the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Oct 2025
SODA4: a mesoscale ocean/sea ice reanalysis 1980–2024
Abstract. This paper describes the new Simple Ocean Data Assimilation version 4 (SODA4) global eddy-resolving ocean/sea ice reanalysis that spans the 45-year period 1980–2024. The reanalysis is constructed using GFDL MOM5/SIS1 numerics and ECMWF ERA5 forcings with surface and subsurface temperature and salinity observations as constraints within an optimal interpolation data assimilation algorithm. The method of construction and resulting output files are described. Comparison of the SODA4 temperature and salinity fields to observations and to the UK Met Office EN4 temperature and salinity analyses in the upper ocean shows SODA4 has marginal bias and exhibits more regional variability, with less of an imprint of the sparse and inhomogeneous distribution of observations. Comparison of transports across major ocean sections and passages are generally consistent with independent moored observations.
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Status: open (until 19 Dec 2025)
- RC1: 'Comment on egusphere-2025-3810', Anonymous Referee #1, 30 Oct 2025 reply
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The paper presents the new eddy-resolving SODA4 reanalysis, which represents a step forward in the reconstruction of the ocean climate with the inclusion of mesoscale signals.
As such, it is broadly interesting for the ocean community, the included verification shows very good performances, and I recommend the publication after a few issues are addressed, mostly, but not all, minor. In some cases, the authors could spend some more time describing the specificities without indicating references, so that the SODA4 description appears self-contained in the manuscript.
Minor points
L74-75: Change of advection scheme may have an impact on temporal consistency of momentum and energy; can you specifically comment on the changes in the main text?
L114: More details needed. Is the difference of heat or freshwater spread/pushed into purely input fields (shortwave and precipitation)? Or are the net fluxes bias corrected with climatological analysis increments corrected to fluxes?
L152: So is assimilation also performed every 10 days? Using observations in the previous 10 days?
Figure 6: The salinity panel is not really informative; the palette should be changed
Section 3: I feel this could go in an appendix, in order not to interrupt the scientific discourse
L203 : It would be useful to quantify the depth below which there are discontinuities due to the change of stream. The upper ocean is likely unaffected, but the deep and abyssal layers are, so users should be aware of such discontinuities.
L204: As mentioned earlier, to say whether this changes energy/momentum
L210: The authors need to mention that this is not an independent dataset (as EN4 relies mostly on WOD). Comparison with altimetry should be carried out, as this is a completely independent dataset, and I recommend including it.
Section 4: I suggest adding a comparison of heat transports where known (RAPID, etc.).
Table 3: Is there a reason why OSNAP-West is excluded?
Also, the time period changes across observation-based estimates. Is this considered in the Table when the mean is computed?
Additionally, simple scores like correlation that quantify the correctness of the temporal evolution of the transports should be included as well (with a statistical significance statement).
Typos/Language
L11-12 “The method of construction” (of what?) is unclear, needs to be reworded
L37 broken sentence, to be rewritten
L108 the use of 15 to indicate ERA5 (considering ERA15 existed) is quite cryptic and unfortunate. Is it not possible to change it?
L155 “Since…” broken sentence
Caption of Figure 8: “monthly means” I guess here
L223 “300-1000”
Figure 9’s caption: “temperature anomaly”