Tidal mixing of the deep ocean does not necessarily occur where energy is lost from the barotropic tide, but rather where the low-mode baroclinic tide dissipates, perhaps thousands of kilometers away. Ocean general circulation models are sensitive to the magnitude and geography of deep mixing, but very few observations of this process exist. Understanding the location and mechanisms of tidal dissipation is necessary for their continued improvement. The Tasmanian Tidal Dissipation Experiment?s (T-TIDE) hypothesis is that significant tidal dissipation takes place where propagating low-mode internal tides impinge on steep continental slopes. Slope geometry affects wave reflection, transmission, scattering, and breaking, which in turn determine the spatial distribution of mixing. Important breaking mechanisms may include lee effects at small-scale bathymetry and nonlinear bore formation near critical slopes. We propose a coordinated observational/modeling effort to understand the physics geometry, and magnitude of dissipation and mixing. The goals are to 1) identify where mixing occurs on the continental slope, 2) determine the dominant dissipative processes, 3) assess the amount of energy transmitted onto the shelf and reflected back into the deep sea to be dissipated elsewhere, and 4) generalize these results as a parameterization of topographic scattering and dissipation in numerical models, which ideally depends on the topography, stratification, and the barotropic and/or low-mode baroclinic tides. The three main components to the experimental plan are 1) intensive 2D and 3D modeling before and after the fieldwork to plan and contextualize the measurements; 2) a glider based Scout Experiment to identify dynamic processes, evaluate models, and finalize mooring sites; and 3) a comprehensive Process Experiment using ships, gliders, and moorings with the time and space resolution to address T-TIDEs hypothesis and achieve its goals. An array of profiling and fixed-sensor moorings will be combined with gliders acting as virtual moorings. Tidally-resolving or higher-frequency observations will measure internal wave motions, energy fluxes, and dissipation and mixing either directly or via finescale parameterizations. An altimetric analysis of internal-tide sea surface height will assess mesoscale refraction of the incident internal tide along with Australian gliders, which will monitor hydrography and internal tidal displacements in the Tasman Sea.
The proposed work will provide global insights on the dissipation of tidal energy. Specifically, inclusion of tidal dissipation on continental slopes may be an important component of the next generation of mixing parameterizations in climate models. Graduate students will be trained as part of the experiment. In addition to its scientific import, T-TIDE will sponsor and participate in two training seminars with high school science teachers who are charged with delivering the earth science program in San Diego Unified School District. This effort will be performed in conjunction with the Birch Aquarium at Scripps. Finally, an essential element of the project is the development of strong collaboration and resource sharing with Australian and Canadian scientists.