Global Abyssal Mixing Inferred from Lowered ADCP Shear and CTD Strain Profiles

Abstract

Internal wave?wave interaction theories and observations support a parameterization for the turbulent dissipation rate ε and eddy diffusivity K that depends on internal wave shear ?Vz2? and strain ??z2? variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 ? 10?4 m2 s?1 within 2° of the equator to (0.4?0.5) ? 10?4 m2 s?1 at 50°?70°. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 ? 10?4 m2 s?1 between 20° and 30°, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation ?ε is contributed from the main pycnocline, hotspots in ?ε show some correlation with small-scale bottom roughness and near-bottom flow at sites where strong surface tidal dissipation resulting from tide?topography interactions has been implicated. Average vertically integrated dissipation rates are 1.0 mW m?2, lying closer to the 0.8 mW m?2 expected for a canonical (Garrett and Munk) internal wave spectrum than the global-averaged deep-ocean surface tide loss of 3.3 mW m?2.