Bottom Stress and the Generation of Vertical Vorticity Over the Inner Shelf

在邻近海岸的浅滩和更深的海洋之间的水交换中，小的千米尺度的涡流起着关键作用。这些涡流在很大程度上受海底摩擦力的控制，而摩擦力本身又取决于许多参数，从海底地形的粗糙度到水柱中的密度分布。最近的证据表明，与更复杂的基于物理机制的公式相比，简单、可调的底部应力公式在数值模型中表现得更好。对于不需要通过观测进行调整的更精确的数值模型，需要更好地理解导致底部应力的小尺度物理。这项研究将在海底附近进行详细而广泛的测量，以加深所需的理解。目前的模型描述了海底应力与近底水流、由表面波和内波产生的近底速度、海床组成、河床形态以及由热、盐和悬浮泥沙引起的层化的相关性。然而，最近的区域尺度模拟表明，与基于更简单的阻力定律的模拟相比，这些应力模型的使用降低了区域尺度的洋流模拟。最近的测量和大涡模拟表明，朗缪尔环流可能会渗透到海底，扰乱近底流动和阻力关系，最近的应力测量和粒子图像测速表明，长期存在的模型可能过度预测底部应力对表面重力波的依赖，并过度简化大型波浪形成的沙纹上的近底流动。这项观测研究将结合对底部应力的直接协方差测量和对假设影响底部阻力定律的量的相应测量。它利用了一项单独资助的、同时在同一地点进行的关于内陆架交换和扩散的研究，该研究将包括用高分辨率高频雷达测量表面流、水柱测量海流和层结、测量风强迫和区域尺度模拟。对综合测量和模拟结果的分析将对海底应力的动力学及其在涡度动力学中的作用产生新的见解，涡度动力学支配着千米级的涡旋和穿过内陆架的输送。智力上的优点：这项研究将产生一个全面的数据集，其中包括海底应力的测量和所有假设影响底部阻力定律的量。对数据集的分析将对地应力的力学产生新的见解，对长期存在的模型和尚未纳入地应力模型的朗缪尔环流等过程的作用进行评估，并最终改进地应力模型的物理学。海底应力的测量和分析将为单独资助的同时进行的区域尺度表面速度、水柱水流和层结以及风力强迫的测量和模拟提供重要的新内容，这将有助于对内陆架上垂直涡度的产生和衰减有一个新的理解。广泛影响：这项研究将导致改进海底应力的模型，这将改进对涡旋和其他运动的预测，这些运动控制着从靠近海岸的浅滩到更大的中、外大陆架区域的水团和包括沉积物、幼虫和营养物在内的水媒物质的运动。改进的预测将提高沿海科学家、工程师和规划者解决社会重要应用的能力，包括有害的藻华、污染物的运输和去向以及搜救行动。私人投资机构有在世界卫生组织和美国地质调查局指导以及与沿海管理人员和规划者互动的历史。这项研究将支持WHOI-USGS博士后学者项目的一名新近毕业的博士，并在广泛的沿海海洋学问题、方法和应用方面提供培训和经验。

Small, kilometers-scale eddies play a critical role in the exchange of water between the shallows adjacent to the coast and the deeper ocean. These eddies are largely controlled by the friction at the sea floor, which itself depends on many parameters ranging from the roughness of the bottom topography to the density distribution in the water column. Recent evidence shows that simple, tunable formulations for bottom stress perform better in numerical models than more sophisticated ones based on physical mechanisms. Better understanding of the small-scale physics leading to bottom stress is needed for more accurate numerical models that do not need tuning with observations. This study will make detailed and extensive measurements near the sea floor to develop the needed understanding.Current models describe the dependence of the bottom stress on the near-bottom current, near-bottom velocities produced by surface and internal waves, seabed composition, bedforms, and stratification caused by heat, salt and suspended sediment. However, recent regional-scale simulations suggest that use of these stress models degrades regional-scale simulations of currents relative to those based on simpler drag laws. Recent measurements and large-eddy simulations suggest that Langmuir Circulations might penetrate to the seafloor and disrupt the near-bottom flow and drag relationships, and recent stress measurements and particle-image velocimetry suggest that the longstanding models might over-predict the dependence of the bottom stress on surface gravity waves and over-simplify the near-bottom flow over large wave-formed sand ripples. This observational study will combine direct-covariance measurements of the bottom stress with corresponding measurements of the quantities that have been hypothesized to influence the bottom drag law. It capitalizes on a separately funded, concurrent, co-located study of exchange and dispersion across the inner shelf that will include measurements of surface currents by high-resolution high-frequency radar, water-column measurements of currents and stratification, measurements of wind forcing, and regional-scale simulations. Analysis of the resulting combined measurements and simulations will produce new insights into the dynamics of the bottom stress and its role in the vorticity dynamics that govern kilometer-scale eddies and transport across the inner shelf.Intellectual Merit :This study will produce a comprehensive data set with measurements of the bottom stress and all of the quantities that have been hypothesized to influence the bottom drag law. Analysis of the data set will produce new insights into the mechanics of the bottom stress, an evaluation of longstanding models and the role of processes such as Langmuir Circulations that have not yet been incorporated into bottom stress models, and ultimately an improvement of the physics of bottom stress models. The measurements and analysis of bottom stress will contribute an important new element to the separately funded concurrent regional-scale measurements and simulations of surface velocities, water-column currents and stratification, and wind forcing, which will lead to a new understanding of the generation and damping of vertical vorticity over the inner shelf.Broader Impacts :This research will lead to improved models of the bottom stress, which will improve predictions of eddies and other motions that control the movement of water masses and water-borne substances, including sediments, larvae and nutrients, from shallows adjacent to the coast to the larger mid- and outer shelf regions. Improved predictions will enhance the ability of coastal scientists, engineers and planners to address societally important applications including harmful algal blooms, transport and fate of pollutants, and search and rescue operations. The PIs have a history of mentoring at WHOI and USGS as well as interacting with coastal managers and planners. The study will support a recent doctoral graduate in the WHOI-USGS Postdoctoral Scholar Program and provide training and experience in a wide range of coastal oceanographic problems, methods, and applications.