Robust changes in global subtropical circulation under greenhouse warming

Domain: earth_science
Published: 2024-01-02
Editorial outcome: minor_revision

Abstract
The lower tropospheric subtropical circulation (SC) is characterized by monsoons and subtropical highs, playing an important role in global teleconnections and climate variability. The SC changes in a warmer climate are influenced by complex and region-specific mechanisms, resulting in uneven projections worldwide. Here, we present a method to quantify the overall intensity change in global SC, revealing a robust weakening across CMIP6 models. The weakening is primarily caused by global-mean surface warming, and partly counteracted by the direct CO2 effect. The direct CO2 effect is apparent in the transient response but is eventually dominated by the surface warming effect in a slow response. The distinct response timescales to global-mean warming and direct CO2 radiative forcing can well explain the time-varying SC changes in other CO2 emission scenarios. The declined SC implies a contracted monsoon range and drying at its boundary with arid regions under CO2-induced global warming.

Introduction
The subtropical circulation (SC) in the lower troposphere consists of subtropical highs over the oceans and monsoons1. The proportion of subtropical highs and monsoons varies seasonally, following the seasonal variation in the descending branches of the Hadley cell and the land–sea thermal contrast2–5. The SC connects the trade winds with the midlatitude westerlies and transports the tropical moisture poleward via the western flank of the subtropical high6–9, playing an important role in global energy and moisture transport. Changes in intensity and location of the SC can impact the tropical cyclone tracks10, modulate the distribution and variability of rainfall over East Asia, North and South America, and South Africa11–14, and bring extreme events such as droughts and heatwaves15,16.

Projection of the SC changes under greenhouse warming has been widely studied17–22. When the divergent SC is projected to weaken due to a weakened tropical overturning circulation under global warming23–26, the change in the dominant rotational component of SC remains inconclusive19,20,22,27–29. The projection for the rotational SC represented by 850 hPa stream function (hereafter referred to as SC) is crucially dependent on the location and season (Fig. 1 and Supplementary Fig. S1). The SC changes are distinct during the boreal summer and winter in the Northern Hemisphere, whereas the seasonal variation of the SC changes in the Southern Hemisphere is relatively weak. The typical seasonal SC changes include the slightly weakened Australian summer monsoon (Fig. 1a), the slightly strengthened East Asian summer monsoon, and the robust westward shifted North Atlantic subtropical high in JJA (Fig. 1b). On closer inspection, the SC change projection even depends on the chosen domain and metrics sensitively. The domain-average low-level stream function in the center of the subtropical high increases under global warming20,29. The eddy stream function in an extended domain exhibits an insignificant change in the North Pacific subtropical high22, but several studies selecting variable-dependent polygonal domains project a weakened North Pacific subtropical high19,28. Furthermore, the responses of SC to global warming often exhibit a mixture of varying intensity and shifting SC location, such as the intensified and westward shifting North Atlantic subtropical high in boreal summer (Fig. 1b)19,22. Considering the center and intensity of background SC seasonally vary, we cannot choose a domain that is suitable for all seasons and distinguish the changes in intensity and location. Due to the mixture of these factors, we did not obtain a robust projection on global SC changes.

The uncertainty of SC changes is also associated with the multiplicity of the influencing factors, including the direct CO2 radiative forcing17,30–32, sea surface temperature (SST) warming28,30,33, and increased atmospheric stability34. The direct CO2 effect can induce enhanced land–sea thermal contrast34 with an intensification of the Asian and northern African monsoons in boreal summer31,32,35. The SST warming comprises two components: a uniform component and a patterned component. The uniform component is closely linked to the magnitude of global-mean surface warming, which weakens the tropical overturning circulation23,24,36 and increases low-level water vapor with further intensification of the hydrological cycle24,37,38. By contrast, the patterned SST warming can induce a shift in tropical convection and circulation39–41. These mechanisms can result in specific local circulation changes27, but the delicate offset of different physical processes differs among the climate models participating in Phase 6 and 5 of the Coupled Model Intercomparison Project (CMIP6 and CMIP5)19,22,28,42, leading to uncertain projections of the global SC changes.

Here, we present a method to extract intensity change in global SC independent of the domain and season selection. A robust weakening of global SC is projected under the Shared Socioeconomic Pathway (SSP) 5-8.5 scenario by the end of the 21st century in 34 CMIP6 models and in the abrupt-4×CO2 experiments forced by an abrupt quadrupling of the pre-industrial CO2 level in 32 CMIP6 models. The robustness of the weakening is verified by the single-model initial-condition large ensemble (SMILE)43 and the ‘Database for Policy Decision Making for Future Climate Change’ (d4PDF) simulations44, which consider the uncertainties from internal variability and SST warming pattern, respectively. We clarify that the robust weakening of global SC is dominated by global-mean surface warming and partly counteracted by the direct CO2 effect on different timescales, confirmed by the third Phase of the Cloud Feedback Model Intercomparison Project (CFMIP-3) in CMIP645. This unified mechanism highlights the dependence of projected SC changes on simulation scenarios, thus providing an explanation for prior uncertainties in projections.

Results
Robust weakening in the subtropical circulation

The overall intensity change of SC is defined by the projection of global (10°–45°S, 10°–45°N) SC changes onto the SC climatology (see the “Methods” section for details). Figure 2 shows the intensity change in global SC in several sets of experiments, including the SSP5-8.5, abrupt-4×CO2, CFMIP-3, SMILEs, and the d4PDF (see the “Methods” section for details). The multiple experiments illustrate the impacts of different mechanisms. The intensity changes in SSP5-8.5 (black dots in Fig. 2) and abrupt-4×CO2 (orange dots in Fig. 2) show a weakened SC in most models and both hemispheres (Supplementary Figs. S2 and S3), although the degree shows a large inter-model spread. The inter-model correlation coefficient between the intensity change in global SC and global-mean surface warming among the models is −0.43 (P < 0.02) in SSP5-8.5 and −0.55 (P < 0.002) in abrupt-4×CO2, suggesting that the degree of weakened global SC is related to the degree of global warming. The weakening is more apparent in the abrupt-4×CO2 experiments (orange dots in Fig. 2) and in both hemispheres, with a stronger global-mean surface warming than SSP5-8.5.

We also confirm the earlier conclusion of a decline in the divergent SC resulting from the weakening of the tropical overturning circulation23–26. As shown in Supplementary Fig. S4, the divergent component of SC, represented by the velocity potential at 850 hPa, demonstrates a more robust decrease in the SSP5-8.5 scenario, aligning with previous findings23–26.

As the internal variability could greatly influence the regional SC changes27, we analyze the SMILEs of five models, CESM1-CAM5, CanESM2, CSIRO-Mk3-6-0, GFDL-CM3, and MPI-ESM, and confirm a limited role of internal variability to the weakened SC (gray hollow markers in Fig. 2). An apparent exception projecting a strengthened global SC is MPI-ESM (Fig. 2), including the 100 members of MPI-ESM in the SMILEs, and the related versions of MPI-ESM in the SSP5-8.5 (MPI-ESM1-2-HR and MPI-ESM1-2-LR) and abrupt-4×CO2 (MPI-ESM-1-2-HAM, MPI-ESM1-2-HR, and MPI-ESM1-2-LR) from the CMIP6. Even though, the results of MPI-ESM-related models follow the negative correlation between global SC change and global-mean surface warming, reflecting the enhanced SC could be related to the lower warming in the models.

The uncertainty from patterned SST warming is also evaluated by the simulations from d4PDF, which are six atmosphere-only experiments forced by 4-K patterned warmings scaled from six representative SST warming patterns (Supplementary Fig. S5) extracted from the CMIP5 models (purple markers in Fig. 2). The spread of the intensity changes in the six d4PDF experiments does not exceed the inter-model spread of SSP5-8.5 and abrupt-4×CO2 experiments, also exhibiting a robust weakening of global SC. We contrast two extreme results in the six d4PDF experiments: one involving slight intensification and the other a considerable weakening (Fig. 2), particularly in the Southern Hemisphere (Supplementary Figs. S2 and S3). These results are associated with HFB_4K_MI (Supplementary Fig. S5d) and HFB_4K_MR (Supplementary Fig. S5f), respectively. In HFB_4K_MI, a rare cooling occurs over the Southern Ocean, enhancing the meridional temperature gradient and strengthening westerlies46. Additionally, there is a relatively weak El Niño-like warming in the tropical Pacific, resulting in a weaker westerly change over the equatorial western Pacific. Both these features in HFB_4K_MI contribute to strengthening the Southern Hemisphere SC.

The role of the three primary processes in CO2 increase, the uniform SST warming, the patterned SST warming, and the direct CO2 radiative forcing22,30, are investigated using a set of atmosphere-only experiments from CFMIP-345, including amip, amip-p4K, amip-future4K, and amip-4×CO2. Uniform (blue dots in Fig. 2) and patterned (light-blue dots in Fig. 2) SST warming both significantly weaken the global SC. By contrast, the direct CO2 effect strengthens the global SC confirmed by the amip-4×CO2 experiment (red dots in Fig. 2), opposite to the effect of SST warming. On a global scale, the diminishing influence of SST warming overcomes the enhancement of the direct CO2 effect, leading to a robust weakening, although these two mechanisms are comparable in some regions22.

The effects of SST warming and direct CO2 radiative forcing have distinct temporal features. The direct CO2 effect is a rapid process with timescales from weeks to months22,34, whereas the SST warming associated with its impact is almost proportional to the relatively slow global-mean surface warming. The opposite roles of SST warming and direct CO2 effect with distinct temporal features can explain the more robust decrease in abrupt-4×CO2 than in SSP5-8.5 experiments (Fig. 2). There is persistent new CO2 emission in SSP5-8.547 but not in abrupt-4×CO2, resulting in more transient response in SSP5-8.5 during the years 2070–2099 than in abrupt-4×CO2 during the years 121–150. Given that the warming of the transient response to a CO2 emission is weaker compared to the response nearing equilibrium, the role of uniform SST warming is underrated in SSP5-8.5 relative to the response nearing equilibrium in abrupt-4×CO2. This process could be the reason for the absence of a robust change obtained in many previous studies utilizing SSP5-8.5 experiments.

