Isopycnal Mixing Controls Deep Ocean Tracer Distributions

In coarse-resolution models, isopycnal mixing by mesoscale eddies is often parameterized using Redi’s 1982 scheme, in which an isopycnal-mixing coefficient, K_redi, is multiplied by a diffusion operator that acts in the isopycnal direction. The magnitude of K_redi is uncertain, with observational estimates ranging from 10 m²/s to 10,000 m²/s. In an idealized-geometry ocean model, K_redi is varied across a similar range without allowing the circulation to change: this leads to large changes in the ventilation of the deep ocean. Passive tracers are used to assess how K_redi influences water mass distributions, both in the modern ocean and at the LGM. In the modern ocean, increasing K_redi from 50 m²/s to 5000 m²/s leads to a 63% reduction in the amount of North Atlantic Deep Water and a doubling in the amount of Antarctic Bottom Water in the deep Pacific ocean. The effect of spatial variations in K_redi is explored.