Tidal Mixing Sustains a Bottom-Trapped River Plume and Buoyant Coastal Current on an Energetic Continental Shelf
Tidal Mixing Sustains a Bottom-Trapped River Plume and Buoyant Coastal Current on an Energetic Continental Shelf
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潮汐混合维持了充满活力的大陆架上的底部捕获的河流羽流和浮力海岸流
DOI：
10.1029/2018jc014105
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发表时间：
2018
影响因子：
3.6
通讯作者：
Wu Hui
中科院分区：
文献类型：
--
作者：
Wu Tianning;Wu Hui
传统的wisdom河羽动力学表明，下架浮力沿岸流最终将被困在一个特定的深度，即捕获深度，限制河流流出和近海底部埃克曼运输。从理论上讲，一个先决条件下的陆架流是必要的，以形成一个稳定的底部被困的河流羽。本文通过对浙闽沿岸流的模拟研究，提出了一种替代方案。来自长江的浮力水是驱动ZMCC的主要因素，这在底部捕获的河流羽流中很常见;然而，捕获深度更多地取决于潮汐混合。当羽水来到倾斜的地形，强大的潮汐混合诱导混合前，向岸的底部埃克曼层占据了整个水柱。这样的潮汐诱导锋维持了一个下陆架锋流，由于热成风平衡，该锋流在表层和由于潮汐整流，在底边界层顶部都得到了加强。直接的风引起的传输只覆盖了ZMCC的一小部分;然而，它重新分配羽流水，从而影响沿岸流。潮汐引起的锋面捕获深度在季节之间的变化比以前的羽流理论预测的要小得多。相反，它在春季-小潮周期中波动很大。即使在夏季上升风盛行时，混合锋仍然维持着下陆架沿岸流。强烈的潮汐混合存在于许多沿海沃茨中，这可能是形成底部捕获的河流羽流及其相关的浮力沿岸流的另一种机制。大型河流，如亚马逊河、密西西比河和长江，向接收水体输出大量的陆地物质。通常情况下，较轻的河流淡水与密度较大的周围海水混合，然后沿着沿着（在北方半球）传播，这被称为浮力海岸流。河流引起的大型浮力沿岸流可以传播数百公里。它经常导致有害的藻类大量繁殖、缺氧和其他环境问题。因此，了解海岸浮力流的形成机制是非常重要的。传统的沿岸浮力流理论是非常成功的，但是，它们很少考虑潮汐混合的影响，而潮汐混合在沿岸沃茨中普遍存在。本文以长江沿岸浮力流为例，通过一系列精心设计的数值试验，对这一问题进行了研究。结果表明，在不利的风条件下，潮汐混合对维持稳定的沿岸浮力流起着至关重要的作用。没有潮混合，稳定的沿岸浮力流就难以存在。本研究的结果有助于加深对海岸动力过程及其他相关过程的认识。
Conventional wisdoms on river plume dynamics suggest that a down-shelf buoyant coastal current will ultimately be trapped at a specific depth, that is, the trapping depth, as constrained by riverine outflow and offshore bottom Ekman transport. Theoretically, a prerequisite down-shelf current is necessary to form a stable bottom-trapped river plume. In this study an alternative is described by carrying out a modeling study on the Zhe-Min Coastal Current (ZMCC). Buoyant water from the Changjiang River is a major factor driving the ZMCC, as is common in bottom-trapped river plumes; however, the trapping depth is more determined by tidal mixing. When the plumewater comes to the sloping topography, strong tidal mixing induces a mixing front, shoreward of which the bottom Ekman layer occupies the entire water column. Such a tidal-induced front maintains a down-shelf frontal current, which is intensified both at the surface due to the thermal wind balance and on the top of bottom boundary layer due to the tidal rectification. Direct wind-induced transport only covers a small fraction of the ZMCC; however, it redistributes the plume water and, thus, affects the coastal current. The tide-induced frontal trapping depth varies much less between seasons than that predicted by previous plume theories. Instead, it fluctuates strongly in the spring-neap cycle. Even in summer when upwelling-favorable winds prevail, the mixing front still sustains a down-shelf coastal current. Intense tidal mixing exists in many coastal waters, which might be an alternative mechanism in forming bottom-trapped river plumes and their associated buoyant coastal current. Plain Language Summary Large rivers, such as the Amazon, Mississippi, and Changjiang, export a huge amount of terrestrial materials to the receiving waterbodies. Typically, the lighter riverine freshwater mixes with the denser ambient seawater and then propagates rightward along the coast (in the Northern Hemisphere), which is known as the buoyant coastal current. Large river-induced buoyant coastal current can propagate hundreds of kilometers. It frequently causes harmful algal blooms, hypoxia, and other environmental problems. Therefore, understanding the formation mechanisms of buoyant coastal currents is very important. Conventional theories on buoyant coastal currents are very successful; however, they rarely consider the effect of tidal mixing, which exists ubiquitously in coastal waters. Here in this study, we investigated this issue by using the Changjiang River-induced buoyant coastal current as an example, through a series of well-designed numerical experiments. The results showed that tidal mixing plays an essential role in maintaining a stable buoyant coastal current under unfavorable wind conditions. Without the tidal mixing, the stable buoyant coastal current can hardly exist. The finding of this study can promote our understandings on the coastal dynamic processes and other relevant processes.