The transport of tracers, such as heat, salt and carbon, within the ocean plays an important role not only in ocean dynamics, thermodynamics and biogeochemistry but also as a method to observe the ocean and infer circulation properties.

Large-scale tracer release experiments, where a synthetic tracer is released in the ocean and observed over a period of several years, have been instrumental in improving our understanding of the role of turbulent mixing in driving ocean circulation. However, there has yet to be an experiment where a tracer is released close to the seafloor in the abyssal ocean, despite the inferred importance of layers of strong turbulent mixing immediately above rough, sloping abyssal topography.

In anticipation of such an experiment, CLEX researchers examined the behaviour of a passive tracer released near the seafloor in an idealised two-dimensional flow driven by bottom-enhanced turbulence. They found the presence of the boundary made it difficult to directly relate the average spreading rate of the tracer across surfaces of constant density to the small-scale turbulent diffusivity important for the ocean’s buoyancy and mass budgets.

The above animation shows a tracer released near a sloping boundary within an idealized abyssal mixing layer. The tracer spreads vertically due to bottom-intensified vertical diffusion and up the slope within a thin Bottom Boundary Layer (BBL) where the buoyancy flux converges and drives diapycnal upwelling. The rate of increase of the variance of the tracer in density space (left panel) reduces once the tracer encounters the boundary.

Surprisingly, the bottom boundary reduced the rate that the tracer diffuses across surfaces of constant density, despite the turbulence being most enhanced at the seafloor. These results have implications for what can be learned about the characteristics of mixing near sloping boundaries from both past and future tracer-release experiments.