A global summary of seafloor topography influenced by internal-wave induced turbulent water mixing

Abstract. Turbulent water motions are important for the exchange of momentum, heat, nutrients, and suspended matter including sediments in the deep-sea that is generally stably stratified in density. To maintain ocean-density stratification, an irreversible diapycnal turbulent transport is needed. The geological shape and texture of marine topography is important for water mixing as most of deep-sea turbulence is generated via breaking internal waves at sloping seafloors. For example, slopes of semidiurnal internal tidal characteristics can ‘critically’ match the mean seafloor slope. In this paper, the concept of critical slopes is revisited from a global internal wave-turbulence viewpoint using seafloor topography- and moored high-resolution temperature sensor data. Observations suggest that turbulence generation via internal wave breaking at 5±1.5 % of all seafloors is sufficient to maintain ocean-density stratification. However most, >90 %, turbulence contribution is found at supercritical, rather than the more limited critical, slopes measured at 1'-scales that cover about 50 % of seafloors at water depths < 2000 m. Internal tides (~60 %) dominate over near-inertial waves (~40 %), which is confirmed from comparison of NE-Atlantic data with East-Mediterranean data (no tides). Seafloor-elevation spectra show a wavenumber (k) fall-off rate of k^-3, which is steeper than previously found. The fall-off rate is even steeper, resulting in less elevation-variance, in a one-order-of-magnitude bandwidth around k_T=0.5 cycle-per-km. The corresponding length is equivalent to the internal tidal excursion. The reduction in seafloor-elevation variance seems associated with seafloor-erosion by internal wave breaking. Potential robustness of the seafloor-internal wave interaction is discussed.