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

Ocean and sea-ice reanalyses are reconstructions of historical ocean and sea-ice states by ingesting observations into simulated model states through data assimilation methods. Reanalysis provides invaluable information for climate monitoring and is an essential component in long-term prediction such as seasonal to decadal forecasts. The Ocean ReAnalysis System-6 (ORAS6) is the 6^th generation of ocean and sea-ice reanalysis system developed at ECMWF. ORAS6 is forced by hourly ERA5 atmospheric fields and uses an ensemble variational ocean data assimilation (EDA) to produce an 11-member ensemble of ocean reanalyses. Both the new EDA system and the use of the latest reprocessed input datasets help making better use of ocean observations to provide ocean states with improved fit to observations both at the surface and the subsurface, a better representation of the daily cycle of the surface temperature and more accurate ocean transports. ORAS6 sea-ice states are produced using a new multi-category sea ice model assimilating sea-ice concentration observations and show improved fit to satellite observations. This increased complexity came with its lot of technical challenges but now opens the door for the future assimilation of new sea-ice variables such as sea-ice thickness and new diagnostics such as snow depth over sea-ice that will benefit from future satellite missions.

Ocean and sea-ice states from ORAS6 will be used to initialise the ocean component of the ECMWF operational coupled ensemble forecasts and provide the lower boundary conditions for the upcoming ERA6 atmospheric reanalysis produced by the Copernicus Climate Change Service (C3S). This presentation will feature results from the first stream of production of ORAS6 covering the most recent decade as well as a preview of ORAS6 streams covering the rest of the satellite-observed decades as well as the pre-satellite period with a focus on performance evaluation against the current ECMWF operational system-5 (ORAS5).