Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration

Domain: earth_science
Published: 2025-11-24
Editorial outcome: major_revision

Abstract
The Indonesian Throughflow (ITF) represents the only tropical inter-ocean pathway of global ocean circulation and plays a central role in the Earth’s climate system. Climate models suggest that the ITF would be weakened under global warming, but historical observations indicate a contradictory trend. Here using CMIP6 simulations, we find that the response of the ITF to increasing atmospheric carbon dioxide (CO2) concentration is nonlinear and exhibits an abrupt rapid decline after surpassing a turning point. The ITF’s decline can be explained by the response of its halosteric and thermosteric components to rising CO2 levels associated with changes in winds and precipitation. The current level of CO2 concentration has reached the turning point, indicating that a rapid decline of ITF and a transition of the Indo-Pacific state is likely imminent despite the remaining uncertainties. These findings have implications for understanding climate tipping elements and global ocean circulation under global warming.

Introduction
The Indo-Pacific region is the only pathway that connects different ocean basins in the tropics and plays a central role in the Earth’s climate system1–3. The cross-basin oceanic flow from the Pacific to the Indian Ocean, termed the Indonesian Throughflow (ITF), controls the inter-ocean exchange of heat and fresh water between the Pacific Ocean and Indian Ocean, affects the heat uptake of the tropical oceans and the storage of extra heat due to greenhouse gas emissions, and hence plays a crucial role in the global climate system4–8. In addition, the ITF serves as the key to the Global Meridional Overturning Circulation (GMOC) system1,9,10. The strengthening and weakening of the ITF lead to a fluctuation of the inter-basin exchange between the Pacific and Indian Oceans, change the GMOC and shape the Indo-Pacific and global marine environment and climate6–8,11.

Previous studies suggest that the ITF possesses significant natural variability on multi-timescales, e.g., intra-seasonal variability triggered by the Madden-Julian Oscillation12, interannual variability associated with El Niño-Southern Oscillation13–17, decadal variability related to the Pacific Decadal Oscillation (PDO) and the Atlantic Meridional Overturning Circulation (AMOC)11,18–23, as well as long-term changes due to external forcing4,24–26. Climate modes affect the ITF through both the wind forcing and salinity changes related to freshwater forcing11,16,27–29. The salinity effect plays an essential role in the interannual variability and decadal trend of the ITF transport and properties11,16. Recent studies suggest that the salinity effect is important in determining the multi-decadal change of the ITF as well30,31.

In the context of global warming, climate models suggest that the ITF will be weakened significantly due to a number of factors including a weakening of the Walker Circulation in the Pacific Ocean4; and/or a decrease in the deep waters entering the Pacific basin at ~40°S with subsequent reduction in the net deep ocean upwelling in the tropical and South Pacific24,32; and/or remote forcing in the North Atlantic Ocean26. It is worth noting that the ITF is in general affected by either wind forcing-related dynamics, and/or buoyancy-forcing related processes that primarily affect the halosteric component30. In contrast to the climate models, historical observations suggest that the ITF transport showed no significant trend during the past century or even an increasing trend during the recent decades when the ocean has been rapidly warming11,30,31,33,34. Given that greenhouse gas concentrations are expected to continue rising over the next century as they have over the past hundred years, the inconsistency between historical ITF trends and future projections raises critical questions. It is therefore essential to understand the response of ITF to rising greenhouse gas concentration, and at what threshold of greenhouse gas concentration will the long-term trend of ITF become discernible.

Several tipping points may be triggered when global temperatures rise above 1.5°C compared with the pre-industrial level35–37. Recent studies suggest that the increase of global temperature will exceed 1.5°C in less than 10 years according to the current emissions trajectory38. The ocean circulation system shows significant changes in response to global warming39, and the collapse of the AMOC is thought to be one of the most prominent climate tipping elements35–37,40,41. A long-term strengthening or weakening of the ITF could lead to state transitions of the Indian Ocean and Pacific Ocean (IPO) system: a strong ITF means extensive exchange of heat, fresh water, nutrients and carbon between the two oceans, while a collapsed ITF represents a relatively isolated IPO system. However, it remains uncertain whether the ITF can be classified as one of the tipping elements or not.

In this study, we aim to investigate the response of the ITF to increasing atmospheric carbon dioxide (CO2), examine the independent responses of halosteric and thermosteric components of the ITF, and explore the role of the ITF as a potential climate tipping element, using numerical simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and in situ observations.

Results
We mainly use simulations from the historical runs (Hist, 1850–2014) and idealized 1% annually increasing CO2 concentrations experiments (1pctCO2, 1850–1999) from 25 CMIP6 coupled climate models to examine the ITF response to increasing CO2 concentration. First, we validated the ITF strength in the CMIP6 models and found that the multi-model mean reasonably reproduces the observed ITF30 and captures its mean structure and seasonal cycle. The vertical structure of the climatological mean zonal currents across the 114°E section and their changes in 1pctCO2 relative to the Hist runs are shown in Supplementary Fig. S1a. The mean ITF in the CMIP6 models shows a core between 11°S and 13°S with maximum velocity greater than 20 cm/s.

The ITF transport averaged over the 150 years in the Hist runs of CMIP6 is about −13 Sv (hereafter negative for westward into the Indian Ocean), quite close to the −15 Sv determined from direct observations42. Supplementary Fig. S1b presents a comparison of the seasonal cycle of ITF transports from the historical runs of CMIP6 models and the geostrophic calculations using observations-based datasets30. The observed seasonal cycle of ITF transports is well reproduced in the CMIP6 Hist runs. The correlation coefficient is 0.82 (significant at a 99% confidence level) between the ensemble mean of CMIP6 models and the observed geostrophic transport.

To separate the halosteric and thermosteric components, we calculated the total transport of the ITF and geostrophic transport of the ITF with modeled temperature and salinity. The time series of ITF transport in the 1pctCO2 experiments and Hist runs of CMIP6 and their halosteric (ITFS) and thermosteric (ITFT) components are presented in Fig. 1.

Although the geostrophic transport is smaller than the total transport, the ITF transports from the geostrophic calculations (Fig. 1a, d) are in good agreement with the total transport in both the 1pctCO2 experiments and the Hist runs (Supplementary Fig. S2), indicating that the geostrophic current is a major component of the ITF at 114°E.

The natural variation of the ITF transports, defined as the standard deviation of the detrended time series, is about 0.45 Sv in both the 1pctCO2 experiments and the Hist runs. However, the long-term trends in the 1pctCO2 experiments are clearly different from that in the Hist runs. As evident, the ITF transport in the Hist runs shows no significant trend during the past 150 years (Fig. 1d). By comparison, the ITF transport in the 1pctCO2 experiments shows a persistent weakening trend of about 0.3 Sv per decade, i.e., 4 Sv over 150 years, which is statistically significant at a 99% confidence level (Fig. 1a).

The trend of the ITF transport in the Hist runs is very weak and much smaller than the uncertainty: the linear trend of the total ITF is −0.01 ± 0.04 Sv decade−1 (positive for decreasing transport), the trend of ITF is −0.02 ± 0.13 Sv decade−1, while the linear trend of ITF is 0.01 ± 0.13 Sv decade−1 (Fig. 1f–h). By contrast, the ITF transport in the 1pctCO2 experiments shows a significant decreasing trend: the linear trend of total ITF is 0.28 ± 0.11 Sv decade−1, with the trend of ITF at −0.32 ± 0.37 Sv decade−1, while the linear trend of ITF is 0.60 ± 0.38 Sv decade−1. Hence, the halosteric and thermosteric components of the ITF transport show significant opposite trends in response to the increasing CO2 concentration.

Although the trends of ITF transport in Hist runs are much weaker than that in the 1pctCO2 experiments, the uncertainties in the two experiments have a similar order of O(0.1) Sv decade−1, and natural variability in the two experiments have the same magnitude (0.45 Sv) as well. This implies that the natural variability in the simulated ITF transport in the two experiments are comparable and reasonable, and the significant difference between the long-term trends in ITF transport is due to their difference in the external forcing, i.e., the CO2 concentrations in the two experiments.

The ITF transport and its halosteric and thermosteric components as a function of CO2 forcing are shown in Fig. 2. It should be noted that in CMIP6 models, the CO2 forcing is represented by the total atmospheric mass of CO2 (measured in Kg), which also serves as a variable indicating the concentration of atmospheric CO2. The thermosteric component of ITF transport shows a decreasing ITF transport trend with increased CO2 forcing at a rate of 0.86 Sv per 1015 Kg (Fig. 2c, b). By comparison, the halosteric ITF transport shows an increasing ITF transport trend with enhanced CO2 forcing at a rate of −0.37 Sv per 1015 Kg (Figs. 2c, b). As a result, the total ITF transport is decreased with CO2 forcing at a rate of approximately 0.62 Sv per 1015 Kg (Fig. 2a, b).

Interestingly, the decline of ITF transport in response to atmospheric CO2 forcing is weak when the CO2 mass is less than approximately 3 × 1015 Kg (i.e., 385 ppm, Supplementary Fig. S3a) or greater than 8 × 1015 Kg (red dots in Supplementary Fig. S3b), but is very rapid in between, indicating a nonlinear response of the ITF transport and its components to external CO2 forcing. The decline trend of total ITF (ITF) in response to CO2 forcing is 0.81 Sv per 1015 Kg when 3 × 1015 Kg < CO2 mass < 8 × 1015 Kg, but drops to 0.31 Sv per 1015 Kg when CO2 mass is greater than 8 × 1015 Kg, and 0.21 Sv per 1015 Kg when CO2 mass is less than 3 × 1015 Kg (Fig. 2b). The decline rate of the ITF shows a peak of 0.93 Sv per 1015 Kg when CO2 mass is about 4.5 × 1015 Kg, which is about 4 times of that when CO2 before 3 × 1015 Kg and 3 times of that when CO2 after 8 × 1015 Kg. These features suggest that a CO2 mass of ~3 × 1015 Kg and 8 × 1015 Kg might provide two important turning points (Fig. 2). The comparison between the ITF response to the CO2 forcing in the 1pctCO2 experiments and the Hist runs may explain why the ITF transport in the Hist runs shows no significant trend. As we can see in Fig. 2, the CO2 forcing in the Hist runs is mostly less than 3 × 1015 Kg below the first turning point, and hence the response of the ITF is certainly much weaker than that in the 1pctCO2 experiments.

In addition, the ITF transport versus global mean surface temperature (GMST), and GMST as a function of CO2 mass, is shown in Supplementary Fig. S4. We find that the GMST increases linearly with the increasing of CO2 mass with no turning points (Supplementary Fig. S4a), but the GMST shows a similar qualitative response as a function of CO2 concentration, with an initial slow response followed by a faster growth and two turning points (Supplementary Fig. S4b). The model spread in the simulated ITF-GMST relationship is notable, but the overall trend of ITF in response to GMST is much greater than the model spread (Supplementary Fig. S4b). The consistency of ITF-CO2 and ITF-GMST is not surprising, considering the linear relationship between GMST and CO2 mass. The ITF anomalies (minus their corresponding mean values) in three CO2 scenarios, including 1pctCO2, Hist, and the Shared Socioeconomic Pathway (SSP) 585, are plotted together for comparison, and these scenarios indicate a similar ITF response to CO2 forcing (Supplementary Fig. S4c).

To further examine the emerging time of the long-term trend of the ITF from its natural variability, we have calculated the running trend of the ITF transport over a 30-year window in both the CMIP historic runs and 1pctCO2 experiments. As shown in Supplementary Fig. S5, the running trend in the Hist runs fluctuates within a range of roughly −2 to 2 Sv/century, which is a result of natural variability. By contrast, in the 1pctCO2 experiments, the 30-year running trend reaches 4 Sv/century rapidly, and the fluctuations are masked by the significant decreasing trend when the CO2 reaches the first turning point, indicating that the first turning point is indeed the emerging time of a weakening ITF trend that is distinct from its natural variability.

Observations suggest that the CO2 mass has increased from 2.46 × 1015 Kg in 1958 to 3.10 × 1015 Kg in 2014, indicating an increasing rate of CO2 mass of about 1.12 × 1015 Kg per century during the past half a century (Supplementary Fig. S3). Observations indicate that the annual mean CO2 in 2023 was approximately 3.30 × 1015 Kg, which implies an increase of about 0.84 × 1015 Kg since 1958. With a mean rate of 0.42 Sv per 1015 Kg, the increase of CO2 by the year 2023 would lead to a decrease of about 0.4 Sv in the ITF transport, which is comparable to the natural variability, i.e., 0.45 Sv. Assuming a future rate of CO2 increase similar to that observed in 2014, the mass of CO2 would exceed 4.33 × 1015 Kg a century later. As shown in Fig. 2a, the ITF will finally be stable at only about 6.9 Sv when the CO2 is beyond 8.5 × 1015 Kg, indicating that the exchange between the Indian Ocean and Pacific Ocean would be nearly half of that at present, and the IPO system would be relatively isolated. This phase transition is hence essential for the Indo-Pacific environment and the global climate system.

A further question arises as to how the wind and buoyancy forcing, which exert direct control over variations in ITF transport, respond to external CO2 forcing. This is specifically relevant given that the ITF is determined by the Indo-Pacific pressure gradient related to the difference in steric height between the western Pacific Ocean and eastern Indian Ocean, which is further regulated by wind forcing and buoyancy forcing (precipitation) in the Indo-Pacific Ocean30.

Given that the ITF’s response is determined by the response of both the thermosteric component and halosteric component, we first examine the response of oceanic temperature and salinity in the Indo-Pacific Ocean to increasing CO2 concentration. Supplementary Fig. S6 presents the ensemble mean trends of temperature and salinity in the 1pctCO2 experiments, and the time series of temperature and salinity averaged over the western Pacific Ocean (WPO, 120–180°E, 15°S–15°N) and the eastern Indian Ocean (EIO, 80–114°E, 22°S–8°S) over the upper 200 m layer. Both the WPO and EIO show a significant warming trend in the 1pctCO2 experiments, but the warming in the EIO is much more rapid than that in the WPO, leading to a decreased temperature difference between the EIO and WPO with the increased CO2 concentration (Supplementary Fig. S6c and S6e). By contrast, the difference in salinity between the EIO and WPO is increased as the CO2 concentration increases due to a significant freshening in WPO but with no significant change in salinity in the EIO (Supplementary Fig. S6d and S6f).

Previous studies suggest that the temperature in the tropical Indo-Pacific Ocean is mainly related to a convergence of warm water due to zonal wind stress, while salinity changes in the upper layer ocean are mainly associated with surface freshwater input due to precipitation43,44. Hence, we further examine the response of the zonal wind stress and precipitation in the tropical Indo-Pacific Ocean to the CO2 forcing in the 1pctCO2 experiments.

As shown in Fig. 3, zonal wind stress in both the EIO and WPO show an increasing trend, i.e., weakening of westward trade winds, in the 1pctCO2 experiments, leading to a significant decreasing trend in the difference of zonal wind stress between the EIO and WPO. Note that, for the wind forcing, the ITF variability on decadal and longer timescales is likely controlled by mostly the Pacific winds and then the southeastern Indian Ocean6. The weakening of trade winds in the western Pacific Ocean leads to weakening convergence of warm water in the upstream region of the ITF in the Pacific and reduces the pressure gradient between the western Pacific Ocean and eastern Indian Ocean that drives the long-term ITF45. In comparison, the trends of precipitation in the two regions are clearly opposite: significantly increased precipitation in the WPO but decreased precipitation in the EIO. This opposite response causes an increasing trend of precipitation difference between the EIO and WPO (Fig. 3d). An intermediate point exists at 5 × 1015 kg of CO2, where the wind stress and precipitation anomaly changes sign, but this is mainly because we display anomalies relative to a mean value over a CO2 range of 2 to 10 × 1015 kg.

Like the ITF transport, the response of wind forcing and precipitation in the Indo-Pacific Ocean to CO2 forcing also shows turning points (Fig. 3b, d). During the stages with atmospheric CO2 mass less than 3 × 1015 Kg or massive atmospheric CO2 mass greater than 8 × 1015 Kg, the responses of both wind forcing and precipitation are weak. But between the two turning points, both the forcings, particularly the wind forcing, are very sensitive to the increasing of CO2 mass (Fig. 3b, d). After exceeding the second turning point, the responses of wind stress and precipitation differences in the western Pacific and eastern Indian Ocean to CO2 are both weakened, leading to a reduced ITF response, which may be associated with changes in the background state and the weakened Indo-Pacific exchange under this regime. Therefore, the responses of wind stress and precipitation to increasing CO2 concentration explain the opposite and nonlinear response of the ITF’s halosteric and thermosteric components to increasing CO2 concentration.

