the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Nov 2025
A dipole pattern of orbital-scale precipitation oxygen isotope variation in North African monsoon region and the driving mechanism
Abstract. Variations in North African (NAF) monsoon have pronounced socioeconomic impacts on many Western African countries. The orbital-scale NAF monsoon variation is reconstructed using speleothem δ18O (δ18Oc) records from caves. However, the climate meaning of these records is not well-known. δ18Oc in tropical monsoon regions is primarily determined by precipitation δ18O (δ18Op). Here, we use the isotope-enabled Community Earth System Model (iCESM) to investigate the climate interpretation of δ18Oc in the NAF monsoon region on the orbital timescale. Our analysis shows that both δ18Op and NAF monsoon rainfall exhibit a distinct precession signal. Enhanced Northern Hemisphere summer insolation (NHSI) intensifies land-sea thermal contrast, leading to strengthened monsoon circulation and increased NAF monsoon rainfall. However, the δ18Op presents a spatially dipole pattern, with depletion in the southern NAF and enrichment in the north. Tagging experiments further reveal that, while depletion in the south is driven by increased rainfall in upstream regions, enrichment in the north results from shifts in moisture sources, with a reduced contribution from distant sources and an increased influence of local sources. This work provides new insights into understanding the climatic meaning of isotopic records in NAF region and underscores the complex spatial features of isotopic signals compared to rainfall responses.
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- RC1: 'Comment on egusphere-2025-5029', Anonymous Referee #1, 20 Dec 2025
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RC2: 'Comment on egusphere-2025-5029', Anonymous Referee #2, 13 Jan 2026
For the most part, the manuscript is interesting, it is easy to read, and it provides a thorough description of the mechanisms underlying the simulated spatial distribution of water isotope ratios in precipitation over the North African (NAF) monsoon region during different states of orbital precession. The choice to use paleoclimate simulations with both water isotopic tracers and water tag diagnostics is well supported. Tying the simulations more closely to observations would strengthen the paper.
I have three major comments.
- I found the order in which material was presented in Figures 5-7 confusing. Each figure duplicates material from another figure, but this is not stated explicitly in the captions. Figure 5 (b-row) shows the changes in absolute precipitation contribution from distinct moisture source regions, while Figure 7 (b-row) shows the changes in relative contribution. Why not put these side-by-side? Both Figure 6 (b-row) and Figure 7 (a-row), which are identical, show changes in relative precipitation contributions scaled by the average isotope ratio from the source region. Therefore, four of the rows across three of the figures show essentially the same type of information: the contribution of changing moisture source regions to precipitation isotope ratios. I recommend re-organizing and consolidating this information.
- Figure 8 and related text argue that much of the isotopic depletion in the southern part of the NAF monsoon region results from distillation (rainout) as air masses “travel” from their source on the African continent to their sink within the NAF monsoon region (boxed on the figures). While this mathematical construct (term 2 on the RHS of Equation 6) makes sense for source regions that do not overlap with the sink region, I’m not sure it works for the African (continental) source. It implies that the isotope ratio of vapor within the NAF monsoon box is lower than the average isotope ratio over the continent. It does not give us any information about whether distillation occurs as air masses “travel” across the continent, as the term “en route” suggests.
- The paper begins by discussing the importance of interpreting isotope ratios in paleoproxies correctly. However, the paper gives no indication whether the dipole pattern simulated in iCESM is representative of isotopic spatial patterns inferred from paleo records. Furthermore, the Introduction suggests that other models simulate a spatially uniform isotopic response to precession forcing. Why should we trust the iCESM results? How well do they inform our ability to interpret paleo records? The Discussion should compare and contrast the simulation results with other models and contextualize the research findings by describing their relevance for paleoclimate interpretation.
Minor comments:
The manuscript has a few grammatical issues, especially with use of past/persent verb tense and pronouns.
Line 71: Despite “these advances…” what does this refer to? The previous sentences discuss simulation output, not necessarily advances.
L 83: “This coherent depletion pattern appears inconsistent with…” I recommend emphasizing the apparent contradiction sooner to help motivate the work.
L 90: Suggest replacing “among different models” with “based on different simulation results.”
L 130: “other factors…do not fundamentally change the perception”. Perception is the wrong word here. Consider rephrasing this paragraph to say that effects from these factors are expected to be negligible because of results from Roe and other papers.
Eqn 2 and 3: w-PDB and p-SMOW should be spelled out in the text and explained for greater accessibility.
L 159: by “integrated” do you mean “run for 40 years?”
L 166: I might suggest “terms” instead of “parts” when discussing the equations.
L 180: Please explain what vertical advection means within a vertically integrated atmospheric column. Why doesn’t Fnet capture MSE gain/loss through the top/bottom?
