Last modified on Jul 30, 2026 18:13

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.

The ORAS5 Analysis Ready Cloud Optimised (ARCO) presented here is a subset of some parameters of the full Climate Data Store (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 a ARCO format. It allows direct access to a subset of the surface and wave variables (see below) without downloading individual files, enabling efficient and scalable data access and retrieval. The ARCO format has a dual-chunking approach which means that data access can be optimised for retrieving long-time periods for a limited area (geo-chunked) or short-time period for large areas (time-chunked).

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 to provide efficient access to time-chunked and geo-chunked ORAS5 data:

The variables available in the Single level and All levels Zarr files are listed in Table 1 and Table 2, respectively.

The Zarr datacubes can be accessed using your CDS API key. Below is a simple plug-and-play code snippet and a more detailed Jupyter Notebook example. For more detailed and advanced access examples, please refer to the main Analysis Ready Cloud Optimised (ARCO) Data.

import xarray as xr

# Geo-chunked data for access optimised along the time dimension (e.g. for time-series at a single point)
# Consolidated, 1958-2014
consolidated_geo_url = "https://arco.datastores.ecmwf.int/cadl-arco-geo-001/arco/reanalysis_oras5/consolidated/geoChunked.zarr"
# Operational, 2015-present
operational_geo_url = "https://arco.datastores.ecmwf.int/cadl-arco-geo-001/arco/reanalysis_oras5/operational/geoChunked.zarr"

# Time-chunked data for access optimised along the time dimension (e.g. for short-time period for large areas)
# Consolidated, 1958-2014
consolidated_time_url = "https://arco.datastores.ecmwf.int/cadl-arco-time-001/arco/reanalysis_oras5/consolidated/timeChunked.zarr"
# Operational, 2015-present
operational_time_url = "https://arco.datastores.ecmwf.int/cadl-arco-time-001/arco/reanalysis_oras5/operational/timeChunked.zarr"


# Open one of the Zarr objects with xarray, the default example opens the geo-chunked surface variables
ds = xr.open_zarr(
    surface_geo_url,
    consolidated=True,
    storage_options={
        "headers": {"Authorization": f"Bearer <CDS-API-KEY>"}
    }
)

# Inspect the variables
print(ds)

Time-series access via the CDS

The ORAS5 global ocean reanalysis monthly time-series data from 1958 to present catalogue entry allows users to download long time series for a single-point or limited area via the CDS download webform or a CDS API request that users can build from the webform. The data download via the time-series entry is as described below with additional aspects:

1. 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. Similarly, selecting an “Area” returns all points within the region defined by the nearest neighborhood corner points to the user-specified S, W, N, and E latitude/longitude values (maximum 5° x 5°).

2. The download form allows users to select a choice of netCDF or CSV. The conversion is done directly with xarray with no additional processing from the Zarr description above.
   • Given the computational cost of writing to csv, users are limited to smaller downloads with this option.
3. The surface and single level parameters (Table 1) and the all levels parameters (Table 2) are in different Zarr stores, therefore they will be served in different output files.

Spatial grid

The parameters have a regular grid of 0.25° x 0.25° (around 25 km in the tropics and 9 km in the Arctic).

Temporal frequency

Temporal frequency of the data is monthly.

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 15^th of the month

Parameters listings 

Table 1 below lists the single level parameters and Table 2 lists the 'All levels' parameters.

Table 1: Single level parameters

Table 2: All Levels parameters

Known issues

Please refer to the ORAS5 documentation.

References

Further ORAS5 references are available from the ECMWF website.

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