Methods
Model outputs from the historical runs (Hist, 1850–2014) and idealized 1% annually increasing CO2 concentrations experiments (1pctCO2, 1850–1999) from 25 CMIP6 coupled climate models49 are used to investigate the response of ITF to external forcing. These models include: ACCESS-CM2, ACCESS-ESM1-5, CAMS-CSM1-0, CAS-ESM2-0, CESM2, CESM2-WACCM, CESM2-WACCM-FV2, CMCC-CM2-SR5, CMCC-ESM2, CanESM5, E3SM-1-0, EC-Earth3-AerChen, EC-Earth3-CC, EC-Earth3-Verg, FGOALS-f3-L, FIO-ESM-2-0, IPSL-CM5A2-INCA, IPSL-CM6A-LR, MCM-UA-1-0, MIROC6, MPI-ESM1-2-HAM, MPI-ESM1-2-HR, MRI-ESM2-0, NESM3, and SAM0-UNICON. The 1pctCO2 is one of the four baseline experiments of the Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, which is one of the three major components of CMIP. We used temperature, salinity, zonal and meridional current velocities, zonal and meridional components of wind stress, and precipitation from these models to investigate the responses of the halosteric and thermosteric components of the ITF to the increasing CO2 forcing.

Both the Hist and 1pctCO2 runs branch the end of the pre-industrial control (piControl runs) in 1850. The piControl reproduces an equilibrium state that lasts 500 years and serves as a baseline for branched experiments like the Hist and 1pctCO2. The forcing for Hist and 1pctCO2 runs are external forcing like emissions of greenhouse gases (GHGs) and aerosols due to human activities and volcanic eruptions. It should be noted that the external forcing for the Hist runs are based on observations over 1850–201450, and the Hist runs are usually used to examine natural variability and compare with historical observations17. For example, Santoso et al.17 found that the CMIP5 models overall simulate aspects of ITF variability. The 1pctCO2 runs branches the piControl as well, but are driven by idealized forcing that is the same as that in piControl except for the prescribed CO2 concentration increasing at 1% yr−1 (month by month or suddenly rises by 1% at the end of the year) since 1850 without considering any observations. So, the historical runs can be approximately regarded as an approximation of “real world”, while the 1pctCO2 runs are idealized experiments. Using the 1pctCO2 runs and their comparison with Hist runs, we are able to emulate the response of the ITF to an increasing of CO2 concentration.

In addition to the Hist runs and 1pctCO2 runs, simulations from the Shared Socioeconomic Pathway (SSP) 585 and forced by an abrupt quadrupling of CO2 (abrupt 4×CO2 runs) when available are employed as well. The geostrophic transport of ITF from Lu et al.30 (L24) is used for comparison to the numerical simulations. The geostrophic transport L24 was estimated using observation-based dataset products.

ITF transport
The ITF transport is calculated as the zonal transport across the 114°E section (8.5°S–22.5°S) bounded by Java and Australia, as defined in previous studies30:

\int_{8.5^{\circ}S}^{22.5^{\circ}S} \int_{0m}^{1100m} u dydz, (1)

where u is zonal current velocity, y and z denote meridional and vertical directions. Current velocities from the CMIP models and geostrophic current velocities calculated with oceanic temperature and salinity from the RG Argo Climatology and CMIP6 model outputs are used to compute the ITF transport. Here, we used the 1100 m as the level of no motion, considering the sill depth of ITF within the Indonesian Seas16. Using deeper depth as the level of no motion lead to no significant difference of the fluctuation of ITF transport.

Geostrophic current and its thermosteric and halosteric components
The zonal geostrophic velocity V between two adjacent stations of A and B is calculated by ref. 51

V = \frac{1}{fL} \int_{P_B}^{P_A} \alpha_A dp - \int_{P_B}^{P_A} \alpha_B dp, (2)

where f is the Coriolis parameter, L is the distance between A and B, P_A and \alpha_A are the pressure and the specific volume anomaly of A, respectively, P_B and \alpha_B are the same for B, P_r is the reference level of no flow and is designated as the 1100-m surface.

The thermosteric (S', T) and halosteric components (S, T') of the ITF are separated following Hu and Sprintall16, where T and S are time-dependent, and T' and S' are climatological means.

Data availability
CMIP6 model outputs, including the historical (Hist) runs, 1% annually increasing CO2 concentration experiments (1pctCO2), Shared Socioeconomic Pathway (SSP) 585 and abrupt quadrupling of CO2 (abrupt 4×CO2) runs, are available at https://aims2.llnl.gov/search/cmip6/. Monthly in-situ CO2 datasets are available at https://scrippsco2.ucsd.edu/. The geostrophic transport of Indonesian Throughflow from Lu et al. (2024) can be found at a repository site at Zenodo (doi:10.5281/Zenodo.17564268).

