Observations of boundary layer dynamics beneath nonlinear internal waves of depression

Andrew Zulberti, University of Western Australia, Perth, WA, Australia, Nicole L Jones, University of Western Australia, Oceans Graduate School and Oceans Institute, Crawley, WA, Australia and Gregory N Ivey, University Western Australia, Oceans Graduate School and Oceans Institute, Crawley, Western Australia, Australia
Abstract:
It has long been suggested that boundary turbulence beneath internal waves may be significant in shaping the world’s continental shelves. More recently it has been suggested that unique boundary stability dynamics beneath nonlinear internal waves (NLIWs) may provide mechanisms for greatly enhanced turbulent transport, yet field observations of such processes remain entirely qualitative. We present detailed observations of the bottom boundary layer beneath mode-1 NLIWs of depression propagating on the continental shelf. Through-the-water column measurements show the waves were highly non-linear, with amplitudes reaching approximately 40% of the total depth. A bottom lander measured the mean flow, turbulent structure, and sediment distribution in the boundary layer beneath propagating NLIWs of depression. The boundary layer was highly turbulent, driven by the combination of the barotropic tide and the NLIW forcing, and influenced by density stratification at the outer edge. Momentum and sediment transport were dominated by intense boundary layer turbulence, similar in many characteristics to the simpler case of a neutrally-stable oscillatory turbulent boundary layer. However, the stratification and associated baroclinic NLIW motions modulated the boundary layer in three ways: 1) The boundary layer was limited to around 5% of the total flow depth, an order of magnitude smaller than the wave amplitude and the unstratified turbulent Stokes layer thickness; 2) Vertical strain associated with the mean (NLIW) velocity field modulated the vertical extent of the boundary layer and turbulent structures within, thus influencing bed stress, and creating an aperiodic pumping mechanism for enhanced sediment transport; and 3) Mean (wave-induced) vertical strain periodically intensified stratification at the edge of the boundary, influencing the energetics of mixing and hence the overall growth of the boundary layer. The waves occurred only on the off-shelf phase of the tide, creating an asymmetry in sediment dynamics, with potentially significant consequences for cross shelf transport.
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