L 227: Suggest replacing “satisfies” with “is consistent with”
L 261: “Since the d18Oc record evolves coherently…” This statement suggests the d18O should decrease everywhere (since precipitation amount increases over the whole box). But there’s a dipole in the spatial isotopic pattern. So, this sentence should be clarified or rephrased.
L 272: I suggest using “MSE” consistently between text, caption, and methods, rather than introducing the term “enthalpy” here.
L 273: Refers to Fig 1 but I think Fig 4 is intended.
L 283: “Moving from low to high” is confusing here. I would suggest replacing with “Comparing high minus low NHSI periods.”
L 285: Please explain why greater shortwave radiation absorption occurs with increased cloud cover. In many regions, the dominant role of more cloudiness is to increase SW reflected.
L 290: Fig 4h is not discussed anywhere.
L 303: Fig 5 caption and axes labels suggest the plotted quantity is 16O. Is this actually the mass flux of 16O? Or H216O? I would strongly recommend using bulk precipitation rate or H216O precipitation rate rather than the flux of light atomic oxygen.
L 316: “To address this, we next employ…tagging…” However, tags are already introduced in the previous section and discussed near L 295. I would skip this sentence.
Citation: https://doi.org/10.5194/egusphere-2025-5029-RC2
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- 1
This manuscript investigates the orbital-scale response of North African (NAF) monsoon rainfall and precipitation water isotopes using an isotope-enabled coupled model, NCAR-iCESM. They conducted a long transient simulation spanning the past 150,000 years forced by orbital parameters (with 100-year orbital acceleration). They also conduct two moisture-tagging fixed-SST iCAM5 experiments for 127 ka (high Northern Hemisphere summer insolation-NHSI) and 116 ka representing a low NHSI, to diagnose the relative roles of moisture source contributions. The authors find that while NAF monsoon rainfall responds uniformly to precessional forcing, δ¹⁸Oₚ exhibits a pronounced north-south dipole (enrichment in the north vs. depletion in the south). They attribute δ¹⁸Oₚ depletion in southern NAF primarily to enhanced upstream rainfall and enrichment in northern NAF to shifts toward more local and relatively enriched moisture sources. The topic is timely and relevant, and the use of transient isotope-incorporated simulations combined with moisture tagging has the potential to provide valuable insights into the climatic interpretation of speleothem δ¹⁸O records.
However, while the scientific idea is promising, the manuscript currently has substantial issues in clarity, structure, and methodological description, particularly in the Introduction and Methods sections. Several key concepts and calculations are either insufficiently defined or ambiguously described. Further, I suggest the authors revise the Introduction to focus on background, motivation, and research questions, and move result-oriented statements to the Results/Discussion sections.
Also, I suggest adding more quantifications and graphics, as discussed in detail below in specific comments.
Hence, I recommend major revision before the manuscript can be considered for publication.
Specific Comments:
In Abstract: Please specify the latitude ranges used to define the southern and northern NAF regions, as this spatial distinction is central to the reported δ¹⁸Oₚ dipole.
Lines 21, 43: Define δ¹⁸Oc with more clarity, is it the oxygen-18 signal of speleothem calcite?
Line 51: Briefly describe what is meant by amount effect.
Line 55: Please clarify “isotope composition during convective activity”.
Line 36 : define The North African (NAF) monsoon region’s bounds
Line 60: middle Holocene- Mid-Holocene, also add the approximate time period in BP
Line 90: Please consider rephrasing this sentence to improve clarity (e.g. “is not yet fully understood”).
Line 91: Add NCAR to the sentence, also cite Brady et al., 2019 for iCESM.
Lines 92-: These lines in the Introduction currently contain key results of the study. These should be moved to the Results/Discussion sections, with this part of the introduction revised to emphasize motivation and research questions of this study.
Lines 102-103: Please rephrase to indicate that the stated CAM5 resolution applies to present simulations rather than being a general model property.
Lines 103-107: This sentence could be rewritten for clarity and readability. Consider simplifying the phrasing and splitting it into two sentences.
Line 111-: The part “albeit with smaller values” is ambiguous. Please clarify what is meant by “smaller values” and relative to which reference.
Line 115: Citation style-The citation should be inside the main bracket.
Line 118: Although the dominant signals can be represented with orbital forcing, please mention the caveats of this approach. There are several papers discussing vegetation feedback on the Mid-Holocene climate, for example.
Lines 128-137: My comment above addresses the same caveats discussed here.
The section needs improvements in terms of structure. Please move these lines to the paragraph starting from Line 118 and rephrase/shorten the paragraph without losing the main information.
