Experiments on the generation of internal waves over continental shelf topography

Abstract

Experiments were performed to examine the generation of internal waves by a barotropic tide forcing a continuously stratified fluid over idealized continental shelf/slope topography. A range of responses was observed, including the generation of both internal wave beams and boundary layer boluses, primarily dependent on the values of both the Reynolds number and the topographic steepness parameter. The formation of beams required a critical bottom slope, whilst for bolus formation a large vertical fluid excursion was necessary. A bolus formed when the non-dimensional vertical excursion parameter ΔhN/W0 > 3.2. Here Δh is the vertical excursion, N is the buoyancy frequency and W0 is the near-bottom vertical velocity associated with the local depth-averaged velocity. We simplified the classification of the observed flow regimes using a generation parameter G, defined as the ratio of a Reynolds number to the topographic steepness parameter. The estimated flow regime boundaries were: for G < 3 only a beam was observed, for 3 < G < 50 there was a transitional regime with both a beam and a bolus observed, for 50 < G < 400 there was another transitional regime with no beam but a bolus observed, and finally for the regime with G > 400 there was no bolus observed. We estimated that approximately 4% of the barotropic energy was converted to baroclinic energy when beams were generated.