The ECMWF OCEAN5 system is a new global eddy-permitting ocean-sea ice ensemble reanalysis analysis system. This Technical Memorandum gives a full description of the OCEAN5 system, with the focus on its Behind-Real-Time (BRT) component, the reanalysis product ORAS5. The OCEAN5 Real-Time (RT) component includes all upgrades developed for ORAS5 and runs daily using the latest observations and forcing fields from the operational Numerical Weather Prediction (NWP).
ORAS5 includes 5 ensemble members and covers from 1979 onwards. Compared to ORAS4, the ocean model resolution has been increased to an eddy-permitting resolution (0.25o) in the horizontal and a near-surface resolution of 1 m in the vertical. ORAS5 also includes a prognostic thermodynamic-dynamic sea-ice model with assimilation of sea-ice concentration data. Both observation data sets and forcing fields have been updated as well. An important novelty in ORAS5 is a generic ensemble generation scheme for perturbing both observations and forcing fields. Other upgrades include revisions to the ensemble-based spin-up and a-priori bias correction scheme. Assessment of ORAS5 and sensitivity experiments suggests that all system components contribute to an improved ocean analysis state, with directly assimilation of temperature and salinity in-situ data as the most influential term. Compared to ORAS4, ORAS5 also shows improved ocean climate variability in terms of Sea Surface Temperature (SST) and sea-level when verified against independent observation data sets. The SST biases in the Gulf Stream region remain large in ORAS5 though, which is associated with a misrepresentation of front positions and overshoot of the northward transport of the Gulf Stream.