Observations of an internal tide beam in the Tasman Sea

Internal-tide energy can propagate away from 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 geography of turbulent mixing rates. Previous studies of low-mode internal tide propagation have observed regions where the internal tide was diffuse and exhibited complex interference patterns. As a result, direct comparisons of observed energy-flux divergence and dissipation rates have been inconclusive. A well-defined beam of internal tide energy originates from the Macquarie Ridge southwest of New Zealand and propagates across the Tasman Sea towards Tasmania, dominating the internal wave field found in the region. Numerical simulations have shown that the internal tide focuses into a “beam” as it propagates northwest across the Tasman Sea before it eventually impinges on the Tasmanian continental slope. During January-February 2015, field observations mapped the structure and variability of this internal-tide beam in the deep ocean before it reached the continental slope using moored, ship-based and glider observations. In-situ observations from the Tasman Sea captured synoptic measurements of incident internal-tide energy flux that are comparable to those inferred from altimetric estimates. As the region is known to have a strong mesoscale which can bias altimetric estimates, comparisons made here can document the extent of this bias. Estimates on how variability of the internal beam as it crosses through an active mesoscale are made using both observational and numerical results.