Boundary currents over shelf and slope topography

Laboratory experiments are presented from a modelling investigation into th e influence of shelf and slope topography on f-plane surface and intermedia te flows along ocean boundaries. The surface flows are formed from an upstr eam source by the release of fresh water into a rotating tank containing sa lt water, while for the intermediate-water counterpart flows, neutrally-buo yant fluid was released from a submerged source into stably-stratified (two -layer) and quiescent receiving waters in solid body rotation. It is shown that the stability of these buoyancy-driven currents can be described satis factorily by a combination of the dimensionless parameters Bu = N-2/f(2), E k = 2 nu/fD(0)(2) and Ro = U-0/fL(0), where N and f are the buoyancy and Co riolis frequencies respectively, D-0, L-0 and U-0 are the initial depth, wi dth and velocity of the currents, respectively, and nu is the kinematic vis cosity of the fluid. Furthermore, comparison with physical models of surfac e and intermediate flows along a vertical wall and over a flat bottom revea ls that the stability regimes are not significantly altered by the presence of shelf topography. Variation of the depth of the surface flows with resp ect to the total fluid depth above the underlying shelf is shown to have a significant effect on the velocity and density structure of these flows. Wh en the depth and width ratios are small, the surface flow is not affected b y the varying topography. However, when the current occupies a considerable height above the shelf and is at least as wide as the shelf, upper layer f luid is transported offshore through the bottom Ekman layer, where it is ar rested above the sloping bottom. At this location, a deepening of the upper layer develops due to potential vorticity conservation of the lower layer, accompanied by a local alongshore velocity maximum. This shelf break front prevents significant offshore transport of upper layer fluid far beyond th e shelf break, even in cases where the flow is unstable. Comparison of the intermediate currents with dynamically-similar currents above a flat bottom does not reveal a stabilising effect of the slope. For unstable intermedia te currents, offshore transport is not prohibited (as it is shown to be for surface currents over narrow shelves, due to the presence of the slope), a nd large scale instability patterns can extend over great distances from th e slope. It is shown that the geostrophic nature of these currents is destr oyed close to the sloping bottom. Here, the upper and lower density interfa ces, denoting the vertical extents of the intermediate current, tilt sharpl y downwards. (C) 1999 Elsevier Science B.V. All rights reserved.