ORAS6¶
ECMWF Ocean ReAnalysis System 6 (ORAS6)
Upcoming
Not yet released
Not yet publicly released. In June 2026 ECMWF announced that the altimeter period from 1993 onward had been produced and would be made available shortly to ECMWF Member States and external collaborators. As of 2026-10-12, ORAS6 is not listed on the CDS or in ECMWF's reanalysis datasets, and no public release date or distribution channel has been announced.
At a glance¶
| Item | Details |
|---|---|
| Section | Ocean › Reanalyses |
| Type | Reanalysis |
| Domains | Ocean, Sea ice |
| Provider | ECMWF (developed with support from C3S and the Copernicus Marine Service) |
| Period | 1950 ~ present |
| Coverage | Global |
| Horizontal resolution | NEMO v4 eORCA025 tripolar grid, ¼° (approximately 27 km at the equator and 12 km in the Arctic; Browne et al. 2026) — the same horizontal resolution as ORAS5. The grid of the public product has not yet been announced |
| Vertical resolution | 75 levels (as in ORAS5) |
| Temporal resolution | For 1993 onward, hourly 2D and daily 3D fields are planned (ECMWF, June 2026). Formats for other periods and for statistics have not been announced |
| Update frequency | Not yet released (update frequency after public release not announced) |
| Licence | To be confirmed (expected at release) |
| Account required | Yes |
Note on period: Planned to cover 1950 to the present, produced in several parallel streams that are joined using analyses from the preceding stream. Per the June 2026 ECMWF announcement, the period from 1993 onward has been produced and the early stream from 1950 was expected to finish in August 2026 (no completion notice seen yet). The July 2024 Newsletter expected the full record back to 1950 by the end of 2024, but the schedule slipped. As of 2026-10-12 no data have been publicly released.
How to read the resolution
- Horizontal resolution lists both the native model grid and the grid you download, as given in the official documentation. The same dataset can come on different grids from different providers or products, so check the grid of your downloaded file.
- Grid spacing is not the effective resolution. A provided grid that is finer than the native one carries no extra information.
- Intervals and whether a value is instantaneous, mean or accumulated differ between variables — see 'Available variables' below.
- The 'provided resolution' in lists and the comparison table is a simplified km/degree value for quick comparison. Use this page for exact values.
Overview¶
ECMWF's sixth-generation global ocean and sea-ice reanalysis and the successor to ORAS5. It uses the NEMO v4.0.6 ocean model with the SI3 multi-category sea-ice model, hourly ERA5 atmospheric forcing and ensemble-based 3D-Var (NEMOVAR) data assimilation of temperature and salinity profiles, reprocessed DUACS sea-level anomalies, OSI SAF sea-ice concentration and reprocessed OSTIA SST. It is the first ECMWF ocean reanalysis to assimilate SST directly, and it produces 11 ensemble members of ocean and sea-ice analyses in GRIB2. It provides the ocean and sea-ice initial conditions for ERA6.
What changed from the previous dataset
Available variables¶
The variable list is not final because the data are not yet released. According to ECMWF, ocean and sea-ice state variables (temperature, salinity, currents, sea surface height, sea-ice concentration and thickness, etc.) will be provided for 1993 onward as hourly 2D and daily 3D fields. Variable names and units will be added on release.
How to get the data¶
- ECMWF news — New ORAS6 ocean reanalysis (2026-06-08) — Announcement page (Member States and collaborators first)
- ECMWF states that the 1993-onward period will be made available first to Member States and external collaborators. No distribution channel for general users has been announced.
- ECMWF reanalysis datasets (expected release location) — ECMWF data dissemination services
- The data availability statement of Browne et al. (2026) says ORAS6 "will be publically available via the ECMWF data dissemination services" at this address. When checked on 2026-10-12, ORAS6 was not yet listed. Newsletter 180 states that the output is archived in MARS as GRIB2.
Practical tips¶
- Even after release, do not join ORAS6 to ORAS5 as one time series. Atmospheric forcing (hourly ERA5), model (NEMO4/SI3) and assimilation (ensemble 3D-Var with direct SST assimilation) all differ.
- ERA6's ocean component starts from ORAS6 initial conditions, but "ERA6 will not ingest ocean observations directly". Do not treat ERA6 ocean fields as equivalent to ORAS6.
