South Asian summer monsoon enhanced by the uplift of Iranian Plateau in Middle Miocene
Abstract. The South Asian summer monsoon (SASM) remarkably strengthened during the Middle Miocene (16–11 Ma), coincident with the rapid uplifts of the Iranian Plateau (IP) and the Himalaya (HM). Although the development of the SASM has long been linked to the topographic changes in the Tibetan Plateau (TP) region, the effects of the HM and IP uplift are still vigorously debated, and the underlying mechanisms remain unclear. Based on Middle Miocene paleogeography, we employ the fully coupled earth system model CESM to perform a set of topographic sensitivity experiments with altered altitudes of the IP and the HM. Our simulations reproduce the strengthening of the SASM in northwestern India and over the Arabian Sea, largely attributing to the thermal effect of the IP uplift. The elevated IP insulates the warm and moist airs from the westerlies in the south of the IP and produces a low-level cyclonic circulation around the IP, which leads to the convergence of the warm and moist air in the northwestern India and triggers positive feedback between the moist convection and the large-scale monsoon circulation, further enhancing the monsoonal precipitation. Whereas the HM uplift produces orographic precipitation without favorable circulation adjustment for the SASM. We thus interpret the intensification of the Middle Miocene SASM in the western part of the South Asia as a response to the IP uplift while the subtle SASM change in eastern India reflects the effects of the HM uplift.
Meng Zuo et al.
Meng Zuo et al.
Meng Zuo et al.
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Reviewer Comments to Author:
Zou et al., South Asian monsoon enhanced by the uplift of the Iranian Plateau in the middle Miocene
This study aimed to understand the intensification of the South Asian monsoon during the middle Miocene by providing a suite of paleoclimate simulations with alternative Asian topographic configurations. The experiments focus on the impact of the Anatolian-Iranian Plateau and Himalayan Mountains and demonstrate how the Anatolian-Iranian Plateau controls the western portion of the circulation while the Himalayan Mountains control the east. The authors determined that the uplift of the Iranian Plateau was the main proponent that enhanced the monsoon during the middle Miocene.
There has been a recent reinvigoration of this topic. Understanding the paleoelevation of the Asian topographies has substantial implications on the regional climate and can give insight into topography-atmosphere interactions. The wide range of topographic scenarios provided by the authors highlights such interest. In this respect, the authors have the correct idea in testing the importance of the IP in the evolution of the Asian monsoon, as previous studies have done. However, I found several shortcomings in the study that should be addressed first. I express my concerns below.
There’s a reason why the paleo community has not fully performed this task. Current Miocene simulations are only performed at 2 degree or lower horizontal resolution, which inevitably does not capture either the Indo-Gangetic LLJ or IP insulation correctly. Previous work, albeit modern-day simulations, performed these experiments at a higher resolution (0.25 km) and demonstrated that low-resolution models are unable to capture orographic blocking mechanisms by either the Himalayan Mountains (see Acosta and Huber, 2017) or by the Iranian Plateau [Boos and Hurley, 2013; Acosta and Huber, 2020]. CESM runs in Figure 2 of the manuscript has the iconic excessive precipitation along the western Himalayan Mountains which ERA5 clearly does not. This is because CESM at low resolution (and many other low-resolution climate models) directly transport moisture flux from the Arabian Sea toward the mountains rather than from the Bay of Bengal via Indo-Gangetic LLJ. This gives the impression that the mountains are pulling the onshore moisture flux toward the region, aka “diabatic effect”, when observations show they simply act as a barrier for moisture to condense on. I believe the manuscript should fully elaborate on this caveat.
If one truly wishes to evaluate the impact of the IP in a “realistic” setting with the current version of the model, one should test the closure of the Tethys as an integral factor leading to the uplift of the Zagros mountains and the greater Anatolian-Iranian Plateau (see Sarr et al., 2022). Additionally, according to the papers sighted by the authors (McQuarrie et al., 2003; Mouthereau, 2011; Ballato et al., 2017; Bialik et al., 2019) one should expect to have a higher rather than a Flat IP which much of the Anatolian mountains are missing due to the paleogeographic constraints. Regardless, I suspect the IP insulation effect is not present in the paleosimulations presented. Also, since the Tethys is open in the current simulations one should check if moisture is added when the gateway is open versus when its closed. The authors should perform analysis on HM0IP0 against a modern run with the HM and IP set to sea level to understand the importance of closure of the Tethys Sea way.
The authors also fail to mention that the early to middle Miocene was a warming period leading up to the Middle Miocene Climatic Optimum then followed by late Miocene cooling. The authors utilized a single CO2 parameter when the CO2 could have been as high as ~800 ppm [Sosdian et al., 2020; Rae et al., 2021] and potentially changing regional precipitation rates without any topography changes. To decouple the impact of topography and climate, comparison between a warm and cold Miocene simulations with a low and high IP should be performed.
I see that the authors tried to convey changes in SAM in figure 3 but I don’t see a difference between the dots in plot c, d and e. For instance, ODP 722B remains “enhanced” with or without HMIP. Does this suggest that by having TP only you can still have an “enhanced SAM”. Since these are fully coupled runs the authors should gather available ocean proxies from early to middle and middle to late [Betzler et al., 2016; Zhuang et al., 2017] and provide a sufficient model-data comparison/evidence for strengthening of Arabian Sea upwelling. Again, this could test whether its topography or CO2 that’s sufficient to produce the proxy signal over the region.
Lastly, the community honestly did not “neglect the geological history of the region” and I don’t believe Boos and Kang 2010 suggested the HM rose before TP. Their study was mainly used to dispute the importance of elevated heating over TP and demonstrate that SASM can exist without the Plateau present. Language such as this gives the impression that prior work did not perform their due diligence and is honestly somewhat insulting.
Line 35 The term “inception” is quite vague since one can go back in literature and show a SASM existing during the early Cenozoic (See Huber and Goldner, 2012; Farnsworth et al., 2019). One could say the formation of a modern-like SASM.
The authors failed to talk about the TP in their analysis and discussion since some form of a proto-Tibet have existed prior to the HM and IP.
Lines 386-387 cite Sarr et al., 2022 and Acosta and Huber 2020
I don’t think it's necessary to provide the moisture budget decomp since it doesn’t add much value to the analysis. A figure like figure 7 with the other simulations is much more intuitive for the paleo community. One should also analyze changes in Arabian Sea moisture flux distribution since that’s a main topic in the discussion section 5.2.
In Figure 2 add all sensitivity studies to the figure.
In figure 3 did you mean plot c as MMIO or MMCO?
In figure 4 are plots b and c I IP100HM100 - IP0HM100 and IP100HM100 - IP100HM0 respectively? If not, I would suspect that this circulation IP100HM100 - IP0HM100 would have a weak impact on the circulation since the insulation is not properly simulated by the model. Please clarify with the exact name of the experiments.
Figure 7 is the anomaly IP0HM0 - IP100HM100 or the reverse order? I would expect IP100HM100 - IP0HM0 would yield a positive surface thetae, higher surface humidity etc over western India not the other way around. Also, I understand what wap, q, zg are since they are common output variables in CESM but you have not mentioned them in the manuscript or figure caption. I would be interested to see if the thetae, LCL is largely similar with IP0HM100.
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