Miocene African topography induces decoupling of Somali Jet and South Asian summer monsoon rainfall

Domain: earth_science
Published: 2025-08-04
Editorial outcome: major_revision

Abstract
The Miocene epoch, marked by significant tectonic and climatic shifts, presents a unique period to study the evolution of South Asian summer monsoon (SASM) dynamics. Previous studies have shown conflicting evidence: wind proxies from the western Arabian Sea suggest a weaker Somali Jet during the Middle Miocene compared to the Late Miocene, while rain-related records indicate increased SASM rainfall. This apparent decoupling of monsoonal winds and rainfall has challenged our understanding of SASM variability. Here, using the fully coupled EC-Earth3 model, we identify a key driver of this decoupling: changes in African topography rather than other external forcings such as CO2 change. Our simulations reveal that changes in Miocene African topography weakened the cross-equatorial Somali Jet and reduced upwelling in the western Arabian Sea, while simultaneously enhancing monsoonal rainfall by inducing atmospheric circulation anomalies over the Arabian Sea. The weakened Somali Jet fostered a positive Indian Ocean Dipole-like warming pattern, further amplifying the monsoonal rainfall through ocean-atmosphere feedbacks. In contrast, CO2 forcing enhances both Somali Jet and rainfall simultaneously, showing no decoupling effect. These findings reconcile the discrepancies between wind and rainfall proxies and highlight the critical role of African topography in shaping the multi-stage evolution of the SASM system.

Introduction
The South Asian summer monsoon (SASM), a major component of the global monsoon system1–3, has a strong impact on the Indo-Pacific moisture transport and governs the stability of the present-day Indian Monsoon. Over geological timescales, the strength of the SASM has fluctuated significantly, as evidenced by multiple proxy records. During the Miocene, significant tectonic activity, atmospheric CO2 fluctuations, and environmental changes4,5, influenced monsoon dynamics, drawing considerable interest in the evolution and key drivers of SASM variability6,7.

Marine proxies indicate a major intensification of SASM circulation, particularly the Somali Jet in the western Arabian Sea during the Late Miocene after 8 Ma8,9. However, sediment archives from the western Arabian Sea10–13 and Maldives archipelago14 suggest that the strengthening of SASM circulation might have commenced as early as the late Middle Miocene (~12.9 Ma). These records link wind-driven coastal upwelling in the western Arabian Sea with low-level tropospheric SASM winds, which drive surface waters offshore, bringing cold, nutrient-rich waters into the euphotic zone and enhancing primary production. Nonetheless, the interpretation of wind-based proxies has been challenged by the emergence of rain-related proxies that present a different perspective on SASM rainfall variability15–21. Studies of oxygen isotopes (δ18O) from pedogenic carbonates15,16 and mammalian teeth22 from the Siwaliks Hills suggest a decline in SASM rainfall since the Middle Miocene. This decline corresponds with a shift in vegetation from forest ecosystems to grassland-dominated landscapes in South Asia15,22. This reduction in the SASM rainfall after around 12 Ma is further supported by proxies indicating a weakening of chemical weathering and erosion, including decreased K/Al ratios and clastic fluxes into the ocean18. Lower chemical weathering rates and sediment transport imply drier conditions and a weakening of SASM.

Recent studies attribute the SASM circulation and rainfall intensity primarily to the uplift of Himalaya-Tibetan Plateau3,23,24, the expansion of polar ice sheet development25,26 or variations in pCO227 during the Miocene. However, discrepancies between oceanic wind-based records and terrestrial rain-related records suggest that additional boundary forcings contributed to the multi-stage evolution of the SASM system. Reconstructions of Africa’s dynamic topography over the past 30 Ma indicate a gradual uplift of the East African Rift System, including the Ethiopian and the East African plateaus, and the concurrent subsidence of the Congo Basin by up to 500 meters28. Notably, these tectonic changes in Africa aligns with the emergence of a “modern-like” SASM during the late Middle Miocene (~13 Ma)10–14,29. Recent modeling studies further highlight the sensitivity of the SASM to the uplift of the East African Highlands since the Miocene6,7, reinforcing the intricate link between African topography, SASM circulation, and regional rainfall. Despite these insights, the mechanisms underlying the apparent decoupling of Somali Jet and rainfall between the Middle to Late Miocene remain poorly understood. Previous modeling efforts have limited by low-horizontal resolution in atmospheric general circulation models or have overlooked the actual African topography of the Miocene, as well as the oceanic feedbacks induced by topographic changes6,7,30–32. These limitations complicate efforts to quantify the role of African topography in SASM evolution.

To address this gap, we investigate the climate response to reconstructed African dynamic topography at ~25 Ma (Late Oligocene; referred to here as Early Miocene simulations), ~15 Ma (Middle Miocene), ~5 Ma (Late Miocene), and present-day (pre-industrial) using a high-resolution configuration of the fully coupled EC-Earth3 model. We also perform sensitivity experiments using a linear baroclinic model (LBM) to identify the dominant mechanisms driving the simulated SASM changes (see Methods and Supplementary Figures for a detailed description). Our results demonstrate that the decoupling of Somali Jet and SASM rainfall is primarily driven by changes in African topography during the Miocene, operating through positive ocean-atmosphere feedbacks.

Results
Miocene African topography changes cause the decoupling of Somali Jet and SASM rainfall

We use the coupled EC-Earth3 model to simulate the response of the SASM to changes in African topography since the Miocene (see Methods). The model successfully replicates the present-day SASM system, which is characterized by a strong cross-equatorial Somali Jet over the Arabian Sea and increased rainfall over the Indian subcontinent during boreal summer (June to August) (Supplementary Fig. 1a, b). Due to the Coriolis force, the climatological southwesterly summer monsoon winds over the Arabian Sea can drive strong ocean upwelling along the coastal region (Supplementary Fig. 1c, d). The agreement between our pre-industrial simulation and contemporary observations validate the use of EC-Earth3 for exploring the influence of the African topography on the SASM.

Our simulations reveal a notable decoupling of the cross-equatorial Somali Jet and rainfall from Middle Miocene to the present-day in response to changes in African topography. The Miocene epoch was characterized by substantial tectonic activity, especially within the East African Rift System (EARS). Reconstructions of Africa’s dynamic topography over the past 30 Ma28 indicate a gradual uplift of the EARS, including the Ethiopian and the East African plateaus, and subsidence of the Congo Basin by up to 500 meters (Fig. 1a–c and Supplementary Fig. 2). Our simulations show a significant weakening of the meridional cross-equatorial flow of the Somali Jet (Fig. 1d–f and Supplementary Fig. 3), with reductions of 0.36 ms-1 and 0.22 ms-1 in the Early and Middle Miocene, respectively, compared to the present-day (Fig. 1g). These findings are largely in agreement with previous modeling studies31–33.

Accompanied by the weakened Somali Jet, upwelling in the western Arabian Sea is significantly reduced in the Early and Middle Miocene (Fig. 1g and Supplementary Fig. 4). This simulated upwelling change aligns with wind-based proxy records (see Methods), which indicate a weakening Somali Jet-induced oceanic upwelling in the western Arabian Sea during the Middle Miocene compared to Late Miocene (Fig. 1i, j). However, recent publication has advanced our knowledge that the weakening of upwelling is more complex than a simple reduction in wind strength and is attributed to changes in both the intensity and structure of the Somali Jet34. Specifically, while the jet weakens in its core, low-level tropospheric winds strengthen along the western Arabian Peninsula (Supplementary Fig. 3). This altered wind pattern decreases the wind-stress curl along the western Arabian Sea coast34–36, where the proxy sites are located, resulting in negative Ekman pumping and weaker upwelling (Supplementary Fig. 4).

In present-day climate, the East African topography acts as a barrier that channels the easterly Somali Jet, yielding intense rainfall over the Indian subcontinent. Given the established relationship between the Somali Jet and SASM rainfall, one would expect the topography-induced weakening of the Somali Jet to result in reduced SASM rainfall due to decreased moisture transport37. However, our results indicate a spatially coherent increase in summer rainfall, with the maxima in the northeast of the Indian subcontinent and south of the Western Ghats region during the Early and Middle Miocene (Fig. 1d, e). Specifically, SASM rainfall increases by 15% and 9% during the Early and Middle Miocene, respectively, compared to the present-day (Fig. 1h). This simulated increase in SASM rainfall is consistent with qualitative rainfall-based proxies for the Miocene (see Methods), which suggest more humid conditions over the South Asia during the Middle Miocene compared to the Late Miocene (Fig. 1k–o).

Our experiments successfully reproduce the major observational features of Somali Jet-induced upwelling and monsoon rainfall decoupling from Middle to Late Miocene, as seen in proxy records. This highlights the significant role of altered African topography in modulating the SASM system during the Miocene, offering a previously unrecognized explanation for the observed discrepancies between wind-based and rainfall-based proxy reconstructions.

During the boreal summer, the East African Highlands act as a major topographic barrier, redirecting low-level easterly winds over the Southern Indian Ocean across the equator, forming the southwesterly Somali Jet38 (Supplementary Fig. 1). Numerous idealized modeling studies have demonstrated that removing or lowering the East African Highlands allow easterly winds south of the equator to penetrate further inland over Africa, creating a broader, less concentrated cross-equatorial flow, rather than channeling it into a strong Somali Jet31–33.

Our simulations support this mechanism, showing a significant weakening of the core of Somali Jet in response to the lowered East African Highlands during the Miocene (Fig. 1, Fig. 2a, d, g, and Supplementary Fig. 3). This highlights the critical role of reduced topographical blocking in weakening the cross-equatorial Somali Jet, consistent with previous modeling studies6,31–33. Consequently, the weakened Somali Jet during the Early and Middle Miocene results in a marked downwelling motion anomaly along the coast of East Africa (Fig. 2c, f). This would have reduced coastal upwelling and primary productivity39, affecting wind-based proxies such as TOC and G. bulloides in the western Arabian Sea (Fig. 1).

Dynamic effects induced by atmospheric circulation anomalies control the changes in SASM rainfall

The significant increase in SASM rainfall during the Early and Middle Miocene, despite the weakened Somali Jet, is primarily associated with anomalous cyclonic circulation over Arabian Sea and an anticyclonic pattern over the Bay of Bengal (red bold vectors in Fig. 1d, e). These circulation patterns enhance meridional moisture transport from the tropical Indian Ocean to the SASM region (Supplementary Fig. 5), providing a dynamic explanation for the increased precipitation.

This hypothesis is further confirmed by a linearized moisture budget analysis (see Methods). The moisture budget decomposition reveals that the combined thermodynamic and dynamic terms (Supplementary Fig. 6a, d and g) effectively capture the primary characteristics of SASM rainfall changes during the Early, Middle and Late Miocene (Fig. 1d–f), with pattern correlation coefficients of 0.90, 0.88, and 0.67, respectively. The increased SASM rainfall is mainly driven by dynamic effects resulting from changes in atmospheric circulation (Supplementary Fig. 6c, f and i), particularly during the Early and Middle Miocene, whereas the thermodynamic effects due to atmospheric moisture changes are negligible (Supplementary Fig. 6b, e, and h). It is important to note that this dynamic response does not contradict the hypothesis of a decoupling between monsoon winds and rainfall, but rather provides a mechanistic explanation for how SASM rainfall increased despite a weaker Somali Jet, emphasizing the importance of atmospheric circulation changes and ocean-atmosphere feedbacks in modulating the SASM system.

The analysis indicates that cyclonic and anticyclonic circulation anomalies over the northern Indian Ocean are the key drivers of increased SASM rainfall during these periods (Fig. 1d–f and Supplementary Fig. 5). This raises a critical question: how do changes in African topography induce these atmospheric circulation anomalies that, in turn, affect the SASM rainfall? The following section explores the role of ocean-atmosphere interaction in this process.

Increased SASM rainfall due to direct atmospheric processes

The anomalous cyclonic circulation over the Arabian Sea in the low-level troposphere is directly linked to a significant reduction in summer rainfall over tropical central Africa. During boreal summer, prevailing southwesterly monsoonal winds transport moisture from the tropical Atlantic Ocean and the Gulf of Guinea into central Africa, sustaining regional rainfall (Supplementary Fig. 7a). However, during the Early to Middle Miocene, large-scale tectonic uplift over the East African Highlands likely resulted in a higher elevation of the Congo basin (Fig. 1a, b, and Supplementary Fig. 2). This elevated topography acted as a barrier, reducing moisture transport from the Atlantic Ocean into central Africa, and resulting in a notable decrease in atmospheric water vapor (Supplementary Fig. 7b and f).

The resulting weakening of the West African summer monsoon causes significantly reduced rainfall over central Africa (Fig. 1d, e). This drying trend is further supported by enhanced descending air anomalies (Supplementary Fig. 7c and g) and a substantial reduction in latent heat release in the region (Supplementary Fig. 7d and h). Additionally, surface warming anomalies over central Africa (likely due to suppressed convection) and increased surface pressure indicate the presence of subsidence driven atmospheric stability (Supplementary Fig. 7e and i). These conditions likely triggered a Kelvin wave response, generating westerly wind anomalies over the tropical Indian Ocean40, which gradually weakened toward the northern Indian Ocean, thereby reinforcing cyclonic circulation in the low-level troposphere (Fig. 1d, e).

To confirm that suppressed precipitation over tropical central Africa could enhance SASM rainfall via atmosphere dynamics, we conducted an experiment using an LBM with prescribed cooling over tropical central Africa (see Methods and Supplementary Fig. 8a). According to Gill’s theory41, localized atmospheric heating and cooling anomalies generate distinct wave responses in the tropics, where Rossby waves propagate westward and Kelvin waves propagate eastward. While Kelvin waves are strongest near the equator (~10°S–10°N), their influence can extend up to ~20°N, depending on the structure and strength of the forcing40. Our LBM experiment demonstrates that suppressed convection over North-Central Africa induces a cooling anomaly, triggering a Kelvin wave response with westerly wind anomalies over the tropical Indian Ocean. This result is consistent with a previous study40. Furthermore, maximum wind speeds are found north of the equator, decreasing with latitude in the northern Indian Ocean, this favors the establishment of a cyclonic circulation over the Arabian Sea, which enhances moisture transport into the SASM region, increasing local rainfall.

However, it is worth noting that the simulated cyclonic circulation in the LBM experiment is positioned slightly northward compared to the EC-Earth simulations. This discrepancy suggests the involvement of additional mechanisms, such as a positive precipitation-atmosphere feedback and the impact of a weakened Somali Jet. To investigate this further, we performed another set of LBM experiments with prescribed heating over the SASM region (see Methods). These experiments show that enhanced SASM rainfall can trigger a Rossby wave response over its northwestern region (Supplementary Fig. 8b), which further enhances cyclonic circulation over the Arabian Sea, highlighting the role of positive precipitation-atmosphere feedback. Moreover, our results demonstrate that the Somali Jet weakens significantly when the East African Highlands are lowered during the Early and Middle Miocene. This reinforces northerly wind anomalies over the western Arabian Sea, shifting the cyclonic circulation further south compared to its position in the LBM experiment results. This displacement of the cyclonic anomaly is a direct result of the altered Somali Jet structure, further confirming the significant role of African topography in modulating the SASM system during the Miocene.

Increased SASM rainfall due to indirect ocean-atmosphere feedbacks

The increased SASM rainfall during the Miocene was further amplified by the weakened Somali Jet through positive ocean-atmosphere feedback mechanisms. Figure 2 shows how the weakened Somali Jet induces a positive Indian Ocean Dipole (IOD)-like warming pattern during the Miocene. The pressure-longitude cross section at the equator shows that lower East African Highlands during the Miocene led to northerly wind anomalies along the East African coast, weakening the climatological southwesterlies summer monsoon circulation (Fig. 2a, d). This weakening, in turn, reduced ocean upwelling along the East African coast, leading to anomalously warming SSTs in the western Indian Ocean (Fig. 2b, e and Supplementary Fig. 9). This warming of the western Indian Ocean lowers sea level pressure (SLP), reinforcing easterly wind anomalies along the equator, which enhance ocean cooling in the eastern Indian Ocean (Fig. 2b, e). These processes are further supported by their seasonal variation (Supplementary Fig. 9). Specifically, warm SST anomalies develop in the western equatorial Indian Ocean by June, accompanied by significant easterly wind anomalies extending from the west to central Indian Ocean. In the following months, this warming intensifies, with easterly wind anomalies migrate further eastward, while the southeastern tropical Indian Ocean begins to cool. These processes are particularly pronounced in the Early and Middle Miocene when the East African Highlands were much lower compared to the Late Miocene (Supplementary Fig. 10).

According to ocean-atmosphere coupling theory42, these anomalous easterly winds enhanced downwelling in the western Indian ocean and upwelling along the Java and Sumatra coasts in eastern Indian Ocean43–45 (Fig. 2c, f). Simultaneously, the thermocline deepens in the equatorial western Indian Ocean while becoming shallower in the eastern Indian Ocean, producing a dipole-like temperature pattern characterized by a “warm West and cold East” structure, indicating a positive IOD-like warming pattern43–46 (Fig. 2c, f). It is important to note that during the Late Miocene, African topography was similar to present-day, resulting in minimal changes in the Somali Jet and Indian Ocean dynamics (Fig. 2g–i). This supports the conclusion that changes in Somali Jet plays a critical role in controlling Indian Ocean dynamics on geological timescales.

The positive IOD-like warming pattern induced by the weakened Somali Jet during the Early and Middle Miocene, further increases SASM rainfall through atmosphere dynamics. Specifically, the reduced gradient of zonal SLP over the Indian Ocean caused by the IOD-like warming pattern leads to a weakened Indian Ocean Walker circulation45, featuring anomalously descending motion over the eastern Indian Ocean and ascending motion over the western Indian Ocean (Fig. 3a, c). This anomalous atmospheric circulation results in reduced latent heat release due to suppressed precipitation over the eastern Indian Ocean, which in turn triggers a Rossby wave response with an anticyclonic circulation pattern over the Bay of Bengal40,41,47,48 (red bold vectors in Fig. 3b, d). Consequently, enhanced southerly winds transport more moisture away from the tropical ocean as a part of the large-scale anticyclonic flow over the Bay of Bengal, further contributing to increased rainfall (Supplementary Fig. 5).

To further investigate the impact of suppressed convection over the eastern Indian Ocean on SASM rainfall, we designed an LBM experiment with prescribed cooling over the eastern Indian Ocean (see Methods). Consistent with Gill’s theory41, cooling in the eastern equatorial Indian Ocean produces a pair of anticyclones in the off-equatorial region, resulting in the equatorial easterly wind anomalies (Supplementary Fig. 8c). This sustains and strengthens positive IOD-like warming anomalies. The LBM experiment supports our EC-Earth simulation results, indicating that the anomalous cyclonic circulation over the Bay of Bengal also contributes to increased SASM rainfall by reinforcing positive ocean-atmosphere feedbacks.

The effects of the pCO2 forcing show no decoupling effect

The Miocene period was generally warmer than today5, particularly during the Middle Miocene Climatic Optimum (~15 Ma), largely due to elevated atmospheric CO2 concentrations4 (Fig. 4a). However, multi-proxy estimates of atmospheric CO2 during this period remain highly uncertain49–68 (Fig. 4a), making it difficult to precisely assess the role of pCO2 in SASM changes. To determine whether the SASM response to African topography changes depends on variations in CO2 concentration, we conducted sensitivity experiments using the EC-Earth3 model (see Methods).

Unlike the decoupling of Somali Jet and SASM rainfall observed in experiments driven by changes in African topography, our simulations with elevated CO2 consistently produce both an enhanced Somali Jet and increased SASM rainfall (Fig. 4), demonstrating no decoupling effect. The increase in SASM rainfall due to increased CO2 concentration (Fig. 4b–d) aligns with previous studies, highlighting the role of increased atmospheric moisture in response to global warming69–72. This follows the “wetter-gets-wetter” mechanism71,73, whereby warmer temperatures intensify moisture availability in already humid regions. As expected, the rise in surface air temperature due to higher CO2 level leads to increased atmospheric moisture content (Supplementary Fig. 11a), consistent with the Clausius-Clapeyron relation71. Thus, while changes in African topography affect SASM rainfall through dynamic circulation anomalies, CO2 forcing primarily drives rainfall changes via thermodynamic effects (Supplementary Fig. 11).

Notably, uncertainties in pCO2 estimates do not significantly alter the SASM circulation response to African topography changes during the Miocene. Simulations that combine CO2 forcing and African topography changes (see Methods) show clear cyclonic and anticyclonic atmospheric circulations over the Arabian Sea and the Bay of Bengal, respectively (Supplementary Fig. 12). This pattern is similar to the atmospheric circulation anomalies due to African topography changes (Fig. 1d, e), indicating that SASM circulation changes during the Miocene are mainly controlled by the African topography rather than pCO2 levels. Furthermore, the decoupling of Somali Jet and SASM rainfall—characterized by a weakened Somali Jet alongside increased SASM rainfall—is also evident in these combined simulations. This reinforces the conclusion that African topography played a dominant role in shaping the evolution of the SASM during the Miocene, whereas CO2 forcing alone is insufficient to drive the observed decoupling.

Methods
EC-Earth3 model and simulations

The EC-Earth3 is a fully coupled Earth system model developed by a consortium of European research institutions78, contributing to the Climate Model Intercomparison Project (CMIP) efforts79–81. It has been widely used to explore the climate dynamics of past, present and future72,79,82. The EC-Earth model incorporates various state-of-the-art components, including atmosphere, ocean, sea ice, land and biosphere. The atmospheric component is the Integrated Forecast System (IFS, version cycle 36r4) model from the European Center for Medium-Range Weather Forecasts (ECMWF), including the land surface hydrology (H-TESSEL) model. The ocean component is the Nucleus for European Modelling of the Ocean (NEMO) version 3.683, which is coupled with a sea ice model (LIM3), and the biogeochemical model PISCES. EC-Earth3 also incorporates the dynamic vegetation component from the Lund-Postsdam-Jena General Ecosystem Simulator (LPJ-GUESS). Detailed descriptions of the EC-Earth3 have been introduced previously79.

In this study, we use model configurations EC-Earth3-veg (standard CMIP6 model name), with coupled dynamic vegetation model. The atmosphere component IFS has a resolution of T255 spherical grid in the horizontal (~0.7°, 85 km) and 91 levels in the vertical; the resolution of the ocean model is ORCA1L75, representing ~1° in the horizontal, and 75 layers in the vertical.

We conducted seven sensitivity experiments to investigate the roles of the African topography and pCO2 in shaping the Miocene climate. Due to computational limitations, transient simulations covering the entire Miocene are not feasible, so we focus on three key time slices representing different stage of African topography change. In addition to a pre-industrial experiment (PI) with modern topography and a CO2 concentration of 284 p.p.m.v., we performed three experiments based on the PI setup, incorporating the African topography changes during the Early Miocene (Oligocene-Miocene transition around 25 Ma ago; MT25), Middle Miocene [Middle Miocene Climate Optimum (MMCO) around 15 Ma ago, MT15], and Late Miocene (Miocene-Pliocene transition around 5 Ma ago, MT05). The African topography details during the Miocene are set according to the geological maps from ref. 28, which reconstructs the evolution of dynamic topography of Africa over the past 30 Ma.

Furthermore, we conducted three additional experiments to clarify the relative impact of high CO2 on SASM rainfall compared to African topography changes. In these experiments, CO2 concentrations are set to 500 p.p.m.v. with African topography in Early, Middle and Late Miocene, referred to as MC25, MC15 and MC05, respectively. Orbital forcing was not considered, and all simulations were run with pre-industrial orbital settings (longitude of perihelion=100.33, obliquity=23.549, eccentricity=0.016764). Thus, climate anomalies due to African topography changes are represented by the difference between the MT25/MT15/MT05 experiments and the PI experiment. The response of SASM changes to higher CO2 concentrations is denoted by the comparisons between the MC25 and MT25, MC15 and MT15, or MC05 and MT05. The combined effects of African topography changes and elevated CO2 levels are indicated by the differences between MC25/MC15/MC05 and PI experiment.

Supplementary Table 1 provides an overview of experiments, detailing the corresponding topography and pCO2 values. The pre-industrial simulation was run for 1000 model years to ensure equilibrium, while the other experiments were run for 200 model years. Since all simulations reach quasi-equilibrium during the last 100 years, with the trend in mean global surface temperature being <0.005 K, the last 100 years of each simulation are used for analysis.

