Isopycnal Mixing Controls Deep Ocean Ventilation

Abstract Climate models often parameterize isopycnal mixing by mesoscale eddies using a diffusion operator that acts in the isopycnal direction, multiplied by an isopycnal‐mixing coefficient. The magnitude of this coefficient is uncertain, with observational estimates ranging from 10 to 10,000 m2/s. In an idealized‐geometry ocean model, the isopycnal‐mixing coefficient 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 the impact of varying the isopycnal‐mixing coefficient on water mass distributions. Increasing the isopycnal‐mixing coefficient from 50 to 5,000 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. This change is associated with a 700‐yr reduction in the ideal age of water in the deep Pacific ocean.