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Created by
Michela Giusti, last updated by Anabelle Menochet on Apr 15, 2026
3 minute read
Introduction
Ocean Reanalysis System 5 (ORAS5) is the global monthly ocean and sea-ice ECMWF reanalysis. It covers the past 68 years from January 1958 to the present.
This dataset presented here is a subset of some parameters of the full CDS ORAS5 global ocean monthly dataset on approximately 0.25° x 0.25° (around 25 km in the tropics and 9 km in the Arctic) resolution and is stored in Analysis Ready Cloud Optimised (ARCO) format, which has been implemented for retrieving long time-series for a single point in a efficient way.
Methodology
The process to generate this dataset involves:
- Fetching ORAS5 data in netCDF format.
- Regridding from original tripolar coordinate system to a regular 0.25º longitude and latitude grid.
- Applying homogenization conventions, as applicable:
- Ensure latitude, longitude ranges are between [-90, +90], [-180, +180].
- Unify time reference. This is to ensure that all files share a common time encoding and a temporal dimension called time.
- Convert depth to elevation. Elevation is sorted in an increasing fashion, from the bottom of the sea up. Values below the sea surface are negative.
- Writing to an ARCO Zarr archive (which is similar in structure to netCDF).
- The CDS requests the data from the Zarr archive using xarray and writes the data to netCDF or CSV (as requested).
Data organization and access
The ARCO data is stored as Zarr datacubes.
The data is now available from the Climate Data Store (CDS), either interactively through its download web form or programmatically using the CDS API service:
ORAS5 global ocean reanalysis monthly time series data from 1958 to present
Spatial grid
The parameters have a regular grid of 0.25 degrees.
Please be aware when you select a "Location" on the form, the latitude and longitude values are rounded to the closest neighborhood point of the 0.25 degrees grid.
Temporal frequency
Temporal frequency of the data is monthly.
Data format
In the CDS, there are the options of retrieving the data in netCDF format or CSV format.
Data update frequency
ORAS5 is updated depending on the product:
- Consolidated product: Possible future extension in time
- Operational product: New near real-time data added monthly on the 15th of the month
Variations in delivery times may occur due to the non-operational nature of this CDS service, as issues may arise which cause delays.
Parameters listings
Table 1 below lists the single level parameters and Table 2 lists the 'All levels' parameters.
Table 1: Single level parameters
| Name | Units | Variable name | Notes | |
|---|---|---|---|---|
| 1 | Depth of 20°C isotherm | m | depth_of_20_c_isotherm | |
| 2 | Meridional wind stress | N m-2 | meridional_wind_stress | |
| 3 | Mixed layer depth 0.01 | m | mixed_layer_depth_0_01 | |
| 4 | Mixed layer depth 0.03 | m | mixed_layer_depth_0_03 | |
| 5 | Ocean heat content for the total water column | J m-2 | ocean_heat_content_for_the_total_water_column | |
| 6 | Ocean heat content for the upper 300m | J m-2 | ocean_heat_content_for_the_upper_300m | |
| 7 | Sea ice concentration | Dimensionless | ||
| 8 | Sea ice meridional velocity | m s-1 | sea_ice_meridional_velocity | |
| 9 | Sea ice thickness | m | sea_ice_thickness | |
| 10 | Sea ice zonal velocity | m s-1 | sea_ice_zonal_velocity | |
| 11 | m | sea_surface_height | ||
| 12 | Sea surface salinity | PSU | sea_surface_salinity | |
| 13 | Sea surface temperature | °C | sea_surface_temperature | |
| 14 | Zonal wind stress | N m-2 | zonal_wind_stress |
Table 2: All Levels parameters
| Count | Name | Units | Variable name |
|---|---|---|---|
1 | Salinity | PSU | salinity |
Known issues
Please refer to the ORAS5 documentation.
References
Further ORAS5 references are available from the ECMWF website.
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