The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.

中文翻译：

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使用示踪方差衰变来量化哈德逊河河口盐分混合的变化

河口中的盐度结构受时间依赖的混合过程控制。然而，人们对混合现象的发生位置和时间的变异性还没有很好的理解。在这里，我们利用示踪方差方法来证明哈德逊河河口盐分混合的时空结构。我们对潮汐，洋流和盐度进行了为期四个月的水动力模拟，以捕获春季潮汐变化以及风和淡水流动事件。在春季至春季的时间尺度上，盐度变化消散（混合）主要发生在从春季至春季潮汐的过渡期间。在潮汐时间尺度上，盐度差异耗散的60％发生在退潮期间，而40％发生在潮汐期间。在空间上 退潮过程中的混合主要发生在侧向底部盐度前沿在从主通道向相邻浅滩的过渡处与床层相交的地方。在退潮期间，这些侧向锋面形成了沿河口位于多个位置的缩颈向海。在洪水期间，混合是由混合底部边界层顶部水柱中升高的剪切层产生的，沿通道密度梯度的变化局部增强了斜压梯度，从而导致了较强的垂直剪切力和更多的混合。对于潮起潮落，混合发生在强垂直分层和涡流扩散率重叠的位置，而不是在这两个量的最大值处。这种理解为河口盐度结构的空间和时间依赖性提供了新的见解。