Inter-basin contrast in the Southern Ocean warming

Domain: earth_science
Published: 2025-10-13
Editorial outcome: major_revision

Abstract
The Southern Ocean (SO) is a major contributor to global ocean heat uptake, exhibiting deep-reaching warming trends within the 35°S-55°S band. Here, we reveal a notable inter-basin contrast in the SO warming rates. Over the past six decades, the warming of the 0-700 m Atlantic-Indian sector was 40.0 ± 5.7% faster than the Pacific sector, nearly doubling at ~44°S. Ocean-only and coupled model experiments suggest that this basin-scale contrast arises from alterations in wind-driven heat redistribution rather than surface heating. Specifically, the intensification and poleward migration of westerly winds are more prominent in the Atlantic-Indian sector, leading to stronger upper-layer heat convergence. The inter-basin warming contrast is projected to persist and amplify throughout the remainder of the 21st century. This study highlights the inter-basin contrast in the past and future SO warming, with useful implications for understanding regional changes in the SO climate, ice mass, and marine ecosystems.

Introduction
The Southern Ocean (SO) has witnessed substantial warming trends since the mid-twentieth century1–6, accounting for ~60% of the global ocean heat storage in the upper 2000 m layer7,8. Notably, the rate of SO warming has also accelerated, reaching 2.72±0.29 ZJ yr⁻¹ during 1991–2019—nearly double the rate of 1.49±0.23 ZJ yr⁻¹ during 1958–19909. The SO warming has significant ramifications on sea ice10 and ice shelves11, the formation of abyssal waters12–14, marine ecosystems and biodiversity around the Antarctic15 and far-reaching impacts on climates over the globe16,17.

The SO warming characterizes a prominent meridional structure with enhanced, deep-reaching warming trends extending to 2000 m between 35°S-55°S and a warming maximum on the northern flank of the Antarctic Circumpolar Current (ACC); by contrast, the surface layer of regions south of the ACC has not warmed significantly2,5,18–20. The “added heat” mechanism—involving heat uptake through surface heat fluxes and heat transport by the mean circulation—has been used to explain the meridional structure of SO warming2,18,21. The “anthropogenic heat” enters the ocean in regions south of the ACC and flows northward through Ekman currents under westerly winds3,19,20,22. The transport of warmed water across the ACC leads to heat convergence within the 35°S-55°S band19,23 and subduction into the subsurface ocean with mode and intermediate waters24–26. The “redistributed heat” mechanism dominated by changes in ocean circulation was believed to play a secondary role in shaping this meridional structure2,19,27. Meanwhile, mesoscale and sub-mesoscale processes within the ACC partly offset the wind-driven northward heat transport28,29. Experiments based on an ocean model suggested that both winds and heat flues contribute to the enhanced heat uptake by the SO30.

Compared to the well-documented meridional structure, the zonal structure of SO warming has been less appreciated. Zonally asymmetric features were often overlooked owing to the common practice of circumpolar3,19 or regional30 mean diagnostics. A recent study based on ocean model simulations revealed complex asymmetric features in the SO warming pattern of 1979–2019 and complex forcing effects of surface winds, heat flux, and freshwater flux, with the dominant driver varying with latitude31. Yet, the results of Ref. 31. for the four-decade period contain both the externally forced long-term changes and internal variability. Particularly, pronounced decadal variability arising from Interdecadal Pacific Oscillation can cause out-of-phase heat content changes between the Pacific and Atlantic–Indian sectors of the SO32; observation-based analysis suggested that wind forcing dominated the decadal warming of the Subantarctic Mode Water during 2005–201525. Therefore, whether these recently revealed asymmetric warming patterns and the underlying mechanisms30–32 are robust features of the SO under anthropogenic greenhouse warming remain uncertain.

Here, based upon established understanding30–32, this study pursues further insights into asymmetric patterns of the SO heat storage under anthropogenic warming and the drivers. We will attempt to reveal a key asymmetric feature of anthropogenic heat storage in the SO—the notable inter-basin contrast with significantly stronger warming rates in the Atlantic and Indian sectors than the Pacific sector—consistently standing out from updated observational datasets7,33–35 since 1960 and climate model simulations under historical and 21st-century scenarios (Fig. 1). Diagnosis using large-ensemble simulations of Community Earth System Model version-1 (CESM1-LE)36 and sensitivity experiments using a standard-alone ocean model37 uniformly suggest that this pattern is primarily shaped by asymmetric changes in westerly winds under anthropogenic climate change, rather than surface heat fluxes. Specifically, strengthening and southward migration of westerly winds are systematically more prominent in the Atlantic-Indian sector, causing stronger upper-layer warming in the 35°S-55°S band of this sector through oceanic heat redistribution. These results highlight the central role of redistributed heat in shaping the zonal structure of SO warming, despite the well-established meridional structure shaped by added heat.

Results
Inter-basin warming contrast
We start by looking at the 0–700m OHC trends during 1960–2020 based on three observational datasets7,33–35 (Fig. 1a). Within the enhanced warming band of 35°S-55°S, we detect a zonal asymmetry with the Atlantic-Indian sector (defined east of 70°E and west of 150°E here) warming evidently faster than the Pacific sector (west of 70°E and east of 150°E). In the Atlantic-Indian sector, a warming maximum emerges at ~45°S along the northern flank of the ACC—a feature unseen in the Pacific sector. Strong warming trends extend down to ~1000m in the Atlantic-Indian sector but are mostly confined within the upper 300m in the Pacific (Fig. 1c). This inter-basin contrast is consistently seen across three observational datasets, despite differences in details (Supplementary Fig. 1). This contrast also exists in the intermediate layer of 700–2000m but much weaker in warming magnitudes (Supplementary Fig. 2). Given more limited observational sampling in deeper layers, hereafter we focus on the warming features in the upper 700m layers.

The ensemble-mean of CESM1-LE largely replicates the warming pattern, although the warming maximum in the Atlantic-Indian sector is weaker and shifted southward compared to observations (Fig. 1b). CESM1-LE also captures the inter-basin contrasts in both warming magnitude and vertical structure (Fig. 1d). Meanwhile, CESM1-LE overestimates the near-surface warming, likely associated with the common biases of stronger upper-ocean stratification and shallower mixed layer in coupled models38,39. The overall consensus between observations and CESM1-LE ensemble mean indicates that this inter-basin contrast in heat storage represents a robust feature of the SO under anthropogenic climate change. In fact, this contrast is discernible in most of the 40 members of CESM1-LE (not shown), reflecting the dominance of external forcing over internal variability in the SO heat storage pattern.

In the observations, the Pacific sector has an average warming rate of 0.05±0.01 K decade−1 in the 0–700m layer (Supplementary Fig. 3), corresponding to a 60-year warming of 0.30±0.04 K by 2020 relative to the 1960–1970 baseline (Fig. 1e). By contrast, the Atlantic-Indian sector warms at a rate of 0.07±0.01 K decade−1 (Supplementary Fig. 3), resulting in a 60-year warming of 0.42±0.07 K (Fig. 1f). The warming of the Atlantic-Indian sector is 46.7±10.7% than that of the Pacific sector. The weaker contrast over 1990–2020 implies the influence of decadal variability on shorter-period change (Supplementary Fig. 3). However, this contrast existed in both the Argo (2004–2020) and pre-Argo (1960–2003) periods. Notably, the Argo era witnessed accelerated warming than before in both sectors, partly owing to enhanced observational sampling that can more sufficiently resolve the warming trends. At the heat storage maximum of ~44°S, the warming of Atlantic-Indian sector was nearly twice as strong as the Pacific sector (Supplementary Fig. 4a, b). The inter-basin contrast is generally insensitive to the choice of the ending year in trend calculation. Although fluctuated by decadal variability32, the stronger warming of the Atlantic-Indian sector is a persistent feature throughout 1960–2020. In CESM1-LE simulations, the Pacific and Atlantic-Indian sectors warm at rates of 0.04±0.01 and 0.07±0.01 K decade−1, respectively, indicating an inter-basin contrast of 68.4±39.0%—greater than the observed contrast of 40.0±5.7%. This difference reflects the weaker subsurface warming in the central Pacific and stronger surface warming in the Atlantic-Indian sector in CESM1-LE ensemble-mean. This can be partly explained by decadal internal variability32 that causes warmer Pacific sector in observation than in CESM1-LE ensemble-mean since 2012 (Fig. 1e).

Surface heat uptake versus wind-driven heat redistribution
Surface heat flux-driven heat uptake and wind-driven ocean heat redistribution are two key mechanisms potentially shaping the SO heat storage pattern2,20,30–32,40–43. To explore their relative importance, we first look at trends of surface net heat flux Qnet and surface winds during 1960–2020 (Fig. 2). The trend of Qnet (Fig. 2a and Supplementary Fig. 5a) suggests prevailing surface heating in the high-latitude seas around Antarctica and heat release toward the atmosphere in the mid-latitudes north of 50°S (Supplementary Fig. 5c and Supplementary Fig. 6a). Interestingly, on average, the Pacific sector (1.08 Wm−2 decade−1) receives stronger surface heating than the Atlantic-Indian sector (0.42 Wm−2 decade−1) in the high-latitudes in ERA5. This feature is further supported by Japanese 55-year Reanalysis (JRA-55) data, with 0.57 Wm−2 decade−1 in the Pacific sector versus 0.29 Wm−2 decade−1 in the Atlantic-Indian sector. These results indicate surface heating cannot explain the asymmetric pattern of heat storage.

Westerlies over the SO have intensified during 1960–2020 (Fig. 2b), due to a combination of stratospheric ozone depletion, greenhouse gas forcing, and tropical natural variability44–48. The strengthened westerlies accompany trends of negative wind stress curls (WSCs) south of 55°S and positive WSCs north of this latitude, which enhances the northward ocean heat transport from high latitudes and heat convergence in the mid-latitudes27. Similar trend patterns are seen in the JRA-55 data (Supplementary Fig. 5). In addition to prevailing intensification across sectors, the inter-basin contrast with a stronger intensification in the Atlantic-Indian sector is significant at 90% confidence level. This contrast in surface winds caused stronger anomalous northward ocean heat transport and heat convergence north of 45°S in the Atlantic-Indian sector than in the Pacific—a candidate explanation for the inter-basin contrast in heat storage.

In CESM1-LE, there are prevailing trends of positive Qnet at all latitudes of the SO (Fig. 2c and Supplementary Fig. 6b), in contrast to the mid-latitude heat loss in ERA5 (Fig. 2a and Supplementary Fig. 6a). This discrepancy is linked to insufficient strengthening of westerly winds compared to observation-based changes and easterly trends in mid-latitude regions (Fig. 2d)—a common bias among IPCC-class climate models39,49, which reduces the total wind speed and suppresses heat release there. The underestimated heat uptake in the high latitudes is likely due to overestimated surface warming and absence of regional cooling trends around the Antarctic in models50. Nevertheless, the surface heating in the Pacific sector is also stronger than in the Atlantic-Indian sector (0.24 versus 0.16 Wm−2 decade−1), unable to account for the SO warming asymmetry. CESM1-LE also produces stronger asymmetric changes in westerly winds, although much weaker than changes in ERA5 and JRA-55.

To better visualize the inter-basin contrast, we compare meridional profiles of zonal-mean Qnet and WSC of the two sectors (Fig. 3). To account for influence of sea-ice cover, here we calculate area-weighted average Qnet only for open-ocean regions. Consistent with Fig. 2a, the Pacific sector receives stronger anomalous heat fluxes than the Atlantic-Indian sector in the high latitudes (south of 55°S) (Fig. 3a, b). The high-latitude Pacific sector loses heat to the atmosphere in the climatology, which weakened at ~1.0 Wm−2 decade−1 since 1960; meanwhile, the heat uptake in the Atlantic-Indian counterpart increased at a rate of ~0.4 Wm−2 decade−1 (Fig. 3b). In CESM1-LE, changes in Qnet can neither explain the inter-basin contrast in heat storage (Supplementary Fig. 6b and Supplementary Fig. 7a). The positive WSCs north of 55°S are enhanced in both sectors (Fig. 3c, d and Supplementary Fig. 5d). This enhancement is more pronounced in the Atlantic-Indian sector (0.61×10−8 Pa m−1 decade−1) than in the Pacific sector (0.49 × 10-8 Pa m−1 decade−1) in ERA5, which is also the case in JRA-55 (0.35× 10−8 versus 0.28 × 10-8 Pa m−1 decade−1). Notably, in the Atlantic-Indian sector, the trend indicates a poleward shift of the climatological WSC distribution51–53. Climate models tend to overestimate this trend54,55, including CESM1-LE56. To summarize, these results suggest that wind changes, rather than Qnet, can explain the observed inter-basin warming contrast in the SO.

To understand how surface wind changes affect heat storage, we calculate ocean heat transports (Methods) (Fig. 4). In the SO, the meridional ocean heat transport (MOHT) driven by westerly winds is primarily facilitated by Ekman currents in the near-surface layer19,20. The 0–100m MOHT, predominantly undertaken by Ekman currents, has strengthened in both sectors and dramatically shifted poleward in the Atlantic-Indian sector in CESM1-LE (Supplementary Fig. 7b). We further calculate the divergence of MOHT (Fig. 4a–d), i.e., ∂MOHT/∂y, which measures the heating effect exerted on the upper SO by MOHT. In the climatology, the MOHT shows divergence and convergence, indicated by positive and negative ∂MOHT/∂y values on the southern and northern flanks of the westerly maximum at ~47°S, respectively. They also correspond to climatologically positive and negative WSCs, respectively (Fig. 3c). Although the heat convergence (negative ∂MOHT/∂y) between 35°S-55°S is overall enhanced in both sectors, the convergent trends in the Atlantic-Indian sector are evidently stronger and cover a wider latitude range than those in the Pacific. Similar patterns are also seen in the heat convergence of Ekman currents, ∂MOHTE/∂y, as estimated using ERA5 winds (Fig. 4c, d). This feature is in line with the inter-basin contrast in surface wind changes (Fig. 2b, d) which shows stronger positive WSC trends and a larger poleward displacement of the WSC structure in the Atlantic-Indian sector (Fig. 3c, d).

Accompanied with heat convergence, there is prevailing downward heat flux (Methods) within the 35°–55°S band, penetrating to the subsurface ocean in both sectors (Fig. 4c, d). This downward heat flux is also evidently stronger in the Atlantic-Indian sector, particularly in the upper 1000m layer. Trends of downward heat flux also penetrate deeper in the Atlantic-Indian sector than in the Pacific sector, consistent with the inter-basin contrast in the vertical structure of warming (Fig. 1d). Overall, these results highlight the essence of the time-varying wind-driven upper-ocean circulation in shaping the salient inter-basin contrast in the SO heat storage.

Zonal exchange between the two sectors may also contribute to the asymmetric warming features25. To examine this process, we further calculate the zonal ocean heat transport (ZOHT) integrated from 35°S to Antarctic coasts using CESM1-LE. The ZOHT from the Indian Ocean to the Pacific and that from the Pacific to the Atlantic are shown at 150°E, south of Australia, and at 70°W, within the Drake Passage, respectively (Supplementary Fig. 8). In CESM1-LE, the ZOHT has increased at both 150°E and 70°W, reflecting the acceleration of some regions in the north of the ACC57 (although not through Drake Passage) and warming of the transported waters carried by the ACC. Since 1960, the heat transport from the Indian Ocean to the Pacific sector has increased by 1.1 × 1013 W decade−1, slower than the heat transport increase of 1.7 × 1013 W decade−1 at Drake Passage, with the difference significant at the 90% confidence level. This suggests a net heat loss of the Pacific sector to the Atlantic-Indian sector through ZOHT. The increase in heat transport has not been confirmed by in-situ observations within the Drake Passage5, possibly owing to insufficient sampling in spatial and temporal coverage of data. In addition, changes in CESM1-LE represent externally forced trends that are still undetectable in the short observational records containing internal variability.

Insights from ocean model experiments
Analysis of observational data and CESM1-LE presented above overall points to the essence of wind-driven heat redistribution in shaping the zonal asymmetry of the SO heat storage pattern. To validate this conclusion and gain further insights, we employ stand-alone ocean models to perform simulations and experiments (Methods). The control simulation (CTRL) of the LASG/IAP Climate Ocean Model (LICOM) version-337 forced with daily surface atmospheric fields of ERA558 has well reproduced the observed heat storage pattern in the SO of 1960-2020, showing enhanced warming within the mid-latitude band of the Atlantic-Indian sector (Fig. 5a), albeit with stronger trends. Compared to the observed structure, the simulated warming band is shifted equatorward, and the heat storage near the western boundary of Pacific sector is overestimated. These discrepancies are likely associated with coarse model resolution (1°) which leads to underestimation of mesoscale eddies. The poleward heat transport induced by eddies act to compensate the wind-driven heat transport and the resultant heat convergence. Therefore, we adopt the central-to-eastern Pacific region of 180°-70°E, 35°S-50°S to quantify its weaker warming trend (0.06 K decade−1) than that of the Atlantic-Indian sector (70°E-150°E, 35°S–-50°S; 0.09 K decade−1) (Fig. 5b). A simulation with the Massachusetts Institute of Technology General Circulation Model (MITgcm)59 forced by ERA5 fields also capture the inter-basin contrast in the SO warming (Supplementary Fig. 9). MITgcm provides an additional support of surface atmospheric changes in shaping the inter-basin contrast. The inter-basin contrast in LICOM and MITgcm is weaker than in observation and CESM1-LE, possibly owing to the lack of air-sea coupling in ocean-only models.

Then, we carried out two sensitivity experiments using LICOM to separate the effects of surface heating and wind forcing (Methods). The heat flux (HTFL) or wind-forcing (WND) simulations retain temporal changes only in surface heat fluxes or winds, respectively, while keeping other forcing fields invariant. The sum of HTFL and WND (Fig. 5c, d) closely resembles CTRL in warming pattern, indicating a negligible nonlinear effect between surface heating and wind-forcing and the robustness of our diagnoses. HTFL represents the storage of added heat41-with warming signatures generated by surface heating and transported by the climatological ocean circulation. This experiment produces quasi-uniform warming over the 35°S-50°S band (Fig. 5e, f). Note that the warming of the Atlantic-Indian sector is slightly weaker than the Pacific sector, 0.05 versus 0.06 K decade−1, owing to inhomogeneous surface heating (Figs. 2 and 3). The results of HTFL confirm that surface heat fluxes cannot account for the observed inter-basin contrast, although heat fluxes can cause some symmetric features in heat storage19,20.

WND largely represent the storage of redistributed heat41 with warming signatures arising from heat redistribution induced by changes in wind-driven ocean circulations. This experiment produces enhanced heat pile-up in the 35°S-50°S band of the Atlantic-Indian Ocean sector (0.03 K decade−1) without additional heat uptake (Fig. 5g, h), in contrast to insignificant trends in the central-to-eastern Pacific sector. The results of WND confirm that the inter-basin contrast in the SO heat storage results mainly from wind-driven upper-ocean heat redistribution. Note that the effect of mode water subduction19,25 is also contained in WND, given that key factors determining the subduction rate, such as Ekman pumping and the mixed layer depth, are primarily controlled by winds. There are cooling trends in many regions in WND, which reflects a vertical redistribution of heat: enhanced northward transport under westerly winds brings cold waters from higher latitudes, while the downwelling moves upper-layer warm waters to the deep (Supplementary Fig. 10). LICOM experiments indicate that wind-driven heat redistribution also plays a role in shaping the meridional structure of warming, in addition to the zonal asymmetry (Supplementary Fig. 11). Specifically, while surface heating leads to prevailing warming in the 35°S-55°S band3,19,20,22, the intensified westerly winds are essential for the warming maximum at ~40°S (at ~44°S in observation) in the Atlantic-Indian sector (Supplementary Fig. 11a).

