ERA-40¶

ECMWF 40-year Reanalysis (ERA-40)

Discontinued

At a glance¶

Note on period: September 1957 to August 2002 (45 years), starting just before the International Geophysical Year (1958).

Overview¶

ECMWF's second-generation reanalysis of the global atmosphere and surface conditions for the 45 years from September 1957 to August 2002. It used 3D-Var (FGAT) data assimilation with a T159, 60-level model and was among the first reanalyses to assimilate satellite radiances (TOVS, SSM/I, etc.) directly. A coupled ocean-wave model provides wave parameters as well. ERA-40 was a standard dataset for climate research until the mid-2000s and has since been superseded by ERA5.

What changes with the successor¶

ERA-40 → ERA-Interim¶

ERA-Interim replaced 3D-Var with 12-hour 4D-Var and variational bias correction of satellite radiances (VarBC) and raised the resolution to T255, addressing ERA-40's problems with the hydrological cycle, the stratospheric circulation and temporal consistency.

Sources: Dee et al. (2011), The ERA-Interim reanalysis, QJRMS · Uppala et al. (2005), The ERA-40 re-analysis, QJRMS · ECMWF — What is the difference between ERA-Interim (1979-2019) and ERA-40 (1957-2002) · ERA-Interim documentation (Copernicus Knowledge Base) · Kållberg et al. (2004), The ERA-40 archive · Uppala (2006), From ERA-15 to ERA-40 and ERA-Interim

Compare the two side by side

Available variables¶

Main parameters from the parameter tables of the ERA-40 archive report (ERA-40 Project Report Series No. 17). Names are ECMWF GRIB short names (matching the parameter codes). The original data are GRIB1; accumulated parameters are forecast accumulations (W m-2 s, etc.).

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

Pressure levels (23)¶

Surface and single level¶

Accumulated forecast parameters¶

Ocean waves¶

Land surface¶

The full variable list is in the official documentation.

How to get the data¶

• NSF NCAR GDEX — ERA-40 Global Surface and Flux Data (d117000) — Web download (HTTPS) / Globus / subsetting and format conversion
  • 6-hourly surface and flux data on the Gaussian grid. Subsetting and format conversion require an NCAR login.
• NSF NCAR GDEX — ERA-40 Monthly Means of 2.5 Degree Upper Air Pressure Level Analysis (d120001) — Web download (HTTPS) / Globus / subsetting
• NSF NCAR GDEX — ERA-40 2.5° surface and flux monthly means (d120000) — Web download (HTTPS) / Globus / subsetting
• NSF NCAR GDEX — ERA-40 Global Upper Air Pressure Level Analysis (d117001) — On request
  • When checked on 2026-10-11 the dataset was marked as containing files that are not currently available, so request them through GDEX Contact Us. The 2.5° 6-hourly pressure-level (d118001) and surface (d118000) datasets are in the same state.
• CEDA Archive — ERA-40 model data — Web download
  • Requires a CEDA account. Holds data on the reduced Gaussian grid and the regular 2.5° grid; ECMWF data conditions apply.

Practical tips¶

• The NCAR Climate Data Guide lists the main weaknesses: tropical moisture (precipitation, total column water) too high after 1991, precipitation well above evaporation, an overly strong Brewer-Dobson circulation and unrealistic Arctic temperature trends. Avoid ERA-40 for hydrological-cycle, stratospheric or Arctic-trend studies.
• The observing system changes sharply around 1979 as satellite data come in. A trend over the full 1957–2002 period can carry that discontinuity.
• With only 23 pressure levels, compare with ERA-Interim or ERA5 (37 levels) on the levels they share.

Notices and change history¶

References for your paper¶

Dataset citation¶

NCAR GDEX surface and flux (d117000) — DOI 10.5065/S4S2-R159

@misc{era40_d117000,
  author    = {{European Centre for Medium-Range Weather Forecasts}},
  title     = {{ERA-40} Global Surface and Flux Data},
  year      = {2004},
  publisher = {NSF National Center for Atmospheric Research},
  doi       = {10.5065/S4S2-R159},
  note      = {Accessed DD MMM YYYY}
}

NCAR GDEX 6-hourly pressure-level analysis (d117001) — DOI 10.5065/WQZB-A142

NCAR GDEX 2.5° pressure-level monthly means (d120001) — DOI 10.5065/R275-GB48

Reference papers¶

ERA-40 reference paper — DOI 10.1256/qj.04.176

@article{uppala2005era40,
  author  = {Uppala, S. M. and K{\"a}llberg, P. W. and Simmons, A. J. and Andrae, U. and Da Costa Bechtold, V. and Fiorino, M. and others},
  title   = {The {ERA-40} re-analysis},
  journal = {Quarterly Journal of the Royal Meteorological Society},
  volume  = {131},
  number  = {612},
  pages   = {2961--3012},
  year    = {2005},
  doi     = {10.1256/qj.04.176}
}

ERA-40 archive report (optional, no DOI)

Acknowledgements¶

Credit ECMWF as the producer and name the archive you actually used (the NCAR GDEX dataset DOI or the CEDA catalogue record). GDEX provides the data on condition that users accept the ECMWF terms of use.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• ECMWF — ECMWF Reanalysis 40 years (ERA-40)
• The ERA-40 archive (parameter tables, PDF)
• ERA-40 Atlas (ECMWF)
• What is the difference between ERA-Interim and ERA-40 (Copernicus Knowledge Base)
• Decommissioning of ECMWF Public Datasets service
• NCAR Climate Data Guide — ERA-40