SMOS sea surface salinity (ESA, CATDS, BEC)¶

ESA Soil Moisture and Ocean Salinity (SMOS) Sea Surface Salinity — ESA Level 2 Ocean Salinity (L2OS v700), CATDS Level 3/Level 4 fields (CPDC operational L3OS, CEC LOCEAN debiased L3 v11, CEC LOPS SMOS/SMAP OI L4) and BEC global L3/L4

Available

At a glance¶

Note on period: SMOS launched on 2009-11-02 and L1 and L2 data start on 2010-01-12. ESA advises against using commissioning data from 2010-01-12 to 05-31; the L2OS v700 reprocessed data (REPR) cover 2010-06-01 to 2021-05-24, and operational data (OPER) run from 2021-05-25. CATDS operational L3OS consists of reprocessing RE07 (2010-01 to 2021-05) and operational data (2021-06 onward); LOCEAN debiased v11 covers 2010-01 to 2025-12 (updated yearly, next update planned for early 2027); BEC global L3/L4 covers 2011 to 2019 plus an extension from 2020-01 to 2021-05.

Overview¶

SMOS is an ESA Earth Explorer that measures soil moisture and sea surface salinity with an L-band (1.4 GHz) interferometric synthetic-aperture radiometer (MIRAS), and it holds the longest satellite salinity record, starting in 2010. ESA produces swath-level Level 2 salinity (L2OS). The French CATDS grids it into the operational L3OS on EASE-Grid 2.0 (updated daily), a research-grade debiased L3 (LOCEAN) and an OI L4 merged with SMAP, while the Spanish BEC builds global L3/L4 fields with its own method for reducing land and radio-frequency interference contamination. Climate studies mostly use the carefully debiased CATDS LOCEAN and BEC products, and monitoring of recent conditions mostly uses the CATDS operational L3OS.

What changes with the successor¶

ESA SMOS L2OS v662 → ESA SMOS L2OS v700¶

Transition: 2021-05

The third mission reprocessing baseline. The anomaly reference changed to a SMOS-derived climatology, a new dielectric model improves retrievals in cold water, and the land–sea contamination correction now uses ISAS-15 reference fields.

Sources: ESA SMOS L2OS v700 release note (PDF) · ESA — new SMOS L1 and L2 baseline announcement (2021-05-28) · ESA SMOS status presentation (R. Sabia, 2026 Ocean Salinity Science & Technology Meeting)

CATDS LOCEAN debiased L3 v10 → CATDS LOCEAN debiased L3 v11¶

Transition: 2026-08

Keeps the v10 debiasing method, adds a Southern Ocean ice-vicinity bias correction and extends the record to December 2025. The products are experimental and updated yearly.

Sources: CATDS — v11 release news · CATDS — CEC LOCEAN L3 Debiased v11 product page

Available variables¶

ESA L2OS variable names follow the L2OS v700 release note, CATDS L3OS names follow the CATDS L3/L4 OS ATBD (Table 10), and LOCEAN variables follow the v11 documentation. Salinity in pss is practical salinity. In CATDS L3OS names, the suffix _1 marks rain-corrected "bulk" salinity and _2 salinity without rain correction.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

ESA L2OS v700 (swath, ISEA 4H9)¶

CATDS operational L3OS 3Q (EASE-Grid 2.0 25 and 50 km, 10-day running and monthly, ascending and descending merged)¶

CATDS LOCEAN debiased L3 v11 (25 km, every 4 days)¶

The full variable list is in the official documentation.

How to get the data¶

• CATDS — operational L3OS 10-day and monthly fields (HTTPS) — Direct HTTPS download (OPER/MIR_CSQ3A_ 25 km and MIR_CSQ3B_ 50 km, reprocessing under RE07/, daily L2Q under MIR_CSF2QA and MIR_CSF2QD) No login needed
  • NetCDF3 files inside .tgz archives. 1-day-delay NRT files are in the OPER/NRT_* folders. Use CATDS SIPAD (https://www.catds.fr/sipad/) for spatial and temporal subsets. Also redistributed by Copernicus Marine (MULTIOBS_GLO_PHY_SSS_L3_MYNRT_015_014).
• CATDS — CEC LOCEAN debiased L3 v11 (HTTPS) — Direct HTTPS download (L3_DEBIAS_LOCEAN_v11/debiasedSSS_09days_v11.0, debiasedSSS_18days_v11.0) No login needed
  • The documentation (documentation/Doc_L3_DEBIAS_LOCEAN_v11.pdf) and data policy (DataPolicy.txt) are in the same folder. The products are experimental and user feedback is requested.
• CATDS — CEC LOPS SMOS/SMAP OI L4 (HTTPS) — Direct HTTPS download (SSS_SMOS-SMAP_OI_L4_v01, 7-day 25 km EASE files) No login needed
  • An L4 merging SMOS and SMAP L3 by optimal interpolation. A new version was posted in January 2026.
• ESA SMOS dissemination service (L1 and L2OS) — HTTPS web / FTPS (Earth Explorer format or NetCDF)
  • Requires a free ESA EO Sign In account. The L2OS product name is MIR_OSUDP2.
• BEC — global and polar SMOS salinity (SFTP) — SFTP (becftp.icm.csic.es, port 27500)
  • Free registration required. Offers global L3 (0.25°, 9-day) and L4 (0.05°, daily) and dedicated Arctic and Southern Ocean products.