Methods
Observational datasets
We use three observational ocean datasets for the 1960-2020 period: the Institute of Atmospheric Physics (IAP) ocean analysis data provided by the Chinese Academy of Sciences7,63, the EN4.2.1 from Met Office Hadley Centre35,64, and the 2017 version of Ishii data from Meteorological Research Institute, Japan Meteorological Agency34. All the three datasets provide monthly fields of analyzed ocean temperature on standard levels with 1°×1° horizontal resolutions. These datasets are based on all-available historical in-situ ocean temperature data and constructed in different quality control and gap-filling methods. Usage of the three datasets allows for a comparative analysis. In this study, the linear trend of the 0–700m ocean heat content (OHC) is computed for the 1960−2020 period, as a quantification of the upper-ocean heat storage. The 0−700m OHC is calculated as

OHC = \int_{-700}^{0} \rho_0 c_p \theta dz

where θ is potential temperature, ρ0 = 1025 kg m−3 is the seawater density, and cp = 3996 J K−1 kg−1 is the specific heat capacity of seawater. Monthly data of surface heat fluxes and winds of 1960–2020 are primarily derived from ERA558, while JRA-5565,66 is used as a supplementary dataset to examine the robustness of the results. The wind stress curl is calculated as ∂xτy − ∂yτx, where τx and τy are zonal and meridional wind stress, respectively. The Ekman transport plays a critical role in northward heat transport in the upper SO3,19,20, whose vertical component is inversely proportional to the wind stress curl at a fixed latitude.

Climate model simulations
The large ensemble of the Community Earth System Model version 1 (CESM1-LE) provides a 40-member run of coupled simulations with distinct initial conditions and forced by the same radiative forcing scenario36. Here we analyze historical run from 1920 to 2005 and a future run under the Representative Concentration Pathway (RCP) 8.5 emission scenario from 2006 to 2100. The ocean component of CESM1 adopt the Parallel Ocean Program (POP) in 1°×1° resolutions. Here, we analyze monthly data of temperature and ocean horizonal and vertical velocities (including Eulerian mean, mesoscale, and sub-mesoscale processes). The ensemble-mean of CESM1-LE represents externally forced changes by anthropogenic greenhouse gases, aerosols, and ozone, while the spread of model members measures the effect of internal natural variability.

Coupled Model Intercomparison Project phase 6 (CMIP6)67 provides more comprehensive simulations from various coupled models, under historical forcing of 1850-2014 and three Shared Socioeconomic Pathway (SSP) scenarios (1-2.6, 2-4.5, and 5-8.5) of 2015-2100. The projected results from the multi-model ensemble mean of 13 CMIP6 models (ACCESS-CM2, BCC-CSM2-MR, CAMS-CSM1-0, CESM2-WACCM, CIESM, EC-Earth3, FGOALS, FIO-ESM, GFDL-CM4, MIROC6, MPI-ESM1-2-LR, MRI-ESM2-0, NorESM2-MM) reveals effects of anthropogenic greenhouse warming. We interpolated model outputs to a regular 1°×1° horizontal grids and analyze annual-mean data of ocean temperature.

Ocean model simulations
Forced ocean models can be utilized to understand specific oceanic responses to global warming-induced changes in surface net heat flux and wind stress via sensitivity experiments. The LASG/IAP Climate Ocean Model (LICOM) version 337 is developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) of the IAP. LICOM features a horizontal resolution of ~1°×1° and 30 vertical layers extending from the surface to 5500m. Surface atmospheric forcing is computed from ERA5 using bulk formula, including surface winds, surface shortwave and longwave radiation, near-surface air temperature, specific humidity, precipitation, air density, and sea level pressure. Forced by the repeated daily forcing of 1941–1950, LICOM is spun up for 500 years to reach a quasi-equilibrium state of the ocean. Following the spin-up, the control run (CTRL) is forced by all atmospheric fields from 1940 to 2020 (Lu et al. 2024).

To investigated distinct roles of surface heat flux and surface winds in OHC change, we designed two sensitivity experiments. The heat flux experiment (HTFL) aims to investigate the effect of surface heat uptake, using the same wind stress and precipitation as the spin-up, with other variables matching those in CTRL. The winds experiment (WND) can isolate the role of wind-driven heat redistribution in the SO warming, which uses the same surface wind stress as CTRL, while other variables are kept consistent with the spin-up. Although these idealized experiments are subject to limitations, such as prescribed surface heat flux in WND may inhibit radiative and turbulent feedback associated with SST changes, they enable us to separately examine the impacts of surface heat uptake and wind-driven redistribution on SO warming, providing insights into the mechanisms driving observed changes in this region.

To confirm the simulation of LICOM, we also performed a simulation with the Massachusetts Institute of Technology General Circulation Model (MITgcm)59 Ocean Comprehensible Atlas 2 (OCCA2)68 also forced with ERA5 reanalysis from 1960 to 2024. The simulation is based on the same model configuration as Estimating the Circulation and Climate of the Ocean Version 4 (ECCO4) release 269 with a nominal horizontal resolution of 1 degree. The only differences compared with ECCO4 release 2 are the initial conditions and forcing fields. Atmosphere forcing was replaced with ERA5 plus climatological monthly mean adjustment, which was calculated to match ECCO4 release 2 forcing over 1992-2011. Initial conditions from 1992 were used to carry out a ten-year spin-up from 1960 to 1970. The result for January 1st 1970 was then used to initialize the OCCA2 simulation which starts January 1st 1960.

Heat transports
The meridional ocean heat transport (MOHT) in the 0–100m layer (almost the Ekman layer) is calculated as

MOHT = \int_{-100}^{0} \int_{x1}^{x2} \rho_0 c_p v \theta dx dz

where v is the “residual” meridional velocity2,70 in CESM1-LE, including contributions from Eulerian mean circulation, mesoscale eddies, and sub-mesoscale processes. The meridional gradient of MOHT, ∂MOHT/∂y, represents divergent cooling (positive) and convergent heating (negative) induced by MOHT. The estimated Ekman MOHT based on τx in ERA5 and temperature in IAP is calculated as

MOHT_E = \int_{-100}^{0} \int_{x1}^{x2} \rho_0 c_p \frac{\tau_x}{f \rho_0} [\theta] dx

where [θ] is the vertical mean temperature of 0–100m. The vertical heat flux is computed as ρ0 cp wθ, where w is the residual vertical velocity in CESM1-LE. All zonal mean figures in this paper are divided by corresponding zonal distance (m−1) to properly compare property differences in the Atlantic-Indian and Pacific sectors. Zonal ocean heat transport (ZOHT) at a given longitude is calculated as

ZOHT = \int_{-700}^{0} \int_{y1}^{y2} \rho_0 c_p u \theta dy dz

where u is the residual zonal velocity in CESM-LE. We calculate the ZOHT of the 0-700m, 90°S-35°S sections at 150°E and 70°W, to represent the heat transport into and out of the Pacific sector, respectively.

Data availability
The minimum dataset required to interpret, verify, and extend the results of this article is available from (https://doi.org/10.5281/zenodo.16116786). The observational and reanalysis data used in this study are publicly available. IAP data are available at (http://www.ocean.iap.ac.cn). EN4.2.1 data are available at (https://www.metoffice.gov.uk/hadobs/en4/). Ishii data are available at (https://www.data.jma.go.jp/kaiyou/english/ohc/ohc_data_en.html). ERA5 reanalysis data were obtained from the ECMWF data server (https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5). CESM1-LE simulations can be found at (https://www.cesm.ucar.edu/community-projects/lens/datasets). LICOM and MITgcm outputs are available upon request. CMIP6 simulations can be obtained from (https://esgf-node.ipsl.upmc.fr/projects/cmip6-ipsl/).

