the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jan 2024
Dynamic interaction of lakes, climate and vegetation over northern Africa during the mid-Holocene
Abstract. During the early to mid-Holocene, about 11,500 to 5,500 years ago, lakes expanded across the Sahel and Sahara in response to enhanced summer monsoon precipitation. To investigate the effect of these lakes on the West African summer monsoon, previous simulation studies prescribed mid-Holocene lakes from reconstructions. By prescribing mid-Holocene lakes, however, the terrestrial water balance is inconsistent with the size of the lakes. In order to close the terrestrial water cycle, we construct a dynamic endorheic lake (DEL) model and implement it into the atmosphere-land model ICON-JSBACH4. For the first time, this allows us to investigate the dynamic interaction between climate, lakes and vegetation over northern Africa. Additionally, we investigate the effect of lake depth changes on the mid-Holocene precipitation, which was neglected in previous simulation studies.
A pre-industrial control simulation shows that the DEL model realistically simulates the lake extent over northern Africa. Only in the Ahnet and Chotts Basin is the lake area slightly overestimated, which likely relates to the coarse resolution of the simulations. The mid-Holocene simulations reveal that both, the lake expansion and the vegetation expansion cause a precipitation increase over northern Africa. The sum of these individual contributions on the precipitation is however larger than the combined effect, when lake and vegetation dynamics interact. Thus, the lake-vegetation interaction causes a relative drying response over the entire Sahel. The main reason for this drying response is that the simulated vegetation expansion cools the land surface more strongly than the lake expansion, mainly the expansion of the Lake Chad area. The resulting warming response over the larger lake area causes local changes in meridional surface-temperature gradient that decelerate the inland moisture transport from the tropical Atlantic into the Sahel, which causes a drying response in the Sahel. An idealized mid-Holocene experiment shows that a similar drying response is induced when the depth of Lake Chad is decreased by about 1–5 m, without changing the horizontal lake area. By reducing the depth of Lake Chad the heat storage capacity of the lake decreases and the lakes warms faster during the summer months. Thus, in the ICON-JSBACH4 model, the lake depth significantly influences the simulated surface temperature and the simulated meridonal surface-temperature gradient between the simulated lakes and vegetation and thus, the mid-Holocene precipitation over northern Africa.
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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
(4547 KB)
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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
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-3081', Shivangi Tiwari, 22 Feb 2024
This is a well-written manuscript addressing a question which is certainly relevant to the scope of CP. It presents a new dynamic lake model implemented to address a pertinent question – the effect of lakes on the mid-Holocene northern African climate. To the best of my knowledge, this is a novel contribution, and it does lead to interesting new insights regarding the synergistic effect between vegetation and lakes as well as regarding the effect of lake depth. I do not have the required expertise to assess the HD and DEL models deeply; on the surface they appear to be reasonably formulated. Overall, I found the manuscript thoughtfully written and would recommend it for publication after addressing some issues.
Specific comments:
Line 265: I’m not sure if the lake and vegetation cover can be claimed to have reached a “close-to-equilibrium state” at 400 years into the simulation. Looking at Figure 5, the choices of last 100 years or the last 400 years as evaluation period seem more logical to me, compared to last 150 years which starts with a declining peak in both covers. The peak in lake cover at the beginning of the evaluation period is around 10,000 km2 and that doesn’t seem trivial to me. Either the authors could explain the quantitative basis behind this choice of evaluation period, or remove the phrase “close-to-equilibrium state” and simply mention that the last 150 years were used as evaluation period. Does the precipitation over northern Africa change significantly if the last 400 years are used instead?
Figure 5: Please mention in the caption what the horizontal dashed lines in subplot a) represent.
Lines 270 – 273: I appreciate this example to put the 18.8% deviation in context.
Line 303: Some newer work has indicated that precipitation could be even higher than that indicated by Bartlein et al. (2011) (which is also the proxy dataset that Braconnot et al., 2012 refer to). For example, Hely et al. (2014) show that Sudanian taxa were present up to 25 N, which would require 500-1500 mm/yr of rainfall, and Guineo-Congolian taxa (requiring >1500 mm/yr) potentially reached 20 N. More “extreme” examples are also provided by Tierney et al. (2017) and Sha et al. (2019). Hence, I suggest changing “seems to underestimate” to “underestimates” since that is a clear and significant under-estimation of precipitation.
Line 312: Braconnot et al. (2012) discussed PMIP1 and PMIP2 model results. Brierley et al. (2020) might be a better reference for state-of-the-art climate models. However, in my understanding, biases are usually discussed in terms of simulation of present day climate, and not while discussing (MH-PI, or similar) anomalies.
