Sensitivity of ventilation rates and radiocarbon uptake to subgrid-scale mixing in ocean models

The sensitivity of ventilation timescales and radiocarbon (C-14) uptake to subgrid-scale mixing parameterization is studied in a global ocean model. S even experiments are examined that are identical in every manner except the ir representation of subgrid-scale mixing of tracers. The cases include (i) two runs with traditional Cartesian mixing (HOR), (ii) a run with enhanced isopycnal mixing (ISO), and (iii) four runs in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams (GM ). Horizontal, isopycnal, and isopycnal-thickness diffusion coefficients ar e varied sequentially in the model runs. Of particular interest is the role of the tracer mixing schemes in influencing longer timescale ventilation p rocesses-centennial and beyond-such as deep water mass renewal and circulat ion. Simulated ventilation timescales and C-14 vary greatly between the three mi xing schemes. The isopycnal mixing run exhibits the most rapid water mass r enewal due to strong diffusion effects and excessive surface convective ove rturn, particularly in the Southern Ocean. In contrast, the GM cases show m uch more gradual renewal of deep and bottom waters, with limited vertical c onvection of surface waters and slower abyssal currents. Under GM, a backgr ound horizontal diffusion or altered isopycnal mixing do not significantly change interior ocean ventilation rates. This means modelers can adjust the se background diffusion coefficients under GM (for numerical purposes) with out significantly changing model ventilation rates. Reducing the GM isopycn al thickness diffusivity, on the other hand, noticeably increases simulated deep water ventilation rates. In comparison with the HOR runs, deep and bo ttom water ventilation timescales are reduced by about 30% in ISO, and incr eased by 30%-40% under GM. Comparison is made between model simulated and o bserved C-14. The GM runs appear to be the least successful in the North At lantic Ocean, exhibiting very gradual and only shallow water-mass renewal c ompared to observations. In the Pacific and Indian Oceans, the HOR and ISO runs are ventilated too rapidly due to strong convection and water-mass con tribution from the Southern Ocean. In contrast, the GM runs simulate spurio usly old and C-14-depleted bottom and middepth water. The GM cases do, howe ver, capture realistic C-14 in the upper 1500 m of the Indian and Pacific O ceans. Overall, none of the model cases reproduce global ocean ventilation rates over centennial timescales (under the chosen set of parameter values) . Higher horizontal resolution and a spatially varying GM thickness diffusi vity may be required before global models capture long timescale ocean rene wal processes with some degree of fidelity.