Code availability
The LICOM source code is available at (https://github.com/yongqiangyu/FGOALS.git); The data generated in this study have been deposited in the Zenodo database under accession MATLAB code: (https://doi.org/10.5281/zenodo.16116786).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Inter-basin contrast in the Southern Ocean warming by Song et al.\nThis paper looks at historical data and the CSESM1-LE, along with some ancillary models, to examine the difference in\nocean basin heat content and temperature change in the Southern Ocean. They show that in the observations and CSEM1\nover the historical period 1960-2020 that there is a statistically significant (just) difference in modelled heat uptake by these\nregions. They argue that these differences are primarily due to differences in the regional winds, specifically wind stress curl,\ndriving differential heating. This is further backed up with a model experiment that separates the heat flux and wind impacts\non ocean heat content.\nOverall, the authors do a reasonable job in delivering this analysis, but I am unsure exactly how novel the results are.\nOverall the idea that the winds are primarily driving ocean heat uptake is well supported by previous observational studies\n(eg Gao et al., 2018) and both Huguenin et al. (2022) and Hague et al., (2024) explicitly run experiments on wind vs surface\nfluxes to determine their roles in driving ocean heat uptake and the basin asymmetry therein. Whilst Hague et al., 2024 is\nnoted in the introduction Huguenin is not, despite its clear relevance to the study. Both could be considerably more heavily\nreferenced in the model forcing experiments, notably to compare the basin wide differences, but this is not done.\nThe authors are clearly aware of the overlap, as they explicitly mention Hague et al., in the introduction, but go onto say that:\n“the inter-basin contrast in the SO heat storage under anthropogenic warming and the underlying drivers remain\nunexplored.”. However only a small portion in the conclusions of this work look at future forcing, and in a fairly superficial\nway, whilst the majority of the analysis focuses on the basin wide warming differences that largely recover similar results to\nHuguenin and Hague studies – Hague et al. in particular break down their results by basin and with even more granularity in\nforcing and latitude domain.\nI think therefore that whilst the findings of this work are supported (in general) by the evidence provided, they are perhaps\nmore of confirmation of previous findings with a greater emphasis on the Indo-Atlantic vs Pacific. I think for Nature family\njournals the authors need to make more effort to demonstrate where the fundamentally new results lie – as it isn’t terribly\nobvious to me.\nScientifically there are some issues that I lay out in detail below, largely to do with questions around the (not observationally\nsupported) transport of heat from the Pacific to the Atlantic, and what implications this may have for the basin differences in\nwarming, as well as some questions about the robustness or representativeness of the single model used as the basis for\nmost result (large ensemble notwithstanding) when the two ancillary models presented give much more statistically vague\nresults for the differences between basins.\nFinally, I think the authors need to considerably rework the conclusions section, where they make a number of poorly\nsupported claims around the implications of their work, with sweeping claims made on the basis of unexplored figures, or\nreferences that do not support (or only qualitatively so) the statements being made.\nOverall the results appear to me to be valid, but perhaps not sufficiently differentiated from several recent papers, and there\nare numerous smaller issues with the analysis and presentation that must be addressed to be publishable. I leave the\nnovelty issue to the editor, but the other issues probably qualify as major changes before publication.\nLine by line comments:\n78: Doesn’t reference 44 deal explicitly with zonal heat distribution and warming?\n93: IAP, EN4, Ishii aren’t explained prior to this figure.\n108: Salient is a slightly odd word choice here.\nFigure 1: CF gao, who note most warming in pacific? SAMW/AAIW export regions\n117: Most climate models overestimate surface warming, often due to SW issues, and often interior, notably CDW warming\ntoo. How does CESM do in this regard, does this impact results? Is it a constant offset, or have its own assym?\n135: The uncertainty means there is almost an overlap in the two ranges. In observations the pacific in particular is heavily\nunderrepresented. How do the trends compare for the comparatively evenly sampled Argo period (~ 2004- present?).\n138: Does it fluctuate in your results too? How much by, again, what confidence can be given prior to Argo?\n145: What is the MLD contrast like in the model vs obs in these regions. SAMW subduction is a primary mechanism for heat\nuptake, and is sensitive to many processes, often poorly represented in models. Does the relative depth/stratification give\nsome clue as to why this asymmetry is stronger in the model?\n160: What is the integrated heat fluxes over the areas – the qualitative spatial patterns seem consistent with your inference,\nbut is this backed up quantitatively in terms of heat uptake by each sector?\n164-165: This is a bit vague, where is south and north when talking about WSC here?\n168: Again, please be quantitative – how much stronger is the wind shift/WSC change in the Atlantic/Indian sector vs the\nPacific.\n172: Broadly consistent is possibly a bit strong, particularly for the heat fluxes. Whilst if you interpret it very generously you\nmight convince yourself that there is a northern cooling and southern warming in the heat flux, it really is nowhere near as\nstrong as in the obs, nor have a similar regional structure. I think this needs to be more carefully treated, and how this poor\nrepresentation may impact the conclusions drawn between model/obs comparisons.\n194: ‘..in the climatology..’ here and elsewhere, ‘the’ is missing in relation to many references to the climatology.\n197: I don’t understand where the weakening by 1.08 Wm-2 came from, this appears to be strengthening by roughly this\namount in the Pacific trend in the high latitudes in figure 3a). Also, how are these values calculated? Are they area weighted\naverages south of some latitude, with things like sea ice accounted for?\n197: I don’t think these are particularly similar…there are significant qualitative and quantitative differences between the two\ndatasets. You need to note these and explain why they do/don’t matter for your subsequent conclusions.\n226-227: Is this pattern supported by the observational data? Obviously, it is harder to assess ocean heat transport from\nobservations, but the Ekman transport and surface heat content over the Argo period should be sufficient to assess the last\ntwo decades of upper ocean MOHT and trends – with the caveats that eddy transports etc will not be captured. However if it\nis primarily wind driven as you suggest the same features should be present.\n231-238: Can you provide comment on why the vertical heat flux trend in the Atlantic-Indian appears to align well with the\nregional divergence trend, but this in not the case in the Pacific, where there is significant positive vertical heat flux where\nthere is apparently a divergence trend around 55S, and the strongest convergence trend is associated with the weakest\nvertical heat flux?\n239: If wind change dominate ocean heat uptake, then the relative difference in surface heat fluxes is probably irrelevant to\nthe relative heating of the two regions, as the Pacific is also taking up extra wind pumped heat, just at a lesser rate. As also\nnoted in Gao et al., 2018, it is the winds that dominate heat uptake in both region, and so probably also dominates the extra\nheat that may be exported from the Pacific to Atlantic sectors.\n241: Or an export to the north from the Pacific (or much less likely, to the Antarctic coast)? How do you exclude these\npossibilities – it is a region of significant mode water formation and export after all.\n247: The transport and temperatures through Drake passage hasn’t increased significantly, e.g. as referenced in your\nreference 67, though the zonal transport to the north of the ACC has. It may have done in the model (you don’t specify or\nquantify this), but this isn’t reflective of the observations and so should be noted. As you are using the CESM1, as ref 67 (Shi\net al., 2021) does, the current change in Drake passage is inconsistent with observations – again to be noted.\n250: This seems reasonable, but without looking at the other three sides of the Pacific box budget (you only look down to\n700 m), it is only suggestive of a reason – and again, is one that isn’t strongly supported by the observations. This also\nneeds addressing in figure 6, which shows significant heat going through drake passage – again, not supported by\nobservations. To what extent does this heat contribute to the net difference in heat content/mean temperature difference in\nthe two basins? Extended data figure 5 suggests 0.02 PW, but how does this translate to the temperature differences\nbetween the basins in for eg Figure 1e/f? Is it a significant amount that may account for the differences? If so, then the fact\nthat it isn’t consistent with the observations may challenge the entire thesis of the paper, and mean that it is more of an issue\nwiht CESM1 than a representative finding. If it is not sufficient to make a big hemispheric difference, then it should be noted\nas such and probably removed from Figure 6 as an important feature. Overall I think this uncertainty makes a case for a more\ncomprehensive budget analysis of the boxes to be mad.\n251-253: This sentence doesn’t make sense. You have previously said the difference was due to surface fluxes. Similarly,\nyou would think generally that if the Atlantic-Indian basin was taking up heat faster than the Pacific due to wind\nconvergence, then there would be more heat available there to flux zonally to the Pacific. The obvious answer would be that\nthe Atlantic-Indian basin is fluxing the heat to the north via the subtropical gyre, but nowhere do you suggest this or calculate\nit.\n263: ‘stand-alone’\n270: Worth noting that the LICOM appears to be somewhat warmer than the observations, and especially the CESM1,\nparticularly in the Pacific.\n271: There does appear to be inter model sensitivity to the Pacific and Atlantic-Indian split though – extended data 6 shows\nthat in LICOM the Pacific actually warms slightly more in total, whilst the MITgcm shows that the two splits are probably\nbarely statistically different. How are the trend lines fitted here? Both of these models appear to have much less basin\ndifference than either the observations or CESM1 (eg fig 1), and should be acknowledged. Also, ‘further confirms the\ndeterministic’ is a poor choice of words. It doesn’t confirm anything, but it does add support to the hypothesis; and I should\nhope the ocean state is deterministic, otherwise we’re all out of a job. This should be changed.\n286: Most of the change is concentrated in the Atlantic sector – do you know why this is? Similarly there seems to be some\nweakness in the indian sector uptake of HTFL + WND compared with the CTRL – suggesting at least some of the enhanced\natlantic-indian uptake is a combination of the effects, which makes intuitive sense. This agrees with for eg Armour et al 2016\nwhere it is a combination of enhanced surface heat uptake and downward mode water pumping (also Gao et al 2018) that\nresults in increased heat storage on the north of the ACC.\nIt is worth noting some of the issues that these experiments presumably have: ie changes in surface SST due to heat\nredistribution by the winds won’t lead to changes in long wave outgoing heat (as the net heat flux is fixed I believe in WND),\nand so presumably there are biases induced here. Similarly the WND experiment won’t result in enhanced turbulent heat\nfluxes (I imagine based on the methods, it doesn’t specify) so these effects aren’t captured either. It would be worth\nexpanding on these a little, though I agree the HTFL + WND does look somewhat similar to CTRL.\n310: This is a bit of a stretch. The wind induced warming is well to the north of regions where Antarctic krill may be expected\n(fisheries concentrate around the peninsula). I suggest removing this statement.\n318: This statement isn’t backed up by by ref 41 (Deng et al., 2022). They focus on future wind scenarios, and importantly\nstate “The projected changes in the SHWs are basically consistent among the three ocean basins”. This may not be the\ncase for historical winds, but they do not show this. They also do not make statements regarding drivers of asymmetry as far\nas I can see, so this should be removed. The other reference is from 2000, and does not directly pertain to trends in the\nwinds. I think more appropriate references should be found, or this section deleted.\n323-324: This is a surprising statement to include at this point. Topography is no doubt incredibly important to the structure of\nthe ACC and westerlies, but not the only factor. This statement is made without much justification (particularly its absolute\nnature) and the supplementary figure does not do much to support it except in the most hand wavy way. I think references\nneed to be given to demonstrate the dominance of topography in the wind stress, curl and surface velocities, or again,\nremove or strongly moderate this statement.\n332: Whilst this appears consistent in the CESM1-LE model, only one reference is given to suggest that this is consistent\nacross the model ensemble. Ref 41 suggests that wind changes are faily even across basins, and otherwise only looks at\nSSTs. Qualitatively there is a pattern in the stronger scenarios that suggests surface warming may have a basin difference,\nbut this is not stated explicitly in the paper, nor is it the same metric as extended data figure 10. Again, I think more caution is\nneeded in these statements, or more appropriate references need to be found. This paper (and possibly the qualitative\ninterpretation of their figure 5) also seems to be sole basis for the rest of this paragraph too, see below.\n336: This paper explicitly says that there aren’t notable basin differences in the winds under strong scenarios. e.g they state:\n“The projected changes in the SHWs are basically consistent among the three ocean basins (Fig. 2).”", "reviewer_2": "This is an interesting paper which seeks to explain the drivers behind the asymmetry in multi-decadal warming of the upper Southern Ocean between the Atlantic-Indian and Pacific sectors. The primary conclusion is that a change in wind forcing rather than air-sea heat flux is the main driver. This would be a significant result, if fully substantiated, that merits publication in Nature Comms. However, the study at present relies too much on qualitative interpretation of figures that is not sufficiently supported by quantitative analysis that would support the claims being made. Furthermore, the observation based heat flux vs wind stress results are dependent on a single reanalysis (ERA5) and thus lack robustness. If the authors, can support their results with further quantitative analysis and the inclusion of results from an additional reanalysis (e.g. MERRA2,JRA) as detailed below then their manuscript may become suitable for publication. I strongly encourage them to do this as I think they would then have a substantial paper that would attract widespread interest. Given the amount of work required my recommendation is major revision.\nRequired changes:\n1) Abstract and various places in main text. ‘Over the past six decades (1960-2020), the 0-700 m warming in the Atlantic-\nIndian sector has been 40.0 ± 5.7% faster than in the Pacific sector, nearly doubling the rate at the warming maximum of\n~44°S.’ The authors need to discuss and acknowledge in the main text that the number of ocean temperature observations\nthat go into the various products they analyse (e.g. EN4) have increased massively through the 60 year period considered.\nThere are very few observations prior to 1990 and a major increase since the early 2000s because of Argo. Would they\nreach the same conclusions if they considered the relatively well observed period 1990-2020?\n2) lines 71-73. ‘Driven by asymmetric changes in surface winds, the Pacific and the Atlantic-Indian sectors exhibit out-of-\nphase variations in the upper-layer heat content on decadal timescales43.’ Is this perhaps related also to an asymmetry in\nnet heat flux between the Pacific and the Atlantic-Indian sectors?\n3) line 83. The authors focus on 0-700 m but what about 700-2000 m? Please comment on what’s happening at lower levels.\nAlso, how are the 0-700m results for OHC asymmetry different from earlier studies that found asymmetric warming and why?\nThe reason I’m asking is that it’s difficult to tell whether the OHC asymmetry results from observations are an incremental\nadvance or a significant development from what has previously been published?\n4) lines 85-87. Can the authors comment more on why the CESM warming is confined to a shallower layer than\nobservations? And why is it stronger than in observations? Is it simply because an external warming signal is confined to a\nshallower layer? Also, do they have any insights into whether other CMIP-class models show similar behaviour to CESM?\n5) lines 90-93. ‘This pattern is primarily driven by asymmetric changes in westerly winds, rather than variations in surface\nheat fluxes. While the added heat dominates the meridional distribution, the redistributed heat plays a central role in shaping\nthe zonal structure of SO warming.’ This is only an assertion at this point in the paper and should be prefixed by ‘We will\nattempt to show that this pattern is primarily driven … ‘.\n6) line 132. ‘much weaker below 700 m’ Could the much weaker behaviour at depth be due to lack of observations at depth\nobscuring a stronger trend?\n7) line 138. Should 0.37 be 0.42? i.e. 60 yr warming should be 6 x the decadal warming of 0.07 K per decade?\n8) line 142. ‘100+/-60%’ this result appears to be insignificant at the two standard deviation level. So, is it worth including?\n9) lines 162-167. These qualitative statements need to be backed up by some quantitative analysis. The asymmetric pattern\nof heat storage in Fig.1 refers to two boxed regions. So, what are the trends in heat flux in these two boxed regions?\nAlso, what does a time series of heat flux look like in the two regions? An extra two panels similar to Fig.1e-f need to be added to\nFig.2 but showing heat flux in the Pacific and Atlantic-Indian sectors (35-55S) from observations and models. These figures\nand quantitative analysis are needed to support the assertion that surface heating cannot explain the asymmetric pattern of\nheat storage in the two sectors.\nNote: I can see that some time series are shown subsequently in Fig.3. but, crucially,\nthe mid-latitudes heat flux time series is not shown and needs to be included as suggested here.\n10) lines 162-167. Also, only one reanalysis is considered (ERA5); the results need to be extended to include other\nreanalyses (e.g. MERRA5, JRA) to determine the robustness of the heat flux trends.\n11) lines 173-174. ‘they are significantly stronger in the Atlantic-Indian sector than in the Pacific, highlighting a prominent\ninter-basin contrast.’ Again, this qualitative assertion needs to be backed up by quantitative analysis of the change in surface\nwind stress magnitude averaged over the two sectors. How much stronger is the Atlantic-Indian sector wind stress change\nthan the Pacific sector? And by what level of significance testing are they stronger? 95%? Two further panels are needed\nshowing the wind stress magnitude time series for each sector for ERA5 and JRA, MERRA2. Overall, the statements here\non heat flux and wind stress contributions are too superficial and lacking in quantitative analysis to merit publication.\n12) line 179.’ showing an evident inter-basin contrast in winds rather than Qnet.’ Again, support this assertion with box-\naveraged numbers.\n13) line 202. ‘heat uptake’ should be ‘heat loss’?\n14) lines 203-204. ‘CESM1-LE shows similar results to ERA5 (Extended Data Fig. 4a).’ This statement is not supported by a\ncomparison of the figures, Fig,4a shows weak positive trends in the mid-latitudes while Figure.3a shows increasingly strong\nnegative trends. This disagreement needs to be noted and raises concerns about the robustness\nof the mid-latitude heat flux trend conclusions as the reanalysis data and the model trends differ in mid-latitudes.\n15) lines 245-247. ‘The Pacific sector has received a stronger surface heating than the Atlantic-Indian sector, particularly in\nthe high latitudes (Fig. 3a, b), which, however, does not lead to a stronger heat storage there. This indicates a heat\nexchange between the two sectors.’ It looks like the surface heating is only stronger in the Pacific at high latitudes and is not\nstronger in the mid-latitudes. So there’s no requirement for heat exchange between the two sectors in the mid-latitudes. The\nauthors need to make this point or show quantitatively that the Pacific mid-latitude sector surface heating is stronger than the\nAtlantic-Indian sector.\n16) lines 253-255. ‘Since 1960, the heat transport from the Indian Ocean to the Pacific sector has increased by 1.1 × 1013\nW/decade, slower than the heat transport increase of 1.7 × 1013 W/decade at Drake Passage.’ Are these numbers\nsignificantly different? Also, the time series are flat from 1960-1990 so fitting linear trends to the whole period may not be\nappropriate. Please comment.\n17) lines 292-294. ‘Therefore, the zonal asymmetry in SO heat storage pattern – with the Atlantic-Indian sector warming\nmore rapidly than the Pacific – results mainly from wind-driven upper-ocean circulation changes. ‘ I’m not fully convinced by\nthis as the WND experiment heat storage pattern shows heating confined to a relatively small region of the Atlantic-Indian\nsector primarily in the Atlantic sector N of 45 S. Need to see a version of Figs 1e-f but with each of the 4 model experiment\ntime series shown in order to be convinced that wind-driven changes are the primary factor. So, please include these panels\non the bottom of Fig.5\n18) lines 340-343.’ Under high and middle emission scenarios, the intensification and poleward migration of westerlies are\nprojected to be more prominent in the Atlantic-Indian sector than in the Pacific sector, posing a greater threat to marine\necosystems and ice sheets there.’ Please add a reference to support this statement."}, "author_response": "Response to Review\nInter-basin contrast in the Southern Ocean warming\nYuanyuan Song1,2, Yuanlong Li1,3*, Gaël Forget2, Aixue Hu4, Qian Li2, Jia-Rui Shi5,6,\nXiaodan Chen7, Kai Ge1, Fan Wang1,3\n1Key Laboratory of Ocean Observation and Forecasting and Key Laboratory of Ocean\nCirculation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao,\nChina\n2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of\nTechnology, Cambridge, MA, USA\n3Laboratory for Ocean Dynamics and Climate, Qingdao Marine Science and Technology\nCenter, Qingdao, China\n4Climate and Global Dynamics Laboratory, National Center for Atmospheric Research,\nBoulder, CO, USA\n5Woods Hole Oceanographic Institution, Woods Hole, MA, USA\n6Courant Institute of Mathematical Sciences, New York University\n7Department of Atmospheric and Oceanic Sciences, Institute of Atmospheric Sciences, Fudan\nUniversity, Shanghai, China\nRevision Submission: July 18th, 2025\nRevision Due: July 19th, 2025\nReviewer #1\nWe are grateful to this reviewer for reading our paper and providing important and\nvaluable comments and suggestions. We are glad to have the opportunity to address\nthese comments and improve the quality of this work by highlighting the novelty and\ndifferences from previous literature and adding more quantitative analysis. After a\nthoughtfully consideration, we made concrete modifications in the paper. We hope the\nrevised paper could be more acceptable.\nThe point-to-point responses to Reviewer #1’s comments are given below:\nResponse to general comments:\nOverall, the authors do a reasonable job in delivering this analysis, but I am unsure\nexactly how novel the results are. Overall the idea that the winds are primarily driving\nocean heat uptake is well supported by previous observational studies (eg Gao et al.,\n2018) and both Huguenin et al. (2022) and Hague et al., (2024) explicitly run\nexperiments on wind vs surface fluxes to determine their roles in driving ocean heat\nuptake and the basin asymmetry therein. Whilst Hague et al., 2024 is noted in the\nintroduction Huguenin is not, despite its clear relevance to the study. Both could be\nconsiderably more heavily referenced in the model forcing experiments, notably to\ncompare the basin wide differences, but this is not done.\nThe authors are clearly aware of the overlap, as they explicitly mention Hague et al., in\nthe introduction, but go onto say that: “the inter-basin contrast in the SO heat storage\nunder anthropogenic warming and the underlying drivers remain unexplored.”.\nHowever only a small portion in the conclusions of this work look at future forcing,\nand in a fairly superficial way, whilst the majority of the analysis focuses on the basin\nwide warming differences that largely recover similar results to Huguenin and Hague\nstudies – Hague et al. in particular break down their results by basin and with even more\ngranularity in forcing and latitude domain.\nI think therefore that whilst the findings of this work are supported (in general) by the\nevidence provided, they are perhaps more of confirmation of previous findings with a\ngreater emphasis on the Indo-Atlantic vs Pacific. I think for Nature family journals the\nauthors need to demonstrate where the fundamentally new results lie – as it isn’t terribly\nobvious to me.\nScientifically there are some issues that I lay out in detail below, largely to do with\nquestions around the (not observationally supported) transport of heat from the Pacific\nto the Atlantic, and what implications this may have for the basin differences in\nwarming, as well as some questions about the robustness or representativeness of the\nsingle model used as the basis for most result (large ensemble notwithstanding) when\nthe two ancillary models presented give much more statistically vague results for the\ndifferences between basins.\nFinally, I think the authors need to considerably rework the conclusions section, where\nthey make a number of poorly supported claims around the implications of their work,\nwith sweeping claims made on the basis of unexplored figures, or references that do not\nsupport (or only qualitatively so) the statements being made.\nOverall the results appear to me to be valid, but perhaps not sufficiently differentiated\nfrom several recent papers, and there are numerous smaller issues with the analysis and\npresentation that must be addressed to be publishable. I leave the novelty issue to the\neditor, but the other issues probably qualify as major changes before publication.\nResponse:\nWe sincerely thank the Reviewer #1 for thoughtful comments and recognize that the\nnovelty and significant differences from existing studies should be stated clearly in the\nmanuscript. To do so, we have made substantial revisions to the manuscript.\n1) We’ve highlighted three new findings compared to existing work: the basin-scale\ncontrast (Atlantic-Indian vs. Pacific), the mechanism insights derived from both\nocean-only and coupled models, and the analysis that spans both historical period\n(1960-2020) and future projections.\n2) The Introduction section has been rewritten by reviewing previous studies (Gao et\nal. 2018; Huguenin et al. 2022; Hague et al. 2024) properly. We’ve stated the\nuncertainty of heat transport from Pacific to Atlantic-Indian sector. We’ve also\ndiscussed simulation discrepancies among ocean model simulations by the end of\nthis paper. The section of Discussion and implications has been rewritten by\ndeleting Extended Data Fig. 9 and those description of Antarctic Krill, ref 41 (Deng\net al., 2022), and topography. Then we’ve added future projection analysis under\nSSP-126, SSP-245, and SSP-585 in CMIP6.\nWe hope these concrete revisions help clarify the contributions of our work and address\nthe reviewer’s concerns regarding novelty, evidence, and interpretation.\nAbstract:\nThe Southern Ocean (SO) is a major contributor to global ocean heat uptake,\nexhibiting deep-reaching warming trends within the 35°S-55°S band. Here, we reveal\na notable inter-basin contrast in the SO warming rates. Over the past six decades, the\nwarming of the 0-700 m Atlantic-Indian sector was 40.0 ± 5.7% faster than the Pacific\nsector, nearly doubling at ~44°S. Ocean-only and coupled model experiments suggest\nthat this basin-scale contrast arises from alterations in wind-driven heat redistribution\nrather than surface heating. Specifically, the intensification and poleward migration of\nwesterly winds are more prominent in the Atlantic-Indian sector, leading to stronger\nupper-layer heat convergence. The inter-basin warming contrast is projected to persist\nand amplify throughout the remainder of the 21st century. This study highlights the\ninter-basin contrast in the past and future SO warming, with useful implications for\nunderstanding regional changes in the SO climate, ice mass, and marine ecosystems.\nLines 57-70 (Introduction): Compared to the well-documented meridional structure,\nthe zonal structure of SO warming has been less appreciated. Zonally asymmetric\nfeatures were often overlooked owing to the common practice of circumpolar3,19 or\nregional30 mean diagnostics. A recent study based on ocean model simulations revealed\ncomplex asymmetric features in the SO warming pattern of 1979-2019 and complex\nforcing effects of surface winds, heat flux, and freshwater flux, with the dominant driver\nvarying with latitude31. Yet, the results of Ref.31 for the four-decade period contain both\nthe externally forced long-term changes and internal variability. Particularly,\npronounced decadal variability arising from Interdecadal Pacific Oscillation can cause\nout-of-phase heat content changes between the Pacific and Atlantic–Indian sectors of\nthe SO32; observation-based analysis suggested that wind forcing dominated the\ndecadal warming of the Subantarctic Mode Water during 2005-201525. Therefore,\nwhether these recently revealed asymmetric warming patterns and the underlying\nmechanisms30–32 are robust features of the SO under anthropogenic greenhouse\nwarming remain uncertain.\n3. Swart, N. C., Gille, S. T., Fyfe, J. C. & Gillett, N. P. Recent Southern Ocean\nwarming and freshening driven by greenhouse gas emissions and ozone depletion.\nNat. Geosci. 11, 836–841 (2018).\n19. Armour, K. C., Marshall, J., Scott, J. R., Donohoe, A. & Newsom, E. R. Southern\nOcean warming delayed by circumpolar upwelling and equatorward transport.\nNat. Geosci. 9, 549–554 (2016).\n25. Gao, L., Rintoul, S. R. & Yu, W. Recent wind-driven change in Subantarctic Mode\nWater and its impact on ocean heat storage. Nat. Clim. Change 8, 58–63 (2018).\n30. Huguenin, M. F., Holmes, R. M. & England, M. H. Drivers and distribution of\nglobal ocean heat uptake over the last half century. Nat. Commun. 13, 4921 (2022).\n31. Hague, M., Münnich, M. & Gruber, N. Zonally Asymmetric Increase in Southern\nOcean Heat Content. J. Clim. (2024) doi:10.1175/JCLI-D-23-0623.1.\n32. Song, Y. et al. Decadal Thermal Variability of the Upper Southern Ocean: Zonal\nAsymmetry. J. Clim. 37, 3117–3132 (2024).\nLines 268-273: This suggests a net heat loss of the Pacific sector to the Atlantic-Indian\nsector through ZOHT. The increase in heat transport has not been confirmed by in-situ\nobservations within the Drake Passage5,64, possibly owing to insufficient sampling in\nspatial and temporal coverage of data. In addition, changes in CESM1-LE represent\nexternally forced trends that are still undetectable in the short observational records\ncontaining internal variability.\nFig. 6 | Schematic and projection of inter-basin warming contrast in the SO. a,\nSchematic of inter-basin warming contrast. Projected temperature change is calculated\nas the time-mean temperature in 2080-2100 minus that in 1960-1980 from CESM1-LE\nensemble mean. Horizontal section shows projected 0-700 m vertical averaged\ntemperature change. Vertical cross-sections demonstrate projected zonal mean\ntemperature change in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nsectors. b, Historical simulations (1960-2014) and future projections (2015-2099) of 0-\n700 m temperature changes of the Pacific and Atlantic-Indian sectors under SSP1-2.6,\nSSP2-4.5, and SSP5-8.5 scenarios based on CMIP6 models. Shading denotes the one\nstandard deviation range of model members.\nLines 336-400 (Discussion and implications): Our work highlights the necessity of\nlooking forward into the future changes in the SO based on projections of climate\nmodels. CESM1-LE project that the Atlantic-Indian sector will continue to warm faster\nthan the Pacific sector in the 21st century under the high-emission RCP 8.5 scenario\n(Fig. 6a). Furthermore, in projections of Coupled Model Intercomparison Project phase\n6 (CMIP6) models, the 0-700 m warming of the Atlantic-Indian sector will reach\n1.01 ± 0.36 °C, 1.63 ± 0.45 °C, and 2.56 ± 0.76 °C by 2100 relative to the 1960-1980\nbaseline under Shared Socioeconomic Pathway (SSP) 1-2.6, 2-4.5, and 5-8.5 scenarios,\nin contrast to the 0.72 ± 0.28 °C, 1.10 ± 0.41 °C, and 1.72 ± 0.65 °C values of the Pacific\nsector. The inter-basin contrast of SO warming will further amplify throughout the 21st\ncentury under SSP5-8.5 and SSP2-4.5 scenarios (Fig. 6b). Under the low-emission\nscenario of SSP1-2.6, the contrast will diminish by the mid-21st century, showing close\nwarming rates in the two sectors; the SO warming will terminate by the 2070s. The\ndependence on emission scenario highlights the essential role of greenhouse gases in\ngenerating the inter-basin warming contrast. Therefore, the inter-basin contrast is a\nrobust feature of the SO under anthropogenic climate change and will probably persist\nor amplify in the upcoming decades.\nTo summarize, this study reveals the critical role of wind-driven heat redistribution\nin shaping the inter-basin warming contrast in the mid-latitude band (35°-55°S) of the\nSO (Fig. 6a). This finding complements the existing view that emphasizes surface heat\nuptake and mean-circulation advection in the meridional structure of the SO warming.\nBased on recent studies that have reported asymmetric features of the SO warming30–\n32, our work achieves the following distinct insights. First, we highlight the inter-basin\ncontrast – with significantly stronger warming of the Atlantic-Indian sector than the\nPacific sector – as a representative asymmetric feature within the enhanced warming\nband of 35°-55°S. Second, inhomogeneous changes in the westerly winds, showing\nmore prominent intensification and poleward migration in the Atlantic-Indian sector,\nplay the dominant role in causing the inter-basin warming contrast through driving\nchanges in ocean circulation and heat transport. Third, through combining climate\nmodel simulations, we show that the inter-basin contrast is a robust feature of the SO\nwarming in response to anthropogenic climate change and will persist or amplify in the\n21st century. These insights into the inter-basin warming contrast are useful in\nunderstanding and predicting regional changes in primary production61, particularly the\nkey component in the SO food chain — the Antarctic krill62, which has a large\nproportion of the population (>50%) in the Atlantic sector63.\nSimulations of ocean models forced with atmospheric reanalysis fields show\nconsiderable discrepancies among each other and with observation in detailed features\nof the SO warming pattern, as suggested by the present (Fig. 5 and Extended Data Fig.\n9) and previous studies30,31. These discrepancies stem from differences in temporal\nperiod, forcing fields, and model physics among these simulations. In addition, the lack\nof air-sea interaction and uncertainties contained in forcing fields, particularly during\npre-satellite era (e.g., prior to 1979), are two major error sources in ocean model\nsimulations. Our LICOM simulations exhibit notable discrepancies from observation.\nFor example, the equatorward shift of the warming band and the overestimated\nwarming near the western boundary of the Pacific sector may result from a warm bias\nlinked to the model’s coarse horizontal resolutions and underestimation of mesoscale\neddy-induced heat transports. Given these uncertainties, our understanding of the SO’s\nwarming structure under greenhouse warming is far from complete. Future work should\ntake advantage of the expanding Argo network to more rigorously assess the robustness\nof observed and modeled temperature changes. Further efforts are also required to\nassess the relative contributions of other anthropogenic forcings, such as industrial\naerosols, biomass burning aerosols, and ozone. With these regards, we anticipate that\nmore extensive investigations of inter-basin contrast and other asymmetric features of\nthe SO warming in the future, which shall significantly improve our understanding of\noceanic storages of anthropogenic heat and carbon.\nLines 433-441 (Methods): Coupled Model Intercomparison Project phase 6 (CMIP6)76\nprovides more comprehensive simulations from various coupled models, under\nhistorical forcing of 1850-2014 and three Shared Socioeconomic Pathway (SSP)\nscenarios (1-2.6, 2-4.5, and 5-8.5) of 2015-2100. The projected results from the multi-\nmodel ensemble mean of 13 CMIP6 models (ACCESS-CM2, BCC-CSM2-MR,\nCAMS-CSM1-0, CESM2-WACCM, CIESM, EC-Earth3, FGOALS, FIO-ESM,\nGFDL-CM4, MIROC6, MPI-ESM1-2-LR, MRI-ESM2-0, NorESM2-MM) reveals\neffects of anthropogenic greenhouse warming. We interpolated model outputs to a\nregular 1°×1° horizontal grids and analyze annual-mean data of ocean temperature.\nResponse to specific comments:\nQ1: 78: Doesn’t reference 44 deal explicitly with zonal heat distribution and warming?\nResponse:\nIndeed, Hague et al. (2024) provided a detailed assessment of zonally asymmetric\ntemperature trends during 1979–2019. This has been stated in the revised paper. Yet the\n40-year analysis contains both internal variability and anthropogenic warming. We have\nclarified this point.\nLines 68-70: Therefore, whether these recently revealed asymmetric warming patterns\nand the underlying mechanisms34–36 are robust features of the SO under anthropogenic\ngreenhouse warming remain uncertain.\nQ2: 93: IAP, EN4, Ishii aren’t explained prior to this figure.\nResponse:\nThanks for pointing out. We have explained these datasets here.\nLines 88-92: Fig. 1 | Temperature change in the SO. a-b Linear trends of 0-700 m\nocean heat content (OHC) for the period of 1960-2020 derived from the average of the\nInstitute of Atmospheric Physics (IAP) ocean analysis data7,38, the version 4.2.2 of the\nMet Office Hadley Centre “EN” series of datasets (EN4)35, and the 2017 version of\nIshii analysis data34; see “Methods”) and CESM1-LE ensemble-mean.\nQ3: 108: Salient is a slightly odd word choice here.\nResponse:\nWe’ve deleted this word.\nQ4: Figure 1: CF gao, who note most warming in pacific? SAMW/AAIW export\nregions\nResponse:\nThe work of Gao et al. (2018) has been cited in the revised paper. They showed stronger\nwarming of SAMW in the Pacific and Indian sectors, particularly in the region of\nmaximum SAMW thickness between 60°E and 120°W during 2005–2015 (Fig. 5),\nbased on Argo data. However, such short-term (<15 years) trends mainly reflect internal\nvariability. This is distinct from the long-term trend pattern governed primarily by\nanthropogenic forcing addressed in this study. This has been stated in Lines 66-68 in\nthe revised paper.\nLines 66-68: observation-based analysis suggested that wind forcing dominated the\ndecadal warming of the Subantarctic Mode Water during 2005-201525.\n25. Gao, L., Rintoul, S. R. & Yu, W. Recent wind-driven change in Subantarctic Mode\nWater and its impact on ocean heat storage. Nat. Clim. Change 8, 58–63 (2018).\nQ5: 117: Most climate models overestimate surface warming, often due to SW issues,\nand often interior, notably CDW warming too. How does CESM do in this regard, does\nthis impact results? Is it a constant offset, or have its own asymmetry?\nResponse:\nWe thank the reviewer for raising this important point. CESM1-LE does overestimate\nSST warming compared to HadISST during 1960–2020 and fails to capture observed\nSST cooling in many regions south of 35°S (Fig. R1a–b). This is likely related to a\nbroader and stronger trend in surface net heat flux (Fig. 2a, c), particularly in the\nshortwave component (Fig. R1c–d), which is a known source of model bias.\nDespite this warm bias, both CESM1-LE and HadISST consistently show an inter-basin\ncontrast in SST warming, with the Atlantic-Indian sector warming faster than the\nPacific sector. However, the surface net heat flux and shortwave trends in both ERA5\nand CESM1-LE do not exhibit a zonal asymmetry that matches the SST pattern,\nimplying that these flux components cannot fully explain the inter-basin contrast either.\nThe warm bias appears not to alter the key findings. This has been explained in Lines\n180-190.\nLines 180-190: In CESM1-LE, there are prevailing trends of positive Q at all\nnet\nlatitudes of the SO (Fig. 2c and Extended Data Fig. 6b), in contrast to the mid-latitude\nheat loss in ERA5 (Fig. 2a and Extended Data Fig. 6a). This discrepancy is linked to\ninsufficient strengthening of westerly winds compared to observation-based changes\nand easterly trends in mid-latitude regions (Fig. 2d) – a common bias among IPCC-\nclass climate models40,50, which reduces the total wind speed and suppresses heat\nrelease there. The underestimated heat uptake in the high latitudes is likely due to\noverestimated surface warming and absence of regional cooling trends around Antarctic\nin models51. Nevertheless, the surface heating in the Pacific sector is also stronger than\nin the Atlantic-Indian sector (0.24 versus 0.16 W m-2 decade-1), unable to account for\nthe SO warming asymmetry.\nFig. R1 Linear trends of SST based on (a) HadISST and (b) CESM1-LE and trends of\nshortwave radiation in (c) ERA5 and (d) CESM1-LE for the period of 1960-2020.\nFig. 2 | Changes in surface heating and winds over the SO. a, b Linear trends in\nsurface net heat flux Q and wind stress curl (WSC; shading) for the period of 1960-\nnet\n2020, based on European Centre for Medium-Range Weather Forecasts (ECMWF)\nreanalysis dataset version 5 (ERA5). In b, trends of wind stress are show as vectors. c-\nd, Same as a-b, but derived from CESM1-LE ensemble-mean. Stippling in a-d\nindicates insignificant changes at the 95% confidence level.\nQ6: 135: The uncertainty means there is almost an overlap in the two ranges. In\nobservations the pacific in particular is heavily underrepresented. How do the trends\ncompare for the comparatively evenly sampled Argo period (~ 2004- present?).\nResponse:\nWe thank the reviewer for this insightful comment.\nThe overlap between the two ranges results from limited numerical precision in the\noriginally presented trend values. After increasing the precision to three decimal places\n(as shown in Extended Data Fig. 3 and corresponding Table R1), the overlap disappears.\nWithin the 35°-55°S zonal band, the Pacific sector exhibits smaller trend uncertainty\nthan the Atlantic-Indian sector. For both sectors, the trend uncertainty during the Argo\nperiod is substantially lower than during the full 1960-2020 period or the pre-Argo era.\nNotably, the inter-basin contrast exists in all the three periods, although the uncertainty\nis larger during the pre-Argo era.\nWe’ve added Extended Data Fig. 3 and corresponding description in Lines 128-137 to\nclarify observational uncertainty.\nLines 128-137: In the observations, the Pacific sector has an average warming rate of\n0.05 ± 0.01 K decade-1 in the 0-700 m layer (Extended Data Fig. 3), corresponding to\na 60-year warming of 0.30 ± 0.04 K by 2020 relative to the 1960-1970 baseline (Fig.\n1e). By contrast, the Atlantic-Indian sector warms at a rate of 0.07 ± 0.01 K decade-1\n(Extended Data Fig. 3), resulting in a 60-year warming of 0.42 ± 0.07 K (Fig. 1f). The\nwarming of the Atlantic-Indian sector is 46.7 ± 10.7% stronger than that of the Pacific\nsector. This contrast existed in both the Argo (2004-2020) and pre-Argo (1960-2003)\nperiods (Extended Data Fig. 3). Notably, the Argo era witnessed accelerated warming\nthan before in both sectors, partly owing to enhanced observational sampling that can\nmore sufficiently resolve the warming trends.\nExtended Data Fig. 3 | Linear trends and uncertainty of 0-700 m temperature in\nthe Pacific and Atlantic-Indian sectors. Averaged linear trends and one standard\ndeviation ranges (colored bars) of averaged 0-700 m temperature in the Pacific and\nAtlantic-Indian sectors over 1960-2020, 2004-2020 (the Argo era), and 1960-2003 (pre-\nArgo era) based on observational datasets. Square, triangle, and circle denote linear\ntrends based on IAP, EN4.2.1, and Ishii, respectively.