ECMWF 6th generation ocean and sea-ice reanalysis system (ORAS6)

Ocean and sea-ice reanalyses are reconstructions of historical ocean and sea-ice states generated by ingesting observations into simulated model states through data assimilation methods. The Ocean ReAnalysis System-6 (ORAS6) is the 6^th generation of ECMWF ocean and sea-ice reanalysis system. ORAS6 is forced by hourly ERA5 atmospheric fields and uses an ensemble variational ocean data assimilation (EDA) together with the latest reprocessed input datasets to produce a state-of-the-art 11-member ensemble of ocean reanalyses. Ocean and sea-ice states from ORAS6 will be used as ocean initial conditions for both the upcoming coupled ERA6 atmospheric reanalysis and ECMWF forecasting activities and will also be crucial for the continuation of climate monitoring activities within the Copernicus Services.

This presentation will address the performance of ORAS6 with respect to its predecessor (ORAS5) in terms of: fit to both in-situ and remotely sensed observations; representation of physical processes such as ocean transport and SST diurnal cycle; and also in terms of quality and consistency of the climate signals. Impacts of the use of an upgraded ocean and sea-ice model, new data assimilation methods and state-of-the-art atmospheric forcings and ocean observing system will also be evaluated in ORAS6.

Finally, the positive impact of ORAS6 ocean states as initial conditions for ECMWF coupled atmosphere-ocean-sea-ice forecasts will be discussed.