CMEMS - Sealevel GLO PHY L4 observations 008 047

Revision as of 18:50, 9 February 2022 by P.petrelli (talk | contribs)

SEALEVEL_GLO_PHY_L4_MY_008_047 is a satellite global sea level product downloaded from the Copernicus Marine Environment Monitoring Service (CMEMS). This product was known as SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_008_047 until December 2021, when the current version, DT-2021,  was introduced with a new name. Different evolutions have been introduced in the altimeter standards and processing methods used. A full description of the changes implemented is available in the QUID document (section V.4.1).

For more information and the official documentation, including the product manual refer to the CMEMS data portal.

This an extract from the official documentation:

Altimeter satellite gridded Sea Level Anomalies (SLA) computed with respect to a twenty-year 2012 mean. The SLA is estimated by Optimal Interpolation, merging the measurement from the different altimeter missions available. This product is processed by the DUACS multimission altimeter data processing system. It processes data from all altimeter missions: Jason-3, Sentinel-3A, HY-2A, Saral/AltiKa, Cryosat-2, Jason-2, Jason-1, T/P, ENVISAT, GFO, ERS1/2. To produce maps of Sea Level Anomalies (SLA) and Absolute Dynamic Topography (ADT) in delayed-time (REPROCESSED), the system uses the along-track altimeter missions from products called SEALEVEL_*PHY_L3_MY_008*. Finally an Optimal Interpolation is made merging all the flying satellites in order to compute gridded SLA and ADT. The geostrophic currents are derived from sla (geostrophic velocities anomalies, ugosa and vgosa variables) and from adt (absolute geostrophic velicities, ugos and vgos variables). Note that the gridded products can be visualized on the LAS (Live Access Data) Aviso+ web page.

We still have the SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_008_047 latest versions. These were previously distributed by Aviso+, and are occasionally still refer as such. All the missions were homogenized with respect to a reference mission which was OSTM/Jason-2 for v2019. The sla is computed with an optimal and centered computation time window (6 weeks before and after the date). Resolution is 0.25X0.25 degrees and has an irregular temporal resolution. The data starts in 1993 and ends in June 2020.

Terms of use

NB You need to register with CMEMS to use this product:

Registration form: .

The full terms are listed on the CMEMS website:

In particular: ... In case of any publication, the Licensee will ensure credit the Copernicus Marine Service in the following manner:“This study has been conducted using E.U. Copernicus Marine Service Information” ...

The dataset DOI is 10.48670/moi-00148 this should be used when referencing the data.


This dataset is available in the ua8 project:


DT-2021 is the most recent version, the orignal files are organised in yearly and monthly folders:


Filenames are:



  • sla - sea_surface_height_above_sea_level 
  • err_sla - sea_surface_height_above_sea_level standard_error
  • ugosa - surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid 
  • vgosa - surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid 
  • err_ugosa - surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid standard_error
  • err_vgosa - surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid standard_error
  • adt - sea_surface_height_above_geoid 
  • ugos - surface_geostrophic_eastward_sea_water_velocity 
  • vgos - surface_geostrophic_northward_sea_water_velocity 
  • flag_ice - Ice Flag for a 15% criterion of ice concentration
  • tpa_correction - sea_surface_height_above_sea_level


The "DT-2021/timeseries" folder contains a concatenated version of the dataset latest version (DT-2021). Each year is concatenated in one file (cmems_sealevel_DT-2021_<year>.nc) and chunksizes have been changed to make accessing the data as a timeseries faster. We try to update the current year regularly but still there might be more days available in the actual dataset folder (DT-2021).

The current chunking values are:

time:31, latitude: 120, longitude: 240

The size of these (~6.8 MB) is optimised for storing files on disk and is much smaller than optimal chunk size for dask which is around 100-200MB.  Defining bigger chunks (as much as posisble multiple of the original) when opening the files in xarray should imporve performance.

The file timeseries_filelist_<concatenation_date>.txt contains a list fo the files used to create the concatenated version.

A timeseries version also exists for the v2019 version



2022-02-09 We downloading the new renamed DT-2021 version and created a new concatenated version available in the same folder

2021-10-20 There hasn't been updates to this dataset for a while, the latest available data is 2020-06-03. We check for updates regularly, so we when updates will resume we will be downloading the new data automatically.

2020-07-17 We updated to the most recent version v2019. As CMEMS doesn't always distinguish between a new version and an extension the year 2018/2019 were previously incorrectly downloaded as part of v4-0 while they are available now in v2019.

For more information check the CMEMS release notes