mentioned above and their relative importance. Some typical cases are also analysized to explain the relationship between stratification processes and ecological processes. The main results of the paper are as follows:(1) The bottom cold water below the thermocline is a stable and significant water feature in summer in the Bohai Sea. The cold waters are located separately in the Liaodong Bay (LDB) and south of the Bohai Sea respectively and they are not equal in temperature, for the former is 1-3℃lower than the latter, which is named as the Asymmetric Dual-Core Cold Bottom Water (ADCCBW). ROMS is applied to investigate the mechanism of the formation and maintenance of the ADCCBW during stratification periods with a series of sensitivity tests. The paper has revealed that the ADCCBW is caused initially by the differences of the net solar radiation into the sea and the different water depth (thus heat content). The long-time endurence of the ADCCBW is mainly due to the inflow of the near bottom water from the deep channel of the northern Yellow Sea. The temperature is relatively high in the central bight of the Bohai Sea between the ADCCBW due to the relative strong tidal mixing and shallow depth. The spatial disparity of stratification yields several density circulation gyres. On the vertical average, the density circulation in the Bohai Sea in summer is much more stronger than tides and wind induced residual flows. (2)The paper has simulated the enhancement of the extension of the Changjiang Diluted Water (CDW) along the Jiangsu Shoal and in the southwestern Yellow Sea induced by the tidal diffusion during stratification periods. The pycnocline is stronger in this area influenced by the CDW extension and air-sea heat flux. The cold bottom water, referred to as the Yellow Sea Cold Water Mass (YSCWM) by historic literatures, is thus a distinct and long-term endured feature in the north and south portion of the Yellow Sea trough where tidal movements and mixing are weak. The near surface is mainly occupied by a large-scale cyclonic circulation above the YSCWM and embedded with a couple of small-scale eddies. While the central YSCWM is nearly at rest, the cold water south of 34.5°N is much easier to escape and reach the vicinity of the Cheju Island.(3)The paper has investigated the spring phytoplankton bloom occurrence in the vicinity of the central Yellow Sea when weak stratification just begins. There are two sites of blooms with different types of species of phytoplankton, which occurred in succession in the western flank and central part of the Yellow Sea trough respectively. Both of these blooms appear as subsurface Chlorophyll a maximums (SCM). The water mass and hydrodynamic environments at these two sites are different which is in favor of the growth of different phytoplankton species and in turn cause different magnitude of blooms (biomass). The bloom at the western flank the trouch is dominated by the northwestward warm and saline water at the bottom, in the meanwhile is under the prevailing cold and fresh southward coastal current at surface. It is relative weak and terminated finally by the advection. The bloom in the central trough is stronger and of long duration where is favored by the weak and periodical tidal motions and stronger stratification.(4)The paper has demonstrated a tight relationship between the stratification and the low dissolved oxygen (DO) conditions in the near bottom water off the Changjiang River Estuary. The strong pycnocline is contributed by the solar radiation, fresh water from the Changjiang River as well as the dense bottom water from the Taiwan Warm Current (TWC). The intense stratification forms a barrier and makes it difficult for the exchange of DO in the vertical direction, which finally cause a remarkable DO deficiency (hypoxia). Furthermore, forcings like wind fields, the TWC and the river runoff are believed to have significant impacts on the hypoxia distribution through the advection and diffusion processes, which influence the distribution of nutrients and dissolved organic matter in addition to the stratification. The case study with in situ observations analysis and numerical model results on the disparity of summer hypoxic zone between 1999 and 2006 presents detailed evidence to support the above hypothesis.(5)The paper has also evaluated the performance of foure different types of turbulence closure schemes in several realistic situations. The comparative studies suggest that their performance differ in different stratification conditions and should be selected carefully.更多还原