Effects of diapycnal and isopycnal mixing on the ventilation of CFCs in the North Atlantic in an isopycnic coordinate OGCM

Simulated distributions of the chlorofluorocarbons CFC-11 and CFC-12 are used to examine the ventilation of the North Atlantic Ocean in a global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). Three simulations are performed: one with a diapycnal diffusivity K_d= 3 × 10⁻⁷/N m² s⁻¹ and an isopycnal diffusive velocity (i.e., diffusivity divided by the size of the grid cell)v_trac= 0.01 m s⁻¹ (Exp. 1); Exp. 2 is as Exp. 1 but with K_d= 5 × 10⁻⁸/N m² s⁻¹ plus increased bottom mixing; and Exp. 3 is as Exp. 2 but with v_trac= 0.0025 m s−1. The main features of the simulated ventilation are strong uptake of the CFCs in the Labrador, Irminger and Nordic Seas, and a topographically aligned geostrophically controlled southward transport of CFC-enriched water in the Atlantic. It is found that the Overflow Waters (OW) from the Nordic Seas, the penetration of the western boundary currents, the ventilation of the subtropical surface waters, the vertical density stratification and the meridional overturning are all critically dependent on the applied isopycnal and diapycnal diffusivities, with Exp. 3 (Exp. 1) yielding the most (least) realistic results. Furthermore, it is the combined rather than the isolated effect of the isopycnal and diapycnal diffusivities that matter. For instance, the strength of the simulated Meridional Overturning Circulation (MOC) is similar in Exps. 1 and 3, but the simulated CFC-distributions are far too diffusive in Exp. 1 and fairly realistic in Exp. 3. It is demonstrated that the simulated distributions of transient tracers like the CFCs can be used to set the strength of the applied isopycnal mixing parameterization, a task that is difficult to conduct based on the simulated hydrography alone.