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