INTERNAL WAVE-TOPOGRAPHY AND FLOW INTERACTIONS IN THE BOTTOM BOUNDARY LAYER

Abstract

Turbulent bottom boundary layers (BBLs) over sloping topography play a central role in modifying water properties along dense overflow pathways and sustaining the upwelling limb of the global overturning circulation. This work examines the mechanisms controlling the transport and mixing of buoyancy, momentum, and tracers in these layers using high-resolution, idealized large-eddy simulations with a grid-fitted immersed boundary method (IBM). The IBM is validated against canonical downslope Ekman flows, reproducing BBL structure and associated transports across grid-fitted topographic resolutions. Internal waves breaking on slopes generate near-bottom bores and overturns in the ``internal swash" zone, driving fluid ejections into the stratified interior. The extent of these overturns, turbulent mixing, and along-isopycnal intrusions depends strongly on the effective wave height, a length scale defined by the ratio of wave velocity to background buoyancy frequency. A volume budget and patterns in buoyancy flux divergence reveal spatial and temporal variability critical for assessing water-mass transformation and restratification. In dense overflows, symmetric instability (SI) and near-inertial waves, modified by a generated baroclinic background field, extract energy from the mean flow, enhance cross-slope transport, and increase dissipation within the BBL. Long after surface cooling is shut off, SI cells continue to propagate towards the shelf break, driving persistent off-shelf export of dense water. In the interior, positive potential vorticity features generated by the propagation of the gravity current cause the reflection and trapping of near- and sub-inertial waves, promoting wave breaking and possibly enhanced diapycnal mixing. These results demonstrate that interactions among topography, mean flow, internal waves, and submesoscale instabilities play a fundamental role in BBL dynamics, tracer transport, and energy redistribution in dense overflow regions and the deep ocean. By characterizing the restratification and mixing by internal wave breaking, persistent SI-driven off-shelf export, and dense overflow-modified near-inertial wave trapping and reflection, this work provides a framework for interpreting boundary-driven exchange and mixing, thereby guiding their future representation and evaluation in global ocean models.