\nTable R1 Linear trends and uncertainty of 0-700 m temperature in the Pacific and\nAtlantic-Indian sectors over 1960-2020, 2004-2020, 1960-2003 based on\nobservational datasets.\nTrend Averaged\nPeriod Sectors Datasets STD\n(K/decade) Trend\nIAP 0.080\n1960-2020 Atl-IO 0.066 0.012\nEN4 0.063\nIshii 0.056\nIAP 0.050\nPac EN4 0.044 0.045 0.005\nIshii 0.040\nIAP 0.124\nAtl-IO EN4 0.144 0.135 0.010\n2004-2020 Ishii 0.135\n(Argo) IAP 0.116\nPac EN4 0.110 0.112 0.003\nIshii 0.110\nIAP 0.079\nAtl-IO EN4 0.037 0.052 0.023\n1960-2003 Ishii 0.042\n(pre-Argo) IAP 0.039\nPac EN4 0.016 0.027 0.012\nIshii 0.028\nQ7: 138: Does it fluctuate in your results too? How much by, again, what confidence\ncan be given prior to Argo?\nResponse:\nThe calculation of linear trend can be influenced by states of the start and end years,\nwhich is less sensible with longer-period calculation. Due to internal variability, Pacific\nsector temperature goes up at the end years, while Atlantic-Indian sector temperature\ngoes down (Fig. 1e-f). These fluctuation leads to a higher estimation of linear trend in\nthe Pacific sector and lower estimation in the Atlantic-Indian sector, which potentially\ndiminishes the inter-basin warming contrast. But the inter-basin warming contrast is\nstill convincing in each dataset, despite the negative effect of the fluctuation.\nPre-Argo trends are more uncertain and warrant additional caution as mentioned in the\nresponse to the previous question.\nWe’ve edited the sentence to emphasize that the inter-basin contrast is still convincing\ndespite the fluctuation.\nLines 139-142: The inter-basin contrast is generally insensitive to the choice of the\nending year in trend calculation. Although fluctuated by decadal variability32, the\nstronger warming of the Atlantic-Indian sector is a persistent feature throughout 1960-\n2020.\nFig. 1 | Temperature change in the SO. a-b, Linear trends of 0-700 m ocean heat\ncontent (OHC) for the period of 1960-2020 derived from the average of the Institute of\nAtmospheric Physics (IAP) ocean analysis data7,42, the version 4.2.2 of the Met Office\nHadley Centre “EN” series of datasets (EN4)39, and the 2017 version of Ishii analysis\ndata38; see “Methods”) and CESM1-LE ensemble-mean. Grey curves indicate the zonal\nbelt of 35°S-55°S where the Pacific and Atlantic-Indian sectors are separated by 70°W\nand 150°E. c-d, Meridional mean (35°S-55°S) temperature trends for the same period\nin observations and CESM1-LE ensemble mean. The horizontal dashed line denotes\nthe depth of 700 m and the vertical black line indicates 70°W. Stippling indicates\ninsignificant changes at the 95% confidence level. e-f, Time series of averaged 0-700\nm temperature in the Pacific and Atlantic-Indian sectors, based on observations and\nCESM1-LE. All variables are shown as anomalies relative to the 1960-1970 baseline.\nGrey shading shows the one standard deviation range of CESM1-LE members. The\nCESM1-LE results analyzed here consist of the historical simulation of 1960-2005 and\nRepresentative Concentration Pathway (RCP) 8.5 projection of 2006-2100.\nQ8: 145: What is the MLD contrast like in the model vs obs in these regions. SAMW\nsubduction is a primary mechanism for heat uptake, and is sensitive to many processes,\noften poorly represented in models. Does the relative depth/stratification give some clue\nas to why this asymmetry is stronger in the model?\nResponse:\nThe linear trend of MLD during 1960–2020 shows large uncertainty across the three\nobservational datasets (Fig. R2), with only two small regions exhibiting consistent\nsignals: deepening near 150°E and shoaling near 90°E. CESM1-LE reproduces only the\nshoaling near 90°E. Given this large observational spread, it is difficult to clearly\nidentify a robust MLD contrast between the two sectors.\nFig. R2 Linear trend of MLD over 1960-2020 based on (a) IAP, (b) EN4.2.1, (c) Ishii,\nand (c) CESM1-LE. We calculated MLD in IAP, EN4.2.1, and Ishii using Matlab\naccording to the international thermodynamic equation of seawater – 2010 (TEOS-10).\nQ9: 160: What is the integrated heat fluxes over the areas – the qualitative spatial\npatterns seem consistent with your inference, but is this backed up quantitatively in\nterms of heat uptake by each sector?\nResponse:\nThe Pacific sector receives an average surface heating of 1.08 W m⁻² per decade in the\nhigh-latitudes, which is notably higher than the 0.42 W m⁻² per decade observed in the\nAtlantic-Indian sector. This quantitative comparison strengthens our interpretation of\nthe spatial pattern of surface heating. We have revised the manuscript text accordingly.\nLines 156-158: Interestingly, on average, the Pacific sector (1.08 W m-2 per decade)\nreceives stronger surface heating than the Atlantic-Indian sector (0.42 W m-2 per decade)\nin the high-latitudes in ERA5.\nQ10: 164-165: This is a bit vague, where is south and north when talking about WSC\nhere?\nResponse:\nHere, “south” and “north” refer to regions relative to 55°S. We have clarified this in the\nmanuscript.\nLines 171-172: The strengthened westerlies accompany trends of negative wind stress\ncurls (WSCs) south of 55°S and positive WSCs north of this latitude.\nQ11: 168: Again, please be quantitative – how much stronger is the wind shift/WSC\nchange in the Atlantic/Indian sector vs the Pacific.\nResponse:\nWe have included this quantitative comparison in Lines 211-214.\nLines 211-214: This enhancement is more pronounced in the Atlantic-Indian sector\n(0.61×10-8 Pa m-1 decade-1) than in the Pacific sector (0.49 × 10-8 Pa m-1 decade-1) in\nERA5, which is also the case in JRA-55 (0.35 × 10-8 versus 0.28 × 10-8 Pa m-1\ndecade-1).\nQ12: 172: Broadly consistent is possibly a bit strong, particularly for the heat fluxes.\nWhilst if you interpret it very generously you might convince yourself that there is a\nnorthern cooling and southern warming in the heat flux, it really is nowhere near as\nstrong as in the obs, nor have a similar regional structure. I think this needs to be more\ncarefully treated, and how this poor representation may impact the conclusions drawn\nbetween model/obs comparisons.\nResponse:\nWe apologize for the previous inappropriate statement. CESM1-LE shows bias in\nsimulating mid-latitude Q trends. However, CESM1-LE captures the observed inter-\nnet\nbasin contrast in temperature (Fig. 1) and wind trends (Fig. 2b, d), suggesting that wind-\ndriven processes—rather than Q —dominate the zonal asymmetry. This supports our\nnet\nconclusion that the modeled warming contrast arises primarily from wind changes, and\nnot from biases or changes in Q .\nnet\nWe have revised the sentence accordingly for clarity and caution.\nLines 180-191: In CESM1-LE, there are prevailing trends of positive Q at all\nnet\nlatitudes of the SO (Fig. 2c and Extended Data Fig. 6b), in contrast to the mid-latitude\nheat loss in ERA5 (Fig. 2a and Extended Data Fig. 6a). This discrepancy is linked to\ninsufficient strengthening of westerly winds compared to observation-based changes\nand easterly trends in mid-latitude regions (Fig. 2d) – a common bias among IPCC-\nclass climate models40,50, which reduces the total wind speed and suppresses heat\nrelease there. The underestimated heat uptake in the high latitudes is likely due to\noverestimated surface warming and absence of regional cooling trends around Antarctic\nin models51. Nevertheless, the surface heating in the Pacific sector is also stronger than\nin the Atlantic-Indian sector (0.24 versus 0.16 W m-2 decade-1), unable to account for\nthe SO warming asymmetry. CESM1-LE also produces asymmetric changes in westerly\nwinds, although much weaker than changes in ERA5 and JRA-55.\nFig. 2 | Changes in surface heating and winds over the SO. a, b Linear trends in\nsurface net heat flux Q and wind stress curl (WSC; shading) for the period of 1960-\nnet\n2020, based on European Centre for Medium-Range Weather Forecasts (ECMWF)\nreanalysis dataset version 5 (ERA5). In b, trends of wind stress are show as vectors. c-\nd, Same as a-b, but derived from CESM1-LE ensemble-mean. Stippling in a-d\nindicates insignificant changes at the 95% confidence level.\nExtended Data Fig. 6 | Changes in surface heating in ERA5 and CESM1-LE. a-b,\n35°S-55°S average Q in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nnet\nsectors based on ERA5 and CESM1-LE.\nQ13: 194: ‘..in the climatology..’ here and elsewhere, ‘the’ is missing in relation to\nmany references to the climatology.\nResponse:\nWe have corrected the phrase to “in the climatology” throughout the manuscript.\nLines 205-207: The high-latitude Pacific sector loses heat to the atmosphere in the\nclimatology, which weakened at ~1.0 W m-2 decade-1 since 1960.\nLines 235-237: In the climatology, the MOHT shows divergence and convergence,\nindicated by positive and negative ∂MOHT/∂y values on the southern and northern\nflanks of the westerly maximum at ~47°S, respectively.\nQ14: 197: I don’t understand where the weakening by 1.08 Wm-2 came from, this\nappears to be strengthening by roughly this amount in the Pacific trend in the high\nlatitudes in figure 3a). Also, how are these values calculated? Are they area weighted\naverages south of some latitude, with things like sea ice accounted for?\nResponse:\nWe apologize for the incorrect statement in the previous version. The term “heat uptake\nweakening” has been revised to “heat loss weakening” to accurately describe the trend\nin the Pacific sector. Surface net heat flux trends were calculated as area-weighted\naverages over the Pacific (150°E–70°W, 55°S–70°S) and Atlantic-Indian (70°W–150°E,\n55°S–70°S) sectors shown in Fig.3.\nTo assess the potential influence of sea ice on upper-ocean heat uptake, we recalculated\nthe area-weighted Q by masking out grid points with sea ice concentration greater\nnet\nthan 50%. The updated analysis confirms the previous findings, showing consistent\ntime series (Fig. 3b) and comparable linear trends of 1.04 W m-2 per decade in the\nPacific sector (vs. 1.08 W m-2 previously) and 0.37 W m-2 per decade in the Atlantic-\nIndian sector (vs. 0.42 W m-2 previously). The manuscript has been revised accordingly.\nLines 202-210: To account for influence of sea-ice cover, here we calculate area-\nweighted average Q only for open-ocean regions. Consistent with Fig. 2a, the Pacific\nnet\nsector receives stronger anomalous heat fluxes than the Atlantic-Indian sector in the\nhigh latitudes (south of 55°S) (Fig. 3a, b). The high-latitude Pacific sector loses heat to\nthe atmosphere in the climatology, which weakened at ~1.0 W m-2 decade-1 since 1960;\nmeanwhile, the heat uptake in the Atlantic-Indian counterpart increased at a rate of ~0.4\nW m-2 decade-1 (Fig. 3b). In CESM1-LE, changes in Q can neither explain the inter-\nnet\nbasin contrast in heat storage (Extended Data Fig. 6b and Extended Data Fig. 7a).\nFig. 3 | Inter-basin contrast in surface net heat flux and wind stress curl. a, c,\nLinear trends (solid curves) and annual climatology (dashed curves) of zonal-mean Q\nnet\nand\nWSC in the Pacific (from 150°E to 70°W) and Atlantic-Indian (from 70°W to 150°E)\nsectors during 1960-2020. Black dashed lines denote 55°S and 35°S. b, d, Average\nhigh-latitude (55°S-70°S) Q in open-ocean regions (annual mean sea ice\nnet\nconcentration < 50%) and average mid-latitude (35°S-55°S) wind stress curl in the\nPacific and Atlantic-Indian sectors based on ERA5 reanalysis. Dashed lines in b, d\ndenote the linear regression.\nExtended Data Fig. 6 | Changes in surface heating in ERA5 and CESM1-LE. a-b,\n35°S-55°S average Q in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nnet\nsectors based on ERA5 and CESM1-LE.\nExtended Data Fig. 7 | Linear trends and climatology of surface net heat flux and\nMOHT in CESM1-LE a, b, Linear trends (solid curves) and climatology (dashed\ncurves) of zonal mean surface net heat flux and 0-100 m MOHT in the Pacific and\nAtlantic-Indian sectors. Black dashed lines denote 55°S and 35°S.\nQ15: 197: I don’t think these are particularly similar…there are significant qualitative\nand quantitative differences between the two datasets. You need to note these and\nexplain why they do/don’t matter for your subsequent conclusions.\nResponse:\nWe apologize for the incorrect statement in the previous version. While CESM1-LE\nand ERA5 show many differences in Q trend (Extended Data Fig. 7a), both\nnet\nconsistently exhibit an inter-basin contrast in upper-ocean warming (Fig. 1). This\nsuggests that, despite uncertainties in the heat flux trends, the large-scale warming\npattern is robust. We have revised the sentence to more cautiously reflect this point.\nLines 180-191: In CESM1-LE, there are prevailing trends of positive Q at all\nnet\nlatitudes of the SO (Fig. 2c and Extended Data Fig. 6b), in contrast to the mid-latitude\nheat loss in ERA5 (Fig. 2a and Extended Data Fig. 6a). This discrepancy is linked to\ninsufficient strengthening of westerly winds compared to observation-based changes\nand easterly trends in mid-latitude regions (Fig. 2d) – a common bias among IPCC-\nclass climate models40,50, which reduces the total wind speed and suppresses heat\nrelease there. The underestimated heat uptake in the high latitudes is likely due to\noverestimated surface warming and absence of regional cooling trends around Antarctic\nin models51. Nevertheless, the surface heating in the Pacific sector is also stronger than\nin the Atlantic-Indian sector (0.24 versus 0.16 W m-2 decade-1), unable to account for\nthe SO warming asymmetry. CESM1-LE also produces asymmetric changes in westerly\nwinds, although much weaker than changes in ERA5 and JRA-55.\nQ16: 226-227: Is this pattern supported by the observational data? Obviously, it is\nharder to assess ocean heat transport from observations, but the Ekman transport and\nsurface heat content over the Argo period should be sufficient to assess the last two\ndecades of upper ocean MOHT and trends – with the caveats that eddy transports etc\nwill not be captured. However if it is primarily wind driven as you suggest the same\nfeatures should be present.\nResponse:\nWe appreciate the reviewer’s insightful comment. We’ve added Ekman MOHT\nestimated from ERA5 winds and IAP temperature in Fig. 4c-d. The divergence pattern\nof MOHT in CESM1-LE (Fig. 4a-b) is indeed supported by the observational Ekman\nMOHT (Fig. 4c-d). Although observational constraints limit the inclusion of eddy\ncontributions, the wind-driven component captures a stronger convergence in the\nAtlantic-Indian sector, as CESM1-LE does. To align with the model analysis, we\ncalculated linear trends over 1960–2020 rather than restricting to the Argo period. The\nmanuscript has been revised accordingly.\nLines 239-243: Although the heat convergence (negative ∂MOHT/∂y) between 35°-\n55°S is overall enhanced in both sectors, the convergent trends in the Atlantic-Indian\nsector are evidently stronger and cover a wider latitude range than those in the Pacific.\nSimilar patterns are also seen in the heat convergence of Ekman currents, ∂MOHT /∂y,\nE\nas estimated using ERA5 winds (Fig. 4c-d).\nLines 482-485 (Methods): The estimated Ekman MOHT based on 𝜏 in ERA5 and\nx\ntemperature in IAP is calculated as\nMOHT = ∫\n0\n∫\n𝑥2\n𝜌 𝑐 (−[𝜃]\n𝜏𝑥\n) 𝑑𝑥,\nE −100 𝑥1 0 𝑝 𝑓𝜌0\nwhere [𝜃] is the vertical mean temperature of 0-100 m.\nFig. 4 | Inter-basin contrast in ocean heat transport. a, b, Linear trends (red and blue\nbars) and climatology (grey bars) of divergence of 0-100 m MOHT, ∂MOHT/∂y,\ndivided by the corresponding area in the Atlantic-Indian and Pacific sectors in CESM1-\nLE. c, d, Same as a, b but linear trends of divergence of 0-100 m Ekman MOHT,\n∂MOHT /∂y, based on ERA5 data (see Methods). In a-d, black dashed lines denote\nE\n55°S and 35°S. e, f, Linear trend of zonal mean vertical heat flux (𝜌 𝑐 𝑤𝜃 ) in the\n0 𝑝\nAtlantic-Indian and Pacific sectors in CESM1-LE. The dashed line shows the depth of\n700 m. Stippling indicates insignificant changes at the 95% confidence level.\nQ17: 231-238: Can you provide comment on why the vertical heat flux trend in the\nAtlantic-Indian appears to align well with the regional divergence trend, but this is not\nthe case in the Pacific, where there is significant positive vertical heat flux where there\nis apparently a divergence trend around 55S, and the strongest convergence trend is\nassociated with the weakest vertical heat flux?\nResponse:\nYes, for sure. In general, the vertical heat flux trend aligns with the MOHT divergence\ntrend (Fig. 4), reflecting the dominant role of meridional overturning in the Southern\nOcean. However, vertical heat flux is also modulated by the divergence of ZOHT. In\nthe Pacific sector near 55°S, the presence of the South American landmass blocks zonal\nflow, leading to anomalous convergence in the ZOHT. This can disrupt the local\nalignment between vertical heat flux and MOHT divergence in this region.\nQ18: 239: If wind change dominate ocean heat uptake, then the relative difference in\nsurface heat fluxes is probably irrelevant to the relative heating of the two regions,\nas the Pacific is also taking up extra wind pumped heat, just at a lesser rate. As also\nnoted in Gao et al., 2018, it is the winds that dominate heat uptake in both region, and so\nprobably also dominates the extra heat that may be exported from the Pacific to Atlantic\nsectors.\nResponse:\nWe understand that by “ocean heat uptake,” you are referring to heat storage, which\nresults from the combined effects of surface heat fluxes and wind-driven transport.\nWhile the Pacific sector shows stronger surface heating (positive Q trends),\nnet\nparticularly at high latitudes (Fig. 3a, b), this does not translate into stronger heat\nstorage, suggesting that wind-driven heat redistribution more likely plays a dominant\nrole. This interpretation aligns with Gao et al. (2018), which emphasizes the central role\nof winds in controlling ocean heat storage across basins. We have revised the\nmanuscript and added the appropriate citation.\nLines 255-257: Zonal exchange between the two sectors may also contribute to the\nasymmetric warming features25. To examine this process, we further calculate the zonal\nocean heat transport (ZOHT) integrated from 35°S to Antarctic coasts using CESM-LE.\n25. Gao, L., Rintoul, S. R. & Yu, W. Recent wind-driven change in Subantarctic Mode\nWater and its impact on ocean heat storage. Nat. Clim. Change 8, 58–63 (2018).\nQ19: 241: Or an export to the north from the Pacific (or much less likely, to the\nAntarctic coast)? How do you exclude these possibilities – it is a region of significant\nmode water formation and export after all.\nResponse:\nIt is well established that surface heat absorbed at high latitudes in the Southern Ocean\nis largely transported northward by wind-driven processes, such as mode water\nsubduction and Ekman transport, resulting in peak warming around 45°S. Based on this\nunderstanding, we suggest that the northward heat export is more efficient in the\nAtlantic-Indian sector, leading to greater heat storage in that region, as previously\nmentioned. In contrast, the Pacific sector, despite experiencing stronger surface heating,\nshows weaker local heat storage, implying that part of the absorbed heat may instead\nbe transferred zonally to other basins. Export to the Antarctic coast is expected to be\nrelatively minor due to prevailing circulation. We have revised the manuscript\naccordingly.\nLines 255-257: Zonal exchange between the two sectors may also contribute to the\nasymmetric warming features25. To examine this process, we further calculate the zonal\nocean heat transport (ZOHT) integrated from 35°S to Antarctic coasts using CESM-LE.\nQ20: 247: The transport and temperatures through Drake passage hasn’t increased\nsignificantly, e.g. as referenced in your reference 67, though the zonal transport to the\nnorth of the ACC has. It may have done in the model (you don’t specify or quantify\nthis), but this isn’t reflective of the observations and so should be noted. As you are\nusing the CESM1, as ref 67 (Shi et al., 2021) does, the current change in Drake passage\nis inconsistent with observations – again to be noted.\nResponse:\nWe agree that the long-term trend of heat transport through Drake Passage is not clearly\nevident in observational datasets. This highlights a key uncertainty in evaluating ZOHT\nfrom observations, which is partly due to the coarse resolution and limited temporal\ncoverage. While CESM1-LE simulates a strengthening of ZOHT across both 150°E and\n70°W, we acknowledge that this may not fully reflect the observational reality.\nTherefore, we have added a statement in the revised manuscript to clarify this\ninconsistency.\nLines 260-270: In CESM1-LE, the ZOHT has increased at both 150°E and 70°W,\nreflecting the acceleration of the ACC58 and warming of waters carried by the ACC.\nSince 1960, the heat transport from the Indian Ocean to the Pacific sector has increased\nby 1.1 × 1013 W decade-1, slower than the heat transport increase of 1.7 × 1013 W\ndecade-1 at Drake Passage, with the difference significant at the 90% confidence level.\nThis suggests a net heat loss of the Pacific sector to the Atlantic-Indian sector through\nZOHT. The increase in heat transport has not been confirmed by in-situ observations\nwithin the Drake Passage5, possibly owing to insufficient sampling in spatial and\ntemporal coverage of data. In addition, changes in CESM1-LE represent externally\nforced trends that are still undetectable in the short observational records containing\ninternal variability.\n5. Böning, C. W., Dispert, A., Visbeck, M., Rintoul, S. R. & Schwarzkopf, F. U. The\nresponse of the Antarctic Circumpolar Current to recent climate change. Nat.\nGeosci. 1, 864–869 (2008).\n58. Shi, J.-R., Talley, L. D., Xie, S.-P., Peng, Q. & Liu, W. Ocean warming and\naccelerating Southern Ocean zonal flow. Nat. Clim. Change 11, 1090–1097\n(2021).\nQ21: 250: This seems reasonable, but without looking at the other three sides of the\nPacific box budget (you only look down to 700 m), it is only suggestive of a reason –\nand again, is one that isn’t strongly supported by the observations. This also needs\naddressing in figure 6, which shows significant heat going through drake passage –\nagain, not supported by observations. To what extent does this heat contribute to the\nnet difference in heat content/mean temperature difference in the two basins?