The role of mountains in shaping the global meridional overturning circulation

Domain: earth_science
Published: 2024-03-23
Editorial outcome: major_revision

Abstract
The meridional overturning circulation (MOC) in the ocean is a key player in the global climate system, while continental topography provides an essential backdrop to the system. In this study, we design a series of coupled model sensitivity experiments to investigate the influence of various mountain ranges on the global thermohaline circulation. The results highlight the influence of the Tibetan Plateau (TP) on the global thermohaline circulation. It emerges as a requisite for establishing the Atlantic MOC (AMOC) and a determining factor for the cessation of the Pacific MOC (PMOC). Additionally, the Antarctic continent plays a vital role in facilitating the TP to form the AMOC. While the formation of the AMOC cannot be attributed to any single mountain range, the TP alone can inhibit the PMOC’s development. By modifying the global hydrological cycle, the TP is likely to have been crucial in molding the global thermohaline circulation.

Introduction
The meridional overturning circulation (MOC) in the ocean plays a pivotal role in both regional and global climate, via large-scale heat and freshwater transports. In the current climate, the MOC exists mainly in the Atlantic, referred to as the Atlantic MOC (AMOC), which is characterized by deep convection and deep-water formation in the Labrador Sea and Greenland-Iceland-Nordic (GIN) seas of the subpolar Atlantic1,2. The observed strength of the AMOC is approximately 18 Sv3. There is no equivalent Pacific MOC (PMOC) in the North Pacific.

Geological evidence reveals that the primary deep-water formation region in the Northern Hemisphere (NH) might have undergone a shift from the Pacific to the Atlantic in the past. Some earlier studies suggested that North Pacific deep-water (NPDW) formation was strong during the Paleocene period (about 65–55 million years ago, or Ma)4, while the North Atlantic deep-water (NADW) formation was weak and likely began to develop at a later stage5,6. However, a recent study from the DeepMIP project found that neither model results nor proxy data suggest NADW formation during the early Eocene, while evidence for NPDW formation remains elusive. Consequently, the modern AMOC might initially develop in the late Miocene (about 12–9 Ma) and not be fully established until the late Pliocene to early Pleistocene (about 4–3 Ma)7. Nonetheless, a recent study from DeepMIP project found that neither model results nor proxy data suggest NADW formation during the early Eocene, while the evidence for NPDW formation remains inconclusive8. The actual evolutionary history of the AMOC and PMOC therefore remains a topic of considerable debate.

The asymmetry of net surface freshwater flux is often cited as the cause of different overturning modes between the Atlantic and Pacific. The North Atlantic has higher sea-surface salinity (SSS) than the North Pacific because the former is a net evaporation basin, while the latter is nearly neutral9. Additionally, ocean basin geometry plays a role in the different overturning modes. Research indicated that narrow basins are more conducive to deep overturning circulation than wide basins2,10,11. Furthermore, ocean gateways also contribute to the different overturning modes. The opening of the Drake Passage/Tasman Seaway in the late Eocene is thought to have promoted the NADW formation and thus the AMOC formation12.

Geological evidence also suggests that the uplift of large continental mountains has had a significant impact on the climate13. The evolution of continental terrain holds the potential to trigger large-scale transitions in the global MOC (GMOC)14–16, and in the following, we briefly review the uplift periods of the major modern mountain ranges. The Rocky Mountains (RMs) rose from the sea level about 80 Ma17, and reached its current elevation about 45 Ma8. Although the Transantarctic and Gamburtsev Mountains over Antarctica were likely already present at the start of the Cenozoic (65 Ma)18,19, the first large-scale glaciation of Antarctica is believed to have occurred during the Eocene-Oligocene transition period (34–33.5 Ma)20,21. The uplift of Andes Mountains (AMs) started in the Late Cretaceous (~70 Ma)22 and matured around 15–10 Ma23,24. The uplift of these mountains predated the onset of the NADW formation. The Greenland (GL) underwent its initial phase of uplift in the late Miocene (11–10 Ma)25. The timeline for the formation of the Tibetan Plateau (TP) is a topic of highly debate. Some studies argue that parts of the TP were already in place during the late Eocene (38–33 Ma)26,27, while other research suggest that the TP’s rapid and main uplift occurred between 10 and 8 Ma28–31. A more recent study proposes that most of the TP had attained its current elevation before the Mid-Miocene (15 Ma)32. This timing coincides with the onset of NADW formation, suggesting a possible link between the TP uplift and the development of NADW. Recent research also indicates that the TP is a critical factor affecting changes in the GMOC33,34. Nevertheless, it is important to recognize that the chronology of the uplift of major mountain ranges remains a subject of intense discussion and investigation.

Here, as a first step, we numerically investigate how the presence or absence of various major mountain ranges affect the GMOC as well as their cumulative impact when they are uplifted sequentially in the model from an initial flat Earth. In order to isolate the topographic effects from plate tectonics and long-term bathymetric and atmospheric changes, we keep the modern bathymetry and continental positions, as well as modern greenhouse gas concentrations, incident solar radiation and orbital parameters. We show that, in our simulations (Methods and Table 1), the TP uplift is the primary driver of the PMOC shutdown and the AMOC initiation, although high Antarctic topography is required to drive a strong, modern-like AMOC. We further discuss the implication of our results to the long-term Cenozoic history of the AMOC/PMOC.

Results
Adding mountains sequentially to flat continent in the model can ultimately lead to the establishment of the AMOC and the collapse of the PMOC (Fig. 1). However, this dramatic change occurs only after the TP is added. In the Flat scenario, the MOC features a modest PMOC of about 109 Sv and a negligible AMOC of less than 1 Sv, consistent with previous findings14–16,34,35. Following the TP uplift in model year 4001 (red curve), the AMOC recovers rapidly, overshoots its realistic value in about 800 years, and reaches an equilibrium state in about 1000 years, which matches the normal state in Real of about 18 Sv (gray line). Overshooting during AMOC recovery is common in freshwater hosing experiments36, CO2 forcing experiments37, and other paleoclimate simulations38, although their mechanisms may differ. Simultaneously, the PMOC collapses quickly (blue curve), and reaches an “off” state in Real of about 4 Sv in about 200 years. It suggests that the AMOC would be difficult to establish in the absence of the TP, while the PMOC would not exist in the presence of the TP.

None of the individual topography uplift from the Flat terrain is capable of initiating the AMOC. However, the TP uplift alone can cause the collapse of the PMOC (Fig. 2a). The presence of individual AT and AM, on the other hand, can result in a stronger PMOC than that in Flat. As demonstrated by Jiang and Yang39, the presence of the RM alone has negligible effects on both AMOC and PMOC. Although the presence of the TP alone leads to a slight increase in the AMOC strength (approximately 2 Sv), it is inadequate for its complete establishment. Conversely, the PMOC is practically abolished in the presence of the TP alone.

Studies have demonstrated that the removal of the TP can result in the collapse of the AMOC33,34,40. This implies that the combination of the RM, AM, AT, and GL is insufficient to sustain the AMOC. Indeed, this is precisely the situation before the TP is added, as illustrated in Fig. 1. Moreover, we investigated the impacts of various combinations of RM, AM, AT, and GL; and none of these arrangements can establish the AMOC, irrespective of the order in which they were introduced to Flat. As the TP alone cannot support the establishment of the AMOC, we are left with the question: what is the minimum topographic requirement, in addition to the presence of the TP, for the establishment of the AMOC?

We find that only the combination of the TP and AT can lead to the establishment of the AMOC from Flat, whereas the combination of the TP with any other mountains is ineffective (Fig. 2b). With TP+AT, the AMOC gradually increases within the first 800 years, followed by a sharp increase to a very high level in a short time, ultimately reaching quasi-equilibrium in approximately 2000 years, with the magnitude equivalent to that in Real. However, in the other combinations, even after an integration of 2200 years, the AMOC does not increase significantly. These experiments suggest that only the AT effectively assists the TP in the formation of the AMOC. Conversely, the PMOC quickly disappears in any experiment where the TP is present.

Figure 3 shows the patterns of the AMOC, PMOC, and GMOC in each experiment. In Flat (Fig. 3a1, a2), only the shallow wind-driven subtropical cells (STCs) are present in the Atlantic; while the PMOC is strong, it has a weak inter-hemispheric structure (Fig. 3b1, b2), consisting of the wind-driven STCs and subpolar thermohaline circulations. In the Pacific, the wind-driven STCs are strong and hemispherically symmetric, occupying the upper 500-m ocean between 30°S and 30°N. The thermohaline circulation originates in the North Pacific north of 30°N, and its lower branch occupies the deep ocean beneath 1000 m, extending to the Southern Ocean. The PMOC in the South Pacific (Fig. 3b2) is much weaker than the AMOC in the South Atlantic in Real (Fig. 3a8). The structure of strong PMOC and weak AMOC is maintained in Flat2Real before the TP uplift, and in the experiments without the TP.

The uplift of the TP appears to have fundamentally changed the GMOC. Based on the experiments we conducted, we conclude that the TP is a necessary condition for the “on” state of the AMOC and a sufficient condition for the “off” state of the PMOC. The AT is also critical to the establishment of the AMOC. Among the various topographic features, the TP stands the most significant factor influencing the GMOC.

Compared to Real, the MOC in Flat results from three key factors: 1) higher net evaporation in the North Pacific and greater precipitation in the North Atlantic, 2) stronger Ekman downwelling in the North Pacific, and 3) weaker Ekman pumping in the Southern Ocean and associated weaker southward water mass transport in the intermediate-to-deep South Pacific. The first two factors contribute to the formation of the NPDW and the shutdown of the NADW, while the third factor weakens the thermohaline component of the PMOC. The impacts of net evaporation in the North Atlantic and Ekman pumping in the Southern Ocean on the AMOC have been well recognized41–46. Here, we quantify their roles using topography experiments.

In Real, the sea-surface density (SSD) in the subpolar Atlantic is over 27.5 \sigma_0, which is sufficient for the NADW formation, even with Ekman upwelling (Fig. 4b). However, in Flat the SSD in the North Atlantic is reduced to 25.5 \sigma_0 (Fig. 4a); and this reduction (\Delta\sigma ~ 2.0 kg/m3) (Fig. 4e) is significant enough to halt the NADW formation. In Real, the North Pacific SSD is 25.5–26 \sigma_0; and in Flat, it is increased to 26.5 \sigma_0 in the Northeast Pacific (Fig. 4a). This increase (\Delta\sigma ~ 0.5 kg/m3) is sufficient for shallow surface-water subduction, but is insufficient for deep-water formation, which requires the assistance from enhanced Ekman downwelling (Fig. 4e)34. Note that the physical processes involved in NADW formation in Real and NPDW formation in Flat are not the same. The former primarily involves thermohaline dynamics, while the latter involves both thermohaline and wind-driven dynamics35.

The change in SSD from Real to Flat can be attributed to the change in SSS (Fig. 4i), in which the virtual salt flux resulting from net evaporation (i.e., evaporation minus precipitation, or EMP) and sea-ice melting (Supplementary Fig. 2b1) playing crucial roles. The contribution of sea-surface temperature change to SSD change is limited34. For a steady state, EMP across the ocean surface is almost equivalent to the vertically integrated water vapor transport divergence (\nabla \cdot vq) throughout the entire atmosphere column, when freshwater flux from land surface and river runoff are disregarded.

In the transition from Real to Flat, aided by anomalous lows over the North Atlantic and North American continent (Supplementary Fig. 3b1), more atmospheric moisture is transported eastward from the North Pacific and north-eastward from the central tropical Pacific to the North Atlantic (Supplementary Fig. 3a1), leading to increased moisture convergence over the North Atlantic. Later, the southward expansion of sea ice in the subpolar Atlantic results in sea-ice melting at the same time, providing a substantial amount of freshwater into the ocean (Supplementary Fig. 2b1), eventually leading to the shutdown of the AMOC. Meanwhile, aided by anomalous highs over the North Pacific and southern China (Supplementary Fig. 3b1), less atmospheric water vapor converges over the western tropical and subtropical Pacific (Supplementary Fig. 3a1), causing an increase in SSS there (Fig. 4i). The resulting high-salinity surface water is transported northward by the Kuroshio Current and further eastward by the Kuroshio Extension, thereby triggering the NPDW formation. The anomalous high over the North Pacific also leads to Ekman downwelling (Fig. 4e), which further aids the NPDW formation. These two processes collectively lead to the establishment of the PMOC in Flat.

In Flat, the thermohaline component of the PMOC in the South Indo-Pacific is weaker than that of the AMOC in Real (Fig. 3b2 vs a8). This can be attributed to the weaker Ekman pumping in the Southern Ocean in Flat in comparison to that in Real (Fig. 5a). The absence of the Antarctic topography induces a high-pressure anomaly over the Antarctic, which produces an anomalous northward pressure gradient, and thus an anomalous easterlies based on the geostrophic balance. This eventually results in an 80% reduction in wind stress and Ekman pumping along the Antarctic continent (Fig. 5a). The impact of this reduction on the MOC is evident in Fig. 3c1–c3, with the Deacon cell being weakened by 50% in Flat. As a result, the southward water mass transport in the intermediate-to-deep ocean across 30°S is about 30% weaker in Flat when compared to that in Real (Supplementary Fig. 4).

Once we recognize the changes in the MOC from Real to Flat, it becomes easy to understand the MOC changes in TP. From Flat to TP, the changes in the NH atmospheric circulation and moisture transport (Supplementary Fig. 3a4, b4) are similar to those from Flat to Real (Supplementary Fig. 3a2, b2). Consequently, we see similar changes in SSD, SSS, and Ekman pumping in TP as those in Real (Fig. 4b, f, j, and c, g, k). Specifically, the TP uplift leads to more net precipitation (Supplementary Fig. 2b3) and thus smaller SSD in the North Pacific, and a stronger Ekman upwelling in the North Pacific (Fig. 4g), which jointly shut down the PMOC. Moreover, the TP uplift causes less atmospheric moisture transport from the central tropical Pacific to the North Atlantic (Supplementary Fig. 3a4), resulting in accumulation of more saline surface water in the North Atlantic. However, the SSD in the North Atlantic (Fig. 4g) is still smaller than in Real (Fig. 4f) at this stage. The AMOC shows only a slight increase (Fig. 2a). Furthermore, the Ekman pumping in the Southern Ocean in TP is much weaker than that in Real (Fig. 5b), which limits further development of the AMOC.

The presence of the AT alone does not alter the surface buoyancy in the NH (figure not shown). Similar to Flat, the NPDW formation occurs instead of the NADW formation. However, the Ekman pumping in the Southern Ocean in AT is as robust as that in Real (Fig. 5c), resulting in a more potent Deacon cell (Fig. 3f1) and a more forceful thermohaline component of the PMOC (Fig. 3e1) than those in Flat. Similar to Flat, the strong southward water mass transport occurs in the South Indo-Pacific, but not in the South Atlantic (Fig. 5g, Supplementary Fig. 4), because the presence of the TP alters the deep-water formation in the NH, which allows only the NADW to reach the Southern Ocean.

With the assistance of the AT, the establishment of the AMOC in TP+AT becomes possible (Fig. 3d8). The presence of the TP shuts down the PMOC, shifting the deep-water formation from the North Pacific to the North Atlantic. Simultaneously, the presence of the AT leads to strong Ekman pumping in the Southern Ocean, which enhances the NADW formation remotely (Fig. 4l). The MOC patterns in TP+AT are almost identical to those in Real (Fig. 3d8 vs a8) since both the surface buoyancy in the North Atlantic and the Ekman pumping in the Southern Ocean are almost identical to those in Real (Figs. 4d and 5d). Note that in TP+AT, there is strong southward water mass transport in the South Atlantic, instead of in the South Indo-Pacific as in AT (Fig. 5g, h, Supplementary Fig. 4), because the presence of the TP alters the deep-water formation in the NH, which allows only the NADW to reach the Southern Ocean.

