the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 May 2024
Hydrological cycle amplification imposes spatial pattern on climate change response of ocean pH and carbonate chemistry
Abstract. Ocean CO2 uptake and acidification in response to human activities are driven primarily by the rise in atmospheric CO2, but are also modulated by climate change. Existing work suggests that this `climate effect' influences the uptake and storage of anthropogenic carbon and acidification via the global increase in ocean temperature, although some regional responses have been attributed to changes in circulation or biological activity. Here, we investigate spatial patterns in the climate effect on surface-ocean acidification (and the closely related carbonate chemistry) in an Earth System Model under a rapid CO2-increase scenario, and identify another culprit. We show that the amplification of the hydrological cycle, a robustly simulated feature of climate change, is largely responsible for the spatial patterns in this climate effect at the sea surface. This `hydrological effect' can be understood as a subset of the total climate effect which includes warming, hydrological cycle amplification, circulation and biological changes. We demonstrate that it acts through two primary mechanisms: (i) directly diluting or concentrating dissolved ions by adding or removing freshwater and (ii) altering the sea surface temperature, which influences the solubility of dissolved inorganic carbon (DIC) and acidity of seawater. The hydrological effect opposes acidification in salinifying regions, most notably the subtropical Atlantic, and enhances acidification in freshening regions such as the western Pacific. Its single strongest effect is to dilute the negative ions that buffer the dissolution of CO2, quantified as `Alkalinity'. The local changes in Alkalinity, DIC, and pH linked to the pattern of hydrological cycle amplification are as strong as the (largely uniform) changes due to warming, explaining the weak increase in pH and DIC seen in the climate effect in the subtropical Atlantic Ocean.
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Status: open (until 10 Jul 2024)
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RC1: 'Comment on egusphere-2024-1189', Anonymous Referee #1, 20 Jun 2024
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This paper assesses the impacts of the hydrological cycle on pH changes due to increasing atmospheric CO2. It focuses on the dilution and concentration effects of dissolved inorganic carbon (DIC) and alkalinity at the ocean surface, and it also considers the cooling effect of an amplified hydrological cycle. These processes are emphasized as key drivers of ocean surface pH trends, a finding that is novel and important, warranting publication.
However, the overall structure of the paper is not well-organized, making it challenging to understand the concepts presented. I recommend a major revision to enhance clarity and readability for the audience.
The authors argue that the hydrological effects primarily dilute the negative ions that buffer CO2 dissolution (as stated in the abstract), leading to acidification in regions where salinity becomes lower. However, it is unclear why positive ions, like H+, are not also diluted in these regions. I may have confused this point. Aren’t the changes in vertical transport due to mixing changes (like w’dC/dz vs w’dA/dz) important? I assumed that salinity-enhanced mixing in these regions would increase surface DIC and alkalinity from deeper, richer waters. However, due to the larger vertical gradient in alkalinity compared to the DIC gradient—since surface water accumulates anthropogenic carbon—this effect should be more pronounced in alkalinity than DIC. Is this effect not important?
Most carbon chemistry research decomposing thermal and salinity effects on DIC and alkalinity have analyzed salinity-normalized DIC and alkalinity, which extracts salinity-driven biogeochemical variability beyond dilution and concentration effects. This paper challenges that approach by using a salinity-constrained experiment instead. The differences between these analyses remain unclear. What is the strong point of this analysis? (It is mentioned in Section 2.2, but not sufficient.) From your analysis, what biases might we be introducing into the salinity normalized analysis referenced to 35-psu? Conversely, what is the bias in your surface salinity-constrained experiment method? More discussion on this would greatly benefit future research.
Minor Revisions:
- Please refer to papers suggesting CO2-concentration feedback. The term "this study" in line 20 is unclear without the citation.
- L56: "These changes in salinity modify ocean circulation and lead to enhanced ocean heat uptake" remains unclear. I believe that an increase in salinity enhances heat uptake but not a decrease. Moreover, why does an increase in salinity enhance heat uptake? Is it because enhanced mixing reduces the surface temperature? Since this concept is key for this paper, please describe this in more detail. The reference for this description in L58 should be Figure 1d, not Figure 1b.
- The paper includes a variety of simulations and decomposition analyses to isolate climate and hydrological effects. The jargon used for simulation names often causes confusion; therefore, I recommend creating a table listing all experiments, their factor decomposition, and short descriptions.
- Figure 1 is introduced in the Introduction and contains simulation names before their descriptions. While it underscores the importance of the cooling effect through the amplified hydrological cycle, its early reference in the Introduction is unfriendly to readers. If used in the Introduction, a more detailed description of the experiments and isolated effects is needed.
- Line 108: The statement "Alkalinity is not directly sensitive to temperature" is confusing. Consider referencing Figure 1 to suggest the cooling effect, discussing solubility changes before this description, and noting that alkalinity does not change even with surface solubility changes.
- Appendix figures should be referenced in the main text. It is unclear which part each appendix figure corresponds to.
Citation: https://doi.org/10.5194/egusphere-2024-1189-RC1 -
RC2: 'Comment on egusphere-2024-1189', Anonymous Referee #2, 25 Jun 2024
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This study uses experiments with an Earth system model to investigate the effects of changes in the hydrological cycle on surface DIC, alkalinity, and pH. The methodology employed is a relatively simple linear approach, yet robust, allowing for the separation of the direct effect of hydrological changes from the indirect effect of the accompanying temperature changes. Highlights and novel insight include that while the effect of changes in hydrology is small in terms of global pH, it plays a significant role in driving the regional variability in pH due to climate change. Specifically, the effect of changes in hydrology opposes acidification in regions that become “saltier”, but enhances acidification in regions that will become fresher (hence the small overall effect on global scale).
