The Tasman Tidal Dissipation Experiment: Tidal Mixing, Scattering, and Reflection on the East Tasman Slope

Robert Pinkel1, Matthew H Alford2, Andrew Lucas3, Shaun Johnston1, Nicole L Jones4, Sam Kelly5, Jody M Klymak6, Jennifer A MacKinnon1, Jonathan D Nash7, Luc Rainville8, Harper L Simmons9, Peter G Strutton10, Amy Frances Waterhouse1 and Zhongxiang Zhao11, (1)University of California San Diego, La Jolla, CA, United States, (2)Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, United States, (3)Scripps Institution of Oceanography, La Jolla, CA, United States, (4)University of Western Australia, Crawley, WA, Australia, (5)University of Minnesota Duluth, Duluth, MN, United States, (6)University of Victoria, Victoria, BC, Canada, (7)Oregon State Univ, Corvallis, OR, United States, (8)University of Washington, Applied Physics Laboratory, Seattle, WA, United States, (9)University of Alaska Fairbanks, Fairbanks, AK, United States, (10)University of Tasmania, Institute for Marine and Antarctic Studies (IMAS), Hobart, Australia, (11)Applied Physics Laboratory University of Washington, Seattle, WA, United States
Abstract:
To maintain the thermohaline circulation of the ocean in steady state, the sinking of dense waters at high latitude must be balanced by mixing processes in the deep sea. Such mixing is anticipated at sites where baroclinic tides dissipate, among other places. To date, however, the primary observational focus has been on baroclinic generation, not dissipation, sites.

During January-March 2015, the NSF TTIDE Experiment investigated deep-mixing processes on the eastern slope of Tasmania. A well-focused semi-diurnal baroclinic tidal beam generated on the Macquarie Ridge south of New Zealand transits the Tasman Sea and impinges on this slope. The objective of TTIDE is to identify the various small-scale processes associated with tidal shoaling, dissipation, and reflection on the slope and to assess the potential role of such processes in the general circulation of the ocean.

Guided by initial glider reconnaissance and numerical simulations, arrays of moorings were deployed at a southern (reflective-supercritical) site and a northern (dissipative-critical) region. Supplemental experiments T-BEAM and T-SHELF mapped the incoming tidal beam and quantified its signature on the continental shelf. Shipboard surveys using LADCPs and a fast-profiling CTD indicate elevated mixing regions 1-300 m above the slope at depths of 500-2200 m, with high-mode wave-beams in mid-water and near-bottom bores. These intense deep-mixing events occur even though the majority of the incoming tidal energy appears to be reflected northeastward, back into the Tasman Sea. Locally generated semi-diurnal and topographically-trapped diurnal tides are also dissipating on the slope. A more complete picture of this tidally forced slope and shelf will emerge as TTIDE data are synthesized.

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