The presence of other large topographies, such as AM, RM, and GL, would not switch the MOC from the Pacific to the Atlantic. However, the PMOC in Exp AM is noticeably stronger than that in Flat (Fig. 2a). The presence of the AM reduces the equatorial trade wind, but amplifies the off-equatorial Ekman pumping (Supplementary Fig. 5a), thereby boosting the wind-driven STC in the South Indo-Pacific (Supplementary Fig. 5b) and augmenting the thermohaline component of the PMOC there, leading to a stronger PMOC (Fig. 3e2). On the other hand, both the RM and GL have minimal effects on the PMOC (Fig. 3a3–f3) because their presences do not significantly alter the global atmospheric moisture pattern30 and the atmospheric circulation in the tropics (Supplementary Fig. 6).

The AMOC gradually increases in the first few hundred years after the TP uplift in both Flat2Real and TP+AT, followed by an acceleration and eventual return to a normal state in Real (Figs. 1 and 2b). The latter process is accompanied by a swift sea-ice loss in the subpolar Atlantic. In response to the TP uplift, more atmospheric moisture converges (diverges) over the North Pacific (Atlantic) (Supplementary Figs. 2b2, b5 and 3a2, a3), shutting down the PMOC and triggering a gradual increase of the AMOC at the same time. The latter enhances the northward heat transport in the Atlantic, leading to a gradual retreat of sea ice in the subpolar Atlantic. The sea ice retreat is also helped by the anomalous northward Ekman flow, forced by the anomalous easterlies over the subpolar Atlantic (Supplementary Fig. 3b2).

The evolution of sea ice in Flat2Real is shown in Fig. 6. The northward retreat of sea ice is illustrated by the sea ice velocity in Fig. 6b, which leads to additional freshwater loss in this region (Supplementary Fig. 2b2, b5). This freshwater loss in turn increases SSD, the NADW formation and thus the AMOC consequently. In the first few hundred years after adding the TP, the sea-ice in the subpolar Atlantic retreats northward slightly (Fig. 6b), and the March mixed layer depth (MLD) deepens slightly (Fig. 6a), corresponding to a gradual increase of the AMOC. About 500 years after the TP uplift, the collective effects of accumulated saline water in the subpolar Atlantic and Ekman pumping in Southern Ocean accelerate the AMOC, so that the sea-ice margin retreats rapidly northward (Fig. 6b, dashed red curve), resulting in a large amount of freshwater flux loss in the subpolar Atlantic, a rapid deepening of the MLD (Fig. 6a), and a further enhancement of the AMOC (Figs. 1 and 2b). The sea-ice margin reaches its quasi-equilibrium roughly 1000 years after the TP uplift (Fig. 6b), accompanied by significant sea-ice melting in the GIN seas. The sea-ice evolution in TP+AT (Supplementary Fig. 7a, b) displays a similar pattern to that in Flat2Real, while changes in sea ice and MLD in TP are minimal (Supplementary Fig. 7c, d). The evolutions of AMOC, the MLD and sea-ice coverage and margin in Flat2Real and TP+AT suggest a positive feedback between the AMOC and sea ice changes, which eventually lead to the establishment of AMOC. This feedback has been shown in many previous studies34,47.

Methods
We use the National Center for Atmospheric Research Community Earth System Model version 1.0 (NCAR CESM1.0) in this study. The CESM1.0 employed here has the grid of T31_gx3v7, which consists of the atmospheric component (CAM4) with 26 vertical levels and T31 horizontal resolution (3.75° × 3.75°); the ocean component (POP2) with 60 vertical levels and gx3v7 horizontal resolution (approximate 3° near the poles to 0.6° at the equator); the land component (CLM4) and the sea-ice component (CICE) with the same horizontal resolution as the CAM4 and POP2, respectively. More details about these model components can be found in Gent et al.54 and Shields et al.55.

To compare the impacts of various mountain ranges on the global MOC, we conduct a few sets of topography experiments (Table 1, Supplementary Fig. 1). These experiments are designed in relation to the “Flat” experiment, which features a globally flat topography at 50 m above the sea level (Supplementary Fig. 1a). Although not precisely representative of any specific Earth epoch, Flat serves as a reference point for the other experiments. Flat is integrated for 1600 years under the preindustrial conditions, starting from a control run with realistic modern topography (Real) that was completed previously56. We examine the effects of five prominent topographies in this study: the Rocky Mountains, the Antarctic continent, the Andes Mountains, Greenland, and the Tibetan Plateau. Except for topography height, all other boundary conditions remain the same as in Flat or Real. The ocean-land configuration is set to modern-day conditions without correction for plate tectonic motion. Atmospheric CO2 concentration is maintained at the preindustrial level (285 ppm). Changes in river routing and vegetation type are not considered. Continental ice sheets are treated as bright rocks in the model. The planetary albedo can adjust itself according to thermal conditions. These experiments are conducted as single-variable (orography) sensitivity tests, rather than paleoclimate simulations involving multiple prescribed geologic boundary conditions.

The first set of experiments includes adding each of the five different topography individually to the Flat. These experiments are named RM (Rocky Mountains), AT (Antarctic), AM (Andes Mountains), GL (Greenland), and TP (Tibetan Plateau), respectively. Each topography is added to Flat starting from year 801; and each experiment is then integrated for at least 800 years. The second set of experiments, named Flat2Real, includes adding the five topography sequentially according to their uplift times. The RM is added to Flat in year 801; and the model is integrated for 1200 years. The AT is added after the RM in year 2001; and the model is integrated for 400 years. The AM is added in year 2401; and the model is integrated for 1000 years. The GL is added in year 3401; and the model is integrated for 600 years. The TP is added in year 4001; and the model is integrated for 2000 years. The third set of experiments combines topography differently, such as TP+RM, TP+AT, TP+AM, and TP+GL, plus Real with all five mountains. All these experiments start from year 801 of Flat, and are integrated for at least 800 years. More detailed information can be found in Table 1.

