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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.