ECMWF Ocean Reanalysis System 4 (ORAS4)

A new operational ocean analysis/reanalysis system (ORAS4) has been implemented at ECMWF.

Data assimilated: Temperature and Salinity profiles from the EN3 v2a XBT bias corrected database (1958-2009), including XBT, CTD, Argo, Mooring, and from realtime GTS thereafter. Along track altimeter sea level anomalies and global trends from AVISO. SST and sea-ice are from the ERA-40 archive prior to November 1981, from the NCEP OI v2 weekly product (1981 until December 2009) and , and from OSTIA analysis from January 2010 onwards.The SST and sea-ice information is used to constrain the upper level ocean temperature via a newtonian relaxation scheme.

Assimilation method: NEMOVAR in its 3D-var FGAT mode. The assimilation window is 10 days. A bias correction scheme is used to correct the model/forcing errors. Forcing and relaxation used: Daily surface fluxes of heat, momentum and fresh water are used. Prior to 1989, the surface fluxes are from the ERA-40 atmospheric reanalysis. From the period 1989-2009, the surface fluxes are from ERA-Interim reanalysis. From 2010 onwards, when ORAS4 started operational running, daily surface fluxes were derived from the operational ECMWF atmospheric analysis.

Disclaimer: There is large uncertainty in the ocean reanalysis products (especially in the transports), difficult to quantify. These web pages are aimed at the research community. Any outstanding climate feature should be investigated futher and not taken as truth.


Balmaseda, M. A., Mogensen, K. and Weaver, A. T. (2012), Evaluation of the ECMWF ocean reanalysis system ORAS4. Q.J.R. Meteorol. Soc..
doi: 10.1002/qj.2063

Mogensen, K., M. Alonso Balmaseda, A. Weaver, 2012: The NEMOVAR ocean data assimilation system as implemented in the ECMWF ocean analysis for System4. ECMWF Technical Memorandum 668. 59 pages.

potential temperature (degC), salinity, ocean velocity (m/s), sea level (m)
MLD: -0.2 temp crit (m) , -0.5 temp crit (m) , 0.03 rho crit (m) , 0.125 rho crit (m)
Zonal Global by 1 deg
Meridional Global by 1 deg
Vertical 5.02159, 15.07854, 25.16046, 35.27829, 45.44776, 55.69149, 66.04198, 76.54591, 87.27029, 98.31118, 109.8062, 121.9519, 135.0285, 149.4337, 165.7285, 184.6975, 207.4254, 235.3862, 270.5341, 315.3741, 372.9655, 446.8009, 540.5022, 657.3229, 799.5496, 967.9958, 1161.806, 1378.661, 1615.291, 1868.071, 2133.517, 2408.583, 2690.78, 2978.166, 3269.278, 3563.041, 3858.676, 4155.628, 4453.502, 4752.021, 5050.99, 5350.272  
Temporal Jan 1958 to Dec 2017 by 1 month(s)
MLD: Jan 1958 to Dec 2011 by 1 month(s)
Static? yes
Volume ~547MB / year
Server public:
Acquired Sep 10, 2012 (Updated Feb 2018)
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