Seasonal variation of the changes in the subtropical circulation

As SC and the associated processes are seasonally and latitudinally varying1, we further investigate the seasonal and latitudinal SC change by projecting the SC onto the climatological SC latitude-by-latitude and month-by-month (see the “Methods” section for details). Figure 3 shows the seasonal and latitudinal intensity changes in SC of the multi-model mean of CMIP6 models. The center of the climatological SC is shown as a reference for the SC changes (see the “Methods” section for details; Supplementary Fig. S6). The Northern Hemisphere SC is weakened around the equatorward flank of the climatological SC center throughout the year, except in JJA (Fig. 3). The Southern Hemisphere SC is weakened mainly on the equatorward flank of the climatological center but strengthened on the poleward flank (Fig. 3), resembling a poleward shift.

The weakened SC is not robust in all months and latitudes (see the definition of robustness in the “Methods” section), implying a more intricate mechanism underlying these changes in SC. In the amip-p4K simulation, uniform SST warming results in a robust weakening of global SC throughout the year (Fig. 4a and Supplementary Fig. S7). This weakening shifts seasonally with the climatological SC center and occurs primarily on its equatorward flank in a few months, possibly linked to the more pronounced circulation weakening in the tropics than in the extratropics23. The impact of the patterned SST warming (Supplementary Fig. S8) on the SC with the impact from uniform SST warming removed (Fig. 4b) is relatively weak compared to the uniform SST warming (Fig. 4a), but it enhances the poleward flank of the SC in the Southern Hemisphere during the austral summer (Fig. 4b and Supplementary Fig. S9). This enhancement is associated with the southeasterly wind changes over the southeastern Pacific (Supplementary Fig. S8), which is modulated by a stronger west-minus-east gradient during the austral summer around 20°S–40°S of the prescribed-SST change pattern (Supplementary Fig. S8)48. The results in the d4PDF simulations with six different SST change patterns (Supplementary Fig. S10) are consistent with those in SSP5-8.5 (Fig. 3), also suggesting a weak influence of the uncertainty in the SST change pattern.

By contrast, direct CO2 radiative forcing strengthens the global SC, especially during the boreal summer monsoon season (June–September; Fig. 4c and Supplementary Fig. S11). Strengthening of the SC is stronger on the poleward flank of the climatological SC center because of the superposition of the poleward shift in the midlatitude jet (Supplementary Fig. S12). Direct CO2 radiative forcing can induce stratospheric cooling and a poleward shift of midlatitude jet throughout the year46,49, favoring the strengthening of the SC in both hemispheres. However, its impact is located around the midlatitude jets, exactly the poleward boundary of the climatological SC in both hemispheres (Fig. 4c and Supplementary Fig. S12), indicating the poleward shift in the midlatitude jet could not be the major factor enhancing SC, especially during the boreal summer.

The combined impact of the uniform and patterned SST warming, and direct CO2 radiative forcing (Fig. 4d) closely resembles the SC changes in the SSP5-8.5 runs (Fig. 3). Some minor discrepancies between Figs. 3 and 4d still appear in Northern Hemisphere during the boreal autumn, implying a relatively weak role of the atmosphere–ocean coupled process that is neglected in the atmosphere-only experiments. Compared with the SC changes in the SSP5-8.5 runs (Fig. 3), the weakening of SC is more robust in most months and latitudes in the Southern Hemisphere (Fig. 4d), which should be associated with the stronger warming in the +4-K amip experiments than the SSP5-8.5 runs (Fig. 2).

The weakened SC under uniform SST warming is robust throughout the subtropics (Fig. 5a), whereas the SC is strengthened in June–September under direct CO2 radiative forcing (Fig. 5b), except over the Atlantic Ocean. The low-level SC weakening under uniform SST warming is spatially homogeneous throughout the subtropics, which is consistent with the decreased vertical velocity in the mid-level troposphere (Fig. 5c)23 and reflects the mechanism of the Sverdrup balance connected with the weakened tropical circulation1. The degree of weakening in low-level SC is weaker than that in mid-level circulation and is close to that in the Hadley cell50, given that the low-level SC is closely related to the descending branch of the Hadley cell. The major contribution to the slowdown in the tropical overturning circulation is from the Walker circulation, but not the Hadley cell23,24,50. The direct CO2 effect mainly strengthens the SC in the boreal summer monsoon season, most apparent in the low level, by enhancing the land–sea thermal contrast22.

The land–sea thermal contrast (Supplementary Fig. S13) could induce tropical diabatic heating and then influence the SC in both hemispheres through stationary barotropic Rossby waves33,51,52. The process is verified by a linear baroclinic model (LBM; see the “Methods” section for details). According to the boreal summer changes in rainfall under direct CO2 effect (Fig. 5d), we select six regions with idealized heating and cooling to force the LBM in the respective experiment (Supplementary Figs. S14 and S15). The steady responses to the heating and cooling show that the diabatic heating in North Africa and cooling in the northeastern Pacific contribute most to the strengthened SC (Fig. 5b and Supplementary Fig. S16). The tropical responses are close to a Matsuno–Gill pattern53 dependent on the location of forcing.

Timescales of responses

The distinct temporal features of SST warming and direct CO2 effect cause their relative contributions to vary across different emission scenarios. This property is used to verify their combined role by explaining the time-varying SC in the abrupt-4×CO2 simulation. In abrupt-4×CO2 (Fig. 6a), the direct CO2 effect—the enhanced land–sea thermal contrast—reaches its peak in the first year strengthening SC. Then the weakening of SC is increasingly robust in the low-level SC, along with the increase of slow SST warming overcoming the direct CO2 effect. The rate of weakening in the low-level SC at the end of the abrupt-4×CO2 experiment is about −1.14% (with a 5–95% range of −2.90% to 0.21%), per 1 K global surface warming.

The enhanced SC by the direct CO2 effect appears in all latitudes and seasons in the first year of the abrupt-4×CO2 experiment (Fig. 6b). The Northern Hemisphere SC changes are slightly shifted from the boreal summer to spring and early summer. One possible reason is that the change in a single year is susceptible to internal variability, while another possibility is that the annual global-mean surface warming is around 1 K in the first year, indicating that the influence of SST warming has already occurred. However, the response in the Southern Hemisphere (Fig. 6b) closely resembles that in the atmosphere-only model (Fig. 4c), confirming the direct CO2 effect on the SC in the coupled model. As greenhouse warming continues, the SC changes during years 21–30 in the abrupt-4×CO2 experiment (Fig. 6c) resemble those at the end of the SSP5-8.5 scenario (Fig. 3). In years 141–150 of the abrupt-4×CO2 experiment, the Southern Hemisphere SC is clearly weakened throughout the year, and the Northern Hemisphere SC is also weakened, except in August–September (Fig. 6d). Overall, the combined roles of SST warming and direct CO2 radiative forcing in the low-level SC changes shown here explain well the time-varying SC changes in the abrupt-4×CO2 simulations.

Methods
CMIP6 simulations
The historical simulation and the SSP5-8.5 experiment from 34 CMIP6 models60 are used to represent the present-day and future climates, respectively. The simulation skills of historical SC in these models are evaluated by comparing them with the European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) reanalysis dataset61. As shown in Supplementary Fig. S17, the Taylor diagram62 for the spatial correlation coefficients and standard deviations of the 850 hPa stream function from 60°S to 60°N illustrates that most models show a good performance on the simulation of SC. Thus, we do not further pick out models in the present analyses. The pre-industrial control simulation (piControl) and the simulation forced by an abrupt quadrupling of the pre-industrial CO2 level (abrupt-4×CO2) from 32 CMIP6 models are also used to study the timescales of different processes.

A set of atmosphere-only experiments45, including the amip, amip-p4K, amip-4×CO2, and amip-future4K from 10 CMIP6 models, are used to investigate the role of different processes. The amip experiment is a control run forced by the observed monthly mean SST and sea ice concentration, whereas the amip-p4K, amip-4×CO2, and amip-future4K experiments are further forced by uniform SST warming, quadrupling of the pre-industrial CO2 level and patterned SST warming derived from the CMIP3 coupled models, respectively. The prescribed SST warming pattern for each month is shown in Supplementary Fig. S8. The present study employs the 30 years, 1979–2008, of all the amip experiments. The details for selected models in the different experiments are shown in Supplementary Table S1.

We only select the first simulation of the ensembles for each model. All the monthly model data are interpolated onto a 2.5° × 2.5° grid (90°S–90°N, 0°–357.5°E) before analysis. This study presents the results as the multi-model mean unless stated otherwise.

Single-model initial-condition large ensembles
To evaluate the role of internal variability, five single-model initial-condition large ensembles (SMILEs) are used in this study43, which are CESM1-CAM5, CanESM2, CSIRO-Mk3-6-0, GFDL-CM3, and MPI-ESM with 40, 50, 30, 20, and 100 members, respectively.

Database for policy decision making for future climate change
The large ensemble simulation named ‘Database for Policy Decision Making for Future Climate Change’ (d4PDF) is used to study the role of uncertainty in future SST warming patterns44. The Meteorological Research Institute AGCM, version 3.2 (MRI-AGCM3.2) used here includes the simulations for a present climate (1951–2010) with 100 members (HPB) and a future climate for 60-year integration with 90 members. In this model, the global-mean surface air temperature of the future climate is about 3.6 K warmer than that of the present climate, corresponding to the condition around the end of the 21st century under the RCP8.5 scenario44. These simulations consist of six experiments, each using a distinct SST change pattern from CMIP5 models (HFB_4K_CC, HFB_4K_GF, HFB_4K_HA, HFB_4K_MI, HFB_4K_MP, HFB_4K_MR; Supplementary Fig. S5) to capture the impacts of SST warming pattern diversity. For each experiment, 15 members for a 60-year future warming climate are conducted using different initial conditions and different small perturbations of SST. The first 15 ensemble members from 1979 to 2008 in historical simulation are used to represent the historical climate in this study. Different from the future patterned-SST forced experiments such as amip-p4K and amip-future4K from the CFMIP-3 in CMIP6, the greenhouse gases are set to the same value as the end of the RCP8.5 scenario in the global warming simulation from the d4PDF44. Thus, the intensity changes of global SC in the 4-K future climate simulations in d4PDF are lower than that in the amip-p4K and amip-future4K experiments and are close to the results in the SSP5-8.5 scenario.

Definition of the SC
In this study, we focus on the dominant rotational component of SC at the low level63, represented by the 850 hPa stream function. The meridional center of SC for each month is defined as the maximum of the climatological zonal-mean 850 hPa stream function in the Northern Hemisphere (20°–40°N) and the minimum of the climatological zonal-mean 850 hPa stream function in the Southern Hemisphere (20°–40°S) in the historical, amip and piControl experiments. The SC center and the climatological 850 hPa stream function in the historical experiment are shown in Supplementary Fig. S6.