Code availability
Code and documentation for the CMIP6 models are publicly available at https://aims2.llnl.gov/search/cmip6/. Model outputs were processed using the software MATLAB R2023a. The Matlab code for Figs. 2 and 3 is available at Github (https://github.com/ShijianHu/ITF). The code for the basemap (M_Map) is available at https://www-old.eoas.ubc.ca/~rich/map.html.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors used two CIMP6 simulations, the historical and 1% CO2 increase, to identify a nonlinear response of the ITF transport to the CO2 levels. They found a turning point in the rapid decline of the ITF at the current CO2 level and one at a high CO2 level when the decline in ITF becomes stable. The findings are new to the climate community and the topic is worthwhile for a quick publication.\nHere are some comments for the authors to address before the manuscript can be finalized.\nThe authors propose that the current CO2 level is a turning point for the ITF. Is it also a turning point for other climate indices? could they be related to the ITF variation and change?\nA few studies have examined the projected ITF changes under the RCP scenarios. For example, the CO2 levels in Fig. 2 may be related to what is expected by the end of this century.\nPrevious studies have related the ITF reduction to deep water mass and deep upwelling. It is useful to understand the connection between the surface circulation and the thermohaline circulation contribution.\nLine 86-88: the CMIP models, with only greenhouse gas forcing, wouldn't be able to capture the interannual variations of the ITF in observation, which can only be captured with realistic wind forcing. By examining Fig. S1, the correlation is mostly due to the annual cycle of the ITF.\nFig. 3: there are studies suggesting the equatorial Indian Ocean winds are important for the ITF variability. So it's not clear the usefulness of using the 22-8S band to represent the Indian Ocean wind response.\nIn any case, the wind forcing may be secondary to the thermohaline forcing?\nThe lower panel of Fig. 4 shows the mean state, which doesn't illustrate the potential changes.\nIt would be nice to pinpoint the causes of a second turning point.", "reviewer_2": "Using results from idealized CMIP6 annually 1% CO2 rise experiments, the authors have assessed the thermosteric and halosteric components of the ITF transport and concluded that the increasing CO2 concentration in the atmosphere shall eventually lead to a decline in ITF transport. The authors also proposed two turning points based on the changing rate of the ITF transport. The manuscript is generally well written except that the citation numbers are not correctly indexed and the figure captions need improvement. The findings are important but should be presented with the right context.\nI have some significant concerns about the findings and the interpretations which are listed below. By addressing these concerns, the manuscript could be more rigorous, appropriately contextualized, and valuable to the scientific community.\nMajor Concerns:\n1) The idealized 1% CO2 rise experiment used in the study neglects other anthropogenic sources of radiative forcing, such as aerosols. Since aerosols, which are on a declining trend, counteract some of the warming effects of increasing CO2, this could bias the results, and the proposed vanishing ITF transport might be expedited under more realistic conditions. The authors could address this concern by acknowledging that the 1% CO2 experiment is a simplification and that, in reality, aerosols and other factors (such as land use changes and different greenhouse gases) might influence the rate and magnitude of ITF transport changes. The authors should clarify that their results represent an idealized scenario and not a direct prediction of future conditions, emphasizing the need for more comprehensive modeling that incorporates multiple forcings. They should also discuss the potential uncertainties in their conclusions, given the absence of realistic time-evolving aerosol and other greenhouse gas forcing in the experiment.\n2) The study assumes a fixed level of no motion at 1100 m, but the bathymetry at 114 E reaches much deeper (~6000 m) between 11S – 22S. Recent studies have shown significant barotropic variabilities in the Indo-Pacific basin (https://doi.org/10.1016/j.dsr.2023.104104), suggesting that the level of no motion should extend to the ocean bottom for more accurate transport estimates. This may become all the more relevant with the impending climate change wherein frequent extreme events are predicted. An atmospheric disturbance can generate barotropic responses if the wavenumber and the frequency meet certain criteria (see M.J. Lighthill, Time-varying currents, Philos. Trans. Roy. Soc. Lond. Math. Phys. Sci., 270 (1971), pp. 371-390). The authors should consider adjusting their ITF transport calculation to account for deeper bathymetry, potentially extending the level of no motion to the ocean bottom. They could discuss how this adjustment would likely improve the accuracy of the total ITF transport estimates. I do understand that any additional experiment with CMIP6 is time and resource consuming. The authors should at the least present a sensitivity analysis based on the revised assumption of the level of no motion and show that extending the bathymetry to the ocean bottom does not significantly alter the findings of this study.\n3) The causality of the declining ITF transport lies in the changing wind stress and precipitation. Figure 3b and 3d represent a turning point (5 x 1015 kg CO2) where the wind stress and precipitation anomaly changes sign. Why the turning point related to wind stress and precipitation anomalies (as shown in Figures 3b and 3d) is not considered more relevant than the two turning points proposed by the authors. The concern is that the authors may have missed a potentially important relationship between the changes in wind stress and precipitation and the ITF transport. The authors could revise their interpretation of the turning points and consider whether the changes in wind stress and precipitation are more directly tied to the decline in ITF transport than the two turning points they originally proposed.\n4) I have concerns that the abstract may sensationalize the findings, suggesting an imminent and rapid decline in ITF transport. The authors should carefully revise the abstract to better reflect the idealized nature of the study and clarify that the findings pertain to a specific experimental scenario rather than an immediate real-world prediction.\nMinor Comments:\n1) Line 39-42 : The authors may wish to apprise the readers about the MJO driven ITF transports.\n2) Line 52-53: If the winds are strong enough, it can affect the entire water depth.\n3) Line 62: It will be good to inform the readers about the time needed for the global temperature to reach 1.5 C given the current trend.\n4) Captions in Fig S1(a) and Fig S1(b) appear to be swapped. Caption mentions cm/s whereas the colorbar appears in m/s.\n5) The authors use the abbreviations in tandem without describing it. For example, MITF is used in the Results section but it is described much later in the Methods.\n6) Line 77: Even though CMIP6 is pretty well known across the earth science community, it is a good practice to expand the initials given the varied reader base of this journal.\n7) Line 88-90: What is the correlation between the ITF transport from CMIP6 and MITF and the level of significance ? The authors should mention it in the caption of Figure S1b if not in the text.\n8) Line 95-97 : The magnitude differs by ~ 3 Sv. The trends appear similar though. The authors should mention it apart from highlighting that the two transports are in good agreement.\n9) Line 108-112: Why does not ITF_T and ITF_S add up to the total ITF given that the residual term is negligible as claimed in Hu and Sprintall 2016.\n10) ITF’ used in Fig S2b and S2d is not defined anywhere.\n11) Fig 2: The shades in Fig 2b should be properly described in the caption.\n12) Fig 2(a) : 1pctCO2 -EN ??? What does it refer to ?\n13) Line 184-187: I suppose that the temperature and salinity is averaged over depth. If yes, it is averaged up to what depth ? These details should be mentioned in the text.\n14) Figure S4 : I don’t see black quadrilaterals in panels (a) and (c).\n15) Fig 3: Captions (b) and (c) should state that the points are from all CMIP6 models. In fact, the authors should be more attentive about the captions across all figures.\n16) Line 241 : Typo.... Change existed to exist.\n17) Figure 4 should be described in some detail in the text rather than leaving it to the interpretation of the readers.\n18) Line 266-268 : “We find that the response of zonal wind stress in the Indo-Pacific Ocean is featured by nonlinear weakening trend with two turning points like ITF’s response”. This statement is based on Figure 3. It’s not clear from the plot why those two points should be called turning points.\n19) Line 329 : The indexed reference is actually number 15 instead of 14.\n20) Line 287-289 : It is important to elaborate how CO2 increment is implemented in the models. Are the 1% increments spread over months or sudden at the end of a year ? What about the other greenhouse gases? Not all readers may be aware of the DECK experiments within the CMIP6 framework. The manuscript will be more complete if these relevant details are mentioned instead of directing the readers to ref no 47.\n21) Since this journal admits 10 display figures, I don’t see a need to include Supplementary Figures in this manuscript.", "reviewer_3": "Review of “Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration”, by Shijian Hu et al.\nSubmitted to Nature Communications\nThis manuscript discusses the response of the Indonesian Throughflow (ITF) to increasing CO2, in 25 CMIP6 climate models. The model runs studied are the historical (HIST) and 1% per annum CO2 increase (1pctCO2) runs. The manuscript notes that when plotted against CO2 concentration the ITF response is nonlinear, with an initially slow rate of weakening, which accelerates and then slows again as a function of CO2 concentration. The second part of the paper begins to unpick some of the mechanisms of the ITF decline, finding that a balance/competition between wind-driven differences in warming, and precipitation-driven changes in salinity, appear to be the main drivers of the ITF weakening. The authors conclude by speculating, based on the 1pctCO2 results, that current CO2 concentrations may be such that a rapid weakening of the ITF may be imminent.\n.\nThe topic of the paper, changes in the ITF in response to increasing greenhouse gas forcing, is interesting and important, and to the best of my knowledge has not been extensively studied in the literature to date. So the study is welcome.\nUnfortunately there are some methodological weaknesses in the analysis which to my mind invalidate the discussion of the nonlinear response and the conclusion that a rapid ITF decline may be imminent. I have noted these under ‘Methodological issues’ below. I did find the discussion of mechanisms of ITF weakening interesting and more convincing.\nOverall, my feeling is that the authors have the basis of a valuable and interesting study, particularly on the mechanisms driving the ITF weakening, but that a large part of the analysis presented, and the conclusions drawn from it, need to be revisited, to the extent that any revised version would essentially be a different paper. Hence I cannot recommend acceptance of this paper, but I hope that the authors will continue with this work and in particular publish and develop further their analysis of mechanisms. Noting the dependence on supplementary figures, it may be that this analysis would work best in a longer format journal.\nI’m sorry I can’t be more positive about the present manuscript, but I hope that these comments are useful and constructive in taking this interesting work forward.\nMethodological issues\nApart from Fig. 1, the authors present their analysis of the 1pctCO2 run in terms of CO2 concentration rather than time (Figs 2-4). They note a levelling off of the ITF weakening at higher concentrations. This is entirely to be expected as the radiative forcing from CO2 increases logarithmically with the concentration. Indeed this is why the standard protocol follows a percentage increase of concentration per year rather than a linear increase. The same levelling off would be seen in any climate variable (e.g. global mean temperature), if plotted against CO2 concentration. Since there is no direct mechanistic link between CO2 concentration and ITF (only the indirect link through the response of the climate system), it really makes no sense to plot the response in this way.\nThe above does not explain the initial slow response of the ITF. This can equally well be seen when plotting the response against time (Fig. 1a). This is most likely an issue of the initial response time to perturbations. Other variables such as global mean temperature show a 2-timescale response to CO2 perturbations, with a fast timescale of a decade or two followed by a longer adjustment time. It is seen most clearly in the instantaneous 4xCO2 experiment but is also there in the 1pctCO2 experiments (see e.g. https://journals.ametsoc.org/view/journals/clim/26/6/jcli-d-12-00195.1.xml ). This initial adjustment time may be different for the ITF than for global mean temperature, and in fact understanding this response time would be a valuable addition to the mechanistic analysis in this study. In any case, it seems a priori far more likely that the initial slow response is related to an inherent timescale of the model dynamics rather than a specific threshold CO2 concentration. The apparent ‘threshold’ CO2 concentration would therefore be entirely dependent on the specific timeseries of concentration used to force the model. I think if the claim that there is a specific threshold CO2 concentration that results in acceleration is to be made, it would at least have to be shown across several different CO2 forcing scenarios.\nFollowing on from the above, the claim in the Discussion section that the real ITF may be on the verge of a rapid decline, based on current CO2 concentrations, is not justified. Even if CO2 concentration were somehow the direct control variable (which hasn’t been demonstrated), what about all the other radiatively active forcing species in the present and future atmosphere? In any case, to assess the claim, why not simply look at the projection runs (ScenarioMIP) to see if the models actually project a rapid ITF decline?"}, "author_response": "Point-by-point response to Reviewer#1\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #1 (Remarks to the Author):\nThe authors used two CIMP6 simulations, the historical and 1% CO2 increase, to\nidentify a nonlinear response of the ITF transport to the CO2 levels. They found a\nturning point in the rapid decline of the ITF at the current CO2 level and one at a high\nCO2 level when the decline in ITF becomes stable. The findings are new to the climate\ncommunity and the topic is worthwhile for a quick publication. Here are some\ncomments for the authors to address before the manuscript can be finalized.\nResponse: Thank you for your encouragement and constructive suggestions. We have\nmade careful revisions according to your comments. Please see our point-by-point\nresponses below.\nThe authors propose that the current CO2 level is a turning point for the ITF. Is it also\na turning point for other climate indices? could they be related to the ITF variation and\nchange?\nResponse: Thank you for these important questions. Yes, it is possible that the current\nCO2 level is also a turning point for other climate indices and especially for those\nrelated to the ITF variation and change. After the current CO2 level, the ITF will be\nrapidly weakened and the Indo-Pacific Ocean is going to be in a state with weak inter-\nbasin exchange, likely leading to rapid heat changes in the Indian Ocean and Pacific\nOceans. The turning point for other climate indices definitely needs further in depth\nstudy, however in this study, we mainly focus on the response of ITF to the CO2 level.\nIn the revised manuscript, we have added some sentences to describe the possibility of\nturning point for other climate indices related to ITF.\nPlease see Lines 321-325: “Given that the rapid changes in the ITF in response to\nchanges in CO2 concentration would lead to changes in the inter-basin exchange of\nheat and freshwater between the Indian Ocean and Pacific Ocean, turning points might\nalso occur for other Indo-Pacific climate indices, and further studies are needed.”\nA few studies have examined the projected ITF changes under the RCP scenarios. For\nexample, the CO2 levels in Fig. 2 may be related to what is expected by the end of this\ncentury. Previous studies have related the ITF reduction to deep water mass and deep\nupwelling. It is useful to understand the connection between the surface circulation and\nthe thermohaline circulation contribution.\n1 / 21\nResponse: Yes, some previous studies have examined the projected ITF changes by the\nend of this century under the RCP scenarios. As you note, several previous studies have\nexamined the relationship between the ITF reduction and deep ocean circulation (i.e.,\nrelated to deep water mass and deep upwelling) as well (Sen Gupta et al. 2016; Feng et\nal. 2017). For example, Feng et al. (2017) examined the annual mean ITF volume\ntransport in the historical and RCP8.5 runs using a near-global 1/10° ocean general\ncirculation model, and suggested that deep ocean circulation, through changes in net\ndeep ocean upwelling in the tropical and South Pacific, is as important as wind-driven\nprocesses to the ITF transport and variability. Sen Gupta et al. (2012) suggested that\nthe ITF reduction in the CMIP3 models might result from a weaker Mindanao Current\ntransport given that the changes in the two transports have similar magnitudes. As\npointed by Sprintall et al (2024), “One alternative hypothesis suggested for the ITF\ntransport weakening is a projected reduction in the upwelling and circulation of the\ndeep waters entering the Pacific from the Southern Ocean that ultimately upwell to exit\nvia the ITF (Sen Gupta et al. 2016; Feng et al. 2017).” So, changes in the deep ocean\ncirculation might indeed play an important role in the low-frequency variability and\nlong-term trend of ITF. In any case, whether ITF changes are generated by the wind or\nby the deep-upwelling related to thermohaline circulation, would be contained in the\nchanges of Indo-Pacific pressure gradient.\nIn the revised manuscript, we have added a few sentences to discuss the connection\nbetween the surface circulation and the thermohaline circulation contribution and the\npotential role of deep-water mass and deep upwelling in the ITF reduction.\nPlease see Lines 305-309: “For example, previous studies suggested that changes in the\ndeep ocean upwelling also plays an important role in the long-term trend of the ITF in\naddition to the changes in the Indo-Pacific wind forcing24,32, implying that the response\nof the halosteric and thermosteric ITF to CO2 levels might occur through both wind\nforcing and deep ocean upwelling.”\nLine 86-88: the CMIP models, with only greenhouse gas forcing, wouldn't be able to\ncapture the interannual variations of the ITF in observation, which can only be captured\nwith realistic wind forcing. By examining Fig. S1, the correlation is mostly due to the\nannual cycle of the ITF.\nResponse: We are sorry for the ambiguity in describing the models. The CMIP\nsimulations mentioned in Lines 86-88 and Fig. S1 are CMIP6 historical simulations.\nAs noted by Eyring et al. (2016), “These simulations are forced, based on observations,\nby evolving, externally imposed forcings such as solar variability, volcanic aerosols,\nand changes in atmospheric composition (GHGs and aerosols) caused by human\nactivities. The CMIP historical simulations provide rich opportunities to assess model\nability to simulate climate, including variability and century timescale trends (e.g. Flato\net al., 2013). These simulations can also be analysed to determine whether climate\nmodel forcing and sensitivity are consistent with the observational record, which\nprovides opportunities to better bound the magnitude of aerosol forcing (e.g. Stevens,\n2 / 21\n2015).” Hence, the CMIP historical simulations are usually used to examine natural\nvariability and compare with historical observations. For example, Santoso et al. (2022)\nexamined “20 models from phase 5 of the Coupled Model Intercomparison Project\n(CMIP5) and the SODA-2.2.4 ocean reanalysis” and “found that the CMIP5 models\noverall simulate aspects of ITF variability, such as spectral and vertical structure, that\nare consistent with the reanalysis, although inter-model differences are substantial”.\nIn the revised manuscript, we have added a few sentences to specify the experiment of\nCMIP6 (i.e., historical run) we used, and discussed why it is proper to examine the\nvariability using CMIP6 historical simulations.\nPlease see Lines 351-354: “It should be noted that the external forcing for the Hist runs\nare based on observations over 1850-201451,52, and the Hist runs are usually used to\nexamine natural variability and compare with historical observations17. For example,\nSantoso et al.17 found that the CMIP5 models overall simulate aspects of ITF variability.”\nFig. 3: there are studies suggesting the equatorial Indian Ocean winds are important for\nthe ITF variability. So it's not clear the usefulness of using the 22-8S band to represent\nthe Indian Ocean wind response. In any case, the wind forcing may be secondary to the\nthermohaline forcing?