\nExtended data figure 5 suggests 0.02 PW, but how does this translate to the\ntemperature differences between the basins in for eg Figure 1e/f? Is it a significant\namount that may account for the differences? If so, then the fact that it isn’t consistent\nwith the observations may challenge the entire thesis of the paper, and mean that it is\nmore of an issue with CESM1 than a representative finding. If it is not sufficient to\nmake a big hemispheric difference, then it should be noted as such and probably\nremoved from Figure 6 as an important feature. Overall I think this uncertainty makes\na case for a more comprehensive budget analysis of the boxes to be made.\nResponse:\nWe thank the reviewer for this important and detailed comment.\nWe focus on the upper 700 m in our analysis because the observed and modeled\nwarming is primarily confined to this layer (Fig. 1 and Extended Data Fig. 2), while\nobservational uncertainties increase substantially below 700 m due to data sparsity and\nresolution issues. We have clarified this in Lines 113–116.\nRegarding the ZOHT shown in CESM1, we acknowledge that the modeled ZOHT\nthrough the Drake Passage is not well supported by available observations. To better\nreflect this uncertainty, we have revised the description in Lines 262–270 and changed\nthe solid arrow to a dashed and narrower one in the updated Fig. 6a to indicate its lower\nconfidence.\nAs the reviewer suggested, we quantified the heat contribution of the CESM1 ZOHT\nto the Atlantic-Indian sector rather than the Pacific, because net ZOHT is transported to\nthe Atlantic-Indian sector. According to the updated Extended Data Fig. 8, the net\nZOHT into the Atlantic-Indian sector high latitudes is 0.6 × 1013 W decade-1 (0.036 PW\nover 60 years), yielding an integrated heat input of 5.86 × 1020 J over 60 years.\nAssuming this heat is fully stored in the upper 700 m of the Atlantic-Indian sector\n(volume: 2.56 × 1016 m³), it would cause a temperature increase of approximately\n0.01 K, which is much smaller than the observed warming of 0.42 ± 0.07 K shown in\nFig. 1e. This confirms that ZOHT plays only a secondary role, while the primary driver\nof the inter-basin contrast is the wind-driven MOHT convergence, as we demonstrated\nthroughout the manuscript.\nWe fully agree that a complete box-based heat budget analysis can offer further insights.\nYet such an analysis cannot be carried out using observations, due to the absence of\nfull-depth current and the large uncertainty of Q . Even when conducted with CESM1\nnet\noutput, the resulting budget terms cannot be directly constrained or validated by\nobservations. Our paper focuses on the key effect of wind-driven MOHT on the inter-\nbasin warming contrast, as previous studies have generally attributed the 45°S warming\nto intensified MOHT. Both CESM1 and LICOM underscore the critical role of wind-\ndriven MOHT in shaping the SO warming patterns.\nWe appreciate the suggestion and intend to pursue a more detailed budget analysis in\nfuture work, while being mindful of the uncertainties that may affect its reliability.\nLines 113-116: This contrast also exists in the intermediate layer of 700-2000 m but\nmuch weaker in warming magnitudes (Extended Data Fig. 2). Given more limited\nobservation sampling in deeper layers, hereafter we focus on the warming features in\nthe upper 700 m layers.\nLines 262-270: Since 1960, the heat transport from the Indian Ocean to the Pacific\nsector has increased by 1.1 × 1013 W decade-1, slower than the heat transport increase\nof 1.7 × 1013 W decade-1 at Drake Passage, with the difference significant at the 90%\nconfidence level. This suggests a net heat loss of the Pacific sector to the Atlantic-Indian\nsector through ZOHT. The increase in heat transport has not been confirmed by in-situ\nobservations within the Drake Passage5, possibly owing to insufficient sampling in\nspatial and temporal coverage of data. In addition, changes in CESM1-LE represent\nexternally forced trends that are still undetectable in the short observational records\ncontaining internal variability.\nExtended Data Fig. 2 | Changes in 700-2000 m temperature. a-b, Linear trends of\n700-2000 m OHC in observations and CESM1-LE. Stippling indicates insignificant\ndifference at 95% confidence level. c-d, Time series of averaged 700-2000 m\ntemperature in the Pacific and Atlantic-Indian sectors, based on observations and\nCESM1-LE. Grey shading shows the one standard deviation range in CESM1-LE.\nFig. 6 | Schematic and projection of inter-basin warming contrast in the SO. a,\nSchematic of inter-basin warming contrast. Projected temperature change is calculated\nas the time-mean temperature in 2080-2100 minus that in 1960-1980 from CESM1-LE\nensemble mean. Horizontal section shows projected 0-700 m vertical averaged\ntemperature change. Vertical cross-sections demonstrate projected zonal mean\ntemperature change in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nsectors. b, Historical simulations (1960-2014) and future projections (2015-2099) of 0-\n700 m temperature changes of the Pacific and Atlantic-Indian sectors under SSP1-2.6,\nSSP2-4.5, and SSP5-8.5 scenarios based on CMIP6 models. Shading denotes the one\nstandard deviation range of model members.\nQ22: 251-253: This sentence doesn’t make sense. You have previously said the\ndifference was due to surface fluxes. Similarly, you would think generally that if the\nAtlantic-Indian basin was taking up heat faster than the Pacific due to wind convergence,\nthen there would be more heat available there to flux zonally to the Pacific. The obvious\nanswer would be that the Atlantic-Indian basin is fluxing the heat to the north via the\nsubtropical gyre, but nowhere do you suggest this or calculate it.\nResponse:\nThank you for raising this point. We would like to clarify that we did not attribute the\ninter-basin warming contrast to differences in surface heat fluxes. Instead, our point is\nthat despite stronger surface heat uptake in the Pacific sector, this region stores less heat\nthan the Atlantic-Indian sector. This inconsistency implies a potential contribution of\nZOHT to the warming contrast.\nRegarding the possibility of northward heat export from the Atlantic-Indian sector via\nthe subtropical gyre, our analysis does not support this mechanism. As shown in\nExtended Data Fig. 7, the MOHT is directed southward in the Atlantic-Indian sector.\nMoreover, the upwelling around 35°S in Fig. 4e further supports a southward MOHT\nin this region.\nExtended Data Fig. 7 | Linear trends and climatology of surface net heat flux and\nMOHT in CESM1-LE a, b, Linear trends (solid curves) and climatology (dashed\ncurves) of zonal mean surface net heat flux and 0-100 m MOHT in the Pacific and\nAtlantic-Indian sectors. Black dashed lines denote 55°S and 35°S.\nFig. 4 | Inter-basin contrast in ocean heat transport. a, b, Linear trends (red and blue\nbars) and climatology (grey bars) of divergence of 0-100 m MOHT, ∂MOHT/∂y,\ndivided by the corresponding area in the Atlantic-Indian and Pacific sectors in CESM1-\nLE. c, d, Same as a, b but linear trends of divergence of 0-100 m Ekman MOHT,\n∂MOHT /∂y, based on ERA5 data (see Methods). In a-d, black dashed lines denote\nE\n55°S and 35°S. e, f, Linear trend of zonal mean vertical heat flux (𝜌 𝑐 𝑤𝜃 ) in the\n0 𝑝\nAtlantic-Indian and Pacific sectors in CESM1-LE. The dashed line shows the depth of\n700 m. Stippling indicates insignificant changes at the 95% confidence level.\nQ23: 263: ‘stand-alone’\nResponse:\nWe appreciate the correction and have updated the sentence accordingly.\nLines 283-284: To validate this conclusion and gain further insights, we employ stand-\nalone ocean models to perform simulations and experiments (Methods).\nQ24: 270: Worth noting that the LICOM appears to be somewhat warmer than the\nobservations, and especially the CESM1, particularly in the Pacific.\nResponse:\nIndeed, the model warming exceeds that in observations. We have noted this in the\nrevised manuscript.\nLines 285-289: The control simulation (CTRL) of the LASG/IAP Climate Ocean\nModel (LICOM) version-337 forced with daily surface atmospheric fields of ERA559\nhas well reproduced the observed heat storage pattern in the SO of 1960-2020, showing\nenhanced warming within the mid-latitude band of the Atlantic-Indian sector (Fig. 5a),\nalbeit with stronger trends.\nQ25: 271: There does appear to be inter model sensitivity to the Pacific and Atlantic-\nIndian split though – extended data 6 shows that in LICOM the Pacific actually warms\nslightly more in total, whilst the MITgcm shows that the two splits are probably\nbarely statistically different. How are the trend lines fitted here? Both of these models\nappear to have much less basin difference than either the observations or CESM1 (eg fig 1),\nand should be acknowledged. Also, ‘further confirms the deterministic’ is a poor choice\nof words. It doesn’t confirm anything, but it does add support to the hypothesis; and I\nshould hope the ocean state is deterministic, otherwise we’re all out of a job. This\nshould be changed.\nResponse:\nWe appreciate the reviewer’s important comments. The slightly higher temperature in\nthe Pacific sector is shown in the end year (2020) of analysis in LICOM in the previous\nversion. However, we emphasize that warming should be assessed based on the long-\nterm linear trend rather than the single endpoint value in 2020, which may be influenced\nby internal variability. In fact, the Pacific sector in LICOM has a smaller linear trend\ncompared to the Atlantic-Indian sector, qualitatively consistent with MITgcm, the\nobservations, and CESM1-LE. The contrast is more evident in the updated version in\nFig. 5b. The trend lines indicate the least squares fit to the time series over 1960-2020,\nwhich has been added to captions in Fig. 5 and Extended Data Fig. 9.\nWe also notice that the trend differences in LICOM and MITgcm are smaller than those\nin the observations and CESM1-LE. The reduced contrast in the ocean-only models\nmay be due to the lack of air-sea coupling. We’ve added corresponding statement in the\nmanuscript.\nWe agree that the original wording “further confirms the deterministic…” was not\nappropriate. The sentence has been revised to “… provides additional support …” in\nthe updated version.\nLines 285-302: The control simulation (CTRL) of the LASG/IAP Climate Ocean\nModel (LICOM) version-337 forced with daily surface atmospheric fields of ERA559\nhas well reproduced the observed heat storage pattern in the SO of 1960-2020, showing\nenhanced warming within the mid-latitude band of the Atlantic-Indian sector (Fig. 5a),\nalbeit with stronger trends. Compared to the observed structure, the simulated warming\nband is shifted equatorward, and the heat storage near the western boundary of Pacific\nsector is overestimated. These discrepancies are likely associated with coarse model\nresolution (1°) which leads to underestimation of mesoscale eddies. The poleward heat\ntransport induced by eddies act to compensate the wind-driven heat transport and the\nresultant heat convergence. Therefore, we adopt the central-to-eastern Pacific region of\n180°-70°E, 35°S-50°S to quantify its weaker warming trend (0.06 K decade-1) than that\nof the Atlantic-Indian sector (70°E-150°E, 35°S-50°S; 0.09 K decade-1) (Fig. 5b). A\nsimulation with the Massachusetts Institute of Technology General Circulation Model\n(MITgcm)60 forced by ERA5 fields also captures the inter-basin contrast in the SO\nwarming (Extended Data Fig. 9). MITgcm provides an additional support of surface\natmospheric changes in shaping the inter-basin contrast. The inter-basin contrast in\nLICOM and MITgcm is weaker than in observation and CESM1-LE, possibly owing\nto the lack of air-sea coupling in ocean-only models.\nFig. 5 | Heat storage and the inter-basin contrast in LICOM experiments. a, c, e, g\nLinear trends in 0-700 m OHC for the period of 1960-2020 in the CTRL (a), HTFL (c),\nand WND (d) experiments of LICOM and the sum HTFL and WND (b). Stippling\nindicates insignificant changes at the 95% confidence level. b, d, f, h The 0-700 m\naverage temperatures for the central-to-eastern Pacific (180°-70°E, 35°S-50°S) and\nAtlantic-Indian (70°E-150°E, 35°S-50°S) sectors in the CTRL (e), HTFL (g), and WND\n(h) experiments of LICOM and the sum HTFL and WND (f). Dashed line in b, d, f, h\nindicates the least squares fit to corresponding time series.\nExtended Data Fig. 9 | Time series of temperature in MITgcm control experiments.\nTime series of averaged 0-700 m temperature in the Pacific and Atlantic-Indian sectors\nfrom 1960 to 2020 in control experiments of MITgcm, forced by ERA5 reanalysis.\nDashed line indicates the least squares fit to the time series.\nQ26: 286: Most of the change is concentrated in the Atlantic sector – do you know why\nthis is? Similarly there seems to be some weakness in the indian sector uptake of HTFL\n+ WND compared with the CTRL – suggesting at least some of the enhanced atlantic-\nindian uptake is a combination of the effects, which makes intuitive sense. This agrees\nwith for eg Armour et al 2016 where it is a combination of enhanced surface heat uptake\nand downward mode water pumping (also Gao et al 2018) that results in increased heat\nstorage on the north of the ACC.\nIt is worth noting some of the issues that these experiments presumably have: ie changes\nin surface SST due to heat redistribution by the winds won’t lead to changes in long\nwave outgoing heat (as the net heat flux is fixed I believe in WND), and so presumably\nthere are biases induced here. Similarly the WND experiment won’t result in enhanced\nturbulent heat fluxes (I imagine based on the methods, it doesn’t specify) so these\neffects aren’t captured either. It would be worth expanding on these a little, though I\nagree the HTFL + WND does look somewhat similar to CTRL.\nResponse:\nThank you for the helpful suggestion. We agree that the enhanced heat storage in the\nAtlantic-Indian sector is likely a result of both increased surface heat uptake and wind-\ndriven downward heat transport, consistent with previous studies (Armour et al., 2016;\nGao et al., 2018). The HTFL experiment also indicates the surface heating in both\nsectors. We have now added the explanation of downward mode water pumping and\nthe relevant citations in the revised manuscript.\nWe also appreciate the reviewer’s thoughtful observations regarding the limitations of\nthe experimental design. In the WND experiment, the net surface heat flux is prescribed,\nand thus the longwave radiative feedback from SST anomalies is not represented.\nAdditionally, the wind-induced changes in turbulent heat fluxes (latent and sensible\nheat) are not included, which may lead to some underestimation of feedbacks. We have\nadded a brief discussion in results and a description in the revised Methods section.\nLines 317-326: WND largely represents the storage of redistributed heat42 - with\nwarming signatures arising from heat redistribution induced by changes in wind-driven\nocean circulations. This experiment produces enhanced heat pile-up in the 35°-50°S\nband of the Atlantic-Indian Ocean sector (0.03 K decade-1) without additional heat\nuptake (Fig. 5g, h), in contrast to insignificant trends in the central-to-eastern Pacific\nsector. The results of WND confirm that the inter-basin contrast in the SO heat storage\nresults mainly from wind-driven upper-ocean heat redistribution. Note that the effect of\nmode water subduction19,25 is also contained in WND, given that key factors\ndetermining the subduction rate, such as Ekman pumping and the mixed layer depth,\nare primarily controlled by winds.\n19. Armour, K. C., Marshall, J., Scott, J. R., Donohoe, A. & Newsom, E. R. Southern\nOcean warming delayed by circumpolar upwelling and equatorward transport.\nNat. Geosci. 9, 549–554 (2016).\n25. Gao, L., Rintoul, S. R. & Yu, W. Recent wind-driven change in Subantarctic Mode\nWater and its impact on ocean heat storage. Nat. Clim. Change 8, 58–63 (2018).\nLines 460-464 (Methods): Although these idealized experiments are subject to\nlimitations, such as prescribed surface heat flux in WND may inhibit radiative and\nturbulent feedback associated with SST changes, they enable us to separately examine\nthe impacts of surface heat uptake and wind-driven redistribution on SO warming,\nproviding insights into the mechanisms driving observed changes in this region.\nQ27: 310: This is a bit of a stretch. The wind induced warming is well to the north of\nregions where Antarctic krill may be expected (fisheries concentrate around the\npeninsula). I suggest removing this statement.\nResponse:\nWe appreciate this suggestion and have removed this statement.\nQ28: 318: This statement isn’t backed up by by ref 41 (Deng et al., 2022). They focus\non future wind scenarios, and importantly state “The projected changes in the SHWs\nare basically consistent among the three ocean basins”. This may not be the case for\nhistorical winds, but they do not show this. They also do not make statements regarding\ndrivers of asymmetry as far as I can see, so this should be removed. The other reference\nis from 2000, and does not directly pertain to trends in the winds. I think more\nappropriate references should be found, or this section deleted.\nResponse:\nThank you for pointing this out. We’ve deleted this paragraph.\nQ29: 323-324: This is a surprising statement to include at this point. Topography is no\ndoubt incredibly important to the structure of the ACC and westerlies, but not the only\nfactor. This statement is made without much justification (particularly its absolute\nnature) and the supplementary figure does not do much to support it except in the most\nhand wavy way. I think references need to be given to demonstrate the dominance of\ntopography in the wind stress, curl and surface velocities, or again, remove or strongly\nmoderate this statement.\nResponse:\nWe appreciate this advice and have deleted this statement.\nQ30: 332: Whilst this appears consistent in the CESM1-LE model, only one reference\nis given to suggest that this is consistent across the model ensemble. Ref 41 suggests\nthat wind changes are faily even across basins, and otherwise only looks at SSTs.\nQualitatively there is a pattern in the stronger scenarios that suggests surface warming\nmay have a basin difference, but this is not stated explicitly in the paper, nor is it the\nsame metric as extended data figure 10. Again, I think more caution is needed in these\nstatements, or more appropriate references need to be found. This paper (and possibly\nthe qualitative interpretation of their figure 5) also seems to be sole basis for the rest of\nthis paragraph too, see below.\nResponse:\nWe acknowledge the previous citation was not appropriate to support our statement. In\nresponse, we have removed the citation and rewritten the text by incorporating new\nanalysis based on CMIP6 simulations under three scenarios (SSP-585, SSP-245, and\nSSP-126), as shown in Fig. 6b. These results demonstrate that the inter-basin warming\ncontrast is projected to persist and even intensify under most emission scenarios.\nLines 336-351: Our work highlights the necessity of looking forward into the future\nchanges in the SO based on projections of climate models. CESM1-LE project that the\nAtlantic-Indian sector will continue to warm faster than the Pacific sector in the 21st\ncentury under the high-emission RCP 8.5 scenario (Fig. 6a). Furthermore, in\nprojections of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the\n0-700 m warming of the Atlantic-Indian sector will reach 1.01 ± 0.36 °C, 1.63 ± 0.45 °C,\nand 2.56 ± 0.76 °C by 2100 relative to the 1960-1980 baseline under Shared\nSocioeconomic Pathway (SSP) 1-2.6, 2-4.5, and 5-8.5 scenarios, in contrast to the\n0.72 ± 0.28 °C, 1.10 ± 0.41 °C, and 1.72 ± 0.65 °C values of the Pacific sector. The\ninter-basin contrast of SO warming will further amplify throughout the 21st century\nunder SSP5-8.5 and SSP2-4.5 scenarios (Fig. 6b). Under the low-emission scenario of\nSSP1-2.6, the contrast will diminish by the mid-21st century, showing close\nwarming rates in the two sectors; the SO warming will terminate by the 2070s. The\ndependence on emission scenario highlights the essential role of greenhouse gases in\ngenerating the inter-basin warming contrast. Therefore, the inter-basin contrast is a\nrobust feature of the SO under anthropogenic climate change and will probably persist\nor amplify in the upcoming decades.\nFig. 6 | Schematic and projection of inter-basin warming contrast in the SO. sa,\nSchematic of inter-basin warming contrast. Projected temperature change is calculated\nas the time-mean temperature in 2080-2100 minus that in 1960-1980 from CESM1-LE\nensemble mean. Horizontal section shows projected 0-700 m vertical averaged\ntemperature change. Vertical cross-sections demonstrate projected zonal mean\ntemperature change in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nsectors. b, Historical simulations (1960-2014) and future projections (2015-2099) of 0-\n700 m temperature changes of the Pacific and Atlantic-Indian sectors under SSP1-2.6,\nSSP2-4.5, and SSP5-8.5 scenarios based on CMIP6 models. Shading denotes the one\nstandard deviation range of model members.\nQ31: 336: This paper explicitly says that there aren’t notable basin differences in the\nwinds under strong scenarios. e.g they state:\n“The projected changes in the SHWs are basically consistent among the three ocean basins (Fig. 2).”\nResponse:\nWe agree the previous citation was inappropriate and have removed it. CMIP6 analysis\nhas been added instead, as noted in the response to Q30.\nReviewer #2\nWe are grateful to this reviewer for reading our paper and providing important and\nvaluable comments and suggestions. We are glad to have the opportunity to address\nthese comments and improve the quality of this work by adding new reanalysis data\nand more quantitative analysis. After a thoughtfully consideration, we made concrete\nmodifications in the paper. We hope the revised paper could be more acceptable.\nThe point-to-point responses to Reviewer #2’s comments are given below:\nThis is an interesting paper which seeks to explain the drivers behind the asymmetry in\nmulti-decadal warming of the upper Southern Ocean between the Atlantic-Indian and\nPacific sectors. The primary conclusion is that a change in wind forcing rather than air-\nsea heat flux is the main driver. This would be a significant result, if fully substantiated,\nthat merits publication in Nature Comms. However, the study at present relies too much\non qualitative interpretation of figures that is not sufficiently supported by quantitative\nanalysis that would support the claims being made. Furthermore, the observation based\nheat flux vs wind stress results are dependent on a single reanalysis (ERA5) and thus\nlack robustness. If the authors, can support their results with further quantitative\nanalysis and the inclusion of results from an additional reanalysis (e.g. MERRA2,JRA)\nas detailed below then their manuscript may become suitable for publication. I strongly\nencourage them to do this as I think they would then have a substantial paper that would\nattract widespread interest. Given the amount of work required my recommendation is\nmajor revision.