Project: Collaborative Research: A study of the energy dissipation of the internal tide as it reaches the continental slope of Tasmania

NSF Award Abstract:

Surface tides supply about one terawatt of power to internal tides as they propagate up and over large topographic features. Most of the energy of these internal tides propagates away from the generation regions in the form of low-mode internal tides. The ultimate fate of this energy is unknown and has a large impact on the global distribution of ocean properties. Previous studies of low-mode internal tide propagation have observed regions where the internal tide was diffuse and exhibited complex interference patterns, making it difficult to close the energy budget. The Tasman Sea differs from previous sites because it is believed to contain one of the most energetic and focused internal-tide beams in the world. The beam is generated south of New Zealand, propagates 1,500 km across the Tasman Sea, and strikes the Tasman continental margin. This project called T-Beam will document the rate of decay of a focused internal tide beam, compare the measured flux convergence with novel in situ measurements of turbulent mixing, and investigate the dynamical processes responsible for the observed decay. The results from T-Beam should lead to significant improvement in parameterizations of internal-wave induced mixing in global climate models. A major goal of the analysis is to compare in situ internal tide fluxes with those inferred from satellite altimetry; the latter are known to be biased low in the presence of strong mesoscale currents but the extent of the bias is not well documented. T-Beam investigators have established collaborations with Australian scientists who will complement the T-Beam measurements with a suite of synergistic geological and biological analyses. During the field campaign, T-Beam investigators will prepare press releases and publish a daily blog. Undergraduate and graduate students in the United States and Australia will be offered the opportunity for at-sea experience, modeling and analysis.

In T-Beam, the investigators will obtain high-resolution estimates of internal-tide energy flux and dissipation rates in the Tasman Sea. The study site is favorable because it has a single strong generation region, contains a long energetic and confined internal-tide "beam", and is sheltered from remotely generated internal tides. The proposed experiment will be highly coordinated with the NSF-funded Tasmanian Tidal Dissipation Experiment (T-TIDE), which will examine the dissipation of the internal tide as it shoals on the Tasmanian continental slope. T-Beam will enhance T-TIDE by providing synoptic measurements of incident internal-tide energy flux that will reduce uncertainties in estimates of the fraction of energy flux that is dissipated over the continental slope. T-TIDE will enhance T-Beam by providing additional observations (adaptive glider sampling and shipboard surveying) to help identify mechanisms and better constrain the open-ocean decay rates observed during T-Beam. A decade ago, the Hawaiian Ocean Mixing Experiment (HOME) provided a comprehensive look at the internal tide generation process. Together, T-Beam and T-TIDE will complete that life cycle by providing the first comprehensive observations of an internal-tide beam as it propagates through the open ocean and dissipates on a continental slope. The Schmidt Ocean Institute is providing 28 days of ship time coincident with T-TIDE. This project will deploy a two-month mooring situated in the center of the observable internal-tide beam, conduct intensive ship-based surveys of density, velocity and turbulence to resolve the along- and across-beam spatial structure, and numerically model the formation, variability, and dissipation of internal-tide beams in the presence of arbitrary topography and mesoscale variability.