ESA Sea State CCI v4¶

ESA Sea State Climate Change Initiative (Sea_State_cci) version 4 — multi-mission altimeter significant wave height L2P, L3 and L4, and Sentinel-1 SAR wave L2P (satellite sea state climate data record)

Available

At a glance¶

Note on period: Altimeter L2P and L3 cover 1991-08-03 to 2023-12-31 and the L4 monthly product 1992-01 to 2023-12 (CEDA). Version 4 adds ERS-1, ERS-2 and TOPEX/Poseidon to reach back into the 1990s (version 3 covered 2002-06 to 2021-01). Sentinel-1 SAR Wave Mode significant wave height runs from 2015-03-19 to 2025-06-11, and the Sentinel-1 WV Integrated Sea State Parameters (ISSP, DLR) from 2014-12-01 to 2024-12-30.

Overview¶

A satellite sea state climate data record from the ESA Climate Change Initiative. Waveforms from twelve altimeters, from ERS-1 to Sentinel-6A, are retracked (WHALES and others), intercalibrated against Jason-3, calibrated against buoys and denoised with empirical mode decomposition (EMD). The resulting significant wave height comes as along-track data (L2P), daily merged along-track files (L3) and 1° monthly gridded statistics (L4). Version 4 spans 1991–2023 and also provides significant wave height and sea state parameters from Sentinel-1 SAR imagery (L2P). It is used for wave climate change studies, validation of wave models and reanalyses, and coastal engineering.

What changes with the successor¶

Sea State CCI v3 → Sea State CCI v4¶

Transition: 2025-08

Extends the record to 1991–2023 (previously 2002–2021) by adding three 1990s missions and three recent ones. Bathymetry, the land mask and the Jason-3 retracked files were updated, and Sentinel-1 IW and EW SAR products were added.

Sources: CCI Sea State Product User Guide v4.0 — Changes · CEDA — v4 L4 record · CEDA — v3 L4 record

Available variables¶

Variable names follow the CCI Sea State Product User Guide v4.0 and the CEDA v4 catalogue records. The guide recommends swh_denoised (cross-mission bias corrected and denoised) for most applications. L3 and L4 use only L2P measurements with quality level 3 (good), from the Ku band (Ka band for SARAL only).

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

Altimeter L2P and L3 (1 Hz along-track)¶

Altimeter L2P ancillary fields (model and sea ice values at the measurement)¶

Altimeter L4 (1° monthly statistics)¶

The full variable list is in the official documentation.

How to get the data¶

• CEDA — altimeter L4 monthly 1° grid, version 4 — HTTPS, OPeNDAP, anonymous FTP (data.ceda.ac.uk/neodc/esacci/sea_state/data/v4_release/altimeter/l4/v4.0/) No login needed
  • L2P is at catalogue.ceda.ac.uk/uuid/baeb32fd74cb49a1b67a6a4164ff0899 and L3 at catalogue.ceda.ac.uk/uuid/82273fe59b784e7c914e1bd3049e3483, downloadable the same way.
• Ifremer CERSAT — Sea State CCI v4 data server — HTTPS / FTP (ftp://ftp.ifremer.fr/ifremer/cersat/data/ocean-waves/cci-seastate/v4/) No login needed
  • No login required. Paths are data/satellite/altimeter/l2p, l3 and l4 (plus data/satellite/sar); L4 holds monthly files under year folders (e.g. l4/2020/ESACCI-SEASTATE-L4-SWH-MULTI_1M-202001-fv01.nc).
• Ifremer THREDDS — remote subsetting of L4 — OPeNDAP / THREDDS (L4 only) No login needed
• ESA CCI Sea State data extraction service — Web extraction by area, mission, period and variable (generates Python, R and cURL code)
• ESA Climate Office — Sea State data list — Web (CEDA catalogue links and DOIs by version)

Example code¶

Downloads an L4 monthly file from the Ifremer server and prints monthly mean significant wave height around Korea. No login is needed.

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

base = "https://data-cersat.ifremer.fr/data/ocean-waves/cci-seastate/v4/data/satellite/altimeter/l4/"
fname = "ESACCI-SEASTATE-L4-SWH-MULTI_1M-202001-fv01.nc"      # monthly file under a year folder
with open(fname, "wb") as f:
    f.write(requests.get(base + "2020/" + fname, timeout=120).content)

ds = xr.open_dataset(fname)
box = ds.where((ds.lat >= 30) & (ds.lat <= 42) & (ds.lon >= 120) & (ds.lon <= 135), drop=True)  # around Korea
print(box["swh_mean"].values.round(2))
print("passes per cell:", box["swh_count"].values)               # be careful where samples are few