Intensity change
The overall intensity change in global SC is defined by the projection of the changes in global SC (10°–45°S and 10°–45°N) onto its 12-month climatology calculated as:

A = \frac{\sum_{k=1}^{n} \sum_{j=1}^{n} \sum_{i=1}^{n} X_{ijk} Y_{ijk}}{\sum_{k=1}^{n} \sum_{j=1}^{n} \sum_{i=1}^{n} Y_{ijk} Y_{ijk}} (1)

where A represents the intensity change (unit: %); X and Y represent the change and climatology of a circulation system, respectively; i, j, and k represent the latitude, longitude, and month dimensions, respectively. This definition is independent of the domain and the seasonal evolution of SC. We also test other latitudinal ranges, including 45°S–45°N, 10°–40°S, and 10°–40°N and 10°–50°S and 10°–50°N, which do not influence the conclusions (Supplementary Fig. S18). The projection method defining the percentage intensity change prevents unreasonable large values in cases where the climatological SC is close to 0, unlike the conventional method that divides changes by climatology.

Considering SC is seasonally and latitudinally varying, we also project the SC change onto the climatological SC latitude-by-latitude and month-by-month calculated as:

A_{ik} = \frac{\sum_{j=1}^{n} X_{ijk} Y_{ijk}}{\sum_{j=1}^{n} Y_{ijk} Y_{ijk}} (2)

Test of robustness
A change is considered robust if it meet two criteria: the ratio of multi-model mean to inter-model standard deviation of the change is > 1 and at least 66% of the models show a change greater than the internal-variability threshold γ. This test of robustness considers both the uncertainty across the models and internal variability, which is similar to the approach used in the IPCC AR6 WG1 report64. The ratio test is stricter than the sign agreement test often used in previous studies to evaluate the inter-model robustness. The internal-variability threshold is defined as γ = 2 × 1.645 × σ30yr, where σ30yr is the standard deviation of 30-year climatology of thirteen periods separated from 500-year simulation in the piControl in which the first 100 years are ignored64.

