Momentum Flux Convergence From Internal Tides in the North Equatorial Countercurrent Upstream of a Submarine Ridge Near Merir Island, Palau

In the presence of a mean flow, theory and models for internal tide generation at small-amplitude topography show (a) internal tides produce a net momentum flux that extends to the surface and (b) fluxes are directed preferentially upstream. Here, we observe internal tides propagating into the North Equatorial Countercurrent during two spatial surveys near a submarine ridge. On the second survey during a diurnal spring tide, we note elevated shear upstream of the ridge below 112 m with corresponding low Richardson numbers, which suggests the internal waves are dissipating due to shear instability. Reduced amplitudes are noted above 150 m. Internal tidal motions were isolated by performing a phase average over a wavelength upstream of the topography. Momentum flux convergence and divergence are noted upstream of the ridge, have a similar magnitude to mean flow acceleration terms, and are about 10% of the largest mean flow terms. The internal tides were noted upstream and appeared weaker downstream, but due to the topographic wake similar calculations could not be performed downstream. Tall, steep topography and strong equatorial currents are found throughout the low-latitude western Pacific, which suggests such infrequently observed internal tide flux divergences may be commonly contributing to the momentum of mean flows.

Automated summary

Investigations found that internal tides continuously extend to the surface and propagate upstream in small-amplitude topography, with momentum flux convergence and divergence observed upstream near the topography.