Definition of South Asian summer monsoon metrics

The South Asian summer monsoon (SASM) region is defined as the area spanning 5°N-30°N, 60°E−95°E84,85. This study focuses on the boreal summer months from June to August, unless otherwise specified. Including May and/or September in the summer season does not significantly alter the results. The intensity of SASM rainfall is measured by averaging summer rainfall over SASM domain. The variation in the Somali Jet significantly impacts the SASM rainfall and produces notable oceanic features in the western Arabian Sea. During summer, these winds force upwelling of colder waters along the coasts of Oman and Somalia, increasing nutrient levels and sustaining distinct floral and faunal groups9. Due to the relationship between the Somali Jet, SASM rainfall, and ocean circulation, the Somali Jet is often used to measure the strength of SASM circulation, especially in the paleoclimate research8–13. In this study, the intensity of Somali Jet is defined as the wind speed at 850 hPa averaged over the region of 10°S–10°N, 40°−50° E during summer32,86. In addition, the intensity of western Arabian Sea upwelling is defined as the average vertical velocity of the surface ocean (0−100 m) over the western Arabian Sea (15°–21° N, 54°–62°E)36.

Proxy-model comparison

The proxy records reflecting the Somali Jet and humid conditions over South Asia during the Miocene are collected based on previous studies9–11,15–17,19–21,87. Total organic carbon (TOC) values and Planktic foraminifer Globigerina bulloides (G. bulloides)10,11 are widely used to estimate the SASM wind intensity, as they predominantly reflect wind-driven oceanic upwelling and primary production in the western Arabian Sea. Both the TOC and G. bulloides values from the western Arabian Sea are lower during the Middle Miocene compared to the Late Miocene9–11 (Fig. 1i, j), suggesting weakened SASM winds-induced oceanic upwelling in the western Arabian Sea during the Middle Miocene. Our African topography experiments can capture this feature in the wind-based proxies, showing significant weakened upwelling in the western Arabian Sea, particular in the MT25 and MT15 simulations compared to the pre-industrial simulation (Fig. 1g and Supplementary Fig. 4). We found that this weakened upwelling induced by African topography changes is due to the changes in both the strength and structure of Somali Jet (Supplementary Fig. 3 and 4), indicating the complex changes in Arabian Sea upwelling34–36.

In contrast, these wind-based proxies are challenged by rainfall-related proxies. For example, δ13C values in paleosols provide a direct measure of the relative contribution of grasses (C4 plants) and woodland or/and forest (C3 plants) to former vegetation, with C4 grasses favoring drier environment and C3 plants favoring moister environment. δ13C in paleosols from Siwalik Group sediments15,87 is relatively depleted before 7 Ma and more enriched after 7 Ma (Fig. 1k), suggesting that C3 trees dominated in the Middle Miocene while C4 grasses became more prevalent in the Late Miocene16,17. The δ18O of pedogenic carbonate from the Indian Siwalik sequences also indicates decreased rainfall in the Late Miocene15–17, coinciding with the rise of C4 plants (Fig. 1l). This indicates a hydroclimatic change that might have played a crucial role in the transition from C3 to C4 vegetation. Our simulations also capture this forest-to-grassland transition, showing less tree coverage and more grass coverage across the Indian continent and Himalayan foreland during the Late Miocene than those in the Early and Middle Miocene (Supplementary Fig. 13). Thus, our results suggest that the dramatic aridification of the Indian continent from the Middle and Late Miocene, driven by changes in African topography, may have been a key mechanism behind the expansion of C4 plants. This hypothesis is supported by other rainfall-related proxies. Reconstructions of chemical weathering and erosion budgets indicate that faster erosion, associated with more humid and warm climates in the Early to Middle Miocene, transitioned to less erosive and drier climates in the Late Miocene19–21 (Fig. 1m–o).

All these qualitative climate proxies for the Miocene, derived from various sources and locations, generally indicate weakened SASM-induced Arabian Sea upwelling and more humid conditions over South Asia during the Middle Miocene compared to the Late Miocene. This significant feature is well captured by our African topography experiments (Fig. 1d–h).

However, there are some uncertainties in proxy records that should be noted. For instance, while δ18O of soil carbonates is often interpreted as a precipitation proxy, it is also influenced by other factors, such as moisture source variability, wind system shifts, and temperature variations88. As for the wind proxies in western Arabian Sea, such as TOC and G. bulloides records, considered as indicators of monsoon winds, can also be influenced by local oceanographic processes and thermocline variability, adding complexity to their interpretation36. Additionally, most proxies come from specific regions (e.g., Arabian Sea, Siwaliks Hills), which may not fully capture spatial monsoon variability. Despite these uncertainties, the consistent multi-proxy changes support the conclusion that these proxies reliably reflect SASM evolution during the Miocene.

Wind-driven upwelling

The monsoon wind can induce ocean upwelling through “Ekman pumping” caused by wind-stress curl36,89. In this study, we calculate the Ekman pumping velocity using monthly wind stress data as follow:

WEkman = 1/(ρw f) ∇×τ (1)

where ρw is seawater density (1024 kg m-3), f is the Coriolis parameter, and ∇×τ is the wind-stress curl.

Latent heat of condensation

In this study, the latent heat of condensation (QL)90,91 is calculated to reveal the anomalous atmospheric cooling due to significant rainfall decrease. The formula of QL is as follows:

QL = -Lω∂qs/∂p, ω<0; 0, ω≥0 (2)

where L, ω, qs, and p indicates the latent heat of condensation, vertical velocity, saturation specific humidity, and pressure, respectively.

Decomposed atmospheric moisture budget

The atmospheric moisture budget is a widely used analysis to reveal the dominant process controlling the rainfall changes69,73,92, which can be written as:

P' = -ω'∂q/∂p - ω∂q'/∂p - V'∇q - V∇q' + E' + Residual (3)

where P, ω, q, p, V, and E denotes the precipitation, vertical velocity, specific humidity, pressure, and evaporation, respectively. The overbars represent the climatological mean in pre-industrial conditions, and the primes indicate deviations from this mean in sensitive experiments. The symbol ⟨ ⟩ indicates the vertical integration, and the Residual term includes the transient eddy and surface boundary effects. For tropical rainfall anomalies, previous studies indicate that the contributions of advection (⟨V'∇q⟩), evaporation (E') and Residual terms are relatively negligible70,93. Hence, Eq. (3) can be simplified as follows70,72,85:

P' ≈ -ω'⟨∂q/∂p⟩ - ω⟨∂q'/∂p⟩ (4)

δTh δDy

where the ω represent the vertical velocity at 500 hPa, and ⟨q⟩ is the vertical integrated atmospheric moisture through the column. Based on the simplified moisture budget equation, changes in SASM rainfall (P') can be decomposed into the effects of atmospheric moisture changes (thermodynamic term; δTh) and the atmospheric circulation change (dynamic term; δDy). Therefore, in this study, the sum of the δTh and δDy can be roughly considered as the diagnosed changes in summer rainfall over the SASM region.

LBM experiments

Compared to the complex Earth System models, the linear baroclinic model (LBM)94 includes only linear processes, providing a more accurate investigation of the atmospheric response to diabatic heating or cooling95,96. To verify the impact of significantly suppressed convection over tropical central Africa and the equatorial eastern Indian Ocean on SASM rainfall, we performed four LBM experiments based on climatological summer mean of the National Centers for Environmental Prediction (NCEP) reanalysis from 1980-2010, using a time integration method for 40 days. Perturbations from the basic state are regarded as the linear response to the forcing. We used an LBM version with 20 vertical sigma levels and a horizontal resolution of T21, taking the last 10 days as the steady-state solution.

