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

中场河流羽流是一个过渡区，在这里，多种地球物理机制作用于约束强惯性喷流，并将其转化为地转沿岸流。近场射流是指河口外流处于超临界状态、平流和切变混合主导动力的区域，河口形状是决定近场羽流结构形状的重要因素，而且水流通常流向近岸。这不同于地转远场河流羽流，地球？S自转、风和背景流主导着羽流的演化，并且主要是沿海岸流动。虽然近场环境水的夹带量很高，但远场羽流往往形成海岸困住的边界流，在没有风和潮汐等其他强迫机制的情况下，夹带量很小。在中场内，必须阻止烟羽的扩散，减少混合，并沿海岸引导气流。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中的进一步混合。锋面过程增强了局部混合，但对羽流核心有未知的影响。这些过程对羽流扩散和混合的贡献预计将在很大程度上取决于羽流的规模。定量地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输运中的动力作用是必要的。这一提议的总体目标是更好地理解在河口附近作业的O(1公里)小尺度过程如何通过O(10公里)的中等尺度向大尺度(O(100公里))浮力驱动的沿岸流转变。中心假设是，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，从而改变了近场和远场羽流之间的过渡。拟议的工作将确定若干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。它将限制羽流最初演化过程中的总水团变化，并有助于理解大陆架规模的沿海洋流是如何形成的。将追求以下三个具体目标：1)量化阻止烟羽扩散的过程，2)将减少的混合率与阻止烟羽扩散联系起来，3)确定近场和中场区域的净混合。该方法将是一种结合现场观测、实验室实验和数值模式实验的方法。这种方法对于准确描述和量化有助于在天然羽流中混合的全部物理过程以及确定如何在广泛的参数空间中修改这些过程是必要的。梅里马克河羽流的详细现场测量将辅之以非旋转和旋转的实验室实验以及实验室和地球物理尺度的数值模型实验。这项工作将填补将淡水排放与大规模沿海海洋环流联系起来的能力的一个重要空白。这项研究的更广泛影响有两个：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片区域的水特性，并为沿海管理人员提供更好的预测能力。它将资助三名研究生，三所参与院校各一名。此外，实地活动不仅将为三名资助的研究生提供独特的学习体验，而且将为其他研究生和本科生提供独特的学习体验。使用一些小船进行天气学研究将需要研究生在个别船上承担首席科学家的责任，这将提供更大规模的物理海洋野外项目所罕见的自主性和责任。

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.