Linear baroclinic model
We use a simple dry model, Linear baroclinic model (LBM)65, to investigate the atmospheric response to prescribed heating66. The LBM applied here consists of primitive equations linearized concerning a climatological state of June–September, with a horizontal resolution of T42 (roughly equivalent to 2.8°) and 20 vertical sigma levels. The spatial patterns of prescribed heating and cooling maximum at 0.45 level are shown in Supplementary Fig. S14. The vertical profile of diabatic heating forcing the LBM is shown in Supplementary Fig. S15, with a maximum (minimum) at sigma = 0.45 level. The LBM is run up to 30 days, and the 850 hPa stream function in the 21–30 days is used to represent the steady response shown in Supplementary Fig. S16.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "\"Exploring Robust Changes in Global Subtropical Circulafion Under Greenhouse Warming\"\nThis arficle invesfigates the changes occurring in the subtropical circulafion (SC) in the lower troposphere\nas a result of a warmer climate. By analyzing CMIP6 models, the study reveals significant global SC\nchanges characterized by a weakening trend in the Northern Hemisphere throughout the year, except\nduring the boreal summer, and a poleward shift in the Southern Hemisphere. The authors aftribute these\nchanges primarily to global-mean warming, which weakens the SC, parficularly in its equatorward flanks.\nHowever, they also note that direct CO2 radiafive forcing strengthens the SC in the poleward flanks,\nspecifically during the boreal summer monsoon season. While the arficle focuses on an intriguing topic,\nfurther clarificafion is necessary before publicafion.\nThere is a need for more detailed explanafions regarding the role of changes in local sea surface\ntemperature (SST) compared to tropical SST and their varying impacts. The study should delve into the\nseasonal variability of the crucial role played by tropical diabafic heafing in SC.\nFurthermore, the changes in SC are not zonally symmetric in both hemispheres. For instance, during\nboreal winter, the SC over the South Pacific exhibits greater strength compared to the South Atlanfic or\nIndian regions. The SC over the South Pacific is known to be influenced by the heafing over SPCZ (Fahad\net al. 2021). The arficle would benefit from an extended discussion focusing on these discrepancies, with\nparficular emphasis on the local forcing and remote influences.\nThe arficle would also benefit from a more comprehensive discussion, specifically in the secfion fitled\n\"Role of SST warming and direct CO2 radiafive forcing,\" which should concentrate on individual\nhemispheres and basins.\nThere is a need for clarificafion on the stafisfical significance of the findings menfioned in lines 173-175.\nIn the summary secfion, it is crucial to incorporate a discussion on how the results of this arficle differ or\nalign with exisfing literature, parficularly in cases where similar research has been previously conducted,\nparficularly in the Southern Hemisphere.", "reviewer_2": "This study examines the changes in global subtropical circulafion (SC) under greenhouse warming using\nthe future projecfions from CMIP6 models. The results suggest a weakening in the Northern Hemisphere\nSC and a poleward shift in the Southern Hemisphere SC. The authors further discuss the contribufions of\nSST warming and direct CO2 radiafive forcing to the projected changes. Although this study provides\nsome insights into future SC changes and their possible mechanisms, there are a few major flaws in the\ncurrent version, making this study not appropriate to be published in Nature Communicafions. I would\nencourage the authors to resubmit the manuscript to other journals which accept longer arficles. The\ndetailed comments are listed below.\nMajor comments:\n1. “Robust changes”. The authors have never defined how they determine “robust” in the SC changes. Is\nit based on a large model agreement, the significance of the change, the seasonal consistency, or any\nothers? This is a piece of important informafion that the authors would need to include.\n2. Abstract. Overall, the abstract is too technical. Please rewrite it in a form that is more understandable\nto the public audience.\n3. Introducfion. The introducfion is very unclear about what is sfill unknown in the SC changes, and what\nis new in this study compared to the previous literature. The introducfion also lacks examples of how SC\nchanges may impact the weather and climate. The mofivafion is unclear. Although the authors have cited\nsome references, their discussion of these previous studies is not thorough enough. It is important to\ninclude a discussion in terms of the significance of this study.\n4. Novelty. There have been a great number of studies using CMIP6 models to study SC changes, so what\nis the novelty of this study?\n5. Method. This secfion does not provide enough informafion. For example, how the 34 models were\nselected. Are they based on the data availability or the model fidelity in simulafing the SC? The baseline\nis the model would need to capture the SC well, as well as the ensemble members the authors used.\nPlease discuss how many ensemble members are used in each model and what ensemble members the\nauthors used. In addifion, I am not sure why a short period is examined in this study. The historical run is\nup to 2014 while the authors only used the runs up to 2000. I would encourage the authors to extend\ntheir period to reduce the uncertainty and have more robust results. Please also clarify why the historical\nruns are used from 1971. Most of the reanalysis data are from 1979. It is befter to match the period\nbetween historical runs and reanalysis data when comparing the model fidelity in simulafing the SC\nwhich is currently missing. I understand the authors want to use the mulfi-model mean for an easier\ninterpretafion of the results, it is sfill with significant scienfific meaning that the authors would need to\ndiscuss the model spread. Especially as pointed out by the authors that there are large inconsistencies\nbetween the studies which imply a large model spread. This is one of the novelfies that I can see from\nthis study. In terms of the definifion of the subtropical circulafion, it is not sure why the center of\nsubtropical circulafion is calculated using a large lafitude spread which includes the tropics. The authors\nwould need to test the sensifivity of their results by adjusfing this lafitude range.\n6. Readability. The authors want to make the manuscript concise, however, they lost clarity. Most of their\ndescripfion or statements are too general and hard to follow. Their figures are generally not\nstraighfforward, so it will be much more helpful if they could elaborate more in the main text. The\nmechanisms they discuss are also not clear and missing details. They would need to refine most of their\nmain text to keep the manuscript concise while also making the statement clear and with enough details\nfor the readers to understand.\n7. Figures. The sfipplings are very hard to see in the figures. For Figure 2, please do not make -4 to 4\nwhite. There is a large model agreement in the tropics but there is no color. Figure 3: The hatched\npaftern makes the figure messy and the sfippling is rather hard to see. I think the authors could think of a\nbefter way to show the local weakening. Figure 4d: how this figure is calculated would need to be\ndiscussed in the Method secfion. And why 40S-40N is used by including the tropics?\n8. Discussion. A comprehensive discussion is necessary following the summary in terms of consistency\nwith the previous studies and the implicafion we can get from this paper.\n9. Please provide a map showing the SST warming paftern in the supplementary. Or provide a more\ndetailed descripfion of the amip experiments such as the value of the warming, and the paftern of the\nwarming.\n10. Because the introducfion is poorly wriften, it is hard to determine the significance and novelty of this\nmanuscript. Please rewrite the introducfion to show the novelty of this study. In addifion, the\nmanuscript is poorly wriften as I menfioned that some important details are missing, also the figures are\nhard to understand. Please try to include as many details in the Method secfion in terms of calculafion\nand analysis methods. Most of the discussion cited mechanisms from previous papers and the main\nfindings seemed to be the role of SST warming and CO2 forcing. Therefore, this paper is more of a\nfurther analysis instead of a breakthrough finding. I would therefore recommend transferring this study\nto another journal. I would also encourage the authors to try to convert the manuscript into a longer\npaper to include more details and thorough discussion.\nMinor comments:\n1. L6-8. L13-16. L18-20. Please rephrase, hard to understand.\n2. L21-22. How do the results carry implicafions for subtropical climate changes?\n3. L33-34. Is this something already found or the authors' hypothesis? Please clarify.\n4. L33. “but no consistent response…” For example?\n5. L34-37. Not linked to the previous sentence as to which mechanisms are in which\nseasons and which regions?\n6. L37-38. It seems that it has been a well-established mechanism from the impact of the direct CO2\neffect. And this mechanism has been used to explain changes in SC during summer. So, what is new in\nthis study?\n7. L41. The mechanism is fairly unclear here.\n8. L43. Same comment as above.\n9. L48. The authors have never explained why there is a need to examine the changes lafitude-by-\nlafitude.\n10. L50-52. Please do not repeat this again. This has been discussed in the abstract.\n11. L53-57. Please do not include a summary of the results here. The introducfion should be more\nfocused on the background of the field and the significance and novelty of this study.\n12. Need some descripfion of what the SC changes are like and how they change in the figure when\ndiscussing Fig. 1.\n13. L65-67. How the changes in the tropical circulafion contribute to the changes in SC. The authors need\nto clarify that.\n14. L65. I do not see the tropical circulafion change as seasonally consistent from Fig. 1a and 1b. They\nneed befter quanfificafion.\n15. L68-70. This kind of discussion is good. The authors would need to have some similar discussion to\nindicate how the SC changes and its implicafion to the changes of the climate systems.\n16. L71. “and lafitude”. I do not see this from the earlier discussion.\n17. L77-78. The SC in the winter season is more of an indicafion of a weakening in the southward flank\nrather than a weakening of the center when comparing the center of SC and the center of SC changes.\n18. L85. Briefly describe\n19. L90-91. Briefly describe their differences.\n20. L99-100. Why this is specifically discussed? Unless this is important to some weather and climate\nsystems.\n21. L101. Not always, only in a few months.\n22. L104. Please elaborate. The authors always make a general statement. They could state in more\ndetail such as which lafitudes and which seasons.\n23. L106-107. Not a convincing mechanism here. The reducfion in midlafitude baroclinicity is dominant\nin which seasons and caused by which SST change pafterns? Please provide more details.\n24. L138-139. Can this mechanism explain why the strengthening most occurred in the poleward flank?\n25. L355. Not straighfforward how the seasonal changes are calculated. Needed a brief clarificafion here\nor in the main text.\n26. L385. Where is the shading?", "reviewer_3": "This study invesfigates changes in the global subtropical circulafion in response to greenhouse warming\nand tries to quanfify the effects of the global-mean warming and the direct CO2 radiafive forcing by\nanalyzing the amip, amip-p4k, amip-future4K and amip-4xCO2 experiments in CMIP6. Authors highlight\nthat combining these two mechanisms results in seasonally and lafitudinally dependent changes in the\nsubtropical circulafion under a warmer climate. This study provides systemafic analyses with different\namip-type experiments in CMIP6. However, the main results are consistent with previous studies and not\nnovel enough for Nature Communicafion. Thus, I cannot recommend the manuscript for publicafion in\nnature Communicafions.\nMajor comments:\n1. Significance: Many studies have already discussed the roles of global-mean warming, the pafterns of\nSST changes, and direct CO2 radiafive forcing on the changes in the subtropical circulafion as referred to\nin the manuscript. The main results of this study, such as the weakening of tropical circulafion, the\nintensificafion of the Asian summer monsoon, and the poleward shift of SH subtropical circulafion,\nconfirm results from many previous studies. Thus, it seems that the main results are not novel and\nsignificant enough for Nature Communicafion.\n2. Validity/Data and Methodology: The descripfion of the data and methodology is well provided.\nHowever, I think the comparison of AMIP-future4K and AIMP-p4K is not enough to address the role of\nthe SST paftern changes. One of the dominant uncertainfies in circulafion changes is rooted in\nuncertainfies in the future SST paftern changes (e.g., Chadwick et al., 2014 referred to in the\nmanuscript). Since the AMIP-future4K uses the same mean SST paftern projected by CMIP simulafions,\nthe current analysis has limitafions in addressing the effect of future SST paftern changes. However,\nmodels tend to show robust responses to global-mean warming and direct CO2 radiafive forcing as\ndiscussed in many previous studies (e.g., Fahad et al., 2021; Bony et al., 2013; Vecchi et al., 2006; and\nmany others referred to in this manuscript).