Example code¶

Downloads one CATDS operational L3OS 25 km 10-day file (no login needed), unpacks it and reads salinity around the Korean Peninsula. Variable names follow the ATBD table; check the coordinate variable names in the file (the ATBD lists lat and Lon).

# pip install xarray netCDF4 requests
import tarfile, glob, requests
import xarray as xr

base = "https://data.catds.fr/cpdc/Ocean_products/GRIDDED/L3OS/OPER/MIR_CSQ3A_/2026/09/"
name = "SM_OPER_MIR_CSQ3A__20260901T000000_20260910T235959_343_003_7.tgz"
with open(name, "wb") as f:
    f.write(requests.get(base + name, timeout=120).content)
tarfile.open(name).extractall("smos_l3")

ds = xr.open_dataset(glob.glob("smos_l3/**/*.DBL.nc", recursive=True)[0])
print(list(ds.data_vars))                      # check variable names
sss = ds["Mean_Sea_Surface_Salinity_1"]        # rain-corrected bulk salinity (pss)
lat = ds["lat"]
print(float(sss.where((lat > 25) & (lat < 40)).mean()))

Practical tips¶

• ESA strongly recommends filtering L2OS with the procedure in the release note (reject Fg_ctrl_ecmwf or Fg_sc_low_wind equal to 0, reject flags for RFI, ice, sun glint and others equal to 1, keep only Dg_af_fov > 130). Applying all of it removes many grid points, and the note allows relaxing some criteria.
• L2OS v700 still has ascending–descending differences, and biases and noise grow south of 50°S and near sea ice. For long-term analyses, the CATDS LOCEAN and BEC gridded products, which correct these systematic biases, are better choices.
• ESA advises against using January to May 2010 commissioning data. Start time series in June 2010 or later.
• The LOCEAN documentation notes that residual biases remain near coasts and dynamic river plumes even after land–sea correction. When studying coastal plumes such as the Changjiang diluted water in the East China Sea, always compare with in situ data.
• The CATDS operational L3OS is updated daily, but its high-latitude ice correction is imperfect and residual RFI may remain. The research-grade LOCEAN v11 ends in December 2025, so check the differences between the two products before appending recent data.
• For the Arctic, CATDS has produced the operational ARCTIC L3G (9-day) since April 2026, and the research CEC ARCTIC V2 runs to 2023-08-30. CATDS recommends using V2 where available and filling recent periods with L3G.
• LOCEAN's eSSS is only a qualitative error indicator and is unrealistically small near coasts. Do not use it as a quantitative uncertainty.

Notices and change history¶

References for your paper¶

Dataset citation¶

ESA SMOS L2OS v700 — DOI 10.57780/SM1-294cb1b

@misc{esa2021smosl2os,
  author    = {{European Space Agency}},
  title     = {{SMOS} {L2} {OS} {V700}},
  year      = {2021},
  version   = {700},
  publisher = {European Space Agency},
  doi       = {10.57780/SM1-294cb1b}
}

CATDS operational L3OS 3Q (10-day and monthly, debiased) — DOI 10.12770/0f02fc28-cb86-4c44-89f3-ee7df6177e7b

@misc{catds2022l3os3q,
  author    = {{CATDS}},
  title     = {{CATDS-PDC} {L3OS} {3Q} mixed - Debiased average 10 days \& monthly salinity field product from {SMOS} satellite (mixed orbits)},
  year      = {2022},
  publisher = {CATDS (CNES, IFREMER, LOCEAN, ACRI)},
  doi       = {10.12770/0f02fc28-cb86-4c44-89f3-ee7df6177e7b}
}

