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To generate this ORAS5 time series dataset, homogenisation conventions have been applied to the ORAS5 source GRIB data to ensure consistency, usability, and alignment across chosen variables and time steps. The processed data were then written to an ARCO Zarr archive, enabling efficient cloud-optimised access and scalable data retrieval.
The dataset presented here is a subset of selected parameters from the full ORAS5 global ocean monthly dataset. Requirements for additional parameters may be considered. Please raise your request with ECMWF Support here.

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 15^th 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
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	Sea surface height	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.

_{This document has been produced in the context of the Copernicus Climate Change Service (C3S).}

_{The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.}

_{The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.}