\nResponse: Thanks for your question. Yes, some studies suggest that the equatorial\nIndian Ocean winds are important for the ITF variability, but this is dependent on the\nspecific time scales. Please note that the ITF flows into the Indian Ocean in the southern\nhemisphere within the latitudes of 22-8°S, and the major mechanism that the equatorial\nIndian Ocean winds influence the ITF is through generating equatorial Kelvin waves\nwhich propagate to the equatorial eastern Indian Ocean and then southward to the ITF\nregion along the coasts. This means that the influence of the equatorial Indian Ocean\nwinds on the ITF variability is relatively confined in the time scales of Kelvin waves\n(typically range from intra-seasonal, seasonal to interannual time scales).\nOn longer time scales, things are quite different. For example, Sprintall et al. (2009)\nfound that “To first order, the seasonal cycle of transport in the thermocline (_100–150\nm) in all three exit straits is dominated by regional monsoon forcing, with maximum\nITF during the southeast monsoon.” On decadal time scales, the ITF variability is likely\ncontrolled by mostly the Pacific winds and then the southeastern Indian Ocean.\nFor example, Lee et al. (2015) carried out sensitivity experiments to understand the relative\nimportance of wind forcing from the different ocean basins in driving the ITF variability,\nand they conclude that “Using real-time surface winds inside the Pacific Ocean and\nclimatological winds outside of the Pacific Ocean (EXP_PAC), the abrupt increase of\nthe Indian OHC700during the 2000s was successfully simulated.”\nAs to the role of the wind forcing and the thermohaline forcing, we did not separate the\nrelative contribution of the wind forcing and the thermohaline forcing to the ITF change.\nThe contribution due to temperature change may include contributions from both the\n3 / 21\nwind forcing and the thermohaline forcing. But this is really an interesting question we\nneed to study in the future.\nIn the revised manuscript, we have added some sentences to describe why we mainly\nexamined the wind forcing in the western Pacific Ocean and eastern Indian Ocean. And\nplease note that we have presented the wind forcing change in the equatorial Indian\nOcean as well, in addition to the time series of wind stress in the eastern Indian Ocean.\nPlease see Lines 219-221: “Note that, for the wind forcing, the ITF variability on\ndecadal and longer time scales is likely controlled by mostly the Pacific winds and then\nthe southeastern Indian Ocean6.”\nThe lower panel of Fig. 4 shows the mean state, which doesn't illustrate the potential\nchanges. It would be nice to pinpoint the causes of a second turning point.\nResponse: Thanks for your suggestions. Yes, the lower panel of Fig. 4 shows the mean\nstate. In the revised manuscript, we have updated the Fig. 4 and illustrate the potential\nchanges of surface winds and precipitation as well. Please see the updated Fig.4. As to\nthe causes of a second turning point, we are sorry that we are not sure about the causes\nof a second turning point by now. But in the revised manuscript, we have discussed the\npotential causes of a second turning point.\nPlease see Lines 246-249: “After exceeding the second turning point, the responses of\nwind stress and precipitation differences in the western Pacific and eastern Indian\nOcean to CO2 are both weakened, leading to a reduced ITF response, which may be\nassociated with changes in the background state and the weakened Indo-Pacific\nexchange under this regime.”.\nThe Fig.4 has been updated.\nThank you again for your insightful comments and helpful suggestions.\n4 / 21\nPoint-by-point response to Reviewer#2\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #2 (Remarks to the Author):\nUsing results from idealized CMIP6 annually 1% CO2 rise experiments, the authors\nhave assessed the thermosteric and halosteric components of the ITF transport and\nconcluded that the increasing CO2 concentration in the atmosphere shall eventually\nlead to a decline in ITF transport. The authors also proposed two turning points based\non the changing rate of the ITF transport. The manuscript is generally well written\nexcept that the citation numbers are not correctly indexed and the figure captions need\nimprovement. The findings are important but should be presented with the right context.\nI have some significant concerns about the findings and the interpretations which are\nlisted below. By addressing these concerns, the manuscript could be more rigorous,\nappropriately contextualized, and valuable to the scientific community.\nResponse: Thank you for your encouragement and constructive suggestions. We have\nmade careful revisions according to your comments, and the citation numbers have been\ncorrected, the figure captions have been improved. Please see our point-by-point\nresponses below.\nMajor Concerns:\n1) The idealized 1% CO2 rise experiment used in the study neglects other\nanthropogenic sources of radiative forcing, such as aerosols. Since aerosols, which are\non a declining trend, counteract some of the warming effects of increasing CO2, this\ncould bias the results, and the proposed vanishing ITF transport might be expedited\nunder more realistic conditions. The authors could address this concern by\nacknowledging that the 1% CO2 experiment is a simplification and that, in reality,\naerosols and other factors (such as land use changes and different greenhouse gases)\nmight influence the rate and magnitude of ITF transport changes. The authors should\nclarify that their results represent an idealized scenario and not a direct prediction of\nfuture conditions, emphasizing the need for more comprehensive modeling that\nincorporates multiple forcings. They should also discuss the potential uncertainties in\ntheir conclusions, given the absence of realistic time-evolving aerosol and other\ngreenhouse gas forcing in the experiment.\nResponse: Thank you for your insightful comments. We totally agree with you that the\nidealized 1% CO2 rise experiment neglects other anthropogenic sources of radiative\nforcing, such as aerosols; and neglecting these anthropogenic sources may counteract\nsome of the warming effects of increasing CO2 and might bias the results. Following\nyour suggestions, we have:\n5 / 21\na) acknowledged that the 1% CO2 experiment is a simplification and that, in reality,\naerosols and other factors (such as land use changes and different greenhouse gases)\nmight influence the rate and magnitude of ITF transport changes in the abstract and\nmain text;\nb) clarified clearly that these results represent an idealized scenario rather than a direct\nprediction of future conditions, and emphasized the need for more comprehensive\nmodeling that incorporates multiple forcings;\nc) in the Discussion section, we have added sentences to discuss the potential\nuncertainties in their conclusions, given the absence of realistic time-evolving aerosol\nand other greenhouse gas forcing in the experiment.\nPlease see:\nLine 21-23: “Here using idealized CMIP6 simulations, we find that the response\nof the ITF to increasing atmospheric carbon dioxide (CO2) concentration is nonlinear\nand exhibits an abrupt rapid decline after surpassing a turning point.”.\nLines 25-27: “The current level of CO2 concentration has reached a turning point,\nindicating that a rapid decline in the ITF and a state of transition in the Indian and\nPacific Oceans is likely imminent.”.\nLines 325-328: “However, as noted, large uncertainty still exists among the\nclimate models, particularly the idealized 1pctCO2 experiment, in which aerosols and\nother factors such as land use changes and other greenhouse gases are not considered.\nThese factors may also result in changes to the rate and magnitude of the ITF transport.”.\nLines 315-318: “In addition, the results represent an idealized scenario rather than\na direct prediction of future conditions, given that other anthropogenic sources of\nradiative forcing such as aerosols are neglected, and hence more comprehensive\nmodeling that incorporates multiple forcings is needed.”\n2) The study assumes a fixed level of no motion at 1100 m, but the bathymetry at 114\nE reaches much deeper (~6000 m) between 11S – 22S. Recent studies have shown\nsignificant barotropic variabilities in the Indo-Pacific basin\n(https://doi.org/10.1016/j.dsr.2023.104104), suggesting that the level of no motion\nshould extend to the ocean bottom for more accurate transport estimates. This may\nbecome all the more relevant with the impending climate change wherein frequent\nextreme events are predicted. An atmospheric disturbance can generate barotropic\nresponses if the wavenumber and the frequency meet certain criteria (see M.J. Lighthill,\nTime-varying currents, Philos. Trans. Roy. Soc. Lond. Math. Phys. Sci., 270 (1971),\npp. 371-390). The authors should consider adjusting their ITF transport calculation to\n6 / 21\naccount for deeper bathymetry, potentially extending the level of no motion to the ocean\nbottom. They could discuss how this adjustment would likely improve the accuracy of\nthe total ITF transport estimates. I do understand that any additional experiment with\nCMIP6 is time and resource consuming. The authors should at the least present a\nsensitivity analysis based on the revised assumption of the level of no motion and show\nthat extending the bathymetry to the ocean bottom does not significantly alter the\nfindings of this study.\nResponse: Thank you for your comment. Yes, we agree that the level of no motion\nshould probably extend to the ocean bottom for more accurate transport estimates.\nWe considered two reasons assuming the 1100 m as the level of no motion: (1) the sill\ndepth of the main core of the ITF within the Indonesian Seas is about 1100 m (e.g.,\nAndersson and Stigebrandt, 2005; Hu and Sprintall, 2016); (2) variability of the ITF\nand Temperature/Salinity is confined within the upper layer ocean as shown in in situ\nobservations (roughly in the upper 700 m layer; Lu et al., 2024; Sprintall et al., 2009);\n(3) in situ observations show that the velocity in the 1000-1200 m layer is close to zero\n(Sprintall et al., 2009); and (4) in situ observations are mainly available in the upper\nlayer (~700 m for XBTs and 2000 m for Argo floats). We really appreciate your\nunderstanding that additional experiment with CMIP6 is time and resource consuming.\nFollowing your suggestion, we have undertaken sensitivity analysis to test if the\nfindings are sensitive to the choice of the level of no motion or not.\nFigure R1 presents a comparison of the ITF transport anomalies (i.e., minus the mean\nITF transport) using 7 assumed levels of no motion, from 1100 m to 2500 m. Here the\ngeostrophic current velocities are calculated using decadal mean temperature and\nsalinity data from WOA18, which is based on in situ observations and has a high\nresolution (0.1°). (Note that there are extremely few data available below 2500 m so\nwe are reluctant to assume level of no motion at the bottom). As we can see, the ITF\ntransports with different assumed levels of no motion show quite similar variability and\nlong-term trend, indicating that the findings are not sensitive to the choice of the level\nof no motion. This is reasonable, because the variability of both geostrophic currents\nand temperature/salinity, is much larger in the upper layer than in the deep layer (Figure\nR2 for example).\n7 / 21\nFigure R1. Comparison of ITF transport anomalies between various assumed level of\nno motion.\n[FIGURE REDACTED]\nFigure R2. Salinity and temperature trends across the 114°E section over the period\n1958–2017 using WOA18 (a, b) and the ensemble mean of five datasets (c, d). This is\na figure from Lu et al. (2024).\nIn the revised manuscript, we have added some sentences to discuss the influences of\nthe assumed level of no motion on the results, described the result from the sensitivity\nanalysis, and discussed the uncertainty of the results due to using an assumed level of\nno motion.\nPlease see Lines 373-375: “Here we used the 1100 m as the level of no motion\nconsidering the sill depth of ITF within the Indonesian Seas16. Using deeper depth as\nthe level of no motion leads to no significant difference of the fluctuation of ITF\ntransport.”\nReferences:\nAndersson, H. C., and A. Stigebrandt (2005), Regulation of the Indonesian throughflow\nby baroclinic draining of the North Australian Basin, Deep Sea Research Part I:\nOceanographic Research Papers, 52(12), 2214-2233.\n8 / 21\nAfroosa, M., B. Rohith, A. Paul, F. Durand, R. Bourdallé-Badie, and S. S. C. Shenoi\n(2023), The MJO-driven Indo-Pacific barotropic see-saw, Deep Sea Research Part\nI: Oceanographic Research Papers, 199, 104104.\nHu, S., and J. Sprintall (2016), Interannual Variability of the Indonesian Throughflow:\nthe Salinity Effect, Journal of Geophysical Research: Oceans, 121, 2596-2615.\nLighthill, M. J. (1971), Time-Varying Currents, Philosophical Transactions of the\nRoyal Society of London. Series A, Mathematical and Physical Sciences,\n270(1206), 371-390.\nLu, X., S. Hu*, and J. Sprintall (2024), The role of precipitation and salinity effect in\nmulti-decadal changes and long-term trends of the Indonesian Throughflow,\nJournal of Climate, 37(4), 1317–1331.\nSprintall, J., S. E. Wijffels, R. Molcard, and I. Jaya (2009), Direct estimates of the\nIndonesian Throughflow entering the Indian Ocean: 2004–2006, Journal of\nGeophysical Research: Oceans (1978–2012), 114(C7), C07001.\n3) The causality of the declining ITF transport lies in the changing wind stress and\nprecipitation. Figure 3b and 3d represent a turning point (5 x 1015 kg CO2) where the\nwind stress and precipitation anomaly changes sign. Why the turning point related to\nwind stress and precipitation anomalies (as shown in Figures 3b and 3d) is not\nconsidered more relevant than the two turning points proposed by the authors. The\nconcern is that the authors may have missed a potentially important relationship\nbetween the changes in wind stress and precipitation and the ITF transport. The authors\ncould revise their interpretation of the turning points and consider whether the changes\nin wind stress and precipitation are more directly tied to the decline in ITF transport\nthan the two turning points they originally proposed.\nResponse: Thank you for your comments. The point you mentioned is indeed a\nnoteworthy one, as it represents an inevitable intermediate stage in the long-term\nvariations of wind stress and precipitation—specifically, the point (5 x 1015 kg CO2,\ni.e., the middle of 2 to 10 x 1015 kg CO2) where the anomaly value is zero and a sign\nchange occurs before and after it. We have carefully considered this issue, but we\nbelieve that this transition point has little impact on understanding the rate of change in\nthe ITF trend we are discussing due to two main reasons:\n(1) Figure 3b and 3d display anomalies in wind stress and precipitation relative to a\nmean value, so there must inherently be an intermediate point where the sign transitions,\ni.e., the zero point, so the existence of the point 5 x 1015 kg CO2 is expected.\n(2) Before (after) this intermediate point (5 x 1015 kg CO2) corresponds to a positive\n(negative) anomaly of ITF transport (i.e., sign of Q, Q is ITF transport), rather than\nlong-term trends of ITF (sign of 𝜕𝑄⁄𝜕𝑡). By contrast, the response of the ITF to the\ntwo turning points we presented in this study indicate a dramatic jump of the ITF trend\n(sign of 𝜕𝑄⁄𝜕𝑡), rather than simply positive or negative anomalies.\n9 / 21\nIn the revised manuscript, we have added a sentence to discuss this point in Lines 227-\n230: “An intermediate point exists at 5 ×1015 kg of CO2 where the wind stress and\nprecipitation anomaly changes sign, but this is mainly because we display anomalies\nrelative to a mean value over a CO2 range of 2 to 10×1015 kg.”\n4) I have concerns that the abstract may sensationalize the findings, suggesting an\nimminent and rapid decline in ITF transport. The authors should carefully revise the\nabstract to better reflect the idealized nature of the study and clarify that the findings\npertain to a specific experimental scenario rather than an immediate real-world\nprediction.\nResponse: Thank you for your comments. We would like to clarify that the conclusions\ncontain both the idealized aspect and a practical aspect. The idealized aspect, as you\nmentioned, is because we used simulations with specific experimental scenario (i.e.,\nCMIP6 experiments). The practical aspect is embedded in the fact that the observed\nCO2 level is indeed passing the first turning point and that the observed wind and\nprecipitation trends agree with the simulations. Anyway, the key findings of this study\nare based on some idealized numerical experiments with large uncertainties. In the\nrevised manuscript, we have adjusted the wording in the abstract and main texts to\nemphasize the idealization and uncertainty of the results while ensuring respect for the\nobjectivity of the data. Please see:\nLine 21-23: “Here using idealized CMIP6 simulations, we find that the response\nof the ITF to increasing atmospheric carbon dioxide (CO2) concentration is nonlinear\nand exhibits an abrupt rapid decline after surpassing a turning point.”.\nLines 25-27: “The current level of CO2 concentration has reached a turning point,\nindicating that a rapid decline in the ITF and a state of transition in the Indian and\nPacific Oceans is likely imminent.”.\nLines 325-328: “However, as noted, large uncertainty still exists among the\nclimate models, particularly the idealized 1pctCO2 experiment, in which aerosols and\nother factors such as land use changes and other greenhouse gases are not considered.\nThese factors may also result in changes to the rate and magnitude of the ITF transport.”.\nLines 315-318: “In addition, the results represent an idealized scenario rather than\na direct prediction of future conditions, given that other anthropogenic sources of\nradiative forcing such as aerosols are neglected, and hence more comprehensive\nmodeling that incorporates multiple forcings is needed.”\nMinor Comments:\n1) Line 39-42 : The authors may wish to apprise the readers about the MJO driven\nITF transports.\n10 / 21\nResponse: Done. Thanks.\n2) Line 52-53: If the winds are strong enough, it can affect the entire water depth.\nResponse: Indeed. We’ve revised this sentence.\n3) Line 62: It will be good to inform the readers about the time needed for the global\ntemperature to reach 1.5 C given the current trend.\nResponse: We have added a sentence here following your suggestion. Thanks.\nLines 64-65: “Recent studies suggest that the increase of global temperature will\nexceed 1.5°C in less than 10 years according to the current emissions trajectory38.”\n4) Captions in Fig S1(a) and Fig S1(b) appear to be swapped. Caption mentions cm/s\nwhereas the colorbar appears in m/s.\nResponse: Thank you, we have corrected the captions.\n5) The authors use the abbreviations in tandem without describing it. For example,\nMITF is used in the Results section but it is described much later in the Methods.\nResponse: Thanks for pointing out this. We have described these abbreviations when\nthey appear for the first time in the manuscript.\n6) Line 77: Even though CMIP6 is pretty well known across the earth science\ncommunity, it is a good practice to expand the initials given the varied reader base of\nthis journal.\nResponse: We agree. We have expanded the initials of CMIP6 in Line 73 where it is\nfirst used.\n7) Line 88-90: What is the correlation between the ITF transport from CMIP6 and\nMITF and the level of significance? The authors should mention it in the caption of\nFigure S1b if not in the text.\nResponse: Please note that the MITF is not a continuous time series, which was\ninterrupted during about 2011-2013. Hence we calculated the correlation between the\nmonthly ITF transport from CMIP6 and MITF during the longest portion of the MITF\ntime series for Jan 2004-Jul 2011. As a result, the correlation between the ITF transport\nfrom CMIP6 and MITF is 0.25 which is significant at 90% confidence level. We have\nincluded this information in the caption of Figure S1b.\n8) Line 95-97: The magnitude differs by ~ 3 Sv. The trends appear similar though.\nThe authors should mention it apart from highlighting that the two transports are in\ngood agreement.\nResponse: Done. Thanks.\n9) Line 108-112: Why does not ITF_T and ITF_S add up to the total ITF given that\nthe residual term is negligible as claimed in Hu and Sprintall 2016.\n11 / 21\nResponse: As noted, the ITF_T (0.60 Sv per decade) and ITF_S (-0.32 Sv per decade)\nexactly add up to the total ITF (0.28 Sv per decade). So the results are in good\nagreement with that claimed by Hu and Sprintall (2016).\n10) ITF’ used in Fig S2b and S2d is not defined anywhere.\nResponse: Done. Thanks.\n11) Fig 2: The shades in Fig 2b should be properly described in the caption.\nResponse: Done. Thanks.\n12) Fig 2(a): 1pctCO2 -EN ??? What does it refer to?\nResponse: “1pctCO2 -EN” refers to the ensemble mean of 1pctCO2 experiments. We\nhave added some words to specify what it means. Thanks.\n13) Line 184-187: I suppose that the temperature and salinity is averaged over depth.\nIf yes, it is averaged up to what depth ? These details should be mentioned in the text.\nResponse: Yes, they are averaged over depth. This information was mentioned in the\ncaption of Supplementary Fig. S4. Now, we have added it to the main text as well\n(Line 192).\n14) Figure S4 : I don’t see black quadrilaterals in panels (a) and (c).\nResponse: The words “black quadrilaterals” have been deleted. Thanks.\n15) Fig 3: Captions (b) and (c) should state that the points are from all CMIP6\nmodels. In fact, the authors should be more attentive about the captions across all\nfigures.