The Dynamics of Uniformly Stratified Flow over Sills, with Application to Coastal Oceanic Flows

The density stratification in the upper ocean is (generally) continuous, and apart from the near surface region, is approximately uniform. The dynamics of (nearly) uniformly stratified flow over topography present different dynamical features when compared with flows where the stratification is described as a small number of homogenous layers. In particular, for subcritical flow upstream, over sufficiently high topography uniformly stratified flows are not able to maintain the condition of critical flow at the highest point, in most cases. Instead, the flow is supercritical there, and an hydraulic drop (to subcritical flow) occurs at a location further downstream. Upstream blocking of low-level fluid can occur under these conditions. Here the nature of such flows is described for a range of parameters: flow speed, density stratification and topographic height. The dynamical consequences are described, and some coastal oceanographic locations where such flows may occur are identified.