Observations of Tidally Driven Turbulence over Steep, Small-Scale Topography Embedded in the Tasman Slope

Abstract

Abstract Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatiotemporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal time scales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump is O(1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at a horizontal scale of O(1) km and may locally drive significant diapycnal mixing. Significance Statement Near-bottom turbulent mixing is a key element of the global abyssal circulation. We present observations of the spatiotemporal variability of tidally driven turbulent processes over a small-scale topographic bump off Tasmania. The semidiurnal tide generates large-amplitude transient lee waves and hydraulic jumps that are unstable and dissipate the tidal energy. These processes are consistent with the scattering of the incident low-mode internal tide on the continental slope of Tasmania. Despite elevated turbulence over the bump, near-bottom energy dissipation is small relative to the incident wave energy flux.