In experiment 1 (Exp_1), we prescribe the idealized cooling in tropical central Africa (5° S-15° N; 20°-40° E) with a vertical profile following a gamma function, peaking at a cooling rate of 6 to 7 K day-1 at about 450 hPa, based on climatology from June to August (ref. 95). Experiments 2 (Exp_2) and 3 (Exp_3) follow the same methodology as in Exp_1, but applied heating over SASM region (5°-30° N; 60°-95° E) and cooling over tropical eastern Indian Ocean (5° S-5° N; 90°-110° E), respectively. Exp_4 combined diabatic cooling over tropical central Africa and eastern Indian ocean, and heating over SASM region. Indeed, Exp_4 (Supplementary Fig. 7d) reproduced the cyclonic and anticyclonic circulation anomalies over the northern Indian Ocean observed in fully coupled EC-Earth3 simulations (Fig. 1d, e), suggesting that suppressed rainfall in the tropics plays a crucial role in driving large-scale atmospheric circulation anomalies.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Han et al. documents the impacts of African topography on the South Asian summer monsoon winds and rainfall during the Miocene. The manuscript is generally well written and claims are substantiated by physical mechanisms. Overall, the mechanism suggested seems plausible. However, there are some significant sources of uncertainty (L338-346). These sources make me wonder whether present-day simulations by changing the orography would give similar results. I am by no means, an expert of the Miocene period. Therefore, my comments are restricted to the monsoon dynamics reported here. I would expect other reviewers to handle this aspect.\nMy primary concerns are listed below:\nMajor Comments:\n• Fig. 1 d-f:\nWhy 700 hPa? 850 hPa will be more appropriate here, also, please draw only significant vectors.\nThe major changes in rainfall are seen only over the oceanic parts in the windward side of the Western Ghats, and the changes over India are very small. The changes in the Somali jet's moisture-carrying capacity also influence the circulation over the Bay of Bengal and the associated land region due to changes in synoptic-scale disturbances and intra-seasonal oscillations. Such changes are not seen here; why?\n• Fig. 2 – IOD type bias – Cold SST anomalies over IOD east pole must result in suppressed rainfall over the region, the same is not clearly seen in Fig. 1 (d-f).\n• Impact on IOD type bias – Is lowering SLP in western Indian Ocean sufficient to justify the enhanced easterlies along the equator? The magnitude of cooling in the IOD east pole region seems to be almost of similar magnitude in all three simulations. Positive IOD-type conditions will possibly enhance the Indian monsoon rainfall in all the three simulations.\n• The impact of IOD dynamics on the model can imply a possible association with ENSO as well. The ENSO-IOD coupling is documented in the literature. Also show a global map of SSTs and indicate whether there is a change in SSTs globally.\n• Conventionally, the Indian Monsoon is impacted by the IOD by modulation of the local Hadley circulation. The discussion here seems to indicate that atmospheric variability is the stronger driver. Are there other observational studies substantiating this? It is most probably coupled variability. How well does the PI simulation capture the IOD-SASM relation?\n• What is the major objective of this study – documenting the impact of orography on SASM or reconciling the proxies? Since there are major changes in other forcing, as mentioned in L338-346, which may significantly add to the uncertainty, is it not better suited just to do present-day simulations with changes to orography?\n• Please confirm if only significant differences are shown in the figures.\nMinor comments\nL73 – other boundary forcings\nL147 – C4 \"plants\"", "reviewer_2": "The Somali Jet, a near-surface air current along the coast of the Horn of Africa, plays a central role in the South Asian summer monsoon (SASM) dynamics. It transports large amounts of moist air from the Indian Ocean to India, where it causes heavy rainfall. A more intense Somali Jet is usually assumed to lead to more precipitation in the SASM domain. For the Miocene, however, reconstructions have shown that these two components appear to be decoupled, as wind proxies show a strengthening of the Somali jet from Middle Miocene to late Miocene, but precipitation in the SASM domain decreased.\nHan et al. employ the EC-Earth3 model to assess the impact of changes in the African topography during the Miocene on this decoupling and the atmospheric and ocean dynamics and the South Asian monsoon precipitation, in general. To this end, they carry out simulations in which they keep all boundary conditions on the pre-industrial settings, but prescribe the African topography in line with reconstructions.\nAccording to the model, the topography is important in two respects: First, due to the lower East African Highlands during Early Miocene, the Somali Jet is less canalized and therefore much weaker. This in turn has the effect of inducing a positive Indian Ocean Dipole-like SST pattern, which leads to more precipitation in the SASM domain and a positive feedback with the circulation. In the course of the Miocene, the East African Highlands are lifted up, which more and more intensifies the jet, explaining the signal seen in the wind-proxies.\nSecond, A higher Congo Basin leads to less ascent and less latent heat release into the atmosphere. This promotes the formation of a cyclonic circulation anomaly above the Arabian sea, redirecting moisture to the Indian continent. The effect of changes in latent heat release on the atmospheric circulation in distinct regions is further investigated by employing a linear baroclinic model.\nIn addition, Han et al. perform simulations with increased (based on reconstructions) CO2 levels and with both forcings (CO2 and topography changes).\nDecomposing the precipitation response into the effect of atmospheric moisture change (thermodynamic) and atmospheric circulation changes (dynamic response) shows that topography changes purely leads to more precipitation due to changes in the circulation, whereas the CO2 effect has a strong thermodynamic contribution to the precipitation change due to its impact on the atmospheric temperature.\nThis study highlights the effect of the African topography in the SASM evolution and underlines the complexity of the South Asian monsoon dynamics. The figures in the main part are well-chosen and helpful, and are also mostly well presented and easy to understand. The text is clearly and comprehensibly formulated and the individual parts seem well thought out. Methods are carefully applied. The main criticism I have is that I missed the “overarching framework”. The individual sub-chapters don't always seem to build on each other and you get the impression that Han et all lose focus on their core topic and their aim of the study in the course of the manuscript. This means that the paper is not easily comprehensible to a wider readership. However, I believe that this can be solved and therefore suggest a publication in Nature Communication after major revisions.\nIn detail:\na) For me, not knowing much about the Miocene so far and therefore not being familiar with the literature, it is not clear what is really new about the study and what is the highlight of the study. According to the title, the main topic is the influence of African topography on the decoupling of wind and precipitation changes during the Miocene. In my opinion, there is no clear final answer to this research question in the study. For instance, in the abstract as main (and only) reason for the circulation and precipitation changes, the lower East African Highlands are mentioned. In contrast, large parts of the result part deal with the effect of the higher Congo basin on the circulation. Later in the text, the CO2 experiments are analyzed, but the results neither enter the abstract nor the conclusion. In the conclusion, the East African highland and Congo basin topography changes are mentioned as explanation for the weakening of the Somali Jet and the feedbacks with the precipitation, which is not directly clear and confusing, because the linear baroclinic model (used for the explanation of the effect of topography changes in the Congo basin) show no weakening of the Somali Jet in any simulation. This conclusion is therefore imprecise. Not giving a clear answer to the main research question, makes the individual parts of the analysis seem more like, we did this and then that, rather than being able to recognize a clear common line in it. This is a pity, because the individual parts are important and interesting. I would like the reader to be guided more through the work, and the rationale for conducting the individual analyses to be explained more and the work to be structured more (and more clearly). Particularly the Abstract and the Conclusion should be revised.\nb) In addition, the author explain the reduction of the Somali Jet via the reduction of the channel effect by the lower East African Highlands. Just looking at the circulation changes in Fig.1, showing a cyclonic circulation anomaly during the Early and Mid Miocene compared to pre-industrial, the pattern does not really look like a reduction in the channel effect. It looks like a thermally induced dynamic response to the decreased SST in the north-western Arabian sea.\nThe CO2-forced simulation shows a stronger Somali Jet compared to the topography forced simulation. Does this imply that in the CO2 simulations the wind and precipitation changes are not decoupled? And that the decoupling is only induced by the topography changes, overcompensating the CO2 effect in the “real world”? Please clarify.\nc) With respect to this, to really understand how the changes in African topography affect the circulation and precipitation pattern, sensitivity studies are necessary in which either the changes in the Congo basin or the changes in the East African Highlands are prescribed, i.e. simulations in which e.g. the Congo basin elevation is increased, but the East African Highlands are prescribed on pre-industrial settings.\nd) What do we know about the climate in the Miocene? About the drivers? Other (but not applied) changes in the boundary conditions are only mentioned in the conclusion and discussion. For example, the Western Ghats were much lower, which probably also had an impact on the circulation and the precipitation in India. The Tibetan Plateau was probably only half as high. Today it acts as a strong barrier and the vertical ascending on the southern site is very strong, leading to large amounts of precipitation. In addition, it plays a very important role for the atmospheric circulation, also via the influence on the subtropical jet. So I suspect that there might be more reasons why the wind-precipitation-decoupling may have occurred and the climate was like as seen in the reconstructions. Please discuss this in more detail. It would be nice if this was already mentioned in the introduction and it was made clear that (presumably) the topographical changes and the higher CO2 levels were the most prominent drivers and therefore only these forcings were used and it is now being investigated how large the proportion of orography is in comparison with the CO2 forcing proportion. Maybe it would also be an idea to present the orography and CO2 simulations together, to work out that the topography changes are more important. In the current version of the paper, the authors prescribe topography changes and the main results is that topography changes are important. But this is already a predetermined result, when topography is the only forcing of the model that have been prescribed.\ne) A large part of the result chapter compares the model with data. For me, this is done very “uncritically”. Although the reconstructions of wind and precipitation agree quite well with the model, the proxies experienced a completely different world than that in the idealised model simulation (see above). I also do not understand, why the reconstructions are at least not compared with the simulation including both forcing (CO2 and topography changes).\nAnd I also miss a critical (at least short) discussion on the uncertainties in the proxies. For instance, d18O is often used as a proxy for precipitation, but it is also very dependent on other things (moisture source, wind system, temperature). I understand why Han et al. compare the simulations with reconstructions, but if it is to be part of the results, then a more critical discussion would be appropriate. Perhaps it would otherwise be an idea to include the evaluation of the model simulation in the methods (e.g. in the model and simulation description).\nf) Please revise the supplementary figures. The captions and the heading of subfigures are not always clear and correct. I listed a few issues in my specific comments below, but please check all figures carefully before re-submission.\nSpecific comments\nAbstract\n- L33-L38: Please be more precise and refer to all results (see comment (a)). For instance, “blocking effect” is misleading here, as it is rather the opposite, a reduction of the blocking effect.\nIntroduction\nL66: “weakening of the SASM”: As in this study, changes in wind and precipitation are discussed, this term is not clear. It could be both, a weakening in the wind and a weakening in the precipitation.\nResults:\nL105: it is: decoupling of monsoon rainfall…\nL166-L169: This sentence should be moved to the Introduction.\nL177-178: “in western Arabian Sea (Fig.1) is written in italics\nL183: \"These circulations\", not \"There circulations\"\nL211-215: Since it is a coupled system, it is difficult to say what triggers the descending air anomaly. The vertical movement in models is often a result of the convection calculation. So it could well be that the weaker West African monsoon leads to less precipitation, e.g. because less moisture is transported to the region and the lower precipitation is causing/accompanied by weaker upscent and less latent heat release. What about the temperature and pressure at the surface? Maybe this would be another (better) line of argumentation.\nL216: I’m not that familiar with Kelvin waves, but I think Kelvin waves are usually confined to the equatorial region (~10°S to ~10°N). I am also not sure whether a reduction in the latent heat release causes a Kelvin wave response, or whether, for example, the amplitude of the Kelvin wave simply changes. Please check this with the literature.\nL243: ...seen in the LBM experiment results. The “seen in” is missing.\nL282: Please add a reference to Gill’s theory.\nL319: … using THE fully coupled EC-Earth3 model.\nL328: This conclusion is imprecise, because the LMB model does not show a weakening of the Somali Jet. Please rephrase!\nL344-346: I find it unfavorable to use this sentence as the final sentence. It implies that the evolution of the SASM was the main motivation for the study. This is not clear in the context of the study and is also not clear from the title. Instead, I would like to see a final answer to the reasons for the decoupling and maybe a statement what it means, e.g. for present or future climate. It is not clear from the study which effect predominates, but climate models project strong changes in the precipitation pattern over Africa. Is it possible that we will experience a decoupling of the Somali Jet and SASM precipitation again in the future, due to changes in the latent heat release in Africa?\nMethods:\nL357: which IS the Nucleus for ...\nL454: “cooling rate of 6 to 7 K/day”... Does it mean that it gets cooler every day? I do not understand this.\nFigures:\ngeneral: I guess units are always written in rectangular bracket, e.g. [m/s], rather than (m/s).\nFig.1: Maybe flip the time-axis in the reconstruction plot so that it is in line with the other plots in this figure. Maybe you could additionally mark or explain in the caption which proxies in i) to o) indicate wind and which indicate precipitation.\nFig.1: Have the precipitation anomalies been tested for significance?\nFig.1: Why do you use 700hPa level and in other plots 1000hPa or 950hPa or 850hPa? Please check if you use this level for each plot referring to the Somali Jet to facilitate the comparison.\nFig.4: The unit for e) is missing… Is e) also showing the difference between MC and MT simulations? It would also be helpful to plot the area mean changes in the Somali Jet, similar to Fig.1\nFig.5a: I’m wondering, if the cyclonic circulation arrows have the wrong position. The winds are directed to the West Ghats and do not flow in western direction over India (cf. Fig.1). Similarly, the arrow for suppressed convection is located over the African Highlands but not over the Congo basin...\nSupplement:\nGeneral: Please check the spelling in all figure captions.\nFig.1: Here, 850hPa winds are shown, why not consistent to the other plots (700hPa). Why are wind vectors shown in the simulation but not in the observation? They experience the same orography…\nFig 2: Please add a note that only African changes in topography are shown here.\nFig.4: Which levels were used to integrate the moisture flux?\nFig.6: Is K/day the correct unit for Q?\nFig.8: Which levels have been used for the integration of the water vapor flux? Do the plots b-c really show anomalies to MT? This would mean that the CO2 has an even stronger dynamic response than seen for the topography change.\nFig.9: This plot also show the 850hPa while other plots use 700 hPa. It would be easier to compare the figures and the effect of the different forcings if the levels were standardized. Also for this plot it would be nice to add the area-mean change in the Somali-Jet intensity (cf. Bar plot in Fig.1)\nFig.9: The headings of the sub-figures are not correct.", "reviewer_3": "Please find attached my comments."}, "author_response": "Reply to Reviewer #1’s comments\nRemarks to the Author:\nThe manuscript by Han et al. documents the impacts of African topography on the\nSouth Asian summer monsoon winds and rainfall during the Miocene. The manuscript\nis generally well written and claims are substantiated by physical mechanisms.\nOverall, the mechanism suggested seems plausible. However, there are some\nsignificant sources of uncertainty (L338-346). These sources make me wonder\nwhether present-day simulations by changing the orography would give similar\nresults. I am by no means, an expert of the Miocene period. Therefore, my comments\nare restricted to the monsoon dynamics reported here. I would expect other reviewers\nto handle this aspect.\nMy primary concerns are listed below:\nOur response: We thank the reviewer for the kind consideration and constructive\ncomments on our manuscript. The original comments are quoted in blue. We have\ncarefully revised the manuscript and provided the point-by-point response below. The\nchanges in the revised manuscript have been highlighted in red. We hope these\nchanges will strengthen our manuscript.\nGeneral Comments:\n#1. Fig. 1 d-f:\nWhy 700 hPa? 850 hPa will be more appropriate here, also, please draw only\nsignificant vectors.\nThe major changes in rainfall are seen only over the oceanic parts in the windward\nside of the Western Ghats, and the changes over India are very small. The changes in\nthe Somali jet's moisture-carrying capacity also influence the circulation over the Bay\nof Bengal and the associated land region due to changes in synoptic-scale\ndisturbances and intra-seasonal oscillations. Such changes are not seen here; why?\nOur response:\n1\n(1) We appreciate the reviewer’s insights regarding the choice of 700 hPa for wind\nvectors. After careful reconsideration, we have decided to retain the 700-hPa wind\nin Figure 1 in the manuscript for the following reasons:\n• The primary weakening of the meridional cross-equatorial flow due to the\nlowering of the East African Highlands is clearly captured at both the 700-hPa\n(Figure R1a-c) and 850-hPa wind patterns (Figure R1d-f). However, the\nthermally induced cyclonic circulation anomalies over the Arabian Sea, which\nsignificantly contribute to increased SASM rainfall, are more prominent at 700\nhPa.\n• This anomalous cyclonic circulation is directly linked to suppressed convection\nover tropical central Africa (Supplementary Figure 5 and 7 in the Supplementary\nmaterials). This is further intensified by the Rossby wave induced by increased\nSASM rainfall (Supplementary Figure 8 in the Supplementary materials). These\ndynamically driven feedbacks are more evident in the 700-hPa wind (Figure R1a-\nc), while the signal is weaker at 850-hPa because of the influence of\ntopographical changes in lower atmospheric levels (Figure R1d-f).\nTo address the reviewer’s concern, we have now included the corresponding 850-hPa\nwind as Supplementary Figure 3 in the revised Supplementary materials for\ncomparison. We have also clarified this in Lines 128-130 of the revised manuscript.\n(2) The initial figure used an inappropriate color bar, made land rainfall changes\nappear less prominent. In response, we have adjusted the color bar scale to better\ncapture these variations and conducted a Student’s t-test to assess statistical\nsignificance (Figure R1a-c). This reveals that significant changes occur not only\nover the Arabian Sea but also over the Indian Continent and Bay of Bengal.\nAccordingly, we have updated Figure 1d-f in the revised manuscript, ensure that\nall displayed wind vectors and rainfall changes are statistically robust.\nat 700 hPa (vectors, m s-1) in the a MT25, b MT15 and c MT05 simulations compared\nto pre-industrial simulations. d-f Same as a-c, but for the changes in low-level\natmospheric circulation at 850 hPa (vectors, m s-1). Gray stippling and vectors are\nonly shown at regions where the changes are significant at the 95% confidence level\naccording to Student’s t-test.\n#2. Fig. 2 – IOD type bias – Cold SST anomalies over IOD east pole must result in\nsuppressed rainfall over the region, the same is not clearly seen in Fig. 1 (d-f).\nOur response: We appreciate the reviewer’s careful observation regarding the\nexpected rainfall suppression over the east pole of Indian Ocean dipole (IOD)-like\npattern. In the initial submission, this effect was not clearly visible due to the\ninappropriate color bar that we chose in Figure 1d-f. To address, we have replotted\nFigure 1d-f (see Figure R2a-c) with: a more color scale to better capture spatial\nvariations in rainfall; and student’s t-test for both rainfall and wind fields to highlight\nstatistically significant changes.\nclearly visible over the eastern IOD pole, especially in Early and Middle Miocene\nsimulations.\nTo further validate this, we analyzed additional key variables associated with\nconvective activity. (1) Outgoing longwave radiation (OLR) (Figure R2d-f): Higher\nOLT values over the eastern IOD pole indicate reduced cloud cover and suppressed\nconvection. (2) Moisture condensation (Figure R2g-i): Reduced latent heat release\nover the same region confirms a decrease in convective activity and precipitation.\nThese results consistently demonstrate that the cold SST over the IOD-like east\npole indeed result in significant rainfall suppression during the Miocene simulations.\nWe have updated Figure 1d-f in the revised manuscript to include the corrected color\nscale and significance test.\ntopography changes during boreal summer. Changes in rainfall (shading, mm day-1)\nlow-level atmospheric circulation at 700 hPa (vectors, m s-1) in the a MT25, b MT15\nand c MT05 simulations compared to pre-industrial simulations. d-f Same as a-c, but\nfor the changes in outgoing longwave radiation (OLR, W m-1). g-i Same as a-c, but\nfor the changes in moisture condensation (Q ; shading, 10-2 m2 s-3; see Methods for\nL\ndetails) at 500 hPa. Gray stippling and vectors are only shown at regions where\nthe changes are significant at the 95% confidence level according to Student’s t-test.\n5\n#3. Impact on IOD type bias – Is lowering SLP in western Indian Ocean sufficient to\njustify the enhanced easterlies along the equator? The magnitude of cooling in the\nIOD east pole region seems to be almost of similar magnitude in all three simulations.\nPositive IOD-type conditions will possibly enhance the Indian monsoon rainfall in all\nthe three simulations.\nOur response: This is really good and important question. To address your questions,\nwe conducted a more detailed analysis of the monthly evolution of sea surface\ntemperature (SST), wind anomalies and sea level pressure (SLP), which is now\npresented in Figure R3 and R4.\nGiven the importance of positive SST anomalies in the western equatorial Indian\nOcean for establishing positive ocean-atmosphere feedbacks, we further examined the\nseasonal cycle of SST and the Somali Jet in this region. Our findings indicate that\nSST changes in the western Indian Ocean are closely linked to variations in the\nintensity of the Somali Jet during the Miocene (Figure R3). Specifically, a weakened\nSomali Jet leads to surface ocean warming by changing surface heat fluxes, consistent\nwith findings from previous studies (Schott et al. 2009; Li et al., 2015). This warming\nemerges in the pre-monsoon months and intensifies during boreal summer when the\nSomali Jet is typically strongest.\nThe seasonal variation presented in Figure R4 confirm that positive SST\nanomalies over the western Indian Ocean significantly decrease SLP, thereby\ninducing easterly wind anomalies along the equator. Specifically, warm SST\nanomalies appear in the western equatorial Indian Ocean by June, accompanied by\nsignificant easterly wind anomalies extending from west to central Indian Ocean. In\nthe following months, this warming intensifies and easterly wind anomalies migrate\nfurther eastward, while the southeastern tropical Indian Ocean begins to cool down.\nThese processes are particularly pronounced in the Early and Middle Miocene when\nthe East African Highlands were much lower compared to the Late Miocene (Figure\nR4). As expected, our analysis demonstrates that IOD-like warming patterns are\nstronger in the Early and Middle Miocene compared to the Late Miocene. This\n6\nsupports the conclusion that lowered East African Highlands played a key role in\namplifying positive IOD-like conditions, which in turn enhanced monsoon rainfall.\nWe have clarified these findings in Lines 261-269 of the revised manuscript and\nincluded the Figure R3 and Figure R4 as Supplementary Figure 9 and Figure 10 in the\nrevised Supplementary materials, respectively. Thank you for guiding us toward a\nmore robust and detailed analysis.\nReference:\nSchott, F.A., Xie, S.P. and McCreary Jr, J.P., 2009. Indian Ocean circulation\nand climate variability. Reviews of Geophysics, 47(1).\nLi, G., Xie, S.P. and Du, Y., 2015. Monsoon-induced biases of climate models\nover the tropical Indian Ocean. Journal of Climate, 28(8), pp.3058-3072.\nFigure R3. Strong coupling of the intensity of Somali Jet and sea surface temperature\n(SST) over the western Arabian Sea. Seasonal variation of changes in intensity of\nSomali Jet (m s-1; see Methods) and SST (℃) over tropical western Indian Ocean (10°\nAugust).\nFigure R4. Seasonal variation.Changes in SST (shading, ℃), sea level pressure\n[contours; hPa; solid (dashed) lines represent the positive (negative) values], and\nsurface wind (vectors, m s-1) from May to September in the MT25 (left column),\nMT15 (middle column) and MT05 (right column) simulations compared to pre-\nindustrial simulation. Gray stippling and vectors are only shown at regions where\nchanges are significant at the 95% confidence level according to Student’s t-test.\n8\n#4. The impact of IOD dynamics on the model can imply a possible association with\nENSO as well. The ENSO-IOD coupling is documented in the literature. Also show a\nglobal map of SSTs and indicate whether there is a change in SSTs globally.\nOur response: We appreciate the reviewer’s thoughtful comment regarding the\npotential link between the IOD and ENSO in our model simulations. We acknowledge\nthat the IOD-ESNO relationship has been extensively studies in the context of\ninterannual climate variability (Saji et al., 1999; Webster et al., 1999; Cai et al., 2013,\n2019). However, it is important to clarify the distinction between the IOD as an\ninterannual phenomenon and the IOD-like warming pattern observed in our study,\nwhich reflects long-term changes in the mean state of the ocean-atmosphere system\ninduced by African topography changes.\nThe positive IOD is initiated by enhanced ocean upwelling along the Java and\nSumatra coasts, leading to cooler SSTs in the eastern Indian Ocean and warmer in the\nwestern Indian Ocean, accompanying by easterly wind anomalies over the tropical\nIndian Ocean (Figure R5). In our simulations, we observe a similar west-warming and\neast-cooling SST pattern, but it is not a manifestation of interannual IOD variability.\nInstead, it represents a tectonically forced, long-term change in the climate mean state\ndue to altered African topography (Figure R6). Our simulations, which span several\nhundred years, reach a quasi-equilibrium state, and we analyze the climatological\nmean of the last 100 years. Therefore, we refer to this pattern as positive IOD-like\nwarming pattern, distinct from interannual varying IOD events.