Abstract: Observations of nonlinear internal waves at the Namibian shelf



Authors: Volker Mohrholz and Toralf Heene

Recent observations of near bottom currents at the Namibian shelf reveal the occurence of highly nonlineary internal waves, that controll to a unknown extent the distribution of sediments and the vertical transports of oxygen in the near bottom layer. Using satellite images it is shown that surface expressions of internal nonlineary waves are a very common feature at the shelf off southwest Africa. It is hypothesized that these waves are generated by internal tides  at the shelf edge.  The sediment distribution at the shelf off central Namibia depicts an accumulation of carbon rich mud in three bandlike patterns parallel to the bathymatry. This distribution is caused most probably by the interaction of enhanced resuspension at critical slope angles and cross shelf advection.  The power spectrum of current velocities in the internal wave range (N...f) is dominated by the M2 (12.4h), diurnal forced motions (24h) and near inertial waves. Supercritical slopes angles for internal wave reflection were found in water depth of 500 to 100m near the shelf edge and on the shelf. An area that compares well to the satellite and in situ observations of nonlinear internal waves.  In situ observations of an internal bolus on the Namibian shelf are presented and analysed, that were obtained by a high resolution current meter (Mode12 ADCP) and a microstructure shear profiler (MSS).  During the passage of the bolus the bed stress exceeded the critical value of 0.05Pa, necessary for resuspension of sediment. ADCP backscatter data showed strongly increased values after the passage of the bolus, indicating an increased particle concentration in the bottom water.  The backscatter remained at high level for more then ten hours. The TS signature of bottom water  changed with the passage of the bolus, pointing to a net on shelf transport of bottom waters.  Implications of the observed processes for the suitability of benthic habitats and carbon transport from the shelf to the adjacent deep ocean are discussed.