From large-scale internal tides to small-scale turbulence: observations on the continental slope off Tasmania
The dissipation of surface tides is thought to be one of the major contributors to small-scale turbulence and mixing in the abyssal ocean (below $>2000~\mathrm{m}$), which is an essential component of the large-scale deep circulation. However, the energy of surface tides is first converted into internal tides and these waves can be transformed through several processes until they break and generated mixing. Over the course of two months in 2015, we made intensive in-situ observations to determine the energy fate of the incident internal tide on the continental slope off Tasmania, Australia. Over the steep slope in the southeast region, the incoming tide mostly reflects back to the deep ocean. We observed a complex interference pattern that is associated with both wave reflection and the alongshore propagation of a slope wave. During our experiment, the incoming internal tide exhibits a predictable spring-neap cycle. However, the mismatch between the observed and predicted phases of the fortnightly variability indicates that local waves are also important in the internal-tide wave field. The irregular northeastern continental slope off Tasmania leads to large scattering of the incoming internal tide. Tidal flow over a steep bump sets nonlinear lee-wave like phenomena on the offshore and onshore sides of the topography. Through these processes, the semidiurnal tide drives most of the observed energy dissipation rate. However, the temporal variability is complex on weekly timescales and may be associated with the interference between remote and local tides. Nevertheless, idealized theoretical predictions roughly agree with the observations and supports that wave scattering is responsible for the near-bottom turbulence. Despite the high reflectivity off southeastern Tasmania, small-scale bathymetric corrugations modify the near-bottom tidal flow, which is in turn coupled with the background current. Large cross-shore tidal and subtidal velocities are observed within the trough of the corrugation. The low-frequency horizontal current is $O(10^{-2})~\mathrm{m}~\mathrm{s}^{-1}$, which may be associated with large vertical velocities of up to $O(10^{-3})~\mathrm{m}~\mathrm{s}^{-1}$. The temporal variability of the near-bottom subtidal current is correlated with turbulence estimates, which do not have a clear fortnight modulation. However, spatially and temporally resolving measurements show that the sloshing of the internal tide is crucial for setting the near-bottom turbulence. The vertical divergence of the semidiurnal Reynolds stress explains the temporal variability of turbulence within the corrugation trough and suggests an internal-tide driven onshore subtidal flow. The onshore acceleration is likely balanced by an offshore pressure gradient, and the associated density field is responsible for the decrease in magnitude of the overlying alongshore flow as it approaches the top of the corrugations.