The changes due to the presence of unique topography are obtained by subtracting the results of Flat from each topography experiment. Some experiments show an initial jump when adding the topography suddenly, but that jump has little effect on the equilibrium state. These experiments indicate that uplift causes a wide array of changes in the global climate. We use student-t test to examine the statistical significance of our results. Most changes are significant at the 95% confidence level, which is expected because altering large topography induces strong mechanical forcing and obvious responses around the globe. For visual clarity, significance test is not included in any figures.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a review of “Which Mountains Mafter to Global Meridional Overturning Circulafion” by Haijun Yang and coauthors for potenfial publicafion in Nature Communicafions.\nUsing numerical simulafions with a low-resolufion Earth System Model (CESM1.0 at ~ 3°), the authors invesfigate how the absence/presence of major mountain ranges (the Tibetan Plateau/Himalaya, the Antarcfic topography, the Andes, the Rockies and the Greenland topography) alters the global meridional overturning circulafion. They show that in a “flat” Earth, the AMOC shuts down in favour of the PMOC, and that adding new reliefs to this flat Earth may spectacularly reorganize the global MOC. In parficular they show that the presence of the Tibetan Plateau is both a sufficient condifion to suppress the PMOC and a necessary (but not sufficient) condifion to the existence of the AMOC. They show that the Andes and Antarcfic Mountains are reinforcing drivers of, respecfively, the PMOC (but not the AMOC) and both the AMOC or PMOC (depending on the presence or not of the Tibetan Plateau). On the contrary, the Rockies and the Greenland topography only exhibit weak impacts on the MOC in the simulafions presented here. Finally, an interesfing aspect is that the authors perform what can be seen as a more realisfic scenario of the Cenozoic mountain evolufion (Flat2Real), in which they sequenfially add the aforemenfioned mountain ranges to a flat Earth and tentafively suggest that the main phases of uplift during the Cenozoic may have at least partly driven the Cenozoic MOC evolufion, though it must be noted that the authors state the limitafions of their approach to the “real” history of the MOC.\nI must say I really enjoyed reading this manuscript. The experiments are nicely designed and the results are robustly demonstrated. The manuscript is also clear and well-wriften. I would quite straighfforwardly recommend this manuscript for publicafion from a physics (mechanisfic) perspecfive. However, and in parficular in the context of a broadly oriented journal like Nature Communicafions, I feel like the rafionale behind the experiments and the contribufion of the findings to the understanding of long-term Cenozoic evolufion should be befter discussed and/or arficulated (see below) and I therefore recommend to return the manuscript to the authors with minor revisions. I hope this can help strengthen the manuscript for broader impact and I would be happy to read a revised version.\nTwo points must be befter treated.\nFirst, while I think the authors nicely show how the Tibetan Plateau and Antarcfic Mountains make the system switch from the PMOC to the AMOC, the chronology of MOC changes and uplift events described in l. 7-14 and 24-33 overlooks the (very) large uncertainfies associated with these. For instance, some studies argue that parts of the Tibetan Plateau were already high in the late Eocene (e.g., Xiong et al. 2020, Su et al. 2019). The Transantarcfic and Gamburtsev Mountains over Antarcfica were likely already present at the start of the Cenozoic (Elliot 2014, Ferraccioli 2011)¬—except if the Antarcfic uplift considered here is in fact the Eocene-Oligocene glaciafion of Antarcfica? In this case, it should be more explicit. The history of the AMOC/PMOC is also quite debated and not really agreed upon. l. 7-14 and 24-33 instead convey another impression upon reading. I agree that it is not at all the job of this manuscript to provide a comprehensive review of these changes but these secfions should be re-wriften with more caveats to reflect these uncertainfies.\nSecond, though this is touched upon on l. 18-19 and 235-236, the impacts of oceanic gateways cannot simply be neglected because these have the potenfial to counterbalance the effects of mountain uplift by providing pathways to efficiently mix water masses. I think that menfioning some relevant work relafive to the Cenozoic evolufion of a few major gateways¬—e.g., Drake Passage/Tasman Seaway, Panama, Arcfic gateways—and how they may have modulated the ocean circulafion would be worthwhile. For instance, before the deep opening of Drake Passage / Tasman Seaway in the late Eocene (e.g., Sauermilch et al. 2021), it is likely that the Ekman pumping due to the Westerlies may have been much weaker. This is especially relevant as some simulafion findings contradict the history of MOC changes as wriften l. 7-14 and could be used to provide alternafive hypothesis: sfill using the Drake/Tasman example, Fig. 2 shows that adding the Antarcfic Mountains substanfially increase the PMOC intensity (by the way this is not seen on Flat2Real because the simulafion is only run for 400 years after the AT addifion and before the AM addifion, which is not enough in parficular because the AM drives a similar increase in PMOC intensity and obscures the AT effect). However, chronologically, the PMOC is expected to decrease during the Cenozoic (l. 10 of the manuscript, see also Ferreira et al. 2018) although this is quite uncertain. In this case, a counterbalancing effect of the Southern Ocean gateways could provide a hypothesis to limit the impact of AT on the MOC.\nNow I am well aware of how delicate it is to speculate about such changes and I am not asking the authors to overly expand on these kinds of hypothesis but I think the authors should sfill use a couple of judiciously-chosen example to make clear that repeafing these experiments with another land-sea mask (e.g. narrow Drake/Tasman and wide Panama gateways) could give a completely different story, including a possibly much weaker role for the Tibetan Plateau.\nMinor points.\nl. 113 Why is the salt flux virtual?\nThroughout. How is computed Ekman pumping/downwelling?\nl. 165-166. Not only surface buoyancy in North Atlanfic but also Ekman pumping in the Southern Ocean.\nl. 176-177. This sentence reads a bit weirdly because the AM does not alter the moisture paftern either but sfill affect the MOC intensity via alterafion of the atmosphere dynamics.\nMethods. Are changes in runoff consequent in the simulafions? What would happen if the runoff was adequately re-routed?\nFig. 4. I get what dots and crosses represent on Fig. 4a1-a4 but not on Fig. 4b1-b4 (the difference figures). Do crosses (dots) represent increased downwelling (upwelling)?\nAlso what do you mean by “mean Ekman pumping/downwelling”? I do not understand how dots and crosses can have units of cm/day.\nFig. 5. Mass transport on Fig. 5b1-b4 should be in Sv to be consistent with the other figures.\nFig. 6. What month or season is reflected in the sea-ice margin/velocity? It should be clearly wriften in the legend.\nExtended data Fig. 1 should be plofted at model resolufion because this is what mafters in the results.\nReferences.\nElliot, D. H. (2013). The geological and tectonic evolufion of the Transantarcfic Mountains: a review. Geological Society, London, Special Publicafions, 381(1), 7-35.\nFerraccioli, F., et al. (2011). East Antarcfic rifting triggers uplift of the Gamburtsev Mountains. Nature, 479(7373), 388-392.\nFerreira, D., et al. (2018). Atlanfic-Pacific asymmetry in deep water formafion. Annual Review of Earth and Planetary Sciences, 46, 327-352.\nSauermilch, I., et al. (2021). Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling. Nature communicafions, 12(1), 6465.\nSu, T., et al. (2019). Uplift, climate and biofic changes at the Eocene–Oligocene transifion in south-eastern Tibet. Nafional Science Review, 6(3), 495-504.\nXiong, Z., et al. (2020). The early Eocene rise of the Gonjo Basin, SE Tibet: From low desert to high forest. Earth and Planetary Science Lefters, 543, 116312.", "reviewer_2": "The manuscript by Haijun Yang et al. presents an invesfigafion into the influence of orography on the Meridional Overturning Circulafion (MOC) using the low-resolufion coupled model CESM. Previous studies, including Fallah et al. (2016), Su et al. (2018), and Yang and Wen (2020), have examined the effects of the Tibetan plateau on oceanic circulafion. Maffre et al. (2017) conducted an analysis on the impact of global orography on oceanic circulafion by comparing two simulafions: one incorporafing present-day geography and another using a flat Earth representafion. In this regard, Haijun Yang et al.'s work combines these two approaches by invesfigafing the effects of a flat Earth representafion and various combinafions of present-day orography on oceanic circulafion. While this study is interesfing, I have a few concerns regarding this approach.\n(L24-L39) One concern with the sensifivity experiments conducted in this study is that they include the enfire topography of the confinent, rather than focusing solely on specific features like the Tibetan plateau or the Rocky Mountains. Previous research has demonstrated that the uplift of the Tibetan plateau influences the Atlanfic Meridional Overturning Circulafion (AMOC). However, by incorporafing the enfire topography of a confinent, it becomes challenging to isolate the effects of individual landforms and determine whether minor features play a role in the establishment or disrupfion of the AMOC/PMOC.\nAddifionally, the study includes the Andes Mountains and the enfire South American, African, Middle Eastern regions up to the Zagros Mountains and Anatolian plateau under the scenario \"AM\". Regarding the ice sheet (Antarcfica and Greenland), the study tests the sensifivity to changes in elevafion, but it is not specified whether the authors maintained the albedo of ice when removing the Antarcfica / Greenland topography (the term ice sheet is never employed in the manuscript). This informafion is crucial as altering the albedo would affect the energy balance and potenfially influence the circulafion pafterns and precipitafion.\nMoreover, when orography is removed, the drainage basin system is expected to undergo changes. The authors indicate that river roufing is kept unchanged. Understanding the modificafions in the drainage basin system is relevant to comprehending the overall impact of orography removal on oceanic circulafion.\nThese concerns regarding the specificity of landforms, albedo adjustments, and drainage basin modificafions should be addressed in the manuscript to provide a comprehensive analysis of the study's findings.\nDespite the fact that the authors exclude to explain the evolufion of AMOC during the Cenozoic and restrain this explanafion to the role of Tibetan plateau in the present day geography, the Figure 1 exhibits the impact of a sequenfial uplift of mountains with a “geological” chronology. The sequence is supposed to mimic the main phase of uplift (or growth of ice sheet).\nAlthough the authors do not try to provide a detailed explanafion of the evolufion of the AMOC/PMOC during the Cenozoic era and instead focus on the role of the topography in present-day geography, Figure 1 demonstrates the impact of sequenfial mountain uplift with a geological chronology. The sequence is intended to simulate the impact on ocean circulafion of the primary phase of uplift or the growth of an ice sheet.\nThe length of the simulafion is a mafter of debate. Following the addifion of Antarcfic, the simulafion is integrated for 400 years. However, this durafion may be considered relafively short to ensure that the deep ocean layers have fully reached equilibrium with the boundary condifions.\n(L69) The authors do not provide an explanafion regarding whether the albedo was modified when removing the topography of Antarcfica. It would be beneficial to clarify whether any adjustments were made to account for changes in albedo resulfing from the modificafion of Antarcfic topography.\n(L93-…) The authors primarily focus on describing the oceanic changes in their study, driven by atmospheric modificafions resulfing from the addifion or removal of topography. However, they do not provide a detailed explanafion of the underlying mechanisms involved. It would indeed be valuable to include a few sentences elucidafing the specific mechanisms by which the evaporafion or precipitafion pafterns are altered when modifying the topography.\nIn this work, the authors do not explicitly menfion conducfing stafisfical significance tests to evaluate the differences between two variables.\n(L112-122) The mechanism causing a change in precipitafion or evaporafion (or both) are discussed. The changes in atmospheric circulafion and moisture transport are associated to changes in GPH. The authors do not explicitly delve into the topic of temperature changes in relafion to GPH variafions, nor do they discuss the potenfial role of Sea Surface Temperature (SST) changes in the North Atlanfic affecfing the North Atlanfic Deep Water (NADW) circulafion. To provide a more comprehensive analysis, it would be beneficial for the authors to address the possible temperature changes resulfing from GPH modificafions and discuss the influence of SST changes on NADW formafion explicitly. Incorporating these aspects would further elucidate the connecfions between atmospheric circulafion, temperature, and oceanic processes, enhancing the overall understanding of the research findings.\n(L122-L123) “The expansion of sea ice (…) carries a substanfial amount of freshwater”: the sentence is not clear to me.\n(L135) “The absence of the Antarcfic topography leads to an 80% decrease in wind stress”: is it due to temperature gradient?\nFigure 5: the Ekman pumping/downwelling is discussed with the 850hPa wind stress. This is not clear. Why don't the authors show the wind stress at the sea surface ?\nPosifive mass transport across 30°S is counted posifively northwards. This is not menfioned in the figure capfion.\n(L202+Extended Data Figure 3a1-4) The authors' conclusion that the Tibetan plateau acts as a significant aftractor of freshwater in the Northern Hemisphere (NH) is potenfially accurate if the boundary condifions used in their scenario are limited to the Tibetan plateau alone. However, it is important to consider that changes in the North Pacific region can also be influenced by the westerlies blowing from Eastern Asia, which may be impacted by the presence or absence of landforms located to the north of the Tibetan plateau.\n(L224) I think that the authors could quote a more recent paper than the Ruddiman’s 1989 paper\n(L232-243) I agree with the observafion regarding the limitafions of the study, parficularly with respect to the spafial resolufion of the atmospheric model. I wonder whether the low resolufion of the model might not be a potenfial factor contribufing to the lack of impact observed for certain mountain ranges, parficularly those with an elongated shape such as the Andes or the Rockies. Mountain ranges with complex topography may not be adequately captured or represented in the model due to the coarse resolufion. In contrast, the Antarcfic ice cap and the Tibetan plateau may be befter resolved within the model. This could explain why their impacts are more apparent in the simulafions.\nOther comment\n(L24-33) The fiming of uplift is also debatable. A part of the Tibetan plateau is largely uplifted before the Late Miocene.\nAntarcfic (ice sheet ?) : the authors explains that “Antarcfic rapidly expanded in the Oligocene and persisted unfil the Late Oligocene”. This is not clear to me what the authors mean by “persisted unfil the Late Oligocene”. Does it mean that the ice sheet melts after the Late Oligocene ? No reference is provided.\nAndes uplift => reference 19 : Zachos et al. 2001 ?\nFigure 5: confinent contours are too thin to be seen when printed\nExtended Data Figure 3 : confinent contours are hard to see"}, "author_response": "Replies to Reviewer #1:\nThank you very much for these constructive comments. We have revised the manuscript\ncarefully based on these suggestions. The following are our point-by-point replies.\nUsing numerical simulations with a low-resolution Earth System Model (CESM1.0 at ~ 3°), the\nauthors investigate how the absence/presence of major mountain ranges (the Tibetan\nPlateau/Himalaya, the Antarctic topography, the Andes, the Rockies and the Greenland\ntopography) alters the global meridional overturning circulation. They show that in a “flat” Earth,\nthe AMOC shuts down in favour of the PMOC, and that adding new reliefs to this flat Earth may\nspectacularly reorganize the global MOC. In particular they show that the presence of the Tibetan\nPlateau is both a sufficient condition to suppress the PMOC and a necessary (but not sufficient)\ncondition to the existence of the AMOC. They show that the Andes and Antarctic Mountains are\nreinforcing drivers of, respectively, the PMOC (but not the AMOC) and both the AMOC or PMOC\n(depending on the presence or not of the Tibetan Plateau). On the contrary, the Rockies and the\nGreenland topography only exhibit weak impacts on the MOC in the simulations presented here.\nFinally, an interesting aspect is that the authors perform what can be seen as a more realistic\nscenario of the Cenozoic mountain evolution (Flat2Real), in which they sequentially add the\naforementioned mountain ranges to a flat Earth and tentatively suggest that the main phases of\nuplift during the Cenozoic (Since 65 Ma) may have at least partly driven the Cenozoic MOC\nevolution, though it must be noted that the authors state the limitations of their approach to the\n“real” history of the MOC.\nI must say I really enjoyed reading this manuscript. The experiments are nicely designed and\nthe results are robustly demonstrated. The manuscript is also clear and well-written. I would quite\nstraightforwardly recommend this manuscript for publication from a physics (mechanistic)\nperspective. However, and in particular in the context of a broadly oriented journal like Nature\nCommunications, I feel like the rationale behind the experiments and the contribution of the\nfindings to the understanding of long-term Cenozoic evolution should be better discussed and/or\narticulated (see below) and I therefore recommend to return the manuscript to the authors with\nminor revisions. I hope this can help strengthen the manuscript for broader impact and I would be\nhappy to read a revised version.\nResponses: Thank you very much for your valuable comments, which help us improve the\nmanuscript tremendously. Considering the comments from all the reviewers, we revised the\nmanuscript primarily in these following aspects:\n1) Introduction to research background is much improved. We rewrote statements and make\nit clear to readers that there are many controversies regarding the on/off state of the\nAMOC/PMOC, as well as the timing of mountain uplift.