Laboratory modelling experiments on the flow generated by the tidal motion of a stratified ocean over a continental shelf

Abstract

Laboratory model experiments have been conducted to investigate the interaction of a tidally forced stratified flow with a continental shelf. Results collected from a wide range of parametric experiments covering sub-, critical and super-critical tidal forcing are presented to show that the resulting flow regimes can be classified conveniently by the dimensionless parameters Fr= U sb / c w and μ= ω 2/ N 0 2 α, where U sb is the maximum velocity at the shelf break of slope angle α, c w is the long-wave speed of the lowest internal mode, ω is the frequency of the forcing and N 0 is the buoyancy frequency of the initial undisturbed flow. Three distinct and characteristic flow regimes are observed, namely (i) shelf break eddies, (ii) intense downslope jets associated with offshore filamentary transport and (iii) pure up- and down-slope oscillatory jet flow, the occurrence of each of which is well-determined within Fr: μ parameter space. Measurements show a monotonic increase in eddy size (normalised by the amplitude A of the tidal forcing) with increasing Fr, while the thickness of the up- and downslope jet is shown to scale with A and to have no significant dependence on Fr. Measurements of the perturbation density field show significant intra-cycle variability in the local mixing and overturning parameters such as the buoyancy anomaly g′( z i ), the available potential energy function ξ and the r.m.s. density ρ r. m. s. , with evidence of weak mixing and overturning (but significant internal wave activity) at slack tide conditions. Maximum values of both ξ and ρ r. m. s. within a cycle show monotonic increases with increasing Fr and a strong dependence upon α and offshore location. Overall qualitative agreement with aspects of related numerical model results of Xing and Davies (J. Phys. Oceanogr. 27 (1997) 2100) and Holloway and Barnes (Cont. Shelf Res. 18 (1998) 31) is shown to be encouraging.