ECMWF's current ocean reanalysis uses a sophisticated data assimilation methodology which includes a model bias correction. The ocean model used is forced by atmospheric daily surface fluxes, relaxed to SST and bias corrected.
The Balmaseda et al (QJRMS 2013) reference provides excellent peer reviewed documentation and evaluation.
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National Center for Atmospheric Research Staff (Eds). Last modified 13 Oct 2016. "The Climate Data Guide: ORAS4: ECMWF Ocean Reanalysis and derived ocean heat content." Retrieved from https://climatedataguide.ucar.edu/climate-data/oras4-ecmwf-ocean-reanalysis-and-derived-ocean-heat-content.
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Created by: Climate Data Guide PIs and Staff