\n3. Clarity and Context: The manuscript is well wriften and clear. However, there are several limitafions in\ncontext. First, the main results are only based on mean of mulfi-model changes. Fidelity of model\nsimulafion in the present and the role of model uncertainfies in addressing future changes should be\ndiscussed. Second, the significant test is only based on the degree of the models agree on the sign of the\nmulfi-model mean in the study. Considering large internal variability in circulafion changes, a significant\ntest should be also applied with respect to internal variability."}, "author_response": "Response to reviewer #1:\nThank you for the insightful comments and detailed instruction on how to improve the\nmanuscript. The quality of the manuscript has been greatly improved based on your\ncomments. Our point-by-point reply follows, where the original comments are quoted\nin italic.\n1. There is a need for more detailed explanations regarding the role of changes in local\nsea surface temperature (SST) compared to tropical SST and their varying impacts. The\nstudy should delve into the seasonal variability of the crucial role played by tropical\ndiabatic heating in SC.\nRe: Thank you for the constructive suggestion.\nIn our original manuscript, the analyses based on amip-p4K and amip-future4K\nsimulations (new Fig. 4, Supplementary Figs. S7 and S8) exhibit that the impact of\nprescribed-SST warming pattern is relatively weak compared to the uniform SST\nwarming on SC. Thus, we did not discuss the role of SST in different locations in the\noriginal manuscript. In the revision, we improve the related statements shown in Lines\n155–162.\nTo further illustrate the role of SST warming pattern, we analyzed the “Database\nfor Policy Decision Making for Future Climate Change” (d4PDF) simulations in the\nrevision. The d4PDF has six experiments using six typical SST change patterns in\nCMIP5 models respectively (HFB_4K_CC, HFB_4K_GF, HFB_4K_HA,\nHFB_4K_MI, HFB_4K_MP, HFB_4K_MR; Supplementary Fig. 4) to produce the\nimpacts of the diversity in SST warming patterns. Results in the d4PDF simulations\n(Supplementary Fig. 9) align with those from SSP5-8.5 (Fig. 4a), indicating a limited\ninfluence of SST change pattern uncertainty. We discuss this in Lines 160–162.\nIn response to your suggestion, we employ a linear baroclinic model (LBM) to\nexplore the contribution of tropical diabatic heating changes across different regions to\nthe enhanced SC due to direct CO2 effects. The LBM employs linearized primitive\nequations based on a June–September climatological state, with T42 horizontal\nresolution (approximately 2.8°) and 20 vertical sigma levels. A detailed LBM\nintroduction is presented in Lines 352–361.\nThe LBM uses prescribed diabatic heating and cooling locations, detailed in\nSupplementary Fig. S13. The model runs for 30 days, with the 850 hPa streamfunction\nduring days 21–30 representing the steady response (Supplementary Fig. 15). Steady\nresponses reveal that diabatic heating in North Africa and cooling in the Northeastern\nPacific contribute predominantly to the strengthened SC (Fig. 5b and Supplementary\nFig. 15). These tropical responses align closely with a Matsuno-Gill pattern, contingent\non the forcing's location. The discussion of these results is presented in Lines 198–205.\nFigure S13. The prescribed diabatic heating and cooling for six regions used in the\nlinear baroclinic model.\nFigure S15. The steady response in 850 hPa streamfunction to the prescribed diabatic\nheating and cooling shown in Fig. S13.\n2. Furthermore, the changes in SC are not zonally symmetric in both hemispheres. For\ninstance, during boreal winter, the SC over the South Pacific exhibits greater strength\ncompared to the South Atlantic or Indian regions. The SC over the South Pacific is\nknown to be influenced by the heating over SPCZ (Fahad et al. 2021). The article would\nbenefit from an extended discussion focusing on these discrepancies, with particular\nemphasis on the local forcing and remote influences.\nThe article would also benefit from a more comprehensive discussion, specifically in\nthe section titled \"Role of SST warming and direct CO2 radiative forcing,\" which\nshould concentrate on individual hemispheres and basins.\nRe: Thank you for your comment. Indeed, SC changes exhibit hemispheric asymmetry,\nwhich has contributed to the challenge of identifying consistent trends across locations.\nIn this study, we introduce an innovative method for quantifying global SC intensity\nchanges, irrespective of regional distinctions. Consequently, we identify a robust\noverall SC change across models. This robustness is rooted in the overall trend observed\namong models, despite the regional variations in SC alterations. To enhance clarity and\nnovelty, we've revised the introduction and bolstered our study's rationale with\nadditional references.\nAdditionally, we've expanded our experimentation, encompassing SSP5-8.5,\nabrupt-4xCO2, CFMIP-3, SMILEs, and d4PDF (outlined in the Methods section). The\nresults, including Fig. 2 and supplementary figures, corroborate the consistent decline\nin global SC. We've also examined SC changes in individual hemispheres\n(Supplementary Figs. S2 and S3).\nAs addressed in response to comment #1, our use of an LBM delves into the role\nof local diabatic heating. In the revised manuscript, we provide an in-depth exploration\nof mechanisms underlying the robust weakening of global SC.\nWe agree with the comment that the regional differences in SC changes are also\nimportant relative to the general changes in global SC. Although the regional change in\nSC is less discussed in this study, the newly developed method provides an efficient\nway to remove the overall changes to highlight the regional features of SC changes in\nfurther study. We have further refined this aspect in the Summary section (Lines 261–\n268).\n3. There is a need for clarification on the statistical significance of the findings\nmentioned in lines 173-175.\nRe: We have revised the sentence as “The rate of weakening in the low-level SC at the\nend of the abrupt-4xCO2 experiment is about −1.14% (with a 5–95% range of −2.90%\nto 0.21%), per 1 K global surface warming.”\n4. In the summary section, it is crucial to incorporate a discussion on how the results\nof this article differ or align with existing literature, particularly in cases where similar\nresearch has been previously conducted, particularly in the Southern Hemisphere.\nRe: Thank you for the comment. To address this comment and related comments from\nother reviewers together, we extensively rewrote the introduction section to emphasize\nthe novelty of this study in relation to prior literature. Some necessary references are\nalso added.\nIn the revision, we further calculated the general intensity change in global SC in\naddition to the global SC changes month-by-month and latitude-by-latitude in the\noriginal manuscript. The overall change computation for global SC omits regional\ndiscrepancies, revealing the robust changes in global SC.\nResponse to reviewer #2:\nThank you for the insightful comments and detailed instruction on how to improve the\nmanuscript. The quality of the manuscript has been greatly improved based on your\ncomments. Our point-by-point reply follows, where the original comments are quoted\nin italic.\nMajor comments:\n1. “Robust changes”. The authors have never defined how they determine “robust” in\nthe SC changes. Is it based on a large model agreement, the significance of the change,\nthe seasonal consistency, or any others? This is a piece of important information that\nthe authors would need to include.\nRe: Thank you for the constructive comment. Indeed, the SC changes vary across\nseasons and regions. The \"robust\" claim in our study relies on inter-model consistency\nin the global SC decrease. This contrasts with earlier research reporting divergent\nregional changes. Our revised introduction underscores this novelty and clarifies the\nspecific facet of robustness.\nAdditionally, we enhance the intermodel significance test in alignment with the\nmethod commonly employed in IPCC AR6. For specifics on the robustness test, refer\nto the Method section (Lines 342–351), provided below for convenience:\n“Test of robustness. A change is considered to be robust if it meets two criteria that\nthe ratio of multi-model mean to inter-model standard deviation of the change is greater\nthan 1 and 66% of the models show a change greater than the internal-variability\nthreshold γ. This test of robustness considers both the uncertainty across the models\nand from internal variability, which is similar to the approach used in IPCC AR6 WG1\nreport. The ratio test is stricter than the sign agreement test often used in previous\nstudies to evaluate the inter-model robustness. The internal-variability threshold is\ndefined as 𝛾 =√2∙1.645∙𝜎 , where 𝜎 is the standard deviation of 30-year\nclimatology of thirteen periods separated from 500-year simulation in the piControl in\nwhich the first 100 years are ignored.”\n2. Abstract. Overall, the abstract is too technical. Please rewrite it in a form that is\nmore understandable to the public audience.\nRe: We have revised the abstract thoroughly.\n3. Introduction. The introduction is very unclear about what is still unknown in the SC\nchanges, and what is new in this study compared to the previous literature. The\nintroduction also lacks examples of how SC changes may impact the weather and\nclimate. The motivation is unclear. Although the authors have cited some references,\ntheir discussion of these previous studies is not thorough enough. It is important to\ninclude a discussion in terms of the significance of this study. 4. Novelty. There have\nbeen a great number of studies using CMIP6 models to study SC changes, so what is\nthe novelty of this study?\nRe for #3 and #4:\nWe've revised the introduction section according to the suggestion, clarifying the\nmotivation and novelty of this study.\nPrevious projections of a specific SC have shown considerable variability based\non location and season, lacking consistent changes. Further examination demonstrates\nthat SC change projections are sensitive to domain and metrics selection. This study\nintroduces a novel metric, assessing overall global SC changes and latitude-by-latitude\nSC changes, and uncovers a robust decline in global SC due to greenhouse warming.\nKey findings include:\n1. Development of a novel method to measure general global SC intensity change\nby mitigating regional disparities.\n2. Identification of a robust decline in global SC, contrary to varied projections in\nearlier research.\n3. Global scale projection indicates the increasing dominance of SST warming\nover direct CO2 forcing, resulting in a consistent decrease.\nThese insights greatly contribute to comprehending subtropical climate changes in\na warmer climate.\n5. Method. This section does not provide enough information. For example, how the 34\nmodels were selected. Are they based on the data availability or the model fidelity in\nsimulating the SC? The baseline is the model would need to capture the SC well, as\nwell as the ensemble members the authors used. Please discuss how many ensemble\nmembers are used in each model and what ensemble members the authors used. In\naddition, I am not sure why a short period is examined in this study. The historical run\nis up to 2014 while the authors only used the runs up to 2000. I would encourage the\nauthors to extend their period to reduce the uncertainty and have more robust results.\nPlease also clarify why the historical runs are used from 1971. Most of the reanalysis\ndata are from 1979. It is better to match the period between historical runs and\nreanalysis data when comparing the model fidelity in simulating the SC which is\ncurrently missing. I understand the authors want to use the multi-model mean for an\neasier interpretation of the results, it is still with significant scientific meaning that the\nauthors would need to discuss the model spread. Especially as pointed out by the\nauthors that there are large inconsistencies between the studies which imply a large\nmodel spread. This is one of the novelties that I can see from this study. In terms of the\ndefinition of the subtropical circulation, it is not sure why the center of subtropical\ncirculation is calculated using a large latitude spread which includes the tropics. The\nauthors would need to test the sensitivity of their results by adjusting this latitude range.\nRe: Thank you for the careful suggestion.\nIn the revision, we have greatly enhanced the clarity of the method presentation.\nOur model selection of 34 models is only based on the availability of the CMIP6 dataset.\nWe have incorporated the fidelity assessment of these models against the ERA5 dataset\nusing a Taylor diagram (Supplementary Fig. 16). Most models exhibit strong\nperformance in simulating subtropical circulation, leading us to maintain the full set of\nmodels for our current analyses.\nFigure S16. Taylor diagram of monthly climatology of the 850 hPa streamfunction in\nthe historical experiment for 34 CMIP6 model covering 60°S–60°N with respect\nto data from ECMWF Reanalysis v5. The time span is from 1979 to 2008.\nIn the revision, the period applied in most experiments is set to 1979–2008\nfollowing the suggestion, which does not influence the conclusions.\nFor the test of robustness across the models, we now include the inter-model\nspread as a metric. All the intensity changes in global SC from different experiments\nused in this study have been shown in one figure in the revised manuscript.