A global summary of seafloor topography influenced by turbulent water mixing

TL;DR

This paper investigates how seafloor topography influences turbulent water mixing in the deep sea, highlighting the role of internal waves and slopes in generating turbulence that maintains ocean stratification.

Contribution

It revisits the concept of critical slopes using global data and high-resolution measurements, revealing that most turbulence occurs at supercritical slopes and is dominated by internal tides.

Findings

Turbulence from internal wave breaking occurs at about 5% of seafloors.

Over 90% of turbulence contribution is at supercritical slopes.

Seafloor elevation spectra follow a k^-3 fall-off, steeper than previously reported.

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 are revisited from a global internal wave-turbulence viewpoint seafloor topography -- and using 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-minute-scales that cover about 50% of seafloors at water depths < 2000 m. Internal tides (about 60%) dominate over near-inertial waves (about 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 erosion by internal wave breaking. Potential robustness of the seafloor-internal wave interaction is discussed.