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

• Main Authors: Matthew H. England, Stefan Rahmstorf
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1999
• Subjects: Southern Ocean
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.681.1806; http://web.science.unsw.edu.au/%7Ematthew/ER99.pdf

The sensitivity of ventilation timescales and radiocarbon (14C) uptake to subgrid-scale mixing parameterization is studied in a global ocean model. Seven experiments are examined that are identical in every manner except their 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 are varied sequentially in the model runs. Of particular interest is the role of the tracer mixing schemes in influencing longer timescale ventilation processes—centennial and beyond—such as deep water mass renewal and circulation. Simulated ventilation timescales and 14C vary greatly between the three mixing schemes. The isopycnal mixing run exhibits the most rapid water mass renewal due to strong diffusion effects and excessive surface convective overturn, particularly in the Southern Ocean. In contrast, the GM cases show much more gradual renewal of deep and bottom waters, with limited vertical convection of surface waters and slower abyssal currents. Under GM, a background horizontal diffusion or altered isopycnal mixing do not significantly change interior ocean ventilation rates. This means modelers can adjust these background diffusion coefficients under GM (for numerical