Images of internal tides near the Norwegian continental slope

Abstract

Internal tides, or internal gravity waves propagating at tidal frequencies, play an important role in ocean mixing but are challenging to detect and map over large spatial sections in the ocean's interior. We present seismic images of oceanic finestructure in the Norwegian Sea that demonstrate that semidiurnal (M2) internal tidal beams can be seismically imaged. We observe bands of seismic reflections that cross isotherms and closely mimic the expected internal tide ray characteristic over hundreds of meters vertically and tens of km laterally, in an area where critical seafloor slopes are common. Coincident temperature and density profiles show that the reflections come from reversible finestructure caused by internal wave strains. Where the beams intersect the seafloor, indications of enhanced mixing are present, including finestructure disruption and enhanced internal wave energy. These results suggest that seismic oceanography can be an effective tool in studies of ocean mixing by internal tides.