Near‐boundary mixing above the flanks of a midlatitude seamount

Fine-scale velocity and density profile data with concurrent turbulent velocity and temperature dissipation estimates obtained above the flanks of Fieberling Guyot, a seamount in the eastern North Pacific Ocean, are examined for evidence of near-bottom boundary mixing. Fine-scale shear and strain spectral levels were elevated over the flanks of the seamount in a 500-m-thick stratified layer above the bottom. The velocity shear was horizontally isotropic, clockwise and counterclockwise-with-depth shear spectral levels were comparable, and no significant correlation between shear and strain was observed. Above the steepest bottom slopes near the seamount summit rim, excess vertical strain relative to shear was observed (as compared to the background internal wave field), suggesting the presence of high-frequency internal waves. These signals may have been the product of wave reflections from the steep flanks of the seamount and/or wave generation from tidal currents flowing over the rough bottom. Associated with the enhanced shears and strains were more frequent occurrences of low 10-m Richardson number events, increased overturning scales, and larger estimated turbulent eddy diffusivity relative to observations 15 km or more from the seamount. In particular, turbulent diffusivity estimates increased from O(0.1×10−4 m2 s−1) in the ocean interior to 1–5×10−4 m2 s−1 within 500 m vertically (1–3 km horizontally) of the seamount flank. A simple geometric scaling argument suggests that boundary mixing of this intensity has relevance to the large-scale circulation at abyssal depths where a large fraction of the ocean waters is in close proximity to the bottom.