Observations of tidally driven turbulence over topographic features on the continental slope of Tasmania.

The surface tides, through the generation and breaking of internal tides, are thought to be one of the main sources of energy for diapycnal mixing in the abyssal ocean. Identifying which processes are the dominant pathways of energy to turbulent dissipation is essential to implement the appropriate parameterizations in large-scale numerical models. Since these processes are often unresolved in spatially and/or temporally sparse turbulence estimates, process-study experiments are essential to test existing theories and highlight the phenomena that should be accounted for.

As part of the Tasman Tidal Dissipation Experiment (TTIDE), we have taken mooring and shipboard measurements to study near-bottom turbulence partly driven by a remote mode-1 internal tide. In particular, two-month long velocity and temperature timeseries resolve tidally driven processes over the bottom few hundred meters. Moreover, moored thermistors provide timeseries of dissipation rate estimates based on temperature variance in the inertial subrange. Our observations highlight the role of small, O(1) km, topographic features in determining elevated bottom-enhanced turbulence. At one site, the primary process leading to turbulence over the bottom few hundred meters is a tidally forced lee wave associated with an isolated supercritical bathymetric feature. In agreement with previous studies, elevated dissipation takes place at times of maximum negative strain, indicating wave breaking by convective instability. A fraction of the incident internal tide energy flux may also be scattered and lead to enhanced near-bottom dissipation at shallower water. Despite the dominant role of the tide, the subtidal temporal variability of near-bottom dissipation rate does not have a spring-neap cycle. This variability suggests that processes that can modulate tidal dissipation at longer timescales are essential for predicting near-bottom mixing.