COSMOS, MPI-M

HAMOCC, the oceanic biogoechemical component of the COSMOS Earth System Model (now renamed to MPI-ESM) was originally designed to investigate the global marine carbon cycle at long time scales (Maier-Reimer, 1993). In earlier versions of HAMOCC the emphasis was rather put onto a representation of sediments than on seasonal variations of the marine biology in the upper ocean (Heinze et al, 1999). The current version of HAMOCC includes a simplified plankton dynamics model treating the comportments nutrients, phytoplankton, zooplankton, DOM (dissolved organic matter) and detritus. Main prognostic variables are the net primary production of the phytoplankton community as well as the export of dead organic material out of the euphotic zone. Limiting nutrients for the marine production are phosphate, nitrate and iron. There is no explicit separation of phytoplankton types, but the mode accounts for production and export of shell material which consists of opaline and calciferous frustales. Silification and calcification rates in the surface layers are adapted in such a way that the resulting export and dissolution of shell material reproduce the observed vertical silicate and alkalinity profiles (Six and Maier-Reimer, 1996). The nitrogen cycle is closely linked to the marine phosphate cycle via a constant stoichiometric ratio. In oxygenated water the remineralisation of organic matter produces phosphate and nitrate in fixed proportions in relation to the consumed oxygen. In regions of low oxygen concentration (OMZ, O₂ > 2 µmol/L) destruction of organic matter occurs by denitrification, i.e., nitrate serves as oxygen donator for denitrifying bacteria. Atmospheric nitrogen fixation occurs in surface waters if the nitrogen to phosphate ratio is lower than the Redfield ratio (N:P = 16).

The model resolution in time and space is adjustable to the scales of interest and reaches from coarse resolution of horizontally 3 × 3 degrees, 40 levels and 2.4 hours time step to 0.4 × 0.4 degrees, 80 levels, and 1 hour time step. The physical forcing is provided by MPIOM (Marsland et al. 2003), a primitive equation ocean model with a free surface, a realistic bottom topography, and a thermodynamic-dynamic sea-ice model.

The model was spun-up for 1650 years in coarse resolution (nominally 3×3 deg) and a further 300 years with the final resolution (nominally 1.5 × 1.5 deg) starting from globally uniform profiles for bgcc-tracers, and Levitus physical parameters in stand alone mode. The fully coupled model was then spun-up for 150 years before the transient experiments were started following the C4MIP protocol.