\nThe interaction between IOD and ENSO has been widely studied, with many\nstudies suggesting that IOD events have been remotely forced by ENSO variability in\nthe Pacific (Xie et al. 2002; Annamalai et al. 2003; Fischer et al. 2005; Zhang et al.\n2015; Fan et al. 2017; Liu et al. 2017; Stuecker et al. 2017). However, some IOD\nevents, for example, the event in 1961, are believed to have been the result of local\nair–sea interactions in the tropical Indian Ocean, independent of ENSO forcing (Saji\net al. 1999; Webster et al. 1999; Ashok et al. 2003; Li et al. 2003; Rao and Behera\n2005; Wang et al. 2016). Additionally, modeling and observational studies suggest\nthat ENSO is not necessarily a prerequisite for IOD events (Fischer et al. 2005;\npattern anomalies as mentioned above, rather than the interannual variability of IOD\nevents. Examining the relationship between ENSO and IOD on tectonic time-scales is\na fascinating topic but falls beyond the scope of our current work. We acknowledge\nthat further investigations are required to explore the possible interaction between\nAfrican topography changes, IOD dynamics, and ENSO over geological time scales,\nand we propose this as a potential avenue for future research.\nIn summary, the IOD-like warming pattern in our study is the change in the\nclimate mean state induced by altered topography but not the IOD of interannual\nvariability. This is clarified in Lines 252-253, 274-276 of revised manuscript. And we\nhave further conducted a full text check, ensure consistent terminology throughout the\ntext, i.e., IOD-like warming pattern instead of IOD pattern, to avoid\nmisunderstandings.\nRegarding global SSTs, remarkably changes are observed in the Northern\nHemisphere, particularly in the North Atlantic (Figure R7). These anomalies may be\nattributed to the fluctuations in the Atlantic Meridional Overturning Circulation\n(AMOC). EC-Earth3 simulations exhibit multi-centennial climate variability, which is\nmost pronounced in the North Atlantic region due to fluctuations in AMOC (Figure\nR8). This phenomenon was identified in our previous study (Cao et al., 2023).\nAdditionally, our recent research indicates that the impact of this multi-centennial\nvariability on subtropical monsoons, such as the SASM, may be less significant\ncompared to its effect on northern high latitudes (Han et al., 2024). However,\nexamining the global SSTs is another fascinating topic but falls beyond the scope of\nour current work. Nonetheless, we have conducted sensitivity tests (Figure not\nshown), examining the last 50 years of the model output, and found that our\nconclusions remain robust and consistent across these different sampling lengths.\n1000-hPa wind (vector, m s-1 ℃-1) from National Centers for Environmental Prediction\nreanalysis 2 (NCEP2) on summer Indian Ocean Dipole (IOD) index from 1980 to 2010.\nThe IOD index is defined as the SST anomaly difference between the regions of 50º-\n70º E, 10º S-10º N and 90º E-110º E, 10º S-0º (Saji et al., 1999). Shading and black\nvectors denote regions in which the anomalies are significant at the 95% confidence\nlevel according to Student’s t-test.\nthe MT25, MT15 and MT05 simulations compared to pre-industrial simulations,\nrespectively. Gray stippling and vectors denote regions in which the changes are\nsignificant at the 95% confidence level according to Student’s t-test.\nsimulations compared to pre-industrial simulations, respectively. Gray stippling and\nvectors denote regions in which the changes are significant at the 95% confidence\nlevel according to Student’s t-test.\nFigure R8. Changes in AMOC index (Sv) in the MT25, MT15, MT05 and pre-\nindustrial simulations.\nAnnamalai, H., R. Murtugudde, J. Potemra, S. P. Xie, P. Liu, and B. Wang,\n2003: Coupled dynamics over the Indian Ocean: Spring initiation of the zonal mode.\nDeep-Sea Res. II, 50, 2305–2330.\nAshok, K., Z. Y. Guan, and T. Yamagata, 2003: A look at the relationship\nbetween the ENSO and the Indian Ocean dipole. J. Meteor. Soc. Japan, 81, 41–56.\nCai, W., Wu, L., Lengaigne, M., Li, T., McGregor, S., Kug, J.-S., Yu, J.-Y.,\nStuecker, M.F., Santoso, A., Li, X., Ham, Y.-G., Chikamoto, Y., Ng, B., McPhaden,\nM.J., Du, Y., Dommenget, D., Jia, F., Kajtar, J.B., Keenlyside, N., Lin, X., Luo, J.-J.,\nMartínRey, M., Ruprich-Robert, Y., Wang, G., Xie, S.-P., Yang, Y., Kang, S.M., Choi,\nJ.-Y., Gan, B., Kim, G.-I., Kim, C.-E., Kim, S., Kim, J.-H., Chang, P., 2019.\nPantropical climate interactions. Science 363, eaav4236.\nCai, W., Zheng, X.-T., Weller, E., Collins, M., Cowan, T., Lengaigne, M., Yu, W.,\nYamagata, T., 2013. Projected response of the Indian Ocean Dipole to greenhouse\nwarming. Nat. Geosci. 6, 999e1007.\nCao, N., Zhang, Q., Power, K.E., Schenk, F., Wyser, K. and Yang, H., 2023. The\nrole of internal feedbacks in sustaining multi-centennial variability of the Atlantic\nMeridional Overturning Circulation revealed by EC-Earth3-LR simulations. Earth\nand Planetary Science Letters, 621, 118372.\nDrbohlav, H. K. L., S. Gualdi, and A. Navarra, 2007: A diagnostic study of the\nIndian Ocean dipole mode in El Niño and non–El Niño years. J. Climate, 20, 2961–\n2977.\nFan, L., Q. Y. Liu, C. Z. Wang, and F. Y. Guo, 2017: Indian Ocean dipole modes\nassociated with different types of ENSO development. J. Climate, 30, 2233–2249.\nFischer, A. S., P. Terray, E. Guilyardi, S. Gualdi, and P. Delecluse, 2005: Two\nindependent triggers for the Indian Ocean dipole/zonal mode in a coupled GCM. J.\nClimate, 18, 3428–3449.\nHan, Z., Power, K., Li, G. and Zhang, Q., 2024. Impacts of Mid‐Pliocene Ice\nSheets and Vegetation on Afro‐Asian Summer Monsoon Rainfall Revealed by EC-\nEarth Simulations. Geophysical Research Letters, 5, p.e2023GL106145.\nOcean dipole-zonal mode. J. Atmos. Sci., 60, 2119–2135.\nLiu, L., G. Yang, X. Zhao, L. Feng, G. Q. Han, Y. Wu, and W. D. Yu, 2017: Why\nwas the Indian Ocean dipole weak in the context of the extreme El Niño in 2015? J.\nClimate, 30, 4755–4761.\nRao, S. A., and S. K. Behera, 2005: Subsurface influence on SST in the tropical\nIndian Ocean: Structure and interannual variability. Dyn. Atmos. Oceans, 39, 103–\n135.\nSaji, N.H., Goswami, B.N., Vinayachandran, P.N. and Yamagata, T., 1999. A\ndipole mode in the tropical Indian Ocean. Nature, 401(6751), pp.360-363.\nStuecker, M. F., A. Timmermann, F. F. Jin, Y. Chikamoto, W. J. Zhang, A. T.\nWittenberg, E. Widiasih, and S. Zhao, 2017: Revisiting ENSO/Indian Ocean dipole\nphase relationships. Geophys. Res. Lett., 44, 2481–2492.\nWang, H., R. Murtugudde, and A. Kumar, 2016: Evolution of Indian Ocean\ndipole and its forcing mechanisms in the absence of ENSO. Climate Dyn., 47, 2481–\n2500.\nWebster, P. J., A. M. Moore, J. P. Loschnigg, and R. R. Leben, 1999: Coupled\nocean–atmosphere dynamics in the Indian Ocean during 1997–98. Nature, 401, 356–\n360.\nXie, S. P., H. Annamalai, F. A. Schott, and J. P. McCreary, 2002: Structure and\nmechanisms of south Indian Ocean climate variability. J. Climate, 15, 864–878\nZhang, W. J., Y. L. Wang, F. F. Jin, M. F. Stuecker, and A. G. Turner, 2015:\nImpact of different El Niño types on the El Niño/IOD relationship. Geophys. Res.\nLett., 42, 8570–8576.\n#5. Conventionally, the Indian Monsoon is impacted by the IOD by modulation of the\nlocal Hadley circulation. The discussion here seems to indicate that atmospheric\nvariability is the stronger driver. Are there other observational studies substantiating\nthis? It is most probably coupled variability. How well does the PI simulation capture\nthe IOD-SASM relation?\nsimulation captures the IOD-SASM relationship, we performed a regression analysis\nof summer SST, 850-hPa wind and rainfall on summer IOD index for both\nobservation data and the PI simulation. The results (Figure R9) show that the PI\nsimulation successfully replicates the observed IOD pattern, which is characterized by\ncooler SSTs off Sumatra and warmer SSTs in the western Indian Ocean, accompanied\nby easterly wind anomalies over tropical Indian Ocean. Increased rainfall over the\nwestern Indian Ocean and decreased rainfall over the eastern tropical Indian Ocean,\nconsistent with the modulation of atmospheric convection (Webster et al., 1999). In\naddition, according to Gill’s theory (Gill, 1980), the suppressed atmospheric\nconvection over the eastern Indian Ocean produces a pair of anticyclonic circulation\nanomalies in the off-equatorial region, indicating a Rossby wave response. This\nfeature is evident in both observation and the PI simulation. These results confirm that\nEC-Earth3 model can capture the interannual variability of the IOD and its impact on\nSASM.\nAs discussed in response to comment 4, our study focuses on the changes in\nclimate mean state driven by African topography changes, rather than the interannual\nvariability of IOD. While our IOD-like warming pattern shares some similarities with\nthe interannual IOD (e.g. west-warming, east-cooling SST pattern), the underlying\nmechanisms are distinct.\nTo address the reviewer’s concern about the impact of IOD-like warming on the\nlocal Hadley circulation, we further examined changes in meridional circulation. Our\nresults (Figure R10) indicate that the local Hadley circulation was significantly\nstronger during the Early and Middle Miocene when compared to the Late Miocene.\nThis strengthening can be attributed to two mechanisms: (1) The southerly wind\nanomalies in the central Indian Ocean were much stronger during the Early and\nMiddle Miocene, enhancing low-level moisture transport toward the SASM region\n(Figure R6). (2) The weakening of the Somali jet linked to the African topography\nchanges, and reduced rainfall over tropical Africa contributed to intensified cyclonic\ncirculation in the Arabian Sea (Figure R1a-c). This, in turn, enhance the low-level\n16\nmeridional circulation, favoring a strengthen local Hadley circulation in the Indian\nOcean.\nFigure R9. a Regression of summer SST (shading, ℃ ℃-1) from the Hadley Centre,\n850-hPa wind (vector, m s-1 ℃-1) from NCEP2 and rainfall (contour, mm day-1 ℃-1;\nsolid lines represent positive values) from the Global Precipitation Climatology\nProject (GPCP) on summer IOD index from 1980 to 2010. b Same as a, but for the\nvariable regression in pre-industrial simulations of EC-Earth3 model. The IOD index\nis defined as the SST anomaly difference between the regions of 50º-70º E, 10º S-10º\nN and 90º E-110º E, 10º S-0º (Saji et al., 1999). Shading, contours and black vectors\ndenote regions in which the anomalies are significant at the 95% confidence level\naccording to Student’s t-test.\nmeridional circulation induced by African topography changes during summer. The\nclimatologies in meridional (m s-1) and vertical (-10-2 Pa s-1) winds (vectors) in the\npre-industrial simulations and the changes in vertical velocity (color shaded; -10-2 Pa\ns-1) in the a MT25, b MT15 and c MT05 simulations relative to the pre-industrial\nsimulation.\nReference:\nGill, A. E. Some simple solutions for heat-induced tropical circulation. Quart. J.\nRoyal. Meteoro. Soc. 106, 447–462 (1980).\ndipole mode in the tropical Indian Ocean. Nature, 401(6751), pp.360-363.\nWebster, P. J., A. M. Moore, J. P. Loschnigg, and R. R. Leben, 1999: Coupled\nocean–atmosphere dynamics in the Indian Ocean during 1997–98. Nature, 401, 356–\n360.\n#6. What is the major objective of this study – documenting the impact of orography\non SASM or reconciling the proxies? Since there are major changes in other forcing,\nas mentioned in L338-346, which may significantly add to the uncertainty, is it not\nbetter suited just to do present-day simulations with changes to orography?\nOur response: Thank you for raising this important point, as it allows us to clarify\nthe primary objective of our study and the rational behand out approach.\nThe main objective of this study is to document the impact of African orography\non SASM evolution during the Miocene, rather than simply reconciling proxy\nrecords. Our aim is to isolate and quantify the role of African topography changes in\nshaping the Somali Jet, monsoon circulation, and associated ocean-atmosphere\nfeedbacks, which remain poorly understood despite their potential significance.\nWe appreciate that multiple boundary forcings impacted the SASM changes\nduring the Miocene, including the uplift of the Himalaya-Tibetan Plateau, tectonic\nchanges in the Anatolian-Iranian plateau, emergence of land in Eastern Arabian\nPeninsula, and development of polar ice sheets and CO fluctuations. However, while\n2\nthese factors played a role, previous studies suggest that African topography was a\nkey driver of SASM evolution. Recent study of Sarr et al. (2022) found that the\nHimalayan and Tibetan Plateau topography influenced early Miocene rainfall\npatterns, its impact on ocean–atmosphere circulation was relatively limited. This\naligns with the evidence indicating that the Himalaya-Tibet uplift was largely\ncomplete by ~20 Ma (Ding et al., 2022), suggesting that its primary influence on\nSASM was in early Miocene or before (Ding et al., 2017; Clif and Webb, 2019;\nTomson et al., 2021). Additionally, Yao et al. (2023) emphasize that interhemispheric\nice-sheet growth has weakened the SASM through atmospheric anomalies since the\nMiddle Miocene. However, their studies cannot explain the strengthening of the wind-\n19\nbased proxies in the western Arabian Sea. Therefore, the discrepancies between\noceanic wind-based records (Kroon et al., 1991; Huang et al., 2007; Gupta et al.,\n2015) and rain-related records (Quade et al., 1989; Quade and Cerling, 1995;\nQuade et al., 1995; Clift 2006; Clift et al., 2008; Lee et al., 2020) suggest that other boundary\nforcings may also play a crucial role in the multi-stage evolution of the SASM system\nduring the Miocene.\nIn contrast, the gradual uplift of the East African highlands and subsidence of the\nCongo Basin occurred between the Middle Miocene to Late Miocene (Moucha and\nForte, 2011), a period that coincides with the onset of a “modern-like” SASM around\nZhuang et al., 2017; Bialik et al., 2020). These findings strongly suggest that the\nevolution of African topography during the Miocene played a critical role in\nreorganizing the Somali Jet and monsoon rainfall patterns during this period.\nIn our study, we test the hypothesis that the impact of African topography\ninfluenced the multi-stage evolution of the SASM system during the Miocene. Using\na high-resolution version of the coupled EC-Earth3 model, we simulate the climate\nresponse to evolution of Miocene African topography based on reconstructed African\ndynamic topography (Moucha and Forte, 2011). By isolating the single boundary\ncondition of African topography change, we can effectively separate its impact from\nother influences on the SASM. Utilizing the actual reconstructed African topography\nallows us to identify potential drivers of some phenomena observed in reconstructed\nproxies. Our results, as anticipated, demonstrate a clear decoupling of monsoon winds\nand rainfall, supporting the findings of Sarr et al. (2022). The key novelty of our study\nis the identification of a previously overlooked tectonic mechanism driving the\ndecoupling of SASM winds and rainfall during the Miocene.\nWe have expanded our discussion of various boundary conditions in the\nintroduction section, emphasizing that the African topography may be the most\nprominent driver. Consequently, this forcing is the primary focus of our investigation.\nThe clarification can be found in Lines 72-76 of the revised manuscript. In addition,\n20\nwe also revised the Conclusion (Lines 356-375) to emphasize the novel mechanistic\ninsights uncovered in our study.\nReference:\nBialik, O. M. et al. Monsoons, upwelling, and the deoxygenation of the\nNorthwestern Indian ocean in response to middle to late miocene global climatic\nshifs. 2020. Paleoceanogr. Paleoclimatol. 35, e2019PA003762.\nBetzler, C. et al. 2016. The abrupt onset of the modern South Asian Monsoon\nwinds. Sci. Rep. 6, 29838.\nClif, P. D. and Webb, A. A. G. 2019. History of the Asian monsoon and its\ninteractions with solid earth tectonics in Cenozoic South Asia. Geol. Soc. Lond. Spec.\nPubl. 483, 875–880.\nClift, P. D. 2006. Controls on the erosion of Cenozoic Asia and the flux of clastic\nsediment to the ocean. Earth Planet. Sci. Lett. 241, 571–580.\nClift, P. D. et al. 2008. Correlation of Himalayan exhumation rates and Asian\nmonsoon intensity. Nat. Geosci. 1, 875–880.\nChakraborty, A., Nanjundiah, R. S. & Srinivasan, J. Impact of African\norography and the Indian summer monsoon on the low-level Somali jet. Int. J. Clim.\n29, 983–992 (2009).\nDing, L., Kapp, P., Cai, F., Garzione, C.N., Xiong, Z., Wang, H. and Wang, C.,\n2022. Timing and mechanisms of Tibetan Plateau uplift. Nature Reviews Earth &\nEnvironment, 3(10), pp.652-667.\nDing, L. et al. 2017. Quantifying the rise of the Himalaya orogen and\nimplications for the South Asian Monsoon. Geology 45, 215–218.\nGupta, A. K., Yuvaraja, A., Prakasam, M., Clemens, S. C. & Velu, A. 2015.\nEvolution of the South Asian monsoon wind system since the late Middle Miocene.\nPalaeogeogr. Palaeoclimatol. Palaeoecol. 438, 160–167.\nHuang, Y., Clemens, S. C., Liu, W., Wang, Y. & Prell, W. L. 2007. Large-scale\nhydrological change drove the late Miocene C4 plant expansion in the Himalayan\nforeland and Arabian peninsula. Geology 35, 531–534.\nupwelling in the western Arabian sea as revealed by planktonic foraminifers\n1. In Proceedings of the ocean drilling program, scientific results 1, 257–263.\nLee, J. et al. 2020. Monsoon-influenced variation of clay mineral compositions\nand detrital Nd-Sr isotopes in the western Andaman Sea (IODP Site U1447) since the\nlate Miocene. Palaeogeogr. Palaeoclimatol. Palaeoecolo. 538, 109339.\nMoucha, R. and Forte, A. M. Changes in African topography driven by mantle\nconvection. Nat. Geosci. 4, 707–712 (2011).\nNigrini, C. 1991. Composition and biostratigraphy of radiolarian assemblages\nfrom an area of upwelling (northwestern Arabian Sea, lag 117). Proc. ODP Sci.\nResults 117, 89–126.\nQuade, J., Cerling, T. E. & Bowman, J. R. 1989. Development of Asian monsoon\nrevealed by marked ecological shift during the latest Miocene in northern Pakistan.\nNature 342, 163–166.\nQuade, J. & Cerling, T. E. 1995. Expansion of C4 grasses in the Late Miocene of\nNorthern Pakistan: evidence from stable isotopes in paleosols. Palaeogeogr.\nPalaeoclimatol., Palaeoecol. 115, 91–116.\nQuade, J., Cater, J. M. L., Ojha, T. P., Adam, J. & Mark Harrison, T. 1995. Late\nMiocene environmental change in Nepal and the northern Indian subcontinent: Stable\nisotopic evidence from paleosols. GSA Bulletin 107, 1381–1397.\nSarr, A.C., Donnadieu, Y., Bolton, C.T., Ladant, J.B., Licht, A., Fluteau, F.,\nLaugié, M., Tardif, D. and Dupont-Nivet, G., 2022. Neogene South Asian monsoon\nrainfall and wind histories diverged due to topographic effects. Nature\nGeoscience, 15(4), pp.314-319.\nSlingo, J., Spencer, H., Hoskins, B., Berrisford, P. & Black, E. The meteorology\nof the Western Indian Ocean, and the influence of the East African Highlands. Phil.\nTrans. R. Soc. A. 363, 25–42 (2005).\nTomson, J. R. et al. 2021. Tectonic and climatic drivers of Asian monsoon\nevolution. Nat. Commun. 12, 4022.\nAsian monsoon. J. Atmos. Sci. 73, 3197–3212 (2016).\nZuo, M., Sun, Y., Zhao, Y., Ramstein, G., Ding, L. and Zhou, T., 2024. South\nAsian summer monsoon enhanced by the uplift of the Iranian Plateau in Middle\nMiocene. Climate of the Past, 20(8), pp.1817-1836.\nZhuang, G., Pagani, M. & Zhang, Y. G. 2017. Monsoonal upwelling in the\nwestern Arabian Sea since the middle Miocene. Geology 45, 655–658.\n#7. Please confirm if only significant differences are shown in the figures.\nOur response: Following your valuable comment, we have replotted all the figures\nand performed additional statistical analyses to assess the significance of the variable\nchanges in the revised manuscript and Supplementary materials. Specifically, we\napplied Student’s t-test to assess the statistical significance, and updated the figures to\nshow only statistically significant differences at 95% significant level.\nMinor comments\n#1. L73 – other boundary forcings\nOur response: Thanks for your carefully reading. We have revised this in Line 71 of\nthe revised manuscript.\n#2. L147 – C4 \"plants\"\nOur response: Really thanks for your carefully reading. We have revised this in Line\n471 of the revised manuscript.\nReviewer #1 (Remarks on code availability):\nCode is not provided.\nOur response: Thanks for your reminder. We have provided the code for the main\nresults in the new submission on Zenodo at https://doi.org/10.5281/zenodo.14905875.\nRemarks to Author:\nThe Somali Jet, a near-surface air current along the coast of the Horn of Africa,\nplays a central role in the South Asian summer monsoon (SASM) dynamics. It\ntransports large amounts of moist air from the Indian Ocean to India, where it causes\nheavy rainfall. A more intense Somali Jet is usually assumed to lead to more\nprecipitation in the SASM domain. For the Miocene, however, reconstructions have\nshown that these two components appear to be decoupled, as wind proxies show a\nstrengthening of the Somali jet from Middle Miocene to late Miocene, but\nprecipitation in the SASM domain decreased.\nHan et al. employ the EC-Earth3 model to assess the impact of changes in the\nAfrican topography during the Miocene on this decoupling and the atmospheric and\nocean dynamics and the South Asian monsoon precipitation, in general. To this end,\nthey carry out simulations in which they keep all boundary conditions on the pre-\nindustrial settings, but prescribe the African topography in line with reconstructions.\nAccording to the model, the topography is important in two respects: First, due\nto the lower East African Highlands during Early Miocene, the Somali Jet is less\ncanalized and therefore much weaker. This in turn has the effect of inducing a\npositive Indian Ocean Dipole-like SST pattern, which leads to more precipitation in\nthe SASM domain and an positive feedback with the circulation. In the course of the\nMiocene, the East African Highlands are lifted up, which more and more intensifies\nthe jet, explaining the signal seen in the wind-proxies.\nSecond, A higher Congo Basin leads to less ascent and less latent heat release\ninto the atmosphere. This promotes the formation of a cyclonic circulation anomaly\nabove the Arabian sea, redirecting moisture to the Indian continent. The effect of\nchanges in latent heat release on the atmospheric circulation in distinct regions is\nfurther investigated by employing a linear baroclinic model. In addition, Han et al.\nperform simulations with increased (based on reconstructions) CO levels and with\n2\nboth forcings (CO and topography changes). Decomposing the precipitation response\n2\ninto the effect of atmospheric moisture change (thermodynamic) and atmospheric\n24\ncirculation changes (dynamic response) shows that topography changes purely lead to\nmore precipitation due to changes in the circulation, whereas the CO effect has a\n2\nstrong thermodynamic contribution to the precipitation change due to its impact on\nthe atmospheric temperature.\nThis study highlights the effect of the African topography in the SASM evolution\nand underlines the complexity of the South Asian monsoon dynamics. The figures in\nthe main part are well-chosen and helpful, and are also mostly well presented and\neasy to understand. The text is clearly and comprehensibly formulated and the\nindividual parts seem well thought out. Methods are carefully applied. The main\ncriticism I have is that I missed the “overarching framework”. The individual sub-\nchapters don't always seem to build on each other and you get the impression that Han\net all lose focus on their core topic and their aim of the study in the course of the\nmanuscript. This means that the paper is not easily comprehensible to a wider\nreadership. However, I believe that this can be solved and therefore suggest a\npublication in Nature Communication after major revisions.\nOur response: Dear reviewer, thank you very much for finding interest in our\nfindings and pointing out the flaws in the analyses/presentation. We have addressed\nyour concerns in a point-by-point manner below, and hope that you will find the\nadded information suitable and sufficient for publication. The original comments are\nquoted in blue and the changes in the revised manuscript have been highlighted in red.\nGeneral comments:\n#1. For me, not knowing much about the Miocene so far and therefore not being\nfamiliar with the literature, it is not clear what is really new about the study and what\nis the highlight of the study. According to the title, the main topic is the influence of\nAfrican topography on the decoupling of wind and precipitation changes during the\nMiocene. In my opinion, there is no clear final answer to this research question in the\nstudy. For instance, in the abstract as main (and only) reason for the circulation and\nprecipitation changes, the lower East African Highlands are mentioned. In contrast,\nlarge parts of the result part deal with the effect of the higher Congo basin on the\n25\ncirculation. Later in the text, the CO experiments are analyzed, but the results neither\n2\nenter the abstract nor the conclusion. In the conclusion, the East African highland and\nCongo basin topography changes are mentioned as explanation for the weakening of\nthe Somali Jet and the feedbacks with the precipitation, which is not directly clear and\nconfusing, because the linear baroclinic model (used for the explanation of the effect\nof topography changes in the Congo basin) show no weakening of the Somali Jet in\nany simulation. This conclusion is therefore imprecise. Not giving a clear answer to\nthe main research question, makes the individual parts of the analysis seem more like,\nwe did this and then that, rather than being able to recognize a clear common line in\nit. This is a pity, because the individual parts are important and interesting. I would\nlike the reader to be guided more through the work, and the rationale for conducting\nthe individual analyses to be explained more and the work to be structured more (and\nmore clearly). Particularly the Abstract and the Conclusion should be revised.\nOur response: We appreciate the reviewer’s detailed and constructive feedback. We\nrecognize that while the individual components of our analysis are scientifically\nvaluable, their integration into a cohesive narrative was not sufficiently clear. Based\non this valuable critique, we have undertaken the following revisions to clarify the\nnovelty of our study, ensure coherence, and strengthen the manuscript’s structure.\n1. About the novelty of this study\nThe key novelty of our study is the identification of a previously\noverlooked tectonic mechanism driving the decoupling of SASM winds and\nrainfall during the Miocene. Unlike previous studies that primarily attribute SASM\nevolution to Himalaya-Tibet uplift, CO₂ changes, or ice sheet dynamics, we provide\ncompelling evidence that (1) African topography changes played a dominant role in\nmodulating monsoon dynamics. (2) Two distinct processes, driven by different\nAfrican topographic features, contributed to the decoupling of SASM winds and\nrainfall: Lower East African Highlands during the Early and Middle Miocene\nweakened the Somali Jet, reducing wind-driven upwelling and inducing a positive\nIOD-like warming pattern, which enhanced monsoonal rainfall via ocean-atmosphere\nfeedbacks. Meanwhile, a higher Congo Basin reduced ascent and latent heat release,\n26\ntriggering a cyclonic circulation anomaly over the Arabian Sea, which redirected\nmoisture transport to the SASM region.\nWhile previous studies (e.g., Sarr et al., 2022) have hinted at the role of African\ntopography in SASM evolution, they did not explore the ocean-atmosphere feedbacks\nthat amplify this effect, nor did they explain how these mechanisms reconciled the\ndiscrepancies between wind and rainfall proxies. Our study is the first to provide\na comprehensive dynamical explanation of this decoupling phenomenon using a high-\nresolution coupled climate model.\nIn the revised manuscript, we have rewritten the Abstract (Lines 29-33, 35-37) to\nhighlight the key findings explicitly, and revised the Conclusion (Lines 342-343, 356-\n375) to emphasize the novel mechanistic insights uncovered in our study.\n2. Improve the structure for clarity and coherence\nWe acknowledge the reviewer’s concern that the manuscript’s structure did\nnot clearly guide the reader through the different analyses. To improve the logical\nflow, we have refined the introduction to explicitly state the study’s objectives and\nhypotheses, reorganized sections so that each analysis builds upon the previous one,\nand moved the discussion of proxy reconstructions from the Results to the Methods\nsection (Lines 446-498) to keep the focus on the model-based analysis.