\nResponse:\nWe sincerely thank the reviewer for the thoughtful and constructive feedback. We are\nencouraged by the recognition of the scientific significance and broad relevance of our\nstudy on the zonal asymmetry of upper Southern Ocean warming.\nWe fully acknowledge the reviewer’s concerns regarding the need for more quantitative\nsupport and the dependence on a single reanalysis dataset. In response, we have\nconducted additional quantitative analyses to strengthen the basis of our conclusions,\nincluding:\n(1) A revised and more detailed quantification of the respective roles of surface\nheat flux and wind forcing in driving the inter-basin warming contrast.\n(2) Supplementary analysis using additional JRA55 alongside ERA5 to assess the\nrobustness of observed surface flux and wind trends. Note that MERRA2 is not applied\nto this study due to the lack of 20-year data from 1960-1979.\nWe hope these enhancements address the key issues and substantially improves the\nclarity and robustness of the manuscript.\nRequired changes:\nQ1: Abstract and various places in main text. ‘Over the past six decades (1960-2020),\nthe 0-700 m warming in the Atlantic-Indian sector has been 40.0 ± 5.7% faster than in\nthe Pacific sector, nearly doubling at ~44°S.’ The authors need to discuss and acknowledge in the main text that the number of ocean temperature observations\nthat go into the various products they analyse (e.g. EN4) have increased massively through the 60 year period considered.\nThere are very few observations prior to 1990 and a major increase since the early 2000s because of Argo. Would they\nreach the same conclusions if they considered the relatively well observed period 1990-2020?\nResponse:\nWe thank the reviewer for highlighting this important point. We fully acknowledge the\ntemporal inhomogeneity in ocean temperature observations, particularly the data\nsparsity prior to the Argo era. In response to a similar comment from Reviewer #1,\nwe have added a separate analysis of the Argo and pre-Argo periods using the three\nobservational datasets, now shown in Extended Data Fig. 3. The inter-basin contrast\nexists in both periods, with more pronounced warming in the Argo era. Relevant\ndiscussion has been added to the revised manuscript.\nWe have also specifically examined the period 1990–2020, which benefits from\nimproved observational coverage. The warming trend in the Atlantic-Indian sector\n(0.097 ± 0.019 K/decade) remains higher than that in the Pacific sector\n(0.084 ± 0.005 K/decade) (Table R2), supporting the same conclusion of inter-basin\ncontrast. While the contrast is smaller than that over the full 1960–2020 period, this\nmay reflect the influence of decadal variability due to the shorter timescale.\nLines 134-137: This contrast existed in both the Argo (2004-2020) and pre-Argo\n(1960-2003) periods (Extended Data Fig. 3). Notably, the Argo era witnessed\naccelerated warming than before in both sectors, partly owing to enhanced\nobservational sampling that can more sufficiently resolve the warming trends.\nExtended Data Fig. 3 | Linear trends and uncertainty of 0-700 m temperature in\nthe Pacific and Atlantic-Indian sectors. Averaged linear trends and one standard\ndeviation ranges (colored bars) of averaged 0-700 m temperature in the Pacific and\nAtlantic-Indian sectors over 1960-2020, 2004-2020 (the Argo era), and 1960-2003 (pre-\nArgo era) based on observational datasets. Square, triangle, and circle denote linear\ntrends based on IAP, EN4.2.1, and Ishii, respectively.\nTable R2 Linear trends and uncertainty of 0-700 m temperature in the Pacific and\nAtlantic-Indian sectors over 1960-2020 and 1990-2020 based on observational\ndatasets.\nTrend Averaged\nPeriod Sectors Datasets STD\n(K/decade) Trend\nIAP 0.080\nAtl-IO EN4 0.063 0.066 0.012\nIshii 0.056\n1960-2020\nIAP 0.050\nPac EN4 0.044 0.045 0.005\nIshii 0.040\nIAP 0.079\nAtl-IO EN4 0.117 0.097 0.019\nIshii 0.094\n1990-2020\nIAP 0.081\nPac EN4 0.090 0.084 0.005\nIshii 0.080\nQ2: lines 71-73. ‘Driven by asymmetric changes in surface winds, the Pacific and the\nAtlantic-Indian sectors exhibit out-of-phase variations in the upper-layer heat content\non decadal timescales43.’ Is this perhaps related also to an asymmetry in net heat flux\nbetween the Pacific and the Atlantic-Indian sectors?\nResponse:\nThe cited result originates from our previous study, which showed that the out-of-phase\ndecadal variations between the Pacific and Atlantic-Indian sectors are mainly driven by\nIPO-related wind changes. Net surface heat flux does not display a corresponding zonal\nasymmetry at decadal timescales.\n36. Song, Y. et al. Decadal Thermal Variability of the Upper Southern Ocean: Zonal\nAsymmetry. J. Clim. 37, 3117–3132 (2024).\nQ3: line 83. The authors focus on 0-700 m but what about 700-2000 m? Please\ncomment on what’s happening at lower levels. Also, how are the 0-700m results for\nOHC asymmetry different from earlier studies that found asymmetric warming and why?\nThe reason I’m asking is that it’s difficult to tell whether the OHC asymmetry results\nfrom observations are an incremental advance or a significant development from what\nhas previously been published?\nResponse:\nThe inter-basin contrast in OHC is much weaker in the 700-2000 m layer compared to\nthe upper 0-700 m, as shown in Extended Data Fig. 2. To clarify this, we have now\nincluded a brief description in the main text stating why we focus on the upper 0-700\nm.\nLines 113-116: This contrast also exists in the intermediate layer of 700-2000 m but\nmuch weaker in warming magnitudes (Extended Data Fig. 2). Given more limited\nobservation sampling in deeper layers, hereafter we focus on the warming features in\nthe upper 700 m layers.\nRegarding the novelty of our observational findings, we emphasize several key\ndistinctions from previous studies:\n1) Zonal basin-scale contrast: Prior studies have discussed the SO warming\nasymmetries in relation to regional or detailed structure, but have not explicitly\nquantified the large-scale contrast between the Atlantic-Indian and Pacific sectors\nas we do here.\n2) Mechanistic insights using both ocean-only and coupled modelling experiments.\n3) Future persistence: We show that the inter-basin contrast not only exists historically\nbut is projected to persist under anthropogenic warming throughout the 21st century,\nbased on both CESM1-LE and CMIP6 models (Fig. 6).\nTogether, these points represent a substantial advancement in understanding the spatial\nstructure and long-term evolution of the SO warming. We have revised the manuscript\naccordingly to better communicate these contributions.\nLines 21-32 (Abstract): The Southern Ocean (SO) is a major contributor to global\nocean heat uptake, exhibiting deep-reaching warming trends within the 35°S-55°S band.\nHere, we reveal a notable inter-basin contrast in the SO warming rates. Over the past\nsix decades, the warming of the 0-700 m Atlantic-Indian sector was 40.0 ± 5.7% faster\nthan the Pacific sector, nearly doubling at ~44°S. Ocean-only and coupled model\nexperiments suggest that this basin-scale contrast arises from alterations in wind-driven\nheat redistribution rather than surface heating. Specifically, the intensification and\npoleward migration of westerly winds are more prominent in the Atlantic-Indian sector,\nleading to stronger upper-layer heat convergence. The inter-basin warming contrast is\nprojected to persist and amplify throughout the remainder of the 21st century. This study\nhighlights the inter-basin contrast in the past and future SO warming, with useful\nimplications for understanding regional changes in the SO climate, ice mass, and\nmarine ecosystems.\nLines 57-70 (Introduction): Compared to the well-documented meridional structure,\nthe zonal structure of SO warming has been less appreciated. Zonally asymmetric\nfeatures were often overlooked owing to the common practice of circumpolar3,19 or\nregional30 mean diagnostics. A recent study based on ocean model simulations revealed\ncomplex asymmetric features in the SO warming pattern of 1979-2019 and complex\nforcing effects of surface winds, heat flux, and freshwater flux, with the dominant driver\nvarying with latitude31. Yet, the results of Ref.31 for the four-decade period contain both\nthe externally forced long-term changes and internal variability. Particularly,\npronounced decadal variability arising from Interdecadal Pacific Oscillation can cause\nout-of-phase heat content changes between the Pacific and Atlantic–Indian sectors of\nthe SO32; observation-based analysis suggested that wind forcing dominated the\ndecadal warming of the Subantarctic Mode Water during 2005-201525. Therefore,\nwhether these recently revealed asymmetric warming patterns and the underlying\nmechanisms30–32 are robust features of the SO under anthropogenic greenhouse\nwarming remain uncertain.\n3. Swart, N. C., Gille, S. T., Fyfe, J. C. & Gillett, N. P. Recent Southern Ocean\nwarming and freshening driven by greenhouse gas emissions and ozone depletion.\nNat. Geosci. 11, 836–841 (2018).\n19. Armour, K. C., Marshall, J., Scott, J. R., Donohoe, A. & Newsom, E. R. Southern\nOcean warming delayed by circumpolar upwelling and equatorward transport.\nNat. Geosci. 9, 549–554 (2016).\n25. Gao, L., Rintoul, S. R. & Yu, W. Recent wind-driven change in Subantarctic Mode\nWater and its impact on ocean heat storage. Nat. Clim. Change 8, 58–63 (2018).\n30. Huguenin, M. F., Holmes, R. M. & England, M. H. Drivers and distribution of\nglobal ocean heat uptake over the last half century. Nat. Commun. 13, 4921 (2022).\n31. Hague, M., Münnich, M. & Gruber, N. Zonally Asymmetric Increase in Southern\nOcean Heat Content. J. Clim. (2024) doi:10.1175/JCLI-D-23-0623.1.\n32. Song, Y. et al. Decadal Thermal Variability of the Upper Southern Ocean: Zonal\nAsymmetry. J. Clim. 37, 3117–3132 (2024).\nFig. 6 | Schematic and projection of inter-basin warming contrast in the SO. a,\nSchematic of inter-basin warming contrast. Projected temperature change is calculated\nas the time-mean temperature in 2080-2100 minus that in 1960-1980 from CESM1-LE\nensemble mean. Horizontal section shows projected 0-700 m vertical averaged\ntemperature change. Vertical cross-sections demonstrate projected zonal mean\ntemperature change in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nsectors. b, Historical simulations (1960-2014) and future projections (2015-2099) of 0-\n700 m temperature changes of the Pacific and Atlantic-Indian sectors under SSP1-2.6,\nSSP2-4.5, and SSP5-8.5 scenarios based on CMIP6 models. Shading denotes the one\nstandard deviation range of model members.\nLines 336-400 (Discussion and implications): Our work highlights the necessity of\nlooking forward into the future changes in the SO based on projections of climate\nmodels. CESM1-LE project that the Atlantic-Indian sector will continue to warm faster\nthan the Pacific sector in the 21st century under the high-emission RCP 8.5 scenario\n(Fig. 6a). Furthermore, in projections of Coupled Model Intercomparison Project phase\n6 (CMIP6) models, the 0-700 m warming of the Atlantic-Indian sector will reach\n1.01 ± 0.36 °C, 1.63 ± 0.45 °C, and 2.56 ± 0.76 °C by 2100 relative to the 1960-1980\nbaseline under Shared Socioeconomic Pathway (SSP) 1-2.6, 2-4.5, and 5-8.5 scenarios,\nin contrast to the 0.72 ± 0.28 °C, 1.10 ± 0.41 °C, and 1.72 ± 0.65 °C values of the Pacific\nsector. The inter-basin contrast of SO warming will further amplify throughout the 21st\ncentury under SSP5-8.5 and SSP2-4.5 scenarios (Fig. 6b). Under the low-emission\nscenario of SSP1-2.6, the contrast will diminish by the mid-21st century, showing close\nwarming rates in the two sectors; the SO warming will terminate by the 2070s. The\ndependence on emission scenario highlights the essential role of greenhouse gases in\ngenerating the inter-basin warming contrast. Therefore, the inter-basin contrast is a\nrobust feature of the SO under anthropogenic climate change and will probably persist\nor amplify in the upcoming decades.\nTo summarize, this study reveals the critical role of wind-driven heat redistribution\nin shaping the inter-basin warming contrast in the mid-latitude band (35°-55°S) of the\nSO (Fig. 6a). This finding complements the existing view that emphasizes surface heat\nuptake and mean-circulation advection in the meridional structure of the SO warming.\nBased on recent studies that have reported asymmetric features of the SO warming30–\n32, our work achieves the following distinct insights. First, we highlight the inter-basin\ncontrast – with significantly stronger warming of the Atlantic-Indian sector than the\nPacific sector – as a representative asymmetric feature within the enhanced warming\nband of 35°-55°S. Second, inhomogeneous changes in the westerly winds, showing\nmore prominent intensification and poleward migration in the Atlantic-Indian sector,\nplay the dominant role in causing the inter-basin warming contrast through driving\nchanges in ocean circulation and heat transport. Third, through combining climate\nmodel simulations, we show that the inter-basin contrast is a robust feature of the SO\nwarming in response to anthropogenic climate change and will persist or amplify in the\n21st century. These insights into the inter-basin warming contrast are useful in\nunderstanding and predicting regional changes in primary production61, particularly the\nkey component in the SO food chain — the Antarctic krill62, which has a large\nproportion of the population (>50%) in the Atlantic sector63.\nQ4: lines 85-87. Can the authors comment more on why the CESM warming is\nconfined to a shallower layer than observations? And why is it stronger than in\nobservations? Is it simply because an external warming signal is confined to a shallower\nlayer? Also, do they have any insights into whether other CMIP-class models show\nsimilar behaviour to CESM?\nResponse:\nCESM1 tends to overestimate upper ocean stratification in the Southern Ocean, which\nsuppresses vertical mixing and confines surface warming to shallower depths. This\nbehavior is consistent with earlier CMIP5 model evaluations, which report\nsystematically shallower mixed layer depths across the model ensemble (Sallée et al.,\n2013). In addition, CESM1 exhibits a known surface heat flux bias and relatively high\nclimate sensitivity, contributing to the stronger surface warming compared to\nobservations. This is also documented in a CMIP5 bias study (Hyder et al., 2018). These\nfactors together lead to enhanced and vertically confined warming in the shallower layer.\nSimilar behavior is found in CMIP6 which also shows relatively strong and shallow\nSouthern Ocean warming (Fig. R3). While the magnitude of the warming varies among\nmodels, the qualitative pattern supports that the CESM results are not model-specific\nartifacts but reflect common biases in coupled models.\nWe’ve added comment in the manuscript.\nLines 119-122: CESM1-LE also captures the inter-basin contrasts in both warming\nmagnitude and vertical structure (Fig. 1d). Meanwhile, CESM1-LE overestimates the\nnear-surface warming, likely associated with the common biases of stronger upper-\nocean stratification and shallower mixed layer in coupled models39,40.\n39. Sallée, J.-B. et al. Assessment of Southern Ocean mixed-layer depths in CMIP5\nmodels: Historical bias and forcing response. J. Geophys. Res. Oceans 118,\n1845–1862 (2013).\n40. Hyder, P. et al. Critical Southern Ocean climate model biases traced to\natmospheric model cloud errors. Nat. Commun. 9, 3625 (2018).\nFig. R3 Meridional mean (35°S-55°S) temperature trends over 1960-2020 in CMIP6.\nThe horizontal dashed line denotes the depth of 700 m and the vertical black line\nindicates 70°W.\nFig. 1 | Temperature change in the SO. a-b, Linear trends of 0-700 m ocean heat\ncontent (OHC) for the period of 1960-2020 derived from the average of the Institute of\nAtmospheric Physics (IAP) ocean analysis data7,42, the version 4.2.2 of the Met Office\nHadley Centre “EN” series of datasets (EN4)39, and the 2017 version of Ishii analysis\ndata38; see “Methods”) and CESM1-LE ensemble-mean. Grey curves indicate the zonal\nbelt of 35°S-55°S where the Pacific and Atlantic-Indian sectors are separated by 70°W\nand 150°E. c-d, Meridional mean (35°S-55°S) temperature trends for the same period\nin observations and CESM1-LE ensemble mean. The horizontal dashed line denotes\nthe depth of 700 m and the vertical black line indicates 70°W. Stippling indicates\ninsignificant changes at the 95% confidence level. e-f, Time series of averaged 0-700\nm temperature in the Pacific and Atlantic-Indian sectors, based on observations and\nCESM1-LE. All variables are shown as anomalies relative to the 1960-1970 baseline.\nGrey shading shows the one standard deviation range of CESM1-LE members.\nQ5: lines 90-93. ‘This pattern is primarily driven by asymmetric changes in westerly\nwinds, rather than variations in surface heat fluxes. While the added heat dominates the\nmeridional distribution, the redistributed heat plays a central role in shaping the zonal\nstructure of SO warming.’ This is only an assertion at this point in the paper and should\nbe prefixed by ‘We will attempt to show that this pattern is primarily driven … ‘.\nResponse:\nWe appreciate this suggestion and have added the prefix.\nLines 73-74: We will attempt to reveal a key asymmetric feature of anthropogenic heat\nstorage in the SO.\nQ6: line 132. ‘much weaker below 700 m’ Could the much weaker behavior at depth\nbe due to lack of observations at depth obscuring a stronger trend?\nResponse:\nWe agree that the much weaker behavior at depth may be due to lack of observations.\nWe’ve added such an explanation in the manuscript.\nLines 113-116: This contrast also exists in the intermediate layer of 700-2000 m but\nmuch weaker in warming magnitudes (Extended Data Fig. 2). Given more limited\nobservation sampling in deeper layers, hereafter we focus on the warming features in\nthe upper 700 m layers.\nQ7: line 138. Should 0.37 be 0.42? i.e. 60 yr warming should be 6 x the decadal\nwarming of 0.07 K per decade?\nResponse:\nThe previously reported value of 0.37 K referred to the 2020 temperature anomaly, not\nthe full 60-year warming. We agree that the long-term warming should be calculated as\nsix times the decadal trend (i.e., 0.42 K for 0.07 K/decade). We have increased the\nprecision and revised the manuscript accordingly.\nLines 130-132: By contrast, the Atlantic-Indian sector warms at a rate of 0.07 ± 0.01\nK decade-1 (Extended Data Fig. 3), resulting in a 60-year warming of 0.42 ± 0.07 K\n(Fig. 1f).\nQ8: line 142. ‘100+/-60%’ this result appears to be insignificant at the two standard\ndeviation level. So, is it worth including?\nResponse:\nThank you for pointing this out. We’ve deleted this value.\nLines 137-139: At the heat storage maximum of ~44°S, the warming of Atlantic-Indian\nsector was nearly twice as strong as the Pacific sector (Extended Data Fig. 4a, b).\nExtended Data Fig. 4 | Changes in 0-700 m temperature around ~44°S. a-b, Time\nseries of averaged 0-700 m temperature around ~44°S (43.5°S-44.5°S) in the Pacific\nand Atlantic-Indian sectors based on observations. Black bold line indicates the least\nsquares fit to the averaged observations. Slope denotes warming rate.\nQ9: lines 162-167. These qualitative statements need to be backed up by some\nquantitative analysis. The asymmetric pattern of heat storage in Fig.1 refers to two\nboxed regions. So, what are the trends in heat flux in these two boxed regions?\nAlso, what does a time series of heat flux look like in the two regions? An extra two panels\nsimilar to Fig.1e-f need to be added to Fig.2 but showing heat flux in the Pacific and\nAtlantic-Indian sectors (35-55S) from observations and models. These figures and\nquantitative analysis are needed to support the assertion that surface heating cannot\nexplain the asymmetric pattern of heat storage in the two sectors.\nNote: I can see that some time series are shown subsequently in Fig.3. but, crucially,\nthe mid-latitudes heat flux time series is not shown and needs to be included as\nsuggested here.\nResponse:\nWe appreciate the suggestion to provide quantitative evidence supporting the surface\nheat flux analysis. We have added the time series of net surface heat flux averaged over\nthe mid-latitude bands (35°–55°S) for both the Atlantic-Indian and Pacific sectors from\nERA5 and CESM1-LE (Extended Data Fig. 6). Heat lose in ERA5 indicates that surface\nnet heat flux cannot account for warming in the two sectors. In CESM1-LE, larger\nsurface heating in the Pacific sector (0.24 W m-2 per decade) cannot explain the inter-\nbasin warming contrast, compared to that in the Atlantic-Indian sector (0.16 W m-2 per\ndecade). The discrepancy of net surface heat flux between observations (heat loss trend)\nand CESM1-LE (heat uptake trend) reflects a known surface heat flux bias in models.\nWe’ve revised the manuscript accordingly.\nLines 153-161: The trend of Q (Fig. 2a and Extended Data Fig. 5a) suggests\nnet\nprevailing surface heating in the high-latitude seas around Antarctica and heat release\ntoward the atmosphere in the mid-latitudes north of 50°S (Extended Data Fig. 5c and\nExtended Data Fig. 6a). Interestingly, on average, the Pacific sector (1.08 W m-2\ndecade-1) receives stronger surface heating than the Atlantic-Indian sector (0.42 W m-2\ndecade-1) in the high-latitudes in ERA5. This feature is further supported by Japanese\n55-year Reanalysis (JRA-55) data, with 0.57 W m-2 decade-1 in the Pacific sector versus\n0.29 W m-2 decade-1 in the Atlantic-Indian sector. These results indicate surface heating\ncannot explain the asymmetric pattern of heat storage.\nLines 180-190: In CESM1-LE, there are prevailing trends of positive Q at all\nnet\nlatitudes of the SO (Fig. 2c and Extended Data Fig. 6b), in contrast to the mid-latitude\nheat loss in ERA5 (Fig. 2a and Extended Data Fig. 6a). This discrepancy is linked to\ninsufficient strengthening of westerly winds compared to observation-based changes\nand easterly trends in mid-latitude regions (Fig. 2d) – a common bias among IPCC-\nclass climate models40,50, which reduces the total wind speed and suppresses heat\nrelease there. The underestimated heat uptake in the high latitudes is likely due to\noverestimated surface warming and absence of regional cooling trends around Antarctic\nin models51. Nevertheless, the surface heating in the Pacific sector is also stronger than\nin the Atlantic-Indian sector (0.24 versus 0.16 W m-2 decade-1), unable to account for\nthe SO warming asymmetry.