Semidiurnal internal tide incoherence in the equatorial Pacific

Abstract

The jets in the equatorial Pacific Ocean of a realistically forced global circulation model with a horizontal resolution of 1/12.5° cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian Islands and Hawaii. This loss of coherence is quantified with a baroclinic energy analysis, in which the semidiurnal‐band terms are separated into coherent, incoherent, and cross terms. For time scales longer than a year, the coherent energy flux approaches zero values at the equator, while the total flux is ∼500 W/m. The time variability of the incoherent energy flux is compared with the internal‐tide travel‐time variability, which is based on along‐beam integrated phase speeds computed with the Taylor‐Goldstein equation. The variability of monthly mean Taylor‐Goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane‐wave fit technique. On monthly time scales, the loss of phase coherence in the equatorward beams from the French Polynesian Islands is attributed to the time variability in the vertically sheared background flow associated with the jets and tropical instability waves. On an annual time scale, the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence. In the model simulations, low‐frequency equatorial jets do not noticeably enhance the dissipation of the internal tide, but merely decohere and scatter it.

Long‐Range Radiation and Interference Pattern of Multisource M2 Internal Tides in the Philippine Sea

“…The convergence of tidal beams from the Luzon‐Miyako area and the Mariana Arc can be speculated and discerned from altimeter SSH observations (Figure 6 of Zhao et al, ). This long‐distance propagation and confluence pattern can also be found in regional and global models but deserves thorough examination in the future (Buijsman et al, ; Kerry et al, ).…”

“…However, in the northeast basin, the baroclinic field of our simulation is completely different from that of Kerry et al's () model, which ignores the internal tide interference between the Tokara Strait and Bonin Ridge. In addition, the coherent portion is suggested to account for the major M 2 baroclinic energy in the Philippine Sea based on global models and altimeter observations (Buijsman et al, ; Zaron, ). The temporally variable portion of M 2 internal tides caused by subtidal currents cannot explain the geographically inhomogeneous pattern of M 2 baroclinic energy in the Philippine Sea.…”

“…Nevertheless, the temporally variable incoherent energy of internal tides caused by the background velocity can further enhance the inhomogeneity of the wavefield (Zaron & Egbert, ) and may shift the interference pattern (Zhao et al, ). The incoherent energy refracted by the background medium could be responsible for the temporally variable and unpredictable proportion of the wavefield (Savage et al, ) and increase with the propagation distance (Buijsman et al, ); these issues deserve specific attention upon further examination of interference processes.…”

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

“…Temporal and spatial variability of stratification, complex bathymetry, the strength of inertial oscillations, geostrophic currents, and mesoscale eddies have all been shown to interact with internal tide generation and propagation and contribute to their unpredictability (e.g., Huang et al, ; Kerry et al, , ; Rainville & Pinkel, ; Zilberman et al, ; Zaron & Egbert, ). In addition, interference from multiple internal tide generation sites and horizontal plane propagation effects may also lead to unpredictability (e.g., Buijsman et al, ; Duda et al, ; Kurapov et al, ). In coastal regions, small‐scale bathymetric variations, stratification variability from wind‐driven processes, buoyancy input from riverine runoff, and shoaling remotely generated internal tides can modify local internal tide generation and propagation (e.g., Martini et al, ; Nash et al, ; Nash, Kelly, et al, ).…”

The Global Mode‐2 M2 Internal Tide

“…The barotropic tide converts to internal tides at 1 TW (Egbert & Ray, , ). A large fraction of the converted tidal energy is transported away by internal tides and dissipate in the open ocean or on the opposite continental slopes (Alford, ; Buijsman et al, ; Kelly et al, ). Different modes have different capabilities of energy transport, because of their different group velocities and decay rates.…”