Processes influencing the circulation and bottom mixing across the shelf slope: the importance of the slope topography and the coast

Abstract

A series of comparisons of across slope current, and growth of a well mixed bottom boundary layer, on a shelf slope using both a single point model and a cross shelf model, with and without a coastal boundary, are performed. The across slope flow is induced by either an upwelling favourable or downwelling favourable along slope flow. Calculations with the single point model show that for a given buoyancy frequency the bottom boundary layer thickness changes with time due to upwelling or downwelling across slope flows produced by near bed shear in the along slope flow. In the single point model, the assumption is made that the flow above the bottom boundary layer is independent of time and not influenced by the boundary layer. In the case of a shelf slope, where the steepness of the slope varies from ocean to shelf, then variations in the flow within the bottom boundary layer due to changes in shelf steepness, lead to regions of convergence and divergence up the slope giving rise to a circulation above the bottom boundary layer. This circulation varies with time and horizontal position. The calculations with the cross slope model show that the assumption in the single point model, that the flow outside the boundary layer is constant, is no longer valid after a few days. Also, the thickness of the bottom boundary layer, the across slope flow and hence cross shelf transport are influenced by the presence or absence of a coastline. The inclusion or omission of the coastline has a significant influence, through the associate free surface elevation gradient, and internal pressure gradients, upon the flow both within and above the bottom boundary layer.