Sea level comparison between TOPEX/POSEIDON altimetric data and a global ocean general circulation model from an assimilation perspective

The TOPEX/POSEIDON altimetric sea level observations during 1993 are used t o validate a simulation made by the Laboratoire d'Oceanographie Dynamique e t de Climatologie ocean general circulation model (OGCM) at global scales. The forcing fields are provided by the European Centre for Medium-Range Wea ther Forecasts reanalysis project. The OGCM is a coarse horizontal resoluti on model (2 degrees longitude by 1.5 degrees latitude with 31 levels in the vertical), mainly used for climate studies. Horizontal mixing is done alon g isopycnal surfaces, whereas the parameterization of vertical mixing is ba sed on a local estimate of the turbulent kinetic energy. The comparison is quantitative in the sense that TOPEX/POSEIDON observations are interpolated onto the model grid by an objective interpolation in view of the assimilat ion case. The model simulation is compared to the observations at large spa tial scales of 500 km or more. The basic features of the variability are si mulated well by the model, in particular with respect to the annual cycle. Major discrepancies are found in the vicinity of western boundary currents and in the Southern Ocean, due in part to the inadequate horizontal resolut ion. In anticipation of a future assimilation of the observed sea level a d ecomposition of the model sea level into baroclinic/barotropic components d iagnosed with some dynamical variables of the model is then examined. It is shown that the baroclinic component explains the large seasonal variations confined in the upper layers, whereas the barotropic component is associat ed with intraseasonal variability. The results of the present study confirm that the model sea level is consistent with the observed large-scale varia bility, providing a basis for future assimilation of the data into the OGCM to improve the representation of the ocean state.