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1. Introduction
Internal tides, or tidally generated internal waves, are a ubiquitous feature of the World Ocean, generated when the large-scale barotropic tide flows over rough seafloor topography. The internal waves are generated at the tidal frequency, or its harmonics (Bell 1975a), and their horizontal wavelength is determined by the scale of the local topography. The structure and behavior of these waves depends on whether their vertical wavelength is small or large compared with the ocean depth. Larger-scale topography (~100 km) generates waves with larger vertical wavelengths that feel the effect of the ocean surface and form low-mode internal tides that, as the name suggests, are well described in terms of a modal decomposition. These low-mode tides propagate large horizontal distances and tend to dissipate their energy remotely-for example, on continental shelves (Nash et al. 2004)-or interact further with topography, scattering, or radiating smaller-scale waves (e.g., St. Laurent and Garrett 2002). Smaller-scale topography (~10 km or smaller) generates tidal waves with vertical wavelengths significantly smaller than the ocean depth. These small-scale internal tides form distinct, vertically propagating tidal beams and are more likely to dissipate and otherwise interact with the local flow. The energy flux associated with these waves, and their breaking and dissipation in the deep ocean, is thought to be important in maintaining the abyssal circulation of the ocean (St. Laurent and Garrett 2002; Kunze 2017). However, not all the small-scale internal tide dissipates in the deep ocean and little attention has been given to the waves that escape this region and propagate into the upper ocean. Waves which span the ocean depth are potentially important, since small-scale internal tides are also associated with vertical fluxes of momentum that can act to accelerate the local flow when the waves dissipate, thereby driving the upper ocean.
The momentum transport by internal waves and the phenomenon of internal wave-driven flow has not been a focus of oceanographic research but has a long history in the atmospheric literature (e.g., White 1949). The waves in question are topographic lee waves, but the principle can be generalized to all internal waves. The seminal work of Eliassen and Palm (1961) showed that waves carry both energy and momentum as they propagate vertically. The momentum is extracted from the solid...