The ORAP6 ocean and sea-ice reanalysis: description and evaluation

Ocean and sea-ice are two essential components of Earth system models. By providing initial conditions of these two system states, ocean and sea-ice analysis play a vital part in the coupled forecasting system of NWP service. A historical reconstruction of ocean and sea-ice states, or reanalysis, can be produced by ingesting observations into simulated model states through data assimilation methods. Ocean and sea-ice reanalyses provide invaluable information for climate monitoring, and also for long-term prediction such as decadal or climatic projections. The Ocean ReAnalysis Pilot system-6 (ORAP6) is a new ocean and sea-ice reanalysis that has been developed based on the ECMWF operational OCEAN5 system. Despite sharing the same model configurations as OCEAN5, ORAP6 uses different Atmospheric forcing and is produced with the most up-to-date reprocessed observation datasets. The data assimilation system has been updated as well, including: i) assimilation of L3 sea-ice concentration data instead of L4 gridded data; ii) a new flow-dependent SST nudging scheme; iii) refined off-line bias correction term for both temperature and salinity. In addition, observation error covariance settings have been revised, especially for observations near the coast and in the high-latitudes. Production of ORAP6 for the full ERA5 period (1979-2019) has been completed. Preliminary evaluation suggests that, in a general sense, ocean and sea-ice states are improved in ORAP6 w.r.t to its predecessor ORAS5, partially due to its more realistic large-scale overturning circulations. The ORAP6 sea-ice performance is better in the sense of both climate signals and spatial distributions of sea-ice thickness and concentration. The ocean heat content tendency in ORAP6 also correlates better with variations of global net energy input derived from independently observed TOA radiation data. A throughout evaluation of ORAP6 is currently underway.