Long-Range Propagation of the Semidiurnal Internal Tide from the Hawaiian Ridge

Abstract

The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.5°–37.1°N (≈1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide’s coherent portion. Together these two approaches provide a unique view of the internal tide’s long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode motions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase and group propagation measured independently from moorings and altimetry agree well with theoretical values. Sea surface height anomalies (SSHAs) measured from moorings and altimetry agree well in amplitude and phase until the northern end of the array, where phase differences arise presumably from refraction by mesoscale flows. Observed variations in SSHA, energy flux, and kinetic-to-potential energy ratio indicate an interference pattern resulting from superposed northeastward radiation from Hawaii and southeastward from the Aleutian Ridge. A simple model of two plane waves explains most of these features.

Tidal Mixing in the South China Sea: An Estimate Based on the Internal Tide Energetics

“…The above dissipation and mixing patterns as well as their magnitudes in the LS region are comparable to the upper bound value of the observational report by Tian et al (2009) based on the finescale shear-strain parameterization and the results given by Alford et al (2011) using the Thorpe-scale method. Waves generated from multiple nearby sources interfere with each other, which can complicate the patterns of wave kinematics and energy fluxes (Rainville et al 2010;Zhao et al 2010) and affect wave scattering and transmission (Klymak et al 2011). Interference between incident baroclinic waves and local barotropic forcing can greatly alter the property and magnitude of local barotropic to baroclinic conversion (Kelly and Nash 2010).…”

Scattering of the low-mode internal tide at the Line Islands Ridge

“…The value of M chosen is the one for which the modal amplitudes determined from the full-depth and partial-depth synthetic data are in agreement. In general, M = 5, 10 is often used based on these analyses (Zhao et al, 2010).…”

“…Over an ocean of constant depth, the internal tides' vertical structure can be represented by a superposition of discrete vertical baroclinic modes that depend only on the buoyancy frequency profile, N (z). Following the work of oceanographic studies by Alford (2003), and Zhao et al (2010), the baroclinic modes for the vertical structure of the internal tide, Φ(z), can be determined by the eigenvalue equation,…”

“…Extensive error analysis was done by Zhao et al (2010) as part of the Internal Waves Across the Pacific (IWAP) study examining the northward propagation of the internal tide generated by Hawaiian Ridge, the same generation site of the southward propagating internal tide that has been observed to scatter at the Line Islands Ridge. Further work needs to be done following the Monte Carlo simulations of Zhao et al…”

Baroclinic Motions and Energetics as Measured by Altimeters

“…(2) or (3) with realistic estimates of N (z), but have commonly employed a rigid-lid boundary condition at z 5 0 instead of the dynamic boundary condition of a free surface (e.g., Zhao et al 2010). Thus, (6) would become G(0) 5 0, and (7) is then dF(0)/dz 5 0.…”