\n2) Mechanisms for global climate changes in response to mountain uplift are explained with\nmore details;\n3) Some relevant references are added; and most of figures are replotted.\nSpecific comments:\n1. First, while I think the authors nicely show how the Tibetan Plateau and Antarctic Mountains\nmake the system switch from the PMOC to the AMOC, the chronology of MOC changes and\nuplift events described in l. 7-14 and 24-33 overlooks the (very) large uncertainties associated\nwith these. For instance, some studies argue that parts of the Tibetan Plateau were already\nhigh in the late Eocene (37.9-33.7 Ma) (e.g., Xiong et al. 2020, Su et al. 2019). The\nTransantarctic and Gamburtsev Mountains over Antarctica were likely already present at the\nstart of the Cenozoic (65 Ma) (Elliot 2014, Ferraccioli 2011), except if the Antarctic uplift\nconsidered here is in fact the Eocene-Oligocene (56-23 Ma) glaciation of Antarctica? In this\ncase, it should be more explicit. The history of the AMOC/PMOC is also quite debated and\nnot really agreed upon. l. 7-14 and 24-33 instead convey another impression upon reading. I\nagree that it is not at all the job of this manuscript to provide a comprehensive review of these\nchanges but these sections should be re-written with more caveats to reflect these\nuncertainties.\nResponses: Thank you very much for raising these important questions. We rewrote this part and\nmade it clear to readers that there are many controversies on the on/off state of the AMOC/PMOC\nin the history in Lines 7-15: “Geological evidence reveals that the primary deep-water formation\nregion in the Northern Hemisphere (NH) might have undergone a shift from the Pacific to the\nAtlantic in the past. Some studies suggest that North Pacific deep-water (NPDW) formation was\nstrong during the Paleocene period (about 65-55 Ma, million years ago), while North Atlantic\ndeep-water (NADW) formation was weak and likely begun to develop during the early Oligocene\nperiod (about 35-33 Ma). Consequently, the modern AMOC might initially develop in the late\nMiocene (about 12-9 Ma) and be fully established until the late Pliocene to early Pleistocene period\n(about 4-3 Ma). Nonetheless, the actual evolutionary history of the AMOC and PMOC remains a\ntopic of considerable debate.”\nAs far as the uplift timings of different mountains are concerned, we rewrote the statement in\nLines 27-40 as follows: “… Although the transantarctic and Gamburtsev Mountains over\nAntarctica were likely already present at the start of the Cenozoic (65 Ma) (Elliot 2014, Ferraccioli\n2011), the glaciation of Antarctica was thought to occur during the Eocene-Oligocene (56-23 Ma),\npredating the uplift of the Andes Mountains (AM) that rose around 24 Ma…” “… Although some\nstudies argue that parts of the TP were already formed in the late Eocene (38-33 Ma) (e.g., Xiong et\nal. 2020, Su et al. 2019), the rapid and main uplift of TP was established about 10 Ma. This timing\ncoincided with the onset of NADW formation, suggesting a potential connection between these two\nmountains uplifts and the development of NADW. Still, it’s important to note that the timing of the\nuplift of major global mountain ranges remains a highly debated topic.”\nPlease refer to Lines 27-40 in the revised manuscript. Due to the article length limited by the\njournal, we apology that we are unable to include detailed description on the uncertainties in the\nmanuscript.\n2. Second, though this is touched upon on l. 18-19 and 235-236, the impacts of oceanic\ngateways cannot simply be neglected because these have the potential to counterbalance the\neffects of mountain uplift by providing pathways to efficiently mix water masses. I think that\nmentioning some relevant work relative to the Cenozoic evolution of a few major gateways,\ne.g., Drake Passage/Tasman Seaway, Panama, Arctic gateways, and how they may have\nmodulated the ocean circulation would be worthwhile. For instance, before the deep opening\nof Drake Passage / Tasman Seaway in the late Eocene (e.g., Sauermilch et al. 2021), it is\nlikely that the Ekman pumping due to the Westerlies may have been much weaker. This is\nespecially relevant as some simulation findings contradict the history of MOC changes as\nwritten l. 7-14 and could be used to provide alternative hypothesis: still using the\nDrake/Tasman example, Fig. 2 shows that adding the Antarctic Mountains substantially\nincrease the PMOC intensity (by the way this is not seen on Flat2Real because the simulation\nis only run for 400 years after the AT addition and before the AM addition, which is not\nenough in particular because the AM drives a similar increase in PMOC intensity and\nobscures the AT effect). However, chronologically, the PMOC is expected to decrease during\nthe Cenozoic (l. 10 of the manuscript, see also Ferreira et al. 2018) although this is quite\nuncertain. In this case, a counterbalancing effect of the Southern Ocean gateways could\nprovide a hypothesis to limit the impact of AT on the MOC.\nResponses: Thank you very much for your insightful comments. You proposed a very interesting\nquestion that deserves an in-depth investigation. Following your comments, we are going to design\nexperiments to study individual contributions of open/closed Drake Passage/Tasman seaway and\nthe AT/AM topography height, and their combined effect on the global MOCs.\nSince the manuscript is limited by words, we only briefly mention that “…ocean basin\ngeometry, ocean gateways and continental topography may contribute to different overturning\nmodes. ... The opening of the Drake Passage/Tasman seaway in the late Eocene may have promoted\nthe NADW formation and AMOC formation...” in lines 19-23.\n3. Now I am well aware of how delicate it is to speculate about such changes and I am not\nasking the authors to overly expand on these kinds of hypothesis but I think the authors should\nstill use a couple of judiciously-chosen example to make clear that repeating these\nexperiments with another land-sea mask (e.g. narrow Drake/Tasman and wide Panama\ngateways) could give a completely different story, including a possibly much weaker role for\nthe Tibetan Plateau.\nResponses: Thank you very much for these insightful comments. This work is just a beginning; and\nwe are actually starting to do lots of other experiments to enhance our understanding of the\ntopography, the ocean and land gateway’s roles in the ocean circulation. There are so much to do.\nWe would like to show you our future work, if possible, based on your suggestion of the\nexperiments.\nThe purpose of this work is to assess the roles of different mountains in the GMOC under\npresent-day situation. We try to focus on comparing the roles of different mountains, not on\ncomparing mountains’ roles with the roles of other factors (such like ocean gateway, land gateway,\netc.). We totally agree with you that there would be many possibilities and the outcomes could be\ncompletely different if we try to mimic “real” evolution of paleoclimate.\nMinor points.\n1. l. 113 Why is the salt flux virtual?\nResponses: It actually is “freshwater flux” across the ocean surface, not really salt flux; so, it is\ncalled “virtual salt flux.”\n2. Throughout. How is computed Ekman pumping/downwelling?\nResponses: The Ekman pumping is calculated using the wind stress at the ocean surface. (cid:2033) =\n(cid:3006)\n(cid:3099)\n(cid:1855)(cid:1873)(cid:1870)(cid:1864)( ), where (cid:2028) is the surface wind stress with the units of N/m2, (cid:2025) is the density of sea water\n(cid:3096)(cid:3033)\n((cid:2025) = 1024 (cid:1863)(cid:1859)/(cid:1865)(cid:2871)), and f is Coriolis parameter with the units of 1/s. So, the units of\n(cid:1840) (cid:1863)(cid:1859)(cid:1865)/(cid:1871)(cid:2870)\n1 (cid:1865)(cid:2870) 1 (cid:1865)(cid:2870)\n(cid:2033) ~ (cid:4684) (cid:4685)~ (cid:3438) (cid:3442) = (cid:1865)/(cid:1871) → (cid:1855)(cid:1865)/(cid:1856)(cid:1853)(cid:1877)\n(cid:3006) (cid:1865) (cid:1863)(cid:1859) (cid:1865) (cid:1863)(cid:1859)\n∙(cid:1871)(cid:2879)(cid:2869) ∙(cid:1871)(cid:2879)(cid:2869)\n(cid:1865)(cid:2871) (cid:1865)(cid:2871)\nIn the caption of Fig. 4, we added how the Ekman pumping is calculated.\n3. l. 165-166. Not only surface buoyancy in North Atlantic but also Ekman pumping in the\nSouthern Ocean.\nResponses: Thank you very much for point out this detail. In the revised manuscript, we changed\nthis statement to “since both the surface buoyancy in the North Atlantic and the Ekman pumping in\nthe Southern Ocean are almost identical to those in Real (Figs. 4a4, 5a4).”\n4. l. 176-177. This sentence reads a bit weirdly because the AM does not alter the moisture\npattern either but still affect the MOC intensity via alteration of the atmosphere dynamics.\nResponses: Thank you very much for pointing out this problem. We should say that the presences\nof RM and GL do not significantly alter the global atmospheric moisture pattern and the tropical\natmospheric circulation, so that they have very limited effects on the AMOC and PMOC.\nIf we agree that the AMOC or PMOC consists of the thermohaline component (mainly\ndetermined by the buoyancy flux in the mid-to-high latitudes) and the wind-driven component\n(mainly determined by the wind stress and Ekman pumping in the tropics), it should be easy to\nunderstand that the RM and GL’s minimal roles in the AMOC and PMOC. However, the AM\naffects the wind-driven component of the PMOC via altering the tropical wind system although it\ndoes not alter the moisture pattern in the mid-to-high latitudes either.\nIn the revised manuscript, we state that “The presence of other large topographies, such as\nAM, RM, and GL, would not switch the MOC from the Pacific to the Atlantic.” (in lines 178-180);\nand “On the other hand, both RM and GL have minimal effects on the PMOC (Figs. 3a3-f3)\nbecause their presences do not significantly alter the global atmospheric moisture pattern and the\natmospheric circulation in the tropics (Extended Data Fig. 6).” (in lines 183-186). Extended Data\nFig. 6 is added to this revision. It is also added here as Fig. R1 for reference.\nFig. R1 Equilibrium changes in atmospheric circulation and moisture transport. (a1)-(a3) changes in vertically\nintegrated moisture transport (vector; units: kgm-1s-1) and its convergence (shading; units: 10-5 kgm-2s-1), (b1)-(b3)\ngeopotential height (shading; units: 10 m) and winds (vector; units: m/s) at 850 hPa. (a1)-(a3) and (b1)-(b3) are changes\nin AM, RM and GL, respectively, with respect to Flat. The atmospheric moisture convergence (divergence) is plotted as\npositive (i.e., −(cid:2008)∙(cid:1874)(cid:3377)(cid:1869)>0) (negative, −(cid:2008)∙(cid:1874)(cid:3377)(cid:1869)<0), representing a gain (EMP<0) (loss, EMP>0) of ocean freshwater\nfrom (to) the atmosphere.\n5. Methods. Are changes in runoff consequent in the simulations? What would happen if the\nrunoff was adequately re-routed?\nResponses: Thank you very much for these questions. The runoff is calculated in Common Land\nModel (CLM) of CESM1.0. It includes the liquid water runoff (R) and ice runoff (I). The changes\nof runoff in the simulations are adjusted according to the River Transport Model (RTM). The RTM\nuses a linear transport scheme at 0.5º resolution to route water from each grid cell to its downstream\nneighboring grid cell. The ocean freshwater liquid and ice fluxes are passed to the flux coupler that\ndistributes the fluxes to the appropriate ocean grid cells.\nIn all experiments, the ice_runoff is set to “False” and the ice runoff is zero. In response to the\ntopographic change, the direction and discharge of river runoff will be changed. In CESM1.0, the\nMR\nLG\n850 hPa winds and GPH\nMA\nMoisture transport and convergence\n(a1) (b1)\n(a2) (b2)\n(a3) (b3)\ndrainage or sub-surface runoff is based on the SIMTOP scheme (Niu et al., 2005). Due to the\nlimited geological data, it is difficult to get the exact distributions of river runoff during the period\nof mountain uplift. There is no artificial modification to the river runoff. The results of runoff\nchanges are based on the model simulations.\nFigure R2 show the runoff flux (units: mSv) anomalies in several experiments relative to Flat.\nPositive value means that the ocean gains fresh water. In general, the effect of river runoff change\non the ocean circulation can be neglected.\nFig. R2 River runoff flux (units: mSv) anomalies in experiments TP, AT, TP+AT, and Real, with respect to Flat.\nPositive value means freshwater gain by the ocean.\nReference:\nNiu, G.-Y., Yang, Z.-L., Dickinson, R.E., and Gulden, L.E. 2005. A simple TOPMODEL-based runoff\nparameterization (SIMTOP) for use in global climate models. 110: D21106. DOI: 10.1029/2005JD006111.\n6. Fig. 4. I get what dots and crosses represent on Fig. 4a1-a4 but not on Fig. 4b1-b4 (the\ndifference figures). Do crosses (dots) represent increased downwelling (upwelling)?\nResponses: Black dots represent Ekman pumping, and crosses, Ekman downwelling (units:\ncm/day). In Fig. 4b1, black dots and crosses denote anomalous Ekman pumping and downwelling,\nrespectively, in Flat, with respect to Real. In Figs. 4b2-b4, black dots and crosses denote Ekman\npumping changes in Flat2Real, TP, and TP+AT, respectively, with respect to Flat. Figure 4’s\ncaption was rewritten, and made clearer in the revised manuscript.\n7. Also what do you mean by “mean Ekman pumping/downwelling”? I do not understand how\ndots and crosses can have units of cm/day.\nResponses: The mean Ekman pumping/downwelling represents the Ekman pumping averaged over\nthe last 100 years of each experiment. Sorry for the mistake in our previous manuscript. The dots\nand crosses do not have units, representing just the region where the upper ocean have positive\n(upward) and negative (downward) movements. The caption of Fig. 4 was rewritten, and made\nclearer in the revised manuscript.\n8. Fig. 5. Mass transport on Fig. 5b1-b4 should be in Sv to be consistent with the other figures.\nResponses: Thank you very much for this suggestion.\nIn the previous Figs. 5b1-b4, the mass transport across 30°S was obtained by zonal integration\nof meridional velocity, V(z)*Lx, which has the units of m2/s.\nFigures 5b1-b4 are replotted in the revised manuscript, in which the mass transport is\ncalculated as V(z)*Lx*dz(z), where dz(z) is the layer depth of V(z), so that the units are m3/s (Sv).\nNow, the mass transport throughout the manuscript has consistent units.\n9. Fig. 6. What month or season is reflected in the sea-ice margin/velocity? It should be clearly\nwritten in the legend.\nResponses: Thank you very much for pointing out this problem. In Fig. 6a, the sea-ice coverage is\nthe annual averaged, and the MLD is for March. In Fig. 6b, all changes are annual averaged. In the\nrevised manuscript, these are clearly stated in figure captions.\n10. Extended data Fig. 1 should be plotted at model resolution because this is what matters in the\nresults.\nResponses: Thank you for this suggestion. In the revised manuscript, Extended Data Fig. 1 is\nreplotted with data at model resolution. Please refer to Fig. R3.\n(a) Flat (b) +RM (c) +AT\n(d) +AM (e) +GL (f) +TP\nFig. R3 Topography configurations in coupled model experiments.\nReferences:\n1. Elliot, D. H. (2013). The geological and tectonic evolution of the Transantarctic Mountains: a review.\nGeological Society, London, Special Publications, 381(1), 7-35.\n2. Ferraccioli, F., et al. (2011). East Antarctic rifting triggers uplift of the Gamburtsev Mountains. Nature,\n479(7373), 388-392.\n3. Ferreira, D., et al. (2018). Atlantic-Pacific asymmetry in deep water formation. Annual Review of Earth\nand Planetary Sciences, 46, 327-352.\n4. Sauermilch, I., et al. (2021). Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling.\nNature communications, 12(1), 6465.\n5. Su, T., et al. (2019). Uplift, climate and biotic changes at the Eocene-Oligocene transition in south-\neastern Tibet. National Science Review, 6(3), 495-504.\n6. Xiong, Z., et al. (2020). The early Eocene rise of the Gonjo Basin, SE Tibet: From low desert to high\nforest. Earth and Planetary Science Letters, 543, 116312.\nResponse: These references are added in the revised manuscript.\nReplies to Reviewer #2:\nThank you very much for these constructive comments. We have revised the manuscript\ncarefully based on these suggestions. The following are our point-by-point replies.\nThe manuscript by Haijun Yang et al. presents an investigation into the influence of orography\non the Meridional Overturning Circulation (MOC) using the low-resolution coupled model CESM.\nPrevious studies, including Fallah et al. (2016), Su et al. (2018), and Yang and Wen (2020), have\nexamined the effects of the Tibetan plateau on oceanic circulation. Maffre et al. (2017) conducted\nan analysis on the impact of global orography on oceanic circulation by comparing two\nsimulations: one incorporating present-day geography and another using a flat Earth\nrepresentation. In this regard, Haijun Yang et al.'s work combines these two approaches by\ninvestigating the effects of a flat Earth representation and various combinations of present-day\norography on oceanic circulation. While this study is interesting, I have a few concerns regarding\nthis approach.\nResponses: Thank you very much for your valuable comments, which help us improve the\nmanuscript tremendously. Considering the comments from all the reviewers, we revised the\nmanuscript primarily in these following aspects:\n1) Introduction to research background is much improved. We rewrote statements and make\nit clear to readers that there are many controversies regarding the on/off state of the\nAMOC/PMOC, as well as the timing of mountain uplift.\n2) Mechanisms for global climate changes in response to mountain uplift are explained with\nmore details;\n3) Some relevant references are added; and most of figures are replotted.\nSpecific comments:\n1. (L24-L39) One concern with the sensitivity experiments conducted in this study is that they\ninclude the entire topography of the continent, rather than focusing solely on specific features\nlike the Tibetan plateau or the Rocky Mountains. Previous research has demonstrated that the\nuplift of the Tibetan plateau influences the Atlantic Meridional Overturning Circulation\n(AMOC). However, by incorporating the entire topography of a continent, it becomes\nchallenging to isolate the effects of individual landforms and determine whether minor features\nplay a role in the establishment or disruption of the AMOC/PMOC.\nResponses: Thank you very much for this question.\nIn previous studies, we also conducted an experiment with only TP removed (Fig. R4a), while\nthe Mongolian Plateau was unchanged. This experiment is called No_RegionalTP. The AMOC\nchange in No_RegionalTP is almost identical to that in NoTibet (Fig. R4b), suggesting that the TP\nis important to the AMOC, while the Mongolian Plateau is not. However, for the PMOC, although\nthe TP is the most important, the Mongolian Plateau also plays a role. This is qualitatively\nconsistent with the finding of White et al. (2017), which disclosed an important role of the\nMongolian Plateau in the Pacific wintertime atmospheric circulation. Since both the wind-driven\nand thermohaline dynamics are important to the PMOC establishment, the Mongolian Plateau can\naffect the wind-driven part of the PMOC through its role in the Pacific atmospheric circulation.