The dataset consists of twin experiments with NorCPM to demonstrate the benefit of vertical localization for the isopycnal coordinate model and to assess the different tapering functions. The “truth” is known and produced from the same model but started from different initial conditions than used in the assimilation experiment. Synthetic SST observations generated from the “truth". Four reanalyzes over the period 1980–2010 were produced with assimilation of SST data.