Preliminary evaluation of the 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. Compared with the current ECMWF operational system-5 (ORAS5), the ocean and sea-ice model has been upgraded and is now driven by hourly atmospheric forcing. A new Ensemble-based variational ocean Data Assimilation (EDA) system has been developed. This new EDA system is constructed with a hybrid covariance model that provides flow-dependent background error variances and correlation scales, both of which are critical for better assimilation of sea surface observations.

Direct assimilation of L4 SST observations with ORAS6 EDA system greatly reduces SST biases, especially in critical regions around the Gulf Stream separation. Assimilation of L3 sea-ice concentration data within a multi-category sea ice model has been implemented as well and shows promising results in terms of sea ice spatial distribution and concentration. ORAS6 also includes a new freshwater budget closure scheme which allows to constrain the atmosphere-ocean freshwater fluxes using an external product.

This presentation will feature results from a prototype ORAS6 reanalysis with a focus on performance evaluation against its predecessor ORAS5 and potential impacts on coupled ECMWF forecasts.