\n(a) (b)\nFig. R4 (a) Topography configuration without Tibetan Plateau (60°-130°E, 20°-45°N) (No_RegionalTP). (b)\nTemporal evolutions of the PMOC (red) and AMOC (blue) in NoTibet (solid curves) and No_RegionalTP (dashed\ncurves).\nWe also answered this question in our previous work on the TP’s effect on the ENSO (Wen et\nal., 2020) (Fig. R5). The results from No_RegionalTP are nearly unchanged from those in NoTibet.\nσ(SST) in No_RegionalTP is roughly of the same magnitude as that in NoTibet (Fig. R6). The\ntropical SST anomaly shows much bigger oscillation after the TP removal (Fig. R6). Also, the mean\nocean climate changes are very close to each other in No_RegionalTP and NoTibet: weakened trade\nwinds, SST warming in the central-eastern Pacific, and SST cooling in the western Pacific, more\nfreshwater gain in the central tropical Pacific, shallower MLD in the central tropical Pacific, and\nflattened thermocline (Fig. R7). In addition, we examined the atmospheric circulation change in\nNo_RegionalTP (Fig. R8). We found that the atmospheric circulation changes over the tropical\noceans are nearly identical in NoTibet and No_RegionalTP. Difference exists only in high latitudes.\nOur work also confirmed that the landform around the TP is much more important than that around\nthe Mongolian Plateau in tropical climate change.\nFig. R5 Topography configuration in coupled model experiments. (a) is for the control simulation with realistic\ntopography (Real), and (b) is for the experiment without the regional Tibetan Plateau (No_RegionalTP; 60°-140°E, 20°-\n45°N) . Units: m.\nFig. R6 Time series of standard deviation of SST anomalies (σ(SST); °C) averaged in the Niño-3 region (150°-\n90°W, 5°S-5°N). The σ(SST) field is smoothed with a sliding window of 11 years. Black line is for Real, red is for\nNoTibet, and orange is for No_RegionalTP.\nFig. R7 Quasi-equilibrium changes in mean tropical climate, including (a): SST (°C) and surface wind stress\n(dyn/cm2), (b): precipitation minus evaporation (PmE; 10-5 kg/m2/s), (c): MLD (m), and (d): thermocline depth (m) in\nNoTibet. (e-h) are the same as (a-d), except for No_RegionalTP.\nFig. R8 Quasi-equilibrium changes in (a) geopotential height (shading; m) and wind (vector; m/s) at 850 hPa and (b)\nvertically integrated water vapor transport ((cid:2025) (cid:1874)(cid:3377)(cid:1869); vectors; kg/m/s) and its convergence (−(cid:2025) (cid:2008)∙((cid:1874)(cid:3377)(cid:1869))); shading; 10-5\n(cid:3028) (cid:3028)\nkg/m2/s) in NoTibet. (c-d) are the same as (a-b), except for No_RegionalTP.\nReferences:\nWen, Q., K. Doos, Z. Lu, Z. Han, and H. Yang, 2020: Investigating the role of the Tibetan Plateau in ENSO variability.\nJ. Climate, 33, doi: 10.1175/JCLI-D-19-0422.1.\nSha, Y., Z. Shi, X. Liu, and Z. An, 2015: Distinct impacts of the Mongolian and Tibetan Plateaus on the evolution of\nthe East Asian monsoon. J. Geophys. Res., 120, 4764–4782, doi:10.1002/2014JD022880.\nShi, Z., X. Liu, Y. Liu, Y. Sha, and T. Xu, 2015: Impact of Mongolian Plateau versus 759 Tibetan Plateau on the\nwesterly jet over North Pacific Ocean. Climate Dyn., 44, 3067–3076, doi:10.1007/s00382-014-2217-2.\nWhite, R. H., D. S., Battisti, and G. H. Roe, 2017: Mongolian mountains matter most: impacts of the latitude and\nheight of Asian orography on Pacific wintertime atmosphere circulation. J. Clim., 30, 4065-4082.\n2. Additionally, the study includes the Andes Mountains and the entire South American, African,\nMiddle Eastern regions up to the Zagros Mountains and Anatolian plateau under the scenario\n\"AM\". Regarding the ice sheet (Antarctica and Greenland), the study tests the sensitivity to\nchanges in elevation, but it is not specified whether the authors maintained the albedo of ice\nwhen removing the Antarctica / Greenland topography (the term ice sheet is never employed\nin the manuscript). This information is crucial as altering the albedo would affect the energy\nbalance and potentially influence the circulation patterns and precipitation.\nResponses: Thank you very much for pointing out these problems.\nIn Flat2Real, it’s true that when adding AM after AT, we actually add the whole South\nAmerican topography, the African topography, the Australian topography, and most of the\nEuropean topography (Extended data Fig. 1d). However, in single topography experiment AM (Fig.\nR9a) and combined-topography experiment TP+AM (Fig. R9b), the AM only includes the South\nAmerican topography.\nFig. R9 Topography configuration in coupled model experiments. (a) is for single AM experiment, and (b) is for the\ncombined experiment TP+AM. Units: m.\nAs far as the AMOC and PMOC are concerned, the combined effect of the South American\ntopography (without the AM), the African topography, the Middle Eastern regions up to the Zagros\nMountains, and the Anatolian Plateau are insignificant, which can be deduced from Fig. 1 and Fig.\n2a. In Fig. 1, the PMOC after adding AM (from year 2400 to year 3400) is about 20 Sv, which\nincludes the combined effect of those topographies. In Fig. 2a, the PMOC after adding AM to Flat\nis about 18 Sv, which does not include the combined effect of those topographies. The AMOC\nduring years 2400-3400 of Fig. 1 is about 2 Sv, while it is about 1 Sv in only-AM experiment in\nFig. 2a. Due to the limitation of computer resources, we did not design experiments to explicitly\ninvestigate the effects of the South American topography (without the AM), the African\ntopography, the Middle Eastern regions up to the Zagros Mountains, and the Anatolian Plateau on\nthe PMOC and AMOC.\nSince this work only focuses on the topography effect, in all experiments we only modify the\nheight of topography, keeping the albedo at the same value of Flat. The albedo can be freely\nadjusted with the integration of simulations. The dynamic ice sheet component in our experiments\nis closed. In the model setting, the land ice component is set to SGLC (stub glacier model),\nwhich\nmeans no dynamic ice sheets. The glacier areas and elevations are taken entirely from CLM’s\nsurface dataset; and no downscaling is done over non-glacier land units. The ice sheets are treated\nas big bright rocks. The bare ice albedo is prescribed to be 0.50 by default. The albedo of glacier is\n0.80 for visible radiation.\nIn the revised manuscript, we explicitly state how the ice sheet is treated in the experiments.\nWe agree that altering the albedo would affect the energy balance and potentially influence\ncirculation patterns and precipitation patterns. However, in such a short manuscript, we cannot\ninvestigate albedo effect in detail. We would like to emphasize that the initial value of albedo over\ntopography region is kept the same as that in FLAT, and it will self-adjusted according to thermal\nconditions during model integration. For example, with the uplift of a mountain, the albedo will\nincrease.\nIn the 2nd paragraph of Methods section of the revised manuscript, we rewrote the related\nsentence as follows: “Except for topography height, all other boundary conditions remain the same\nas in Flat and Real. The ocean-land configuration is set to modern-day conditions without\ncorrection for plate tectonic motion. Atmospheric CO concentration is maintained at the\n2\npreindustrial level (285 ppm). Changes in river routing and vegetation type are not considered.\nContinental ice sheets are treated as bright rocks in the model. The planetary albedo can adjust by\nitself according to thermal conditions. These experiments are conducted as single-variable\n(orography) sensitivity tests, rather than paleoclimate simulations involving multiple prescribed\ngeologic boundary conditions.”\n3. Moreover, when orography is removed, the drainage basin system is expected to undergo\nchanges. The authors indicate that river routing is kept unchanged. Understanding the\nmodifications in the drainage basin system is relevant to comprehending the overall impact of\norography removal on oceanic circulation.\nResponses: Thank you very much for raising this question.\nIn the actual process of topographic uplift, the direction and discharge of river runoff will be\nchanged. In CESM1.0, drainage or sub-surface runoff is based on the SIMTOP scheme (Niu et al.,\n2005). Due to limited geological data, it is difficult to get the exact distributions of river runoff\nduring the period of mountains uplift. There is no artificial modification of river runoff. The results\nof runoff changes are based on model simulations.\nFigure R10 shows the runoff flux (units: mSv) anomaly in several experiments (TP, AT,\nTP+AT, Real) relative to Flat. We can conclude that the anomalous freshwater flux due to the\nchanges in the drainage basin system has a very limited impact on the global-scale ocean\ncirculation.\nFig. R10 River runoff flux (units: mSv) anomaly in experiments TP, AT, TP+AT, and Real, with respect to Flat.\nPositive value means the ocean gain fresh water.\nReference:\nNiu, G.-Y., Yang, Z.-L., Dickinson, R.E., and Gulden, L.E. 2005. A simple TOPMODEL-based runoff\nparameterization (SIMTOP) for use in global climate models. 110: D21106. DOI: 10.1029/2005JD006111.\n4. These concerns regarding the specificity of landforms, albedo adjustments, and drainage\nbasin modifications should be addressed in the manuscript to provide a comprehensive\nanalysis of the study's findings.\nResponses: Thank you very much for this suggestion. In the revised manuscript, we added some\ndetail on how we treat the changes in landforms, albedo, drainage basin, and river runoff flux in\nMethod. Please refer to the replies to Q.2 above.\nDue to space limited by the journal, a comprehensive analysis of these factors on ocean\ncirculations is not feasible. We are working on a longer manuscript, in which comprehensive\nanalysis will be conducted.\n5. Despite the fact that the authors exclude to explain the evolution of AMOC during the\nCenozoic and restrain this explanation to the role of Tibetan plateau in the present-day\ngeography, the Figure 1 exhibits the impact of a sequential uplift of mountains with a\n“geological” chronology. The sequence is supposed to mimic the main phase of uplift (or\ngrowth of ice sheet). Although the authors do not try to provide a detailed explanation of the\nevolution of the AMOC/PMOC during the Cenozoic era and instead focus on the role of the\ntopography in present-day geography, Figure 1 demonstrates the impact of sequential\nmountain uplift with a geological chronology. The sequence is intended to simulate the impact\non ocean circulation of the primary phase of uplift or the growth of an ice sheet.\nResponses: Thank you very much for this comment. To some extent, we can learn from experiment\nFlat2Real about how the AMOC/PMOC could be established in the past. However, the actual\nevolution processes of the AMOC/PMOC during the Cenozoic era were much more complicated,\nbecause they involved so many boundary conditions. In a short manuscript like this one, we can\nonly focus on a narrow topic, namely, the role of some giant topography in the AMOC in present-\nday climate.\n6. The length of the simulation is a matter of debate. Following the addition of Antarctic,\nthe simulation is integrated for 400 years. However, this duration may be considered relatively\nshort to ensure that the deep ocean layers have fully reached equilibrium with the boundary\nconditions.\nResponses: Thank you very much for pointing out this problem.\nWe should have run +AT of Flat2Real for a much longer time (at least 1000 years). In the very\nearly stage of this work, we did not realize this problem and just kept running the model, since the\nAMOC does not increase at this stage. We were just eager to see when the AMOC can get up.\nTo make up this problem, in the later stage of this project we ran the experiment OnlyAT from\nFlat for 1600 years (Fig. 2a) and found that the PMOC in OnlyAT could reach 18 Sv and the\nAMOC was still not established. Therefore, we can say that during years 2000-4000 of Flat2Real,\nthe AT did contribute to the strong PMOC in the later stage; and at the same time, it did not lead to\nthe AMOC formation in the later stage.\n7. (L69) The authors do not provide an explanation regarding whether the albedo was modified\nwhen removing the topography of Antarctica. It would be beneficial to clarify whether any\nadjustments were made to account for changes in albedo resulting from the modification of\nAntarctic topography.\nResponses: Thank you very much for raising this question. Please also refer to our replies to Q.2.\nIn all experiments, we only modify the height of topography, keeping the albedo at the same\nvalue of Flat. The albedo can freely adjust during the integration of the simulation. The dynamic ice\nsheet component in our experiments is turned off, which means no dynamic ice sheets. The ice\nsheets are treated as big bright rocks. The bare ice albedo is prescribed to be 0.50 by default. The\nalbedo of glacier is 0.80 for visible radiation.\nFigure R11 shows albedo in different experiments. The small difference over the Antarctic\ncontinental is from model’s adjustments. It’s not caused by changes in ice sheets. Albedo is\ncalculated using (FSDS-FSNS)/FSDS, where FSDS represents downwelling solar flux at the\nsurface, and FSNS represents net solar flux at the surface.\nFig. R11 Surface albedo in different experiments\nIn our previous study (Wen et al., 2021), we conducted a detailed study of albedo effects and\nfound that changes in albedo over the TP (thermal effect) have a much smaller impact on the ocean\ncirculation, when compared with TP’s dynamic effect.\nReference:\nWen Q, Yang H, Yang K, et al. Possible thermal effect of Tibetan Plateau on the Atlantic meridional\noverturning circulation[J]. Geophysical Research Letters, 2022, 49(4): e2021GL095771.\n8. (L93-…) The authors primarily focus on describing the oceanic changes in their study, driven\nby atmospheric modifications resulting from the addition or removal of topography. However,\nthey do not provide a detailed explanation of the underlying mechanisms involved. It would\nindeed be valuable to include a few sentences elucidating the specific mechanisms by which\nthe evaporation or precipitation patterns are altered when modifying the topography.\nResponses: Thank you very much for this suggestion.\nThe underlying mechanisms are detailed in the following paragraph (Lines 108-148). In our\nprevious studies (Fig. 4 in Wen and Yang, 2020; Fig. 12 in Yang and Wen, 2020), we discussed the\nmechanisms thoroughly. Removing the TP leads to enhanced atmospheric water vapor transport\nfrom the Pacific to the Atlantic, resulting in decreased freshwater flux over the western Pacific and\nincreased freshwater flux over the North Atlantic, which lead to the PMOC formation and AMOC\ndecline.\n9. In this work, the authors do not explicitly mention conducting statistical significance tests to\nevaluate the differences between two variables.\nResponses: Thank you very much for pointing out this problem.\nWe have done statistical tests for all variables, but did not mention them in this manuscript\nexplicitly due to the limitation of word count.\nMost changes seen in the experiment results are statistically significant. We did Mann-Kendall\ntest for all figures related to the differences between two variables. For the clarity of visual effect,\nwe decided not to show the significance test in the figures.\nIn the last paragraph of Methods section of the revised manuscript, we state that “The student-t\ntest is used to examine the statistical significance of our results. Most changes are significant at the\n95% confidence level, which is expected because altering large topography induces strong\nmechanical forcing and obvious responses around the globe. For clarity, significance test is not\nprovided in any figures.”\nFigure R12 shows the SSS change in TP, AT, TP+AT, and Real, with respect to FLAT, in\nwhich the crosses represent the area that exceeds the 95% significance level.\nFig. R12 Changes in annual mean SSS in TP, AT, TP+AT, and Real, with respect to FLAT. Stippling indicates\nchanges exceeding the 95% significance level according to the Mann-Kendall trend test.\n10. (L112-122) The mechanism causing a change in precipitation or evaporation (or both) are\ndiscussed. The changes in atmospheric circulation and moisture transport are associated to\nchanges in GPH. The authors do not explicitly delve into the topic of temperature changes in\nrelation to GPH variations, nor do they discuss the potential role of Sea Surface Temperature\n(SST) changes in the North Atlantic affecting the North Atlantic Deep Water (NADW)\ncirculation. To provide a more comprehensive analysis, it would be beneficial for the authors\nto address the possible temperature changes resulting from GPH modifications and discuss\nthe influence of SST changes on NADW formation explicitly. Incorporating these aspects\nwould further elucidate the connections between atmospheric circulation, temperature, and\noceanic processes, enhancing the overall understanding of the research findings.\nResponses: Thank you very much for raising this question.\nIn this work, we state that “The change in SSD from Real to Flat can be primarily attributed to\nthe change in SSS” and did not discuss the SST change in the North Atlantic, because we had a\nthorough discussion on this topic in our previous papers (e.g., Yang and Wen, 2020). Due to the\nlimitation of paper length, in the revised manuscript we added a brief statement that the temperature\neffect on the AMOC is not important.\nFigure R13 (i.e., Fig. 2 of Yang and Wen, 2020) shows that after 50 years of the TP removal,\nthe AMOC exhibits a roughly linear decline for about 200 years. The AMOC is finally weakened\nby more than 80% and practically shut down in about 300 years.\nAs the first step to understand the AMOC change, the temporal evolution of surface buoyancy\nchange averaged in the NADW region is examined in Fig. R13b. The sea-surface density (SSD)\nincreases during the first 50 years and then decreases linearly (black curve), consistent with the\nchange of the AMOC index (Fig. R13a). The SSD change consists of SST-induced change (dashed\nred) and sea-surface salinity (SSS)-induced change (dashed blue). Note that the surface ocean keeps\ncooling in the whole 400 years (red curve), which always increases the SSD (dashed red). In\ncontrast, the SSS increases first and then decreases (blue curve), followed closely by the SSD\nchange (dashed blue and black curves). Based on Fig. R13b, we quantify that the increase of SSD\nduring stage I is contributed by surface cooling (30%) and surface salinization (70%), while the\nSSD decrease later on is totally contributed by surface freshening. It clearly illustrates that the\nweakening of the AMOC should be attributed to the freshwater increase in the NADW region\nFig. R13 [REDACTED]\nFigure R14 (i.e., Fig. 4 in Yang and Wen, 2020) shows the horizontal patterns of buoyancy\nchange in the North Atlantic in both stages. In stage I, the SST change has a tripolar structure (Fig.