Diapycnal Mixing Induced by Rough Small‐Scale Bathymetry

Abstract

Abstract Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non‐tidal flow over multiple small‐scale (<1 km) bathymetric features penetrating a strongly‐stratified density interface in a coastal region. We combine high‐resolution broadband acoustic observations of turbulence microstructure with traditional shear microstructure profiling, to resolve the variability and intermittency of stratified turbulence related to the rough bathymetry. Scale analysis and acoustic imaging suggest that underlying mixing mechanisms are related to topographic wake eddies and breaking internal waves. Depth averaged dissipation rates (1.1 × 10−7 Wkg−1) and turbulent vertical diffusivities (7 × 10−4 m2s−1) in the halocline exceed reference values by two orders of magnitude. Our study emphasizes the importance of rough small‐scale bathymetric features for the vertical transport of salt in coastal areas.

Keywords

• diapycnal mixing
• rough small‐scale bathymetry
• stratified flow over obstacles
• broadband acoustic observations of turbulent mixing
• microstructure profiler turbulence measurements
• mixing across halocline