CATDS CEC LOCEAN debiased L3 v11 (SEANOE) — DOI 10.17882/52804

@misc{boutin2026debiasv11,
  author    = {Boutin, Jacqueline and Vergely, Jean-Luc and Khvorostyanov, Dmitry},
  title     = {De-biased {SMOS} {SSS} {L3} {V11} maps generated by {LOCEAN}/{ACRI-ST} Expertise Center},
  year      = {2026},
  publisher = {SEANOE},
  doi       = {10.17882/52804},
  note      = {https://doi.org/10.17882/52804\#133087}
}

CATDS CEC LOPS SMOS/SMAP OI L4 (SEANOE) — DOI 10.17882/73142

@misc{kolodziejczyk2020oil4,
  author    = {Kolodziejczyk, Nicolas and Hamon, Mathieu and Boutin, Jacqueline and Vergely, Jean-Luc and Supply, Alexandre and Reverdin, Gilles},
  title     = {{SSS} {SMOS}/{SMAP} {OI} {L4} maps},
  year      = {2025},
  publisher = {SEANOE},
  doi       = {10.17882/73142}
}

Reference papers¶

CATDS LOCEAN debiasing method — DOI 10.1016/j.rse.2018.05.022

@article{boutin2018smos,
  author  = {Boutin, J. and Vergely, J. L. and Marchand, S. and D'Amico, F. and Hasson, A. and Kolodziejczyk, N. and Reul, N. and Reverdin, G. and Vialard, J.},
  title   = {New {SMOS} Sea Surface Salinity with reduced systematic errors and improved variability},
  journal = {Remote Sensing of Environment},
  volume  = {214},
  pages   = {115--134},
  year    = {2018},
  doi     = {10.1016/j.rse.2018.05.022}
}

SMOS/SMAP OI L4 method — DOI 10.1175/JTECH-D-20-0093.1

@article{kolodziejczyk2021oi,
  author  = {Kolodziejczyk, Nicolas and Hamon, Mathieu and Boutin, Jacqueline and Vergely, Jean-Luc and Reverdin, Gilles and Supply, Alexandre and Reul, Nicolas},
  title   = {Objective Analysis of {SMOS} and {SMAP} Sea Surface Salinity to Reduce Large-Scale and Time-Dependent Biases from Low to High Latitudes},
  journal = {Journal of Atmospheric and Oceanic Technology},
  volume  = {38},
  number  = {3},
  pages   = {405--421},
  year    = {2021},
  doi     = {10.1175/JTECH-D-20-0093.1}
}

BEC global SMOS salinity (L3 DOI 10.20350/digitalCSIC/12601, L4 DOI 10.20350/digitalCSIC/12600) — DOI 10.5194/essd-13-857-2021

@article{olmedo2021bec,
  author  = {Olmedo, Estrella and Gonz{\'a}lez-Haro, Cristina and Hoareau, Nina and Umbert, Marta and Gonz{\'a}lez-Gambau, Ver{\'o}nica and Mart{\'i}nez, Justino and Gabarr{\'o}, Carolina and Turiel, Antonio},
  title   = {Nine years of {SMOS} sea surface salinity global maps at the {Barcelona} {Expert} {Center}},
  journal = {Earth System Science Data},
  volume  = {13},
  number  = {2},
  pages   = {857--888},
  year    = {2021},
  doi     = {10.5194/essd-13-857-2021}
}

Acknowledgements¶

For CATDS LOCEAN products, the data policy (DataPolicy.txt) asks for an acknowledgement that the maps were produced by LOCEAN/IPSL and ACRI-st within the CATDS Ocean Salinity Expertise Center (CECOS) and distributed by IFREMER (Plouzané, France). If you combine ESA L2OS, CATDS and BEC products, cite each DOI.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• ESA Earth Online — SMOS mission
• ESA — SMOS science products (L1 and L2 list, documents)
• CATDS — ocean products
• CATDS — differences between operational and research L3 (PDF)
• CATDS L3/L4 OS ATBD (PDF)
• BEC — global SMOS salinity
• Pi-MEP satellite salinity validation reports
• ESA SMOS status presentation (2026-05, extended until end of 2028)
• CNES — SMOS project page
• This site — SMAP sea surface salinity
• This site — ESA CCI sea surface salinity (merged SMOS, SMAP and Aquarius)
• This site — SMOS thin sea ice thickness (sea ice products from the same satellite)
• This site — CS2SMOS sea ice thickness (merged CryoSat-2 and SMOS)