Tidal straining, density currents, and stirring in the control of estuarine stratification
Tidal straining, density currents, and stirring in the control of estuarine stratification
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DOI：
10.2307/1351581
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发表时间：
1990-06
期刊：
影响因子：
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通讯作者：
J. Simpson;J. Brown;J. Matthews;G. Allen
中科院分区：
文献类型：
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作者：
J. Simpson;J. Brown;J. Matthews;G. Allen
淡水输入的浮力在河口及邻近沿海水域产生分层作用。预测这种分层的形成和消散，相较于由表面加热引起的类似分层情况，本质上是一个更困难的问题，因为浮力输入源自侧向边界。在此采用的方法中，我们调整了用于表面加热问题的能量考量，以描述淡水的稳定作用与潮汐及风力搅拌所带来的垂直混合之间的竞争。淡水输入导致水平梯度，驱动河口环流，即表层较轻的流体向海流动，而下方较重的流体向陆流动。这种对分层的影响预计会随着潮汐周期中湍流程度的变化而随时间变化。密度梯度也与潮流中的垂直切变直接相互作用，导致对分层产生周期性的输入，在落潮阶段为正。将这种输入与可用的搅拌能量进行比较，得出了一个关于应变诱导分层存在的简单判据。在利物浦湾一个满足该判据的区域进行的观测证实，分层存在强烈的半日变化，在高潮位附近除了小潮时可能会出现更持久的分层外，明显存在完全的垂直混合。基于一个包含应变、搅拌和河口环流的模型对月周期进行的模拟，在定性上与观测的主要特征相符。
Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations.