Tidally Dominated Flows past a Three-Dimensional Topography: Wake Vortices, Turbulence, and Mixing

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Abstract Oceanic turbulence influences the transport and mixing of freshwater, heat, nutrients, and other biogeochemical tracers. It also has broader implications for oceanic and atmospheric circulations. Tides contribute substantially to the mechanically driven turbulent ocean mixing through the internal waves resulting from tide–topography interactions. Tidal currents also drive turbulent wakes and shear layers when the topography is three-dimensional (3D). The hypothesis that seamounts are the “stirring rods” of the ocean has motivated considerable recent interest in turbulent flow features near 3D topography. It also motivates the present large-eddy simulations (LES) of tidally dominated flows (tidal oscillations superposed on a weaker mean) past an idealized steep seamount. Complex interactions occur between the topography, the near wake, and previously shed vortices, especially during the tidal phases when the flow direction is reversed. The topographic wake is shown to be a hotspot for mixing, featuring large dissipation rates in the attached shear layers, hydraulic jet, recirculation region in the near wake, and peripheries of shed vortices. The majority of the observed dissipation is due to the vertical shear. Over a tidal cycle, the volume-integrated local dissipation within the wake is at least 4 times greater than the internal wave flux that may be dissipated elsewhere. Furthermore, normalized dissipation rates are maximized for the purely tidal setting. Within the tidal cycle, bulk mixing efficiency η varies substantially and is maximized at η ≈ 0.25 around flow reversals. Significance Statement Tidal forcing, primarily due to the gravitational attraction from the moon and the sun, constitutes a major source of mechanical energy input into the World Ocean. Delineating the energy pathways that ultimately lead to dissipation of the input is crucial to understanding the ocean circulation and mixing, which in turn have broader implications. A significant portion of energy dissipation is attributed to breaking of internal waves generated due to flow–topography interactions. We show that steep seamounts, hypothesized to be stirring rods of the ocean, not only generate internal waves but also act as local hotspots of energy dissipation. The local dissipation in tidally dominated flows is at least 4 times greater than the energy transferred to the internal waves, to potentially be dissipated elsewhere.

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