Evidence of sediment resuspension by nonlinear internal waves on the western Portuguese mid-shelf

Abstract

In-situ observations of nonlinear internal waves (NIWs) propagating over the northern shelf of Portugal, near the Nazaré submarine canyon (39°47.4′N/009°11.4′W), and evidence of their action on the bottom, are presented and discussed. Synthetic Aperture Radar (SAR) images from this margin suggest the generation of NIWs over the canyon head (39°35′N/009°25′W) and the subsequent propagation directly to the mid-shelf. An observational program was conducted in this area in the summer of 2004, between August 29th and September 1st, to study these short-period internal waves. Temperature and current high resolution measurements were made with thermistor chains and acoustic Doppler current profilers (ADCPs), moored along the canyon generated NIW track, at the 82 m isobath. A simultaneous ENVISAT SAR image was obtained during the experiment. These observations were complemented by a CTD survey and bottom sediment sampling. Collected data show the propagation of large amplitude internal solitons forcing strong bottom current pulses. Turbidity profiles and ADCP echo variability suggest the remobilization of the mid-shelf bottom sediments induced by these short-period events. Calculations of bottom shear stress, induced by the observed canyon generated NIWs, reveal an energetic internal wave activity that can complement, during summer, the role of winter swell in sediment resuspension over the mid-shelf. This work is a contribution to the European project EUROSTRATAFORM, whose objectives include the study of the specific canyon systems dynamics and related sedimentary impacts on the European continental margin.