Satellite Investigation of the M2 Internal Tide in the Tasman Sea

Abstract

The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

Internal Tidal Modal Ray Refraction and Energy Ducting in Baroclinic Gulf Stream Currents

“…The southern group shows much less refraction than the northern group because the model property gradients are weaker. The existence of relatively straight rays in areas with weaker eddy activity is in agreement with observations (Zhao and Alford 2009). FIG.…”

“…The world's barotropic tides drive baroclinic tides by the action of moving stratified water across gravitational potential surfaces at sloping bathymetric features. In general, this effect tends to form internal tidal beams that are vertically narrow, composed of many modes (Vlasenko et al 2005;Garrett and Kunze 2007;Zhang and Duda 2013), but observations show that modal dispersion and dissipation lead to mode-one waves being strong and identifiable away from the source regions, both in deep ocean basins and on continental shelves (e.g., Sherwin 1988;Ray and Mitchum 1996;Alford and Zhao 2007;Dushaw et al 2011;Klymak et al 2011;Li and Farmer 2011;Zhao et al 2016). At a given location and time, mode-one waves have vertical displacements throughout the water column that are all of the same sign, and behave much like interfacial internal waves in a two-layer environment, the simplest archetype environment that can support baroclinic waves.…”

“…Demonstrations of an ability to measure and study real-ocean internal tide signals were made by Dushaw (2002Dushaw ( , 2003Dushaw ( , 2006. This was followed by altimetric mapping of mode-one internal tides in the Pacific Ocean (Zhao and Alford 2009;Zhao et al 2011), a data-based study of the Hawaii area (Zhao et al 2010), and a study using both data and computational modeling to analyze generation and propagation south of Hawaii (Rainville et al 2010).…”

“…In many situations the mode coupling is weak because very pronounced bathymetry is required for strong coupling (Johnston and Merrifield 2003;Johnston et al 2003;Kelly et al 2016) and the equations can be decoupled under the adiabatic approximation (Milder 1969). In this approximation the modes do not exchange energy and the propagation of each mode can be described by the Helmholtz equation (Rainville and Pinkel 2006;Zhao et al 2016). The coupled-mode approach requires that modes be orthogonal, which the full modes are not, so that approach can only be approximate if full modes in baroclinic flows are used.…”

“…Note that long-range ray calculations of the type shown in Fig. 15 must include a curvature term to satisfy the constraint that the rays remain on the geoid; in other words, for an ocean of no current, uniform stratification, and uniform depth, modal ray trajectories would lie near geodesics, departing due to the beta effect (Zhao et al 2018). It has been shown that propagating wave ray trajectories on the Earth can be computed by separately computing the angular deviation due to the geoid constraint and the angular deviation due to media effects (Heaney et al 1991).…”