the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Dec 2025
Brief communication: Uncertainties in Southern Ocean sea surface conditions and their impact on Antarctic climate over 1958–1978
Abstract. Atmospheric reanalyses extending before 1979, including ERA5, show a substantial cold bias over Antarctica during 1958–1978. Because Southern Ocean sea surface conditions (SSCs) are poorly constrained in this period, we assess their role using two ensembles of a global atmospheric model forced by different SSC datasets. The first uses the widely employed HadISST product, while the second is based on a recent reconstruction that assimilates continental observations. Our results show that SSC differences strongly alter the Antarctic surface climate, but explain only about 30 % of the ERA5 cold bias ERA5, indicating that additional factors contribute to this bias.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(13213 KB) - Metadata XML
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Supplement
(1027 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-5634', David Bromwich, 08 Jan 2026
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AC1: 'Reply on RC1', Quentin Dalaiden, 05 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5634/egusphere-2025-5634-AC1-supplement.pdf
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AC1: 'Reply on RC1', Quentin Dalaiden, 05 Feb 2026
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RC2: 'Comment on egusphere-2025-5634', John King, 20 Jan 2026
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AC2: 'Reply on RC2', Quentin Dalaiden, 05 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5634/egusphere-2025-5634-AC2-supplement.pdf
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AC2: 'Reply on RC2', Quentin Dalaiden, 05 Feb 2026
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-5634', David Bromwich, 08 Jan 2026
Overview Comments
The authors set out to explain the cold bias in ERA5 temperatures over Antarctica prior to 1979 reported by Bromwich et al. (2024) and Dalaiden et al. (2025). The uncertainty in the SST and sea ice conditions (SSC) over the Southern Ocean during this period is explored to determine the SSC contribution to the cold bias in ERA5. About 30% of the ERA5 cold bias is attributed to Southern Ocean SSC uncertainty. The desire for a reliable SSC prior to 1979 is well justified. This is a valuable study that just needs some more contextual content.
Specific Comments:
- Line 30: It is not strictly accurate to call the period prior to 1979 pre-satellite. There were a few primitive satellite sensors in orbit back to the 1960s, as described by Hersbach et al. (2020). It is true that satellite observations became much more extensive starting in late 1978.
- The reconstruction by Fogt et al. (2022) https://doi.org/10.1038/s41558-021-01254-9 produced a much smaller sea-ice decrease across 1979 than shown by ANT-REC. The seasonal sea ice extent reconstructions were primarily based on monthly mean pressure and temperature records across the Southern Hemisphere extratropics and midlatitudes from 1905 to 2020, similar to ANT-REC. ANT-REC does provide a much more comprehensive analysis, Fig. 5 for example.
- Some more discussion of the results of Bromwich et al. (2024) is desirable. Their estimate of the cold bias in ERA5 was ~1C for all of Antarctica, compared to 0.7C here based on station observations. Also those authors discussed the widespread Southern Hemisphere issues with assimilation of satellite (atmospheric) observations prior to 1979 and the jump in performance of the Southern Hemisphere forecasts across 1979 (Fig. 8(e) in Soci et al. (2024) https://doi.org/10.1002/qj.4803).
Citation: https://doi.org/10.5194/egusphere-2025-5634-RC1 -
AC1: 'Reply on RC1', Quentin Dalaiden, 05 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5634/egusphere-2025-5634-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-5634', John King, 20 Jan 2026
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AC2: 'Reply on RC2', Quentin Dalaiden, 05 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5634/egusphere-2025-5634-AC2-supplement.pdf
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AC2: 'Reply on RC2', Quentin Dalaiden, 05 Feb 2026
Peer review completion
Journal article(s) based on this preprint
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Quentin Dalaiden
Ingo Bethke
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(13213 KB) - Metadata XML
-
Supplement
(1027 KB) - BibTeX
- EndNote
- Final revised paper
Overview Comments
The authors set out to explain the cold bias in ERA5 temperatures over Antarctica prior to 1979 reported by Bromwich et al. (2024) and Dalaiden et al. (2025). The uncertainty in the SST and sea ice conditions (SSC) over the Southern Ocean during this period is explored to determine the SSC contribution to the cold bias in ERA5. About 30% of the ERA5 cold bias is attributed to Southern Ocean SSC uncertainty. The desire for a reliable SSC prior to 1979 is well justified. This is a valuable study that just needs some more contextual content.
Specific Comments: