Internal wave reflection and mixing at Fieberling Guyot

Abstract

The structure of internal wave reflection off a sloping bottom on the steep flank of a tall North Pacific Ocean seamount is observed in year‐long moored array records to differ substantially from the form predicted by linear theory, although linear theory accounts for several qualitative features of the process. This study documents new features of wave reflection as described below. Wave reflection is detectable as far as 750 m above the bottom. Motions are dominated by a single empirical mode whose phase structure obeys linear internal wave dispersion but whose amplitude decays with a scale comparable to the wavenumber. While the dominant mode has scales appropriate to the reflection of a first baroclinic mode wave incident from the open ocean, its decay from the bottom is such that current vectors in the vertical plane rotate clockwise in time when viewed with shallow water to the right. This flow resembles the lower half of the deepest cell pattern predicted by linear reflection from a uniform slope. At the local internal wave critical frequency, the dominant mode has nearly a vanishing wavenumber rather than the infinite wavenumber predicted by linear reflection. Reflected waves are aligned parallel to the bottom slope measured on wave spatial scales, rather than shorter ones. Wave reflection causes large, frequent density overturns, implying mixing. The rate and strength of these overturns imply a local vertical eddy viscosity of 2–6 × 10−4 m2/s over the bottom few hundred meters. The contribution of bottom‐intensified mixing to the open deep ocean is roughly equivalent to that found in situ, although reflection from gentler slopes or at lower latitude may produce greater contribution from internal wave‐reflection‐induced mixing.