\n3. Strengthening the conclusion for prevision and consistency\nThe reviewer correctly noted that the linear baroclinic model (LBM) does not\nshow any weakening of the Somali Jet, yet our conclusion suggested otherwise. We\nrecognize this inconsistency and have now clarified in the conclusion that the LBM\nresults specifically assess the impact of latent heat release changes on regional\natmospheric circulation rather than the direct weakening of the Somali Jet itself. We\nhave emphasized that the Somali Jet weakening is primarily driven by changes in East\nAfrican topography in our fully coupled EC-Earth3 simulations. And we integrated\nthe CO₂ experiment findings into the discussion and conclusion, making it clear\n27\nthat topography-induced changes were the dominant driver of SASM decoupling,\nwhile CO₂ changes primarily affected the thermodynamic response.\nThe clarification can be found in Lines 311-314, 331-336, 342-343, and 344-349\nof the revised manuscript.\n#2. In addition, the author explains the reduction of the Somali Jet via the reduction of\nthe channel effect by the lower East African Highlands. Just looking at the circulation\nchanges in Fig.1, showing a cyclonic circulation anomaly during the Early and Middle\nMiocene compared to pre-industrial, the pattern does not really look like a reduction\nin the channel effect. It looks like a thermally induced dynamic response to the\ndecreased SST in the north-western Arabian sea. The CO -forced simulation shows a\n2\nstronger Somali Jet compared to the topography forced simulation. Does this imply\nthat in the CO simulations the wind and precipitation changes are not decoupled?\novercompensating the CO effect in the “real world”? Please clarify.\ncirculation anomaly over the Arabian Sea (Figure R1) during the Early and Middle\nMiocene compared to pre-industrial is resulted from following two key processes:\n1. Weakened meridional cross-equatorial flow due to the lowering of the East\nAfrican Highlands. To further support this, we have now included 850-hPa wind\nanomalies (Figure R2), which are more directly influenced by the East African\nHighlands. These show a clear weakening of the Somali Jet core along East Africa\nwith lower highlands, confirming the channel effect mechanism.\n2. Thermally induced dynamic response to SST anomalies. As the reviewer correctly\npointed out, the cyclonic circulation patterns over the Arabian Sea are also linked\nto SST driven atmospheric dynamics. Our analysis reveals that a substantial\nreduction in summer rainfall over tropical central Africa leads to lower latent heat\nrelease, which enhances the cyclonic circulation over the Arabian Sea\n(Supplementary Figure 5 and 7 in the revised Supplementary materials). This\n28\nprocess is further intensified by a Rossby wave response induced by the increase\nin SASM rainfall (as shown in the LBM results in Supplementary Figure 8 in the\nrevised Supplementary materials).\nThus, while the lower East African Highlands primarily weakened the Somali Jet via\nthe channel effect, the Arabian Sea cyclonic anomaly is largely driven by ocean-\natmosphere feedbacks. To provide a comprehensive understanding of our research\nfindings, we decide to keep the 700-hPa winds in Figure 1d-f in the revised\nmanuscript to illustrate the large-scale circulation changes. We have added the 850-\nhPa wind anomalies (identical to Figure R2) as Supplementary Figure 3 to explicitly\nshow the weakening of the Somali Jet due to the reduced channel effect, and clarified\nthe discussion in Lines 128-130 in the revised manuscript.\nTo address your question about the relationship between the changes in Somali\nJet and SASM rainfall under CO forcing, we have performed additional analysis and\n2\ncalculated the area mean changes in the intensity of Somali Jet and rainfall in Figure\nR3. In our simulations of combined effect of African topography and CO forcings,\n2\nthe Somali Jet is weakened while SASM rainfall is increased (Figure R3, and\nSupplementary Figure 12 in Supplementary materials), demonstrating a clear\ndecoupling of monsoon winds and rainfall. When we only consider the African\ntopography forcing, we can reproduce this decoupling feature (Figure R3, and Figure\n1g-h in the manuscript), indicating African topography change is the primary driver of\nSASM evolution during the Miocene. When we only consider the CO forcing, there\n2\nis no decoupling of monsoon winds and rainfall in our simulations (Figure R3, and\nFigure 4 in the manuscript). Instead, both Somali Jet strength and SASM rainfall\nincrease simultaneously, confirming that CO alone does not cause the decoupling\n2\nphenomena. The corresponding discussion are added in Lines 311-314, 331-336, 342-\n343 of the revised manuscript. In addition, we also include the area mean changes in\nthe intensity of Somali Jet in Figure 4 and Supplementary Figure 12 in the revised\nmanuscript and Supplementary materials, respectively.\ncirculation at 700 hPa (vectors, m s-1) in the a MT25, b MT15 and c MT05\nsimulations relative to the pre-industrial simulations. Black boxes (5° N-30° N; 60° E-\n95° E) mark the SASM region, and blue boxes (10º S-10º N, 40º -50º E) mark the\ncross-equatorial Somali Jet. Gray stippling denotes regions in which the changes are\nsignificant at the 95% confidence level according to Student’s t-test.\nduring boreal summer. Climatological mean 850-hPa winds (vectors, m s-1) and its\nwind speed (shading, m s-1) in the a pre-industrial, b MT25, c MT15 and d MT05\nsimulations. Responses to Miocene African topography changes in e MT25, f MT15\nand g MT05 simulations compared to the pre-industrial simulation. Black boxes mark\nthe SASM region, and blue boxes mark the cross-equatorial Somali Jet. In e-g, gray\nstippling denotes regions in which the changes are significant at the 95% confidence\nlevel according to Student’s t-test.\nsummer rainfall (%) over the SASM region due to forcings of F (combined effect of\nall\nAfrican topography and increased CO concentration), F (changes in African\n2 top\ntopography), and F (increased CO concentration) during Miocene. The asterisks\nCO2 2\nabove or below the bars denoting changes that are significant at the 95% confidence\nlevel according to Student’s t-test.\n#3. With respect to this, to really understand how the changes in African topography\naffect the circulation and precipitation pattern, sensitivity studies are necessary in\nwhich either the changes in the Congo basin or the changes in the East African\nHighlands are prescribed, i.e. simulations in which e.g. the Congo basin elevation is\nincreased, but the East African Highlands are prescribed on pre-industrial settings.\nOur response: Thanks for your suggestions regarding the need for additional\nsensitivity experiments. While such experiments could provide further insights, our\n32\nobjective in this study is to assess the real-world impact of reconstructed African\nMiocene topography on SASM evolution rather than conducting idealized sensitivity\nexperiments. Below, we outline how our current approach effectively isolates the\nroles of different African topographic features using robust comparative analyses,\naligning with existing literature.\n1. Our results show that the uplift of the Congo Basin during the Miocene\nsignificantly weakened the African monsoon, leading to a substantial decrease in\nprecipitation over tropical Africa (Supplementary Figure 7 in the Supplementary\nmaterials). This drying trend, in turn, influenced SASM circulation through\natmospheric teleconnections. This relationship between the rainfall anomaly over\nNorth-central Africa and SASM circulation has been well documented in previous\nstudies (e.g., Li et al., 2022). Using CMIP6 outputs and sensitivity experiments, Li et\nal. (2022) demonstrated that significant rainfall reductions over tropical Africa can\ninduce a Kelvin wave response over the northern Indian Ocea, altering SASM\ndynamics. Our results from topography simulations (Figure 1d-f in the manuscript)\nand LBM experiments (Supplementary Figure 8 in the Supplementary materials) are\nconsistent with their findings, confirming the significant role of African convective\nchanges in modulating SASM circulation and rainfall. Thus, while additional\nsensitivity experiments could provide further confirmation, our topography-forced\nsimulations already capture the key teleconnections during SASM variability.\n2. The East African Highlands play a crucial role in shaping the Somali jet, and our\nsimulations align with previous idealized modeling studies (Slingo et al., 2005;\nChakraborty et al., 2009; Wei et al., 2016). These works demonstrated that removing\nthe East African Highlands significantly weakens the Somali Jet by reducing the\ntopographic blacking effect. In our altered topography simulations, the most notable\nfeature of the meridional cross-equatorial flow is a significant weakening of the core\nof Somali Jet due to the lowering of the East African Highlands during the Miocene\n(Figure R2, and Figure 1 in the manuscript). The consistency of our simulations with\n33\nprevious idealized modeling studies gives us confidence that the East African\nHighlands are a key factor in controlling Somali Jet strength and monsoon dynamics.\n3. In the revised manuscript, our further analysis highlights that the weakened Somali\nJet leads to positive SST anomalies in the western equatorial Indian Ocean, triggering\na positive IOD-like warming pattern that enhances SASM rainfall. To examine this\nprocess in more detail, we analyzed the monthly evolution of changes in intensity of\nthe Somali Jet, SST, wind anomalies, and sea level pressure (SLP) (Figures R4 and\nR5). We found SST changes in the western Indian Ocean are closely linked to\nvariations in the intensity of the Somali Jet during the Miocene (Figure R4).\nSpecifically, a weakened Somali Jet can cause rapid surface ocean warming via\nchanging surface heat flux (Schott et al. 2009; Li et al., 2015) in the western\nequatorial Indian Ocean, particularly in the pre-monsoon months. These SST\nanomalies intensify during boreal summer, reinforcing the positive IOD-like warming\npattern that strengthens SASM rainfall. In addition, the seasonal variations in Figure\nR5 confirm that the development of positive SST anomalies over the western Indian\nOcean decrease SLP there, inducing easterly anomalies along the equator, sustaining\nthe IOD-like pattern. These processes are prominent in the Early and Middle\nMiocene, when the East African Highlands were lower, leading to a stronger IOD-like\nwarming pattern compared to the Late Miocene. This, in turn, further increase SASM\nrainfall through atmospheric teleconnections, as confirmed by our LBM experiments\n(Supplementary Figure 8 in the Supplementary materials).\nIn summary, while conducting separate simulations for the Congo Basin and East\nAfrican Highlands could provide additional insights, we argue that our current\nMiocene simulations already isolate the individual roles of these regions by\ncomparing their effects on circulation and precipitation patterns. Our results are\nconsistent with previous idealized modelling studies and have explicitly examined\nthese mechanisms. Further analysis of seasonal variation (Figures R4 and R5)\nsupports the validity of the mechanisms proposed in our study, making additional\nexperiments beyond the scope of this study. To ensure this is clearly conveyed, we\n34\nhave added the Figure R4 and Figure R5 as Supplementary Figure 9 and Figure 10 in\nthe revised Supplementary materials, respectively. The corresponding findings have\nalso been clarified in Lines 261-269 in the revised manuscript.\nFigure R4. Strong coupling of the intensity of Somali Jet and sea surface temperature\n(SST) over the western Arabian Sea. Seasonal variation of changes in intensity of\nSomali Jet (m s-1; see Methods) and SST (℃) over tropical western Indian Ocean (10°\nAugust).\nFigure R5. Seasonal variation.Changes in SST (shading, ℃), sea level pressure\n[contours; hPa; solid (dashed) lines represent the positive (negative) values], and\nsurface wind (vectors, m s-1) from May to September in the MT25 (left column),\nMT15 (middle column) and MT05 (right column) simulations compared to pre-\nindustrial simulation. Gray stippling and vectors are only shown at regions where\nchanges are significant at the 95% confidence level according to Student’s t-test.\nLi, T. et al. Distinctive South and East Asian monsoon circulation responses to\nglobal warming. Sci. Bull. 67, 762–770 (2022).\nSlingo, J., Spencer, H., Hoskins, B., Berrisford, P. and Black, E. The\nmeteorology of the Western Indian Ocean, and the influence of the East African\nHighlands. Phil. Trans. R. Soc. A. 363, 25–42 (2005).\nChakraborty, A., Nanjundiah, R. S. and Srinivasan, J. Impact of African\norography and the Indian summer monsoon on the low‐level Somali jet. Int. J.\nClimatol. 29, 983–992 (2009).\nWei, H.-H. and Bordoni, S. On the Role of the African Topography in the South\nAsian Monsoon. J. Atmos. Sci. 73, 3197–3212 (2016).\nSchott, F.A., Xie, S.P. and McCreary Jr, J.P. Indian Ocean circulation\nand climate variability. Reviews of Geophysics, 47(1) (2009).\nLi, G., Xie, S.P. and Du, Y. Monsoon-induced biases of climate models over the\ntropical Indian Ocean. Journal of Climate, 28(8), 3058-3072 (2015).\n#4. (1) What do we know about the climate in the Miocene? About the drivers? Other\n(but not applied) changes in the boundary conditions are only mentioned in the\nconclusion and discussion. For example, the Western Ghats were much lower, which\nprobably also had an impact on the circulation and the precipitation in India. The\nTibetan Plateau was probably only half as high. Today it acts as a strong barrier and\nthe vertical ascending on the southern site is very strong, leading to large amounts of\nprecipitation. In addition, it plays a very important role for the atmospheric\ncirculation, also via the influence on the subtropical jet. So I suspect that there might\nbe more reasons why the wind-precipitation-decoupling may have occurred and the\nclimate was like as seen in the reconstructions. Please discuss this in more detail. It\nwould be nice if this was already mentioned in the introduction and it was made clear\nthat (presumably) the topographical changes and the higher CO levels were the most\n2\nprominent drivers and therefore only these forcings were used and it is now being\ninvestigated how large the proportion of orography is in comparison with the CO\n2\n37\nforcing proportion. (2) Maybe it would also be an idea to present the orography and\nCO simulations together, to work out that the topography changes are more\n2\nimportant. In the current version of the paper, the authors prescribe topography\nchanges and the main results is that topography changes are important. But this is\nalready a predetermined result, when topography is the only forcing of the model that\nhave been prescribed.\nOur response: (1) We acknowledge that a broader discussion of Miocene climate\ndrivers—including tectonic, paleogeographic, and CO₂-related changes—was lacking\nin the introduction. To address this, we have now expanded the introduction to\nprovide clearer justification for focusing on topographic changes and CO₂ levels as\nthe dominant drivers of SASM evolution during the Miocene.\n• About key boundary condition changes and their relevance to SASM system.\nThe Himalaya-Tibetan Plateau uplift significantly shaped monsoon dynamics\nbefore the Miocene. However, previous studies indicate its uplift occurred before ~20\nMa, with southern Tibet reaching elevations ≥4 km by ~55 Ma and central Tibet by\n~45 Ma (Ding et al., 2022). The western Chats were much lower than today, likely\naffecting orographic rainfall over the Indian subcontinent. The Iranian Plateau and\nEastern Arabian Peninsula were still evolving, which may have influenced regional\natmospheric circulation.\n• Why we focus on African topography and CO as key SASM drivers?\npredominantly influenced early Miocene rainfall, its impact on ocean-atmosphere\ncirculation was relatively limited in the Middle and Late Miocene. In contrast,\nthe gradual uplift of the East African Highlandsand the subsidence of the Congo\nBasin from the Middle Miocene to Late Miocene aligns with the onset of a “modern-\nlike” SASM in the late middle Miocene (~13 Ma) (Nigrini, 1991; Huang et al., 2007;\nGupta et al., 2015; Betzler et al., 2016; Zhuang et al., 2017; Bialik et al., 2020). The\nEast African Highlands uplift shaped the Somali Jet, while the Congo Basin uplift\nmodified regional convection, make African topography a key influence on SASM\n38\nwind-precipitation decoupling. Although CO levels fluctuated during the Miocene,\n2\nthey remained moderately higher than modern values with large uncertainties\n(Beerling and royer, 2011; Foster et al., 2012; Super et al., 2018; Sosdian et al., 2020;\nSteinthorsdottir et al., 2020).\nFollowing your valuable suggestions, we have included a more detailed\ndiscussion as mentioned above in the introduction of the revised manuscript to\nhighlight that topographical changes were the most prominent drivers, specifically in\nLines 72-76 of revised manuscript.\n(2) Thanks for your insightful suggestion regarding the idea of presenting the\norography and CO simulations together to emphasize the importance of topography\n2\nchanges. After carefully consideration, we believe that our current structure –\nfocusing first on African topography and then on CO effects – is more suitable for\n2\nthe following reasons.\nThe core objective of this paper is to evaluate the climate effects of realistic\nMiocene African topography evolution, rather than conducting idealized sensitivity\ntests. This is why we focus on three key time slices (early, middle and late Miocene)\nusing reconstructed African dynamic topography. Unlike African topography, which\nfollows a clear geological reconstruction, CO estimates vary widely. During the\n2\nmodern values (Beerling and royer, 2011; Foster et al., 2012; Super et al., 2018;\nSosdian et al., 2020; Steinthorsdottir et al., 2020). However, multi-proxy estimates of\natmospheric CO during this period show large uncertainties (Figure 4a in\n2\nmanuscript). Thus, we idealized CO forcing to 500 ppm in sensitivity experiments so\n2\nisolate its effect without imposing additional uncertainties from varying proxy\nreconstructions.\nSeparate mechanisms:\n• African topography causes a decoupling of monsoon winds and rainfall, where the\nSomali Jet weakens while SASM rainfall increases.\n39\n• CO forcing, in contrast, enhances both monsoon winds and rainfall\n2\nsimultaneously, showing no decoupling effect.\n• When both forcings are combined, the decoupling effect persists, reinforcing that\nthe African topography is the dominant driver of Miocene SASM evolution.\nThese are clarified in Lines 29-30, 36-37, 142-151, 311-314, 331-336, 342-343, and\n412 of the revised manuscript.\nReference:\nBialik, O. M. et al. Monsoons, upwelling, and the deoxygenation of the\nNorthwestern Indian ocean in response to middle to late miocene global climatic\nshifs. Paleoceanogr. Paleoclimatol. 35, e2019PA003762 (2020).\nBetzler, C. et al. The abrupt onset of the modern South Asian Monsoon winds.\nSci. Rep. 6, 29838 (2016).\nBeerling, D. J., and Royer, D. L. Convergent cenozoic CO2 history. Nature\nGeoscience, 4, 418–420 (2011).\nClif, P. D. and Webb, A. A. G. History of the Asian monsoon and its interactions\nwith solid earth tectonics in Cenozoic South Asia. Geol. Soc. Lond. Spec. Publ. 483,\n875–880 (2019).\nChakraborty, A., Nanjundiah, R. S. and Srinivasan, J. Impact of African\norography and the Indian summer monsoon on the low-level Somali jet. Int. J. Clim.\n29, 983–992 (2009).\nDing, L., Kapp, P., Cai, F., Garzione, C.N., Xiong, Z., Wang, H. and Wang, C..\nTiming and mechanisms of Tibetan Plateau uplift. Nature Reviews Earth &\nEnvironment, 3(10), pp.652-667 (2022).\nDing, L. et al.. Quantifying the rise of the Himalaya orogen and implications for\nthe South Asian Monsoon. Geology 45, 215–218 (2017).\nand climate during the middle Miocene. Earth and Planetary Science Letters, 341,\n243–254 (2012).\nGupta, A. K., Yuvaraja, A., Prakasam, M., Clemens, S. C. & Velu, A. Evolution\nof the South Asian monsoon wind system since the late Middle Miocene. Palaeogeogr.\nPalaeoclimatol. Palaeoecol. 438, 160–167 (2015).\nHuang, Y., Clemens, S. C., Liu, W., Wang, Y. & Prell, W. L. Large-scale\nhydrological change drove the late Miocene C4 plant expansion in the Himalayan\nforeland and Arabian Peninsula. Geology 35, 531–534 (2007).\nMoucha, R. and Forte, A. M. Changes in African topography driven by mantle\nconvection. Nat. Geosci. 4, 707–712 (2011).\nNigrini, C. Composition and biostratigraphy of radiolarian assemblages from an\narea of upwelling (northwestern Arabian Sea, lag 117). Proc. ODP Sci. Results 117,\n89–126 (1991).\nQuade, J., Cerling, T. E. & Bowman, J. R. 1989. Development of Asian monsoon\nrevealed by marked ecological shift during the latest Miocene in northern Pakistan.\nNature 342, 163–166.\nQuade, J. & Cerling, T. E. 1995. Expansion of C4 grasses in the Late Miocene of\nNorthern Pakistan: evidence from stable isotopes in paleosols. Palaeogeogr.\nPalaeoclimatol., Palaeoecol. 115, 91–116.\nQuade, J., Cater, J. M. L., Ojha, T. P., Adam, J. & Mark Harrison, T. 1995. Late\nMiocene environmental change in Nepal and the northern Indian subcontinent: Stable\nisotopic evidence from paleosols. GSA Bulletin 107, 1381–1397.\nSarr, A.C., Donnadieu, Y., Bolton, C.T., Ladant, J.B., Licht, A., Fluteau, F.,\nLaugié, M., Tardif, D. and Dupont-Nivet, G.. Neogene South Asian monsoon rainfall\nand wind histories diverged due to topographic effects. Nature\nGeoscience, 15(4), pp.314-319 (2022).\nSlingo, J., Spencer, H., Hoskins, B., Berrisford, P. & Black, E. The meteorology\nof the Western Indian Ocean, and the influence of the East African Highlands. Phil.\nTrans. R. Soc. A. 363, 25–42 (2005).\nNat. Commun. 12, 4022 (2021).\nWei, H.-H. & Bordoni, S. On the role of the African topography in the South\nAsian monsoon. J. Atmos. Sci. 73, 3197–3212 (2016).\nZuo, M., Sun, Y., Zhao, Y., Ramstein, G., Ding, L. and Zhou, T.. South Asian\nsummer monsoon enhanced by the uplift of the Iranian Plateau in Middle\nMiocene. Climate of the Past, 20(8), pp.1817-1836 (2024).\nZhuang, G., Pagani, M. & Zhang, Y. G. 2017. Monsoonal upwelling in the\nwestern Arabian Sea since the middle Miocene. Geology 45, 655–658.\n#5. A large part of the result chapter compares the model with data. For me, this is\ndone very “uncritically”. Although the reconstructions of wind and precipitation agree\nquite well with the model, the proxies experienced a completely different world than\nthat in the idealized model simulation (see above). I also do not understand, why the\nreconstructions are at least not compared with the simulation including both forcing\n(CO and topography changes). And I also miss a critical (at least short) discussion on\n2\nthe uncertainties in the proxies. For instance, d18O is often used as a proxy for\nprecipitation, but it is also very dependent on other things (moisture source, wind\nsystem, temperature). I understand why Han et al. compare the simulations with\nreconstructions, but if it is to be part of the results, then a more critical discussion\nwould be appropriate. Perhaps it would otherwise be an idea to include the evaluation\nof the model simulation in the methods (e.g. in the model and simulation description).\nOur response: We appreciate this thoughtful comment, as it highlights the need for a\nmore critical evaluation of data-model comparisons and a clearer discussion of proxy\nuncertainties. To improve the manuscript, we have restructured the presentation of\nproxy data, refined our discussion on uncertainties, and provided a clearer\njustification for our comparison approach.\n(1) Following your valuable suggestions, to enhance clarity and avoid an uncritical\ntone, we have changed the title of the section from “Data-model comparison for\nmonsoon winds and rainfall”, emphasizing the mechanistic focus rather than an overly\ndirect model-data validation. And then we moved the detailed proxy discussion to the\nMethods section (Lines 446-498), ensuring that the core results remain focused on\nmodel-based mechanisms. We also explicitly stated in the Methods that proxy\nreconstructions provide qualitative constraints rather than exact validation of the\nsimulations. This restructuring helps to make it clear that our primary goal is to\nexplore the physical mechanisms behind monsoon evolution, while proxy\ncomparisons serve as supporting evidence rather than definitive proof.\n(2) The reviewer raises a fair point about whether the reconstructions should be\ncompared with the simulation that includes both CO₂ and topography changes. We\nacknowledge this concern and clarify our rationale below:\n• Our primary objective is to isolate the role of African topography in SASM\nevolution. The topography-only simulations allow us to separate the direct impact\nof African tectonic changes without interference from CO₂-related thermodynamic\neffects. If we were to compare proxies with the combined CO₂ + topography\nsimulations, it would be harder to disentangle the specific influence of\ntopography.\n• CO₂ levels in the Miocene have high uncertainties, making direct model-data\ncomparisons difficult. Multi-proxy CO₂ reconstructions vary significantly (e.g.,\nBeerling and Royer, 2011; Foster et al., 2012; Super et al., 2018; Sosdian et al.,\n2020; Steinthorsdottir et al., 2020). Since we idealized CO₂ to 500 ppm in\nsensitivity experiments, direct comparison with proxy data could be misleading.\nDespite these limitations, we have expanded the discussion in Lines 308-309, 311-\n314, 331-336, and 342-343, explicitly acknowledging this potential issue and\nclarifying that while CO₂ may modulate the monsoon, our findings indicate that\nAfrican topography is the dominant driver of wind-rainfall decoupling.\n43\n(3) The reviewer is right that proxy records have inherent uncertainties, which should\nbe acknowledged in the manuscript. We have now incorporated a dedicated\ndiscussion in the Methods section (Lines 488-498).\nWe expanded discussion on δ¹⁸O limitations: While δ¹⁸O of soil carbonates is\noften interpreted as a precipitation proxy, it is also influenced by (1) moisture source\nvariability (changes in the origin of monsoon moisture); (2) wind system shifts\n(affecting moisture transport pathways); and (3) temperature variations (altering\nfractionation processes) (Wen et al., 2024a). This means that while δ¹⁸O data suggest\ndrying trends, they do not provide a direct measure of precipitation amount.\nWe acknowledged uncertainties in wind proxies (e.g., TOC and G. bulloides\nrecords): these proxies are indirect indicators of monsoon winds, as they reflect\nupwelling-driven productivity changes rather than wind speed itself. Upwelling can\nalso be influenced by local oceanographic processes and thermocline variability,\nadding complexity to their interpretation (Wen et al., 2024b).\nWe clarified the broader limitations of proxy-model comparisons: (1) Temporal\naveraging differences: Proxy records often reflect multi-millennial averages, while\nmodels analyze mean state differences; (2) Spatial sampling bias: Most proxies come\nfrom specific regions (e.g., Arabian Sea, Siwaliks), which may not fully capture\nspatial monsoon variability.\nThese discussions ensure that the model-data comparison is not overly\ndeterministic but instead framed as a qualitative consistency check. Additionally,\ndespite these uncertainties, the consistent multi-proxy changes support the conclusion\nthat these proxies reliably reflect SASM evolution during the Miocene.\nReference:\nWen, Q., Liu, Z., Jing, Z., Clemens, S.C., Wang, Y., Yan, M., Ning, L. and Liu, J.,\n2024a. Grand dipole response of Asian summer monsoon to orbital forcing. npj\nClimate and Atmospheric Science, 7(1), 202.\nL., Yuan, L. & Gao, Y., 2024b. Contrasting responses of Indian summer monsoon\nrainfall and Arabian Sea upwelling to orbital forcing. Commun. Earth Environ 5,\n409.\n#6. Please revise the supplementary figures. The captions and the heading of\nsubfigures are not always clear and correct. I listed a few issues in my specific\ncomments below, but please check all figures carefully before re-submission.\nOur response: Thanks for the careful attention to the clarity and accuracy of our\nsupplementary figures. To ensure consistency, correctness, and readability, we have\nmade systematic review and revision of captions and subfigure headings. And we\nhave performed additional statistical analyses for all relevant figures to ensure that\nreported changes are robust and significant. Specifically, we applied Student’s t-test\nto assess statistical significance, with only differences at the 95% confidence level are\nshown in the figures.\nOther specific comments:\nAbstract\n#1. - L33-L38: Please be more precise and refer to all results (see comment (a)). For\ninstance, “blocking effect” is misleading here, as it is rather the opposite, a reduction\nof the blocking effect.