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

摘要

The TOPEX/POSEIDON altimetric sea level observations during 1993 are used to validate a simulation made by the Laboratoire d'Oceanographie Dynamique et de Climatologie ocean general circulation model (OGCM) at global scales. The forcing fields are provided by the European Centre for Medium-Range Weather Forecasts reanalysis project. The OGCM is a coarse horizontal resolution model (2 deg longitude by 1.5 deg latitude with 31 levels in the vertical), mainly used for climate studies. Horizontal mixing is done along isopycnal surfaces, whereas the parameterization of vertical mixing is based 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 assimilation case. The model simulation is compared to the observations at large spatial scales of 500 km or more. The basic features of the variability are simulated 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 resolution. In anticipation of a future assimilation of the observed sea level a decomposition of the model sea level into baroclinic/barotropic components diagnosed 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 associated with intraseasonal variability. The results of the present study confirm that the model sea level is consistent with the observed large-scale variability, providing a basis for future assimilation of the data into the OGCM to improve the representation of the ocean state.