\nResponse: Sorry for the unclear description. We originally intended to express that it\nis from all models by using the words “Ensemble mean”. Now we have added a\nsentence to be clearer: “Note the dots in panels b and d are ensemble mean of all\nmodels.”\n16) Line 241: Typo.... Change existed to exist.\nResponse: Done. Thanks!\n17) Figure 4 should be described in some detail in the text rather than leaving it to the\ninterpretation of the readers.\nResponse: Good idea. We have added some sentences to described in more detail\nabout Figure 4. Please see Lines 260-262: “Figure 4a presents a schematic diagram\nillustrating the response of the ITF to CO2 forcing, concisely demonstrating the\nnonlinear response process of the ITF across different CO2 phases as observed in\nCMIP6 models.”\nLines 289-291: “Figure 4b schematically illustrates the effect of the Indo-Pacific\noceanic dynamics on the ITF during the “Exchange state”, highlighting the substantial\ninfluences of wind forcing and freshwater forcing on ITF variations.”\n12 / 21\n18) Line 266-268: “We find that the response of zonal wind stress in the Indo-Pacific\nOcean is featured by nonlinear weakening trend with two turning points like ITF’s\nresponse”. This statement is based on Figure 3. It’s not clear from the plot why those\ntwo points should be called turning points.\nResponse: Yes, this statement is based on Figure 3. The reason we call them as\n“turning points” is: before and after the two points, the zonal wind stress is not quite\nsensitive to CO2 changes (i.e., its variation is minimal with changing CO2);\nhowever,\nwhen CO2 is in between the two points, the zonal wind stress is rapidly decreased\nwith increasing CO2.\n19) Line 329 : The indexed reference is actually number 15 instead of 14.\nResponse: Corrected. Thanks.\n20) Line 287-289: It is important to elaborate how CO2 increment is implemented in\nthe models. Are the 1% increments spread over months or sudden at the end of a\nyear? What about the other greenhouse gases? Not all readers may be aware of the\nDECK experiments within the CMIP6 framework. The manuscript will be more\ncomplete if these relevant details are mentioned instead of directing the readers to ref\nno 47.\nResponse: Thank you for your suggestions. Yes, it is better to mention these important\ndetails.\nWe have communicated with some of the model development teams, and it appears that\nthere are slight differences between the various models. In some models, CO2 increases\nmonth by month, while in others, it suddenly rises by 1% at the end of the year. Other\ngreenhouse gases in the 1pctCO2 experiment is supposed to be the same with the\ncontrol run, which means the experiment conditions (e.g. greenhouse gas\nconcentrations, ozone concentration, surface land conditions) should be representative\nof Earth around the year 1850.\nThe 1pctCO2, i.e., a simulation forced by a 1%yr-1 CO2 increase, is one of the four\nbaseline experiments of the Diagnostic, Evaluation and Characterization of Klima\n(DECK) experiments (klima is Greek for “climate”), which is one of the three major\ncomponents of CMIP.\nIn the revised manuscript, we have added more descriptions about the relevant details\nof the experiments. Please see:\nLines 341-343: “The 1pctCO2 is one of the four baseline experiments of the Diagnostic,\nEvaluation and Characterization of Klima (DECK) experiments, which is one of the\nthree major components of CMIP.”\nLines 354-357: “The 1pctCO2 runs branches the piControl as well but are driven by\nidealized forcing that is the same as that in piControl except for the prescribed CO\n2\n13 / 21\nconcentration increasing at 1% yr-1 (month by month or suddenly rises by 1% at the end\nof the year) since the 1850 without considering any observations.” Thanks!\n21) Since this journal admits 10 display figures, I don’t see a need to include\nSupplementary Figures in this manuscript.\nResponse: Yes, this journal admits 10 display figures, but we feel like the selected\nfigures in the main manuscript tell the salient points of our story, whereas the\nsupplementary figures are more secondary supporting evidence.\nAgain, we would like to express our sincere gratitude for your valuable comments and\nsuggestions on our manuscript. Your insightful feedback has significantly improved\nthe quality of our paper.\n14 / 21\nPoint-by-point response to Reviewer#3\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #3 (Remarks to the Author):\nThis manuscript discusses the response of the Indonesian Throughflow (ITF) to\nincreasing CO2, in 25 CMIP6 climate models. The model runs studied are the historical\n(HIST) and 1% per annum CO2 increase (1pctCO2) runs. The manuscript notes that\nwhen plotted against CO2 concentration the ITF response is nonlinear, with an initially\nslow rate of weakening, which accelerates and then slows again as a function of CO2\nconcentration. The second part of the paper begins to unpick some of the mechanisms\nof the ITF decline, finding that a balance/competition between wind-driven differences\nin warming, and precipitation-driven changes in salinity, appear to be the main drivers\nof the ITF weakening. The authors conclude by speculating, based on the 1pctCO2\nresults, that current CO2 concentrations may be such that a rapid weakening of the ITF\nmay be imminent.\nThe topic of the paper, changes in the ITF in response to increasing greenhouse gas\nforcing, is interesting and important, and to the best of my knowledge has not been\nextensively studied in the literature to date. So the study is welcome. Unfortunately\nthere are some methodological weaknesses in the analysis which to my mind invalidate\nthe discussion of the nonlinear response and the conclusion that a ralpid ITF decline\nmay be imminent. I have noted these under ‘Methodological issues’ below. I did find\nthe discussion of mechanisms of ITF weakening interesting and more convincing.\nOverall, my feeling is that the authors have the basis of a valuable and interesting study,\nparticularly on the mechanisms driving the ITF weakening, but that a large part of the\nanalysis presented, and the conclusions drawn from it, need to be revisited, to the extent\nthat any revised version would essentially be a different paper. Hence I cannot\nrecommend acceptance of this paper, but I hope that the authors will continue with this\nwork and in particular publish and develop further their analysis of mechanisms. Noting\nthe dependence on supplementary figures, it may be that this analysis would work best\nin a longer format journal. I’m sorry I can’t be more positive about the present\nmanuscript, but I hope that these comments are useful and constructive in taking this\ninteresting work forward.\nResponse: Thank you for your comments, suggestions, and encouragement. As per\nyours and other reviewer recommendations, we have revised the manuscript, with\nparticular emphasis on refining the methodology and analysis sections you\n15 / 21\nhighlighted. We hope that our revisions will help address your concerns and lead to a\nmore favorable evaluation of our manuscript. Please see our point-by-point responses\nbelow.\nApart from Fig. 1, the authors present their analysis of the 1pctCO2 run in terms of\nCO2 concentration rather than time (Figs 2-4). They note a levelling off of the ITF\nweakening at higher concentrations. This is entirely to be expected as the radiative\nforcing from CO2 increases logarithmically with the concentration. Indeed this is why\nthe standard protocol follows a percentage increase of concentration per year rather\nthan a linear increase. The same levelling off would be seen in any climate variable (e.g.\nglobal mean temperature), if plotted against CO2 concentration. Since there is no direct\nmechanistic link between CO2 concentration and ITF (only the indirect link through\nthe response of the climate system), it really makes no sense to plot the response in this\nway.\nResponse: Thank you for your critical and valuable comments.\nFirst of all, please note that the ITF’s response to CO2 increases is not simply “a\nlevelling off of the ITF weakening at higher concentrations”, but rather, a nonlinear\nprocess with a rapid weakening between two turning points and slow changes at\nconcentrations lower than the first turning point or higher than the second turning\npoint.\n[FIGURE REDACTED]\nFig. R3. Response of global mean surface temperature to CO2 (From Nicholls et al.,\n2020).\nSecond, such a nonlinear response is not really expected and previous studies suggest\nthat same levelling off would be NOT necessarily be seen in other climate variable such\nas global mean temperature. For example, Nicholls et al. (2020) reported the response\nof global mean surface temperature to CO2 forcing using 1pctCO2 simulations. As we\ncan see in Fig.R3, the global mean temperature shows a linear increasing as the CO2\nincreases and no turning points can be found in the response of global mean temperature\nto CO2 increasing. So, the response of the ITF is very different from that of other\n16 / 21\nclimate variable such as global mean temperature. It is really surprising to see the ITF\nshows such interesting and unique response to CO2 increasing.\nNonetheless, we agree with you that “there is no direct mechanistic link between CO2\nconcentration and the ITF”. This does not mean that the CO2 concentration is not\nessential to the ITF, but is simply because the Earth’s climate system is super complex.\nAs the primary external forcing driving long-term changes in the Earth system, rising\ngreenhouse gas emissions affect all key processes and physical parameters within the\nsystem - and the ITF is no exception. CO2 is one of the most representative greenhouse\ngases and an effective indicator of external forcing. Studying the ITF's response to the\nprojected CO₂ increase helps predict the ITF future behavior and the impact that those\nchanges might have on both the Indian and Pacific Oceans.\nNicholls, Z. R. J., et al. (2020), Reduced Complexity Model Intercomparison Project\nPhase 1: introduction and evaluation of global-mean temperature response, Geosci.\nModel Dev., 13(11), 5175-5190, doi:10.5194/gmd-13-5175-2020.\nThe above does not explain the initial slow response of the ITF. This can equally well\nbe seen when plotting the response against time (Fig. 1a). This is most likely an issue\nof the initial response time to perturbations. Other variables such as global mean\ntemperature show a 2-timescale response to CO2 perturbations, with a fast timescale of\na decade or two followed by a longer adjustment time. It is seen most clearly in the\ninstantaneous 4xCO2 experiment but is also there in the 1pctCO2 experiments (see\ne.g. https://journals.ametsoc.org/view/journals/clim/26/6/jcli-d-12-00195.1.xml). This\ninitial adjustment time may be different for the ITF than for global mean temperature,\nand in fact understanding this response time would be a valuable addition to the\nmechanistic analysis in this study. In any case, it seems a priori far more likely that the\ninitial slow response is related to an inherent timescale of the model dynamics rather\nthan a specific threshold CO2 concentration. The apparent ‘threshold’ CO2\nconcentration would therefore be entirely dependent on the specific timeseries of\nconcentration used to force the model. I think if the claim that there is a specific\nthreshold CO2 concentration that results in acceleration is to be made, it would at least\nhave to be shown across several different CO2 forcing scenarios.\nResponse: Thank you for raising this crucial point regarding the existence of both fast\nand slow response processes in the ocean system - this idea provides extremely valuable\ninsights for understanding the ITF's response to CO2 increasing!\nWe agree with you that the nonlinear response of the ITF to CO2 forcing might be\nrelated to the inherent timescale of the model dynamics. However, the relationship\nbetween the ITF and CO2 level, i.e., ITF=ITF(CO2), is actually different from that\nbetween ITF and time, i.e., ITF=ITF(t), when the CO2 increases independently with\ntime. In the time-domain relationship ITF=ITF(t), the ITF’s response, i.e., 𝜕𝐼𝑇𝐹\n𝜕𝑡 , might\n17 / 21\nbe similar to that discussed by Geoffroy et al. (2013), when the changing rate of ITF is\ndetermined by the fast and slow response processes, and hence the initial slow response\nis attributed to the slow response process.\nBy contrast, in the CO2-domain relationship ITF=ITF(CO2), the changing rate of ITF,\ni.e., 𝜕𝐼𝑇𝐹\n𝜕𝐶𝑂2 , is not related directly to time, and hence is not necessary directly associated\nwith the fast and slow response processes. The CO2-domain relation ITF=ITF(CO2)\nsimply reflects the sensitivity of ITF to CO2 changes. We suppose that at any specific\nCO2 level, the adjustment of the ITF might have been completed, i.e., the fast and slow\nresponse processes has been completed. Actually, in the 1pctCO2 experiment, the CO2\nincreases 1% per year, indicating that the time from the first value for the CO2 level to\nreach the first turning point is about 40 years, much longer than the known adjustment\ntime scales of ITF (e.g., wind, Rossby waves, precipitation, deep ocean currents)\naccording to previous studies (e.g., Sprintall et al., 2019).\nIn the revised manuscript, we have added new sentences to discuss the potential role of\nthe fast and slow response processes, previous studies including Geoffroy et al. (2013)\nare cited, and in particular, the role of the inherent timescale of the model dynamics in\nthe initial slow response is discussed.\nLines 309-315: “In addition, the response of global mean temperature to CO2\nperturbations shows a fast timescale of a decade or two followed by a slow timescale\n(Geoffroy et al., 2013). Even though the response of the ITF to CO2 is expected to be\ndifferent from that of global mean temperature, the fast and slow response processes\nmight be important in determining the turning points, especially the initial slow\nresponse before the first turning point. Further in-depth research is still needed to\nunderstand the physical processes and mechanisms underlying these two turning points.”\nFollowing on from the above, the claim in the Discussion section that the real ITF\nmay be on the verge of a rapid decline, based on current CO2 concentrations, is not\njustified. Even if CO2 concentration were somehow the direct control variable (which\nhasn’t been demonstrated), what about all the other radiatively active forcing species\nin the present and future atmosphere? In any case, to assess the claim, why not simply\nlook at the projection runs (ScenarioMIP) to see if the models actually project a rapid\nITF decline?\nResponse: Thanks for your comments.\nFollowing your suggestion, we examined the changes of the ITF in different projection\nruns. Fig.R4 presents a comparison of projected changes of ITF velocity between\nssp585 and 1pctCO2 runs. As you can see, the ITF changes in the ssp585 is quite similar\nto that in the 1pctCO2 runs, and the weakening of ITF in the ssp585 is slightly stronger\nthan that in the 1pctCO2 runs. Most importantly, the ssp585 experiment shows a turning\npoint at about 1026 ppm as well, after which the decline of ITF is much weaker than\n18 / 21\nbetween 400-1026 ppm. Please note that the starting CO2 concentration is about 400\nppm, which exceeds the first turning point already, so we are not able to examine the\nfirst turning point in the ssp585 experiments. Nonetheless, the ITF response to the CO2\nin the ssp585 experiments is similar to that in the 1pctCO2 experiments. Fig. R4. Left\npanels: Comparison of changes of the ITF velocity in ssp585 (a) and\n1pctCO2 (b) runs. Right panel: response of ITF to CO2 concentration in the ssp585\nexperiments from 16 models (black dots for each model and black line for ensemble\nmean of all models). The solid red dot denotes the second turning point, while the\ndashed red line and red cycle denote the first turning point.\nAs to the decline of ITF, some previous studies have shown that the CMIP models\nproject a significant ITF decline under the business-as-usual emission scenario RCP8.5\n(see Fig. R5 from Hu et al., 2015, Nature). As shown in Fig. R5, the ITF averaged over\nthe 2050–2100 period under RCP8.5 is about 3 Sv weaker than the historical mean,\nindicating a future decline of ITF under RCP8.5. Under representative concentration\npathways (RCP) 4.5 scenario, a robust ITF transport slowdown is found as well at\ndepths below 100 to 1200 m (Santoso et al., 2022).\n19 / 21\n[FIGURE REDACTED]\nFig. R5. Modelled transports of annual-mean Pacific WBCs and their projected changes.\na, Historical mean transport averaged over the twentieth century (positive values\nindicate northward, and negative, southward transports), and b, projected change from\nthe historical mean, averaged over the 2050–2100 period under RCP8.5, for selected\nPacific currents. The ITF is highlighted by the blue box. (From Hu et al., 2015, Nature).\nHence, the decline of the ITF presented in our study is in agreement with the projection\nruns and previous studies that show a weakening trend of the ITF in the context of\nglobal warming. However the turning points and the rapid decline after the first turning\npoint have not been reported previously.\nAs to other radiatively active forcings, we agree that the CO2 concentration is definitely\nnot the only external forcing. But as you mentioned, the CO2 concentration was the\ndirect control variable in the 1pctCO2 experiments, and this is the reason why 1pctCO2\nexperiments are suitable for studying the response of ITF to greenhouse gas emissions.\nOur paper aims to investigate the response of ITF to the CO2 concentration: the\nresponse to “all the other radiatively active forcing species”, which is obviously more\ncomplex, is beyond the scope of this study.\n20 / 21\nIn the revised manuscript, we have added sentences in the Discussion section to discuss\nthe potential uncertainties, given the absence of other radiatively active forcing such as\naerosol and other greenhouse gas forcing in the experiment.\nPlease see Lines 273-276: “The decline of the ITF presented in our study is in\nagreement with the projection runs and previous studies that show a weakening trend\nof the ITF in the context of global warming4,17, but the turning points and the rapid\ndecline after the first turning point have not been reported previously.”\nLines 315-318: “It should be noted that the results represent an idealized scenario rather\nthan a direct prediction of future conditions, given that other anthropogenic sources of\nradiative forcing such as aerosols are neglected, and hence more comprehensive\nmodeling that incorporates multiple forcings is needed.”.\nAgain, we thank you so much for your insightful comments, which are helpful and\nenlightening for not only this study, but also our future studies.\n21 / 21"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have improved the manuscript based on the previous review comments. I believe the manuscript is valuable for the climate research community and is worth a quick publication. Still, there are some minor revisions necessary before the manuscript can be considered for publication.\n1. Regarding the previous comment on Fig. S1:\nI noted that the comparison was with the historical simulations. Even in the \"historical simulations\", unlike the pacemaker model experiment, the models will not be able to generate ENSO events in the same years as in the observations. I checked the Santoso paper, and they only compared the ITF annual cycle with observations. I suggest that the authors separate the annual and interannual signals in the ITF and then compare them with the observed ones. What you would find is that for the annual cycle, it is very similar to Santoso et al. (2022), but for the interannual part, the correlations would be insignificant.\nI note that this is not a critical part of the text. However, it is a fundamental concept of the CMIP models, so it needs to be correctly stated.\n2. First \"turning point\":\nThe turning point would be of interest to most readers. There is a dilemma. The climate models project a weakening ITF; however, in observations, we haven't captured a weakening trend so far. The ITF transport has strengthened over the recent decades. We somehow know that this ITF trend is due to climate variations, such as the phase of the PDO. It is a fair question to ask when the weakening trend of the ITF will emerge from its natural variability.\nAs this study is mostly statistical, it would be useful to compare the greenhouse effect-driven ITF trend with that of the natural variability. I don't know how the authors derive trends in Fig. 2b. For natural variability, they probably can calculate 30-yr trends in the ITF transport in the CMIP historic runs.\nAnyway, I would be happy to see this work being further improved.", "reviewer_2": "The authors have responded to all the queries satisfactorily. The manuscript can now be considered for publication.", "reviewer_3": "Please see review in attached pdf"}, "author_response": "Point-by-point response to Reviewer#1\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #1 (Remarks to the Author):\nThe authors have improved the manuscript based on the previous review comments. I\nbelieve the manuscript is valuable for the climate research community and is worth a\nquick publication. Still, there are some minor revisions necessary before the manuscript\ncan be considered for publication.