Impact of a mean current on the internal tide energy dissipation at the critical latitude

Abstract

revious numerical studies of the dissipation of internal tides in idealized settings suggest the existence of a critical latitude (~29°) where dissipation is enhanced. But observations only indicate a modest enhancement at this latitude. To resolve this difference between observational and numerical results, we study the latitudinal dependence of internal tides dissipation in more realistic conditions. In particular, the ocean is not a quiescent medium, the presence of large-scale currents or mesoscale eddies can impact the propagation and dissipation of internal tides. We investigate the impact of a weak background mean current in numerical simulations. We focus on the local dissipation of high-spatial-mode internal waves near their generation site. The vertical profile of dissipation and its variation with latitude without mean current are consistent with earlier studies. But adding a weak mean current has a major impact on the latitudinal distribution of dissipation. The peak at the critical latitude disappears and the dissipation is closer to a constant, albeit two weak peaks at ~25° and ~35° latitude. This disappearance results from the Doppler shift of the internal tides frequency, which hinders the nonlinear transfer of energy to small-scale secondary waves via the Parametric Subharmonic Instability (PSI). The new two weak peaks correspond to the Dopplershifted critical latitudes of the left and right propagating waves. The results are confirmed in simulations with simple sinusoidal topography. Thus, although nonlinear transfers via PSI are efficient at dissipating internal tides, the exact location of the dissipation is sensitive to large-scale oceanic conditions.