Diapycnal motion, diffusion, and stretching of tracers in the ocean

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Measurements of in-situ turbulence reveal that mixing is bottom-enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. However, in-situ mixing estimates are indirect and the inferred vertical velocities have not yet been confirmed. Purposeful releases of inert tracers, and their subsequent spreading, have been used to independently infer turbulent diffusivities; however, these Tracer Release Experiments (TREs) provide estimates in excess of in-situ ones. In an attempt to reconcile these differences, Ruan and Ferrari (2021) derived exact buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. We show in a numerical simulation that tracer-averaged diapycnal motion is directly driven by the tracer-averaged buoyancy velocity, a convolution of the asymmetric upwelling/downwelling dipole. Diapycnal spreading, however, involves both the expected contribution from the tracer-averaged in-situ diffusion and an additional non-linear diapycnal stretching term. These diapycnal stretching effects, caused by correlations between buoyancy and the buoyancy velocity, can either enhance or reduce tracer spreading. Diapycnal stretching in the stratified interior is compensated by diapycnal contraction near the bottom; for simulations of the Brazil Basin Tracer Release Experiment these nearly cancel by coincidence. By contrast, a numerical tracer released near the bottom experiences leading-order stretching that varies in time. These results suggest mixing estimates from TREs are not unambiguous, especially near topography, and that more attention should be paid towards the evolution of tracers' first moments.

The Influence of a Suspended Cage Aquaculture Farm on the Hydrodynamic Environment in a Semienclosed Bay, SE China

Hydrodynamic responses of the aquaculture farm structures have been increasingly studied because of their importance in informing the aquaculture carrying capacity and ecological sustainability. The hydrodynamical effect of the suspended cage farm on flow structures and vertical mixing in the Sansha Bay, SE China, is examined using observational data of two comparative stations inside and outside the cage farm. The results show that current velocities are relatively uniform in the vertical except a bottom boundary layer outside the cage farm. Within the cage farm, the surface boundary layer produced by the cage-induced friction is obvious with current velocities decreasing upward, combining the classic bottom boundary layer to form a “double-drag layers” structure in the water column. The cage-induced drag decreases with water depth in the surface boundary layer with a maximum thickness of 3/4 the water column, and the current velocities can be reduced by 54%. The cage-induced friction can also significantly hinder the horizontal water exchange in the farm. Periodic stratification phenomena exist at both stations under the influence of lateral circulation. However, the subsurface (5–10 m below the sea surface) water column below the cage facilities is well-mixed as indicated by the vertical density profile, where the velocity shear (10–3 m–2) is about 10 times higher than that of the subsurface layer outside the cage farm. Therefore, we speculate that the well-mixing of the subsurface water column results from the local turbulence induced by the velocity shear, which in turn is produced by the friction of cage structures.