Evaluation of the ECMWF ocean reanalysis system ORAS4

Abstract A new operational ocean reanalysis system (ORAS4) has been implemented at ECMWF. It spans the period 1958 to the present. This article describes its main components and evaluates its quality. The adequacy of ORAS4 for the initialization of seasonal forecasts is discussed, along with the robustness of some prominent climate signals. ORAS4 has been evaluated using different metrics, including comparison with observed ocean currents, RAPID‐derived transports, sea‐level gauges, and GRACE‐derived bottom pressure. Compared to a control ocean model simulation, ORAS4 improves the fit to observations, the interannual variability, and seasonal forecast skill. Some problems have been identified, such as the…