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Dynamics of freshwater plumes: observations and numerical modeling of the wind-forced response and alongshore freshwater transport

机译:淡水羽流的动态:风力响应和沿岸淡水运输的观测和数值模拟

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摘要

A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross shore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion.
机译:当河流或河口向大陆架排放水时,通常会形成淡水羽流。淡水羽流是沿海海洋无处不在的特征,通常在沿海水文学中具有显着的特征。本研究结合了水文数据和理想的数值模拟,以检验环境电流和风如何影响羽状水的输送和混合。本文的第一部分使用理想化的数值模拟方法研究了淡水的近岸运输。在没有任何环境水流的情况下,下游沿海水流仅携带排出的淡水的一小部分。其余部分在河口附近以不断增长的“凸起”淡水再循环。在沿海水流中运输的淡水比例取决于河口的水源条件。环境电流的存在增加了羽流中的运输,因此其淡水运输与淡水源匹配。对于与地转沿海电流方向相同的任何环境电流,羽流将演化为稳态宽度。关键结果是,需要外部强迫剂才能使河流排放的全部淡水作为沿海水流运输。论文的下一部分使用缅因州西部海湾地区收集的水文数据,研究了河羽的风向平流。观测结果表明,羽流的跨岸结构随着沿岸风力的波动而显着变化。与Ekman动力学一致,上升的有利风将烟羽散布到近海,有时将其扩展到近海50多公里,而下降的有利风将烟羽宽度缩小到几个Rossby半径。与Ekman理论一致,近地表电流表显示出跨岸电流与沿岸风应力之间的显着相关性。由系泊电流表阵列计算出的沿岸动量方程中各项的估计值也表明了羽状流中的近似埃克曼平衡。沿岸潮流与沿岸风应力之间的显着相关性表明界面阻力可能很重要。本文的最后一部分是在简单的矩形区域内使用三维模型研究在上升气流有利风应力作用下表面被困河羽的平流和混合。模型模拟表明,羽流变稀,并通过跨岸埃克曼运输向近海平流。由于垂直剪切的水平电流,稀薄的羽流易于显着混合。一阶羽流响应由Ekman动力学和Richardson数混合标准解释。

著录项

  • 作者

    Fong Derek Allen;

  • 作者单位
  • 年度 1988
  • 总页数
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类

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