长江河口北槽的周期性层化对潮流椭圆垂向结构的影响鏯

The Effect of Periodic Stratification on the Vertical Structure of the Tidal Ellipse at the North Passage in the Changjiang River Estuary

基于现场观测流速、盐度资料,通过采用整体理查森计算公式、潮汐调和分析理论,本项研究定量分析了长江河口北槽的周期性层化对M2半日分潮潮流椭圆垂向结构的影响。结果显示:小潮期间层化显著强于大潮,仅小潮出现密度跃层。在向陆端位于导堤和丁坝内的各站位,各层的潮流椭圆均退化为往复流形式;然而,在向海端位于导堤和丁坝外的站位,各层的潮流椭圆均呈顺时针旋转。小潮期间整体理查森数与表、底层潮流椭圆率之差呈现清晰的正线性相关关系,表明层化对潮流椭圆的垂向结构有显著影响。在向海端位于导堤和丁坝外的站位,小潮期间表、底层潮流椭圆倾角差达到约40°,而大潮为10°以下。小潮期间大多数站位的表、底层潮流椭圆相角差达到20°~50°,而大潮为10°以下,小、大潮周期性变化明显。潮流椭圆倾角和相角都在密度跃层处存在明显的垂向突变。在强层化的小潮期间,层化导致的密度跃层处垂向涡动粘性系数的剧烈衰减可能是潮流椭圆椭圆率、倾角和相角垂向变化的主要物理因素。

Based on the field data of current velocities and salinities,by using the equation of the bulk Richardson number and the theory of harmonic tidal analysis,this study quantitatively analyzes the possible effect of stratification on the vertical structure of the M2 semidiurnal tidal ellipse at the North Passage in the Changjiang River estuary.Results show that stratification is much stronger on a neap tide than that on a spring tide.A pycnocline occurs at each station on a neap tide only.The tidal ellipses degenerate to rectilinear motion at each layer of the water column at landward stations within the dike-groyne system,while they rotate clockwise at seaward stations outside the dike-groyne system.The surface-to-bottom ellipticity difference appears to have a clear positive and linear relationship with the bulk Richardson number mainly on a neap tide,indicating the significant effect of stratification on vertical structure of the tidal ellipse.The surface-to-bottom inclination angle difference of the tidal ellipse could reach up to about 40°at a seaward station outside the dike-groyne system on a neap tide but less than 10°on a spring tide.Vertical phase shifts of the tidal ellipses range from 20°to 50°at most stations on a neap tide,while they are less than 10°on a spring tide,indicating significant neap-spring variability.Sudden inclination and vertical phase shift of the tidal ellipses,which account for the most part of the entire vertical variability,occur at the pycnocline.A significant reduction in the eddy viscosity at the pycnocline seems to be the major physical factor responsible for the vertical variability of ellipticity,inclination and phase of the tidal ellipses on a strongly stratified neap tide.