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，它们与更咸的海水混合在一起。这一过程始于河流排放(羽流)的向海边界，并影响河口附近和整个邻近沿海地区的环流和流动。尽管这种河流-海洋界面的区域只是一小片海洋，但它对海洋混合非常重要，在用于预测沿海地区环境条件的环流模型中往往分辨不佳。作为这项调查的一部分，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向海边界结构的可变性，以及那里发生的混合。这些过程将在不同的河流流量和潮汐条件下进行检查。此外，这项研究将评估退潮的河流外流有多少循环通过向海边界，并评估该区域对将总河流流量与近海盐度混合的总体重要性。这项工作的结果将揭示发生在河流-海洋界面上的物理过程的细节，并提高大型模型的预测能力。此外，从这项研究中获得的进展最终将使人们能够更好地预测污染物、营养物和海洋中水团的去向和迁移。在该项目期间，将支持三名研究生为建设科学能力做出贡献。河流羽流的前缘区域是将浮力的海岸排放与周围的海洋水分开的朝向大海的边界。所有尺度的海洋锋面都通过鲜为人知的机制，在几个数量级上充当着重要的能量汇。尽管过去几十年来对羽流动力学的认识有了很大的进步，但在我们对锋面演化的理解以及锋面在确定羽流内部和总体羽流混合收支中的作用方面仍然存在根本差距。这一建议的目的是了解锋面过程在一系列尺度上的演变和意义，从小尺度传播的流量锋到边界落潮脉冲的中尺度陆架锋面，并确定控制锋面从工程尺度演变到地转尺度的机制。我们的假设是，锋面混合在羽流中总体混合中的相对重要性由代表与锋面相互作用的水团的锋面长度尺度与羽流总长度尺度的比率控制，并随着羽流锋面的演变而减弱，这是由于落潮期间羽流锋面与源之间的联系减弱所致。我们假设，连通性减弱的机制既包括通过羽流的正压梯度的减弱，也包括随着羽流变为地转而产生的科里奥利效应。这项工作将使用21世纪羽流前沿和核心的水流、水文学和微结构测量来进行，以使用各种技术以及流体静力和非流体静力真实和理想化的数值模型来评估混合。实地工作的重点是中等规模的梅里马克河和康涅狄格河，这些河流得到了很好的研究，为观测有或没有强潮流地区的锋面演变提供了理想的环境。实地研究的结果将允许校准和验证模型，然后可以用来理解大范围参数空间的类似锋面过程。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.