Vertical structure of barotropic‐to‐baroclinic tidal energy conversion on a continental slope

Abstract

Horizontal distribution of the vertically integrated barotropic‐to‐baroclinic energy conversion has been widely studied to examine the generation of internal tides at steep topography. The vertical structure of the energy conversion that provides insights into the associated dynamics, however, is masked by the often used depth‐integrated approach. Here, we reveal the vertical profile of barotropic‐to‐baroclinic energy conversion by employing an idealized ocean model in a slope‐shelf context forced by M2 barotropic tidal flow. The model shows two vertically separated hotspots of energy conversion, one near the sloping bottom and the other at the thermocline, resulting from the stronger vertical velocity and enhancement of the density perturbation, respectively. Isolation of the hotspots demonstrates that baroclinic energy generated in the bottom layer radiates toward onshore and offshore primarily in the form of internal wave beams, whereas that generated at the thermocline propagates away in the form of internal wave modes. Although energy converted at the thermocline contributes to only a small portion of the total energy conversion, it plays an important role in onshore baroclinic energy radiation and can be significantly affected by the internal wave activity at the bottom layer. With a fixed bottom topography, the percentage of baroclinic energy generated at the thermocline is linearly related to a body force exerted by the barotropic tidal flow over topography that can be estimated analytically. This provides a convenient way to estimate the overall barotropic‐to‐baroclinic energy conversion over a continental slope in the real ocean by measuring the energy conversion in the thermocline only.Plain Language SummaryInternal waves propagating in the interior of the stratified ocean are linked to energy redistribution and mixing. They are often generated when the surface tides flow over sloping topography. In this process, a portion of tidal energy is converted to internal waves. The horizontal distribution of this converted energy has been well‐studied, while its vertical structure has not. Here, we reveal the vertical profile of the energy conversion by employing an idealized ocean model to provide insights into the associated dynamics. The model shows two vertically separated hotspots of energy conversion, one near the seafloor and the other at the thermocline where the temperature and salinity change dramatically in the vertical direction. Although the upper hotspot contributed to only a small portion of the total energy conversion, it plays an important role for the baroclinic energy radiating onto the shallow continental shelf. By isolating the hotspots and comparing the modeled energy conversion with theoretical predictions, we find a linear relation between them. This provides a convenient way for energy conversion estimation in the real ocean that only the parameters in the thermocline need to be measured, instead of the whole water column.Key PointsVertical structure of barotropic‐to‐baroclinic energy conversion shows two hotspots, near the sloping bottom and at the thermoclineNear thermocline hotspot provides a small portion of the total energy conversion but is essential in onshore baroclinic energy radiationTotal energy conversion can be estimated from a linear relation between the thermocline energy conversion and an analytical prediction