Public access to this dataset will be closed on June 1st, 2023. For more information and possible alternatives, please consult the dedicated page on Decommissioning of ECMWF Public Datasets service.

ERA-40 is a global reanalysis dataset covering the period from September 1957 to August 2002.

The atmospheric model used for ERA-40 is known as IFS CY23r4. A comprehensive documentation about the ECMWF forecasting system (the code of which has been developed jointly with Météo-France.) can be found on the ECMWF website at

ERA-40 had the following resolution:

  • 60 levels in the vertical;
  • T159 spherical-harmonic representation for basic dynamical fields;
  • a reduced Gaussian grid with approximately uniform 125km spacing for surface and other grid-point fields.

The atmospheric model was coupled to an ocean-wave model which resolved 25 wave frequencies and 12 wave directions at the nodes of its 1.5° grid.

The ERA-40 data assimilation and forecast suite produced:

  • four analyses per day, for 00, 06, 12, and 18 UTC;
  • two forecasts per day to 6 hours ahead, from 06 and 18 UTC;
  • two forecasts per day to 36 hours ahead, from 00 and 12 UTC;

In addition, forecasts have been subsequently run to ten days ahead from 00 and 12 UTC.

ERA-40 generated supported datasets, archived in ECMWF’s Meteorological Archive and Retrieval System (MARS). The data comprises:

  • observations;
  • analysis and forecast fields from the assimilating atmospheric model at full resolution;
  • analysis and forecast fields from the atmospheric model evaluated at standard pressure levels;
  • analysis and forecast fields from the atmospheric model evaluated on isentropic and PV = ± 2 surfaces;
  • analysis and forecast fields from the coupled ocean-wave model on its regular 1.5° x1.5° latitude/longitude grid;
  • monthly means, variances and covariances;
  • feedback from the data assimilation for each observation supplied to it;
  • selected diagnostic and derived data.


Kållberg, P.,  A. Simmons, S. Uppala, and M. Fuentes, 2007: ERA-40 Project Report Series, 17. The ERA-40 Archive, 31pp,

Some ERA-40 references available from the ECMWF website.