On the Fraction of Internal Tide Energy Dissipated Near Topography

Internal tides have been implicated as a major source of mechanical energy for mixing in the ocean interior. Indeed, microstructure and tracer measurements have indicated that enhanced turbulence levels occur near topography where internal tides are generated. However, the details of the energy budget and the mechanisms by which energy is transferred from the internal tide to turbulence have been uncertain. It now seems that the energy levels associated with locally enhanced mixing near topography may constitute only a small fraction of the available internal tide energy flux. In this study, the generation, radiation, and energy dissipation of deep ocean internal tides are examined. Properties of the internal tide at the Mid-Atlantic Ridge and Hawaiian Ridge are considered. It is found that most internal tide energy is generated as low modes. The Richardson number of the generated internal tide typically exceeds unity for these motions, so direct shear instability of the generated waves is not the dominant energy transfer mechanism. It also seems that wave-wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux. Instead, it seems that much of the generated energy radiates away from the generation site in low mode waves. These low modes must dissipate somewhere in the ocean, though this likely occurs over O(1000 km) distances as the waves propagate. Additionally, a small fraction of energy flux is generated as high mode waves. These have slower group speeds, higher shear, and faster wave-wave interaction rates than the low modes, and so are likely to dissipate as turbulence close to the topography of the generation site. Preliminary estimates suggest that turbulent dissipation near sites of generation accounts for 30% or less of the total generated energy flux. Based on these estimates, a parameterization of the mixing sustained by high mode internal tide generation is proposed.