Mean flow due to momentum flux convergence from internal tides upstream
of a submarine ridge near Merir Island, Palau
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.