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Mean flow due to momentum flux convergence from internal tides upstream of a submarine ridge near Merir Island, Palau
  • Shaun Johnston,
  • Celia Yun Ou
Shaun Johnston
Scripps Institution of Oceanography

Corresponding Author:[email protected]

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Celia Yun Ou
Scripps Institution of Oceanography
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Abstract

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) energy and momentum 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. Reduced amplitudes are noted above 150 m. In a similar but not identical depth range to the elevated shear, vertical momentum flux convergence and divergence are noted upstream of the ridge and are sufficient to produce a mean flow of 2–6 cm s$^{-1}$. The mean flow is 10–30\% of the tidal current. The upstream current below 112 m increases by about 10 cm s$^{-1}$ between surveys. Other unresolved terms in the momentum equation may contribute to this change in current speed, but it is notable that internal tide momentum fluxes can contribute a mean flow of similar magnitude. Tall, steep topography and strong equatorial currents are found throughout the low-latitude western Pacific, which suggests this infrequently observed process of internal wave flux divergences forcing mean flows may actually be common there.