Global and regional ocean and sea ice reanalysis products (ORAs) are increasingly used in polar research, but their quality remains to be systematically assessed. To address this, the Polar ORA Intercomparison Project (Polar ORA-IP) has been established following on from the ORA-IP project. Several aspects of ten selected ORAs in the Arctic and Antarctic were addressed by concentrating on comparing their mean states in terms of snow, sea ice, ocean transports and hydrography. Most polar diagnostics were carried out for the first time in such an extensive set of ORAs. For the multi-ORA mean state, we found that deviations from observations were typically smaller than individual ORA anomalies, often attributed to offsetting biases of individual ORAs. The ORA ensemble mean therefore appears to be a useful product and while knowing its main deficiencies and recognising its restrictions, it can be used to gain useful information on the physical state of the polar marine environment.

We acknowledge Dr. Benjamin Rabe and the two anonymous reviewers for their comments that significantly improved the manuscript. EU-COST EOS-1402 Ocean Synthesis action is acknowledged for their support, in particular to assist the organisation of the Polar ORA-IP meetings, both physical and virtual, which were crucial for the study. Work of Uotila was supported by the Finnish Academy (Grants 264358 and 283034) and by the EU MCSA grant 707262-LAWINE. Chevallier, Fučkar, Haines and Massonnet have received funding from the European Union’s Horizon 2020 Research and Innovation programme through Grant agreement No. 727862 APPLICATE. Fučkar was a Juan de la Cierva-incorporacion fellow supported by the Spanish government. Goosse is a research director and Massonnet a post-doctoral researcher with the FRS/FNRS, Belgium. The ORA and MMM data used in this study are provided by Hamburg University on the ORA-IP web-site at https ://icdc.cen.uni-hambu rg.de/1/daten /reana lysis -ocean /oraip .html.

Peer Reviewed