Section 2.2:
This section needs substantial revision for clarity, beginning with the section title, and why annual mean is chosen. It is unclear whether δ¹⁸Oc is explicitly simulated by iCESM or if it’s speleothem calcite δ¹⁸Oc as in the introduction. It needs to be clear whether the values discussed (including precipitation, δ¹⁸Op) are model outputs or observational cave records, or approximated-δ¹⁸Oc from δ¹⁸Op. Please clearly define all isotope-related terms (e.g., δ¹⁸Oc, δ¹⁸Ow-PDB) and variables such as cave temperature. If cave temperature is approximated using model-simulated surface air temperature, this assumption should be explicitly stated and justified, including whether a single grid point or an average over nearby grid points is used. In its current form, the section lacks sufficient details on the methodology.
Line 150: Replace target region with the name of the monsoon region.
Line 154: “and the climate response between these two periods can infer the climate evolution at the orbital timescale”. Please rephrase for clarity.
Line 155:Consider rephrasing, for example: “The boundary conditions are derived from 1000-year climatological mean states corresponding to each period, from the coupled iCESM simulation.” Also what about the sea surface isotope ratios? Are they also derived from the 1000-year means?
Line 160: “to study the NAF” -Specify that it is NAF rainfall.
Lines 160-: Please provide a supporting reference for grouping the original 25 source regions into five aggregated regions [Are they the major sources for NAF rainfall from observations/modeling?]. It is unclear how these groupings were determined and why they reflect relative importance for NAF hydroclimate.
Section 2.3: If appropriate, cite Tabor et al. (2018) for the decomposition framework
Section 2: Add a paragraph on the observational data to be compared with the transient simulation
Line 170: The first term in “Eqn. 6”
Figure 1: Suggest to add a scatter plot of annual average GNIP δ¹⁸Oₚ versus annual average model-simulated δ¹⁸Oₚ to better quantify model-observation agreement. Also, please specify the model years or simulation period used for this plot. If there was a modern-simulation, please add the description to the methods.
Line 197: Consider citing Sultan&Janicot 2003.
Line 221: Please add reference for ODP Site 968 data
Figure 2 Caption: “(b) Simulated δ18Oc in the NAF region” As the model does not simulate this value, I suggest rewriting the caption to specify that.
Line 225: Please include a figure to substantiate the reported correlation between δ¹⁸Oc and rainfall.
Figure 3: Please specify in the caption that solid contours denote positive values and dashed contours denote negative values.
Line 256: “..Sahara, thereby reducing dust emissions.” Please clarify with a figure whether there is model-based evidence supporting this.
Line 260: “Furthermore, orbitally forced Northern Hemisphere summer warming
amplifies the interhemispheric temperature gradient, shifting the ITCZ northward (Schneider et al., 2014). “ You may quantify the simulated ITCZ shift following methodologies from previous studies and provide numerical values or a figure to support this statement.
Line 262: ..However, “as discussed”..
Line 284: The mechanisms are discussed but not shown. Please include figures in the supplementary material to support these results.
Line 291: Please clarify whether “horizontal moist enthalpy advection” is equivalent to horizontal MSE advection (Fig. 4g), and use consistent terminology throughout the manuscript.
Line 294: Like the comparisons with modern precipitation and water isotope ratios in Figure 1, it’d be appropriate to discuss how the simulated percentage contributions of moisture sources compare with present-day observations, and identify the dominant moisture sources for the region.
Line 295: “While the mean state rainfall in the NAF region is primarily supplied by the
African continent” - Please quantify the contribution.
Conclusions Section:
The study’s limitations related to orbital-only forcing, and implications if any, should be briefly discussed in the Conclusions, along with any comparisons with other transient climate model studies. Furthermore, in both results and conclusions, please consider adding quantifications (percentage contribution of source effects vs. upstream rainout) to strengthen the main conclusions.
Line 384 : “Our results indicate that the “amount effect” does not work in the southern NAF region”
I suggest the authors add comparisons with a few more studies, such as Risi et al. (2008), and also contrast their results with these existing literature. Also I suggest rephrasing “does not work”.
Further, please discuss in detail how your findings related to the amount effect would affect the usage of speleothem archives as a proxy for African rainfall.
Comment on technical difficulty in locating line numbers: Line numbers are shown only every five lines in the file, making referencing difficult.
References:
Risi, C., S. Bony, F. Vimeux, L. Descroix, B. Ibrahim, E. Lebreton, I. Mamadou, and B. Sultan (2008), What controls the isotopic composition of the African monsoon precipitation? Insights from event-based precipitation collected during the 2006 AMMA field campaign, Geophys. Res. Lett., 35, L24808, doi:10.1029/2008GL035920.
Sultan, B. and Janicot, S., 2003. The West African monsoon dynamics. Part II: The “preonset” and “onset” of the summer monsoon. Journal of climate, 16(21), pp.3407-3427.