\nOur response: Done.\nIntroduction\n#2. L66: “weakening of the SASM”: As in this study, changes in wind and\nprecipitation are discussed, this term is not clear. It could be both, a weakening in the\nwind and a weakening in the precipitation.\nof the SASM rainfall. We have descripted it more clear in Line 62 of the new revised\nmanuscript as follows:\n“This reduction in the SASM rainfall …” (Line 62)\nResults:\n#3. L105: it is: decoupling of monsoon rainfall…\nOur response: Revised.\n#4. L166-L169: This sentence should be moved to the Introduction.\nOur response: Thanks for the suggestions. We have moved this sentence to the\nintroduction section in Lines 72-75 of the revised manuscript.\n#5. L177-178: “in western Arabian Sea (Fig.1) is written in italics\nOur response: Revised.\n#6. L183: \"These circulations\", not \"There circulations\"\nOur response: Revised.\n#7. L211-215: Since it is a coupled system, it is difficult to say what triggers the\ndescending air anomaly. The vertical movement in models is often a result of the\nconvection calculation. So it could well be that the weaker West African monsoon leads\nto less precipitation, e.g. because less moisture is transported to the region and the lower\nprecipitation is causing/accompanied by weaker ascent and less latent heat release.\nWhat about the temperature and pressure at the surface? Maybe this would be another\n(better) line of argumentation.\nOur response: We appreciate the reviewer’s insightful suggestions. To strengthen\nour argument, we have reanalyzed surface changes in temperature and pressure and\n46\nadded this information into the revised manuscript. Our findings support the\nreviewer’s hypothesis: The weakened West African monsoon circulation leads to\nreduced moisture transport into central Africa, resulting in lower precipitation. The\ndecreased precipitation is directly associated with weakened ascent and reduced latent\nheat release, reinforcing the descending motion. Surface analysis reveals a warming\nanomaly in central Africa, likely linked to suppressed convection, while surface\npressure increases in response to reduced latent heating (Figure R6).\nFollowing this additional analysis, we have added the Figure R6 in\nSupplementary Figure 7, and rewritten the relevant section in Lines 207-213 of the\nrevised manuscript as follows:\n“The resulting weakening of the West African summer monsoon cause\nsignificantly reduced rainfall over central Africa (Fig. 1d and e). This drying trend is\nfurther supported by enhanced descending air anomalies (Supplementary Fig. 7c and\ng) and a substantial reduction in latent heat release in the region (Supplementary Fig.\n7d and h). Additionally, surface warming anomalies over central Africa (likely due to\nsuppressed convection) and increased surface pressure indicate the presence of\nsubsidence driven atmospheric stability (Supplementary Fig. 7e and i).” (Lines 207-\n213)\nAfrican topography changes in a MT25, c MT15 and e MT05 simulations compared\nto the pre-industrial simulation. b, d, and f Same as a, c, and e, but for the responses\nof surface pressure (hPa). Gray stippling denotes regions in which the changes are\nsignificant at the 95% confidence level according to Student’s t-test.\n#8. L216: I’m not that familiar with Kelvin waves, but I think Kelvin waves are\nusually confined to the equatorial region (~10°S to ~10°N). I am also not sure\n48\nwhether a reduction in the latent heat release causes a Kelvin wave response, or\nwhether, for example, the amplitude of the Kelvin wave simply changes. Please check\nthis with the literature.\nOur response: Thanks for your carefully assessment of our discussion on Kelvin\nwaves. To ensure the scientific robustness of our explanation, we have revisited the\ntheoretical framework, checked relevant literature, and validated our results with\nprevious studies.\nAccording to the Gill’s theory (Gill, 1980), the response of the tropical\natmosphere to localized heating or cooling follows a distinct wave pattern, which\nincludes Rossby waves on the western side of the heat source, and Kelvin waves on\nthe eastern side of the heat source. The spatial extent of the Kelvin wave response\ndepends on the latitudinal position and strength of the heating/cooling anomaly.\nMathematically, Gill’s model represents a heating rate Q is symmetric about the\nequator in the form:\n1 2 (1)\n−4(𝑦+𝑑)\n𝑄(𝑥,𝑦) = 𝐴∙𝑔(𝑥)∙𝑒\n𝜋\n𝑐𝑜𝑠𝑘𝑠 |𝑥| ≤ 𝐿\n𝑔(𝑥) = { , 𝑘 = 2𝐿\nwhere A represents heat in0t e n s i t y |,𝑥 d| r>ep𝐿resents the longitude distance from the equator\nto the center of the heat source. d>0 and d<0 denote the center of the heat source is\nlocated in the southern and northern hemisphere, respectively. 2L denotes the zonal\nwidth of the heat source.\nThis formulation helps explain why atmospheric responses to heating sources are\nstrongest near the equator but can extend poleward depending on the structure of the\nforcing. While Kelvin waves are strongest at the equator, they can extend up to\n~20°N/S, particularly when the heating or cooling source is asymmetric about the\nequator (Figure R7), as in Nan et al. (2014). Our LBM experiments confirm that a\ncooling source over North-Central Africa generates an atmospheric response\nconsistent with Kelvin wave dynamics (Figure R8 and R9), similar to findings by Li\n49\net al. (2022). Thus, our LBM results align well with established Kelvin wave theory\nand previous modelling studies.\nTo ensure clarity and scientific rigor, we have rewritten the relevant section in\nthe revised manuscript as follows (Lines 221-229):\n“According to Gill’s theory41, localized atmospheric heating and cooling\nanomalies generate distinct wave responses in the tropics, where Rossby waves\npropagate westward and Kelvin waves propagate eastward. While Kelvin waves are\nstrongest near the equator (~10° S–10° N), their influence can extend up to ~20° N,\ndepending on the structure and strength of the forcing40. Our LBM experiment\ndemonstrates that suppressed convection over North-Central Africa induces a cooling\nanomaly, triggering a Kelvin wave response with easterly wind anomalies over the\ntropical Indian Ocean. This result is consistent with previous study40.”\nFigure R7. Same as Figure 1 of Nan et al. (2014). Distributions of the isolated\nequatorially asymmetric heating and cooling sources given in Eq. (1). (a) The heating\nsource with the heating center at d=1; (b) as in (a) but for d=−1; (c) as in (a) but for\nthe cooling source; (d) as in (b) but for the cooling source. Here d represents the\nmeridional distance between the equator and heating center, d＞0 and d＜0 represent\nthat the heating centers are located in the southern and northern hemispheres,\n50\nrespectively. The thick solid and dashed lines represent heating and cooling sources,\nrespectively, and the contour interval is 0.2\nFigure R8. Same as Figure 3 of Nan et al. (2014). Horizontal wind (arrows), vertical\nvelocity (shading), and pressure (thick contours) solutions in the lower layer for the\nisolated asymmetric heating cases shown in Fig. 1. (a) Analytical solutions of\natmospheric variables by the heating in Fig. 1a; (b) as in (a) but for the heating in Fig.\n1b; (c) as in (a) but for the heating in Fig. 1c; (d) as in (a) but for the heating in Fig.\n1d. Thick solid (dashed) lines represent positive (negative) pressure, and contour\ninterval is 0.3.\nFigure R9. Same as Figure 5b of Li et al. (2022).The response of low-level\natmospheric wind field (m s-1) to the prescribed heating anomaly over the North\nAfrica (0º –20º N, 0º –55º E) (b) with the maximum heating rate of 1 K day-1.\nLi, T., Wang, Y., Wang, B., Ting, M., Ding, Y., Sun, Y., He, C. and Yang, G..\nDistinctive South and East Asian monsoon circulation responses to global warming.\nScience Bulletin, 67(7), 762-770 (2022).\nGill, A. E. Some simple solutions for heat-induced tropical circulation. Quart. J.\nRoyal. Meteoro. Soc. 106, 447–462 (1980).\nNan Xing, Li Jianping, Li Yaokun. Response of the tropical atmosphere to\nisolated equatorially asymmetric heating. Chinese Journal of Atmospheric Sciences\n(in Chinese), 38 (6): 1147−1158 (2014).\n#9. L243: ...seen in the LBM experiment results. The “seen in” is missing.\nOur response: Revised.\n#10. L282: Please add a reference to Gill’s theory.\nOur response: Following your valuable comment, we have added the reference of\n“Gill et al. (1980)” in Line 296 of the revised manuscript.\nGill, A. E. Some simple solutions for heat-induced tropical circulation. Quart. J.\nRoyal. Meteoro. Soc. 106, 447–462 (1980).\n#11. L319: … using THE fully coupled EC-Earth3 model.\nOur response: Revised.\n#12. L328: This conclusion is imprecise, because the LMB model does not show a\nweakening of the Somali Jet. Please rephrase!\nOur response: Following your valuable comment, we have rephrased this confusing\nsentence in Lines 344-349 of the revised manuscript.\n#13. L344-346: I find it unfavorable to use this sentence as the final sentence. It\n52\nimplies that the evolution of the SASM was the main motivation for the study. This is\nnot clear in the context of the study and is also not clear from the title. Instead, I\nwould like to see a final answer to the reasons for the decoupling and maybe a\nstatement what it means, e.g. for present or future climate. It is not clear from the\nstudy which effect predominates, but climate models project strong changes in the\nprecipitation pattern over Africa. Is it possible that we will experience a decoupling of\nthe Somali Jet and SASM precipitation again in the future, due to changes in the latent\nheat release in Africa?\nOur response: Thank you for raising this insightful suggestion. We agree that the\nconclusion should provide a definitive explanation for the decoupling mechanism and\nhighlight its broader relevance for both past and future climate. To address this, we\nhave revised the final section of the manuscript by clearly stating that African\ntopographic changes were the key driver of the Somali Jet-monsoon rainfall\ndecoupling; emphasizing the role of latent heat release in shaping large-scale\natmospheric circulation, reinforcing the importance of understanding past and future\nAfrican climate dynamics and providing a forward-looking statement on potential\nfuture implications, given projected shifts in African precipitation patterns under\nglobal warming.\nThis is clarified in Lines 356-375 of the revised manuscript as follows:\n“The key novelty of our study is the identification of a previously overlooked\ntectonic mechanism that drives the decoupling of SASM winds and rainfall during the\nMiocene. Unlike previous studies that primarily attribute SASM evolution to the\nchanges in land/ocean configuration74,75, changes in polar ice sheet development25,26,\nchanges in ocean gateways23,76, closure of the Tethys Seaway13,74,77, and uplift of\nHinalaya-Tibetan and other regions6,7,24, our results provide compelling evidence that\nAfrican topography changes were a dominant driver of SASM evolution.\nevidence, resolving the discrepancy between wind-based proxies in the Arabian Sea\nand rain-related proxies over the Indian continent and nearby Indian Ocean for SASM\nchanges from the Middle to Late Miocene. This is the first study to provide a\ncomprehensive dynamical explanation of this decoupling phenomenon using a state-\nof-the-art coupled climate model, highlighting the importance of accurately\nreconstructing the timing and magnitude of African topographic changes when\ninterpreting past monsoon evolution. Moreover, as climate models project significant\nchanges in precipitation over Africa due to global warming, similar latent heat-\ninduced atmospheric circulation changes could influence future SASM dynamics.\nThis raises the possibility that a decoupling of the Somali Jet and monsoonal rainfall\ncould occur again in a warming world, with potential consequences for regional\nhydroclimate, monsoon predictability, and water resource management in South\nAsia.”\nMethods:\n#14. L357: which IS the Nucleus for ...\nOur response: Revised\n#15. L454: “cooling rate of 6 to 7 K/day”... Does it mean that it gets cooler every\nday? I do not understand this.\nOur response: Thanks for the comment. The \"cooling rate of 6 to 7 K/day\" does not\nimply a continuous daily drop in temperature. Instead, it refers to the steady-state\ncooling imposed in the LBM experiments, which remains constant over time rather\nthan getting cooler every day.\nexplore the steady-state atmospheric response to a prescribed diabatic heating or\ncooling anomaly. The applied cooling rate of 6–7 K/day represents a fixed\nthermodynamic forcing at a given pressure level, rather than a progressive daily\ndecrease in temperature.\nIn the thermodynamic energy equation (Yanai et al., 1973; Knutson and Manabe,\n1995), the diabatic heating (or cooling) can be written as:\n(2)\n𝑄 𝜕𝑇 𝑝 𝜕𝜃\n𝑐𝑝 = 𝜕𝑡 +𝑉 ⃑ ∙∇𝑇−(𝑝0)𝜔𝜕𝑝\nwhere T, , , p, and are the temperature, potential temperature, horizontal wind,\npressure, a𝜃nd 𝑉 v⃗ertical pr𝜔essure velocity, respectively; is the specific heat of dry air\nat constant pressure, and is the reference pressure 𝑐(1𝑝000 hPa). The term\ndenotes the total diabatic h𝑝e 0 ating, with unit of K/day, indicating the prescribe𝑄d /ra 𝑐𝑝 te of\nthermal forcing rather than a cumulative temperature decrease.\nFor instant, Figure R10 shows the vertical profile of the specific cooling rate (K\nday-1) in the cooling center for Exp_1. In the LBM experiment, we prescribed the\nidealized cooling in tropical central Africa with a vertical profile following a gamma\nfunction, peaking at a cooling rate of 6 to 7 K day-1 at about 450 hPa (Figure R10).\nfor Exp_1.\nReference:\nKnutson, T.R. and Manabe, S.. Time-mean response over the tropical Pacific to\nincreased CO in a coupled ocean-atmosphere model. Journal of Climate, 8(9),\n2\npp.2181-2199 (1995).\nWatanabe, M. and Kimoto, M.. Atmosphere-ocean thermal coupling in the North\nAtlantic: A positive feedback [Software]. Quarterly Journal of the Royal\nMeteorological Society, 126, (570), 3343–3369 (2000).\nYanai, M., Esbensen, S. and Chu, J.H.. Determination of bulk properties of\ntropical cloud clusters from large-scale heat and moisture budgets. Journal of\nAtmospheric Sciences, 30(4), 611-627 (1973).\nFigures:\n#16. general: I guess units are always written in rectangular bracket, e.g. [m/s],\nrather than (m/s).\nOur response: Thanks for your reminder. After reviewing other articles in the journal\nof Nature Communications, we observed that the units are presented in parentheses\n(•) rather than square brackets [•]. Therefore, we have adhered to using parentheses\n(•) for units in our manuscript.\n#17. Fig.1: Maybe flip the time-axis in the reconstruction plot so that it is in line with\nthe other plots in this figure. Maybe you could additionally mark or explain in the\ncaption which proxies in i) to o) indicate wind and which indicate precipitation.\nOur response: Following your valuable, we have reversed the time-axis in the\nreconstruction plot as follows Figure R11 of the revised manuscript. Additionally, we\nhave labeled the proxies in Figure 1i-o to indicate whether they are wind-based\nproxies or rainfall-based proxies in the revised manuscript.\nFigure R11 Same as Figure 1i-o in the manuscript. i Total organic carbon (TOC, wt%)\nvalues from ODP Sites 722B, 728B, 730A, and 731A. j Planktic foraminifer\nGlobigerina bulloides (G. bulloides; %) from ODP 722B and 730A. k δ13C (PDB) of\n57\npaleosol carbonate (‰) from the Potwar Plateau and NW India. l δ18O (PDB) of soil\ncarbonate (‰) from the Potwar Plateau. m Chemical weathering proxies (CIA, %)\nfrom the Indus Marine A-1 well in the northern Arabian Sea. n Kaolinite/chlorite ratio\nfrom IODP Site U1447 in the western Andaman Sea and K/Al ratio from ODP Site\n718 in the Bengal fan. o Total sediment flux into the Indus fan (103 km3 Ma-1). The\nvertical shaded areas in i-o represent the time intervals of Middle Miocene Climate\nOptimum (MMCO, ~15 Ma) and Late Miocene (~5 Ma).\n#18. Fig.1: Have the precipitation anomalies been tested for significance?\nOur response: Thanks for your carefully reading and reminder. Following your\nvaluable comment, we have replotted and tested all figures in the revised manuscript\nand Supplementary materials using Student’s t-test. In these figures, the gray stippling\nand vectors are only shown at regions where changes are significant at the 95% confidence level.\n#19. Fig.1: Why do you use 700hPa level and in other plots 1000hPa or 925hPa or\n850hPa? Please check if you use this level for each plot referring to the Somali Jet to\nfacilitate the comparison.\nOur response: Thanks for your comments. The reasons for using 700-hPa wind\nanomaly were discussed in Question 2 in detail. According to your valuable comment,\nwe have further checked the levels of 1000-hPa to 850-hPa winds in Figure R12. As\nexpected, in all levels of 850-hPa, 925-hPa, and 1000-hPa, there is the significant\nweakening of the core of Somali Jet along the East Africa with the lowering of the\nEast African Highlands during the Miocene. Therefore, the results are consistent with\neach other when using different levels at the low-level troposphere.\nduring summer. Changes in wind speed (shading, m s-1) and horizontal wind (vectors,\nm s-1) at 850 hPa in the a MT25, b MT15 and c MT05 simulations relative to the pre-\nindustrial simulations. d-f Same as a-c, but for the 925-hPa winds. g-i Same as a-c,\nbut for the 1000-hPa winds. The black solid boxes mark the SASM region (10°-30°\nN, 60°-95° E). The blue solid boxes (10° S-10° N, 40°-50° E) mark the Somali Jet.\n#20. Fig.4: The unit for e) is missing… Is e) also showing the difference between MC\nand MT simulations? It would also be helpful to plot the area mean changes in the\nSomali Jet, similar to Fig.1\nOur response: Thanks for your comments. We have added the unit for Figure 4e in\nthe revised manuscript.\nrainfall over SASM region mean the difference between MC and MT simulations.\nFollowing your valuable suggestions, we have included the area averaged\nchanges in the Somali Jet as Figure R13e in the revised manuscript. Corresponding\ndescription is clarified in Lines 311-314 and 331-336 of the revised manuscript.\nFigure R13 Responses of atmospheric circulation to increased CO concentration\n2\nduring boreal summer. a Multi-proxy atmospheric CO levels (p.p.m.v) compiled\n2\nfrom previous literature, including stomata (black circles), δ13C of alkenones (red\ntriangles) and marine boron (black crosses). Changes in rainfall (shading, mm day-1)\nand 700-hPa wind (vectors, m s-1) in the b MC25, c MC15 and d MC05 simulations\n60\nrelative to the MT25, MT15 and MT05 simulations, respectively. In b-d, gray\nstippling and vectors denote regions in which the changes are significant at the 95% confidence level according to Student’s t-test, and the solid boxes mark the SASM\nregion. e Area-averaged changes in Somali Jet intensity (m s-1; see Methods) and\nratios of summer rainfall (%) over the SASM region in b-d due to increased CO\n2\nconcentration in b-d, with asterisks above the bars denoting changes that are\nsignificant at the 95% confidence level according to Student’s t-test.\n#21. Fig.5a: I’m wondering, if the cyclonic circulation arrows have the wrong position.\nThe winds are directed to the West Ghats and do not flow in western direction over\nIndia (cf. Fig.1). Similarly, the arrow for suppressed convection is located over the\nAfrican Highlands but not over the Congo basin...\nOur response: We apologize for the wrong position of the arrows. According to your\nsuggestion, we have replotted these two figures in the revised manuscript.\nSupplement:\n#22. General: Please check the spelling in all figure captions.\nOur response: Thank you for your reminder. We have checked and revised the\nspelling in all figure captions in the revised Supplementary materials.\n#23. Fig.1: Here, 850hPa winds are shown, why not consistent to the other plots\n(700hPa). Why are wind vectors shown in the simulation but not in the observation?\nThey experience the same orography…\nOur response: Thanks for the comments. We have discussed this in detail in\nQuestion 2. The primary reason is that the 850-hPa wind in low-level troposphere is\nsignificantly influenced by the topographical changes, which can subsequently affect\nthe cross-equatorial Somali Jet and upwelling in the western Arabian Sea.\nWe apologize for this mistake, and we have replotted the Supplementary Figure\n1 as follows in Figure R14a and b.\nand 850-hPa wind (m s-1), and c surface wind (vectors, m s-1) and its induced Ekman\npumping (cm day-1) during boreal summer from 1980 to 2010. b and d are the same\nas a and c, but for the results of pre-industrial simulation from EC-Earth2 model.\nMonthly rainfall in a is from the Global Precipitation Climatology Project (GPCP),\nand 850-hPa and surface horizontal winds are from the National Centers for\nEnvironmental Prediction reanalysis 2 (NCEP2). In a and b, red dots or boxes denote\nthe locations of the proxy records in Figure 1i-o, black boxes mark the SASM region\n(60° E-95° E, 5° N-30° N), and blue boxes (10º S-10º N, 40º -50º E) mark the cross-\nequatorial Somali Jet. In c and d, gray circles in western Arabian Sea denote the\nlocations of the proxy records in Figure 1i and j (same as in a and b), gray boxes (15º\n-21º N, 54º -62º E) denote the western Arabian Sea upwelling region that proxy\nrecords located, and blue boxes mark the cross-equatorial Somali Jet.\n62\n#24. Fig 2: Please add a note that only African changes in topography are shown here.\nOur response: Following your valuable suggestion, we have added this in the revised\nmanuscript.\n#25. Fig.4: Which levels were used to integrate the moisture flux?\nOur response: In this study, the vertically integrated moisture transport is from\nsurface to 10 hPa in our study, including the levels of 1000, 925, 850, 700, 500, 400,\n300, 200, 100, 50, and 10 hPa. This is clarified in Supplementary Figure 5 of the\nrevised Supplementary materials.\n#26. Fig.6: Is K/day the correct unit for Q?\nOur response: We apologize for the wrong unit for Q , and the unit should be m2 s-3.\nL\nWe have corrected the units of Q in the revised manuscript, and checked the whole\nL\nmanuscript to avoid this mistake.\n#27. Fig.8: Which levels have been used for the integration of the water vapor flux?\nDo the plots b-c really show anomalies to MT? This would mean that the CO has an\n2\neven stronger dynamic response than seen for the topography change.\nOur response: Thanks for the comment. We use the levels from surface to 10 hPa for\nthe integration of the water vapor flux in our study, including the levels of 1000, 925,\n850, 700, 500, 400, 300, 200, 100, 50, and 10 hPa. This is clarified in the\nSupplementary Figure 11 of the revised Supplementary materials.\nYes, Supplementary Figure 11b-d of the Supplementary materials illustrate\nanomalies between the MC and MT simulations, reflecting the impact of CO\n2\nchanges. However, these anomalies represent the response to increased CO alone, not\n2\nthe relative contribution of CO and African topography. As clarified in the Methods\n2\nsection, the MC simulations (MC25, MC15 and MC05) incorporate both CO changes\n2\n(500 ppm) and the corresponding African topography for each Miocene time slice.\nThe climate response to CO forcing alone is determined by the comparing MC25\n2\n63\nwith MT25, MC15 with MT15, and MC05 with MT05 simulations. Following your\ncomment, we further examined the relative contribution of dynamic effect on SASM\nrainfall changes induced by African topography changes and elevated CO\n2\nconcentration in Figure R15. It is evident that African topography changes have a\nsignificantly stronger dynamic effect on SASM rainfall than CO concentration\n2\nchanges, supporting our previous findings in the manuscript.\nFigure R15. The dynamic effect (see Methods in detail) averaged over the SASM\nregion. Blue and red bars mean the dynamic effect induced by African topography\nchanges and elevated CO concentration, respectively. Units: Pa kg m-2 s-1.\n2\n#28. Fig.9: This plot also shows the 850hPa while other plots use 700 hPa. It would\nbe easier to compare the figures and the effect of the different forcings if the levels\nwere standardized. Also for this plot it would be nice to add the area-mean change in\nthe Somali-Jet intensity (cf. Bar plot in Fig.1)\nOur response: Following your valuable comments, we have used 700-hPa winds in\nthe figures and also added the area-mean changes in the Somali Jet intensity in the\nrevised Supplementary Figure 12 as follows in Figure R16. This is clarified in Lines\n331-336 in the revised manuscript.\neffects of African topography changes and elevated CO levels. Changes in rainfall\n2\n(shading, mm day-1) and 850-hPa wind (vectors, m s-1) in the a MC25, b MC15 and c\nMC05 simulations relative to the pre-industrial simulation. d Area-averaged changes\nin Somali Jet intensity (m s-1; see Methods) and ratios of summer rainfall (%) over the\nSASM region in a-c due to combined effect of African topography and CO forcings,\n2\nwith asterisks above the bars denoting changes that are significant at the 95% confidence level according to Student’s t-test. In a-c, gray stippling and vectors\ndenote regions in which the changes are significant at the 95% confidence level\naccording to Student’s t-test.\n#29. Fig.9: The headings of the sub-figures are not correct.\nOur response: Thanks for the comment. After checking the headings of the sub-\nfigures carefully, we found it is correct. As clarified in the Methods section, MC25,\nMC15 and MC05 are the simulations with the CO concentrations set at 500 p.p.m.v\n2\nand African topography configurations corresponding to the Early, Middle and Late\nMiocene, respectively. Therefore, the climate response to total forcing, i.e., combined\n65\neffects of African topography changes and elevated CO concentrations, is denoted by\n2\nthe changes in MC25, MC15 and MC05 simulations relative to pre-industrial. This is\nclarified in Line 412 of the revised manuscript.\nReviewer #2 (Remarks on code availability):\nThe code is not available, only upon request.\nOur response: Thanks for your reminder. We have provided the code for the main\nresults in the new submission on Zenodo at https://doi.org/10.5281/zenodo.15605414.\nRemarks to the Author:\nRecent studies have shown that monsoon winds in the Arabian Sea and the South\nAsian monsoon are positively correlated on interannual timescales but not necessarily\non longer orbital timescales. In this study, Z. Han et al. extend this concept,\nsuggesting a similar decoupling over much longer tectonic timescales, supported by\nproxy evidence. They propose a mechanism based on climate model experiments,\nlinking the uplift of African orography during the middle to late Miocene to the South\nAsian Summer Monsoon (SASM). According to their model, the less elevated East\nAfrican orography induces anomalous latent heating in the atmosphere, triggering\ncirculation changes via Gill’s response. These circulation shifts alter low-level\nmonsoon winds, with the resulting SST anomalies (resembling a positive IOD) further\namplifying both monsoon winds and rainfall.\nInitially, I was very interested in the study and looked forward to reading the\nmanuscript. However, the results seem to suggest quite the opposite of the authors’\nprimary claim—that monsoon winds and rainfall are, in fact, decoupled on these\ntectonic timescales. There are three key issues:\n1. The regions chosen to represent monsoon winds do not overlap with the proxy\nregions, and in the proxy regions, the changes in winds shown by the climate model\nexperiments are not robust.\n2. Their moisture budget analysis indicates that SASM rainfall changes are\nstrongly linked to the dynamic component (i.e., monsoon winds), directly\ncontradicting their central hypothesis of decoupling.\n3. The authors interpret upwelling proxies as being linked to monsoon wind speed.\nHowever, it is well known that changes in wind speed, the latitude, or width of the\nlow-level jet (LLJ) can all influence upwelling. The correct interpretation should be\nthat upwelling is not coupled with rainfall.\nDue to these issues, I do not find the author’s primary claim convincing.\nHowever, I find their detailed mechanism linking African orography to SASM rainfall\nquite interesting and could offer an alternative focus for the study. Unfortunately, this\nconnection between African orography and SASM rainfall is already well-established\nin the literature. Moreover, the connection between changes in African rainfall and its\nimpact on upwelling in the northern Arabian Sea and SASM rainfall has been\ndemonstrated in previous studies. While the authors’ mechanism is well-constructed,\nit lacks the novelty required for publication in a high-impact journal like Nature\nCommunications.\nFor these reasons, I recommend rejecting the manuscript in its current form. I\nencourage the authors to refine their focus on the proposed mechanism, enhance the\nrigor of their experiments. The current experiments need refinement to better align\nwith their claims, and addressing these issues will significantly improve the paper’s\noverall impact. My major and minor concerns, outlined below, should provide helpful\nguidance for their revisions.