Line 328: Could the authors expand a bit on why lake expansion leads to higher precipitation south of Megalake Chad?
Line 390: I think a plot similar to Figure 7 but for mean annual temperature would help visualize this point. Additionally, could the authors provide estimates for changes in albedo due to vegetation and lakes? What exactly causes the change in the meridional temperature gradient?
Line 430: I’m not sure the results of this study strictly “contradict” the previous studies, since none of the previous studies discussed the synergistic effect, but only the net effect of vegetation and lakes. If I’ve understood correctly, the drying response is only due to the synergistic effect, and this study agrees with previous ones regarding the net effect.
Line 456: It seems to me that one of the main conclusions of the study (overturning circulation over Megalake Chad, and associated changes) is sensitive to the choice of the lake surface albedo. How much would this change with the use of a dynamic lake albedo? For example, what could be a plausible albedo range for Megalake Chad if a dynamic scheme were to be used?
Line 460-463: The switch from surface warming to surface cooling due to lakes is confusing.
Technical comments:
The authors should consider merging Figures 2 and 3 into one figure with the differences highlighted. Results should be moved into a separate section (Section 2.4 Section 3), moving Conclusions forward (Section 3 Section 4). The authors could consider moving Figure 11 into the Supplementary Information.
Lastly, I found some grammatical/typographical errors which should be corrected:
Line 21: warms→warm
Line 67: In the following section?
Lines 79-81: Consider replacing the phrase “a too” with “overly” or “unrealistically”
Line 89: albedo scheme represents changes
Line 91: over the northern Africa → over northern Africa
Line 110: on the climate variables e.g. the precipitation?
Line 142: thr → the
Figure 1 caption: In the last sentence – endorheic catchments were generated in a way that (remove "it") suits
Line 214: represented by → represented as a
Line 245: shirk → shrink?
Line 263: different → difference
Figure 5 caption: In the first sentence – northern. Last sentence – subplot c) or d)?
Line 264: 550 years
Line 273: Considering this → such large changes?
Line 290: orbit → orbital
Lines 334, 341: predominately → predominantly
Line 340: note the different → difference in the scaling
Lines 345, 348: meridonal → meridional
Lines 348 onwards: please check all references to various subplots of Figure 9
Line 358: is → in the Sahel
Line 410: transitional → transient?
Line 424: for → in
Line 466: the effect of exorheic lakes
I appreciate the work put in by the authors and hope they find these comments helpful. I thank Prof. Buizert for considering me for this review.
Citation: https://doi.org/10.5194/egusphere-2023-3081-RC1 - AC1: 'Reply on RC1', Nora Specht, 10 Apr 2024
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RC2: 'Comment on egusphere-2023-3081', Stefan Hagemann, 04 Mar 2024
- AC2: 'Reply on RC2', Nora Specht, 10 Apr 2024
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EC1: 'Editor Comment on egusphere-2023-3081', Christo Buizert, 19 Mar 2024
Dear Authors,
Your manuscript has now been seen by two reviewers. As you can see from their reports, they are overall supportive of publication after several comments have been addressed. The next step is for you to respond to these reviewer comments, ideally in the form of proposed changes to the manuscript. If your responses are appropriate and sufficient, you will be invited to submit a revised manuscript.
Please let me know if you have questions, and good luck preparing your responses.
Best, Christo Buizert (CP editor)
Citation: https://doi.org/10.5194/egusphere-2023-3081-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-3081', Shivangi Tiwari, 22 Feb 2024
This is a well-written manuscript addressing a question which is certainly relevant to the scope of CP. It presents a new dynamic lake model implemented to address a pertinent question – the effect of lakes on the mid-Holocene northern African climate. To the best of my knowledge, this is a novel contribution, and it does lead to interesting new insights regarding the synergistic effect between vegetation and lakes as well as regarding the effect of lake depth. I do not have the required expertise to assess the HD and DEL models deeply; on the surface they appear to be reasonably formulated. Overall, I found the manuscript thoughtfully written and would recommend it for publication after addressing some issues.
Specific comments:
Line 265: I’m not sure if the lake and vegetation cover can be claimed to have reached a “close-to-equilibrium state” at 400 years into the simulation. Looking at Figure 5, the choices of last 100 years or the last 400 years as evaluation period seem more logical to me, compared to last 150 years which starts with a declining peak in both covers. The peak in lake cover at the beginning of the evaluation period is around 10,000 km2 and that doesn’t seem trivial to me. Either the authors could explain the quantitative basis behind this choice of evaluation period, or remove the phrase “close-to-equilibrium state” and simply mention that the last 150 years were used as evaluation period. Does the precipitation over northern Africa change significantly if the last 400 years are used instead?
