Observations of the Tasman Sea Internal Tide Beam

Abstract

Low-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea are generated at Macquarie Ridge, south of New Zealand, and propagate northwest as a focused beam before impinging on the Tasmanian continental slope. In situ observations from the Tasman Sea capture synoptic measurements of the incident semidiurnal mode-1 internal-tide, which has an observed wavelength of 183 km and surface displacement of approximately 1 cm. Plane-wave fits to in situ and altimetric estimates of surface displacement agree to within a measurement uncertainty of 0.3 cm, which is the same order of magnitude as the nonstationary (not phase locked) mode-1 tide observed over a 40-day mooring deployment. Stationary energy flux, estimated from a plane-wave fit to the in situ observations, is directed toward Tasmania with a magnitude of 3.4 ± 1.4 kW m−1, consistent with a satellite estimate of 3.9 ± 2.2 kW m−1. Approximately 90% of the time-mean energy flux is due to the stationary tide. However, nonstationary velocity and pressure, which are typically 1/4 the amplitude of the stationary components, sometimes lead to instantaneous energy fluxes that are double or half of the stationary energy flux, overwhelming any spring–neap variability. Despite strong winds and intermittent near-inertial currents, the parameterized turbulent-kinetic-energy dissipation rate is small (i.e., 10−10 W kg−1) below the near surface and observations of mode-1 internal tide energy-flux convergence are indistinguishable from zero (i.e., the confidence intervals include zero), indicating little decay of the mode-1 internal tide within the Tasman Sea.

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

“…Estimates of the vertically integrated energy dissipation rates at the 12 in situ stations from the strain‐based finestructure parameterization range from 0.5 to 3.3 mW m −2 with the highest dissipation rate at the station closest to the Plato Seamount and the lowest values at the two northernmost stations between the Plato Seamount and the Azores (Figure ). At the two southernmost stations within the tidal beam but away from the generation sites, the energy dissipation rate is approximately 1 mW m −2 which is in good agreement with what was observed for low‐mode internal tides in the Tasman Sea (Waterhouse et al, ).…”

Coastal Semidiurnal Internal Tidal Incoherence in the Santa Maria Basin, California: Observations and Model Simulations

“…A quantitative measure of internal tide predictability can be achieved through decomposition into “coherent” and “incoherent” variability with respect to the barotropic tide. In the time domain, variability that is phase locked to a distinct astronomical tidal frequency (e.g., as determined by harmonic analysis) is termed coherent or stationary, while the remaining components of the signal are termed incoherent or nonstationary (e.g., Nash, Kelly, et al, ; Nash, Shroyer, et al, ; Pickering et al, ; Waterhouse et al, ). A similar decomposition in the frequency domain distinguishes between spectral “line” and “band” variability within a tidal range (e.g., Colosi & Munk, ).…”

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

“…Larapuna station) were predominantly toward the north or south, but were strong and variable with a 50th and 95th percentile wind stress (speed) of 0.052 N m 22 (5.5 m s 21 ) and 0.23 N m 22 (11.6 m s 21 ), respectively. In the open ocean, a low-mode internal tide beam at the semidiurnal (M2) frequency is generated at Macquarie Ridge south of New Zealand (approximately 1500 km southeast of our sampled transect), and propagates northwestward to the Tasman continental slope (Simmons et al 2004;Boettger et al 2015;Johnston and Rudnick 2015;Waterhouse et al 2018). It is estimated that around 65% of the energy is reflected at the Tasmanian continental slope, and up to 30% propagates along-slope as a superinertial partially trapped wave (Klymak et al 2016).…”