\nResponse: Thank you for your encouragement and constructive suggestions. We have\nfurther revised the manuscript. Please see our point-by-point responses below\n1. Regarding the previous comment on Fig. S1:\nI noted that the comparison was with the historical simulations. Even in the \"historical\nsimulations\", unlike the pacemaker model experiment, the models will not be able to\ngenerate ENSO events in the same years as in the observations. I checked the Santoso\npaper, and they only compared the ITF annual cycle with observations. I suggest that\nthe authors separate the annual and interannual signals in the ITF and then compare\nthem with the observed ones. What you would find is that for the annual cycle, it is very\nsimilar to Santoso et al. (2022), but for the interannual part, the correlations would be\ninsignificant. I note that this is not a critical part of the text. However, it is a fundamental\nconcept of the CMIP models, so it needs to be correctly stated.\nResponse: Thank you for your valuable feedback. We better understand now your\nprevious comments about the errors with respect to the HIST RUNs. Indeed, as you\npointed out, in the multi-model ensemble mean ITF time series, its variations primarily\nreflect the response to external forcing, while signals related to internal variability (such\nas ENSO-related interannual variability) are essentially negligible.\nAs you have stated, for this reason, previous studies (e.g., Santoso et al., 2022) primarily\nfocused on evaluating CMIP models' performance in simulating the climatology\n(including seasonal cycles) rather than validating interannual signals—especially the\npositive/negative phases of interannual variability. This was the case, even though the\nfocus of their manuscript was largely on ENSO and IOD variability in the CMIP models.\nIn the revised manuscript, we have now removed the interannual signal comparison in\nFig. S1 and replaced it with climatological and seasonal cycle analyses (reproduced\n1 / 17\nbelow as Fig. RR1). Additionally, all related discussions on interannual variability\ncomparisons have been deleted. We sincerely appreciate your feedback!\nFig. RR1. Updated Fig. S1 with a new panel b to present the comparison of the seasonal\ncycle of the ITF in CMIP6 and observations. Left panel (a): Climatological mean zonal\nvelocity in the Hist runs at the 114°E section (contour lines, cm s-1, positive for eastward)\nand its change (mean in 1pctCO2 runs minus mean in historical runs; shading color).\n(b) The seasonal cycle of ITF transports (Sv) from the historical runs of CMIP6 models\n(multi-model ensemble mean; blue line) and geostrophic calculations with\nobservational datasets from Lu et al. (2024) (red line). Black dots in the left panel\nindicate positions where change is greater than standard deviation.\nLines 80-94: “We mainly use simulations from the historical runs (Hist, 1850-2014)\nand idealized 1% annually increasing CO2 concentrations experiments (1pctCO2,\n1850-1999) from 25 CMIP6 coupled climate models to ITF response to increasing CO2\nconcentration. First, we validated the ITF strength in the CMIP6 models and found that\nthe multi-model mean reasonably reproduces the observed ITF30 and captures its mean\nstructure and seasonal cycle. The vertical structure of the climatological mean zonal\ncurrents and their change in 1pctCO2 relative to the Hist runs across the 114°E section\nis shown in Figure S1a. The mean ITF in the CMIP6 models shows a core between\n11°S and 13°S with maximum velocity greater than 20 cm/s. The ITF transport\naveraged over the 150 years in the Hist runs of CMIP6 is about -13 Sv (hereafter\nnegative for westward into the Indian Ocean), quite close to the -15 Sv determined from\ndirect observations42. Figure S1b presents a comparison of the seasonal cycle of ITF\ntransports from the historical runs of CMIP6 models and the geostrophic calculations\n2 / 17\nusing observations-based datesets30. The observed seasonal cycle of ITF transports is\nwell reproduced in the CMIP6 Hist runs. The correlation coefficient is 0.82 (significant\nat a 99% confidence level) between the ensemble mean of CMIP6 models and the\nobserved geostrophic transport.”\nLines 20-25 in the Supplementary Information: “Supplementary Fig. S1. (a)\nClimatological mean zonal velocity in the Hist runs at the 114°E section (contour lines,\ncm s-1, positive for eastward) and its change (mean in 1pctCO2 runs minus mean in\nhistorical runs; shading color). (b) The seasonal cycle of ITF transports (Sv) from the\nhistorical runs of CMIP6 models (multi-model ensemble mean; blue line) and\ngeostrophic calculations with observational datasets from Lu et al. (2024) (red line).\nBlack dots in the left panel indicate positions where change is greater than standard\ndeviation.”\n2. First \"turning point\": The turning point would be of interest to most readers. There\nis a dilemma. The climate models project a weakening ITF; however, in observations,\nwe haven't captured a weakening trend so far. The ITF transport has strengthened over\nthe recent decades. We somehow know that this ITF trend is due to climate variations,\nsuch as the phase of the PDO. It is a fair question to ask when the weakening trend of\nthe ITF will emerge from its natural variability. As this study is mostly statistical, it\nwould be useful to compare the greenhouse effect-driven ITF trend with that of the\nnatural variability. I don't know how the authors derive trends in Fig. 2b. For natural\nvariability, they probably can calculate 30-yr trends in the ITF transport in the CMIP\nhistoric runs. Anyway, I would be happy to see this work being further improved.\nResponse: Thank you. Indeed, it is an essential question when the weakening trend of\nthe ITF will emerge from its natural variability. The trends in Fig.2b are 𝜕𝐼𝑇𝐹 ⁄\n𝜕𝐶𝑂2\n𝜕𝐼𝑇𝐹\n𝜕𝑡\nrather than ⁄ , in order to examine the response to ITF to increasing CO2 and\nalso considering that the CO2 in the experiments is not increasing linearly with time.\nFollowing your suggestion, we have calculated the running trends of the ITF transport\nin a 30-yr window in both the CMIP historic runs and 1pctCO2 experiments, in order\nto compare the greenhouse effect-driven ITF trend with that of the natural variability.\nAs shown in the Fig.RR2, the running trend fluctuates within the range of roughly -2 to\nexperiments, the 30-yr running trend reaches 4 Sv/century rapidly, and the fluctuations\nof the 30-yr running trend are masked by the significant decreasing trend at the model\nmonth of about 370, when the CO2 reaches the first turning point. The trends of ITF in\n3 / 17\nboth the time domain (Fig. RR2) and the CO2 domain (Fig.2b) agree that the first\nturning point is the time of emerging of weakening trend from its natural variability.\nIn the revised manuscript, we have added a new figure, Supplementary Fig. S5, to\npresent the running trend of ITF in a 30-yr window in the 1pctCO2 runs and historical\nruns. The comparison between natural variability and the long-term trend and the\nemerge time of the weakening trend of the ITF is discussed in more detail in the main\ntext and Supplementary Figures, as indicated below.\nFig. RR2. Running trend of the ITF in 30-yr windows in the 1pctCO2 runs (blue curves)\nand historical runs (red curves). The red dot denotes the first turning point as detected\nin the change in ITF transport over change in CO , as shown in Figure 2b. (This is the\n2\nnewly added Supplementary Fig. S5).\nLines 175-182: “To further examine the emerging time of the long-term trend of the\nITF from its natural variability, we have calculated the running trends of the ITF\ntransport over 30-year windows in both the CMIP historic runs and 1pctCO2\nexperiments. As shown in Figure S5, the running trend in the Hist runs fluctuates within\na range of roughly -2 to 2 Sv/century, which is a result of natural variability. By contrast,\nin the 1pctCO2 experiments the 30-year running trend reaches 4 Sv/century rapidly,\nand the fluctuations are masked by the significant decreasing trend when the CO2\nreaches the first turning point, indicating that the first turning point is indeed the\nemerging time of a weakening ITF trend that is distinct from its natural variability.”.\n4 / 17\nPoint-by-point response to Reviewer#2\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #2 (Remarks to the Author):\nThe authors have responded to all the queries satisfactorily. The manuscript can now\nbe considered for publication.\nResponse: Thank you for your feedback.\n5 / 17\nPoint-by-point response to Reviewer#3\nWe thank the Reviewer#3 for his/her helpful suggestions and insightful comments. In\nthe\nfollowing, black indicates the comments by the reviewer; blue is our response to\nthe\nreviewer; grey is our last response to the reviewer’s first round of review; and\nchanges in the manuscript are highlighted by red. To facilitate reading, I have added\nnumbering to your review comments.\nReviewer #3 (Remarks to the Author):\nThanks to the authors for their responses to my earlier review. As indicated in my earlier\nreview, I think the work in this paper is interesting and discusses an under-researched\ncomponent of the climate system’s response to increasing greenhouse gases. My main\nconcern, as in my earlier review, is with the framing of the analysis in terms of CO2\nconcentration, and the conclusions that the authors draw from that analysis, which I\nbelieve are misleading. Unfortunately the authors’ responses and text revisions haven’t\nresolved these concerns, so my overall assessment is as before, that I don’t feel able to\nrecommend the paper for publication in its present form. I do think the investigation of\nthe mechanisms of change is valuable, and I hope the authors will be able to reframe\ntheir work in response to the points in my earlier review (and the further comments\nbelow). I believe this will be in their interests.\nAs it stands I think the issues raised around interpretation and framing will be\nimmediately obvious to many climate scientists reading the paper, leading them to\ndismiss the whole paper and so miss the very worthwhile analysis of mechanisms. I’m\nsorry I can’t be more positive about the paper as it stands, but I emphasise that I do see\nsome very valuable analysis here, and I hope these comments are constructive and\nultimately helpful to the authors.\nResponse: Thank you for your valuable critical comments and encouragement. We\nhave carefully reviewed and discussed your feedback. New calculations have been\nconducted (every additional calculation you proposed has been performed), and\nrevisions have been made to the descriptions of certain conclusions. While we highly\nrespect all of your comments, for a few points, after repeated analysis of extensive data,\nwe have decided to maintain our original views. Below, we provide a point-by-point\nresponse to each of your comments.\nComment 1:\n“Response: First of all, please note that the ITF’s response to CO2 increases is not\nsimply “a levelling off of the ITF weakening at higher concentrations”, but rather, a\nnonlinear process with a rapid weakening between two turning points and slow changes\n6 / 17\nat concentrations lower than the first turning point or higher than the second turning\npoint. Fig. R3. Response of global mean surface temperature to CO2 (From Nicholls et\nal., 2020). Second, such a nonlinear response is not really expected and previous\nstudies suggest that same levelling off would be NOT necessarily be seen in other\nclimate variable such as global mean temperature. For example, Nicholls et al. (2020)\nreported the response of global mean surface temperature to CO2 forcing using\n1pctCO2 simulations. As we can see in Fig.R3, the global mean temperature shows a\nlinear increasing as the CO2 increases and no turning points can be found in the\nresponse of global mean temperature to CO2 increasing. So, the response of the ITF is\nvery different from that of other 17 / 21 climate variable such as global mean\ntemperature. It is really surprising to see the ITF shows such interesting and unique\nresponse to CO2 increasing. Nonetheless, we agree with you that “there is no direct\nmechanistic link between CO2 concentration and the ITF”. This does not mean that the\nCO2 concentration is not essential to the ITF, but is simply because the Earth’s climate\nsystem is super complex. As the primary external forcing driving long-term changes in\nthe Earth system, rising greenhouse gas emissions affect all key processes and physical\nparameters within the system - and the ITF is no exception. CO2 is one of the most\nrepresentative greenhouse gases and an effective indicator of external forcing.\nStudying the ITF's response to the projected CO₂ increase helps predict the ITF future\nbehavior and the impact that those changes might have on both the Indian and Pacific\nOceans. Nicholls, Z. R. J., et al. (2020), Reduced Complexity Model Intercomparison\nProject Phase 1: introduction and evaluation of global-mean temperature response,\nGeosci. Model Dev., 13(11), 5175-5190, doi:10.5194/gmd-13-5175-2020.”\nIn the figure R3 provided (from Nicholls et al 2020), I see exactly the same qualitative\nresponse as a function of CO2 concentration (or cumulative emissions which is used as\nthe x-axis in that figure). There is an initial slow response (quite hard to see but clearly\npresent in the MCE curves over the first 100 years), followed by a faster growth that\ngradually levels off over time (when plotted against cumulative emissions). This is just\nthe same as Fig. 2a of the manuscript. When plotted against time (approximately the\nsame as plotting against radiative forcing, for the 1pctCO2 runs) the trend looks initially\nslow, and thereafter linear, for both the ITF (Fig. 1a of this manuscript) and for GMST\n(Fig. 2b of Nicholls et al., or also see Geoffroy et al).\nThe timescales of this behaviour would not necessarily be the same for ITF and GMST,\nand the different behaviours of the thermal and haline components for the ITF (Figs. 1b\nand 1c of this paper) contain valuable information about these timescales and the\nmechanisms of the response. But the null hypothesis has to be that the response is\nbroadly linear to radiative forcing (which is the physical driver of the changes). A\nsimple way to demonstrate that some different inherent timescales are operating for the\nITF compared to GMST, would be to plot ITF against GMST. It wouldn’t surprise me\nif that was the case, but it needs to be demonstrated. I agree that studying the idealised\nCO2-only scenarios is a valuable contribution to understanding the response to more\ncomplex forcings. I have no problem with the choice of runs to analyse, just the\ninterpretation of the results.\nResponse: Thank you for your additional comments which are helpful for us to better\nunderstand this issue. Following your suggestion, we have plotted ITF against GMST\n(Figure RR3a, added to the revised manuscript as Supplementary Figure S4). Just as\nyou pointed out, the ITF shows a similar qualitative response as a function of GMST to\nthat with CO2, with an initial slow response followed by a faster growth (Figure RR3a).\nWe have also plotted the GMST as a function of CO2 mass to examine the relationship\nbetween the two (Figure RR3b). Triangles in panel (a) roughly denote the turning points\nin GMST, which is determined by the GMST-CO2 relationship shown in Figure RR3b.\nAs we can see, the GMST increases linearly with the increasing of CO2 mass and no\nturning points can be found in the GMST-CO2 (Figure RR3b). The GMST-CO2\nrelationship is not at all surprising because CO2 concentration—through the\ngreenhouse effect—is a physical factor that directly influences temperature changes.\nTherefore, the consistency of the ITF-CO2 and ITF-GMST relationships is also not\nsurprising, considering the linear relationship between GMST and CO2 mass. The\nnonlinear relationship between ITF and GMST reflects the same ITF response to long-\nterm external forcing, and to some extent serves as additional evidence supporting the\nnonlinear response of ITF.\nFigure RR3. Relationship of ITF-GMST and GMST-CO2. Panel (a) presents the ITF\ntransport averaged over bins of GMST with a bin width of 0.2°C. Panel (b) presents the\nGMST averaged over bins of CO2 mass with a bin width of 0.5 (1015 Kg). The ITF,\nGMST and CO2 mass are ensemble means from the 1pctCO2 experiments of CMIP6.\n8 / 17\nThe triangles represent the rough position of the two turning points and the circled areas\nrepresent the phases outside the two points. (This is the newly added Supplementary\nFig. S4).\nWe agree with you that the ITF’s response is broadly linear to radiative forcing,\nparticularly between the two points (Figure RR3a). However, as you mentioned, there\nare indeed turning points, before and after which the ITF response is different (Figure\nRR3a).\nIn the revised manuscript, we have added a new paragraph to discuss the relationships\nof ITF-GMST and GMST-CO2.\nLines 169-174: “In addition, the ITF transport versus global mean surface temperature\n(GMST), and GMST as a function of CO2 mass, is shown in Figure S4. We find that\nthe GMST increases linearly with the increasing of CO2 mass with no turning points\n(Figure S4b), but the GMST shows a similar qualitative response as a function of CO2\nconcentration, with an initial slow response followed by a faster growth and two turning\npoints (Figure S4a). The consistency of ITF-CO2 and ITF-GMST is not surprising,\nconsidering the linear relationship between GMST and CO2 mass.”\nComment 2:\n“Response: Thank you for raising this crucial point regarding the existence of both fast\nand slow response processes in the ocean system - this idea provides extremely valuable\ninsights for understanding the ITF's response to CO2 increasing! We agree with you\nthat the nonlinear response of the ITF to CO2 forcing might be related to the inherent\ntimescale of the model dynamics. However, the relationship between the ITF and CO2\nlevel, i.e., ITF=ITF(CO2), is actually different from that between ITF and time, i.e.,\nITF=ITF(t), when the CO2 increases independently with time. In the time-domain\nrelationship ITF=ITF(t), the ITF’s response, i.e., 𝜕𝐼𝑇𝐹 𝜕𝑡 , might 18 / 21 be similar to\nthat discussed by Geoffroy et al. (2013), when the changing rate of ITF is determined\nby the fast and slow response processes, and hence the initial slow response is\nattributed to the slow response process. By contrast, in the CO2-domain relationship\nITF=ITF(CO2), the changing rate of ITF, i.e., 𝜕𝐼𝑇𝐹 𝜕𝐶𝑂2 , is not related directly to\ntime, and hence is not necessary directly associated with the fast and slow response\nprocesses. The CO2-domain relation ITF=ITF(CO2) simply reflects the sensitivity of\nITF to CO2 changes.”\nIt is simply not possible to make such a deduction from analysing a group of runs, all\nof which have the same CO2 as a function of time.\nResponse: We're sorry our earlier reply didn't address this concern to your satisfaction.\nWith respect to this comment in the first review, you suggested \"I think if the claim that\nthere is a specific threshold CO2 concentration that results in acceleration is to be made,\nit would at least have to be shown across several different CO2 forcing scenarios. \". So,\nwe have looked at the TPs from different CO2 forcing scenarios. As you can see in the\nfollowing from our response to your comments as well as in our response to Reviewer\n1 (see new Figure S5), we have examined this issue using multiple experiments—\nincluding HIST, SSP585, abrupt 4×CO2, and 1pctCO2 (each contains dozens of\nmembers) in both the domain of CO2 (ITF as a function of CO2) and the time domain\n(ITF as a function of time). It seems that, across multiple experiments, the ITF\nconsistently demonstrates a nonlinear response to CO₂, the existence of two turning\npoints, and a significant weakening trend between them.\" Please see our response to\nyour other comments below for further support.\nComment 3:\nWe suppose that at any specific CO2 level, the adjustment of the ITF might have been\ncompleted, i.e., the fast and slow response processes has been completed.\nThere’s no evidence presented to support this, and as far as I can see no a priori reason\nto believe it. The initial adjustment timescale would be the slower of the inherent\nadjustment timescale of the ITF, and the response timescale of the climate variables\nthat force the ITF, to the forcing. The latter is hard to estimate for the 1pctCO2 scenario.\nIt would help to sort out the different timescales, to look at the instantaneous 4xCO2\nruns.\nResponse: Thank you for your comment. We agree with you that it is hard to estimate\nthe response timescale of the climate variables that force the ITF to the forcing for the\n1pctCO2 scenario. So, following your suggestion, we have looked at the ITF change in\nthe abrupt 4xCO2 runs combining the Hist runs, to examine the response timescales\n(Fig. RR4).\nAs we can see in Fig. RR4, the ITF shows a rapid decrease after the CO2 abruptly shifts\nto 8.88x1015 kg, corresponding to a significant adjustment phase. This adjustment lasted\nfor more than 40 years and then the ITF reaches a dynamically stable state, indicating\nthat the time scale of the ITF in response to abrupt CO2 change is roughly 40 years. In\nthe Hist runs, the CO2 increases from 2.22 x1015 kg to 3.11 x1015 kg, but that is by and\nlarge below the first turning point; while in the 4xCO2 runs, the CO2 is a constant and\ngreater than the second turning point of ~8x1015 kg. The non-significant trends in the\nHist runs and the significant decreasing trend in the 4xCO2 run agree with what we find\nin the 1pctCO2 runs.\n10 / 17\nFigure RR4. ITF in the Hist Runs (red lines, bold line for 13-month running mean)\nand Abrupt4xCO2 experiments (blue lines, bold line for 13-month running mean).\nNote that the CO2 in the 4xCO2 experiments is constant. (This is the newly added\nSupplementary Fig. S7).\nIn the revised manuscript, we discuss this issue on the basis of the above new\ncalculations.\nLines 326-329: “The time scales of the ITF’s response to abrupt increasing of CO2 is\nexamined using the abrupt 4xCO2 runs combining the Hist runs. We found that the\nadjustment of the ITF lasted for about 40 years and then the ITF reaches a dynamic\nstable state (Figure S7).”