A global model of silicon cycling: Sensitivity to eddy parameterization and dissolution

A simple model of silicon cycling has been embedded in a coarse-resolution ocean general circulation model. The modeled distribution of silicate was extremely sensitive to the parameterization of the effects of mesoscale eddies, and was somewhat sensitive to the distribution of dissolution within the water column. The modeled export flux of biogenic silica varied by a factor of 2 with respect to both the range of dissolution and eddy parameterizations. The best results were found when eddies were parameterized as mixing along isopycnals while simultaneously driving an advective flux so as to homogenize the depth of isopycnal surfaces (the so-called Gent-McWilliams parameterization). The Gent-McWilliams scheme produced the most realistic stratification field in the Southern Ocean, reducing vertical exchange and hence export fluxes of biogenic silica. This scheme did not solve the model's tendency to underpredict the formation of the North Atlantic Deep Water and to overpredict the importance of Antarctic source waters in the abyss, especially in the Atlantic. When used with a temperature-dependent dissolution scheme, this model predicted a global production of 89 Tmol yr(-1) with about 40% occurring in both the Southern Ocean and the tropics. In both the North Pacific and Southern Oceans, local silicate maxima are maintained by near-surface inflow of silicate with outflow at depth. The modeled production of biogenic silica showed little sensitivity to polar freshening when the Gent-McWilliams parameterization was used. The results emphasize the impact of including both the diffusive and advective effects of eddies, as in the Gent-McWilliams parameterization.