Observation of Near-Inertial Waves in the Bottom Boundary Layer of an 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 ob-served. 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 in-teractions in the BBL. Energy loss from the near-bottom flows to radiating NIWs (-8 X 10-4 W m-2) is estimated to be sub-stantially larger than that due to bottom drag dissipation (-2 X 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.

Automated summary

The study investigates the energy dissipation mechanism of low-frequency flows in the abyssal ocean, specifically focusing on the role of the bottom boundary layer (BBL). In situ data collected near an abyssal seamount in the western Pacific Ocean shows that strong bottom-trapped flows over sloping topography can lose their energy to near-inertial waves (NIWs) generated by the adjustment of the bottom Ekman layer. These waves have near-resonant frequencies corresponding to internal waves and are strongest in the BBL. The generation of internal waves via Ekman transport adjustment plays an important role in damping the subinertial flows over the sloping seafloor.