\nR14a): a significant cooling (more than 2°C) occurs in the midlatitudes between 40° and 60°N,\nsaddled by two warming regions located in the GIN seas and tropics, respectively. The SSS change\nhas an east-west dipole structure (Fig. R14b), with significant salinization in the Labrador Sea and\nsouth of the Greenland Sea and a weak freshening in the eastern North Atlantic. The combined\neffect of changes in SST and SSS results in SSD increase (Fig. R14c), particularly in the NADW\nregion; this increases the MLD and thus the deep-water formation, leading to a stronger AMOC in\nstage I. In stage II, the quasi-equilibrium changes in SST, SSS, and SSD show rather simple\nstructures (Figs. R14d-f), that is, significant cooling, freshening, and thus a lighter surface ocean in\nthe entire North Atlantic, consistent with the ‘‘off’’ state of the AMOC. It is clear that the SSD\nchanges in the North Atlantic in both stages are mostly determined by SSS change.\nFig. R14 [REDACTED]\nReference:\nYang, H., and Q. Wen, 2020: Investigating the role of the Tibetan Plateau in the formation of Atlantic\nmeridional overturning circulation. J. Climate, 33(9), 3585-3601, doi: 10.1175/JCLI-D-19-0205.1.\n11. (L122-L123) “The expansion of sea ice (…) carries a substantial amount of freshwater”:\nthe sentence is not clear to me.\nResponses: Thank you very much for this question.\nDuring the southward expansion of sea ice, there is a remarkable amount of sea-ice melting at\nthe same time, which provides fresh water to the ocean, leading to the AMOC shutdown. In the\nrevised manuscript, we revised this sentence and made it clearer in lines 129-131.\n12. (L135) “The absence of the Antarctic topography leads to an 80% decrease in wind stress”:\nis it due to temperature gradient?\nResponses: Thank you very much for this question.\nThis is mainly due to the change of meridional pressure gradient. Absence of the Antarctic\ntopography causes an anomalous high pressure over the Antarctic, which produces an anomalous\nnorthward pressure gradient, and thus an anomalous easterlies based on the geostrophic balance. In\nthe revised manuscript, we made this point clearer in lines 141-144.\n13. Figure 5: the Ekman pumping/downwelling is discussed with the 850hPa wind stress. This is\nnot clear. Why don't the authors show the wind stress at the sea surface?\nResponses: Thank you very much for raising this question.\nIn Fig. 5, the Ekman pumping/downwelling is calculated by using surface wind stress: (cid:2033) =\n(cid:3006)\n(cid:3099)\n(cid:1855)(cid:1873)(cid:1870)(cid:1864)( ), where (cid:2028) is surface wind stress. The wind field we used is at 850 hPa, because at this\n(cid:3096)(cid:3033)\nlevel the geostrophic balance is satisfied, so that the wind field can also represent the pressure field.\nFigure R15 shows Ekman pumping and wind stress at sea-surface level. We can see that the\nageostrophic component of the wind is strong, compared to that in Fig. 5. For the Ekman pumping,\nboth geostrophic and ageostrophic components of the wind are important to produce vertical\nmovement in the upper ocean.\nWind stress and Ekman pumping\nReal AT\nTP TP+AT\nFig. R15 Surface wind stress (units: dyn/cm2) and Ekman pumping (units: cm/day). The values less than 0.015\ndyn/cm2 are not shown.\n14. Positive mass transport across 30°S is counted positively northwards. This is not mentioned\nin the figure caption.\nResponses: Thank you very much for this suggestion. We revised it in Fig. 5.\n15. (L202+Extended Data Figure 3a1-4) The authors' conclusion that the Tibetan plateau acts as\na significant attractor of freshwater in the Northern Hemisphere (NH) is potentially accurate\nif the boundary conditions used in their scenario are limited to the Tibetan plateau alone.\nHowever, it is important to consider that changes in the North Pacific region can also be\ninfluenced by the westerlies blowing from Eastern Asia, which may be impacted by the\npresence or absence of landforms located to the north of the Tibetan plateau.\nResponses: Thank you very much for raising this question.\nPlease refer to our reply to Q.1 and Figs. R4, R5 and R8. It is true that changes in the North\nPacific can also be influenced by the westerlies from East Asia, which can be impacted by the\npresence or absence of landforms (such like the Mongolian Plateau) to the north of the TP.\nHowever, the TP’s impact dominates; so we did not separately consider the impact from the\nMongolian Plateau.\n16. (L224) I think that the authors could quote a more recent paper than the Ruddiman’s 1989\npaper\nResponses: Thank you very much for this suggestion. We added a recent review paper published in\nNature as reference #46 in the revised manuscript.\nWu, F., Fang, X., Yang, Y. et al. Reorganization of Asian climate in relation to Tibetan Plateau\nuplift. Nat Rev Earth Environ 3, 684–700 (2022).\n17. (L232-243) I agree with the observation regarding the limitations of the study, particularly\nwith respect to the spatial resolution of the atmospheric model. I wonder whether the low\nresolution of the model might not be a potential factor contributing to the lack of impact\nobserved for certain mountain ranges, particularly those with an elongated shape such as the\nAndes or the Rockies. Mountain ranges with complex topography may not be adequately\ncaptured or represented in the model due to the coarse resolution. In contrast, the Antarctic\nice cap and the Tibetan plateau may be better resolved within the model. This could explain\nwhy their impacts are more apparent in the simulations.\nResponses: Thank you very much for these insightful thoughts.\nWe are going to repeat some experiments using high-resolution version of CESM2.0. This will\nbe very resource-consuming and take a long time. We hope to have some new fascinating findings.\nOther comments:\n1. (L24-33) The timing of uplift is also debatable. A part of the Tibetan plateau is largely\nuplifted before the Late Miocene.\nResponses: Thank you very much for this comment. We mentioned this problem in the revised\nmanuscript in lines 38-40.\n2. Antarctic (ice sheet?): the authors explains that “Antarctic rapidly expanded in the Oligocene\nand persisted until the Late Oligocene”. This is not clear to me what the authors mean by\n“persisted until the Late Oligocene”. Does it mean that the ice sheet melts after the Late\nOligocene ? No reference is provided.\nResponses: Thank you very much for raising this question. Here, we mean the glaciation of the\nAntarctic persists from the early Oligocene to the late Oligocene (34-26 Ma). In the revised\nmanuscript, we rewrote this sentence as follows: “…Although the transantarctic and Gamburtsev\nMountains over the Antarctica were likely already present at the start of the Cenozoic (65 Ma)\n(Ferraccioli 2011; Elliot 2014), the glaciation of the Antarctica was thought to occur during the\nEocene-Oligocene (56-23 Ma)…”\n3. Andes uplift => reference 19 : Zachos et al. 2001 ? Revised\n4. Figure 5: continent contours are too thin to be seen when printed Replotted.\n5. Extended Data Figure 3 : continent contours are hard to see Replotted.\nReplies to Reviewer #1:\nThank you very much for giving us the second chance to improve this manuscript. We have\nrevised the manuscript carefully based on your constructive suggestions. The following are our\npoint-by-point replies.\nThis is the second review and I first apologize for taking so long to complete this review.\nIn my first set of comments, I recommended the manuscript to be returned to the authors with\nminor revisions. The experimental setup was sound and the results generally robustly demonstrated\nbut the context in which the study is integrated was not very well described and the relevance of the\nfindings to the understanding of the MOC evolution remained unclear.\nI do not think the manuscript has been much improved from this perspective.\nTo be more specific, I still think that the core of the paper, that is, the physical results, is a\nvaluable addition to the literature on the role of mountains in the climate system, and I would like\nto see these results published. However, the Discussion and, to a lesser extent, the Introduction, are\nstill not up to the job, in particular for publication in a journal like Nature Communications.\nThe Introduction, which mostly reviews the Cenozoic evolution of the AMOC/PMOC and of the\nmountain ranges that are investigated later in the manuscript (l. 66-99), has been slightly expanded\nand reads better, though some errors remain (see below). My issue in this section is mainly with the\nlast paragraph, which needs to be re-written to 1) state more clearly why the authors chose to keep\nthe modern land-sea mask and 2) not elude the fact that your Flat2Real simulation is an attempt of\nsimulating the Cenozoic AMOC/PMOC evolution.\nSomething along these lines for example: “Here, as a first step, we numerically investigate how\nthe presence or absence of various major mountain ranges affect the GMOC as well as their\ncumulative impact when they are uplifted sequentially in the model in an initial flat Earth\nsimulation. In order to isolate the topographic effects from plate tectonics and long-term\nbathymetric and atmospheric changes, we keep the modern bathymetry and continental positions,\nas well as modern greenhouse gas concentrations, incident solar radiation and orbital parameters.\nWe show that, in our simulations, the TP uplift is the primary driver of the shutdown of a PMOC\nand initiation of an AMOC, although high Antarctic topography is required to drive a strong,\nmodern-like, AMOC. We further discuss the implications of our results to the long-term Cenozoic\nhistory of the AMOC/PMOC.”\nResponses: Thank you very much for your valuable comments. The last paragraph of the\nintroduction is revised following your suggestion (see below). Please also refer to lines 45-53 of the\nrevised text.\n“Here, as a first step, we numerically investigate how the presence or absence of various major mountain\nranges affect the GMOC as well as their cumulative impact when they are uplifted sequentially in the model\nfrom an initial flat Earth. In order to isolate the topographic effects from plate tectonics and long-term\nbathymetric and atmospheric changes, we keep the modern bathymetry and continental positions, as well as\nmodern greenhouse gas concentrations, incident solar radiation and orbital parameters. We show that, in our\nsimulations (Methods and Table 1), the TP uplift is the primary driver of the PMOC shutdown and the\nAMOC initiation, although high Antarctic topography is required to drive a strong, modern-like AMOC. We\nfurther discuss the implications of our results to the long-term Cenozoic history of the AMOC/PMOC.”\nThe Discussion is perhaps more problematic, as it currently stands, and I note that it has only\nbarely been revised, in spite of comments from both reviewers. The work presented here may well\nbe “just a beginning” as the authors claim in their rebuttal, yet this must not preclude them to\nbetter discuss their results.\nHere are a couple of comments to improve it:\n1) the 3rd paragraph (l. 289-300) reads quite weird. It starts with “rather than mimicking past\nclimate evolution” but the next sentence ends with “The results […] provide insight into […] the\nclimate development in the Late Cenozoic”. A few lines later “this study establishes the direction of\nMOC change as a function of increasing uplift over time”. What is the goal of the study in this\ncase? I do not see it problematic that the authors attempt to interpret their results in light of long-\nterm Cenozoic climate change but it needs to be discussed: what are the implications of a modern\nland-sea mask? And of modern gateways and/or greenhouse gas?\nResponses: Thank you very much for your valuable comments. This paragraph is rewritten based\non your suggestion (see below). Please also refer to lines 248-264 of the revised text.\n“This research marks the beginning of our investigation into the effects of the presence or absence of\nvarious major mountain ranges on the GMOC, as well as their cumulative impact under modern conditions.\nOur research aims to specifically discern the impact of topography on ocean circulation from other\ncontributing factors, by incorporating modern bathymetry and continental configurations, current greenhouse\ngas levels, incident solar radiation, and orbital parameters in our experimental setup. The individual\nmountain uplift experiments in this study allow us to explore the linkages between uplift regions and climate\nchanges in remote areas of the modern world. The sequential mountain uplift experiment Flat2Real provides\ninsight into specific periods of paleoclimate, such as the climate development in the late Cenozoic. Previous\nstudies suggested that the TP uplift played a key role in shaping Cenozoic climate through circulation\nchanges and weathering13,49,50: over the past 40 million years, this uplift led to substantial deflection of the\natmospheric jet stream, intensified monsoonal circulation, increased rainfall on the front slopes of the\nHimalayas, and conductive conditions for the formation of deep and intermediate waters in the North\nAtlantic. By using experiment Flat as a starting point and tracing the changes due to various uplifts, we can\nquantify the linkages between uplifts and climate changes and understand how the MOC changes as the\nprogressive uplift of continental mountains.”\nThe implications of a modern land-sea mask, ocean gateways and greenhouse gas in our\nexperiments implies that the TP impact on the GMOC could be exaggerated. We have added the\ndiscussion in the following paragraph.\n2) Just a single line stating that the authors “did not consider the effects of continental drift and\noceanic gateway switches” is just simply not enough, in particular as the interpretation of the\nresults as consistent with the general direction of Cenozoic climate change is repeated several\ntimes in the Discussion (see comment above and again l. 316).\nResponses: Thank you very much for your valuable comments. In this paragraph, more discussions\nare added (see below). Please also refer to lines 270-282 of the revised text\n“Additionally, we did not consider the effects of continental drift and oceanic gateway switches, nor did\nwe treat Greenland and Antarctic glaciation dynamically. The setup of modern land-sea mask and ocean\ngateways in our experiments implies that the TP impact on the GMOC could be exaggerated. After all, the\nTethys Seaway closing, the Panama Isthmus closing and the Bering Strait opening all occurred after the TP\nuplift8, all of which may lead to significant increases in the continental ice sheets in both hemispheres and\nenhance the NADW formation51-53. Furthermore, the background climate in the experiments uses the\npreindustrial conditions with constant CO , and the effect of chemical erosion in rapidly uplifted areas on\n2\natmospheric CO is not accounted for. Typically, atmospheric CO levels cannot remain in a steady state\n2 2\nduring the time of intense tectonism, which can last for millions of years, due to the temperature-weathering\nfeedback mechanism. The TP uplift may have resulted in a decrease of atmospheric CO level, which can\n2\nalso enhance the NADW formation due to the growth of large continental ice sheets in the NH49.”\n3) The list of other studies referenced in the last paragraph should be better integrated. For\nexample, the authors state that “Maroon discovered that the RM can have a significant effect on\nGMOC through its impact on hydrology”, which is opposite to what the authors found in their\nexperiments (l. 242-245). Why?\nResponses: Thank you very much for this suggestion. In the revised manuscript, we added several\nnew references and more analyses in several places. Due to the limitation of paper length and\nreferences required by the journal, we deleted these lines and three references. However, here we\nwould like to provide an answer to why Maroon’s finding is difference from ours.\nIn Maroon (2016), removing only RM leads to a significant weakening of the AMOC by about\n30%, however, removing the orography globally only leads to the AMOC weakened slightly. The\nlatter result is not consistent with results of Schmittner et al. (2011), Sinha et al. (2012) and our\nstudy.\nMaroon herself stated in her PhD. thesis that “The AMOC collapses due to the removal of global\norography in the models of Schmittner et al. (2011) and Sinha et al. (2012), a result which our\nsimulations do not produce. We performed an additional simulation (not shown) in which we\nremoved orography globally and found that the AMOC weakened slightly …”. “The AMOC in our\nmodel is not as sensitive to orography as in the models used in other studies. One possible\nexplanation for the weak response of the AMOC in CM2Mc is that the region of net freshening (P-\nE>0) extends too far south in our Control simulation when compared to the freshwater input from\nobserved storm track. This bias likely originates from the low resolution of the atmospheric model\n(Brayshaw et al., 2009) and is reflected in the SST biases in the North Atlantic”.\nMaroon also stated that “Warren (1983) argued that the North Pacific does not have a PMOC\nbecause P - E > 0. Following his argument, the freshwater biases in the North Atlantic of the\nControl simulation should make the North Atlantic more similar to the North Pacific and prevent an\nAMOC. And yet, there is an AMOC in the Control simulation, and it is relatively insensitive to the\nremoval of the Rocky Mountains. These results suggest that the Rockies are not the primary\nreason why there is an AMOC.”\nSpecific comments in the Introduction:\n1. l. 68-70. Worth integrating DeepMIP results? Most models do not simulate an AMOC or\nPMOC in the Early Eocene (Zhang et al. 2022).\nResponses: Thank you very much for this suggestion. The results from DeepMIP project are very\ninteresting and provide us with new insight.\nIn the revised manuscript, we removed the old reference 5 and cited Zhang et al. (2022) as the\nreference 8. In lines13-16 of the revised text, we state that “a recent study from DeepMIP project\nfound that neither model results nor proxy data suggest NADW formation during the early Eocene,\nwhile the evidence for NPDW formation remains inconclusive8.”\n2. l. 70-71. “during the Early Oligocene period (about 35-33 Ma)” => “at a later stage”. By the\nway, the 35-34 Ma interval is still Eocene. Revised as suggested.\n3. l. 77. “the former has a net evaporation” => “the former is a net evaporative basin”. Revised\n4. l. 81. “is believed” => “is also believed” Revised\n5. l. 82. Incorrect reference. Ref. 12 does not discuss the impact of Drake and Tasman gateways\non the AMOC. Could cite Hutchinson et al. (2019) instead.\nResponses: Thank you very much for pointing out this mistake. The reference is changed to\nHutchinson et al. (2019).\n6. l. 84-85. “can precipitate contrasting shifts in the global MOC” => “holds the potential to\ntrigger large-scale transitions in the global MOC”. Revised.\n7. l. 87 and throughout. “over the Antarctica” => “over Antarctica” or “over the Antarctic\ncontinent”. Revised.