Sediment-induced slope convection: Two-dimensional numerical case studies

The potential role of sediment in oceanic slope convection is examined by means of a rotational numerical model applied in a vertical ocean slice. The model couples the hydrodynamics with transport, settling, deposition, and resuspension of fine-grained silty muds. Sediment plumes (turbidity currents), descending on an idealized continental slope with constant bottom slope, are driven from an initial density anomaly caused by an assumed suspension of sediment in shelf water. A number of case studies were conducted in order to understand the effects of (1) different suspended sediment concentrations in shelf water as compared to an equivalent salinity anomaly (salt brine release), (2) different oceanic density stratifications, and (3) resuspension of bed sediment. It is demonstrated that sediment plumes may account for a downslope transport of water, which, once void of its sediment load, becomes lighter than water above. Then, sedimentation along the slope, with a maximum adjacent to the foot of the slope, drives vigorous upward convection (parameterized in the model), stirring slope water over a depth range of several hundred meters. This is in agreement with field observations from a tropical ocean. Detrainment associated with sediment settling constitutes an important mechanism inherent in sediment plumes. It not only induces upward convection but also prevents the rapid increase in plume thickness caused by entrainment as compared to “water mass plumes.” Owing to a balance between entrainment and detrainment, the sediment plume, while descending on the slope, attains constant height and bed shear velocities. In order to facilitate the detection of sediment plumes in (historical or future) field data, we describe their simulated traces in terms of water mass properties and flow anomalies.