Stratification and mixing on sloping boundaries

In lakes as well as in the ocean vertical transport is, to a large extent, controlled by localized mixing along the boundaries of the respective basin. Direct measurements of turbulent mixing and density stratification in the bottom-boundary layer of a large and deep lake reveal the importance of cross-slope advection near sloping boundaries for local and basin-scale vertical transport. The periodic, internal wave driven, occurrence of up- and downslope directed currents results in periodic heating and cooling of the boundary layer. Cooling of the boundary layer during periods of up-slope flow is associated with unstable stratification and turbulent diffusivities exceed those observed during periods of down-slope flow by almost one order of magnitude. The periodic replacement of water in this layer with water from the interior of the basin maintains vertical density gradients and thus enhances the overall efficiency of the boundary mixing.