\n8. l. 88-89. “the glaciation of the Antarctica is believed to have occurred during the Eocene-\nOligocene (56-23 Ma).” This is very vague. The consensus today is that the first large-scale\nglaciation of Antarctica occurred at the EO transition (34-33.5 Ma).\nResponses: Thank you very much for this suggestion.\nIn lines 31-32 of the revised text, this statement is changed to “… the first large-scale glaciation\nof Antarctica is believed to have occurred during the Eocene-Oligocene transition period (34-33.5\nMa)20,21…”.\nWe added two relevant references as ref. 20, 21 in the revised manuscript:\n20. Jamieson, S. S. R., Sugden, D. E., & Hulton, N. R. J. The evolution of the sub-glacial landscape of\nAntarctica. Earth and Planetary Science Letters 293, 1-27 (2010).\n21. Jamieson, S. S. R., Ross, N., Paxman, G. J. G., Clubb, F. J. An ancient river landscape preserved beneath\nthe East Antarctic Ice Sheet. Nat Commun 14, 6507 (2023).\n9. l. 90. Incorrect reference. Ref. 20 is not about the Andes. I suggest that the authors use the\nBoschmann (2021) paper as their new reference on the uplift of the Andes, and revise their\nchronology in consequence.\nResponses: Thank you very much for pointing out this mistake.\nThe reference 20 is changed to Boschmann (2021). In lines 32-33 of the revised text, the\nstatement about AMs is changed to “The uplift of Andes Mountains (AMs) started in the Late\nCretaceous (~70 Ma)22 and matured around 15-10 Ma23,24.”\n10. l. 93-94. “the rapid and main uplift of the TP was realized between 10 and 8 Ma”. Not sure this\nis correct. At least two of the references (25-29) are > 30 years old and they should be updated,\ngiven the dynamism existing about the TP uplift history. Latest reviews suggest that only small\nparts of the TP reached their modern elevation after the Early (20 Ma) or Mid (15 Ma)\nMiocene. See, e.g., Tardif et al. (2023).\nResponses: Thank you very much for this comment. We rewrote the statement on the TP based on\nyour suggestion. Please also refer to lines 35-44 of the revised text.\n“The timeline for the formation of the Tibetan Plateau (TP) is a topic of highly debate. Some studies\nargue that parts of the TP were already in place during the late Eocene (38-33 Ma)26,27, while other research\nsuggest that the TP's rapid and main uplift occurred between 10 and 8 Ma28-31. A more recent study proposes\nthat most of the TP had attained its current elevation before the Mid-Miocene (15 Ma)32. This timing\ncoincides with the onset of NADW formation, suggesting a possible link between the TP uplift and the\ndevelopment of NADW. Recent research also indicates that the TP is a critical factor affecting changes in the\nGMOC33,34. Nevertheless, it is important to recognize that the chronology of the uplift of major mountain\nranges remains a subject of intense discussion and investigation.”\nHere we removed the old reference 26 and added Tardif et al (2023) as reference 32.\nIn the end, I think that the revisions required are still quite minor, but necessary, and I trust the\nauthors to adequately revise their manuscript.\nResponses: Thank you very much for your invaluable comments, which help us to improve this\nmanuscript greatly. All 4 references you provided are cited in the revised manuscript.\nReferences\n1. Boschman, L. M. (2021). Andean mountain building since the Late Cretaceous: A paleoelevation\nreconstruction. Earth-Science Reviews, 220, 103640. https://doi.org/10.1016/j.earscirev.2021.103640\n2. Hutchinson, D. K., Coxall, H. K., OʹRegan, M., Nilsson, J., Caballero, R., & de Boer, A. M. (2019).\nArctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition. Nature\nCommunications, 10(1), 3797. https://doi.org/10.1038/s41467-019-11828-z\n3. Tardif, D., Sarr, A. C., Fluteau, F., Licht, A., Kaya, M., Ladant, J. B., ... & Banfield, W. (2023). The role\nof paleogeography in Asian monsoon evolution: Associated a review and new insights from climate\nmodelling. Earth-Science Reviews, 104464. https://doi.org/10.1016/j.earscirev.2023.104464\n4. Zhang, Y., de Boer, A. M., Lunt, D. J., Hutchinson, D. K., Ross, P., van de Flierdt, T., ... & Huber, M.\n(2022). Early Eocene ocean meridional overturning circulation: The roles of atmospheric forcing and\nstrait geometry. Paleoceanography and Paleoclimatology, 37(3), e2021PA004329.\nhttps://doi.org/10.1029/2021PA004329\nReplies to Reviewer #2:\nThank you very much for these constructive comments. We have revised the manuscript\ncarefully based on these suggestions. The following are our point-by-point replies.\nThe manuscript by Haijun Yang et al. offers a comprehensive description of the physical\nmechanisms influenced by the presence / absence of large landform systems. The article is\ncommendably clear and well-written. The responses to the reviewers' inquiries provide valuable\ninsights that enhance our understanding of the authors' methodology.\nThe authors appropriately acknowledge the study's limitations in the conclusion, notably\nhighlighting factors such as the paleogeography anchored in the present and the absence of\natmospheric CO2 variation, preventing direct comparison with the evolution of Cenozoic climates.\nI fully concur with this assessment.\nHowever, in the conclusion, certain sentences imply that Tibet played a dominant role in the\nevolution of deep-water formation. To enhance clarity, it is advisable to ensure that the key points\nand the summary explicitly convey the nuanced nature of the study's findings and the limitations\nacknowledged in the conclusion. This adjustment will contribute to a more precise and\ncomprehensive understanding of the research presented.\nResponses: Thank you very much for these suggestions.\nIn Section summary and discussion, we added statement in lines 248-253 of the revised text to\nexplicitly address the goal of this study as “This research marks the beginning of our investigation\ninto the effects of the presence or absence of various major mountain ranges on the GMOC, as well\nas their cumulative impact under modern conditions. Our research aims to specifically discern the\nimpact of topography on ocean circulation from other contributing factors, by incorporating modern\nbathymetry and continental configurations, current greenhouse gas levels, incident solar radiation,\nand orbital parameters in our experimental setup.”\nAnd we also added statements to address the limitation of this study. In lines 272-276 of the text\nwe state that “The setup of modern land-sea mask and ocean gateways in our experiments implies\nthat the TP impact on the GMOC could be exaggerated. After all, the Tethys Seaway closing, the\nPanama Isthmus closing and the Bering Strait opening all occurred after the TP uplift8, all of which\nmay lead to significant increases in the continental ice sheets in both hemispheres and enhance the\nNADW formation51-53.” Further in lines 280-282 we state that “The TP uplift may have resulted in a\ndecrease of atmospheric CO level, which can also enhance the NADW formation due to the growth\n2\nof large continental ice sheets in the NH49.”\n1. Key 1 : The authors contrast their results with previous studies, but the conditions of the Jones\nand Cecci (2017) study are different, so the comparison seems risky to me.\nResponses: Thank you very much for this comment.\nIn lines 18-25 of the revised text, we listed three reasons that cause the different overturning\nmodes between the Atlantic and Pacific: the asymmetry of net surface freshwater flux, the basin\ngeometry and the ocean gateways. The key point #1 is also revised.\nJones and Cecci (2017)’s study focused on the effect of ocean basin geometry on the GMOC.\nThey showed that the North Atlantic has higher salinity than the North Pacific because the Atlantic\nbasin is narrower than the Pacific basin. They further explained that, because the southward western\nboundary current associated with the wind-driven subpolar gyre has higher velocity in the wide\nbasin than in the narrow basin, it overwhelms the northward western boundary current associated\nwith the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain\nsouthward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses\nlocal convection and spreads eastward, leading to low salinities in the north of the wide basin for\nwide-basin sinking. While in the narrow basin, the northward MOC western boundary current\novercomes the southward western boundary current associated with the wind-driven subpolar gyre,\nbringing salty water from lower latitudes northward and enabling deep-water mass formation.\nJobes and Cecci (2017) did a perfect work to understand the different overturning modes from\nthe point of view of ocean dynamics, particularly the wind-driven circulation dynamics, through\nsensitivity experiments using an ocean model (MITgcm). In their work, readers can learn a lot about\nhow the wind-driven circulation affects the thermohaline circulation and their interplay.\nOur work focused on the effect of the surface freshwater flux on the GMOC under the condition\nof current realistic ocean geometry. We just would like to show that, provided with an ocean\ngeometry, the continental orography may significantly affect the global hydrological cycle and thus\nthe surface freshwater fluxed over different oceans, which can cause remarkable changes of\noverturning modes in different oceans.\nOur conclusion does not have to contradict to that of Jones and Cecci (2017), neither we\ncompare our results with that of Jones and Cecci (2017). Our results are obtained from a coupled\nEarth system model while Jones and Cecci’s results were obtained from an ocean GCM.\nIn lines 18-25 of the revised text, this paragraph is rewritten as follows.\n“The asymmetry of net surface freshwater flux is often cited as the cause of different overturning modes\nbetween the Atlantic and Pacific. The North Atlantic has higher sea-surface salinity (SSS) than the North\nPacific because the former is a net evaporation basin, while the latter is nearly neutral9. Additionally, ocean\nbasin geometry plays a role in the different overturning modes. Research indicated that narrow basins are\nmore conducive to deep overturning circulation than wide basins2,10,11. Furthermore, ocean gateways also\ncontribute to the different overturning modes. The opening of the Drake Passage/Tasman Seaway in the late\nEocene is thought to have promoted the NADW formation and thus the AMOC formation12.”\n2. L84: add more recent references\nResponses: Thank you very much for this suggestion. We have replaced ref. 12 with a more\nrelevant reference “Hutchinson, D. K., Coxall, H. K., OʹRegan, M. et al. Arctic closure as a trigger\nfor Atlantic overturning at the Eocene-Oligocene Transition. Nat Commun 10, 3797 (2019)”. This is\nalso suggested by Reviewer #1.\n3. L89: The authors are encouraged to update the age of glacial inception in Antarctica,\nrecognizing that the onset of glaciation in Antarctica is typically dated to the Late Eocene. This\ntiming is attributed to a CO2 atmospheric level that was considered too high before the Late\nEocene. Please consider incorporating a relevant reference to support this updated information,\nproviding a solid foundation for the revised age of glacial inception in Antarctica.\nResponses: Thank you very much for this comment.\nIn lines 29-32 of the revised text, this statement is rewritten as follows.\n“Although the transantarctic and Gamburtsev Mountains over Antarctica were likely already present at\nthe start of the Cenozoic (65 Ma)18,19, the first large-scale glaciation of Antarctica is believed to have\noccurred during the Eocene-Oligocene transition period (34-33.5 Ma)20,21”\nWe added two relevant references as ref. 20, 21 in the revised manuscript:\n20. Jamieson, S. S. R., Sugden, D. E., & Hulton, N. R. J. The evolution of the sub-glacial landscape of\nAntarctica. Earth and Planetary Science Letters 293, 1-27 (2010).\n21. Jamieson, S. S. R., Ross, N., Paxman, G. J. G., Clubb, F. J. An ancient river landscape preserved beneath\nthe East Antarctic Ice Sheet. Nat Commun 14, 6507 (2023).\n4. L90: Change the reference.\nResponses: Thank you very much for this suggestion. The old reference 20 is changed to\nBoschmann (2021) (ref. 22 in the revised manuscript). And the statement about AMs is changed to\n“The uplift of Andes Mountains (AMs) started in the Late Cretaceous (~70 Ma)22 and matured\naround 15-10 Ma23,24.”\n5. L83-99: The paragraph in question lacks clarity as the authors attempt to draw a comparison\nbetween the age of Antarctic glaciation and the age of the Andes uplift. It is recommended that\nthe authors revisit and revise this paragraph to articulate the comparison more explicitly,\nproviding a clearer and more coherent presentation of the relationship between the two\ngeological events.\nResponses: Thank you very much for this comment. This paragraph is revised significantly\nfollowing suggestions from you and Reviewer#1. The references within are also changed and\nupdated. The revised paragraph reads as follows:\n“Geological evidence also suggests that the uplift of large continental mountains has had a significant\nimpact on the climate13. The evolution of continental terrain holds the potential to trigger large-scale\ntransitions in the global MOC (GMOC)14-16. The Rocky Mountains (RMs) rose from the sea level about 80\nMa17, and reached its current elevation about 45 Ma8. Although the transantarctic and Gamburtsev\nMountains over Antarctica were likely already present at the start of the Cenozoic (65 Ma)18,19, the first\nlarge-scale glaciation of Antarctica is believed to have occurred during the Eocene-Oligocene transition\nperiod (34-33.5 Ma)20,21. The uplift of Andes Mountains (AMs) started in the Late Cretaceous (~70 Ma)22\nand matured around 15-10 Ma23,24. The uplift of these mountains predated the onset of the NADW formation.\nThe Greenland (GL) underwent its initial phase of uplift in the late Miocene (11-10 Ma)25. The timeline for\nthe formation of the Tibetan Plateau (TP) is a topic of highly debate. Some studies argue that parts of the TP\nwere already in place during the late Eocene (38-33 Ma)26,27, while other research suggest that the TP's rapid\nand main uplift occurred between 10 and 8 Ma28-31. A more recent study proposes that most of the TP had\nattained its current elevation before the Mid-Miocene (15 Ma)32. This timing coincides with the onset of\nNADW formation, suggesting a possible link between the TP uplift and the development of NADW. Recent\nresearch also indicates that the TP is a critical factor affecting changes in the GMOC33,34. Nevertheless, it is\nimportant to recognize that the chronology of the uplift of major mountain ranges remains a subject of\nintense discussion and investigation.”\n6. Leier et al. 2013: First uplift Early Miocene, second uplift Late Miocene – Andes Bolivia\nResponses: Thank you very much for this suggestion. We added the newer study of Leier et al.\n(2013) as the new reference 24.\n7. L142-151: The manuscript does not thoroughly discuss the intensification of the Pacific\nMeridional Overturning Circulation (PMOC) in relation to the uplift of the Andes\nResponses: Thank you very much for this comment. In this study, we did not provide detailed\nanalyses on the PMOC change in response to the uplift of the Andes mountains (AMs), which, we\nthink, deserve a full-length paper. We are working intensely on this issue through more sensitivity\nexperiments. On page 2 of the revised manuscript, we delete the key point #5.\nIn lines 188-192 of the revised text, we briefly discussed the mechanism of the PMOC\nintensification in response to the uplift of the AMs as follows.\n“The presence of the AM reduces the equatorial trade wind, but amplifies the off-equatorial Ekman\npumping (Extended Data Fig. 5a), thereby boosting the wind-driven STC in the South Indo-Pacific\n(Extended Data Fig. 5b) and augmenting the thermohaline component of the PMOC there, leading to a\nstronger PMOC (Fig. 3e2).”\n8. Figure 4 -Line 8: Does the case Flat2Real correspond to the end of the experiment when all\nreliefs are uplifted (year 5601-6000) ?\nResponses: Thank you very much for this comment. Yes, the equilibrium change in Flat2Real with\nrespect to Flat corresponds to year 5601-6000 of Flat2Real.\n9. L201: During the summer in Antarctica, the flat topography of the continent is likely to promote\nconvection. Additionally, the presence of a high-pressure anomaly can indeed be seasonal.\nResponses: Thank you very much for this comment. We agree that “the flat topography in\nAntarctica during the summer is likely to promote convection”, however, this convection is much\nweaker than that in the presence of the AT mountains. It is also likely that “the presence of a high-\npressure anomaly can be seasonal”. We would like to say that on seasonal timescale, the effect of\nconvection or high-pressure anomaly cannot lead to significant change in AABW and\nAMOC/PMOC, which has to rely on the accumulated annual changes.\n10. Figure 5a1: the figure 5a1 represents the difference Real minus Flat (on the plot) or Flat2Real\nminus flat (in the caption)\nResponses: Thank you very much for this comment. Fig. 5a1 represents the difference between\nFlat2Real (year 5601-6000) and Flat. which is practically identical to the difference between Real\nand Flat. We corrected the small mistake on the plot.\n11. L246-262: The manuscript describes the retreat of sea ice in the subpolar Atlantic, but the\nunderlying cause of this retreat is not clearly explained. It is crucial to address this gap by\nproviding a detailed and explicit discussion on the factors driving the retreat of sea ice in the\nsubpolar Atlantic.\nResponses: Thank you very much for this comment.\nThis paragraph is revised and states explicitly what initialize the sea ice change in the subpolar\nAtlantic (see below). Please also refer to lines 195-221 of the revised text.\n“The AMOC gradually increases in the first few hundred years after the TP uplift in both Flat2Real and\nTP+AT, followed by an acceleration and eventual return to a normal state in Real (Figs. 1, 2b). The latter\nprocess is accompanied by a swift sea-ice loss in the subpolar Atlantic. In response to the TP uplift, more\natmospheric moisture converges (diverges) over the North Pacific (Atlantic) (Extended Data Figs. 2b2, 2b5,\n3a2, 3a3), shutting down the PMOC and triggering a gradual increase of the AMOC at the same time. The\nlatter enhances the northward heat transport in the Atlantic, leading to a gradual retreat of sea ice in the\nsubpolar Atlantic. The sea ice retreat is also helped by the anomalous northward Ekman flow, forced by the\nanomalous easterlies over the subpolar Atlantic (Extended Data Fig. 3b2).\nThe evolution of sea ice in Flat2Real is shown in Fig. 6. The northward retreat of sea ice is illustrated by\nthe sea ice velocity in Fig. 6b, which leads to additional freshwater loss in this region (Extended Data Figs.