SIMULATION OF THE ATLANTIC CIRCULATION WITH A COUPLED SEA ICE-MIXED LAYER-ISOPYCNIC GENERAL-CIRCULATION MODEL .1. MODEL DESCRIPTION

A diabatic ocean general circulation model based on primitive equation s is described. It uses isopycnals as Lagrangian coordinates in the ve rtical and predicts a free surface. Prognostic fields of temperature a nd salinity enter the dynamics as active tracers through a realistic e quation of state. The surface boundary layer is parameterized by a det ailed mixed-layer model. A sea ice model with a viscous-plastic rheolo gy is coupled to the mixed layer. Thermal forcing, wind stress, and su rface input of turbulent kinetic energy are determined from monthly me an values of atmospheric quantities, while the freshwater flux still i s parameterized by a Newtonian relaxation towards the observed surface salinity. The model equations are written in layer formulation. Each interface represents an isopycnal. As the equations are written in flu x form, the mass flux and the content of mass, heat, and salt are cons erved in the model domain. A potential vorticity conserving scheme is included. Except for the mixed layer, all layers are kept at a prescri bed potential density that is different for each layer. In the uppermo st layer, potential density is allowed to develop arbitrarily. A metho d is developed that treats vanishing layers by making the horizontal b oundaries time dependent in each layer. The time integration scheme co nsists of a predictor-corrector technique combined with a semi-implici t scheme. The model is formulated in spherical coordinates with variab le, but still orthogonal, grid resolution in longitude and latitude an d allows for any irregular geometry.