Abstract Water exchange, featured by bottom inflow and surface outflow in a typical estuary, determines the transport and redistribution of salt, nutrients, pollutants, and suspended sediments and organisms. Water exchange in Chesapeake Bay, the largest estuary in the US, has been extensively studied, yet its long-term interannual variability and its relation with the external forcings are not fully understood. Based on a long-term (1980–2011) 3D numerical simulation, this study examines the water exchanges between Chesapeake Bay and the adjacent coastal shelf, between different regions within the bay, as well as their relationship with river discharge, wind, and residence time. Through an EOF analysis of the bottom inflow and surface outflow at seven selected cross-bay sections, we found that over 90% of the spatiotemporal variations of water exchange can be explained by the first two EOF modes, which are highly correlated with the freshwater discharge and northwesterly wind, respectively. Unlike the outflow that increases linearly with river discharge as commonly expected, the inflow responds non-monotonically to river discharge. The relationship between the river discharge and inflow can be described by a combination of the Michaelis–Menten/Monod equation and a linearly decreasing function, i.e., the inflow initially increases with river discharge due to enhanced gravitational circulation and then levels, and gradually declines due to overwhelming seaward barotropic current. The persistent reflux of surface outflow occurring in the lower-middle bay due to the irregular geometry and rapid shoaling in the channel bathymetry results in the locally enhanced water exchange. The water exchange and the mean residence time can be connected reciprocally through the bay volume, yet the validation of this relationship depends on the timescale to be considered since the residence time looks into the future depending on the forward dynamics to transport the water parcel out of the embayment. A delay effect should be considered when using it to estimate outflow interchangeably with the direct computation of flux.

中文翻译：

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切萨皮克湾水交换及其与外强迫和停留时间的关系

摘要 典型河口以底入、地表出流为特征的水交换，决定了盐分、养分、污染物、悬浮物和生物体的迁移和再分配。美国最大的河口切萨皮克湾的水交换已得到广泛研究，但其长期年际变化及其与外强迫的关系尚未完全了解。本研究基于长期（1980-2011 年）3D 数值模拟，研究了切萨皮克湾与相邻海岸架之间、湾内不同区域之间的水交换，以及它们与河流流量、风和居住的关系时间。通过对七个选定的跨海湾部分的底部流入和表面流出进行 EOF 分析，我们发现 90% 以上的水交换时空变化可以用前两种 EOF 模式解释，它们分别与淡水流量和西北风高度相关。与通常预期的随河流流量线性增加的流出量不同，流入量对河流流量的响应非单调。河流流量与流入量之间的关系可以用Michaelis-Menten/Monod方程和线性递减函数的组合来描述，即流入量最初因重力环流增强而随河流流量增加，然后水平下降，然后逐渐下降。压倒性的向海正压流。由于不规则几何形状和航道水深测量中的快速浅滩，中下海湾发生的地表流出物的持续回流导致局部增强的水交换。水交换和平均停留时间可以通过海湾体积相互联系，但这种关系的验证取决于要考虑的时间尺度，因为停留时间展望未来取决于将水包运出的前向动力学海湾。当使用它来估计流出量与直接计算通量互换时，应考虑延迟效应。然而，这种关系的验证取决于要考虑的时间尺度，因为停留时间展望未来取决于将水包运出海湾的前向动力。当使用它来估计流出量与直接计算通量互换时，应考虑延迟效应。然而，这种关系的验证取决于要考虑的时间尺度，因为停留时间展望未来取决于将水包运出海湾的前向动力。当使用它来估计流出量与直接计算通量互换时，应考虑延迟效应。