Time-space variability of the near bottom nepheloid layer at the West Sahara continental slope and shelf

Turbidity vertical profiles, being good indicators of sediment concentration in the near bottom oceanic layer, were obtained by Sea Tech Mark nephelometer simultaneously with Neil Brown temperature, salinity, and density profiles. Measurements were carried out at the continental slope and shelf off the Western Sahara at the regular grid (45 miles×41 miles) of one hundred stations separated by five minutes each other along the latitudes and longitudes. Fine structure of the near bottom nepheloid layer (BNL) was revealed at the most stations over the slope. Its origin mainly related to intermittent down slope advection. BNL near the shelf break is more homogeneous owing to the turbulent mixing caused by short period nonlinear internal wave instability. Increasing of BNL, thickness was approximately proportional to depth of the continental rise, accompanying by decreasing of the mean sediment concentration. Well-mixed high energetically near bottom layer (MBL) was found at the several stations, where the bottom slope exceeds the characteristic slope for semidiurnal internal tides. At the shelf an MBL has been revealed in the upwelling zone associated with submesoscale cyclonic eddy