长江河口北槽弯道横向次生流、混合与层化

2013年2月25至26日(枯季/大潮)、7月23至24日(洪季/大潮)分别在长江河口北槽弯道沿着3条横向测线CS6、CSW和CS3(每条测线上有北、中、南3个站位)测得水位、流速、盐度和含沙量的时间序列资料。通过这些资料的定量计算、分析,理解弯道横向次生流、混合与层化的时、空变化和各种物理机制及其相对重要性。3条横向测线均存在横向次生流,且横向测线CS3还出现横向次生环流。枯、洪季,仅在横向测线CS6、CS3出现环状欧拉余流。枯、洪季,沿着横向测线CS3,3个站位的横向斜压梯度比离心加速度和科氏加速度都大1~2个数量级,而后两项大小接近且数量级都是10-4,罗斯贝数在1左右。这些表明:横向次生流受横向斜压梯度、离心加速度和科氏加速度共同驱动,前一项相对于后两项更加重要。沿着3条横向测线:1)枯、洪季大潮,平均势能差异分别约为54.23、66.56 J/m3,表明洪季层化强于枯季;2)枯季涨潮,平均的势能差异普遍小于落潮,而洪季涨潮,平均的势能差异普遍大于落潮,表明枯、洪季湍流混合均存在潮汐不对称性;3)枯季,由横向、纵向水深平均应变(ΦS-y、ΦS-x)引起的势能差异变化率的范围分别是-67×10~(-3)~37×10~(-3)、-7×10~(-3)~11×10~(-3)W/m~3,而洪季,相应的范围分别是-45×10~(-3)~30×10~(-3)、-14×10~(-3)~13×10~(-3)W/m~3,表明枯、洪季差异不明显,横向水深平均应变(ΦS-y)均大于纵向水深平均应变(ΦS-x),前项对水体混合与层化的影响更大;4)枯季大潮,纵向平流(ΦA-x)、横向平流(ΦA-y)、纵向水深平均应变(ΦS-x)和横向水深平均应变(ΦS-y)的潮汐平均绝对值占四项总和之比例分别为26%、33%、18%和23%,而洪季大潮,相应的值的比例分别为13%、9%、22%和56%,表明枯季,平流项(ΦA-y最大)对混合与层化的控制可能占主导地位;洪季,应变项(ΦS-y最大)可能占主导地位。无量纲数(m)被用于判别横向平流(ΦA-y)、横向水深-平均应变(ΦS-y)的相对重要性。一个概念性模式被用于显示层化与横向次生流/环流的相互关系。

Lateral secondary flow, mixing, and stratification within the curved channel of the North Passage in the Changjiang River estuary

Time series measurements of water level, current velocity, salinity and suspended sediment concentration were made along three cross-channel lines (CS6, CSW, and CS3; three stations for each line) within the curved channel of the North Passage in the Changjiang River estuary on 25 to 26 February (spring tide/dry season) and 23 to 24 July 2013 (spring tide/wet season), respectively. Quantitative analyses of those data are made to understand the temporal and spatial variability of lateral secondary flow, mixing, and stratification, the various physical mechanisms and the relative importance of each mechanism. Lateral secondary flows are present along three cross-channel lines and lateral secondary circulation is also present along the cross-channel line CS3. Circulating Eulerian residual flows are present along the cross-channel line CS6 in the dry season and along the cross-channel line CS3 in the wet season. Along the cross-channel line CS3 in the dry and wet seasons, lateral baroclinic pressure gradient is larger than both the centrifugal and Coriolis accelerations by the order of 1~2, while the latter two ones are nearly the same with the order of 10-4. The Rossby number is around 1. These suggest that lateral secondary flow is jointly driven by lateral baroclinic pressure gradient together with the centrifugal and Coriolis accelerations, and the former is relatively more significant than the latter two ones. Along three cross-channel lines, tidal mean potential energy anomalies are about 54.23 and 66.56 J/m3 over the spring tide in the dry and wet seasons, respectively. It is suggested that stratification in the wet season is stronger than that in the dry season. Tidal mean potential energy anomaly over the flood tide is generally smaller than that over the ebb tide in the dry season, while tidal mean potential energy anomaly over the flood tide is generally larger than that over the ebb tide in the wet season. It is suggested that asymmetries in turbulent mixing occur in both the dry and wet seasons. Time derivatives of the potential energy anomalies caused by the cross-channel and along-channel depth-mean strainings are approximately in the range of -67×10-3~37×10-3 to -7×10-3~11×10-3 W/m3 in the dry season, and -45×10-3~30×10-3to -14×10-3~13×10-3 W/m3 in the wet season, respectively. Furthermore, the cross-channel depth-mean straining in the dry/wet seasons is larger than the along-channel one, suggesting that the former has a greater impact on lateral mixing and stratification than the latter there. Along three cross-channel lines, the absolute tidal mean values of the along-channel advection (A-x), the cross-channel advection (A-y), the along-channel depth-mean straining (S-x), and the cross-channel depth-mean- straining (S-y) account for 26, 33, 18 and 23 percentages of the sum of the four terms over the spring tide in the dry season, respectively. The absolute tidal mean values of A-x, A-y, S-x and S-y account for 13, 9, 22 and 56 percentages of the sum of the four terms over the spring tide in the wet season, respectively. It is suggested that the advection terms (maximum A-y) may be dominant in controlling mixing and stratification in the dry season, while the straining terms (maximum S-y) may be dominant in the wet season. The dimensionless number (m) is derived to examine the relative importance of the cross-channel advection (A-y) and the cross-channel depth-mean straining (S-y). A conceptual model is proposed to show the relationship between stratification and lateral secondary flow/circulation.