In my opinion the study is novel, interesting and provides much needed insight for the drivers of the spatial variability in the response of the DIC, Alkalinity and pH to climate change. In my opinion, the methodology and analysis is thorough. While some necessary assumptions are made, the study provides useful information regarding the first-order effects of changes in the hydrological cycle. I recommend the study to be accepted for publication after addressing some minor queries/suggestions/comments detailed below (mostly in terms on clarifications to further strengthen the study).
General comment:
- I appreciate that assumptions are necessary for separating the different drivers in terms of changes in pH. However, I think it will be useful to be able to see how robust is your assumptions in section 2.3. I suggest you include a figure (maybe in the appendix) that shows maps of the actual DeltapH_hydro (pH_standard – pH_fix-sss) and DeltapH_clim (pH_standard – pH_fix-clim) from the model experiments versus the DeltapH_hydro and DeltapH_clim estimated from pyCOSYS.
Specific comments:
- Lines 111-113. I am not sure I agree with this. Based on comparisons of the panels in Figure A5 to me it appears that the hydrological cycle leads to equally large “anomalous CO2 fluxes” as the climate effect. In my opinion, this makes sense as in my understanding a large part of the effect from changes in the circulation is encapsulated to this “hydrology cycle”. Please, clarify and correct me if I have misunderstood.
- Lines 125-126. “This approach includes the influence of mixing and transport …” This is a little misleading, as I agree with you in terms of what you describe/discuss in lines 115 and 127-129 that this approach does not include and cannot account for the influence of mixing and transport as the vertical and horizontal gradients of salinity and DIC and alkalinity are not the same. Please consider erasing this part or rephrase it to avoid confusion.
- Section 2.3 and equations. Throughout this section you use different symbols/notation for the runs than in the previous section which makes it a little confusing. I suggest, for consistently, to use in all equations/notation: “Standard” instead of “Std” and “thermal” instead of “T” (such as DeltaDIC_thermal,hydro in line 151).
- Section 2.3 estimates of DeltapH. In my opinion, it makes more sense to reference the DeltapH in a pH_fix-SSS that has been estimated using a consistent approximation (the CO2SYS algorithm and not the direct pH from the fix-sss and fix-clim runs), which does not seem to be the case looking at the equations in lines 148-153. To clarify, I propose that the pH_fix-sss in the equations in lines 148-153 should be estimated as pH(DIC_fix-sss,Alk_fix-sss,SST_fix-sss,SSS_fix-SSH) from the CO2SYS algorithm rather than using the pH directly from the fix-sss run. Else, in my opinion, you could be introducing additional unrealistic changes in the DeltapH. Please correct me if I have misunderstood.
- Lines 141-142. I am not sure what you mean by “… ESM2M does not reach chemical equilibrium because it is constrained by conservation of heat and mass”. Maybe I have misunderstood but just to clarify I believe that your assumption is that the surface ocean is in a saturated state? In my understanding this does not have to be the case not just because the model is an ESM that conserves heat and mass. Please, consider clarifying.
- Lines 165-166. I believe that your residual includes not only biological or circulation effects but also the effect of the non-linearity (i.e., errors associated with the linear assumption of your methodology). I suggest that you should explicitly discuss/mention the inclusion of this non-linear effect within the residual here to avoid any confusion.
- Line 200-201 (links to my comment above), I believe that the residual will also include the effect of the nonlinearity of the carbonate system. I suggest you add this in the text here.
- Line 211-212. I am not sure what you mean by the “efficiency of the 3D mixing”, maybe consider rephrasing to something along the lines “… the effect of vertical and horizontal mixing, and circulation on the near-surface ocean…”.
- Figure A4. I am a little confused by this figure. (i) I suggest to clarify in the caption-text what the bars correspond to (I think is the freshwater component/effect but I may have misunderstood). (ii) I think there must be a typo here as you reference Figure 4b and e but figure 4 has no panel e, maybe you mean c (please check). (iii) In the legend you state that the “lollipops” correspond to the sum of thermal and freshwater. I believe you probably mean the thermal + freshwayer + residual, else I am unsure how you separated the thermal contribution in the alkalinity from the residual (and in my understanding this residual in alkalinity includes the effect of circulation, mixing, nonlinearity etc. rather than only the thermal contribution). Please consider clarifying in the caption and update the legend.
Typos:
Line 108. I believe you mean Figures A1 and A2 (not S1 and S2).
Lines 174-175, decrease appears twice (rephrase to something along the lines “… decrease DIC and Alkalinity by -9 …”).
Line 185-186. I believe you mean Figure A3 c and f.
Figures 4 and 6 caption-text. I believe you mean “… hatching indicates DSSS_hydro< 0.1” not <0 (based on the rest of the discussion and the actual legends in the plots). Please check.
Figure A3, for consistency with the rest of the text and the equations, I suggest you change the title in c and f to DeltaDIC_thermal,clim and DeltaDIC_thermal,hydro (rather than _SST).
Citation: https://doi.org/10.5194/egusphere-2024-1189-RC2
Model code and software
GFDL ESM2M NOAA-GFDL https://github.com/mom-ocean/MOM5/blob/master/doc/web/quickstart.md
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