\nOur response: We would like to thank you for professional review work,\nconstructive comments, and valuable suggestions on our manuscript. Your time and\nefforts are greatly appreciated. We have strived to improve the manuscript\naccordingly as listed in details below, and hope that you will find the added\ninformation suitable and sufficient for publication. The original comments are quoted\nin blue and the changes in the revised manuscript have been highlighted in red.\nGeneral Comments:\n#1. Choice of regions for analysis: (1) The authors use proxy data to demonstrate\nthat upwelling in the northern Arabian Sea and SASM rainfall were not coupled\nduring the middle to late Miocene monsoon evolution. However, in their model\nanalysis, they selected a study region significantly south of the proxy region.\nAnalyzing this area in isolation leads them to conclude that the low-level jet (LLJ)\nwind speed and monsoon rainfall are not coupled. However, they overlook an\nimportant detail: the LLJ, and consequently the monsoon winds, show a significant\nincrease in speed in the region immediately west of their chosen area. This stronger\nLLJ impacts the western coast of India, producing substantial rainfall over the\nWestern Ghats and a slight increase in rainfall across mainland India. Therefore, the\nauthors’ claim that the LLJ/monsoon winds and SASM rainfall are decoupled is not\nfully supported. In fact, a more careful examination of the wind changes in the proxy\nregion reveals a much more complex response, which the authors have not addressed\nin their analysis.\n(2) Furthermore, I find the authors’ choice of region for defining the SASM\nproblematic. The precipitation response plots indicate a pronounced change in rainfall\nalong the western coast, extending into the adjacent Arabian Sea, while most of inland\nIndia—the core SASM region—shows insignificant changes in rainfall. The rainfall\nsignal used by the authors to support their claim is predominantly a result of offshore\nprecipitation along the western coast of India, which is a direct consequence of\nchanges in the LLJ/monsoon winds.\nThus, I find the authors’ conclusion in the model analysis unconvincing and\nsensitive to their choice of region. Their claim appears somewhat overstated and does\nnot fully align with the complexity of the wind-rainfall relationship seen in the data.\nOur response: Thank you for these important observations. We agree that a more\ndetailed analysis of Somali Jet dynamics and its regional impact on rainfall is\nessential to strengthen our conclusions. Below, we address both points in detail.\n(1) Our further analysis confirms that the cross-equatorial Somali Jet is significantly\nweakened in the tropical western Indian Ocean, while winds along the western\nArabian Sea are strengthened in the MT25 and MT15 simulations compared to pre-\nindustrial simulation (Figure R1). This south weakened-north strengthened wind\npattern exactly leads to weakened Arabian Sea upwelling (Please see Question 3 for\nmore details), particularly along the proxy sites (Figure R2).\nThe weakened Somali Jet results in reduced upwelling along the western Arabian\nSea coast, in agreement with wind-proxy records (Figure 1i and j in the manuscript).\nThis upwelling response is not simply a function of wind intensity but is also driven\nby structural changes in the Somali Jet, support the idea that the proxy-based\ninterpretation must account for both effects (Please see Question 3 for more details).\nRecent work by Sarr et al. (2022) supports our findings, showing that the uplift\nof the East African topography played a pivotal role in the establishment of the\nmodern Somali Jet structure and upwelling dynamics from the Early Miocene\n(~20Ma) to the Late Miocene (~10Ma).\nIn summary, the further considering of wind structure changes and ocean\nupwelling in western Arabian Sea can’t change the conclusions we have drawn from\nour original results, confirming the robustness of our primary findings. In the revised\nmanuscript, we have expanded the discussion of Somali Jet structural changes and\ntheir impact on upwelling in Lines 121-132, 146-151, 442-445, 458-461, and 499-\n505. Additionally, we have included Figure R1 and R2 as Supplementary Figure 3\nand 4 in the revised Supplementary materials, and Figure R3 as Figure 1g and 1h in\nthe revised manuscript. Thank you again for guiding us toward a more robust and\ndetailed analysis.\n(2) We have adjusted the color bar range and performed student’s t-test on\nrainfall and wind data to enhance the visibility of significant rainfall changes over the\ninland Indian (Figure R4a-c). These updated figures show statistically significant\nincreases in inland SASM rainfall, particularly in Early and Middle Miocene\nsimulations.\nTo further confirm these rainfall anomalies, we examined the changes in\noutgoing longwave radiation (OLR) (Figure R4d-f). The significant decrease in OLR\nover inland India confirms that deep convection and enhance rainfall occurred in these\nregions during the Early and Middle Miocene. We separately calculated the changes\nin SASM rainfall over ocean and land (Figure R5) to determine the spatial distribution\nof precipitation anomalies. Our results confirm that both land and oceanic SASM\nrainfall increased during the Early and Middle Miocene, coinciding with the period of\nlower East African Highlands.\nFigure R1. Changes in atmospheric circulation due to African topography changes\nduring boreal summer. Climatological mean 850-hPa winds (vectors, m s-1) and its\nwind speed (shading, m s-1) in the a pre-industrial, b MT25, c MT15 and d MT05\nsimulations. Responses to Miocene African topography changes in e MT25, f MT15\nand g MT05 simulations compared to the pre-industrial simulation. Black boxes mark\nthe SASM region, and blue boxes mark the cross-equatorial Somali Jet. In e-g, gray\nstippling denotes regions in which the changes are significant at the 95% confidence\nlevel according to Student’s t-test.\nFigure R2. Responses of ocean upwelling over western Arabian Sea to African\ntopography changes during boreal summer. Changes in vertical velocity of surface\nocean (0-100m average, cm day-1) in the a MT25, b MT15 and c MT05 simulations\ncompared to pre-industrial simulations. The gray circles in western Arabian Sea\ndenote the locations of the proxy records in Figure 1i and j in the manuscript. The\nblack boxes denote the western Arabian Sea upwelling region that proxy records\nlocated. Gray stippling denotes regions in which the changes are significant at the\n95% confidence level according to Student’s t-test.\nFigure R3. Response of simulated SASM to African topography changes during the\nMiocene in the coupled EC-Earth3 experiments. The changes in a intensity of Somali\nJet (m s-1) and western Arabian Sea upwelling (cm day-1), and b ratios of SASM\nrainfall (%), with asterisks above or below the bars denoting changes that are\nsignificant at the 95% confidence level according to Student’s t-test. The solid blue\nboxes (15 S-10 N, 37.5 -62.5 E) and solid purple boxes (15 -21 N, 54 -62 E)\nin Figure R1 are used to define the intensity of Somali Jet and western Arabian Sea\nupwelling, respectively.\nFigure R4. Responses of SASM to African topography changes during boreal\nsummer. Changes in rainfall (shading, mm day-1) low-level atmospheric circulation at\n700 hPa (vectors, m s-1) in the a MT25, b MT15 and c MT05 simulations compared to\npre-industrial simulations. d-f Same as a-c, but for the changes in outgoing longwave\nradiation (OLR, W m-1). Gray stippling and vectors are only shown at regions where\nthe changes are significant at the 95% confidence level according to Student’s t-test.\nFigure R5. Response of simulated SASM rainfall (mm day-1) to African topography\nchanges during the Miocene, with asterisks below the bars denoting changes that are\nsignificant at the 95% confidence level according to Student’s t-test.\n#2. Moisture budget analysis: My concerns are further heightened by the authors’\nmoisture budget analysis. Their results clearly indicate that changes in precipitation\nover the SASM region are driven by the dynamic component (i.e., changes in\nmonsoon winds). This presents a significant contradiction: on one hand, the authors\nclaim that monsoon rainfall and winds are decoupled, yet their own analysis shows\nthat the changes in rainfall are directly linked to wind dynamics. This inconsistency\nundermines their central argument and raises questions about the validity of their\nconclusions.\nOur response: We appreciate this insightful comment, as it allows us to further\nclarify the concept of monsoon wind-rainfall decoupling in the context of tectonic-\nscale changes.\nDecoupling does not imply a complete lack of relationship between monsoon\nwinds and rainfall but rather signifies a nonlinear and regionally varying response of\nmonsoonal precipitation to changes in Somali Jet intensity. In present-day climate,\nthere is a strong correlation between Somali Jet and SASM rainfall, as the jet directly\ncontrols moisture transport (Chakraborty et al., 2009; Chen et al., 2023). However,\npaleo-proxy records suggest that this relationship did not hold consistently over\ntectonic timescales (Sarr et al., 2022). Wind-based proxies (e.g., Kroon et al., 1991;\nHuang et al., 2007; Gupta et al., 2015) indicate that the Somali Jet strengthened after\n~13 Ma. Rainfall-related proxies indicate the increased rainfall during the Middle\nMiocene (~15 Ma) (Quade et al., 1989; Quade and Cerling ,1995; Clift and Webb,\n2019; Lee et al., 2020) despite weaker Somali Jet intensity. Therefore, this apparent\ncontradiction is what led to the hypothesis of wind-rainfall decoupling during the\nMiocene.\nThe decoupling refers to the reversal of the expected relationship—instead of a\nstronger Somali Jet leading to increased rainfall, our simulations show that a\nweakened Somali Jet can still lead to enhanced SASM rainfall via indirect ocean-\natmosphere feedbacks.\nThe dynamic term in the moisture budget analysis captures the effect of large-\nscale circulation changes on rainfall. Our results indicate that changes in African\ntopography trigger large-scale atmospheric circulation anomalies, which in turn\nenhance moisture transport and increase SASM rainfall. This dynamic response does\nnot contradict the decoupling hypothesis but rather provides a mechanistic\nexplanation for why rainfall increased despite a weaker Somali Jet. The key process\ninvolves ocean-atmosphere interactions: A weakened Somali Jet leads to a positive\nIndian Ocean Dipole (IOD)-like warming pattern, which weakens the Indian Ocean\nWalker circulation and enhances the moisture supply to the SASM region. This\nprocess compensates for the reduction in moisture transport due to a weaker Somali\nJet, ultimately increasing rainfall in the SASM region.\nOur results are supported by recent study of Sarr et al. (2022), which highlight\nthat the East African topography played a pivotal role in the establishing the modern\nSomali Jet structure above the western Indian Ocean during the Miocene.\nIn the revised manuscript, we expanded discussion on the role of dynamic effects\nin rainfall changes and clarified how moisture budget analysis supports the\ndecoupling hypothesis. We highlighted the indirect ocean-atmosphere feedbacks that\ndrive increased SASM rainfall despite a weaker Somali Jet (Lines 185-189 and 192-\n195).\nReference:\nChakraborty, A., Nanjundiah, R. S. and Srinivasan, J. Impact of African\norography and the Indian summer monsoon on the low‐level Somali jet. Int. J.\nClimatol. 29, 983–992 (2009).\nChen, S. et al. Impact of interannual variation of the spring Somali Jet intensity\non the northwest–southeast movement of the South Asian High in the following\nsummer. Clim. Dyn. 60, 1583–1598 (2023).\nClift, P. D. and Webb, A. A. G. A history of the Asian monsoon and its\ninteractions with solid Earth tectonics in Cenozoic South Asia. SP 483, 631–652\n(2019).\nGupta, A. K., Yuvaraja, A., Prakasam, M., Clemens, S. C. and Velu, A. Evolution\nof the South Asian monsoon wind system since the late Middle Miocene. Palaeogeogr.\nPalaeoclimatol. Palaeoecol. 438, 160–167 (2015).\nHuang, Y., Clemens, S. C., Liu, W., Wang, Y. and Prell, W. L. Large-scale\nhydrological change drove the late Miocene C4 plant expansion in the Himalayan\nforeland and Arabian Peninsula. Geology 35, 531 (2007).\nKroon, D., Steens, T. and Troelstra, S. R. Onset of monsoonal related\nupwelling in the western Arabian sea as revealed by planktonic foraminifers 1. In\nProceedings of the ocean drilling program, scientific results 1, 257–263 (1991).\nLee, J. et al. Monsoon-influenced variation of clay mineral compositions and\ndetrital Nd-Sr isotopes in the western Andaman Sea (IODP Site U1447) since the late\nMiocene. Palaeogeogr. Palaeoclimatol. Palaeoecolo. 538, 109339 (2020).\nQuade, J., Cerling, T. E. and Bowman, J. R. Development of Asian monsoon\nrevealed by marked ecological shift during the latest Miocene in northern Pakistan.\nNature 342, 163–166 (1989).\nQuade, J. and Cerling, T. E. Expansion of C4 grasses in the Late Miocene of\nNorthern Pakistan: evidence from stable isotopes in paleosols. Palaeogeogr.\nPalaeoclimatol., Palaeoecol. 115, 91–116 (1995).\nTurner, A. G. & Annamalai, H. Climate change and the South Asian summer\nmonsoon. Nat. Clim. Change 2, 587–595 (2012).\nSarr, A.C., Donnadieu, Y., Bolton, C.T., Ladant, J.B., Licht, A., Fluteau, F.,\nLaugié, M., Tardif, D. and Dupont-Nivet, G.. Neogene South Asian monsoon rainfall\nand wind histories diverged due to topographic effects. Nature Geoscience, 15(4),\n314-319 (2022).\n#3. Interpretation of upwelling proxies:\n(1) In this study, the authors claim that monsoon winds and rainfall are decoupled, based on\nproxy records. However, the correct interpretation should be that upwelling in the northern\nArabian Sea and rainfall are decoupled. Upwelling in this region is influenced by various factors,\nincluding wind speed, the latitude, and width of the low-level jet (LLJ). Even slight changes to\none or more of these LLJ characteristics can modulate upwelling and, by extension, rainfall (see\nrecent studies on impact of orbital forcing on upwelling and rainfall for example). Therefore, the\nprimary claim of this study—that monsoon winds and rainfall are decoupled—seems questionable.\nThe relationship between winds, upwelling, and rainfall is more complex than the authors suggest,\nand their conclusions may oversimplify these dynamics.\n(2) This is further supported by their figures of SST anomalies. The SST anomalies during\nthe middle to late Miocene are negative in the northern Arabian Sea, indicating enhanced\nupwelling. This region, which spans the coast of Yemen and Oman and extends into the open sea,\nencompasses most of the proxy locations. This again indicates that upwelling-rainfall are\ndecoupled not necessarily monsoon winds and rainfall.\nOur response: These are really good and important questions that we have not\nconsidered in our previous manuscript version. Indeed, the relationship between\nwinds, upwelling, and rainfall is more complex than we initially thought. Following\nyour comments, we further analyzed the western Arabian Sea upwelling and\ncalculated the Ekman pumping to evaluate the complex relationship. Additionally, we\nexamined the sea surface temperature (SST) anomalies in the Arabian Sea by\ncalculating the heat budget equation for the upper-ocean mixed layer. Our further\nanalysis confirms that SASM wind-induced upwelling and rainfall are indeed\ndecoupled during the Miocene, supporting our previous results.\n(1) Indeed, upwelling in the western Arabian Sea is more complicated than\ninitially thought, which is controlled by changes in both the strength and the patterns\nof Somali Jet (Le Mézo et al., 2017; Jalihal et al., 2022; Wen et al., 2024). These\nchanges in monsoon winds can lead to wind-stress curl anomaly, driving the ocean\nupwelling through “Ekman pumping” (Rykaczewski and Checkley, 2008; Wen et al.,\n2024). To check these processes caused by the Miocene African topography changes,\nwe further use monthly wind stress to calculate the Ekman pumping velocity as\nfollows:\n(1)\n1 𝜏 1\n𝑊𝐸𝑘𝑚𝑎𝑛 = 𝜌𝑤∇×(𝑓) ≈ 𝜌𝑤𝑓∇×𝜏\nwhere is seawater density (1024 kg m-3), is the Coriolis parameter, and\nis the w\n𝜌\nin𝑤d-stress curl.\n𝑓 ∇×𝜏\nAs expected, our model successfully replicates the present-day SASM system,\ncharacterized by a strong cross-equatorial Somali Jet and increased rainfall over the\nIndian subcontinent during the boreal summer (Figure R6a and b). Due to the Coriolis\nforce, the climatological southwesterly monsoon wind over the Arabian Sea drive\nstrong ocean upwelling along the coastal region during summer (Figure R6c and d).\nThe agreement between our pre-industrial simulation and contemporary observations\nvalidates the use of EC-Earth3 for exploring the influence of the African topography\non the SASM.\nWe agree to you that our claim should not be simply interpreted as \"monsoon\nwinds and rainfall are decoupled\", but rather that the structure of the Somali Jet has\nchanged, leading to regionally complex upwelling responses, which in turn affected\nSASM rainfall. Our further analysis found that the African topography changes can\nlead to a remarkably weakened cross-equatorial Somali Jet while strengthened winds\nalongshore the Arabian Peninsula at low-level troposphere, particularly in the Early\nand Middle Miocene (Figure R1e and f). This south weakened-north strengthened\nwind patterns decrease the wind-stress curl alongshore the Arabian Peninsula\n( <0), leading to negative Ekman pumping and weaker upwelling there in the\nEarly and Middle Miocene (Figure R3 and R7). Despite weaker Somali Jet intensity\n∇×𝜏\noverall, monsoon rainfall increases due to ocean-atmosphere feedbacks, explaining\nthe decoupling observed in proxies (Figure 1i and j in the manuscript).\nProxy records, i.e., Total organic carbon (TOC) values and Planktic foraminifer\nGlobigerina bulloides (G. bulloides), indicate weakened upwelling during the Middle\nMiocene (Figure 1i and j in the manuscript), aligning with our model results (Figure\nR3 and R7). That is, the changes in Somali Jet induced by African topography\nchanges alter the wind-stress curl, reducing upwelling in the western Arabian Sea\nduring the Miocene (Figure R7). Additionally, TOC values and G. bulloides are\nwidely used to estimate the SASM wind intensity (Huang et al., 2007; Gupta et al.,\n2015), as they predominantly reflect wind-driven oceanic upwelling and primary\nproduction in the western Arabian Sea. Our further analysis suggests that the wind-\nbased proxies, which is reflected the Arabian Sea upwelling changes, should be\ncarefully considered, particularly on geological timescales, when used as an indicator\nof the changes in the intensity of SASM.\nTherefore, the further considering of wind structure changes and upwelling in\nwestern Arabian Sea can’t change the conclusions we have drawn from our original\nresults, confirming the robustness of our primary findings. In the revised manuscript,\nwe clarified that our claim is about the structural change of the Somali Jet and its\nimpact on regional upwelling, not a simple decoupling of monsoon winds and rainfall.\nWe expanded discussions on the complexities of Somali Jet variability and upwelling\n(Lines 31, 104-106, 121-132, 442-445, 458-461, and 499-505), and provided a more\ncomprehensive and coherent analysis. Additionally, Figures R1, R6, and R7 has been\nadded as Supplementary Figures 3, 1 and 4 in the revised Supplementary materials.\n(2) Regarding the question about negative SST anomaly in western Arabian Sea\nwhere the upwelling is weakened.\nSSTs are governed by both atmospheric and oceanic processes. On the\natmospheric side, wind speed, air temperature, cloudiness, and humidity are the\ndominant factors regulating the exchange of energy at the sea surface. On the oceanic\nside, heat transport by currents, vertical mixing, and boundary layer depth influence\nSST. To explore the mechanism of negative SST anomaly in the western Arabian Sea,\nwe further calculate the heat budget equation as follows.\nMathematically, the heat budget for the upper-ocean mixed layer may be written\nas:\n(2)\n𝜕𝑇 𝑄𝑛𝑒𝑡 𝜕𝑇\nwhere T is the mixed l𝜕a𝑡y=er 𝜌te𝐶m𝑝𝐻pe−ra𝑉 t ⃑ u∙re∇,𝑇−𝜔 𝜕i𝑧s +the𝑅 n𝑒e𝑠t surface energy flux, is the\ndensity of seawater, is the specific he\n𝑄\na𝑛t 𝑒o𝑡f seawater, H is the mixed layer\n𝜌\ndepth,\nis the horizontal cu𝐶r 𝑝 rent, and is the vertical velocity. is defined as:\n𝑉 ⃑ 𝜔 𝑄(𝑛3𝑒)𝑡\n𝑄𝑛𝑒𝑡 = 𝑄𝑠𝑏 +𝑄𝑙𝑏 +𝑄𝑠𝑤 +𝑄𝑙𝑤\nwhere Q is the sensible heat flux, Q is the latent heat flux, Q is the net shortwave\nsb lb sw\nradiation at surface, Q is the net longwave radiation at surface.\nlw\nThe terms on the right-hand side of Eq. (2) are net surface heat flux, horizontal\ntemperature advection, vertical temperature advection, and residual terms. In\nclimatology, there is merely a balance between the terms on the right hand of Eq. (2)\nas the temporal variation can be ignored (Deser et al., 2010; Schneider et al., 2012;\nLong et al., 2014; Wen et al., 2018):\n(4)\n𝑄𝑛𝑒𝑡 𝜕𝑇\nAccording to Eq.0 (4=), 𝜌o𝐶u𝑝r𝐻 fu−rt𝑉h ⃑ e∙r ∇h𝑇ea−t b𝜔ud𝜕g𝑧e+t a𝑅n𝑒al𝑠ysis confirms that the simulated\nnegative SST anomalies are primarily driven by horizontal advection rather than\nupwelling anomalies (Figure R8). The weakened upwelling should theoretically cause\nwarming (Figure R8g-i), but horizontal advection compensates (Figure R8d-f),\nleading to net cooling in the western Arabian Sea (Figure 2b and e in the revised\nmanuscript). Thus, negative SST anomalies in the Arabian Sea do not indicate\nenhanced upwelling. Instead, they result from changes in ocean heat transport and\nlarge-scale circulation.\nTherefore, the simulated negative SST anomalies during the Middle to Late\nMiocene in the western Arabian Sea are primarily due to horizontal temperature\nadvection anomalies rather than upwelling anomalies. The negative SST anomalies do\nnot contradict our findings of the weakened upwelling during the Early and Middle\nMiocene.\nFigure R6. Model validation. a Observed climatological mean rainfall (mm day-1) and\n850-hPa wind (m s-1), and c surface wind (vectors, m s-1) and its induced Ekman\npumping (cm day-1) during boreal summer from 1980 to 2010. b and d are the same\nas a and c, but for the results of pre-industrial simulation from EC-Earth3 model.\nMonthly rainfall in a is from the Global Precipitation Climatology Project (GPCP),\nand 850-hPa and surface horizontal winds are from the National Centers for\nEnvironmental Prediction reanalysis 2 (NCEP2). In a and b, red dots or red boxes\ndenote the locations of the proxy records in Figure 1i-o in manuscript, and black\nboxes (5° N-30° N; 60° E-95° E) mark the SASM region. In c and d, gray circles in\nwestern Arabian Sea denote the locations of the proxy records (same as in Figure R1a\nand b). In a-d, blue boxes (10º S-10º N, 40º -50º E) mark the cross-equatorial Somali\nJet and purple boxes (15º -21º N, 54º -62º E) mark the western Arabian Sea upwelling\nregion that proxy records located.\nFigure R7. Responses of ocean upwelling over western Arabian Sea to African\ntopography changes during boreal summer. Changes in vertical velocity of surface\nocean (0-100m average, cm day-1) in the a MT25, b MT15 and c MT05 simulations\ncompared to pre-industrial simulations. d-f Same as a-c, but for the changes in Ekman\npumping (see Methods, cm day-1). The gray circles in western Arabian Sea denote the\nlocations of the proxy records in Figure 1i and j. The black boxes (same as in Figure\nR1 and R2) denote the western Arabian Sea upwelling region that proxy records\nmainly located. The gray stippling denotes regions in which the changes are\nsignificant at the 95% confidence level according to Student’s t-test.\nFigure R8. Changes in SST equation terms over western Arabian Sea due to African\ntopography changes during boreal summer (calculated by Equation 4). Changes in net\nsurface heat flux in the a MT25, b MT15 and c MT05 simulations compared to pre-\nindustrial simulations. d-f Same as a-c, but for the changes in horizontal temperature\nadvection. g-i Same as a-c, but for the changes in vertical temperature advection. j-l\nSame as a-c, but for the residual term. The black boxes (same as in Figure R1, R2 and\nR4) denote the western Arabian Sea upwelling region that proxy records located.\nUnits: ℃ month-1.\nReference:\nDeser, C., Alexander, M.A., Xie, S.P. and Phillips, A.S., 2010. Sea surface\ntemperature variability: Patterns and mechanisms. Annual review of marine\nscience, 2(1), pp.115-143 (2010).\nGupta, A. K., Yuvaraja, A., Prakasam, M., Clemens, S. C. and Velu, A. Evolution\nof the South Asian monsoon wind system since the late Middle Miocene. Palaeogeogr.\nPalaeoclimatol. Palaeoecol. 438, 160–167 (2015).\nHuang, Y., Clemens, S. C., Liu, W., Wang, Y. & Prell, W. L. Large-scale\nhydrological change drove the late Miocene C4 plant expansion in the Himalayan\nforeland and Arabian Peninsula. Geology 35, 531 (2007).\nJalihal, C., Srinivasan, J. and Chakraborty, A. Response of the Low‐Level Jet\nto Precession and Its Implications for Proxies of the Indian Monsoon. Geophysical\nResearch Letters, 49(2), p.e2021GL094760 (2022).\nLe Mézo, P., Beaufort, L., Bopp, L., Braconnot, P. and Kageyama, M. From\nmonsoon to marine productivity in the Arabian Sea: insights from glacial and\ninterglacial climates. Climate of the Past, 13(7), 759-778 (2017).\nRykaczewski, R.R. and Checkley Jr, D.M.. Influence of ocean winds on the\npelagic ecosystem in upwelling regions. Proceedings of the National Academy of\nSciences, 105(6), 1965-1970 (2008).\nSchneider, E. K., and Fan, M. Observed decadal north Atlantic tripole SST\nvariability. Part II: Diagnosis of mechanisms. Journal of the Atmospheric Sciences,\n69 (1): 51-64 (2012).\nWen, Q., Yao, J., Döös, K. and Yang, H., 2018. Decoding hosing and heating\neffects on global temperature and meridional circulations in a warming climate.\nJournal of Climate, 31(23), 9605-9623 (2018).\nWen, Q., Liu, Z., Liu, J., Clemens, S., Jing, Z., Wang, Y., Lv, G., Yan, M., Ning,\nL., Yuan, L. and Gao, Y., 2024. Contrasting responses of Indian summer monsoon\nrainfall and Arabian Sea upwelling to orbital forcing. Communications Earth &\nEnvironment, 5(1), 409 (2024).\nZhuang, G., Pagani, M. and Zhang, Y. G. Monsoonal upwelling in the western\nArabian Sea since the middle Miocene. Geology 45, 655–658 (2017).\n#4. Missing experiments: The authors conducted experiments where global\norography is set to pre-industrial levels, except over Africa, where it is adjusted to 25\nMa, 15 Ma, and 5 Ma. However, for a well-constructed study, I would first expect\nresults from experiments where global orography is set to match these time periods\nfully. Following that, the authors should demonstrate that modifying only African\norography is sufficient to capture the majority of wind and rainfall responses. This\napproach would strengthen their case for the impact of African orography—although,\nI still believe it won’t support the claim that monsoon winds and rainfall are\ndecoupled.\nOur response: Thank you for raising this important point. Although this is a valuable\nidea, conducting fully global topography experiments poses a challenge due to the\nlack of high spatiotemporal resolution paleo-geographic and topography\nreconstructions. Nonetheless, we have carefully reassessed our approach and findings,\nand we demonstrate below why our methodology remains robust in addressing the\nrole of African topography in modulating the SASM system.\nIn this study, the topography reconstruction is based on Moucha and Forte\n(2011), which provide dynamic topography estimates only for Africa for the Early\n(25Ma), Middle (15Ma) and Late Miocene (5Ma). In contrast, global orography\nreconstructions for the Miocene are limited and mainly available for the Middle\nMiocene (~15Ma) (Frigola et al., 2018; Poblete et al., 2021). Therefore, the lack of\nthe reconstructed global orography in all three time slices that we focused is not\nfeasible to implement consistent global topography experiments for our study.\nSarr et al. (2022) assessed the impact of multiple topography changes, including\nthe Himalayan-Tibetan Plateau and East African Highlands, and found that\nHimalayan uplift primarily influenced early Miocene rainfall, while East African\nuplift played a dominant role from the Middle to Late Miocene. Given the significant\nuplift of the Himalaya-Tibetan Plateau mainly occurred before ~20 Ma (Ding et al.,\n2022), its impact on monsoon dynamics in our study period (15–5 Ma) is likely\nsecondary compared to African topography changes. These findings suggest that\nfocusing on African topography as the primary boundary forcing is a reasonable and\nwell-supported approach. Several recent studies examining Miocene climate change\nhave focused on specific topographic regions rather than global orography changes\n(Burls et al., 2011; Chakraborty et al., 2009; Wei and Bordoni, 2016; Sarr et al., 2022;\nZuo et al., 2024). These studies highlight that African topography plays a key role in\nshaping modern atmospheric circulation, supporting our focus on isolating its effects\nin our model simulations.