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title	Table of Contents

Table of Contents

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Note

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

Excerpt

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

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	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

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Introduction

Methodology

Data organization and access

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Old Version 7

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Introduction

Methodology

Data organization and access