Observation of Near-Inertial Waves in the Bottom Boundary Layer of an Abyssal Seamount

Abstract

Abstract The bottom boundary layer (BBL) contributes significantly to the global energy dissipation of low-frequency flows in the abyssal ocean, but how this dissipation occurs remains poorly understood. Using in situ data collected near the BBL at an abyssal seamount in the western Pacific Ocean, we demonstrate that strong bottom-trapped flows over sloping topography can lose their energy to near-inertial waves (NIWs) generated via the adjustment of the bottom Ekman layer. The NIWs with near-resonant frequencies corresponding to internal waves with propagation direction parallel to the topographic slope are observed. These waves are strongest in the BBL and have a correlation with the off-seamount subinertial flows largely attributed to the Ekman transport driven by the bottom-trapped anticyclonic circulation over the seamount. The bottom-intensified NIWs are observed to have dominant upward-propagating energy and hypothesized to be generated via Ekman flow–topography interactions in the BBL. Energy loss from the near-bottom flows to radiating NIWs (∼8 × 10−4 W m−2) is estimated to be substantially larger than that due to bottom drag dissipation (∼2 × 10−4 W m−2), suggesting the important role of internal-wave generation via the Ekman transport adjustment in damping the subinertial flows over the sloping seafloor. Significance Statement Dissipation of geostrophic currents and eddies via the oceanic bottom boundary layer (BBL) plays an important role in modulating the global oceanic mechanical energy budget. The bottom drag has long been considered a key process in inducing such dissipation, but it is recently suggested to be less effective at a sloping bottom. This study suggests another potentially important mechanism that can remove energy from the geostrophic flows at the sloping bottom. This mechanism is depicted as the resonant generation of near-inertial internal waves by near-bottom flows. Generation of internal waves due to near-bottom flows over sloping topography should be common in the global ocean and therefore have significant implications for the oceanic mechanical energy budget.