Collaborative Research: Creation of a coastal current - The transition of an energetic river discharge from buoyant jet to geostrophic plume

中部河流羽流是一个过渡区，在这里，许多地球物理机制约束并将强惯性射流转化为地转海岸流。近场射流是河口流出流处于超临界状态的区域，其中平流和剪切混合在动力学中占主导地位，河口的形状对近场羽流结构的形状起重要作用，并且流动通常指向近海。这与地转远场河羽哪里的地球不同？S旋转、风和背景流主导了羽流的演变，并且主要沿海岸流动。虽然近场环境水的夹带很大，但远场羽流倾向于形成海岸捕获的边界流，在没有风和潮汐等其他强迫机制的情况下，夹带很小。在中部地区，必须阻止羽流扩散，减少混合，并使其沿海岸流动。旋转或背景流可以阻止羽流扩散，防止羽流加速并减少羽流的进一步混合。锋面过程增强了局部的混合，但对羽流核心有未知的影响。这些过程对烟羽扩散和混合的贡献很大程度上取决于烟羽的规模。定量了解这些过程的相对作用，对于确定不同尺度的河流羽流在陆架环流和运输中的动力学作用是必要的。本提案的总体目标是更好地理解在河口附近运行的小尺度O（1公里）过程如何通过O（10公里）的中间尺度转变为大尺度（O（100公里））浮力驱动的沿海流。核心假设是，在中场羽流中运行的一系列过程改变了羽流的扩散和混合，从而改变了近场羽流和远场羽流之间的过渡。拟议的工作将确定一些关键机制在确定近场和中场羽流区域的空间和时间尺度方面的相对影响。这将限制烟柱初始演化过程中总水团的变化，并有助于了解大陆架规模的海岸流是如何形成的。以下三个具体目标将被追求：1)量化阻止羽流扩散的过程，2)将减少的混合率与阻止羽流扩散联系起来，3)确定近场和中场区域的净混合。该方法将是一种集野外观测、实验室实验和数值模型实验于一体的方法。这种方法对于准确描述和量化导致自然羽流混合的全部物理过程以及确定这些过程如何在广泛的参数空间中被修改是必要的。梅里马克河羽流的详细现场测量将辅以非旋转和旋转实验室实验以及实验室和地球物理尺度数值模型实验。这项工作将填补将淡水排放与大尺度沿海海洋环流联系起来的能力方面的一个重要空白。这项研究的广泛影响有两个方面：更好地理解河流羽流在沿海海洋学中的作用，以及对研究生和本科生的教育。这项研究将确定河流流入如何影响受羽流影响的大面积水域的水质，并为沿海管理者提供更好的预测能力。它将资助三名研究生，三所参与机构各一名。此外，实地考察活动不仅将为三位获得资助的研究生提供独特的学习体验，还将为其他研究生和本科生提供独特的学习体验。使用许多小型船只进行天气研究将要求研究生承担单个船只的首席科学家职责，这为更大规模的物理海洋学野外项目提供了罕见的自主权和责任。

The mid-field river plume is a transition area where a multitude of geophysical mechanisms act to constrain and transform a strongly inertial jet into a geostrophic coastal current. The near-field jet is the area over which the estuarine outflow is supercritical, where advection and shear mixing dominate the dynamics, where the shape of the estuary mouth is important in determining the shape of the near-field plume flow structure, and where the flow is often directed offshore. This is distinct from the geostrophic far-field river plume where the earth?s rotation, wind, and background flow dominate the evolution of the plume, and the flow is primarily along-shore. Although entrainment of ambient water is high in the near-field, the far-field plume tends to form a coastally-trapped boundary current where entrainment is quite small in the absence of other forcing mechanisms such as winds and tides. Within the mid-field, plume spreading must be arrested, mixing reduced, and the flow directed along-shore. Rotation or background currents may arrest plume spreading, preventing plume acceleration and reducing further mixing in the plume. Frontal processes enhance mixing locally, but have an unknown effect on the core of the plume. The contribution of each of these processes to the spreading and mixing of the plume is expected to depend strongly on the scale of the plume. A quantitative understanding of the relative role of these processes is necessary for determining the dynamical role of river plumes of different scales on shelf circulation and transport.The overall objective of this proposal is to better understand how small scale processes of O(1 km) operating near an estuary mouth transition to larger-scale (O(100 km)) buoyancy driven coastal currents through the intermediate scales of O(10 km). The central hypothesis is that a suite of processes operating in the mid-field plume modify the plume spreading and mixing and thus the transition between the near- and far-field plumes. The proposed work will establish the relative influence of a number of key mechanisms in setting the spatial and temporal scales of the near- and mid-field plume regions. It will constrain the total water mass modification in the initial evolution of the plume and help understand how continental shelf-scale coastal currents are formed. The following three specific objectives will be pursued:1) Quantify the processes that arrest plume spreading,2) Relate reduced mixing rates to arrested plume spreading and3) Determine net mixing in the near- and mid-field regions.The approach will be one that integrates field observations, laboratory experiments and numerical model experiments. Such an approach is necessary to accurately describe and quantify the full array of physical processes that contribute to mixing in natural plumes and to determine how these processes are modified across a broad parameter space. Detailed field measurements in the Merrimack River plume will be complimented with non-rotating and rotating laboratory experiments and laboratory and geophysical scale numerical model experiments. This work will fill an important gap in the ability to relate fresh water discharge to large scale coastal ocean circulation.The broader impacts of the study are two-fold: better understanding the role of river plumes in coastal oceanography, and education of graduate and undergraduate students. The study will determine how river inflows affect water properties over a large area influenced by plumes and provide better predictive capability to coastal managers. It will fund three graduate students, one at each of the three participating institutions. In addition, the field campaigns will provide a unique learning experience not only for the three funded graduate students, but for additional graduate and undergraduate students. The use of a number of small vessels for synoptic research will require graduate students to assume chief scientist responsibilities on individual vessels, providing a level of autonomy and responsibility rare for larger scale physical oceanographic field programs.