\nMoreover, we have introduced two additional sets of experiments, SMILEs and\nd4PDF, to assess the influence of internal variability and patterned SST warming\nuncertainty. Notably, these factors do not overshadow the projections.\nRegarding the definition domain of SC, we have determined that the latitude range\ndoes not alter our conclusions. Therefore, in the revised manuscript, we have refined\nour scope to encompass 20°–40°N for the Northern Hemisphere and 20°–40°S for the\nSouthern Hemisphere in calculating the center of subtropical circulation.\n6. Readability. The authors want to make the manuscript concise, however, they lost\nclarity. Most of their description or statements are too general and hard to follow. Their\nfigures are generally not straightforward, so it will be much more helpful if they could\nelaborate more in the main text. The mechanisms they discuss are also not clear and\nmissing details. They would need to refine most of their main text to keep the manuscript\nconcise while also making the statement clear and with enough details for the readers\nto understand.\nRe: We agree with the comment regarding the omission of certain details in the initial\nmanuscript, which was originally tailored for a different journal before its transfer to\nNature Communications. Given the relatively flexible length constraints of Nature\nCommunications, we have incorporated additional information in the revision.\nSpecifically, the word count of the main text has been expanded from approximately\n2100 to around 2800 words. Furthermore, the Method section has grown from about\n235 words to approximately 1100 words. The inclusion of two more figures brings the\ntotal figure count to six, and supplementary materials have been enriched with\nsupplementary information.\nThe key revisions encompass the following aspects:\n1. A comprehensive rework of the introduction to elucidate the motivation and\nnovelty of our study.\n2. Incorporation of an analysis on the global SC's general intensity change.\n3. Introduction of supplementary experiments aimed at assessing the impact of\ninternal variability and the uncertainty associated with the pattern of SST change.\n4. Integration of LBM (Large-scale Biogeochemical Model) experiments to explore\nthe influence of SST forcing across distinct geographical regions.\nAll these supplementary experiments serve to further reinforce the robustness of our\nfindings.\n7. Figures. The stipplings are very hard to see in the figures. For Figure 2, please do\nnot make -4 to 4 white. There is a large model agreement in the tropics but there is no\ncolor. Figure 3: The hatched pattern makes the figure messy and the stippling is rather\nhard to see. I think the authors could think of a better way to show the local weakening.\nFigure 4d: how this figure is calculated would need to be discussed in the Method\nsection. And why 40S-40N is used by including the tropics?\nRe: Apologies for any unclear figures; this may be due to the submission system's\ntransformation impacting figure quality. In the revised version, we've made\nimprovements: changing stippling to hatching, redrawing figures with suggested\ncolormaps, and enhancing the presentation of local weakening.\nWe've also included a comprehensive explanation of the method for calculating\nintensity changes in global SC in the Methods section.\nRegarding Figure 4d, we acknowledge that the previous domain selection (40S-\n40N) wasn't appropriate. In the updated manuscript, we compute the overall change in\nglobal SC and present new Figure 6a with a range of 10°–45°N for the Northern\nHemisphere and 10°–45°S for the Southern Hemisphere.\nTo gauge how changes in the chosen domain's scope affect intensity change in\nglobal SC, we tested various ranges: 45°S–45°N, 10°–40°S and 10°–40°N, as well as\n10°–50°S and 10°–50°N. The outcomes based on these different scopes (see\nSupplementary Fig. 17) exhibit minimal differences, indicating that the calculation of\nintensity change in global SC isn't significantly affected by the chosen domain's scope.\nFigure S17. Intensity changes in the global subtropical circulation with respect to\nglobal-mean surface warming in SSP5-8.5. The intensity changes in the global\nsubtropical circulation in SSP5-8.5 runs (years 2070–2099; 34 models) relative to\nhistorical runs (years 1979–2008) are calculation over 10°–45°S and 10°–45°N\n(black dots), 45°S–45°N (red dots), 10°–40°S and 10°–40°N (blue dots), and 10°–\n50°S and 10°–50°N (purple dots).\n8. Discussion. A comprehensive discussion is necessary following the summary in terms\nof consistency with the previous studies and the implication we can get from this paper.\nRe: We have revised the introduction thoroughly to clarify the motivation and novelty\nof this study and added two paragraphs in Summary discussing the implications (Lines\n249–268).\nThe main implications include: a contracted monsoon range and drying at its\nboundary with arid regions, an importance of CO emission trajectory and projection\n2\nterm, the major uncertainty sources, and the application of new method for regional SC\nchanges.\n9. Please provide a map showing the SST warming pattern in the supplementary. Or\nprovide a more detailed description of the amip experiments such as the value of the\nwarming, and the pattern of the warming.\nRe: We have added Supplementary Fig. 7 to show the changes in the sea surface\ntemperature and 850 hPa vector wind in the amip-future4K experiment relative to the\namip-p4K experiment in each month.\n10. Because the introduction is poorly written, it is hard to determine the significance\nand novelty of this manuscript. Please rewrite the introduction to show the novelty of\nthis study. In addition, the manuscript is poorly written as I mentioned that some\nimportant details are missing, also the figures are hard to understand. Please try to\ninclude as many details in the Method section in terms of calculation and analysis\nmethods. Most of the discussion cited mechanisms from previous papers and the main\nfindings seemed to be the role of SST warming and CO2 forcing. Therefore, this paper\nis more of a further analysis instead of a breakthrough finding. I would therefore\nrecommend transferring this study to another journal. I would also encourage the\nauthors to try to convert the manuscript into a longer paper to include more details and\nthorough discussion.\nRe: We've extensively revised our manuscript to enhance clarity regarding the study's\nmotivation and novelty. The key revisions encompass the following aspects:\n1. A comprehensive rework of the introduction to elucidate the motivation and\nnovelty of our study.\n2. Incorporation of an analysis on the global SC's general intensity change.\n3. Introduction of supplementary experiments aimed at assessing the impact of\ninternal variability and the uncertainty associated with the pattern of SST change.\n4. Integration of LBM (Large-scale Biogeochemical Model) experiments to explore\nthe influence of SST forcing across distinct geographical regions.\nMinor comments:\n1. L6-8. L13-16. L18-20. Please rephrase, hard to understand.\n2. L21-22. How do the results carry implications for subtropical climate changes?\nRe. for #1 and #2: We have rewritten the abstract as in major comment #2. The main\nimplications summarized in the abstract are: “The declined SC implies a contracted\nmonsoon range and drying at its boundary with arid regions under CO -induced global\n2\nwarming, and the varied impact of processes across timescales highlights the essential\nrole of assessing CO emission trajectory and projection term.”\n2\n3. L33-34. Is this something already found or the authors' hypothesis? Please clarify.\n4. L33. “but no consistent response…” For example?\n5. L34-37. Not linked to the previous sentence as to which mechanisms are in which\nseasons and which regions?\n6. L37-38. It seems that it has been a well-established mechanism from the impact of\nthe direct CO2 effect. And this mechanism has been used to explain changes in SC\nduring summer. So, what is new in this study?\n7. L41. The mechanism is fairly unclear here.\n8. L43. Same comment as above.\nRe. for #3 to #8: We have rewritten the introduction. Please see the revised introduction.\n9. L48. The authors have never explained why there is a need to examine the changes\nlatitude-by-latitude.\n10. L50-52. Please do not repeat this again. This has been discussed in the abstract.\n11. L53-57. Please do not include a summary of the results here. The introduction\nshould be more focused on the background of the field and the significance and novelty\nof this study.\nRe. for #9 to #11: We have rewritten this part (Lines 68–79) as follows:\n“Here, we develop a novel method to extract intensity change in global SC independent\nof the domain and season selection. A robust weakening of global SC is projected under\nthe Shared Socioeconomic Pathway (SSP) 5-8.5 scenario by the end of the 21st century\nin 34 CMIP6 models and in the abrupt-4xCO2 experiments forced by an abrupt\nquadrupling of the pre-industrial CO level in 32 CMIP6 models. The robustness of the\n2\nweakening is verified by the single-model initial-condition large ensemble (SMILE)\nand the “Database for Policy Decision Making for Future Climate Change” (d4PDF)\nsimulations, which consider the uncertainties from internal variability and SST\nwarming pattern, respectively. The robust weakening of global SC is dominated by\nglobal-mean surface warming and partly counteracted by the direct CO effect,\n2\nconfirmed by the third Phase of the Cloud Feedback Model Intercomparison Project\n(CFMIP-3) in CMIP6.”\n12. Need some description of what the SC changes are like and how they change in the\nfigure when discussing Fig. 1.\nRe: We have added the description of what the SC changes are like in Lines 35–41:\n“The projection for a regional SC is crucially dependent on the location and season (Fig.\n1 and Supplementary Fig. 1). The SC changes is distinct during the boreal summer and\nwinter in the Northern Hemisphere, whereas the seasonal variation of the SC changes\nin the Southern Hemisphere is relatively weak. The typical seasonal SC changes include\nthe weakened Australian summer monsoon (Fig. 1a), the almost unchanged East Asian\nsummer monsoon, and the westward shifted North Atlantic subtropical high in JJA (Fig.\n1b).”\n13. L65-67. How the changes in the tropical circulation contribute to the changes in\nSC. The authors need to clarify that.\nRe: We have clarified the contribution of changes in the tropical circulation to changes\nin SC in the second paragraph from the bottom of the section “Seasonal variation of the\nchanges in the subtropical circulation” (Lines 185–189) as follows:\n“The low-level SC weakening under uniform SST warming is spatially homogeneous\nthroughout the subtropics, which is consistent with the decreased vertical velocity in\nthe mid-level troposphere (Fig. 5c) and reflects the mechanism of the Sverdrup balance\nconnected with the weakened tropical circulation.”\n14. L65. I do not see the tropical circulation change as seasonally consistent from Fig.\n1a and 1b. They need better quantification.\nRe: This sentence has been removed from the revised manuscript.\n15. L68-70. This kind of discussion is good. The authors would need to have some\nsimilar discussion to indicate how the SC changes and its implication to the changes of\nthe climate systems.\nRe: We have rewritten the summary section, adding two paragraphs to discussion the\nimplications in Lines 249–268.\n16. L71. “and latitude”. I do not see this from the earlier discussion.\nRe: We have added an introduction about the change in SC in the revised manuscript.\nHere is the introduction added in Lines 35–36:\n“The projection for a regional SC is crucially dependent on the location and season (Fig.\n1 and Supplementary Fig. 1).”\n17. L77-78. The SC in the winter season is more of an indication of a weakening in the\nsouthward flank rather than a weakening of the center when comparing the center of\nSC and the center of SC changes.\nRe: We have revised this sentence in Lines 141–143 as:\n“The Northern Hemisphere SC is weakened around the equatorward flank of the\nclimatological SC center throughout the year, except in JJA (Fig. 3).”\n18. L85. Briefly describe\nRe: This paragraph has been removed in the revised manuscript.\n19. L90-91. Briefly describe their differences.\nRe: The different impacts of the mechanisms have been shown in Fig. 2 in the revised\nmanuscript. These mechanisms have also been introduced in the introduction.\n20. L99-100. Why this is specifically discussed? Unless this is important to some\nweather and climate systems.\nRe: This sentence has been removed in the revised manuscript.\n21. L101. Not always, only in a few months.\nRe: We have revised this sentence in Lines 149–151 as:\n“This weakening shifts seasonally with the climatological SC center and occurs\nprimarily on its equatorward flank in a few months.”\n22. L104. Please elaborate. The authors always make a general statement. They could\nstate in more detail such as which latitudes and which seasons.\n23. L106-107. Not a convincing mechanism here. The reduction in midlatitude\nbaroclinicity is dominant in which seasons and caused by which SST change patterns?\nPlease provide more details.\nRe for #22 and #23: We have revised here (Lines 152–160) as “The impact of the\npatterned SST warming (Supplementary Fig. 7) on the SC with the impact from uniform\nSST warming removed (Fig. 4b) is relatively weak compared to the uniform SST\nwarming (Fig. 4a), but it enhances the poleward flank of the SC in the Southern\nHemisphere during the austral summer (Fig. 4b and Supplementary Fig. 8). This\nenhancement is associated with the southeasterly wind changes over the southeastern\nPacific (Supplementary Fig. 7), which is modulated by a stronger west-minus-east\ngradient during the austral summer around 20°S–40°S of the prescribed-SST change\npattern (Supplementary Fig. 7).”