Collaborative Research: Mixing of river water into the coastal ocean and the role and structure of the outer edge of the discharge

当河水进入沿海海洋时，它们与较咸的海水混合。这一过程始于河流排放（羽流）的向海边界，它影响河口附近和整个邻近沿海地区的循环和流量。虽然这种河海界面发生的区域只占海洋的一小部分，但它对海洋混合非常重要，并且在用于预测沿海地区环境条件的环流模式中往往无法很好地解决。作为调查的一部分，研究人员将进行实地测量，并利用数值模型来确定淡水排放的向海边界结构的可变性，以及在那里发生的混合。这些过程将在不同的河流流量和潮汐条件下进行检验。此外，本研究将评估有多少退潮的河流流出物通过向海边界循环，并评估该区域对将河流总流量混合到近海盐度的总体重要性。这项工作的结果将揭示在河流-海洋界面发生的物理过程的细节，并提高大规模模型的预测能力。此外，从这项研究中获得的进展将最终使我们能够更好地预测海洋中污染物、营养物质和水团的命运和运输。在此项目期间，将支持三名研究生为科学能力建设做出贡献。河流羽流的锋面区是向海的边界，它将沿海浮力排放物与周围的海水分开。各种尺度的海洋锋面通过人们知之甚少的机制，在几个数量级上起到了重要的能量吸收作用。尽管在过去的几十年里，人们在羽流动力学方面做出了巨大的努力，但我们对锋面演化的理解仍然存在根本性的差距，以及锋面在建立羽流内部动力学和整体羽流混合预算中的作用。本研究的目的是了解锋面过程在不同尺度上的演变及其意义，从小规模的传播流锋面到中等尺度的陆架锋面，并确定控制锋面从工程尺度到地转尺度演变的机制。我们的假设是，锋面混合对羽流整体混合的相对重要性是由代表水团与锋面相互作用的锋面长度尺度与羽流整体长度尺度的比值控制的，并且随着羽流锋面的演变而减小，这是由于通过退潮减少了羽流锋面与源之间的连连性。我们假设连通性减弱的机制包括通过羽流的正压梯度减弱以及当羽流变为地转时的科里奥利效应。这项工作将使用21世纪的水流、水文和羽流前沿和核心的微观结构测量来评估混合，使用各种技术，以及流体静力学和非流体静力学的现实和理想数值模型。野外工作的重点是中等大小的梅里马克河和康涅狄格河，它们被很好地研究过，并提供了一个理想的环境，可以在有或没有强潮流的地区观察锋面演变。实地研究的结果将允许对模型进行校准和验证，然后可以使用这些模型来了解在广泛的参数空间中类似的锋面过程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

As river waters enter the coastal ocean they mix with the saltier sea water. This process starts at the seaward boundary of the river discharge (plume) and it affects circulation and flow near the river mouth and throughout the adjacent coastal region. Although the area where this river-ocean interface occurs represents a small region of the ocean, it is very important to ocean mixing and is often poorly resolved in circulation models used to predict environmental conditions in coastal regions. As part of this investigation the researchers will carry out field measurements and utilize numerical models to define variability in the structure of the seaward boundary of the freshwater discharge, and the mixing that occurs there. These processes will be examined for different river discharge and tidal conditions. In addition, this study will assess how much of an ebbing riverine outflow is cycled through the seaward boundary and evaluate the overall importance of this region to mixing the total river discharge to a near-ocean salinity. The outcomes from this work will reveal the details of the physical processes taking place at the riverine-ocean interface and improve the predictive capabilities of large-scale models. In addition, the advances gained from this study will ultimately allow better prediction of the fate and transport of pollutants, nutrients and water masses in the ocean. During the period of this project three graduate student will supported contributing to building scientific capacity.The frontal region of a river plume is the seaward boundary that separates a buoyant coastal discharge from ambient ocean water. Oceanic fronts of all scales act as significant energy sinks over several orders of magnitude through mechanisms that are poorly understood. Although significant efforts have advanced knowledge of plume dynamics over the last several decades, fundamental gaps still exist in our understanding of frontal evolution, and the role of the front in establishing the dynamics in the interior of the plume and in the overall plume mixing budget. The objective of this proposal is to understand the evolution and significance of frontal processes across a range of scales, from small scale propagating discharge fronts to intermediate scale shelf fronts bounding an ebb pulse, and to identify the mechanisms controlling the evolution of a front from engineering scales to geostrophic scales. Our hypothesis is that the relative importance of frontal mixing to overall mixing in the plume is controlled by the ratio of a frontal length scale representing water mass interaction with the front, to the overall plume length scale, and diminishes as the plume front evolves, due to diminishing connectivity between the plume front and the source through the ebb tide. We hypothesize that mechanisms of diminishing connectivity include both weakening of barotropic gradients through the plume as well as Coriolis effects as the plume becomes geostrophic. This work will be pursued using 21st century current, hydrography and microstructure measurements in the plume front and core to assess mixing using various techniques, as well as hydrostatic and non-hydrostatic realistic and idealized numerical models. The field effort is focused on the mid-sized Merrimack and Connecticut rivers, which are well studied and provide an ideal setting to observe frontal evolution in regions with and without strong tidal currents. Results from the field study will allow calibration and validation of models, which can then be used to understand similar frontal processes across a wide swath of parameter space.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.