the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Assessing heat and freshwater changes in the Southern Ocean using satellite-derived steric height
Abstract. The Southern Ocean plays a central role in regulating the global overturing circulation, ventilating the deep ocean, and driving sea level rise by delivering heat to Antarctic ice shelves. Understanding heat and freshwater content in this region is key to monitoring these global processes and identifying multiyear changes; however, in-situ observations are limited, and often do not offer the spatial or temporal consistency needed to study long-term variability. Perturbations in steric height can reveal changes in oceanic heat and freshwater content inasmuch as they impact the density of the water column. Here, we show for the first time that the monthly steric height anomaly of the Southern Ocean south of 50° S can be assessed using altimetry and GRACE gravimetry data from 2002 to 2018. Steric height anomalies are validated against in-situ Argo float and CTD data from tagged elephant seals. We find good agreement in the ice-free ocean and parts of the seasonal ice zone, but that the uncertainty of steric height increases on the Antarctic continental shelf and within the permanent ice zone due to leakage error and anti-aliasing in GRACE. The open-ocean steric height anomalies exhibit spatio-temporally coherent patterns that: (i) capture the expected seasonal cycle of low (high) steric height in winter (summer); and (ii) reflect interannual anomalies in surface heat and freshwater content and wind forcing associated with positive and negative phases of the two major modes of Southern Hemisphere climate variability (the El Niño – Southern Oscillation and Southern Annular Mode).
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RC1: 'Comment on egusphere-2023-3050', Anonymous Referee #1, 11 Feb 2024
This manuscript presents an analysis of steric height changes in the Southern Ocean in the last 20 years based on altimetry data. One major enabler of this study is the use of altimetry data in sea-ice covered areas, which have been made possible only very recently. Steric height anomalies are computed from the difference between Dynamic Ocean Topography (DOT) and barystatic height (BH) anomalies. Steric height estimates are validated using in situ data. Then, seasonal and interannual variability are presented, and correlations with climate indices are presented.
While the study is useful and would eventually deserve to be published, it first requires some revisions.
Major comment:
- The title appears broadly misleading judging by the content of results. Heat and freshwater changes are not estimated, so their contribution to steric height changes is discussed only in a speculative way. This is fine, but the title should reflect core results, not discussion.
- A more quantitative estimate of errors would be useful. You are merging information from two very different products. Altimetry has a better spatial resolution; GRACE has some problem of “leakage” (contamination by continental surfaces); temporal resolution between the two products might also differ. What would be the combined uncertainty on steric height estimates and is this expected to be sufficient to capture different scales of variability?
- There is a problem with the smoothing applied on temporal curves in Fig. 4, 6 and 7. Judging by eye, the rolling mean is not centered, so it appears lagged by half the smoothing window (i.e. 6 months). This need to be checked, and I apologize if my eye is inaccurate.
- The potential of the presented dataset appears under-used in the study. Why not add a map of steric height anomaly RMS? EOF of steric height changes would also be useful. A regression of steric height changes to SAM and SOI would help demonstrate a potential statistical link, together with some quantification of significance. At the moment, we struggle to see any clear novelty in the analysis, despite the obvious value of the dataset and potential of the method.
Minor comments:
- l. 10: “The Southern Ocean *circulation* plays …”
- l. 27: Reference Haumann et al 2023 absent from reference list. Check all references.
- l. 29: “The South Atlantic [..] Oceans *waters* …”
- l. 31: “extreme and unpredictable”, vague and potentially misleading. The weather might become more extreme, not sure about the climate. Not sure about the future of climate predictability either.
- l. 172: I do not find the agreement between SHA and GHPA particularly striking in Fig. 3e. Fig. 3b seems more convincing. Can you be more specific/quantitative?
- We politely suggest the authors to read and incorporate the citation Kolbe et al. 2023 in their work (a paper that went unnoticed as so many other studies from the Covid era), as they will find much information about steric height variability in the Southern Indian Ocean during a period overlapping their period of interest, that may be compared with their results. Notably, the relative importance of heat and freshwater changes is discussed in detail.
- There is too much information in the Appendix. Appendix A is useful to interpret the main results. Appendix B could be shortened and put back into the main text. The validation of GRACE against ocean bottom pressure is useful and could be part of the main text. Fig. S4 should also be in the main manuscript. Section 13.5 has nothing to do in an Appendix.
References:
Kolbe, Marlen, Fabien Roquet, Etienne Pauthenet, and David Nerini. “Impact of Thermohaline Variability on Sea Level Changes in the Southern Ocean.” Journal of Geophysical Research: Oceans 126, no. 9 (2021): e2021JC017381. https://doi.org/10.1029/2021JC017381.
Citation: https://doi.org/10.5194/egusphere-2023-3050-RC1 -
RC2: 'Comment on egusphere-2023-3050', Anonymous Referee #2, 22 Apr 2024
This manuscript combines almost 20-years of altimeter and gravity satellite data to investigate steric height anomalies in the Southern Ocean south of 50oS. The steric height anomalies are used to investigate mean, seasonal, and regional variability and correlation with SAM and ENSO. While the study is of potential interest to the community the methods used, presentation and interpretations of results are not always clearly presented. Thus, diminishing the potential impact of the main results. I suggest the manuscript requires a major review before it may be acceptable for publication.
Rather than provide line by line comments and suggestions, this review will provide overarching manuscript comments.
Major Comments.
I agree with Anonymous Referee #1 that title is not appropriate. The manuscript does not assess the Southern Ocean heat and freshwater changes, it does estimate the steric height changes.
I suggest the authors revise how information is written in the manuscript. It is important that clear and concise language that correctly describes the applied data methods is used. For example, but not limited to, section 2.4 explains how in-situ data are used to calculate geopotential height anomaly. It is unclear what is the value of the statement “ in which there are multiple pressure level” when the depth criteria are used to determine what profiles are used (Pmax > 500 dbar and Pmin<25 dbar and quality flag 1). Rather than beginning the sentence with “The maximum depth of 500 dbar…..”, perhaps it is better to begin with “ We calculate GHA relative to 500 dbar, where 500 dbar is sufficiently deep……
Another example, in figure 3 b the correlation coefficient is mostly greater than 0.25 but there are region where the correlation coefficient is less than 0 indicating strong negative correlation, and regions of 0 correlation coefficient (no relationship between SHA and GHPA). The white color in figure 3b may not actually show regions of no relationship but rather region with less than 6 in situ profiles. Here it is important to use a mask for regions where the Pearson correlation coefficient was not calculated. Similarly, in 3c the color bar axis should have a minimum value of 6 and a mask applied to regions with less that 6 profiles. While these are rather small correction, the accurate display of data is important. Finally, what is the value of figure 4d and 4e. A reader may ask why you show a region with anomalous in situ observations. Also, for 4d there appear to be data that in water depth less than 500 m.
Lines 176 to 180 need to be revised. How you validate the SHA is the sea-ice zone is important. What is meant by “ open Southern Ocean”? 60oS to the seasonal sea-ice zone?
While the interannual SHA variability shows a potentially interesting signal, it is somewhat complicated, and the analysis undertaken needs further investigation and refinement. This more detailed consideration of the causes of the variability would enable the authors to reach much more conclusive findings.
I would suggest that the authors consider only the SHA and the relationship with the large-scale SAM and ENSO forcing for this current manuscript and continue to investigate the interannual and seasonal signals for inclusion in another manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-3050-RC2
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