\n24. L138-139. Can this mechanism explain why the strengthening most occurred in the\npoleward flank?\nRe: The strengthening most occurred in the poleward flank is explained by the\nparagraph (Lines 163–173) as follows:\n“By contrast, direct CO radiative forcing robust strengthens the global SC, especially\n2\nduring the boreal summer monsoon season (June–September; Fig. 4c and\nSupplementary Fig. 10). Strengthening of the SC is stronger on the poleward flank of\nthe climatological SC center because of the superposition of the poleward shift in the\nmidlatitude jet (Supplementary Fig. 11). Direct CO radiative forcing can induce\n2\nstratospheric cooling and a poleward shift of midlatitude jet throughout the year,\nfavoring strengthening of the SC in both hemispheres. However, its impact is located\naround the midlatitude jets, exactly the poleward boundary of the climatological SC in\nboth hemispheres (Fig. 4c and Supplementary Fig. 11), indicating the poleward shift in\nthe midlatitude jet could not be the major factor enhancing SC, especially during the\nboreal summer.”\n25. L355. Not straightforward how the seasonal changes are calculated. Needed a brief\nclarification here or in the main text.\nRe: We have added a detailed introduction to how the intensity change in global SC\nand seasonal and latitudinal intensity change are calculated in the Methods.\n26. L385. Where is the shading?\nRe: The shading was lost due to unknown technical problems of the manuscript system.\nWe have solved this issue in another way in current submission.\nResponse to reviewer #3:\nThank you for the insightful comments and detailed instructions on how to improve the\nmanuscript. The quality of the manuscript has been greatly improved based on your\ncomments. Our point-by-point reply follows, where the original comments are quoted\nin italics.\nMajor comments:\n1. Significance: Many studies have already discussed the roles of global-mean warming,\nthe patterns of SST changes, and direct CO2 radiative forcing on the changes in the\nsubtropical circulation as referred to in the manuscript. The main results of this study,\nsuch as the weakening of tropical circulation, the intensification of the Asian summer\nmonsoon, and the poleward shift of SH subtropical circulation, confirm results from\nmany previous studies. Thus, it seems that the main results are not novel and significant\nenough for Nature Communication. \nRe: Thank you for the comments. We are sorry for some unclear representation in the\noriginal manuscript. In the revision, we have rewritten the introduction section to clarify\nthe motivation and novelty of this study. Moreover, we have added a new section to\nemphasize the general decrease in global SC in the revision.\nThe main findings of this study include:\n1. Development of a novel method to measure general global SC intensity change\nby mitigating regional disparities.\n2. Identification of a robust decline in global SC, contrary to varied projections in\nearlier research.\n3. Global scale projection indicates the increasing dominance of SST warming\nover direct CO2 forcing, resulting in a consistent decrease.\nThese insights greatly contribute to comprehending subtropical climate changes in\na warmer climate.\n2. Validity/Data and Methodology: The description of the data and methodology is well\nprovided. However, I think the comparison of AMIP-future4K and AIMP-p4K is not\nenough to address the role of the SST pattern changes. One of the dominant\nuncertainties in circulation changes is rooted in uncertainties in the future SST pattern\nchanges (e.g., Chadwick et al., 2014 referred to in the manuscript). Since the AMIP-\nfuture4K uses the same mean SST pattern projected by CMIP simulations, the current\nanalysis has limitations in addressing the effect of future SST pattern changes. However,\nmodels tend to show robust responses to global-mean warming and direct CO2\nradiative forcing as discussed in many previous studies (e.g., Fahad et al., 2021; Bony\net al., 2013; Vecchi et al., 2006; and many others referred to in this manuscript).\nRe: Thank you for the constructive comments. We agree with the comment that the SST\nchange pattern is an important source inducing great uncertainties in regional climate\nchanges under global warming. This aspect was not adequately addressed in the original\nmanuscript.\nFollowing the suggestion, we conducted an additional analysis using a series of\natmosphere-only experiments referred to as the Database for Policy Decision-Making\nfor Future Climate Change (d4PDF). These experiments consist of six atmosphere-only\nsimulations, each driven by a 4 K patterned warming derived from six distinct\nrepresentative SST warming patterns (Supplementary Fig. 4). The corresponding\nfindings are presented in Fig. 2, Supplementary Figs. 2, 3, 4, and 9.\nThe variability in intensity changes across the six d4PDF experiments remains\nwithin the range of inter-model variations observed in the SSP5-8.5 experiments. This\noutcome is consistent with a robust weakening of the global SC. Further elaboration on\nthe outcomes of the d4PDF analyses can be found in Lines 98–104 and 160–162 of the\nmanuscript.\nFigure S4 Annual-mean changes in the surface air temperature in the a, HFB_4K_CC\n(CCSM4), b, HFB_4K_GF (GFDL-CM3), c, HFB_4K_HA (HadGEM2-AO), d,\nHFB_4K_MI (MIROC5), e, HFB_4K_MP (MPI-ESM-MR) and f, HFB_4K_MR\n(MRI-CGCM3) experiments relative to the HPB (present climate) experiment\nfrom the d4PDF. There are 15 members for each future experiments. Only the\nresults of 15-member ensemble mean are shown at here. Because the SST warming\npattern is not provided in the d4PDF, the change in surface air temperature is\nshown here as a reference for the SST forcing pattern.\n3. Clarity and Context: The manuscript is well written and clear. However, there are\nseveral limitations in context. First, the main results are only based on mean of multi-\nmodel changes. Fidelity of model simulation in the present and the role of model\nuncertainties in addressing future changes should be discussed. Second, the significant\ntest is only based on the degree of the models agree on the sign of the multi-model mean\nin the study. Considering large internal variability in circulation changes, a significant\ntest should be also applied with respect to internal variability.\nRe: Thank you for the kind suggestion. Indeed, some certain details were omitted in the\ninitial manuscript, which was originally tailored for a different journal before its\ntransfer to Nature Communications. Given the relatively flexible length constraints of\nNature Communications, we have incorporated additional information in the revision.\nSpecifically, the word count of the main text has been expanded from approximately\n2100 to around 2800 words. Furthermore, the Method section has grown from about\n235 words to approximately 1100 words. The inclusion of two more figures brings the\ntotal figure count to six, and supplementary materials have been enriched with\nsupplementary information.\nThe key revisions encompass the following aspects:\n1. A comprehensive rework of the introduction to elucidate the motivation and\nnovelty of our study.\n2. Incorporation of an analysis on the global SC's general intensity change.\n3. Introduction of supplementary experiments aimed at assessing the impact of\ninternal variability and the uncertainty associated with the pattern of SST change.\n4. Integration of LBM (Large-scale Biogeochemical Model) experiments to explore\nthe influence of SST forcing across distinct geographical regions.\nAll these supplementary experiments serve to further reinforce the robustness of our\nfindings.\nTest of robustness:\nIn the revision, the definition of inter-model robustness across the models is revised\nto a more reasonable method in the revision, which is widely used in IPCC AR6. The\ndetails for the robustness test are shown in the Method section (Lines 342–351),\nprovided below for convenience:\n“Test of robustness. A change is considered to be robust if it meets two criteria that\nthe ratio of multi-model mean to inter-model standard deviation of the change is greater\nthan 1 and 66% of the models show a change greater than the internal-variability\nthreshold γ. This test of robustness considers both the uncertainty across the models\nand from internal variability, which is similar to the approach used in IPCC AR6 WG1\nreport. The ratio test is stricter than the sign agreement test often used in previous\nstudies to evaluate the inter-model robustness. The internal-variability threshold is\ndefined as 𝛾 =√2∙1.645∙𝜎 , where 𝜎 is the standard deviation of 30-year\nclimatology of thirteen periods separated from 500-year simulation in the piControl in\nwhich the first 100 years are ignored.”\nModel selection:\nOur model selection of 34 models is only based on the availability of the CMIP6\ndataset. We have incorporated the fidelity assessment of these models against the ERA5\ndataset using a Taylor diagram (Supplementary Fig. 16). Most models exhibit strong\nperformance in simulating subtropical circulation, leading us to maintain the full set of\nmodels for our current analyses.\nFigure S16. Taylor diagram of monthly climatology of the 850 hPa streamfunction in\nthe historical experiment for 34 CMIP6 model covering 60°S–60°N with respect\nto data from ECMWF Reanalysis v5. The time span is from 1979 to 2008.\nPossible uncertainties from internal variability\nWe examined five initial-condition large ensembles from a single model to explore\ninternal variability's potential influence. Figure 2 in the revised manuscript displays our\nfindings. The internal variability's spread remains within the bounds of inter-model\nuncertainties."}, {"round": 2, "reviewer_comments": {"reviewer_1": "no further comments from my end.", "reviewer_2": "The manuscript has largely been improved from its previous version by incorporafing comments from all\nthe reviewers. I appreciate all the effort and work the authors have done to strengthen their study. I am\npleased with the revision and do not have any crifical comments. I only have one suggesfion is to further\nimprove their introducfion to emphasize the significance and novelty of this study. The novelty of a\npaper published in this journal cannot be just a novel method, only if that method can be widely applied.\nThe authors would need to highlight what findings are new in their study compared to the previous\nliterature. For example, is this the first study to compare the influence of SST warming and CO2? Also, is\nthe different fimescales of these two impact factors new finding? And is the seasonal variafion new?\nIt would greatly enhance the novelty of this study if the authors could provide evidence that their findings\nare new.", "reviewer_3": "The revised manuscript has been substanfially improved and has addressed most of my comments.\nHowever, some issues sfill require further clarificafion.\nMajor comments:\n1. Representafion of the subtropical circulafion: This study uses the 850-hPa streamfuncfion to represent\nlow-level subtropical circulafion. Thus, this study may miss contribufions from divergent circulafion,\nwhich is also an important part of the monsoon system. A similar approach may apply to the 850-hPa\nvelocity potenfial to check whether robust changes are also detected in the divergent circulafion.\n2. Definifion of the intensity of Subtropical Circulafion: I think the new definifion for the intensity change\nin SC in this study is quite useful since it is independent on the domain and the seasonal evolufion of\nsubtropical circulafion. However, I am concerned about calculafing the intensity changes in global\nsubtropical circulafion. The rafio change may be too large over the region with a very small climatological\nvalue. Figure 1 indicates that large changes tend to occur over the edge region where sign changes. It\nwould be useful to show a map of the intensity change (%, shading) on top of the climatology of the 850-\nhPa streamfuncton(contour) in the historical experiment as a supplementary figure (Similar to Figure 1\nexcept for the intensity change with unit of %). The new figure will be useful to check the validity of the\ndefinifion used in this study.\n3. An expansion and shift of circulafion: As discussed in the Summary secfion, the new definifion of\nglobal subtropical circulafion cannot address an expansion or shift of circulafion, which may be more\nrelevant to society. For example, the posifive anomaly over the Arabian Sea, India, and the Bay of Bengal\ncan be seen as a huge reducfion in cyclonic circulafion, but it can also be interpreted as a northward shift\nand intensificafion of the Indian Ocean anficyclonic circulafion. This aspect may require more discussion\nin the Summary secfion.\nMinor comments:\n1. Lines 39~40: Figure 1 shows the weakening of the Australian summer monsoon and the strengthening\nof the East Asian summer monsoon (the intensificafion and eastward extension of the monsoon trough\nbut the weakening of the western North Pacific anficyclonic circulafion). However, both changes are not\nrobust with respect to the inter-model spread. Interesfingly, the text reads ‘the weakened Australian\nsummer monsoon’ and the almost unchanged East Asia summer monsoon.’ Should it be replaced with\n‘the slightly weakened Australian summer monsoon in DJF (Fig. 1a), the slightly strengthened East Asian\nsummer monsoon and the robust westward shifted North Atlanfic subtropical high in JJA (Fig. 1b)’?