Simulation of density-driven frictional downslope flow in Z-coordinate ocean models

An important component of the ocean's thermohaline circulation is the sinki ng of dense water from continental shelves to abyssal depths. Such downslop e flow is thought to be a consequence of bottom stress retarding the alongs lope flow of density-driven plumes. In this paper the authors explore the p otential for explicitly simulating this simple mechanism in z-coordinate mo dels. A series of experiments are performed using a twin density coordinate model simulation as a standard of comparison. The adiabatic nature of the experiments and the importance of bottom slope make it more likely that the density-coordinate model will faithfully reproduce the solution. The diffi culty of maintaining the density signal as the plume descends the slope is found to be the main impediment to accurate simulation in the z-coordinate model. The results of process experiments suggest that the model solutions will converge when the z-coordinate model has sufficient vertical resolutio n to resolve the bottom viscous layer and horizontal grid spacing equal to its vertical grid spacing divided by the maximum slope. When this criterion is met it is shown that the z-coordinate model converges to an analytical solution for a simple two-dimensional flow.