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


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 sub-inertial 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/m2) is estimated to be substantially larger than that due to bottom drag dissipation (~2 × 10−4 W/m2), suggesting the important role of internal wave generation via the Ekman transport adjustment in damping the sub-inertial flows over the sloping seafloor.