An ocean circulation model developed at MPI. It is based on the primitive equations and designed specifically to model global scale geostrophic circulation. The nonlinear advection of momentum is neglected and fast gravity waves are strongly damped via an implicit time integration scheme that uses a time step of 30 days. An upstream advection scheme us used for salinity and temperature transport and vertical convective mixing is applied when the stratification becomes unstable. The Arakawa E-grid is used for horizontal discretization and a small horizontal diffusion specified to alleviate the inherent mode-splitting problems of this grid scheme. Sea ice is computed from a heat balance and by advection by ocean currents with a simplified viscous rheology.