A global map of tidal dissipation over abyssal hills

Abstract

Internal gravity waves are complex oscillations widely found in fluids exhibiting density stratification, such as the ocean. Such waves may be produced by tidal currents flowing over rough seafloors; they are then called internal tides. As they travel away from the seafloor, internal tides tend to attain high amplitudes and break into turbulent eddies. Their ability to carry and dissipate part of energy make them of utmost importance in physical oceanography. Their intensive study in the past decades revealed that the breaking of internal tides may provide a large part of the power needed to mix the deep ocean and sustain the deep part of the meridional overturning circulation. The fraction of energy that those waves locally dissipate and the resulting vertical mixing distribution are crucial data for the parametrization of oceanic models, yet they remain poorly quantified. In this work, we provide the first estimate of the worldwide three-dimensional distribution of internal tide dissipation by means of large-scale numerical simulations. We couple linear wave theory with a simple nonlinear breaking scheme and use global topography, stratification and tidal current data to reproduce the generation, upward propagation and energy dissipation of internal tides. We show that exceptionally high dissipation is commonly attained at a few hotspots, confined above mid-ocean spreading ridges in the Southern Hemisphere. As expected by earlier studies, we find that a large part of the resulting mixing follows an exponential decay with height, with strong bottom intensities. We however show that substantial mixing over smoother flanks follows a so far unreported non-monotonic vertical profile with significant mid-depth maxima.