the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Feb 2024
South Asian summer monsoon enhanced by the uplift of Iranian Plateau in Middle Miocene
Abstract. The South Asian summer monsoon (SASM) significantly intensified during the Middle Miocene (17–12 Ma), but the driver to this change remains an open question. The uplift of the Himalaya (HM) and Iranian Plateau (IP), and global CO2 variation are prominent factors among suggested drivers. Particularly the impact of high CO2 on the Miocene SASM is little studied despite a large range of reconstructed CO2 values around this period. Here we investigate their effects on the SASM using the fully coupled Ocean-Atmosphere Global Climate Model CESM1.2 through a series of 12 sensitivity experiments. Our simulations show that the IP uplift plays a dominant role in the intensification of the SASM, mainly in the region around northwestern India. The effect of the HM uplift is confined to the range of the HM and its vicinity, producing orographic precipitation change. The topography forcing overall out-competes CO2 variation in driving the intensification of the SASM. In the case of extremely strong CO2 variation, the effects of the two factors are comparable in the core SASM region while in the western region, the topographic forcing is still the dominant driver. A thermodynamical process is proposed to link the uplift of the IP and enhanced SASM through latent heating release. Compared with reconstructions, the response of SASM to the IP uplift is in good agreement with observed precipitation and wind while the effects of the HM uplift and CO2 variation are inadequate to interpret the proxies.
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RC1: 'Comment on cp-2024-8', Anonymous Referee #1, 19 Mar 2024
This manuscript is well written and tires to clarify the role of the Himalaya uplift, Iranian Plateau (IP) uplift and atmospheric CO2 on SASM evolution in the mid-Miocene. They suggest IP uplift to be the main factor causing enhancement of SASM. Overall, I do recommend this study for publication after some revisions. Some suggestions and comments are listed below.
- There are several uncertainties in the Middle Miocene paleogeographic boundary conditions (Frigola et al., 2018) used in the simulation, such as the eastern Tethys seaway and Greenland-Scotland Ridge are deep water gateways, and the Bohai Bay and Yellow Sea basins in East Asia are shallow sea environments, which are inconsistent with many geological records (e.g., Sun J.M. et al., 2021, Paleo-3; Tan M.X. et al., 2020, Marine and Petroleum Geology; Stoker M.S. et al., 2005, Marine and Petroleum Geology). Some uncertainties of the boundary conditions are discussed in Section 5.3.
- In this study, authors lumped together all the mountain ranges west of the Himalayan, including the Hindu Kush and Pamir as the IP. The uplift history of the Iranian plateau, Hindu Kush, Pamir and East Africa remains controversial. Some studies suggested that the IP and East Africa began to uplift in the late Oligocene–early Miocene and rapidly uplifted in the middle–late Miocene (e.g., Macgregor, 2015, Journal of African Earth Sciences; Mouthereau et al., 2012). Uncertainties of topographic uplift can be appropriately added to the discussion.
- Table 2: there are some errors and inappropriate references. For example, Zhuang et al. (2017) interpreted the late Miocene (11–10 Ma) ocean cooling as representing the establishment of monsoonal upwelling in the western Arabian Sea, which may not be suitable as evidence for the enhancement of SASM in the Middle Miocene. Betzler et al. (2016): “deposit” changes to “sedimentary and geochemical record”. Bialik et al. (2020): “Precip” changes to “wind”; Ai et al. (2021): “Precip” changes to “wind”. In these papers, authors mainly talked about wind/monsoonal upwelling, not precipitation.
- Table 2: “sample” changes to “proxies”.
- Lines 507 and 523: “geography” changes to “land-sea distribution”.
Citation: https://doi.org/10.5194/cp-2024-8-RC1 -
AC1: 'Reply on RC1', yan zhao, 28 Mar 2024
Dear Referee,
Thank you very much for your positive recommendation of our manuscript. We truly appreciate your time and effort in reviewing it. Your comments help us to improve the quality of this paper. The responses point by point to your comments are in the attached document.
