Tidal Mixing Fronts: Stability and Cross-frontal Transport in the Presence of Tides, Topography and Bottom Stress

潮汐混合锋形成在分层水域和被潮汐湍流均化的水域之间的分界线上。因此，它们的发展机制要求底部摩擦和潮汐平流在最低层是重要的。此外，通常会出现倾斜的底部(因为潮汐幅度随水深而变化)。利用海洋观测、模拟模型、简化的线性稳定性模型和实验室对这些锋面进行了研究。目前还没有任何研究将广泛的、现实的参数空间与简单的过程导向相结合来研究潮汐混合锋线性和非线性稳定性，本项目将系统地研究潮汐混合锋稳定性。该方法将使用模型的层次结构，从数值连续分层线性稳定性模型(可包括倾斜底部和底部摩擦)开始。下一步将使用原始方程数值模型来处理类似的有限振幅稳定性问题。在这一阶段，将在现有方法的基础上，对热量和营养物质的跨锋面涡流通量进行量化和参数化。第三个阶段使用更真实的模式，由潮汐强迫，用均匀层结进行初始化，然后发展出潮汐混合锋面(因此需要一个真实的湍流闭合模式)。这个锋面模式将被用来处理实际存在大耗散和潮汐平流时的有限振幅稳定性。同样，不稳定率将与跨锋面涡旋通量一起被量化。根据潮汐强迫和地面加热，这个强迫模式可能会达到统计上稳定的锋面形态。最后，这个第三层模式将被用来(以二维和三维形式)评估四种潜在的跨锋面输送的重要机制(涡旋输送、平均跨锋气流、潮汐切变弥散和风驱动)。营养输送和热量输送的表现很可能不同，因此将分别进行评估。智力价值：该项目的智力价值在于它对潮汐混合锋面稳定性和随后的涡动输送进行了系统的处理。稳定性模型将首次在模拟模型之外处理海底应力、湍流混合和振荡潮汐平流的重要性。通量将被量化和参数化，并将与其他潜在影响(包括风)相关的模拟通量进行比较。了解在给定的背景下哪些传输机制是重要的，将有助于我们更好地利用观测和模拟模型。广泛的影响：这项研究将阐明推动潮汐混合地区相关的高生物生产力的过程，如乔治斯滩。因此，它应该对生物海洋学家有用，并有可能改进海洋预测。此外，该项目将支持一名研究生追求该项目更现实的物理和生物学分支。

Tidal mixing fronts form at the boundary between stratified waters and waters that are homogenized by tidal turbulence. The mechanism for their development thus requires that bottom friction and tidal advection are important at lowest order. In addition, a sloping bottom (since tidal amplitude varies with water depth) will generally be present. These fronts have been studied using ocean observations, simulation models, simplified linear stability models and in the laboratory. There does not appear to be any study in the literature that combines a broad, realistic parameter space with a simple, process orientation to investigate the linear and nonlinear stability of tidal mixing fronts.This project will study tidal mixing frontal stability systematically. The approach will be to use a hierarchy of models, starting with a numerical continuously stratified linear stability model (that can include a sloping bottom and bottom friction). The next step will be to treat a comparable finite-amplitude stability problem using a primitive equation numerical model. At this stage, cross-frontal eddy fluxes of heat and nutrients will be quantified and parameterized, building on existing approaches. The third stage uses a more realistic model, forced by tides, that is initialized with uniform stratification, and then develops a tidal mixing front (and so requires a realistic turbulence closure model). This frontal model will be used to treat finite-amplitude stability in the presence of realistically large dissipation and tidal advection. Again, the instability rates will be quantified, along with cross-frontal eddy fluxes. This forced model may, depending on tidal forcing and surface heating, reach a statistically steady frontal configuration.Finally, this third level of model will be used (in two- and three-dimensional forms) to evaluate four potentially important mechanisms of cross-frontal transport (eddy transport, mean cross- frontal flow, tidal shear dispersion and wind driving). It is very likely that nutrient and heat transport behave differently, and so they will be evaluated separately.Intellectual Merit: The intellectual merit of this project lies in its systematic treatment of tidal mixing frontal stability and consequent eddy-driven transports. The stability models will, for the first time outside of a simulation model, deal with the importance of bottom stresses, turbulent mixing and oscillating tidal advection. The fluxes will be quantified and parameterized, and will be compared with modeled fluxes associated with other potential effects, including winds. Knowing which transport mechanisms are important in a given context will help us make better use of observations and of simulation models.Broader impact: This research will shed light on the processes that drive the high biological productivity associated with tidally mixed areas, such as Georges Bank. As such, it should be useful both to biological oceanographers, and, potentially, for improved ocean prediction. In addition, the project will support a graduate student to pursue the more realistic physical and biological ramifications of this project.