Evaluating the role of inertial and tidal internal wave dynamics on a narrow continental shelf: An assessment of the dominant physical dynamics influencing mixing and the energy budget

Mesoscale, wind, inertial, tide and internal tide dynamics determine the vertical mixing, horizontal transport of mass and energy, and dissipation of energy on the continental shelf. We investigate the relative contribution of each of these processes, with emphasis on the internal wave dynamics in particular, to shelf mixing and to the cross- and along-shelf energy budgets on the Tasmanian Eastern Shelf, Australia. We deployed three traditional moorings and two autonomous profilers (WireWalkers) along a transect on this relatively narrow shelf. Estimates of mixing rates were derived from both high-frequency temperature measurements (chi-pods) on the WireWalkers and near-bed continuous measurements of temperature and velocity. The region is unique with a locally generated sub-inertial diurnal internal tide and a super-inertial semidiurnal internal tide, the potential for remotely generated energetic internal tides to be scattered onto the shelf, strong winds that force inertial waves and a persistent along-shore current. Although the diurnal internal tide is sub-inertial, the total horizontal kinetic energy (HKE) is the same order of magnitude as the super-inertial internal semidiurnal tide at O(10 Jm-3). In comparison, the near-inertial HKE is O(100 Jm-3) and dominates the baroclinic tides. Frequent but irregular bore-like nonlinear waves with amplitudes O(10 m) are measured at the shelf-break, but do not propagate to the mid-shelf moorings ( 10 km west). Estimated turbulent dissipation rates ɛ varied from 1e-9 to 1e-6 W kg-1 near the seabed with increased values near the surface following large wind events (>0.6 N m-2). Mixing rates were often in excess of 10-4 m2s-1. The implications of the topographically trapped internal diurnal tide for local dissipation of energy will be discussed.