Evaluation of the ECMWF ocean reanalysis system ORAS4
Article first published online: 31 DEC 2012
© 2012 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society
Volume 139, Issue 674, pages 1132–1161, July 2013 Part A
How to Cite
Balmaseda, M. A., Mogensen, K. and Weaver, A. T. (2013), Evaluation of the ECMWF ocean reanalysis system ORAS4. Q.J.R. Meteorol. Soc., 139: 1132–1161. doi: 10.1002/qj.2063
- Issue published online: 22 JUL 2013
- Article first published online: 31 DEC 2012
- Manuscript Accepted: 1 OCT 2012
- Manuscript Revised: 14 SEP 2012
- Manuscript Received: 15 JUN 2012
- ocean reanalysis;
- climate variability;
- quality metric;
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 underestimation of meridional overturning at 26°N, the magnitude of which is shown to be sensitive to the treatment of the coastal observations.
ORAS4 shows a clear and robust shallowing trend of the Pacific Equatorial thermocline. It also shows a clear and robust nonlinear trend in the 0–700 m ocean heat content, consistent with other observational estimates. Some aspects of these climate signals are sensitive to the choice of sea-surface temperature product and the specification of the observation-error variances. The global sea-level trend is consistent with the altimeter estimate, but the partition into volume and mass variations is more debatable, as inferred by discrepancies in the trend between ORAS4- and GRACE-derived bottom pressure.