\nOur results successfully reproduce the well-documented transition from a\nweakened to a stronger Somali Jet between the Middle and Late Miocene, in\nagreement with wind-based proxies (Gupta et al., 2015; Betzler et al., 2016). We also\ncapture the increased SASM rainfall during the Middle Miocene, as indicated by\nrainfall-based proxies (Quade et al., 1989; Clift and Webb, 2019). These findings\ndemonstrate that modifying only African orography is sufficient to explain key\nfeatures of monsoon evolution and reconcile the proxy records.\nWe expanded our analysis to examine Arabian Sea upwelling and Ekman\npumping, confirming that the changes in the Somali Jet directly modulate ocean\ndynamics, aligning with wind-proxy reconstructions (Figure R3, Supplementary\nFigure 4). Our moisture budget analysis (Figure 1) further supports that African\ntopography changes alone induce the observed SASM rainfall changes via dynamic\ncirculation adjustments. We emphasize that decoupling does not mean a complete\nlack of relationship between monsoon winds and rainfall but rather a shift in their\nresponse due to changes in the Somali Jet structure and ocean-atmosphere\ninteractions. Even when we account for potential uncertainties in CO levels, our\n2\nsimulations with elevated CO (MC experiments) do not show the same decoupling,\n2\nreinforcing the dominant role of African topography.\nAlthough additional fully global topography experiments are valuable, such\nexperiments would require global reconstructions that do not currently exist, they are\nbeyond the scope of our paper. Nonetheless, both previous studies and our further\nanalysis confirm that African topography is the primary control on the SASM\nevolution during the studied period (15–5 Ma). Our model results are consistent with\nproxy records, supporting the validity of our approach.\nIn the revised manuscript, we expanded discussion on the rationale for focusing\non African topography (Lines 72-76), and emphasized that our results align with prior\nstudies that similarly focused on African topography without requiring global\norography changes (Sarr et al., 2022) (Lines 76-79, 119-120). And we clarified that\nour further analysis conforms that the decoupling of Somali Jet and rainfall induced\nAfrican topography changes are robust (Lines 121-132, and 142-151).\nReference:\nBialik, O. M. et al. Monsoons, upwelling, and the deoxygenation of the\nNorthwestern Indian ocean in response to middle to late Miocene global climatic\nshifs. Paleoceanogr. Paleoclimatol. 35, e2019PA003762 (2020).\nBetzler, C. et al. The abrupt onset of the modern South Asian Monsoon winds.\nSci. Rep. 6, 29838 (2016).\nBurls, N.J., Bradshaw, C.D., De Boer, A.M., Herold, N., Huber, M., Pound, M.,\nDonnadieu, Y., Farnsworth, A., Frigola, A., Gasson, E. and von der Heydt, A.S..\nSimulating miocene warmth: insights from an opportunistic multi‐model ensemble\n(MioMIP1). Paleoceanography and Paleoclimatology, 36(5), p.e2020PA004054\n(2021).\nClif, P. D. and Webb, A. A. G. History of the Asian monsoon and its interactions\nwith solid earth tectonics in Cenozoic South Asia. Geol. Soc. Lond. Spec. Publ. 483,\n875–880 (2019).\nChakraborty, A., Nanjundiah, R. S. and Srinivasan, J. Impact of African\norography and the Indian summer monsoon on the low-level Somali jet. Int. J. Clim.\n29, 983–992 (2009).\nDing, L., Kapp, P., Cai, F., Garzione, C.N., Xiong, Z., Wang, H. and Wang, C..\nTiming and mechanisms of Tibetan Plateau uplift. Nature Reviews Earth &\nEnvironment, 3(10), 652-667 (2022).\nDing, L. et al.. Quantifying the rise of the Himalaya orogen and implications for\nthe South Asian Monsoon. Geology 45, 215–218 (2017).\nFrigola, A., Prange, M. and Schulz, M.. Boundary conditions for the middle\nMiocene climate transition (MMCT v1. 0). Geoscientific Model Development, 11(4),\n1607-1626 (2018).\nGupta, A. K., Yuvaraja, A., Prakasam, M., Clemens, S. C. & Velu, A. Evolution\nof the South Asian monsoon wind system since the late Middle Miocene. Palaeogeogr.\nPalaeoclimatol. Palaeoecol. 438, 160–167 (2015).\nHuang, Y., Clemens, S. C., Liu, W., Wang, Y. & Prell, W. L. Large-scale\nhydrological change drove the late Miocene C4 plant expansion in the Himalayan\nforeland and Arabian peninsula. Geology 35, 531–534 (2007).\nMoucha, R. and Forte, A. M. Changes in African topography driven by mantle\nconvection. Nat. Geosci. 4, 707–712 (2011).\nNigrini, C. Composition and biostratigraphy of radiolarian assemblages from an\narea of upwelling (northwestern Arabian Sea, lag 117). Proc. ODP Sci. Results 117,\n89–126 (1991).\nPoblete, F., Dupont-Nivet, G., Licht, A., Van Hinsbergen, D.J., Roperch, P.,\nMihalynuk, M.G., Johnston, S.T., Guillocheau, F., Baby, G., Fluteau, F. and Robin,\nC.. Towards interactive global paleogeographic maps, new reconstructions at 60, 40\nand 20 Ma. Earth-Science Reviews, 214, 103508 (2021).\nSarr, A.C., Donnadieu, Y., Bolton, C.T., Ladant, J.B., Licht, A., Fluteau, F.,\nLaugié, M., Tardif, D. and Dupont-Nivet, G.. Neogene South Asian monsoon rainfall\nand wind histories diverged due to topographic effects. Nature Geoscience, 15(4),\n314-319 (2022).\nSlingo, J., Spencer, H., Hoskins, B., Berrisford, P. & Black, E. The meteorology\nof the Western Indian Ocean, and the influence of the East African Highlands. Phil.\nTrans. R. Soc. A. 363, 25–42 (2005).\nTomson, J. R. et al. Tectonic and climatic drivers of Asian monsoon evolution.\nNat. Commun. 12, 4022 (2021).\nWei, H.-H. & Bordoni, S. On the role of the African topography in the South\nAsian monsoon. J. Atmos. Sci. 73, 3197–3212 (2016).\nZuo, M., Sun, Y., Zhao, Y., Ramstein, G., Ding, L. and Zhou, T.. South Asian\nsummer monsoon enhanced by the uplift of the Iranian Plateau in Middle\nMiocene. Climate of the Past, 20(8), 1817-1836 (2024).\nZhuang, G., Pagani, M. and Zhang, Y. G. Monsoonal upwelling in the western\nArabian Sea since the middle Miocene. Geology 45, 655–658 (2017).\nMinor comments:\n#1. Evolution of SST in the Indian Ocean: The development of positive SST\nanomalies in the western equatorial Indian Ocean is a critical component of the\nauthors’ proposed mechanism. However, the manuscript does not provide sufficient\nexplanation for why or how these anomalies form. A detailed examination of the\nseasonal cycle of SST and winds in this region could offer valuable insights into this\nprocess. Additionally, understanding the impact of East African orography on the\nseasonal cycle (pre-monsoon in particular) of the East African monsoon may be key\nto explaining the emergence of these positive SST anomalies during the monsoon\nseason. Addressing these points would significantly strengthen the authors’ argument.\nOur response: Following your valuable comments, we examined the seasonal\nvariation of Somali Jet and SST changes over the western Indian Ocean (Figure R9).\nOur results indicate that the Somali Jet weakens from the pre-monsoon months (April\n-May), reaching its lowest intensity during boreal summer (June-July-August, JJA) in\nthe Early and Middle Miocene simulations. This weakened Somali Jet reduces wind-\ndriven surface cooling, allowing for rapid warming in the western equatorial Indian\nOcean starting from the pre-monsoon season. The warm SST anomalies intensify\nduring JJA, coinciding with the period of strongest monsoon winds. This pattern\nsuggests that the lower East African Highlands contribute to a weaker Somali Jet,\nwhich in turn promotes surface ocean warming through reduced heat loss from the\nocean to the atmosphere. In the Late Miocene and pre-industrial periods, the higher\nEast African Highlands enhance the Somali Jet, leading to stronger wind-induced\nocean cooling and less pronounced SST anomalies. The results confirm that\ntopography-driven weakening of the Somali Jet is the primary driver of SST changes in\nSST in the western Indian Ocean.\nMoreover, the seasonal variations depicted in Figure R10 further highlight the\ncrucial role of positive SST anomalies in the western Arabian Sea in establishing the\npositive air-ocean feedbacks. In Figure R10, warm SST anomalies first appear in the\nwestern equatorial Indian Ocean by June, concurrent with a significant reduction in\nSomali Jet intensity. As the monsoon season progresses, the warm SST anomaly\nintensifies, and easterly wind anomalies propagate eastward across the equatorial\nIndian Ocean, this results in further cooling of eastern Indian Ocean, enhancing the\nwest-east SST gradient – a pattern characteristic of a positive IOD-like state. The\nlargest SST anomalies occur during the Early and Middle Miocene, when the East\nAfrican Highlands are much lower, confirming the critical role of African topography\nin shaping the seasonal evolution of SST in the Indian Ocean.\nAll above results, indeed, indicate that the development of positive SST\nanomalies in the western equatorial Indian Ocean is a critical component of\nestablishing positive ocean-atmosphere feedbacks. This is clarified in Lines 261-269\nin the revised manuscript, providing a more detailed discussion of the seasonal\nevolution of SST anomalies and their connection to East African topography. And\nwe have also included the Figure R9 and Figure R10 as Supplementary Figure 9 and\n10 in the revised Supplementary materials. We clarified the role of pre-monsoon\nconditions in setting up SST anomalies, highlighting how changes in East African\ntopography alter the monsoon onset and subsequent air-sea interactions.\nFigure R9. Strong coupling of the intensity of Somali Jet and sea surface temperature\n(SST) over the western Arabian Sea. Seasonal variation of changes in intensity of\nSomali Jet (m s-1; see Methods for detail) and SST (℃) over tropical western Indian\nOcean (10° S-10° N, 40° -55°E). The vertical shaded areas represent the boreal\nsummer (June to August).\nFigure R10. Seasonal variation. Changes in SST (shading, ℃), sea level pressure\n[contours; hPa; solid (dashed) lines represent the positive (negative) values], and\nsurface wind (vectors, m s-1) from May to September in the MT25 (left column),\nMT15 (middle column) and MT05 (right column) simulations compared to pre-\nindustrial simulation. Gray stippling and vectors in each panel denote regions in\nwhich the changes are significant at the 95% confidence level according to Student’s\nt-test.\n#2. Line 147: “Fig. 1l” it took me sometime to understand that it is not Fig. eleven.\nMaybe authors should refer to it in capital letter to avoid confusion ß\nOur response: Revised.\nReply to Reviewer #1’s comments\nRemarks to the Author:\nThanks for addressing my comments, I am satisfied with the responses.\nOur response: We would like to thank you for professional review work,\nconstructive comments, and valuable suggestions on our manuscript. Your time and\nefforts are greatly appreciated.\n#1. Fig. R1 – I am a bit confused looking at the response at 850 and 700 hPa. The\nchanges to circulation should be somewhat coherent at these levels. At 850 hPa, (Fig.\nR1 d-f), the changes to circulation seem to be emanating from equatorial Indian\nOcean to Western Ghats. This circulation feature changes abruptly at 700 hPa. The\nchanges in rainfall are concentrated over Western Ghats and Northwest India (which\nreceives less rainfall anyway). No changes are seen along the monsoon trough.\nOur response: Thanks for the comment. The apparent inconsistency in circulation\npatterns between 700 hPa and 850 hPa indeed reflects the influence of multiple\ndynamical processes operating at different vertical levels of the troposphere. At 850\nhPa, the circulation anomalies (Figure R1d-f) are more directly affected by surface\nforcing, particularly the modifications to African topography. These lower-level\nanomalies exhibit a coherent southwesterly anomaly from the equatorial Indian Ocean\ntoward the Western Ghats, aligning well with near-surface anomalies at 925 hPa\n(Figure R1 a-c). This reflects the immediate influence of reduced pyrographic\nblocking by the East African Highlands on the Somali Jet and associated moisture\ntransport pathways.\nIn contrast, the anomalous cyclonic circulation is more distinct at 700 hPa than at\n850 hPa, influenced not only by topographic changes but also by mid-tropospheric\nhearing gradients arising from convection anomalies. Specially, suppressed\nconvection over tropical central Africa reduces latent heat releases (Supplementary\nFigure 5 and 7), inducing subsidence and initiating equatorial wave responses are\nshown in the LBM results with imposed cooling over Africa (Supplementary Figure\n8), generates a Rossby wave response that modifies the mid-level circulation structure\nacross the Indian Ocean sector.\nThe discrepancy in circulation coherence between these two levels therefore\nreflects the vertical structure of the atmospheric response: while the 850 hPa winds\nreflect terrain-induced mechanical effects, the 700 hPa anomalies are dynamically\nmodulated by remote diabatic heating and associated wave responses.\nRegarding the spatial distribution of rainfall, we agree that increased SASM\nrainfall is concentrated over the Western Ghats and Northwest India – regions where\ntopographic enhancement and convergence of moisture-laden winds are most\npronounced. Our results demonstrate that anomalous cyclonic and anticyclonic\ncirculation over the Arabian Sea and Bay of Bengal are key in strengthening moisture\nconvergence into these regions (Figure 1d-f in the revised manuscript and\nSupplementary Figure 5 in the revised Supplementary materials). Figure R2 further\nillustrates this convergence structure.\nMoreover, our moisture budget analysis confirms that the dominant contribution\nto increased SASM rainfall during the Miocene arises from dynamically induced\ncirculation changes rather than local thermodynamic (moisture content) effects\n(Supplementary Figure 6 in the revised Supplementary materials). These results\nunderscore the role of both direct atmospheric processes and indirect ocean-\natmosphere feedbacks in driving the observed rainfall response (as synthesized in\nFigure 5 of the revised manuscript).\nFigure R1. Changes in atmospheric circulation due to African topography changes\nduring summer. Changes in wind speed (shading, m s-1) and horizontal wind (vectors,\nm s-1) at 925 hPa in the a MT25, b MT15 and c MT05 simulations relative to the pre-\nindustrial simulations. d-f Same as a-c, but for the 850-hPa winds. The black solid\nboxes mark the SASM region (10°-30° N, 60°-95° E). The blue solid boxes (10° S-\n10° N, 40°-50° E) mark the Somali Jet.\nFigure R2. Changes in moisture transport due to African topography changes during\nsummer. Changes in vertical integral water vapor flux from 1000 hPa to 10 hPa\n(vectors, kg m-1 s-1) and its divergence (shading, mm day-1) in the a MT25, b MT15\nand c MT05 simulations relative to the pre-industrial simulations. Red bold vectors in\na and b indicate the moisture channel anomalies. In a-c, gray stippling and vectors\ndenote regions in which the changes are significant at the 95% confidence level\naccording to Student’s t-test, and solid boxes mark the SASM region.\nReply to Reviewer #2’s comments\nRemarks to Author:\nThe authors responded very carefully and very extensively to the reviewers'\ncomments, implemented most of the comments and otherwise convincingly explained\nwhy they did not take the comments into account. As a result, the manuscript has\nimproved substantially. The structure and general statements are much clearer.\nAdditional analysis and verification of the significance of the results reinforce the\nstatements. I have rarely seen such a carefully prepared reply. My compliments to the\nauthors. I recommend the publication after a few minors, mostly technical revisions.\nOur response: Your positive assessment is a tremendous source of motivation for our\nteam. We are delighted that the revised manuscript now meets with your approval.\nWe have addressed your concerns in a point-by-point manner below, and hope that\nyou will find the added information suitable and sufficient for publication. The\noriginal comments are quoted in blue and the changes in the revised manuscript have\nbeen highlighted in red.\nGeneral Comments:\n#1. The section starting L152 (Weakened Somali Jet... is now somewhat redundant, as\nit doesn't bring much new compared to the section before. In the previous section, the\nweakening of the jet and the effects on the upwelling are already presented. So you\ncould simply merge this section with the previous one.\nOur response: Thanks for your comment. We have merged this section with previous\none.\n#2. I like that some sections have the main message formulated as a heading. Perhaps\nthis style could be applied to all headings.\nOur response: Thanks for raising this important point. We have further applied the\nstyle to all headings as follows:\n1) Changed heading of “Moisture budget analysis of changes in SASM rainfall” to\n“Dynamic effects induced by atmospheric circulation anomalies control the\nchanges in SASM rainfall” (Lines 168-169)\n2) Changed heading of “The effects of the pCO forcing versus topographic forcing”\n2\nto “The effects of the pCO forcing show no decoupling effect” (Line 303)\n2\n#3. Other comments.\n#(1) L229 \"consistent with a previous study\" -> \"a\" is missing\nOur response: Revised.\n#(2) L235 \"such as a positive precipitation... \" -> \"a\" is missing\nOur response: Revised.\n#(3) L254 \" a lower East African Highlands\" -> delete the \"a\"\nOur response: Revised.\n#(4) L290 it is \"enhanced southerly winds\"\nOur response: Revised.\n#(5) L345 \"the ... Somali Jet ...\" -> add \"the\"\nOur response: Revised.\n#(6) L349 it is \"increasing local ...\"\nOur response: Revised.\n#(7) L361 it is \"Himalaya-Tibetan\"\nOur response: Revised.\n#(8) Fig1: why is there no bar for M05?\nOur response: Thanks for your comment. In fact, there is a bar for M05. However,\nbecause the changes in the ratios of SASM rainfall are much smaller for the MT05\nsimulation (0.2%) compared to the MT25 (15.3%) and MT15 (11.4%) simulations,\nthe bar for M05 is not clearly visible in Figure 1h.\n#(9) Suppl. Fig 13 seems to be a bit blurry.\nOur response: Thanks for your reminder. We have reviewed all the figures in the\nSupplementary materials and have replaced them with clearer versions.\nReviewer #2 (Remarks on code availability):\nThe code is now available, but I have not checked it, because I'm not familiar with\nncl. A README file is not included\nOur response: Thanks for your reminder. We have provided the README file in the\nnew submission on Zenodo at https://doi.org/10.5281/zenodo.15605414.\nReply to Reviewer #3’s comments\nRemarks to the Author:\nI have reviewed the authors’ responses and the revised manuscript. Overall, the\nauthors have addressed most of my concerns satisfactorily. However, one important\nissue remains unresolved—the matter of the decoupling between monsoon winds and\nSouth Asian Summer Monsoon (SASM) rainfall.\nMy original concern was that the authors seem to suggest a decoupling between\nmonsoon winds and SASM rainfall during the Miocene, while their moisture budget\nanalysis clearly indicates a strong relationship between atmospheric dynamics and\nSASM precipitation. Based on their response, it appears that the authors are actually\nreferring to a decoupling between the Somali jet and SASM rainfall, rather than\nbetween large-scale monsoon winds and SASM rainfall.\nIn fact, their results show that large-scale monsoon winds—modulated by\nMiocene topography—play a key role in driving changes in SASM rainfall,\nindependent of changes in the Somali jet. This implies that the modern-day coupling\nbetween the Somali jet and SASM rainfall does not necessarily hold on geological\ntimescales.\nThe current phrasing—\"decoupling between monsoon winds and SASM\nrainfall\"—used in both the title and the main text is therefore misleading and\ntechnically inaccurate. I strongly recommend that the authors revise this wording to\n\"decoupling between the Somali jet and SASM rainfall\".\nWith this correction, I am happy to recommend the manuscript for publication.\nOur response: We sincerely appreciate your professional and valuable comments on\nour manuscript. Following your suggestions, we have thoroughly reviewed the entire\nmanuscript and phrased “decoupling between monsoon winds and SASM rainfall” to\n“decoupling between the Somali jet and SASM rainfall”. This is clarified in Lines 1-\n2, 36, 80, 93, 97, 124, 148, 312, 333, 358, and 449 of the revised manuscript.\nReviewer’s comments on\n“Impacts of African topography on the decoupling of South Asian sum-\nmer monsoon winds and rainfall during the Miocene ”\nby Z. Han et al.\nA) General comments\nRecent studies have shown that monsoon winds in the Arabian Sea and the South Asian monsoon\nare positively correlated on interannual timescales but not necessarily on longer orbital timescales.\nIn this study, Z. Han et al. extend this concept, suggesting a similar decoupling over much longer\ntectonic timescales, supported by proxy evidence. They propose a mechanism based on climate\nmodelexperiments, linkingtheupliftofAfricanorographyduringthemiddletolateMiocenetothe\nSouth Asian Summer Monsoon (SASM). According to their model, the less elevated East African\norography induces anomalous latent heating in the atmosphere, triggering circulation changes via\nGill’s response. These circulation shifts alter low-level monsoon winds, with the resulting SST\nanomalies (resembling a positive IOD) further amplifying both monsoon winds and rainfall.\nInitially, Iwasveryinterestedinthestudyandlookedforwardtoreadingthemanuscript. However,\nthe results seem to suggest quite the opposite of the authors’ primary claim—that monsoon winds\nand rainfall are, in fact, decoupled on these tectonic timescales. There are three key issues:\n1. The regions chosen to represent monsoon winds do not overlap with the proxy regions, and\nin the proxy regions, the changes in winds shown by the climate model experiments are not\nrobust.\n2. Their moisture budget analysis indicates that SASM rainfall changes are strongly linked to\nthe dynamic component (i.e., monsoon winds), directly contradicting their central hypothesis\nof decoupling.\n3. The authors interpret upwelling proxies as being linked to monsoon wind speed. However, it\nis well known that changes in wind speed, the latitude, or width of the low-level jet (LLJ)\ncan all influence upwelling. The correct interpretation should be that upwelling is not coupled\nwith rainfall.\nDue to these issues, I do not find the author’s primary claim convincing. However, I find their\ndetailed mechanism linking African orography to SASM rainfall quite interesting and could offer an\nalternativefocusforthestudy. Unfortunately,thisconnectionbetweenAfricanorographyandSASM\nrainfall is already well-established in the literature. Moreover, the connection between changes in\nAfrican rainfall and its impact on upwelling in the northern Arabian Sea and SASM rainfall has\nbeen demonstrated in previous studies. While the authors’ mechanism is well-constructed, it lacks\nthe novelty required for publication in a high-impact journal like Nature Communications.\nFor these reasons, I recommend rejecting the manuscript in its current form. I encourage the\nauthors to refine their focus on the proposed mechanism, enhance the rigor of their experiments.\nThe current experiments need refinement to better align with their claims, and addressing these\nissues will significantly improve the paper’s overall impact. My major and minor concerns, outlined\nbelow, should provide helpful guidance for their revisions.\nB) Major concerns:\n1. Choice of regions for analysis: The authors use proxy data to demonstrate that upwelling\nin the northern Arabian Sea and SASM rainfall were not coupled during the middle to late\nMiocene monsoon evolution. However, in their model analysis, they selected a study region\nsignificantlysouthoftheproxyregion. Analyzingthisareainisolationleadsthemtoconclude\nthat the low-level jet (LLJ) wind speed and monsoon rainfall are not coupled. However,\nthey overlook an important detail: the LLJ, and consequently the monsoon winds, show a\nsignificantincreaseinspeedintheregionimmediatelywestoftheirchosenarea. Thisstronger\nLLJimpactsthewesterncoastofIndia, producingsubstantialrainfallovertheWesternGhats\nand a slight increase in rainfall across mainland India.\nTherefore, the authors’ claim that the LLJ/monsoon winds and SASM rainfall are decoupled\nis not fully supported. In fact, a more careful examination of the wind changes in the proxy\nregion reveals a much more complex response, which the authors have not addressed in their\nanalysis.\nFurthermore, I find the authors’ choice of region for defining the SASM problematic. The\nprecipitation response plots indicate a pronounced change in rainfall along the western coast,\nextending into the adjacent Arabian Sea, while most of inland India—the core SASM re-\ngion—shows insignificant changes in rainfall. The rainfall signal used by the authors to sup-\nport their claim is predominantly a result of offshore precipitation along the western coast of\nIndia, which is a direct consequence of changes in the LLJ/monsoon winds.\nThus, I find the authors’ conclusion in the model analysis unconvincing and sensitive to their\nchoice of region. Their claim appears somewhat overstated and does not fully align with the\ncomplexity of the wind-rainfall relationship seen in the data.\n2. Moisture budget analysis: My concerns are further heightened by the authors’ moisture\nbudget analysis. Their results clearly indicate that changes in precipitation over the SASM\nregion are driven by the dynamic component (i.e., changes in monsoon winds). This presents\na significant contradiction: on one hand, the authors claim that monsoon rainfall and winds\nare decoupled, yet their own analysis shows that the changes in rainfall are directly linked to\nwind dynamics. This inconsistency undermines their central argument and raises questions\nabout the validity of their conclusions.\n3. Interpretation of upwelling proxies: Inthisstudy, theauthorsclaimthatmonsoonwinds\nandrainfallaredecoupled,basedonproxyrecords. However,thecorrectinterpretationshould\nbe that upwelling in the northern Arabian Sea and rainfall are decoupled. Upwelling in this\nregion is influenced by various factors, including wind speed, the latitude, and width of the\nlow-level jet (LLJ). Even slight changes to one or more of these LLJ characteristics can mod-\nulate upwelling and, by extension, rainfall (see recent studies on impact of orbital forcing on\nupwelling and rainfall for example). Therefore, the primary claim of this study—that mon-\nsoon winds and rainfall are decoupled—seems questionable. The relationship between winds,\nupwelling, and rainfall is more complex than the authors suggest, and their conclusions may\noversimplifythesedynamics. ThisisfurthersupportedbytheirfiguresofSSTanomalies. The\nSST anomalies during the middle to late Miocene are negative in the northern Arabian Sea,\nindicating enhanced upwelling. This region, which spans the coast of Yemen and Oman and\nextends into the open sea, encompasses most of the proxy locations. This again indicates that\nupwelling-rainfall are decoupled not necessarily monsoon winds and rainfall.\n4. Missing experiments: The authors conducted experiments where global orography is set\nto pre-industrial levels, except over Africa, where it is adjusted to 25 Ma, 15 Ma, and 5 Ma.\nHowever, for a well-constructed study, I would first expect results from experiments where\nglobal orography is set to match these time periods fully. Following that, the authors should\ndemonstrate that modifying only African orography is sufficient to capture the majority of\nwind and rainfall responses. This approach would strengthen their case for the impact of\nAfrican orography—although, I still believe it won’t support the claim that monsoon winds\nand rainfall are decoupled.\nC) Minor points:\n1. EvolutionofSSTintheIndianOcean: ThedevelopmentofpositiveSSTanomaliesinthe\nwestern equatorial Indian Ocean is a critical component of the authors’ proposed mechanism.\nHowever,themanuscriptdoesnotprovidesufficientexplanationforwhyorhowtheseanomalies\nform. A detailed examination of the seasonal cycle of SST and winds in this region could offer\nvaluable insights into this process. Additionally, understanding the impact of East African\norography on the seasonal cycle (premonsoon in particular) of the East African monsoon may\nbekeytoexplainingtheemergenceofthesepositiveSSTanomaliesduringthemonsoonseason.\nAddressing these points would significantly strengthen the authors’ argument.\n2. Line 147: ”Fig. 1l” it took me sometime to understand that it is not Fig. eleven.\nMaybe authors should refer to it in capital letter to avoid confusion ß"}]

Decision letter