Isopycnal Mixing Controls Deep Ocean Ventilation.

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. Plain Language Summary: The ocean is full of eddies at the 100‐km scale that stir tracers along surfaces of constant density. Climate models generally have coarse grids that are not able to resolve these ocean eddies. Instead, climate models represent this stirring using a diffusion operator along surfaces of constant density: This diffusion operator smooths out gradients in ocean tracers like temperature, salinity, oxygen, and nutrients. Based on currently available observations, it is unclear how large this diffusion operator should be in climate models. The results shown in this paper suggest that the amount of diffusion along surfaces of constant density influences the pathway along which water reaches the deep Pacific. Improving estimates of the size of this diffusion operator would probably increase the accuracy of climate models. Key Points: Passive tracers are used to quantify the amount of deep water that originated in each region of the surface oceanIncreasing the isopycnal‐mixing coefficient reduces the fraction of NADW in the deep PacificIncreasing the isopycnal‐mixing coefficient reduces the ideal age of water in the deep Pacific [ABSTRACT FROM AUTHOR]