Diapycnal Mixing Induced by Rough Small‐Scale Bathymetry.

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. Plain Language Summary: Mixing of water across density interfaces is important for ecosystems and the circulation between basins. However, mixing related to rough small‐scale bathymetry is often not resolved in models and difficult to measure. In this study, we show high‐resolution acoustic observations of intense vertical mixing across a strong density interface, that separates the saltier bottom water from the fresher surface water in the northern Baltic Sea. In the study region, steep underwater hills and ridges extend into the density interface. As water flows over the region, the hills and ridges cause the water to mix. Measured values of mixing and vertical salt fluxes in this region are up to two orders of magnitude higher than at a nearby reference station with smooth bathymetry. Our analysis suggests that the observed high mixing is mainly caused by eddies in the wake of obstacles and secondarily by breaking internal waves, which are waves within the water that occur on interfaces between layers with different properties. Understanding mixing mechanisms and estimating their contribution to the total mixing is needed to implement mixing into ocean models. This study highlights the importance of rough small‐scale (<1 km) seafloor features for mixing and vertical transport of salt. Key Points: High‐resolution turbulence observations in a coastal, strongly stratified region with extremely rough seafloor topographyAcoustic turbulence imaging shows highly intermittent and localized mixing due to wake eddies and internal‐wave breaking near obstaclesTurbulent diffusivities and transport rates of salt increased by two orders of magnitude inside mixing hotspots near rough bathymetry [ABSTRACT FROM AUTHOR]