A SIGMA-COORDINATE PRIMITIVE EQUATION MODEL FOR STUDYING THE CIRCULATION IN THE SOUTH ATLANTIC - PART I - MODEL CONFIGURATION WITH ERROR-ESTIMATES

This paper describes the configuration of a topography-following (sigm a) coordinate, numerical ocean model for studying the circulation in t he South Atlantic. An analysis is performed (i) to ensure that the mod el configuration does not introduce a numerical bias in the model solu tion and (ii) to give estimates of numerical errors. The model is the Semi-spectral Primitive Equation Model (SPEM) from Rutgers University (Haidvogel et al., 1991). Two important issues relating to the sigma-c oordinate are investigated: the pressure gradient calculation and the diffusion of tracers. Errors in the pressure gradient calculation are investigated by simulating an ocean at rest, and the choice is made to reduce errors by smoothing the bathymetry. A smoothing criterion is d erived that permits a limitation of the errors in the pressure gradien t calculation to an acceptable level (i.e. maximum errors on velocitie s below a millimeter per second).It is applied to define the model bot tom topography. Errors in the tracer fields, induced by a diffusion sc heme operating along constant sigma surfaces, generates large unrealis tic velocities (of the order of 10 cm/s). A rotation of the diffusion tensor into geopotential coordinates is proposed. Tests show that erro rs are then reduced to an insignificant level. The rotation of the dif fusion tensor is therefore retained. The numerical treatment of the op en boundaries and the flux conditions that yields the most realistic c irculation is also described. Open boundary conditions are based on ra diation conditions and relaxation to climatology. They appear to be nu merically robust, and to be able to bring into the South Atlantic basi n the necessary information from the outer oceans. A configuration of the SPEM model to study the large scale circulation in the South Atlan tic is then obtained. Errors due to model configuration are shown to b e small compared to the signal one wants to simulate, and their spatia l pattern is known, which will facilitate the interpretation of the mo del simulations presented in following papers. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.