Scales, Composition, and Oceanographic Forcing of Dunes Beneath Internal Solitary Waves and Tides on the Upper Slope in the South China Sea

Le résumé fourni par la source

Dunes with lengths of hundreds of meters and heights up to 16 m have been discovered on the upper slope in the South China Sea, at a site exposed to forcing by large internal solitary waves and energetic internal tides. Similar dunes occur beneath internal solitary waves elsewhere in the South China Sea and in other ocean basins. Previous work has characterized dune scales, provided evidence for formation by internal solitary waves, and indicated dependence of dune characteristics on the underlying seafloor slope. Here we report an analysis of multibeam measurements, sediment cores, and water column measurements at the dune discovery site. The calculated dune statistics are wavelength, height, skewness, and asymmetry. The results confirm that subtle but statistically significant changes in dune metrics coincide with a change in underlying seafloor slope from supercritical to critical for the predominantly diurnal internal tide. Dune orientations and wave directions coincide, and the mean dune wavelength is nearly equal to the mean wave excursion amplitude. However, the modest variations in dune scales are unrelated or at most weakly related to the scales of the overlying forcing by internal tides and waves. Sediments range in size from clay to gravel and are coarse on dune crests and fine on troughs. Estimates of bottom stress and sediment transport indicate that tides alone cannot move coarse sediments on dune crests, but waves and tides combine to cause offshore sediment transport and dune migration.

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