- ECMWF Newsletter 180 notes that ORAS6 Arctic sea ice can be biased thin in some regions. For sea-ice thickness work, compare with satellite thickness products.
- The system produces 11 ensemble members, so once the release format is known, check whether uncertainty information (ensemble spread) is provided.
Notices and change history¶
2026-06-08 · Info · New ORAS6 ocean reanalysis supporting forecasting and climate monitoring — 1993 onward first
ECMWF announced it is preparing to release ORAS6. Hourly 2D and daily 3D fields for 1993 onward have been produced and will be made available shortly to Member States and external collaborators; the early stream from 1950 was expected to finish in August 2026. No public release date, distribution channel or end date for ORAS5 has been announced.
2026-06-03 · Info · "Sea ice data assimilation in ORAS6" published in The Cryosphere
Browne et al. (2026) describe how sea-ice concentration increments are distributed across the thickness categories of the SI3 multi-category model and add a sea ice–sea water temperature relationship. The paper documents the ORAS6 set-up: eORCA025 grid, an ensemble of one control and 10 perturbed members, and OSI SAF L3 sea-ice concentration assimilation.
2026-03-06 · Info · Copernicus Climate's ERA6 reanalysis production starts (initialised from ORAS6)
Copernicus announced the start of ERA6 production. ERA6's coupled ocean takes its ocean and sea-ice initial conditions from ORAS6, developed together with the Copernicus Marine Service.
2024-07 (day not given) · Info · ECMWF Newsletter 180 — ECMWF's next ensemble reanalysis system for ocean and sea ice: ORAS6
The first detailed description of the ORAS6 configuration: NEMO v4.0.6 with SI3, hourly ERA5 forcing, ensemble-based 3D-Var, direct assimilation of OSTIA SST and 11-member GRIB2 output. It expected the full record back to 1950 by the end of 2024, but the schedule has slipped.
References for your paper¶
Double-check before using in a paper
The citations below were compiled by the maintainer and may contain errors. Always double-check DOIs, authors, years and access-date formats against the provider's official dataset page and your journal's guidelines.
Reference papers¶
ORAS6 system description (ECMWF Newsletter 180) — DOI 10.21957/hzd5y821lk
Zuo, H., Balmaseda, M. A., de Boisseson, E., Browne, P., Chrust, M., Keeley, S., Mogensen, K., Pelletier, C., de Rosnay, P., Takakura, T. (2024): ECMWF's next ensemble reanalysis system for ocean and sea ice: ORAS6. ECMWF Newsletter No. 180, 30–36.
Show BibTeX
@article{zuo2024oras6,
author = {Zuo, Hao and Balmaseda, Magdalena Alonso and de Boisseson, Eric and Browne, Philip and Chrust, Marcin and Keeley, Sarah and Mogensen, Kristian and Pelletier, Charles and de Rosnay, Patricia and Takakura, Toshinari},
title = {{ECMWF}'s next ensemble reanalysis system for ocean and sea ice: {ORAS6}},
journal = {ECMWF Newsletter},
number = {180},
pages = {30--36},
year = {2024},
doi = {10.21957/hzd5y821lk}
}
ORAS6 sea-ice data assimilation paper — DOI 10.5194/tc-20-3299-2026
Browne, P., de Boisseson, E., Keeley, S., Pelletier, C., Zuo, H. (2026): Sea ice data assimilation in ORAS6. The Cryosphere, 20(6), 3299–3311.
Show BibTeX
@article{browne2026oras6seaice,
author = {Browne, Philip and de Boisseson, Eric and Keeley, Sarah and Pelletier, Charles and Zuo, Hao},
title = {Sea ice data assimilation in {ORAS6}},
journal = {The Cryosphere},
volume = {20},
number = {6},
pages = {3299--3311},
year = {2026},
doi = {10.5194/tc-20-3299-2026}
}
Acknowledgements¶
ORAS6 has no dataset DOI yet because it has not been released. Cite the references above when describing the system, and after release follow the dataset citation given by the distributor.
If this page helped, please acknowledge the Korean Climate Data Navigator
Please add the sentence below to the Acknowledgements of your paper.