Simulation of the Atlantic Circulation with a Coupled Sea Ice-Mixed Layer-Isopycnal General Circulation Model. Part I: Model Description

A diabatic ocean general circulation model based on primitive equations is described. It uses isopycnals as Lagrangian coordinates in the vertical and predicts a free surface. Prognostic fields of temperature and salinity enter the dynamics as active tracers through a realistic equation of state. The surface boundary layer is parameterized by a detailed mixed-layer model. A sea ice model with a viscous-plastic rheology is coupled to the mixed layer. Thermal forcing, wind stress, and surface input of turbulent kinetic energy are determined from monthly mean values of atmospheric quantities, while the freshwater flux still is 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 flux form, the mass flux and the content of mass, heat, and salt are conserved in the model domain. A potential vorticity conserving scheme is included. Except for the mixed layer, all layers are kept at a prescribed potential density that is different for each layer. In the uppermost layer, potential density is allowed to develop arbitrarily. A method is developed that treats vanishing layers by making the horizontal boundaries time dependent in each layer. The time integration scheme consists of a predictor-corrector technique combined with a semi-implicit scheme. The model is formulated in spherical coordinates with variable, but still orthogonal, grid resolution in longitude and latitude and allows for any irregular geometry