Best regards,
Yan Zhao on behalf of AC
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RC2: 'Comment on cp-2024-8', Anonymous Referee #2, 19 Mar 2024
This manuscript discusses the enhancement of the South Asian summer monsoon during the Miocene and explores the drivers of that change. By using an ocean-atmosphere global climate model, the response of the South Asian summer monsoon to the uplift of the Iranian Plateau coincides with observed precipitation and wind speeds compared to reconstructions. The uplift of the Iranian Plateau is found to play a dominant role in the enhancement of the South Asian summer monsoon, especially in north-west India. The research work carries out a large number of simulation experiments, a large amount of analysis and simulation, the article is readable, and the findings are important for understanding the long-term evolution of the South Asian monsoon since the Miocene. It is recommended for publication after detailed revision of the following comments.
General:
- The authors have emphasised that the time period of the study is MMIO, 17-12 Ma, and from the authors' very limited collection of record sites, it appears that the age of all the records is concentrated in the 14-12 Ma range, and the trend of the state of the South Asian monsoon indicated by these sites is "increasing".
Here, the authors need to be more explicit about the motivation for the Iranian Plateau, Himalayan and CO2 modelling. The reasons are as follows. Firstly, according to the general concept, the atmospheric CO2 concentration peaked around 15 Ma in the Miocene and then declined (e.g., Toward a Cenozoic history of atmospheric CO2, THE CENOZOIC CO2 PROXY INTEGRATION PROJECT (CENCO2PIP) CONSORTIUM 2023). Therefore, it is unlikely that CO2 is responsible for the intensification of the monsoon during this period from 14 to 12 Ma. Secondly, the authors have cited and elaborated that the Himalayas have reached their present height at 15 Ma, which also seems unlikely to be a factor in the 14~12 Ma monsoon intensification, and thus seems to be excluded. Finally, the authors mention that the Iranian Plateau uplifted at 15~12 Ma, but do not give clearer paleo-height constraints, and there is a lack of geographically reconstructed paleo-height evidence for the study.
The authors need to explicitly give the above information. This relates to the design rationale for the height of the Iranian Plateau in the authors' experimental design, as well as the attribution of monsoon intensification in the geological record.
- The authors excessively cite previous research findings in the experimental analysis section, e.g., P9L227~L228, P11L256~L257, and P14L318~319. which tends to confuse the reader: is this the result of your experiment or the result of previous work? It may even lead to the misunderstanding: your experiment is exactly the same model and experimental design as the previous work? It needs to be revised.
Specific issues:
P4L79~82 Therefore, it is worthy to revisit the response of the SASM to the IP and HM uplift under Miocene boundary conditions with a fully coupled Ocean-Atmosphere Global Climate Model (OAGCM) and investigate the underlying physical processes.
The authors emphasise the coupled experiments in the introduction, but they do not mention the advantages of the coupled ocean-air experiments throughout the analysis and discussion. In addition, considering that each experiment only runs for 200 years and that thousands of years of credits are generally required to carry out a coupled experiment simulation, 200 years is too short a credit for a coupled experiment simulation. Considering the high horizontal resolution used in all experiments, 200 years seems to be acceptable. However, whether the authors have considered biases due to SST disequilibrium or uncertainties in the conclusions of the study due to additional feedbacks would be best discussed briefly in the Discussion section.
P6L136~138 Is the topographic palaeoheight design of the Iranian Plateau supported by relevant cited literature?
P7L182~183 (2) Webster-Yang Index (WYI; Webster and Yang, et al., 1992): meridional wind stress 183 shear between 850 hPa and 200 hPa averaged over 40-110°E, 0-20°N during June-August.
Did the authors use the WY index with the same selected area as in the original work? Did you take into account the uncertainty associated with the index's indication of monsoon circulation due to the inconsistency of the land and sea distribution in the Middle Miocene with the modern era, and did you correct the computed area? Please provide a brief description.
P2L22: Global Climate Model 22 CESM1.2 through a series of 12 sensitivity experiments, the authors stated 12 sets of experiments, while in Table 1 (P32L845) only 10 sets are shown. Please change!