\n2. Lines 98-101: It is good to see results from d4PDF. Although the six SST warming pafterns are quite\nsimilar to each other, mostly having El Nino-like warming (Figure S4), the spread among SST pafterns\nseems very large (e.g., Figure 2, Figure S2, and Figure S3). There are two extremes in the result: one is\nslight intensificafion, and the other is considerable weakening, as shown in Figure 2. The extreme SC\nintensificafion is especially considerable in the Southern Hemisphere, as shown in Figure S3. It may\nbe good to check the two extremes and further discuss the role of the SST warming paftern on changes in\nsubtropical circulafion.\n3. Figure 3: It may be good to add a line for the center of the subtropical circulafion in the future."}, "author_response": "Response to reviewer #2:\nThank you for the insightful comments and detailed instructions on how to improve the\nmanuscript. The quality of the manuscript has been greatly improved based on your\ncomments. Our point-by-point reply follows, where the original comments are quoted\nin italics.\n1. The manuscript has largely been improved from its previous version by incorporating\ncomments from all the reviewers. I appreciate all the effort and work the authors have\ndone to strengthen their study. I am pleased with the revision and do not have any\ncritical comments. I only have one suggestion is to further improve their introduction\nto emphasize the significance and novelty of this study. The novelty of a paper published\nin this journal cannot be just a novel method, only if that method can be widely applied.\nThe authors would need to highlight what findings are new in their study compared to\nthe previous literature. For example, is this the first study to compare the influence of\nSST warming and CO2? Also, is the different timescales of these two impact factors\nnew finding? And is the seasonal variation new? It would greatly enhance the novelty\nof this study if the authors could provide evidence that their findings are new.\nRe: Thank you very much for recognizing our work and giving constructive suggestions\nto improve the presentation of the introduction further.\nWe have revised the last paragraph in the Introduction section to highlight our novelty\nin Lines 79–85.\nResponse to reviewer #3:\nThank you for the insightful comments and detailed instructions on how to improve the\nmanuscript. The quality of the manuscript has been greatly improved based on your\ncomments. Our point-by-point reply follows, where the original comments are quoted\nin italics.\nThe revised manuscript has been substantially improved and has addressed most of\nmy comments. However, some issues still require further clarification.\nRe: Thank you very much for recognizing our efforts. We have implemented all your\nadditional comments, which further help us improve the results and presentation.\nMajor comments:\n1. Representation of the subtropical circulation: This study uses the 850-hPa\nstreamfunction to represent low-level subtropical circulation. Thus, this study may miss\ncontributions from divergent circulation, which is also an important part of the\nmonsoon system. A similar approach may apply to the 850-hPa velocity potential to\ncheck whether robust changes are also detected in the divergent circulation.\nRe: Thank you very much for this reminder. Indeed, the divergent subtropical\ncirculation is not dispensable, particularly during the summer monsoon seasons,\nalthough the rotational component is dominant in the subtropical circulation. Here, we\nonly considered the rotational component mainly based on the progress of previous\nstudies that reasonably demonstrated a weakening of the divergent SC due to a\nweakened tropical overturning circulation (Held and Soden 2006; Tanaka et al. 2004;\nUeda et al. 2006; Vecchi and Soden 2007), whereas the change in the dominant\nrotational component of SC remains inconclusive (Cherchi et al. 2018; He et al. 2017;\nHe and Zhou 2022; Li et al. 2013; Li et al. 2012; Shaw and Voigt 2015). As a result,\nmost previous studies on the SC change focused on the dominant rotational component\nby analyzing the streamfunction of SC (He et al. 2017; Li et al. 2012; Shaw and Voigt\n2015), as examined by default in our previous manuscript.\nTo clarify this point, we have revised the introduction section in Lines 35–38 as:\n“When the divergent SC is projected to weaken due to a weakened tropical overturning\ncirculation under global warming, the change in the dominant rotational component of\nSC remains inconclusive. The projection for the rotational SC represented by 850 hPa\nstreamfunction (hereafter referred to as SC) ……”\nFollowing your suggestion, we also confirm the earlier conclusion of a decline in the\ndivergent SC resulting from the weakening of the tropical overturning circulation by\nanalyzing the velocity potential at 850 hPa. As shown in Fig. R1 (also shown in\nSupplementary Fig. 4), the divergent component of SC demonstrates a more robust\ndecrease in the SSP5-8.5 scenario, aligning with previous findings. The results support\nour present conclusion of a robust weakening in SC.\nWe have added a short paragraph in Lines 101–105 and a supplementary figure to\ndiscuss this point.\nFigure R1 As in Fig. 2, but for the divergent component of subtropical circulation\nrepresented by the velocity potential at 850 hPa in the SSP5-8.5 runs.\nReferences:\nCherchi, A., T. Ambrizzi, S. Behera, A. C. V. Freitas, Y. Morioka, and T. Zhou, 2018:\nThe response of subtropical highs to climate change. Current Climate Change\nReports, 4, 371-382, https://doi.org/10.1007/s40641-018-0114-1.\nHe, C., B. Wu, L. Zou, and T. Zhou, 2017: Responses of the summertime subtropical\nanticyclones to global warming. J. Climate, 30, 6465-6479,\nhttps://doi.org/10.1175/jcli-d-16-0529.1.\nHe, C., and T. J. Zhou, 2022: Distinct responses of North Pacific and North Atlantic\nsummertime subtropical anticyclones to global warming. J. Climate, 35, 4517-\n4532, https://doi.org/10.1175/Jcli-D-21-1024.1.\nHeld, I. M., and B. J. Soden, 2006: Robust responses of the hydrological cycle to global\nwarming. J. Climate, 19, 5686–5699, https://doi.org/10.1175/jcli3990.1.\nLi, W., L. Li, M. Ting, and Y. Liu, 2012: Intensification of northern hemisphere\nsubtropical highs in a warming climate. Nat. Geosci., 5, 830-834,\nhttps://doi.org/10.1038/ngeo1590.\nLi, W., and Coauthors, 2013: Intensification of the southern hemisphere summertime\nsubtropical anticyclones in a warming climate. Geophys. Res. Lett., 40, 5959-\n5964, https://doi.org/10.1002/2013gl058124.\nShaw, T. A., and A. Voigt, 2015: Tug of war on summertime circulation between\nradiative forcing and sea surface warming. Nat. Geosci., 8, 560-566,\nhttps://doi.org/10.1038/ngeo2449.\nTanaka, H. L., N. Ishizaki, and A. Kitoh, 2004: Trend and interannual variability of\nwalker, monsoon and Hadley circulations defined by velocity potential in the\nupper troposphere. Tellus A: Dynamic Meteorology and Oceanography, 56,\nhttps://doi.org/10.3402/tellusa.v56i3.14410.\nUeda, H., A. Iwai, K. Kuwako, and M. E. Hori, 2006: Impact of anthropogenic forcing\non the Asian summer monsoon as simulated by eight GCMs. Geophys. Res.\nLett., 33, L06703, https://doi.org/10.1029/2005gl025336.\nVecchi, G. A., and B. J. Soden, 2007: Global warming and the weakening of the tropical\ncirculation. J. Climate, 20, 4316–4340, https://doi.org/10.1175/jcli4258.1.\n2. Definition of the intensity of Subtropical Circulation: I think the new definition for\nthe intensity change in SC in this study is quite useful since it is independent on the\ndomain and the seasonal evolution of subtropical circulation. However, I am concerned\nabout calculating the intensity changes in global subtropical circulation. The ratio\nchange may be too large over the region with a very small climatological value. Figure\n1 indicates that large changes tend to occur over the edge region where sign changes.\nIt would be useful to show a map of the intensity change (%, shading) on top of the\nclimatology of the 850-hPa streamfuncton(contour) in the historical experiment as a\nsupplementary figure (Similar to Figure 1 except for the intensity change with unit\nof %). The new figure will be useful to check the validity of the definition used in this\nstudy.\nRe: Thanks for your comments. Indeed, some centers of the SC changes represented by\nstreamfunction are misaligned with those of the climatology. Moreover, the\nclimatological SC streamfunction has sign changes. As a result, the ratio change relative\nto the climatology SC is not suitable for the girds with a climatology SC close to zero.\nThat is the reason that previous studies had to select a relatively large domain to define\nthe changes and/or the percentage. The projection method in this study defining the\nintensity change is developed to avoid this issue.\nWe have rephrased the “Intensity change” section in the Methods as follows:\n“The projection method defining the percentage intensity change prevents unreasonable\nlarge values in cases where the climatological SC is close to 0, unlike the conventional\nmethod that divides changes by climatology.”\n3. An expansion and shift of circulation: As discussed in the Summary section, the new\ndefinition of global subtropical circulation cannot address an expansion or shift of\ncirculation, which may be more relevant to society. For example, the positive anomaly\nover the Arabian Sea, India, and the Bay of Bengal can be seen as a huge reduction in\ncyclonic circulation, but it can also be interpreted as a northward shift and\nintensification of the Indian Ocean anticyclonic circulation. This aspect may require\nmore discussion in the Summary section.\nRe: Following the suggestion, we have added more discussion about the limitations of\nour method in the Summary section as follows (Lines 284–288):\n“Some shifts occur in regions where multiple subsystems of SC are interconnected,\nmaking them difficult to interpret solely in terms of overall intensity change. Despite\nthis, the new method can efficiently isolate overall intensity changes in global SC or in\na relatively large domain, aiding the focus on more specific changes in regional SC in\nfuture research.”\nMinor comments:\n1. Lines 39~40: Figure 1 shows the weakening of the Australian summer monsoon and\nthe strengthening of the East Asian summer monsoon (the intensification and eastward\nextension of the monsoon trough but the weakening of the western North Pacific\nanticyclonic circulation). However, both changes are not robust with respect to the\ninter-model spread. Interestingly, the text reads ‘the weakened Australian summer\nmonsoon’ and the almost unchanged East Asia summer monsoon.’ Should it be\nreplaced with ‘the slightly weakened Australian summer monsoon in DJF (Fig. 1a), the\nslightly strengthened East Asian summer monsoon and the robust westward shifted\nNorth Atlantic subtropical high in JJA (Fig. 1b)’?\nRe: Corrected.\n2. Lines 98-101: It is good to see results from d4PDF. Although the six SST warming\npatterns are quite similar to each other, mostly having El Nino-like warming (Figure\nS4), the spread among SST patterns seems very large (e.g., Figure 2, Figure S2, and\nFigure S3). There are two extremes in the result: one is slight intensification, and the\nother is considerable weakening, as shown in Figure 2. The extreme SC\nintensification is especially considerable in the Southern Hemisphere, as shown in Figure S3. It may\nbe good to check the two extremes and further discuss the role of the SST warming\npattern on changes in subtropical circulation.\nRe: We have added more discussion on these two extreme results. The result of slightly\nintensified SC and considerably weakened SC are from HFB_4K_MI and\nHFB_4K_MR experiments, respectively. In HFB_4K_MI, a rare cooling occurs over\nthe Southern Ocean, enhancing the meridional temperature gradient and strengthening\nwesterlies (Ceppi et al. 2018). Additionally, there is a relatively weak El Niño-like\nwarming in the tropical Pacific, resulting in a weaker westerly change over the\nequatorial western Pacific. Both these features in HFB_4K_MI contribute to\nstrengthening the Southern Hemisphere SC.\nThe discussions are added in Lines 123–131 as follows:\n“We contrast two extreme results in the six d4PDF experiments: one involving slight\nintensification and the other a considerable weakening (Fig. 2), particularly in the\nSouthern Hemisphere (Supplementary Figs. 2 and 3). These results are associated with\nHFB_4K_MI (Supplementary Fig. 5d) and HFB_4K_MR (Supplementary Fig. 5f),\nrespectively. In HFB_4K_MI, a rare cooling occurs over the Southern Ocean,\nenhancing the meridional temperature gradient and strengthening westerlies.\nAdditionally, there is a relatively weak El Niño-like warming in the tropical Pacific,\nresulting in a weaker westerly change over the equatorial western Pacific. Both these\nfeatures in HFB_4K_MI contribute to strengthening the Southern Hemisphere SC.”\nReferences:\nCeppi, P., G. Zappa, T. G. Shepherd, and J. M. Gregory, 2018: Fast and slow\ncomponents of the extratropical atmospheric circulation response to CO\n2\nforcing. J. Climate, 31, 1091–1105, https://doi.org/10.1175/jcli-d-17-0323.1.\n3. Figure 3: It may be good to add a line for the center of the subtropical circulation in\nthe future.\nRe: Dashed curves for the future centers of the subtropical circulation have been added\nin Fig. 3."}]

Decision letter
REVIEWERS' COMMENTS
Reviewer #3 (Remarks to the Author):
The authors have clarified my concerns and addressed my comments well. I recommend to accept the
manuscript for publicafion.