Laboratory observations of enhanced entrainment in dense overflows in the presence of submarine canyons and ridges

The continental slopes in the oceans are often covered by small-scale topographic features such as submarine canyons and ridges. When dense plumes, flowing geostrophically along the slope, encounter such features they may be steered downslope inside and alongside the topography. A set of laboratory experiments was conducted at the rotating Coriolis platform to investigate the effect of small-scale topography on plume mixing. A dense water source was placed on top of a slope, and experiments were repeated with three topographies: a smooth slope, a slope with a ridge, and a slope with a canyon. Three flow regimes were studied: laminar, waves, and eddies. When a ridge or a canyon were present on the slope, the dense plume was steered downslope and instabilities developed along the ridge and canyon wall. This happened regardless of the flow characteristics on the smooth slope. Froude and Reynolds numbers were estimated, and were found to be higher for the topographically steered flow than for flow on smooth topography. The stratification in the collecting basin was monitored and the mixing inferred. The total mixing and the entrainment rate increased when a ridge or a canyon were present. The difference in mixing levels between the regimes was smaller when topography was present, indicating that it was the small-scale topography and not the large-scale characteristics of the flow that determined the properties of the product water.