\n2b2, 2b5). This freshwater loss in turn increases SSD, the NADW formation and thus the AMOC\nconsequently. In the first few hundred years after adding the TP, the sea-ice in the subpolar Atlantic retreats\nnorthward slightly (Fig. 6b), and the March mixed layer depth (MLD) deepens slightly (Fig. 6a),\ncorresponding to a gradual increase of the AMOC. About 500 years after the TP uplift, the collective effects\nof accumulated saline water in the subpolar Atlantic and Ekman pumping in Southern Ocean accelerate the\nAMOC, so that the sea-ice margin retreats rapidly northward (Fig. 6b, dashed red curve), resulting in a large\namount of freshwater flux loss in the subpolar Atlantic, a rapid deepening of the MLD (Fig. 6a), and a\nfurther enhancement of the AMOC (Figs. 1, 2b). The sea-ice margin reaches its quasi-equilibrium roughly\n1000 years after the TP uplift (Fig. 6b), accompanied by significant sea-ice melting in the GIN seas. The sea-\nice evolution in TP+AT (Extended Data Figs. 7a-b) displays a similar pattern to that in Flat2Real, while\nchanges in sea ice and MLD in TP are minimal (Extended Data Figs. 7c-d). The evolutions of AMOC, the\nMLD and sea-ice coverage and margin in Flat2Real and TP+AT suggest a positive feedback between the\nAMOC and sea ice changes, which eventually leads to the establishment of AMOC. This feedback has been\nshown in many previous studies (e.g., Brady and Otto-Bliesner 2011; Yang and Wen 2020).”\nReference:\n47. Brady, E. C., and B. L. Otto-Bliesner, 2011: The role of meltwater induced subsurface ocean warming in\nregulating the Atlantic meridional overturning in glacial climate simulations. Climate Dyn., 37, 1517–\n1532, https://doi.org/10.1007/s00382-010-0925-9\n12. L289-291: the authors explained clearly on lines 289-291 that this study is based on sensitivity\nexperiments (presence or absence of relief) and does not mimic past climate evolution. I fully\nagree with that. I regret that the key points and the abstract are less explicit.\nResponses: Thank you very much for this comment.\nIn the revised manuscript, on page 2 the key point #1 is changed to “A series of coupled model\nsensitivity experiments with/without continental orography suggest that the thermohaline\ncirculation could occur either in the Pacific or in the Atlantic, depending much on how the\ncontinental giant mountains affect the pattern of global hydrological cycle.”, which explicitly state\nthat all results are based on sensitivity experiments.\nIn abstract, we explicitly state that “we design a series of coupled model sensitivity experiments\nto investigate …”, and tone down the TP’s effect by removing the word “paramount”.\n13. Then the authors compare with their results with the climate evolution during the Cenozoic.\nI disagree with the sentence (L300) suggesting that “this study establishes the direction of\nMOC change as a function of increasing uplift over time”. The position of continents (and relief)\nremains unchanged, as are the gateways (these limitations are indicated on L307).\nResponses: Thank you very much for this comment. In the revised manuscript, this paragraph is\nrevised significantly. Please refer to lines 248-264 of the revised text.\nParticularly, the last sentence of this paragraph is changed to “By using experiment Flat as a\nstarting point and tracing the changes due to various uplifts, we can quantify the linkages between\nuplifts and climate changes and understand how the MOC changes as progressive uplift of\ncontinental mountains.”\n14. The clarification provided in lines 289-291 regarding the study's reliance on sensitivity\nexperiments rather than a direct mimicry of past climate evolution is appreciated. However,\nthere seems to be a discrepancy in the key points and abstract, where the explicitness might be\nlacking.\nResponses: Thank you very much for this comment. Please refer to our replies to Q.12-13, the key\npoints, the abstract and the old paragraph in lines 289-300 are rewritten in the revised manuscript,\nin which both the consistency and explicitness are improved significantly.\n15. Concerning the comparison with the climate evolution during the Cenozoic, there is\ndisagreement with the sentence on line 300 suggesting that \" this study establishes the direction\nof MOC change as a function of increasing uplift over time \". The disagreement arises from the\nunderstanding that the positions of continents (including relief) and gateways remain\nunchanged, as stated on line 307, and this should be reflected in the conclusions. A careful\nreview and adjustment of these statements in the key points and abstract will contribute to a\nmore accurate representation of the study's findings and limitations.\nResponses: Thank you very much for these suggestions. Please also refer to our replies to Q.12-14,\nthe key points, the abstract and Section “Summary and discussion” are rewritten to better state the\nfindings and limitations of this study.\nReplies to Reviewer #1:\nThank you very much for invaluable suggestions to this manuscript.\nThis is the third review of “Which Mountains Matter to Global Meridional Overturning\nCirculation” by Haijun Yang and coauthors for potential publication in Nature Communications.\nI am happy with the responses to the comments and the corrections made by the authors. In my\nopinion, the manuscript is now suitable for publication.\nI still give a couple of wording suggestions below.\nI am also wondering whether the model outputs used in the manuscript will be made publicly\naccessible to comply with latest standards in climate modelling and to promote open science.\nResponses: Yes. All model results in this study are available from the corresponding authors upon\nrequest. We also add Data availability in the article.\n1. l. 66-71. “and likely began to develop at a later stage. Consequently, the modern AMOC might\ninitially develop in the late Miocene (about 12-9 Ma) and not be fully established until the late\nPliocene to early Pleistocene (about 4-3 Ma). Nonetheless, a recent study from DeepMIP\nproject found that neither model results nor proxy data suggest NADW formation during the\nearly Eocene, while the evidence for NPDW formation remains inconclusive.” => and likely\nbegan to develop at a later stage. However, a recent study from the DeepMIP project found that\nneither model results nor proxy data suggest NADW formation during the early Eocene, while\nevidence for NPDW formation remains elusive. Consequently, the modern AMOC might initially\ndevelop in the late Miocene (about 12-9 Ma) and not be fully established until the late Pliocene\nto early Pleistocene (about 4-3 Ma). Revised.\n2. l. 71. “AMOC and PMOC remains a topic” => AMOC and PMOC therefore remains a\ntopic Revised.\n3. l. 83. “large-scale transitions in the global MOC (GMOC).” => large-scale transitions in\nthe global MOC (GMOC), and in the following, we briefly review the uplift periods of the major\nmodern mountain ranges. Revised.\n4. l. 303. “This research marks the beginning of our investigation into the effects of the presence\nor absence…” => The results presented here are a first step toward a better understanding of\nthe coupled impacts of the presence or absence… Revised.\n5. l. 305. “Our research aims to specifically discern” => Our experiments are specifically\ndesigned to disentangle Revised.\n6. l. 306. “by incorporating […] in our experimental setup” => thereby keeping the\nbathymetry and continental configuration, as well as greenhouse gases, incident solar radiation\nand orbital parameters, to their modern state in our experimental setup. Revised.\n7. l. 309. “explore the linkages between uplift regions and climate changes in remote areas of\nthe modern world”. Consider changing to: explore the links and teleconnections between uplift\nregions and climate? Revised.\n8. l. 310. “The sequential mountain uplift experiment Flat2Real provides insight into specific\nperiods of paleoclimate, such as the climate development in the late Cenozoic.” This sentence\nreads weirdly. Consider changing to: Moreover, the sequential mountain uplift experiment\nFlat2Real may offer interesting insights about the long-term impact of mountains on Cenozoic\nclimate change, notwithstanding significant caveats.\nThen I suggest that you start a new paragraph with the next sentence (“Previous studies suggested\nthat the TP uplift played a key role in shaping Cenozoic climate…\")\nResponses: Thank you very much. We have revised the sentences according to your suggestions.\n9. l. 320-321. If you follow the comment just above, you can remove the following: “It should be\nnoted […] the credibility of model's results” because you already mentioned that there are\ncaveats (or limitations). Just change “For instance” with “However” on line 322 and\nstart the paragraph with it.\nResponses: Thank you very much for this comment. We have revised the sentences according to\nyour suggestions.\n10. l. 328. Remove “After all”. Revised.\n11. l. 330. “lead to significant increases in continental ice sheets in both hemispheres and\nenhance the NADW formation” => lead to significant changes in ocean circulation and\nmodulate NADW formation. Revised.\n12. l. 335-336. Why only the TP uplift and not that of the other mountain ranges discussed?\nResponses: Thank you for your comments. I guess other mountains’ uplift have the same effect on\nthe atmospheric CO2. The TP’s effect, not other mountains, was explicitly discussed in Ref. 49. So\nhere it is safer to just mention the TP’s effect on CO2.\n13. l. 341. “We want to emphasize” => We still want to emphasize. Revised.\n14. l. 345-347. “of debate. Understanding which mountains exert control over the MOC is critical\nfor comprehending oceans’ roles in past and future climate transitions. We hope that this\nstudy will help clarify this issue and reduce the controversy surrounding it.”\n=> of debate, and our results contribute to a better understanding of the controls exerted by mountain\nranges over the MOC, which is critical for a better comprehension of the role of the ocean in past and\nfuture climate transitions. Revised.\nReplies to Reviewer #2:\nThank you very much for invaluable comments to this manuscript, which help to improve the\nmanuscript tremendously.\nThe authors have diligently incorporated the reviewers' comments to refine the manuscript.\nFrom my perspective, the approach utilized demonstrates that the presence or absence of a Tibetan\nplateau influences the Atlantic Meridional Overturning Circulation (AMOC) and Pacific\nMeridional Overturning Circulation (PMOC) in these sensitivity experiments. However, it appears\nchallenging to extrapolate these findings to the actual evolution of the AMOC in the Cenozoic.\nThe uplift of landforms significantly shapes Earth's landscape over tens of millions of years,\nwith major temporal overlaps that diverge from the events discussed here. Additionally, factors\nsuch as changes in ocean passages, variations in atmospheric carbon dioxide (pCO2) levels, and\nthe glaciation of Antarctica and Greenland also play pivotal roles in Earth's climate evolution.\nWhile the study provides valuable insights into the analyzed mechanisms, it does not\nconclusively establish that the presence of the Tibetan plateau (along with Antarctica topography)\nis the sole triggering factor for the AMOC and the demise of the PMOC in the Cenozoic.\nResponses: Thank you very much for the comments. We agree your assessment on this work.\nActually, in Section summary and discussion, we have discussed the problems you concerned very\ncarefully in three paragraphs.\nFirst, we state explicitly that “The results presented here are a first step toward a better\nunderstanding of the coupled impacts of the presence or absence of various major mountain ranges\non the GMOC ...” There are lots of other factors that should be considered if we want to reach a\nfully understanding of the climate evolution in the Cenozonic. There is a very long way to go.\n1. L28: I find the wording of the key point 1 to be questionable as it refers to the thermohaline\ncirculation in general, whereas it should specifically address the location of convection, which\nultimately impacts the Atlantic Meridional Overturning Circulation (AMOC) and Pacific\nMeridional Overturning Circulation (PMOC).\"\nResponses: Thank you for your suggestions. We have changed key point 1 to “A series of coupled\nmodel sensitivity experiments with/without continental orography suggest that the deep-water\nformation could occur either in the Pacific or in the Atlantic, depending much on how the\ncontinental giant mountains affect the pattern of global hydrological cycle.”\n2. L42: The presence of Greenland's topography is unsurprising. Convection in the North Atlantic\nOcean is intensified by the Greenland ice sheet, with surface albedo playing a significant role.\n(Pillot et al. 2022).\nResponses: Thank you very much for this comment. In this study, we just explored the effect of the\nGreenland topography, and did not study the effect of the Greenland ice sheet. The albedo effect\nrelated to the Greenland ice sheet is actually a very important issue that we are going to study in the\nnear future.\n3. L39: Key points should focus on the main results of the work itself and not reference other\npapers from the literature. (ref 1 and 14 called in key point 4)\nResponses: Thank you very much. We have changed key point 4 to “The Rocky Mountains have no\nsignificant effect on either the PMOC or the AMOC.”\n4. L62: The authors assert that geological evidence elucidates the location of deep water\nformation. What evidences? add a reference.\nResponses: Thank you very much for the comment. The references 4-7 is labeled in the following\nsentences in the same paragraph.\n5. L79: It refers more to the paper by Toggweiler, J. R. & Bjornsson, H. Drake passage and\npalaeoclimate. J. Quat. Sci. 15, 319–328 (2000) as indicated in Hutchinson et al. 2019\nResponses: Thank you very much for this suggestion. We have changed the ref. 12 in this sentence\nto “Toggweiler, J. R. & Bjornsson, H. Drake passage and palaeoclimate. J. Quat. Sci. 15, 319–328\n(2000)”.\n6. L83-84: The authors can cite the paper by Chamberlain et al. (2012) to discuss the evolution of\nthe Rocky Mountains. According to their findings, the current shape of the Rocky Mountains\nwas established during the Oligocene. However, it is worth noting that some landforms within\nthe range have undergone collapse since that time.\nResponses: Thank you very much for this suggestion. We have replaced the old ref. 17 with\nChamberlain et al. (2012).\n7. L85: add a capital to Transantarctic Mountain. It appears that the authors did not include\nAntarctic landforms in their simulations. They incorporate the modern elevation of the Antarctic\nice sheet into their runs (after the Fig 1 in SI).\nResponses: Thank you for your suggestion. We have added capital to “Transantarctic Mountain”.\nIn this study, we did not include Antarctic landforms in our simulations.\n8. L89: “The uplift of these mountains predated the onset of the NADW formation”. The\nauthors should recall that the simulated Meridional Overturning Circulation (MOC) responds\nto the absence or presence of mountain ranges. While it is agreed that the Rocky Mountains\nwere largely uplifted before the onset of the North Atlantic Deep Water (NADW) formation, the\nsituation is more debatable for the Andes. The uplift of the Andes may not have occurred\nlinearly over time, and it is possible that the largest uplift took place during the Late Cenozoic.\nResponses: Thank you very much for the invaluable comments. We also found that the effect of the\nAndes mountains on the MOC is not that obvious. We are working intensely on this issue using a\nhigh-resolution version of CESM2.0. Since the Andes mountains is narrow in the east-west\ndirection, the model resolution might be an important issue in simulating its effect on global\nclimate. We also made discussion on this in section Summary and discussion.\n9. L105: The added Antarctic (AT) topography during the simulation primarily represents that of\nthe Antarctic ice sheet, rather than the landforms (Transantarctic and Gamburtsev Mountains).\nThis also applies to the Greenland (GL) ice sheet. As explained in the \"Methods\" section, the\ncontinental ice sheets are treated as bright rocks with an albedo similar to that of a bare soil.\nThis contrasts with the \"REAL\" case, where the albedo of the surface should be realistic. It\nwould be intriguing to observe the Pacific Meridional Overturning Circulation (PMOC) and\nAtlantic Meridional Overturning Circulation (AMOC) when the realistic albedo of the surface\nis applied in the \"REAL\" case. It is surprising to note that the intensity of the AMOC reaches 18\nSv without considering the albedo of the Antarctic and Greenland ice sheets, as observed in the\npreindustrial control run with ice sheets conducted by Shields et al. (2012), although it should\nbe noted that the models may not be exactly the same.\nResponses: Thank you very much for the invaluable comments. Based on our simulations, we feel\nthat the landforms of Antarctic, as well as the detailed albedo patterns appear to have very limited\neffect on the mean strength of the PMOC and AMOC. This issue needs to be explored in-depth\nusing latest version of coupled models. Actually, we are working intensely on the topographic\nclimate effect using CESM2.1. It is a very time-consuming job and we may not have results until\nthe late 2024.\n10. L134: “irrespective of the order in which they were introduced to Flat”. Thus the authors\nhave tested different scenarios (not shown in extended data ?)\nResponses: Thank you for the comment. Yes, we tested lots of different scenarios, which were not\nshown in extended data. The basic concept is that the equilibrium response has to be independent\non the order of the mountains added, otherwise the response in the final “Real” simulation will not\nbe unique.\n11. L288: It is indeed challenging to draw conclusions about the role of the Tibetan Plateau (TP)\non modern-day thermohaline circulation. The abrupt addition of the TP in the experiment is far\nfrom realistic, as acknowledged by the authors. These are sensitivity experiments designed to\nexplore potential impacts rather than replicate real-world conditions.\nMoreover, it's important to note that the uplift of the TP did not commence at 8-10 million years ago\n(Ma). Evidence suggests that at least parts of the Tibetan plateau, such as South and Central Tibet,\nwere already elevated during the Mid-Miocene, thus predating the establishment of a modern\nAtlantic Meridional Overturning Circulation (AMOC).\nResponses: Thank you very much for the comment. We agree that it is indeed challenging to draw\nconclusion about the role of the TP in modern-day thermocline circulation, given the challenges of\naccurately simulating the entire evolutionary process that occurred in reality. It's essential to\nrecognize that among all the world's major mountain ranges, the TP holds the greatest potential to\nimpact the AMOC. While we concur that the TP's uplift began well before 8-10 Ma, there is also\nevidence to suggest that its principal elevation occurred around this time frame.\nThis study represents an initial step towards understanding the significant influence of\ncontinental orography on Earth's climate formation. We are diligently advancing our research on the\nevolution of Earth’s climate since the Cenozoic era through meticulously designed sensitivity\nexperiments.\n12. L481 : Shields et al => ref 55 not 56 Revised."}]

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