Practical tips¶

• The guide recommends swh_denoised for most uses. swh has no cross-mission bias correction, so mixing missions with it can create steps when the satellite changes.
• L4 is a monthly statistic built only from the moments satellites passed. In small regions such as the seas around Korea, where the number of missions changes over time, treat means and maxima from cells and months with a small swh_count with caution.
• Cross-mission bias correction is less certain at high sea states (5–10 m, where comparison pairs are sparse; above 10 m it is extrapolated). For typhoon extremes, check against buoys and models.
• Measurements with sea ice concentration above 10% get quality level 1 and those with 0–10% level 2, so they drop out of L3 and L4. Seas that freeze in winter, such as the Sea of Okhotsk and the Bohai Sea, have gaps or few samples in that season.
• The 1990s (ERS-1, ERS-2, TOPEX) have few missions, and the guide notes that TOPEX cycles up to 16 are excluded, Sides A and B are treated separately, and recorder degradation left gaps. For long-term trends, check how sample numbers change over time.
• Retrackers differ by mission — WHALES for Jason-1/2/3, Envisat, CryoSat-2 and SARAL; MLE3 for ERS and TOPEX; MLE4 for Sentinel-3A/B and Sentinel-6A. Keep this in mind when comparing missions.
• L2P carries ERA5 and WAVEWATCH III wave heights colocated with each measurement (era5_swh, ww3_swh), so model validation can be done without downloading model data separately.
• Version 3 (2002-06 to 2021-01) and version 4 differ in retracker versions, bathymetry and land masking, so do not mix them. When comparing with earlier results based on v1.1 or v3, recompute with v4.

Notices and change history¶

References for your paper¶

Dataset citation¶

Sea State CCI v4 altimeter L4 monthly 1° grid (CEDA) — DOI 10.5285/315d8e013696420192f1ea1d9191ca9c

@misc{piolle2026seastatel4,
  author    = {Pioll{\'e}, Jean-Fran{\c{c}}ois and Dodet, Guillaume and Ardhuin, Fabrice and Passaro, Marcello},
  title     = {{ESA} Sea State Climate Change Initiative (Sea\_State\_cci): Global remote sensing merged multi-mission monthly gridded significant wave height from altimetry, {L4} product, version 4},
  year      = {2026},
  publisher = {NERC EDS Centre for Environmental Data Analysis},
  doi       = {10.5285/315d8e013696420192f1ea1d9191ca9c}
}

Sea State CCI v4 altimeter L3 daily merged along-track (CEDA) — DOI 10.5285/82273fe59b784e7c914e1bd3049e3483

@misc{piolle2026seastatel3,
  author    = {Pioll{\'e}, Jean-Fran{\c{c}}ois and Dodet, Guillaume and Ardhuin, Fabrice and Passaro, Marcello},
  title     = {{ESA} Sea State Climate Change Initiative (Sea\_State\_cci): Global remote sensing daily merged multi-mission along-track significant wave height from altimetry, {L3} product, version 4},
  year      = {2026},
  publisher = {NERC EDS Centre for Environmental Data Analysis},
  doi       = {10.5285/82273fe59b784e7c914e1bd3049e3483}
}

Sea State CCI v4 altimeter L2P along-track (CEDA) — DOI 10.5285/baeb32fd74cb49a1b67a6a4164ff0899

@misc{piolle2026seastatel2p,
  author    = {Pioll{\'e}, Jean-Fran{\c{c}}ois and Dodet, Guillaume and Ardhuin, Fabrice and Passaro, Marcello},
  title     = {{ESA} Sea State Climate Change Initiative (Sea\_State\_cci): Global remote sensing multi-mission along-track significant wave height from altimetry, {L2P} product, version 4},
  year      = {2026},
  publisher = {NERC EDS Centre for Environmental Data Analysis},
  doi       = {10.5285/baeb32fd74cb49a1b67a6a4164ff0899}
}

Reference papers¶

Sea State CCI dataset description (v1) — DOI 10.5194/essd-12-1929-2020

@article{dodet2020seastate,
  author  = {Dodet, Guillaume and Pioll{\'e}, Jean-Fran{\c{c}}ois and Quilfen, Yves and Abdalla, Saleh and Accensi, Micka{\"e}l and Ardhuin, Fabrice and Ash, Ellis and Bidlot, Jean-Raymond and Gommenginger, Christine and Marechal, Gwendal and Passaro, Marcello and Quartly, Graham and Stopa, Justin and Timmermans, Ben and Young, Ian and Cipollini, Paolo and Donlon, Craig},
  title   = {The Sea State {CCI} dataset v1: towards a sea state climate data record based on satellite observations},
  journal = {Earth System Science Data},
  volume  = {12},
  number  = {3},
  pages   = {1929--1951},
  year    = {2020},
  doi     = {10.5194/essd-12-1929-2020}
}

Acknowledgements¶

Under the ESA CCI terms, presentations and publications must acknowledge the ESA Climate Change Initiative, the Sea State CCI project and the data providers, and cite the dataset DOIs used. If CCI data are a major component of the work, consider offering co-authorship, and send copies of resulting publications to the CCI.

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 Climate Office — Sea State project
• ESA Climate Office — Sea State data (by version)
• CCI Sea State Product User Guide v4.0 (web)
• Product User Guide v4.0 (PDF)
• Processing details (retrackers, intercalibration, quality control)
• ESA CCI Sea State terms and conditions (PDF)
• This site — SWOT KaRIn sea surface height and significant wave height
• This site — DUACS multi-mission altimetry L4 sea level