\nComment 4:\n“Actually, in the 1pctCO2 experiment, the CO2 increases 1% per year, indicating that\nthe time from the first value for the CO2 level to reach the first turning point is about\n40 years, much longer than the known adjustment time scales of ITF (e.g., wind, Rossby\nwaves, precipitation, deep ocean currents) according to previous studies (e.g., Sprintall\net al., 2019). In the revised manuscript, we have added new sentences to discuss the\npotential role of the fast and slow response processes, previous studies including\nGeoffroy et al. (2013) are cited, and in particular, the role of the inherent timescale of\nthe model dynamics in the initial slow response is discussed. Lines 309-315: “In\naddition, the response of global mean temperature to CO2 perturbations shows a fast\ntimescale of a decade or two followed by a slow timescale (Geoffroy et al., 2013). Even\nthough the response of the ITF to CO2 is expected to be different from that of global\n11 / 17\nmean temperature, the fast and slow response processes might be important in\ndetermining the turning points, especially the initial slow response before the first\nturning point. Further in-depth research is still needed to understand the physical\nprocesses and mechanisms underlying these two turning points.”\nI’m not saying this new text is wrong (indeed I suspect it is right to some extent), but\nthere is no evidence presented to support it. Again, plotting ITF against GMST would\nshed some light on this, as would plotting the ITF response in the instantaneous 4xCO2\nruns.\nResponse: Thank you for your comment. The new text in the first revision was added\nfor discussion about what we should do in the future based on the Geoffroy et al. (2013)\nreference you mentioned. Nonetheless, in response to the reviewer’s remarks above,\nthe ITF against GMST is now shown in Figure RR3a and the ITF response to the\ninstantaneous 4xCO2 runs is presented in Figure RR4. As we can see in Fig.RR4, this\nadjustment of the ITF to the abrupt 4xCO2 lasted for more than 40 years and then the\nITF reaches a dynamically stable state, indicating that the time scale of the ITF in\nresponse to an abrupt CO2 change is roughly 40 years. By contrast, the ITF in the Hist\nrun when CO2 increase gradually shows no significant trend. Figure RR4 seems to\nindicate that the ITF’s response to the CO2 increase is nonlinear and very different from\nthat of GMST, and also different from what Geoffroy et al. (2013) concluded.\nIn the revised manuscript, we have rephrased these sentences.\nLines 322-330: “Previous studies suggest that the response of global mean surface air\ntemperature to CO2 perturbations shows a fast timescale of a decade or two followed\nby a slow timescale47. Even though the response of the ITF to CO2 is expected to be\ndifferent from that of global mean temperature, the fast and slow response processes\nmight be important in determining the turning points. The time scales of the ITF’s\nresponse to the abrupt increasing of CO2 is examined using the abrupt 4×CO2 runs\ncombining the Hist runs. We found that the adjustment of the ITF lasted for about 40\nyears and then the ITF reaches a dynamic stable state (Figures S7). Further in-depth\nresearch is still needed to understand the physical processes and mechanisms\nunderlying these two turning points.”\nComment 5:\n“Response: Following your suggestion, we examined the changes of the ITF in different\nprojection runs. Fig.R4 presents a comparison of projected changes of ITF velocity\nbetween ssp585 and 1pctCO2 runs. As you can see, the ITF changes in the ssp585 is\nquite similar to that in the 1pctCO2 runs, and the weakening of ITF in the ssp585 is\nslightly stronger than that in the 1pctCO2 runs. Most importantly, the ssp585\n12 / 17\nexperiment shows a turning point at about 1026 ppm as well, after which the decline of\nITF is much weaker than 19 / 21 between 400-1026 ppm.”\nFig. R4. Left panels: Comparison of changes of the ITF velocity in ssp585 (a) and\n1pctCO2 (b) runs. Right panel: response of ITF to CO2 concentration in the ssp585\nexperiments from 16 models (black dots for each model and black line for ensemble\nmean of all models). The solid red dot denotes the second turning point, while the\ndashed red line and red cycle denote the first turning point.\nPlease note that the starting CO2 concentration is about 400 ppm, which exceeds the\nfirst turning point already, so we are not able to examine the first turning point in the\nssp585 experiments.\nSorry, I simply don’t see this in the figure (Fig.R4) provided. It just looks like a minor\nblip at just above 1000ppm, similar to others that appear in the curve.\nIn that case, and there is a special point below 400ppm, why is there no evidence of any\nacceleration of the ITF decline in the HIST runs (Fig. 1c-e)? If you stitch the HIST runs\nto their corresponding SSP585 members do they show an imminent rapid ITF decline?\nResponse: In order to present the ITF response to CO2 in SSP585 more clearly, we\nhave plotted the ITF in the Hist runs and SSP585 runs in two forms: as a function of\ntime and as a function of CO2 concentration. The change rate of ITF (∂ITF/∂CO2,\nunits in Sv per 100 ppm) in the Hist runs (CO2<400 ppm) and the SSP585 (CO2>400\nppm) run was also calculated (Figure RR5). As you can see, the ITF decline shows no\nacceleration in the HIST runs, which is reasonable because the CO2 in the HIST runs\nis less than the first turning point. We have stitched the HIST runs to their\ncorresponding SSP585 members, and we can see that the ITF in the SSP585 after the\nfirst turning point shows an imminent rapid ITF decline (Fig. RR4).\nWe may more intuitively observe the response of the ITF to varying CO2 levels through\nthe rate of change in ITF transport. The rate of change in ITF transport to CO2\n(∂ITF/∂CO2) in the SSP585 run indicates that the ITF decline between the two turning\n13 / 17\npoints is much more rapid than before the first turning point or after the second turning\npoint (Fig. RR4c). The ITF response to CO2 in the SSP585 run agrees with the\nconclusion from the 1pctCO2 runs.\nFigure RR5. ITF transport (Sv) in the Hist runs and SSP585 runs. Panel a presents the\nITF transport anomaly in the Hist runs (red) and SSP585 runs (blue). Panel b presents\nthe ITF transport as a function of CO2 concentration (ppm) in the Hist runs and SSP585\nruns. Panel c shows the rate of change of ITF transport (∂ITF/∂CO2, units in Sv per\n100 ppm) in the Hist runs (CO2<400 ppm) and SSP585 (CO2>400 ppm). The red\ndashed lines in panel (b) and the two red circles in panel (c) denote the two turning\npoints, between which the ITF change rate is significantly positive (i.e., the ITF is\ndecreasing). A new figure Supplementary Fig. S8 has been added to display the above\nresults in the revised manuscript.\n14 / 17\nLines 346-353: “What’s more, we have examined the ITF response to CO2 in the\nShared Socioeconomic Pathway (SSP) 585 as well through stitching the Hist runs to\ntheir corresponding SSP585 members. The ITF in the SSP585 scenarios shows an\nimminent rapid decline after the first turning point, the rate of change in ITF transport\nas a function of CO2 concentration between the two turning points is much more rapid\nthan before the first turning point or after the second turning point, indicating that the\nITF response in the SSP585 is the same as that in the 1pctCO2 experiments (Figure S8).\nHowever, regardless, it must be noted that these results are numerical simulations under\nideal conditions and carry significant uncertainties.”.\nComment 6:\n“Nonetheless, the ITF response to the CO2 in the ssp585 experiments is similar to that\nin the 1pctCO2 experiments. Fig. R4. Left panels: Comparison of changes of the ITF\nvelocity in ssp585 (a) and 1pctCO2 (b) runs. Right panel: response of ITF to CO2\nconcentration in the ssp585 experiments from 16 models (black dots for each model\nand black line for ensemble mean of all models). The solid red dot denotes the second\nturning point, while the dashed red line and red cycle denote the first turning point. As\nto the decline of ITF, some previous studies have shown that the CMIP models project\na significant ITF decline under the business-as-usual emission scenario RCP8.5\n(see Fig. R5 from Hu et al., 2015, Nature). As shown in Fig. R5, the ITF averaged over\nthe 2050–2100 period under RCP8.5 is about 3 Sv weaker than the historical mean,\nindicating a future decline of ITF under RCP8.5. Under representative concentration\npathways (RCP) 4.5 scenario, a robust ITF transport slowdown is found as well at\ndepths below 100 to 1200 m (Santoso et al., 2022).” Hence, the decline of the ITF\npresented in our study is in agreement with the projection runs and previous studies\nthat show a weakening trend of the ITF in the context of global warming.\nI think this decline is uncontroversial.\nResponse: Thank you for your reply. We agree.\nComment 7 (summary):\n“However the turning points and the rapid decline after the first turning point have not\nbeen reported previously. As to other radiatively active forcings, we agree that the CO2\nconcentration is definitely not the only external forcing. But as you mentioned, the CO2\nconcentration was the direct control variable in the 1pctCO2 experiments, and this is\nthe reason why 1pctCO2 experiments are suitable for studying the response of ITF to\ngreenhouse gas emissions. Our paper aims to investigate the response of ITF to the\n15 / 17\nCO2 concentration: the response to “all the other radiatively active forcing species”,\nwhich is obviously more complex, is beyond the scope of this study.”\nAs indicated above I agree that there is value in considering the idealised CO2 forcing.\nBut I can’t think of any reason to assume that the ITF will respond to CO2 in any way\nother than through radiative forcing. So for the real world, with multiple forcings, it is\nnot possible to deduce that the ITF may be about to decline rapidly, based purely on\nthese model results and the observed CO2 level (and an implicit assumption that the\nITF is always in equilibrium with the forcing – see discussion of timescales above).\nThis unjustified deduction is presented as a key result of the paper in both the\ndiscussion section (lines 265-268 of the revised manuscript)) and the abstract (lines\n25-27) and is likely to detract from the interesting mechanistic analysis that the authors\npresent. I strongly recommend that the authors carefully consider what conclusions can\nvalidly be inferred from their analysis, and remove other claims, or add some analysis\nto support them. It’s an interesting study and doesn’t need to be spiced up by\nunsupported and potentially misleading claims.\nResponse: Thank you for your positive feedback and valuable comments. As the\nprimary greenhouse gas emitted by human activities, changes in CO₂ concentration\nprimarily affect the Earth's climate system through the radiative balance and energy\nbudget, as the reviewer points out. The ITF, as one component of the climate system,\ntherefore responds to CO₂ indirectly—through adjustments such as the modified\nregional differences in warming and also through salinity changes. Therefore, when we\nstate that CO₂ is the fundamental driver of long-term ITF changes, we do not exclude\nthe possibility that direct causes may involve other dynamic or thermodynamic factors.\nHowever, at least in the CMIP6 model simulations, increasing CO₂ remains the root\ncause of the ITF's long-term evolution (please see e.g. revised Lines 343-347).\nBased on the above analysis, we find that across multiple experiments—including HIST,\nSSP585, abrupt 4×CO2, and 1pctCO2 (each contains dozens of members)—the ITF\nconsistently demonstrates a nonlinear response to CO₂, the existence of two turning\npoints, and a significant weakening trend between them. While climate models\ninevitably have their limitations, the ones used here represent the current state-of-the-\nart in simulating the Earth's climate system. We strive to understand future climate\nchanges and pursue perfectly accurate results, but this is the most advanced level we\ncan achieve at present.\nIf we accept the numerical simulation results—particularly the existence of the two\ntipping points—while observational data simultaneously indicate that CO₂\nconcentrations have already reached or are approaching the first turning point, then we\ninevitably arrive at the conclusion that the ITF will soon undergo rapid decline. As we\nreplied to the Reviewer#1 above, we have added a new figure, Supplementary Fig. S5,\n16 / 17\nto present the running trend of ITF in a 30-yr window in the 1pctCO2 runs and historical\nruns to investigate the emerge time of the weakening trend of the ITF and also support\nthat the first turning point is the time of emerging of weakening trend from its natural\nvariability.\nIn response to your valuable suggestions, we have thoroughly revised the manuscript\nto explicitly address these uncertainties. We have made clear to readers that: (1) we are\npresenting a bold and potentially contentious inference; (2) our conclusions carry\ninherent limitations; and (3) this situation mirrors the intellectual risks confronted by\nmany pioneering studies in their early stages of publication—where theoretical\nadvances often precede definitive observational confirmation.\nWe have made the following changes in our manuscript to better explain our\nconclusions.\nLines 25-27: “The current level of CO2 concentration has reached the turning point,\nindicating that a rapid decline of ITF and a transition Indo-Pacific state is likely\nimminent despite the remaining uncertainties.”\nLines 175-182: “To further examine the emerging time of the long-term trend of the\nITF from its natural variability, we have calculated the running trends of the ITF\ntransport over 30-year windows in both the CMIP historic runs and 1pctCO2\nexperiments. As shown in Figure S5, the running trend in the Hist runs fluctuates within\na range of roughly -2 to 2 Sv/century, which is a result of natural variability. By contrast,\nin the 1pctCO2 experiments the 30-year running trend reaches 4 Sv/century rapidly,\nand the fluctuations are masked by the significant decreasing trend when the CO2\nreaches the first turning point, indicating that the first turning point is indeed the\nemerging time of a weakening ITF trend that is distinct from its natural variability.”.\nLines 342-353: “Therefore, when we state that CO₂ is the fundamental driver of long-\nterm ITF changes, we do not exclude the possibility that direct causes may involve other\ndynamic or thermodynamic factors. However, at least in the CMIP6 model simulations,\nincreasing CO₂ remains the root cause of the ITF's long-term evolution.\nWhat’s more, we examined the ITF response to CO2 in the Shared Socioeconomic\nPathway (SSP) 585 as well through stitching the Hist runs to their corresponding\nSSP585 members. The ITF in the SSP585 scenarios shows an imminent rapid decline\nafter the first turning point, the rate of change in ITF transport as a function of CO2\nconcentration between the two turning points is much more rapid than before the first\nturning point or after the second turning point, indicating that the ITF response in the\nSSP585 is the same as that in the 1pctCO2 experiments (Figure S8). However,\nregardless, it must be noted that these results are numerical simulations under ideal\nconditions and carry significant uncertainties.”\n17 / 17"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed my previous comments, and the manuscript can be considered for publication in the journal.", "reviewer_3": "Review of “Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration”, by Shijian Hu et al.\nSubmitted to Nature Communications\nThis is my third review of this manuscript. I thank the authors for their careful responses to my last set of comments, including performing substantial additional analyses. I think these analyses, shown in new supplementary figures, shed further light on what is going on. Unfortunately I still find the framing of the results in the main text of the manuscript unconvincing, so I can’t recommend publication it its current form. I do think there is a way forward to produce a highly impactful paper, but it would be more of a rewrite than a revision.\nFirst I have some comments on the new supplementary figures:\nFig. S4(a) (plot of ITF against GMST), This apparently shows an initial slow response of the ITF to increasing GMST. If real, to me this is more unexpected and more striking than the low-CO2 response in the plot of ITF against CO2 concentration (Fig. 2a), which as discussed in my earlier reviews might be expected due to the initial slow warming in the 1% runs. Presumably a slow ITF response at lower warming levels is due to lags in the system such as the multi-decadal adjustment time of the ITF to instantaneous radiative forcing (new Fig. S7). However as it stands I don’t find Fig. S4 convincing. It shows no data for GMST <3.5 deg C. I would expect it to go right down to GMST=0, as there is plenty of data at these GMST values available in the 1pctCO2 runs. The figure also needs an indication of the ensemble spread or confidence interval for the ensemble mean values shown. Because there is a large range in the Transient Climate Response (TCR) between the models (see IPCC AR6 WGI Chapter 7), a given GMST value is likely to be reached at very different CO2 concentrations between models, so a fairly large spread is to be expected. These additions are needed to convince the sceptical reader that the apparent slow initial response is real.\nFig. S7 (Response of ITF in the instantaneous 4xCO2 runs). The response is very interesting here, showing that the (ensemble mean) ITF stabilises at ~6Sv after a few decades, even though the GMST is still warming at this point in most/all of the runs. This does provide evidence (more convincing evidence than Fig. 2a) that a levelling off of the ITF at a non-zero value can be expected at some future point. I think this behavior potentially sheds valuable light on the response dynamics of the ITF. A more in-depth process analysis to determine what sets the timescale and the stabilisation level would be very valuable, building on the insights to the wind and buoyancy forcings in the manuscript. To facilitate this it would be important to show how much the stabilisation time and level vary among the ensemble members/models.\nFig. S8 (ITF as a function of CO2 in HIST and SSP585 runs). By eye I can’t see a clear discontinuity in the slope between the HIST and SSP585 segments. Given the large ensemble spread (evident from the individual plots in the figure), and noise even in the ensemble mean, I doubt that there is any statistically or practically significant difference. The final drop in the timeseries (and in the rate of change in panel b) looks like an edge effect (noting that similar drops are seen at earlier points in the figure, e.g. around CO2 = 650 and 850 ppm). To me, this figure does not as it stands support the assertion in the title of “imminent rapid decline of the ITF”.\nThe new figures have started to unpack some potentially very interesting behavior, and have the potential to really move forward understanding of the future of the ITF. The evidence presented suggests that the response of the ITF to increasing CO2 is dependent on the history of CO2 forcing as well as the current concentration, due to the significant delay timescales of both surface temperature and the ITF itself. This means that conclusions drawn purely from analysis of the 1% CO2 runs are strongly contingent on that specific forcing scenario. It might be more convincing if the SSP585 and 1% timeseries could be plotted together, using consistent processing methods (e.g. it looks as if Figs 2a and S8 use different smoothing), and with a realistic estimate of uncertainty/ensemble spread.\nIn short, I think the figures (including the new supplementary figures) contain a lot of valuable information about the response of the ITF to a warming climate, although some need a little more work to be convincing. The initial response seems to be a combination of more than one dynamical delay timescales, and so is particularly dependent on forcing scenario, while the later levelling off of the ITF response looks like a more robust feature. By combining the information on the response to different CO2 forcing scenarios with the process analysis of the response (a strength of the manuscript) I think there is the potential to really move this topic forward. Unfortunately I still feel that the current framing of the paper in terms of instantaneous CO2 concentration as a primary control on ITF strength, and the claim in the title of imminent rapid weakening, are unconvincing. I’m sorry I can’t recommend publication in its present form. I think the authors have produced most of the analysis needed for a really valuable paper, with a reframing to focus on the processes and timescales that determine the transient ITF response. But it would be a rewrite rather than a revision of the current manuscript."}, "author_response": "Point-by-point response to Reviewer#1\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #1 (Remarks to the Author):\nThe authors have addressed my previous comments, and the manuscript can be\nconsidered for publication in the journal.\nResponse: Thank you for your feedback.\n1 / 11\nPoint-by-point response to Reviewer#3\nWe thank the reviewers for their helpful suggestions and insightful comments. In the\nfollowing, black indicates the comments by the reviewer; blue is our response to the\nreviewer; and changes in the manuscript are highlighted by red.