Distribution of TKE dissipation and turbulent mixing across at the central Namibian shelf

Shelf areas are hot spots of turbulent mixing in the Ocean. A number of smaller scale processes like breaking internal tides at the shelf edge, generation of nonlinear internal waves, shoaling of swell, and current shear causes enhanced vertical mixing at particular locations across the continental shelf. Most of these processes are not well represented in numerical models, and need an improved parameterization. In frame of the GENUS project a series of field observations were carried out off the central Namibian coast, to gather detailed information on the distribution of turbulent kinetic energy (TKE) dissipation and turbulent mixing across at the Namibian shelf. Hot spots and shadow zones were identified. The Interaction of internal tide with the bottom topography leads to enhanced TKE levels at critical slope angles, mainly located at the shelf edge. Here the bottom mixed layer can reach up to 100m thickness, characterized by high suspended matter concentration. Patches with enhanced TKE dissipation rates of about 10^-8 to 10^-7 Wkg^-1 were observed throughout the water column. At the shelf edge nonlinear internal waves are generated frequently. A statistical analysis of satellite images revealed a region of enhanced NLIW generation near the Walvis Ridge. In contrast to the shelf edge the inner shelf off Namibia depict low TKE dissipation rates outside the boundary layers. Based on time series observations with moored instruments it is shown, that near bottom the eddy viscosity is mainly controlled by the mean currents and the law of the wall. Off the Namibian coast the locations of hot spots and shadow zones of TKE correlate with the distribution of carbon rich surface sediments, which points to a high impact of enhanced TKE near sea bed on resuspension of particulate matter.