In order to improve the Mercator Assimilation System, SAM2, different approaches are developed to analyse the system performance. Sensitivity analysis is one of them. It should help to point out the important regions and variables to control to reduce the forecast error. The forecast error is measured with the observation misfit. Its sensitivity to the initial conditions of the ocean state and the forcing field is computed with the adjoint model. This will also give us an indication on how the initial conditions are important to optimize compared to forcing fields. This analysis is applied to the Glorys reanalysis. Glorys is based on the global configuration ORCA1/4°, the model version was NEMO1.9. The next simulation will be based on the NEMO3.0 version. The first reanalysis stream covers the period from 2002 to 2008. Along track SLA (AVISO/SALTO), in-situ temperature and salinity profiles (CORA02) and SST observations are assimilated. The ocean forcing field computation is based on bulk formulae, the atmospheric fields are coming from the ECMWF operational model.