P17L367 Fig7 extra experiment 560ppm 800ppm, but the experiment description clearly states that.
P7L182: Webster and Yang, et al., 1992 is incorrectly cited; it should be Webster and Yang, 1992.
P12L269: 722B, monsoonal signal is absent in IPHM0 (Fig.3d). What is IPHM0? Is it a writing error?
P16L6 The figure is labelled IPHM0 as well?
P18L399~400 north Africa to North Africa
P18L402 2 Medina et al., 2010?The literature is not cited.
P19L414 Regarding to change to Regarding or Regard to.
P21L463 bewteen to between. Words are spelled incorrectly, please double-check the entire text
Citation: https://doi.org/10.5194/cp-2024-8-RC2 -
AC2: 'Reply on RC2', yan zhao, 03 Apr 2024
Dear Referee,
Thank you very much for your positive recommendation of our manuscript. We truly appreciate your time and effort in reviewing it. Your comments and valuable inputs help us to improve the quality of this paper. The point-by-point responses to your comments are in the attached document.
Best regards,
Yan Zhao on behalf of AC
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RC3: 'Comment on cp-2024-8', Anonymous Referee #3, 22 Mar 2024
The manuscript uses climate modeling to compare the effect from the uplift of the Himalaya (HM) and Iranian Plateau (IP) and the increase of atmospheric CO2 to the intensified SASM during the Middle Miocene (17-12 Ma) with coupled atmosphere-ocean global climate model CESM. The results indicate the IP uplift plays a dominant role in the intensification of the SASM, and the effect of the HM uplift is confined to the range of the HM and its vicinity. In the case of extremely atmospheric CO2 variation, the effects of two factors are comparable in the SASM region. Although some results are similar to previse modeling studies, this study compares the effect from topographic forcing and atmospheric CO2 variation, which are also interesting and important. However, there are still several limitations. Particularly, because the main purpose of this study is to compare the effect from topographic forcing and atmospheric CO2 variation, the uncertainties in used topographic change and CO2 variation should be mentioned, these uncertainties may affect the main conclusions of this study.
Lines 60-67, Can you give an estimate of the raised height of the IP during the Middle Miocene?
Lines 85-87, during mid- to late Miocene, the CO2 decreases according to the reconstruction, why increased CO2 is considered here?
Line 111, is the ice sheet model active during the simulation?
Lines 154 and 175, it’s better to show the times series of the surface temperature and net top of the atmosphere radiation imbalance in these experiments.
Lines 159-160, why the reference height is different between HM and IP?
Lines160-164, the experimental design for the uplift of HM and IP from flat to 100% is too idealized compared to the geological evidence (Lines 53-56, 60-67) during the Middle Miocene.
Lines 180-181, annual or seasonal precipitation?
Figure 1, in the experimental design, how do the authors determine the extent of the HM and IP? The extent of the HM looks larger in Fig. S2.
Lines 228-229, it’s better to point out the inconsistencies between model results and records. How about the ODP 359 and 758?
Line 279, where is the ‘core’ region?
Line 307, “to its west” should be “to its east”?
Line 399, “southeasterly” should be “southwesterly”?
Line 404, from Figure 9d, LCL is increased over the IP.
Line 468, where is “American region”?
Lines 484-486, the different used extent of the HM between these studies can explain the disagreement the author mentioned.
Line 506, the uncertainties in used topographic change and CO2 variation should be discussed.
Line 539, Zhang et al., 2017 is not a modeling study.
Table 2 No 3, the change in ODP 722 at 11 Ma is out of the Middle Miocene (17-12 Ma).Citation: https://doi.org/10.5194/cp-2024-8-RC3 -
AC3: 'Reply on RC3', yan zhao, 06 Apr 2024
Dear Referee,
Thank you very much for your positive recommendation of our manuscript. We truly appreciate your time and effort in reviewing it. Your comments and valuable inputs help us to improve the quality of this paper. The responses point by point to your comments are in the attached document.
Best regards,
Yan Zhao on behalf of AC
-
AC3: 'Reply on RC3', yan zhao, 06 Apr 2024
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