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CERA-20C is a climate reanalysis dataset of the period 1901 to 2010.

CERA-20C is produced by ECMWF, it is a 10-member ensemble, and it is based on the CERA assimilation system, which assimilates only surface pressure and marine wind observations as well as ocean temperature and salinity profiles. CERA-20C is an outcome of the ERA-CLIM2 project.  For further details please see the article below from the ECMWF Newsletter 150, page 25, and/or the description on the ECMWF product page.

Known issues

1. Sea-ice accumulation

In CERA-20C, there are several issues with settings of the sea-ice coupling with the IFS that result in too cold atmospheric surface temperature over the sea-ice regions. The summer melting is not strong enough and sea-ice is accumulating over the years, particularly in the Arctic. The sea surface temperature relaxation allows to limit the impact on the sea-ice extent.

2. Discontinuity between streams

CERA-20C was conducted in 14 streams of 8 years with two-year overlap with initial conditions from uncoupled ocean and atmosphere reanalyses. There is an initialization shock at the start of each stream. While the atmosphere is adjusting relatively fast, the ocean needs much more than two years to reach equilibrium. The ocean model drift is visible in the CERA-20C record particularly in the early 20th century and in the ocean interior where the observational constraint is low.

Data access and download

Data is archived at ECMWF and available for download from the ECMWF data archive.

CERA-20C is an ensemble product, and the 'number' on the data download selection page identifies the 10 ensemble members.

The 10 ensemble members are 10 slighly different initial conditions and model parametrizations. This results in 10 slightly different forecasts, but they are all quality controlled against observations, so that none of them is 'better' than the others, they are just different.

If you need a single value for a specific meteorological parameter, we recommend you download all 10 ensemble members and then calculate the average.


  • Laloyaux, P., Balmaseda, M., Dee, D., Mogensen, K. and Janssen, P. (2016), A coupled data assimilation system for climate reanalysis. Q.J.R. Meteorol. Soc., 142: 65-78. doi:10.1002/qj.2629
  • Laloyaux, P., de Boisséson, E., Dahlgren, P. (2016), CERA-20C: An Earth system approach to climate reanalysis. ECMWF newsletter, 150: 25-30. doi:10.21957/ffs36birj2
  • Additional references available from the ECMWF e-Library