Lerczak James A, Geyer W Rockwell, Ralston David K
College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon.
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts.
J Phys Oceanogr. 2009 Apr;39:915-933. doi: 10.1175/2008JPO3933.1. Epub 2009 Apr 1.
The temporal response of the length of a partially-mixed estuary to changes in freshwater discharge, , and tidal amplitude, , is studied using a 108 day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 year) record of , , and near-surface salinity. When was moderately high, the tidally-averaged length of the estuary, , here defined as the distance from the mouth to the up-estuary location where the vertically-averaged salinity is five psu, fluctuated by more than 47 km over the spring-neap cycle, ranging from 28 km to >75 km. During low flow periods, varied very little over the spring-neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in and relative to average discharge, , and tidal amplitude, , and predicts the linear response time scale, τ, and the steady-state length, , for average forcing. Two vertical mixing schemes are considered, in which a) mixing is proportional to and b) dependence of mixing on stratification is also parameterized. Based on least-squares fits between and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge ( = 750 ms, τ = 4.2 days) to a period of low discharge ( = 170 ms, τ = 40.4 days). Over the range of observed discharge, ∝ , consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated τ was proportional to the discharge advection time scale (/, where is the cross-sectional area of the estuary). However, τ was three to four times larger than the theoretical prediction. The model with stratification dependent mixing predicted variations in with higher skill than the model with mixing proportional to . This model provides insight into the time dependent response of a partially-stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring-neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel and because the underlying dynamics are nonlinear and the forcing, and , can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over timescales comparable to or shorter than the response timescale of the estuary.
利用2004年春夏沿哈得逊河河口全长收集的108天时间序列以及关于淡水流量、潮汐振幅和近地表盐度的长期（13.4年）记录，研究了部分混合型河口长度对淡水流量（ ）和潮汐振幅（ ）变化的时间响应。当淡水流量适中偏高时，河口的潮汐平均长度（ ，此处定义为从河口到垂直平均盐度为5个实用盐度单位的河口上游位置的距离）在大潮小潮周期内波动超过47公里，范围从28公里到大于75公里。在低流量时期， 在大潮小潮周期内变化很小，并接近一个稳定长度。对这种响应进行了量化，并与从全球河口盐平衡推导出来的线性化模型的预测结果进行了比较。该模型由相对于平均流量（ ）和潮汐振幅（ ）的 和 的波动驱动，并预测了平均强迫情况下的线性响应时间尺度（τ）和稳态长度（ ）。考虑了两种垂直混合方案，其中a）混合与 成正比，b）混合对分层的依赖性也进行了参数化。基于 与模型预测的河口长度之间的最小二乘拟合，估计的τ从高平均流量时期（ = 750毫秒，τ = 4.2天）到低流量时期（ = 170毫秒，τ = 40.4天）变化了一个数量级。在观测到的流量范围内， ∝ ，这与陆地盐通量由垂直河口交换环流驱动的河口的理论尺度一致。估计的τ与流量平流时间尺度（ / ，其中 是河口的横截面积）成正比。然而，τ比理论预测值大三到四倍。具有分层依赖混合的模型预测 的变化时比混合与 成正比的模型具有更高的技巧性。该模型深入了解了部分分层河口对强迫变化的时间依赖性响应，并解释了大潮小潮响应幅度对淡水流量的强烈依赖性。然而，线性模型的效用是有限的，因为它假设渠道是均匀的，并且由于潜在的动力学是非线性的，而且强迫因素 和 可能会经历大幅度变化。特别是河流流量在与河口响应时间尺度相当或更短的时间尺度上可能会变化一个数量级以上。