Figure 5: Please mention in the caption what the horizontal dashed lines in subplot a) represent.
Lines 270 – 273: I appreciate this example to put the 18.8% deviation in context.
Line 303: Some newer work has indicated that precipitation could be even higher than that indicated by Bartlein et al. (2011) (which is also the proxy dataset that Braconnot et al., 2012 refer to). For example, Hely et al. (2014) show that Sudanian taxa were present up to 25 N, which would require 500-1500 mm/yr of rainfall, and Guineo-Congolian taxa (requiring >1500 mm/yr) potentially reached 20 N. More “extreme” examples are also provided by Tierney et al. (2017) and Sha et al. (2019). Hence, I suggest changing “seems to underestimate” to “underestimates” since that is a clear and significant under-estimation of precipitation.
Line 312: Braconnot et al. (2012) discussed PMIP1 and PMIP2 model results. Brierley et al. (2020) might be a better reference for state-of-the-art climate models. However, in my understanding, biases are usually discussed in terms of simulation of present day climate, and not while discussing (MH-PI, or similar) anomalies.
Line 328: Could the authors expand a bit on why lake expansion leads to higher precipitation south of Megalake Chad?
Line 390: I think a plot similar to Figure 7 but for mean annual temperature would help visualize this point. Additionally, could the authors provide estimates for changes in albedo due to vegetation and lakes? What exactly causes the change in the meridional temperature gradient?
Line 430: I’m not sure the results of this study strictly “contradict” the previous studies, since none of the previous studies discussed the synergistic effect, but only the net effect of vegetation and lakes. If I’ve understood correctly, the drying response is only due to the synergistic effect, and this study agrees with previous ones regarding the net effect.
Line 456: It seems to me that one of the main conclusions of the study (overturning circulation over Megalake Chad, and associated changes) is sensitive to the choice of the lake surface albedo. How much would this change with the use of a dynamic lake albedo? For example, what could be a plausible albedo range for Megalake Chad if a dynamic scheme were to be used?
Line 460-463: The switch from surface warming to surface cooling due to lakes is confusing.
Technical comments:
The authors should consider merging Figures 2 and 3 into one figure with the differences highlighted. Results should be moved into a separate section (Section 2.4 Section 3), moving Conclusions forward (Section 3 Section 4). The authors could consider moving Figure 11 into the Supplementary Information.
Lastly, I found some grammatical/typographical errors which should be corrected:
Line 21: warms→warm
Line 67: In the following section?
Lines 79-81: Consider replacing the phrase “a too” with “overly” or “unrealistically”
Line 89: albedo scheme represents changes
Line 91: over the northern Africa → over northern Africa
Line 110: on the climate variables e.g. the precipitation?
Line 142: thr → the
Figure 1 caption: In the last sentence – endorheic catchments were generated in a way that (remove "it") suits
Line 214: represented by → represented as a
Line 245: shirk → shrink?
Line 263: different → difference
Figure 5 caption: In the first sentence – northern. Last sentence – subplot c) or d)?
Line 264: 550 years
Line 273: Considering this → such large changes?
Line 290: orbit → orbital
Lines 334, 341: predominately → predominantly
Line 340: note the different → difference in the scaling
Lines 345, 348: meridonal → meridional
Lines 348 onwards: please check all references to various subplots of Figure 9
Line 358: is → in the Sahel
Line 410: transitional → transient?
Line 424: for → in
Line 466: the effect of exorheic lakes
I appreciate the work put in by the authors and hope they find these comments helpful. I thank Prof. Buizert for considering me for this review.
Citation: https://doi.org/10.5194/egusphere-2023-3081-RC1 - AC1: 'Reply on RC1', Nora Specht, 10 Apr 2024
-
RC2: 'Comment on egusphere-2023-3081', Stefan Hagemann, 04 Mar 2024
- AC2: 'Reply on RC2', Nora Specht, 10 Apr 2024
-
EC1: 'Editor Comment on egusphere-2023-3081', Christo Buizert, 19 Mar 2024
Dear Authors,
Your manuscript has now been seen by two reviewers. As you can see from their reports, they are overall supportive of publication after several comments have been addressed. The next step is for you to respond to these reviewer comments, ideally in the form of proposed changes to the manuscript. If your responses are appropriate and sufficient, you will be invited to submit a revised manuscript.
Please let me know if you have questions, and good luck preparing your responses.
Best, Christo Buizert (CP editor)
Citation: https://doi.org/10.5194/egusphere-2023-3081-EC1
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Nora Farina Specht
Martin Claussen
Thomas Kleinen
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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