\nReviewer #3 (Remarks to the Author):\nReview of “Imminent rapid decline of the Indonesian Throughflow after reaching a\nturning point of CO2 concentration”, by Shijian Hu et al. Submitted to Nature\nCommunications\nThis is my third review of this manuscript. I thank the authors for their careful responses\nto my last set of comments, including performing substantial additional analyses. I think\nthese analyses, shown in new supplementary figures, shed further light on what is going\non. Unfortunately I still find the framing of the results in the main text of the manuscript\nunconvincing, so I can’t recommend publication it its current form. I do think there is\na way forward to produce a highly impactful paper, but it would be more of a rewrite\nthan a revision.\nResponse: Thank you for your comments and suggestions. We have adopted most of\nthe suggestions you raised this time, such as enhancing the analysis of ensemble spread,\nand have further revised the manuscript accordingly. Specifically, we have made\nsubstantial modifications to Figures S4, S7, and S8. While we highly respect all of your\ncomments, for a few points, after repeated analysis of extensive data, we have decided\nto maintain our original views. Below, we provide a point-by-point response to each of\nyour comments.\nFirst I have some comments on the new supplementary figures:\nFig. S4(a) (plot of ITF against GMST), This apparently shows an initial slow response\nof the ITF to increasing GMST. If real, to me this is more unexpected and more striking\nthan the low-CO2 response in the plot of ITF against CO2 concentration (Fig. 2a),\nwhich as discussed in my earlier reviews might be expected due to the initial slow\nwarming in the 1% runs. Presumably a slow ITF response at lower warming levels is\ndue to lags in the system such as the multi-decadal adjustment time of the ITF to\ninstantaneous radiative forcing (new Fig. S7). However as it stands I don’t find Fig. S4\nconvincing. It shows no data for GMST <3.5 deg C. I would expect it to go right down\nto GMST=0, as there is plenty of data at these GMST values available in the 1pctCO2\nruns. The figure also needs an indication of the ensemble spread or confidence interval\nfor the ensemble mean values shown. Because there is a large range in the Transient\n2 / 11\nClimate Response (TCR) between the models (see IPCC AR6 WGI Chapter 7), a given\nGMST value is likely to be reached at very different CO2 concentrations between\nmodels, so a fairly large spread is to be expected. These additions are needed to\nconvince the sceptical reader that the apparent slow initial response is real.\nResponse: Thank you very much for your comment. In particular, your suggestion\nregarding adding the ensemble spread or confidence interval for the ensemble mean\nvalues has been very helpful to us.\nYes, Fig. S4 (a) shows an initial slow response of the ITF to increasing GMST.\nHowever, as we discussed in the previous round of review, the response of ITF to\nGMST is inherently expected to be the same as its response to CO2 because the GMST\nand CO2 exhibit an almost linear relationship. Precisely for this reason, there is nothing\nunexpected in Fig. 4S—it is a reasonable result.\nHowever, we do not believe that the slow ITF response at lower warming levels is due\nto lags in the system, for the following reason: the initial slow response of the ITF at\nlower warming levels is not only shown in the 1pctCO2 experiment (Fig. S4a), but also\nshown in the Hist runs (Figs. S7 and S8a). Although this phase is confined to a relatively\nshort segment of CO2 levels, it is important to note that the corresponding duration of\nthis brief CO2 increase is quite long—particularly in the Hist runs, where this phase\nlasts for more than 150 years, far exceeding the time required for natural decadal\nadjustments.\nYes, there is no data shown for GMST <3.5 deg C. Please note that this is GMST rather\nthan GMST anomaly relative to a certain value, so we would NOT expect it to go right\ndown to GMST=0. Out of caution and with full respect for your opinion, we have\ncalculated the minimum GMST (by and large equivalent to the initial value) for each\nmodel. As shown in Table R1, the minimum GMST for these models ranges from 12.6\nto 14.1°C, with an average of 13.6°C. (We apologize that in the previous Fig4S, the\ncalculation of the global mean GMST did not incorporate area-weighting to account for\ngrid cell variations. This error has been corrected in the current version, but the\ncorrection does not affect the conclusions.)\nTable R1. Minimum GMST (°C) in the 1pctCO2 runs of various models\nModel name Minimum Model name Minimum Model name Minimum\nGMST GMST GMST\nCESM2 13.6 CanESM5 13.3 MPI-ESM-1-2- 13.5\nHAM\nCESM2-WACCM 13.5 CESM2-WACCM- 14.0 MPI-ESM1-2-HR 13.8\nFV2\nIPSL-CM6A-LR 12.6 SAM0-UNICON 12.7 CMCC-ESM2 13.5\nCMCC-CM2-SR5 13.7 ACCESS-ESM1-5 14.1 ACCESS-CM2 13.7\nCAMS-CSM1-0 13.8 EC-Earth3- 13.8 EC-Earth3-CC 13.9\nAerChem\n3 / 11\nFollowing your suggestion regarding adding the ensemble spread, we have calculated\nthe model spread (the standard deviations among the models), added this information\nto Fig.S4, and revised the Fig.S4. As you can see in the updated Fig.S4, the model\nspread (shading) is indeed large and should be noted. However, the overall trend is\nmuch greater than the model spread, no matter the ITF-GMST relationship or the\nGMST-CO2 relationship.\nIn addition, we have plotted the ITF anomaly (minus their corresponding mean values)\nin the three CO2 scenarios (1pctCO2, SSP585 and Hist) together (Fig.S4c). As we can\nsee in Fig.S4c, these scenarios indicate similar ITF response to CO2 forcing.\nFigure R1 (revised Fig.S4). Relationships of: (a) global mean surface temperature\n(GMST) and CO2 mass, and (b) GMST and ITF. Panel (a) presents the GMST averaged\nover bins of CO2 mass with a bin width of 0.5 (1015 Kg). Panel (b) presents the ITF\ntransport averaged over bins of GMST with a bin width of 0.2°C. Panel (c) plots the\nITF anomaly (minus their corresponding mean values) in the three CO2 scenarios\n(1pctCO2, SSP585 and Hist) together. The ITF, GMST and CO2 mass are ensemble\nmean of the 1pctCO2 and/or SSP585, Hist experiments of CMIP6. The triangles\nrepresent the rough position of the two turning points and the circled areas represent\n4 / 11\nthe phases outside the two points. Seasonality is removed. The shaded color with thin\nblack lines denotes the standard deviations among the models. (This is the newly added\nSupplementary Fig. S4).\nWe have calculated the model spread (the standard deviations among the models),\nrevised the Fig.S4, and added discussion on the model spread in the ITF-GMST and\nGMST-CO2 relationship.\nPlease see updated Fig.S4, Lines 174–176: “The model spread in the simulated ITF-\nGMST relationship is notable, but the overall trend of ITF in response to GMST is\nmuch greater than the model spread (Figure S4b).”;\nand Lines 177-180: “The ITF anomalies (minus their corresponding mean values) in\nthe three CO2 scenarios (1pctCO2, SSP585 and Hist) are plotted together for\ncomparison, and these scenarios indicate similar ITF response to CO2 forcing (Figure\nS4c).”.\nFig. S7 (Response of ITF in the instantaneous 4xCO2 runs). The response is very\ninteresting here, showing that the (ensemble mean) ITF stabilises at ~6Sv after a few\ndecades, even though the GMST is still warming at this point in most/all of the runs.\nThis does provide evidence (more convincing evidence than Fig. 2a) that a levelling off\nof the ITF at a non-zero value can be expected at some future point. I think this behavior\npotentially sheds valuable light on the response dynamics of the ITF. A more in-depth\nprocess analysis to determine what sets the timescale and the stabilisation level would\nbe very valuable, building on the insights to the wind and buoyancy forcings in the\nmanuscript. To facilitate this it would be important to show how much the stabilisation\ntime and level vary among the ensemble members/models.\nResponse: Thank you for your comments and suggestions.\nIndeed, we also found the results of the 4xCO2 runs presented in Fig. S7 to be very\ninteresting. This experiment clearly demonstrates that after an abrupt increase in CO2\nfrom historical levels to 4xCO2, the ITF rapidly weakens. This weakening reaches a\nrelatively stable equilibrium of a weaker ITF after approximately 40 years.\n5 / 11\nFigure R2 (revised Fig.S7). ITF in the Hist Runs (red lines, bold line for 13-month\nrunning mean) and abrupt 4xCO2 experiments (blue lines, bold line for 13-month\nrunning mean). Note that the CO2 in the 4xCO2 experiments is constant. The shaded\ncolor with thin black lines denotes the standard deviations among the models.\nFollowing your suggestion, we further analyzed the differences among various\nnumerical models and updated Fig. S7 accordingly (i.e., Figure R2 here). As shown in\nthe revised Fig. S7, during the rapid weakening of the ITF after the abrupt CO2 increase,\nthe differences among the models are minimal relative to other phases, indicating that\ndifferent models undergo roughly the same adjustment period.\nIn the revised manuscript, we have updated Figure S7, integrated Fig. S7 with other\nresults, such as Fig. 2a, to provide further discussion and analysis on this issue. As for\nwhat exactly determines the timescale of ITF adjustment, more systematic analysis and\nresearch will be needed in the future to reach a definitive conclusion.\nThe relevant revisions can be found in the revised Fig. S7 and Line 332-339: “We found\nthat the adjustment of the ITF lasted for about 40 years and then the ITF reaches a\ndynamic stable state (Figure S7), indicating that the projected rate of ITF weakening in\nthese models is also influenced by multidecadal variability. It should be noted that,\nduring the rapid weakening of the ITF after the abrupt CO2 increase in the 4×CO2 runs,\nthe differences among the models are minimal relative to other phases, indicating that\ndifferent models undergo roughly the same adjustment period (Figure S7). The results\nfrom the 4×CO2 experiment are consistent with and corroborate the findings of the\nsecond turning point observed in the 1pctCO2 experiment (Figures 2 and S7).”.\nFig. S8 (ITF as a function of CO2 in HIST and SSP585 runs). By eye I can’t see a clear\ndiscontinuity in the slope between the HIST and SSP585 segments. Given the large\n6 / 11\nensemble spread (evident from the individual plots in the figure), and noise even in the\nensemble mean, I doubt that there is any statistically or practically significant difference.\nThe final drop in the timeseries (and in the rate of change in panel b) looks like an edge\neffect (noting that similar drops are seen at earlier points in the figure, e.g. around CO2\n= 650 and 850 ppm). To me, this figure does not as it stands support the assertion in the\ntitle of “imminent rapid decline of the ITF”.\nResponse: Thank you for your further comments.\nFigure S8 combines the experimental results from the HIST and SSP585 scenarios. In\nthe previous version of Figure S8, the discontinuity at the two turning points was indeed\nnot particularly pronounced. As you noted in your comments, the lack of clear\ncharacterization at these turning points may be due to the absence of smoothing to filter\nout high-frequency variations, as was done in Fig. 2. Therefore, following your\nsuggestion, we further revised Figure S8 by applying a 3-point smoothing process to\nthe time series and replotted the figure (Fig. R3). As shown in Fig. R3a, after smoothing\nout the high-frequency variations, the ITF shows very significant differences near the\ntwo turning points: the ITF weakens rapidly between the two turning points but remains\nrelatively stable and insensitive to CO₂ changes outside these points.\nRegarding the model ensemble spread, we have previously presented and discussed the\ncalculation results. While the ensemble spread is indeed not small, it is considerably\nsmaller than the rapid weakening of the ITF between the two turning points. For\nexample, the ensemble spread of ITF transport in the SSP585 experiment is\napproximately 3 Sv, whereas the change in ITF between the two turning points exceeds\n4.5 Sv, significantly larger than the ensemble spread. Additionally, it is important to\nnote that the ensemble spread primarily reflects differences in the phases of natural\noscillations due to factors such as initial conditions, rather than differences in the\nresponse to external forcing. Nearly all models consistently show rapid weakening\nbetween the two turning points. Therefore, the observed ITF response is highly\nsignificant.\n7 / 11\nFigure R3 (revised Fig.S8). ITF transports in the Hist runs and SSP585 runs (a). The\nright panel presents the change rate of ITF transport as a function of CO2 concentration\n(∂ITF/∂CO2, unit in Sv per 100 ppm). The red circles denote the two turning points,\nbetween which the ITF change rate is significantly positive (i.e., the ITF is decreasing).\nNote that the time series here has been subjected to a 3-point moving average to focus\non the low-frequency response.\nAs for the final drop in the time series, as mentioned earlier, natural oscillations and\nfluctuations may indeed exist in the evolution of the ITF. Thus, weakening at other\npoints in the time series, e.g., around CO₂ = 650 and 850 ppm, is not unexpected.\nHowever, this does not imply that the final drop is a \"boundary effect.\" The reasons are\nas follows:\n(1) The duration of the final drop is significantly longer than other brief drops,\nespecially after smoothing;\n(2) The stabilization after the second turning point is observed not only in SSP585\nruns but also in the 1pctCO2 experiment, and is even more pronounced in the previously\nshown 4XCO2 experiment;\n(3) The calculated change rate of the ITF indicates that the final drop does not\nonly occur in the last bin but begins to decrease to near zero even before the last bin,\nfurther demonstrating that it is not a boundary effect.\nThe consistency across multiple experimental datasets fully confirms the robustness of\nthe final drop. We highly value your critical feedback. In the revised manuscript, we\nhave followed your suggestions by smoothing the time series, evaluating inter-model\ndifferences, updating Figure S8, and discussing the robustness of the results and related\nfindings.\n8 / 11\nIn the revised manuscript, we have updated Fig. S8 and added corresponding texts.\nPlease see the revised Fig. S8 and Lines 68 – 72 of the Supplementary Information:\n“Supplementary Fig. S8. ITF transports in the Hist runs and SSP585 runs (a). The\nright panel presents the change rate of ITF transport as a function of CO2 concentration\n(∂ITF/∂CO2, unit in Sv per 100 ppm). The red circles denote the two turning points,\nbetween which the ITF change rate is significantly positive (i.e., the ITF is decreasing).\nNote that the time series here has been subjected to a 3-point moving average to focus\non the low-frequency response.”\nThe new figures have started to unpack some potentially very interesting behavior, and\nhave the potential to really move forward understanding of the future of the ITF. The\nevidence presented suggests that the response of the ITF to increasing CO2 is dependent\non the history of CO2 forcing as well as the current concentration, due to the significant\ndelay timescales of both surface temperature and the ITF itself. This means that\nconclusions drawn purely from analysis of the 1% CO2 runs are strongly contingent on\nthat specific forcing scenario. It might be more convincing if the SSP585 and 1%\ntimeseries could be plotted together, using consistent processing methods (e.g. it looks\nas if Figs 2a and S8 use different smoothing), and with a realistic estimate of\nuncertainty/ensemble spread.\nResponse: Thank you for your positive feedback and suggestions.\nIndeed, after revising our work based on your comments, we have obtained more in-\ndepth and interesting findings. Particularly in the 4×CO2 experiment, we observed how\nthe ITF responds to rapid changes in CO2 and reaches a dynamic equilibrium after\nseveral decades of adjustment. This is of great significance for improving our\nunderstanding of the ITF’s response to external forcing. We also agree with your point\nthat using both the 1pctCO2 experiment and other experiments for cross-validation\nhelps enhance the credibility of our conclusions. At the same time, estimating the\nuncertainty and model ensemble spread is crucial for readers to evaluate the results.\nFollowing your suggestion, as we mentioned in our previous response, we have\nintegrated the 1pctCO2 experiment, historical runs, SSP585 experiment, and 4×CO2\nexperiment. We also applied similar smoothing to the SSP585 experiment and found\nthat the ITF response in the SSP585 experiment (Fig. S8) aligns very well with that in\nthe 1pctCO2 experiment (Fig. 2a), both in terms of the time series and the ITF change\nrate. Additionally, we assessed the uncertainty by calculating the model ensemble\nspread, which indicates that the rapid response of the ITF is significantly greater than\nthe uncertainty and is thus statistically significant. For more details, please refer to our\nprevious response.\n9 / 11\nPlease see the revised Fig. S8 and Lines 68 – 72 of the Supplementary Information:\n“Supplementary Fig. S8. ITF transports in the Hist runs and SSP585 runs (a). The\nright panel presents the change rate of ITF transport as a function of CO2 concentration\n(∂ITF/∂CO2, unit in Sv per 100 ppm). The red circles denote the two turning points,\nbetween which the ITF change rate is significantly positive (i.e., the ITF is decreasing).\nNote that the time series here has been subjected to a 3-point moving average to focus\non the low-frequency response.”\nIn short, I think the figures (including the new supplementary figures) contain a lot of\nvaluable information about the response of the ITF to a warming climate, although\nsome need a little more work to be convincing. The initial response seems to be a\ncombination of more than one dynamical delay timescales, and so is particularly\ndependent on forcing scenario, while the later levelling off of the ITF response looks\nlike a more robust feature. By combining the information on the response to different\nCO2 forcing scenarios with the process analysis of the response (a strength of the\nmanuscript) I think there is the potential to really move this topic forward.\nUnfortunately I still feel that the current framing of the paper in terms of instantaneous\nCO2 concentration as a primary control on ITF strength, and the claim in the title of\nimminent rapid weakening, are unconvincing. I’m sorry I can’t recommend publication\nin its present form. I think the authors have produced most of the analysis needed for a\nreally valuable paper, with a reframing to focus on the processes and timescales that\ndetermine the transient ITF response. But it would be a rewrite rather than a revision of\nthe current manuscript.\nResponse: Thank you for your comments.\nRegarding the nonlinear response of the ITF to external CO₂ forcing, we generally agree\nwith your perspective that the ITF exhibits relative stability in both the initial and later\nresponse phases. By integrating different CO₂ forcing scenarios, we can better\ncharacterize the response features and mechanisms of the ITF to CO2. We also\nappreciate your remark that our work has the potential to \"really move this topic\nforward.\"\nHowever, as previously discussed in our earlier responses, the initial response of the\nITF is independent of the specific forcing scenarios. We clearly observed that the\nadjustment period of the ITF to CO₂ changes is approximately 40 years, and the so-\ncalled \"initial response\" phase is prolonged—lasting over 150 years in historical runs,\nhence any dynamical delay timescales involved have already completed their\nadjustment. The response characteristics observed in the \"initial response\" phase in the\nITF–CO2 space correspond to fully adjusted outcomes in the ITF–time space. These\nrepresent the ITF’s response to external forcing and show no dependence on the specific\n10 / 11\nCO2 increase pathways, as further supported by the consistency between the SSP585\nand 1pctCO2 experiments.\nRegarding the issue of treating CO2 concentration as the primary controlling factor of\nITF strength, we have discussed this point in previous rounds of review as well. We\nwould like to emphasize once again, in the numerical experiments we conducted, CO2\nis indeed the dominant external forcing and the principal driver of climate change,\ntherefore, attributing the ITF response to changes in external CO₂ forcing is well\njustified.\nAs for the phrasing \"imminent rapid weakening\" in the title, we have also elaborated\non this in earlier replies. On one hand, the two turning points in the ITF’s response to\nCO2 have been consistently identified through multiple datasets and approaches.\nCurrent atmospheric CO2 levels are supported by observational evidence. Unless the\ntheoretical or model-predicted turning points are invalidated, we cannot ignore the fact\nthat current CO2 concentrations are approaching or are about to exceed the first turning\npoint. Given the pronounced rapid weakening of the ITF beyond this point, it is\nreasonable to conclude that such a decline may be imminent. On the other hand, we\nhave thoroughly addressed the associated uncertainties. In the title, we explicitly state\nthat the \"imminent rapid decline\" is conditional—\"after reaching a turning point of CO2\nconcentration\". Moreover, throughout the article (in both the abstract and main text),\nwe have repeatedly highlighted remaining uncertainties, referring to the phenomenon\nas \"likely imminent despite the remaining uncertainties.\"\nOnce again, we thank you very much for your insightful comments and constructive\nsuggestions.\n11 / 11"}]

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