\nFig. 2 | Changes in surface heating and winds over the SO. a, b Linear trends in\nsurface net heat flux Q and wind stress curl (WSC; shading) for the period of 1960-\nnet\n2020, based on European Centre for Medium-Range Weather Forecasts (ECMWF)\nreanalysis dataset version 5 (ERA5). In b, trends of wind stress are show as vectors. c-\nd, Same as a-b, but derived from CESM1-LE ensemble-mean. Stippling in a-d\nindicates insignificant changes at the 95% confidence level.\nExtended Data Fig. 5 | Changes in surface heating and winds in JRA-55. a-b, Linear\ntrends in Q and wind stress curl for the period of 1960-2020 based on JRA-55\nnet\nreanalysis. In b, trends of wind stress are shown as vectors. c-d, Average mid-latitude\n(35°S-55°S) Q and wind stress curl in the Pacific and Atlantic-Indian sectors. e,\nnet\nAverage high-latitude (55°S-70°S) Q in the Pacific and Atlantic-Indian sectors.\nnet\nDashed lines in c, d, e denote the linear regression.\nExtended Data Fig. 6 | Changes in surface heating in ERA5 and CESM1-LE. a-b,\n35°S-55°S average Q in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nnet\nsectors based on ERA5 and CESM1-LE.\nQ10: lines 162-167. Also, only one reanalysis is considered (ERA5); the results need\nto be extended to include other reanalyses (e.g. MERRA2, JRA) to determine the\nrobustness of the heat flux trends.\nResponse:\nWe thank the reviewer for this important suggestion. We have included results from the\nJRA55 reanalysis (Extended Data Fig. 5). Although there are differences in magnitude\nbetween ERA5 and JRA55, qualitative results are consistent, including stronger surface\nheating in the Pacific sector in the high-latitudes and mid-latitude heat release in both\nsectors, which cannot explain the stronger heat storage in the Atlantic-Indian sector.\nMERRA2 was not included due to its limited temporal coverage (starting in 1980),\nwhich prevents a consistent 60-year trend comparison.\nWe’ve revised the manuscript accordingly.\nLines 153-161: The trend of Q (Fig. 2a and Extended Data Fig. 5a) suggests\nnet\nprevailing surface heating in the high-latitude seas around Antarctica and heat release\ntoward the atmosphere in the mid-latitudes north of 50°S (Extended Data Fig. 5c and\nExtended Data Fig. 6a). Interestingly, on average, the Pacific sector (1.08 W m-2\ndecade-1) receives stronger surface heating than the Atlantic-Indian sector (0.42 W m-2\ndecade-1) in the high-latitudes in ERA5. This feature is further supported by Japanese\n55-year Reanalysis (JRA-55) data, with 0.57 W m-2 decade-1 in the Pacific sector versus\n0.29 W m-2 decade-1 in the Atlantic-Indian sector. These results indicate surface heating\ncannot explain the asymmetric pattern of heat storage.\nExtended Data Fig. 5 | Changes in surface heating and winds in JRA-55. a-b, Linear\ntrends in Q and wind stress curl for the period of 1960-2020 based on JRA-55\nnet\nreanalysis. In b, trends of wind stress are shown as vectors. c-d, Average mid-latitude\n(35°S-55°S) Q and wind stress curl in the Pacific and Atlantic-Indian sectors. e,\nnet\nAverage high-latitude (55°S-70°S) Q in the Pacific and Atlantic-Indian sectors.\nnet\nDashed lines in c, d, e denote the linear regression.\nQ11: lines 173-174. ‘they are significantly stronger in the Atlantic-Indian sector than\nin the Pacific, highlighting a prominent inter-basin contrast.’ Again, this qualitative\nassertion needs to be backed up by quantitative analysis of the change in surface wind\nstress magnitude averaged over the two sectors. How much stronger is the Atlantic-Indian sector wind stress change\nthan the Pacific sector? And by what level of significance testing are they stronger? 95%? Two further panels are needed\nshowing the wind stress magnitude time series for each sector for ERA5 and JRA, MERRA2. Overall, the statements here\non heat flux and wind stress contributions are too superficial and lacking in quantitative analysis to merit publication.\nResponse:\nThank you for raising this important point. The difference of winds in ERA5 is\nsignificant at 90% significance level. We’ve added the quantitative analysis of wind\nstress in the manuscript.\nWind stress magnitude time series for ERA5 is shown in Fig. 3d. We’ve added similar\nanalysis for JRA in Extended Data Fig. 5d, which also shows a stronger wind\nintensification in the Atlantic-Indian sector, despite different magnitude in ERA5 and\nJRA55. MERRA2 was not included due to its limited temporal coverage (starting in\n1980), which prevents a consistent 60-year trend comparison.\nWe’ve added quantitative analysis on heat flux (in the response to Q10) and wind stress\nboth in ERA5 and JRA55.\nLines 171-177: The strengthened westerlies accompany trends of negative wind stress\ncurls (WSCs) south of 55°S and positive WSCs north of this latitude, which enhances\nthe northward ocean heat transport from high latitudes and heat convergence in the mid-\nlatitudes27. Similar trend patterns are seen in the JRA-55 data (Extended Data Fig. 5).\nIn addition to prevailing intensification across sectors, the inter-basin contrast with a\nstronger intensification in the Atlantic-Indian sector is significant at 90% confidence\nlevel.\nLines 210-214: The positive WSCs north of 55°S are enhanced in both sectors (Fig. 3c,\nd and Extended Data Fig. 5d). This enhancement is more pronounced in the Atlantic-\nIndian sector (0.61×10-8 Pa m-1 decade-1) than in the Pacific sector (0.49 × 10-8 Pa m-\n1 decade-1) in ERA5, which is also the case in JRA-55 (0.35 × 10-8 versus 0.28 × 10-\nFig. 3 | Inter-basin contrast in surface net heat flux and wind stress curl. a, c,\nLinear trends (solid curves) and annual climatology (dashed curves) of zonal-mean Q\nnet\nand\nWSC in the Pacific (from 150°E to 70°W) and Atlantic-Indian (from 70°W to 150°E)\nsectors during 1960-2020. Black dashed lines denote 55°S and 35°S. b, d, Average\nhigh-latitude (55°S-70°S) Q in open-ocean regions (annual mean sea ice\nnet\nconcentration < 50%) and average mid-latitude (35°S-55°S) wind stress curl in the\nPacific and Atlantic-Indian sectors based on ERA5 reanalysis. Dashed lines in b, d\ndenote the linear regression.\nExtended Data Fig. 5 | Changes in surface heating and winds in JRA-55. a-b, Linear\ntrends in Q and wind stress curl for the period of 1960-2020 based on JRA-55\nnet\nreanalysis. In b, trends of wind stress are shown as vectors. c-d, Average mid-latitude\n(35°S-55°S) Q and wind stress curl in the Pacific and Atlantic-Indian sectors. e,\nnet\nAverage high-latitude (55°S-70°S) Q in the Pacific and Atlantic-Indian sectors.\nnet\nDashed lines in c, d, e denote the linear regression.\nQ12: line 179.’ showing an evident inter-basin contrast in winds rather than Qnet.’\nAgain, support this assertion with box-averaged numbers.\nResponse:\nThank you for pointing this out. We’ve added box-averaged numbers.\nLines 205-214: The high-latitude Pacific sector loses heat to the atmosphere in the\nclimatology, which weakened at ~1.0 W m-2 decade-1 since 1960; meanwhile, the heat\nuptake in the Atlantic-Indian counterpart increased at a rate of ~0.4 W m-2 decade-1 (Fig.\n3b). In CESM1-LE, changes in Q can neither explain the inter-basin contrast in heat\nnet\nstorage (Extended Data Fig. 6b and Extended Data Fig. 7a). The positive WSCs\nnorth of 55°S are enhanced in both sectors (Fig. 3c, d and Extended Data Fig. 5d).\nThis enhancement is more pronounced in the Atlantic-Indian sector (0.61×10-8 Pa m-1\ndecade-1) than in the Pacific sector (0.49 × 10-8 Pa m-1 decade-1) in ERA5, which is\nalso the case in JRA-55 (0.35 × 10-8 versus 0.28 × 10-8 Pa m-1 decade-1).\nQ13: line 202. ‘heat uptake’ should be ‘heat loss’?\nResponse:\nWe apologize for the incorrect statement and have revised the manuscript.\nLines 205-206: The high-latitude Pacific sector loses heat to the atmosphere in the\nclimatology, which weakened at ~1.0 W m-2 decade-1 since 1960.\nQ14: lines 203-204. ‘CESM1-LE shows similar results to ERA5 (Extended Data Fig.\n4a).’ This statement is not supported by a comparison of the figures, Fig,4a shows weak\npositive trends in the mid-latitudes while Figure.3a shows increasingly strong\nnegative trends. This disagreement needs to be noted and raises concerns about the robustness\nof the mid-latitude heat flux trend conclusions as the reanalysis data and the model\ntrends differ in mid-latitudes.\nResponse:\nWe apologize for the incorrect statement in the previous version. CESM1-LE and ERA5\nshow some disagreement in mid-latitude surface net heat flux trends, with CESM1-LE\nexhibiting weak heat uptake and ERA5 showing more pronounced heat loss. This\ndiscrepancy reflects a known surface heat flux bias in many models. While this limits\nthe robustness of the heat flux trend comparison across reanalysis and models, both\ndatasets consistently show no significant inter-basin contrast in mid-latitude heat flux\ntrends, which is the key point supporting our conclusion that surface heating alone\ncannot explain the asymmetric heat storage. We have revised the manuscript to\nexplicitly acknowledge this model-data inconsistency.\nLines 180-190: In CESM1-LE, there are prevailing trends of positive Q at all\nnet\nlatitudes of the SO (Fig. 2c and Extended Data Fig. 6b), in contrast to the mid-latitude\nheat loss in ERA5 (Fig. 2a and Extended Data Fig. 6a). This discrepancy is linked to\ninsufficient strengthening of westerly winds compared to observation-based changes\nand easterly trends in mid-latitude regions (Fig. 2d) – a common bias among IPCC-\nclass climate models40,50, which reduces the total wind speed and suppresses heat\nrelease there. The underestimated heat uptake in the high latitudes is likely due to\noverestimated surface warming and absence of regional cooling trends around Antarctic\nin models51. Nevertheless, the surface heating in the Pacific sector is also stronger than\nin the Atlantic-Indian sector (0.24 versus 0.16 W m-2 decade-1), unable to account for\nthe SO warming asymmetry.\nFig. 2 | Changes in surface heating and winds over the SO. a, b Linear trends in\nsurface net heat flux Q and wind stress curl (WSC; shading) for the period of 1960-\nnet\n2020, based on European Centre for Medium-Range Weather Forecasts (ECMWF)\nreanalysis dataset version 5 (ERA5). In b, trends of wind stress are show as vectors. c-\nd, Same as a-b, but derived from CESM1-LE ensemble-mean. Stippling in a-d\nindicates insignificant changes at the 95% confidence level.\nExtended Data Fig. 6 | Changes in surface heating in ERA5 and CESM1-LE. a-b,\n35°S-55°S average Q in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nnet\nsectors based on ERA5 and CESM1-LE.\nQ15: lines 245-247. ‘The Pacific sector has received a stronger surface heating than the\nAtlantic-Indian sector, particularly in the high latitudes (Fig. 3a, b), which, however,\ndoes not lead to a stronger heat storage there. This indicates a heat exchange between\nthe two sectors.’ It looks like the surface heating is only stronger in the Pacific at high\nlatitudes and is not stronger in the mid-latitudes. So there’s no requirement for heat\nexchange between the two sectors in the mid-latitudes. The authors need to make this\npoint or show quantitatively that the Pacific mid-latitude sector surface heating is\nstronger than the Atlantic-Indian sector.\nResponse:\nThank you for pointing this out. We agree that Fig. 3 shows stronger surface heating in\nthe Pacific primarily at high latitudes rather than mid-latitudes. We’ve deleted this\nsentence.\nQ16: lines 253-255. ‘Since 1960, the heat transport from the Indian Ocean to the Pacific\nsector has increased by 1.1 × 1013 W/decade, slower than the heat transport increase of\n1.7 × 1013 W/decade at Drake Passage.’ Are these numbers significantly different?\nAlso, the time series are flat from 1960-1990 so fitting linear trends to the whole period\nmay not be appropriate. Please comment.\nResponse:\nThe difference between the heat transport trends at Drake Passage (1.7 × 10¹³ W/decade)\nand from the Indian to Pacific sector (1.1 × 10¹³ W/decade) is statistically significant at\nthe 90% confidence level. We’ve revised the manuscript accordingly.\nRegarding the relatively flat behavior from 1960 to 1990, we note that linear trend\nfitting remains appropriate here because all other variables in the study—such as\ntemperature, wind stress, and surface heat flux—are also analyzed using linear trends\nover the full 1960–2020 period. This provides a consistent framework for comparing\nlong-term basin-scale changes.\nLines 262-265: Since 1960, the heat transport from the Indian Ocean to the Pacific\nsector has increased by 1.1 × 1013 W/decade, slower than the heat transport increase\nof 1.7 × 1013 W/decade at Drake Passage, with the difference significant at the 90%\nconfidence level.\nQ17: lines 292-294. ‘Therefore, the zonal asymmetry in SO heat storage pattern – with\nthe Atlantic-Indian sector warming more rapidly than the Pacific – results mainly from\nwind-driven upper-ocean circulation changes. ‘ I’m not fully convinced by this as the\nWND experiment heat storage pattern shows heating confined to a relatively small\nregion of the Atlantic-Indian sector primarily in the Atlantic sector N of 45 S. Need to\nsee a version of Figs 1e-f but with each of the 4 model experiment time series shown in\norder to be convinced that wind-driven changes are the primary factor. So, please\ninclude these panels on the bottom of Fig.5\nResponse:\nWe appreciate this thoughtful and constructive suggestion. We’ve added the\ncorresponding 4 time series of temperature in Fig. 5, as requested. These additions\nallow for clearer comparison of inter-basin warming contrast across experiments.\nLines 285-296: The control simulation (CTRL) of the LASG/IAP Climate Ocean\nModel (LICOM) version-337 forced with daily surface atmospheric fields of ERA559\nhas well reproduced the observed heat storage pattern in the SO of 1960-2020, showing\nenhanced warming within the mid-latitude band of the Atlantic-Indian sector (Fig. 5a),\nalbeit with stronger trends. Compared to the observed structure, the simulated warming\nband is shifted equatorward, and the heat storage near the western boundary of Pacific\nsector is overestimated. These discrepancies are likely associated with coarse model\nresolution (1°) which leads to underestimation of mesoscale eddies. The poleward heat\ntransport induced by eddies act to compensate the wind-driven heat transport and the\nresultant heat convergence. Therefore, we adopt the central-to-eastern Pacific region of\n180°-70°E, 35°S-50°S to quantify its weaker warming trend (0.06 K decade-1) than that\nof the Atlantic-Indian sector (70°E-150°E, 35°S-50°S; 0.09 K decade-1) (Fig. 5b).\nLines 306-323: The sum of HTFL and WND (Fig. 5c, d) closely resembles CTRL in\nwarming pattern, indicating a negligible nonlinear effect between surface heating and\nwind-forcing and the robustness of our diagnoses. HTFL represents the storage of added\nheat42 - with warming signatures generated by surface heating and transported by the\nclimatological ocean circulation. This experiment produces quasi-uniform warming\nover the 35°-50°S band (Fig. 5e, f). Note that the warming of the Atlantic-Indian sector\nis slightly weaker than the Pacific sector, 0.05 versus 0.06 K decade-1, owing to\ninhomogeneous surface heating (Figs. 2 and 3). The results of HTFL confirm that\nsurface heat fluxes cannot account for the observed inter-basin contrast, although heat\nfluxes can cause some symmetric features in heat storage19,20.\nWND largely represents the storage of redistributed heat42 - with warming signatures\narising from heat redistribution induced by changes in wind-driven ocean circulations.\nThis experiment produces enhanced heat pile-up in the 35°-50°S band of the Atlantic-\nIndian Ocean sector (0.03 K decade-1) without additional heat uptake (Fig. 5g, h),\nin contrast to insignificant trends in the central-to-eastern Pacific sector. The results of\nWND confirm that the inter-basin contrast in the SO heat storage results mainly from\nwind-driven upper-ocean heat redistribution.\nFig. 5 | Heat storage and the inter-basin contrast in LICOM experiments. a, c, e, g\nLinear trends in 0-700 m OHC for the period of 1960-2020 in the CTRL (a), HTFL (c),\nand WND (d) experiments of LICOM and the sum HTFL and WND (b). Stippling\nindicates insignificant changes at the 95% confidence level. b, d, f, h The 0-700 m\naverage temperatures for the central-to-eastern Pacific (180°-70°E, 35°S-50°S) and\nAtlantic-Indian (70°E-150°E, 35°S-50°S) sectors in the CTRL (e), HTFL (g), and WND\n(h) experiments of LICOM and the sum HTFL and WND (f). Dashed line in b, d, f, h\nindicates the least squares fit to corresponding time series.\nQ18: lines 340-343.’ Under high and middle emission scenarios, the intensification and\npoleward migration of westerlies are projected to be more prominent in the Atlantic-\nIndian sector than in the Pacific sector, posing a greater threat to marine ecosystems\nand ice sheets there.’ Please add a reference to support this statement.\nResponse:\nWe’ve deleted this statement. Instead, we’ve added CMIP6 analysis under three\nemission scenarios: SSP5-8.5, SSP2-4.5, and SSP1-2.6 in Fig. 6b.\nLines 336-351: Our work highlights the necessity of looking forward into the future\nchanges in the SO based on projections of climate models. CESM1-LE project that the\nAtlantic-Indian sector will continue to warm faster than the Pacific sector in the 21st\ncentury under the high-emission RCP 8.5 scenario (Fig. 6a). Furthermore, in\nprojections of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the\n0-700 m warming of the Atlantic-Indian sector will reach 1.01 ± 0.36 °C, 1.63 ± 0.45 °C,\nand 2.56 ± 0.76 °C by 2100 relative to the 1960-1980 baseline under Shared\nSocioeconomic Pathway (SSP) 1-2.6, 2-4.5, and 5-8.5 scenarios, in contrast to the\n0.72 ± 0.28 °C, 1.10 ± 0.41 °C, and 1.72 ± 0.65 °C values of the Pacific sector. The\ninter-basin contrast of SO warming will further amplify throughout the 21st century\nunder SSP5-8.5 and SSP2-4.5 scenarios (Fig. 6b). Under the low-emission scenario of\nSSP1-2.6, the contrast will diminish by the mid-21st century, showing close\nwarming rates in the two sectors; the SO warming will terminate by the 2070s. The\ndependence on emission scenario highlights the essential role of greenhouse gases in\ngenerating the inter-basin warming contrast. Therefore, the inter-basin contrast is a\nrobust feature of the SO under anthropogenic climate change and will probably persist\nor amplify in the upcoming decades.\nFig. 6 | Schematic and projection of inter-basin warming contrast in the SO. sa,\nSchematic of inter-basin warming contrast. Projected temperature change is calculated\nas the time-mean temperature in 2080-2100 minus that in 1960-1980 from CESM1-LE\nensemble mean. Horizontal section shows projected 0-700 m vertical averaged\ntemperature change. Vertical cross-sections demonstrate projected zonal mean\ntemperature change in the Pacific (150°E-70°W) and Atlantic-Indian (70°W-150°E)\nsectors. b, Historical simulations (1960-2014) and future projections (2015-2099) of 0-\n700 m temperature changes of the Pacific and Atlantic-Indian sectors under SSP1-2.6,\nSSP2-4.5, and SSP5-8.5 scenarios based on CMIP6 models. Shading denotes the one\nstandard deviation range of model members.\nResponse to Review\nInter-basin contrast in the Southern Ocean warming\nYuanyuan Song1,2, Yuanlong Li1,3*, Gaël Forget2, Aixue Hu4, Qian Li2,5, Jia-Rui Shi6,7,\nXiaodan Chen8, Kai Ge1, Fan Wang1,3\n1Key Laboratory of Ocean Observation and Forecasting and Laboratory of Ocean\nCirculation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao,\nChina\n2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of\nTechnology, Cambridge, MA, USA\n3Laboratory for Ocean Dynamics and Climate, Qingdao Marine Science and Technology\nCenter, Qingdao, China\n4Climate and Global Dynamics Laboratory, National Center for Atmospheric Research,\nBoulder, CO, USA\n5Department of Earth, Ocean, and Atmospheric Science, Florida State University,\nTallahassee, FL, USA\n6Woods Hole Oceanographic Institution, Woods Hole, MA, USA\n7Courant Institute of Mathematical Sciences, New York University\n8Department of Atmospheric and Oceanic Sciences, Institute of Atmospheric Sciences, Fudan\nUniversity, Shanghai, China\nSeptember 1th 2025\nReviewer #1\nWe are grateful to the reviewer for the thoughtful and valuable feedback, and we have\nrevised the manuscript accordingly.\nThe point-to-point responses to Reviewer #1’s comments are given below:\nThe authors have done a good job in responding to my, and rev2’s, questions. Whilst I always\nfelt that the main results were robust, largely because they had been shown in other papers, I\nwas concerned that this was not a significant enough advance to warrant publication in Nature\nComms. However, the authors have now explicitly taken into account the previous work in this\narea and strengthened the presentation of their primary new contributions. I particularly\nappreciate the new work done with CMIP6 (new Figure 6b), which both provides a significant\namount of independent support for the historical period results largely focused on, as well as a\nvaluable future perspective. It would have been nice to show whether the regional wind/heat\nflux differences in these models match the other datasets presented too, but I accept that this is\nprobably a bit much to ask.\nThe other novelty introduced by this paper is the comparison between climatologies, forced and\nair-sea coupled models. Whilst I think it is fair to say the differences between these datasets\nmake the quantitative comparisons somewhat marginal (significant at the 90% level, here and\nelsewhere in the text is certainly touch and go), the qualitative difference is clearly there and\nshows that this is a result independent of the dataset/models – and the CMIP6 ensemble analysis\nprovides further support in this. The authors do acknowledge this in their conclusions now, and\nnote some of the causes of this. These are possibly to be expected due to the poor present state\nof modelling of the Southern Ocean air-sea fluxes and the authors do a good job in drawing\nattention to this important area of research.\nThe authors have taken on my, and Rev 2’s, suggestions for increasing the quantitativeness of\ntheir analysis, notably through the provision of averages over their regional boxes (eg Fig 5).\nThese are significantly more convincing now, though I worry that the fact that the regions\nchosen for averaging do not match up with the actual regional boxes suggests that these have\nbeen chosen with a degree of ‘p-hacking’ in mind. That said, the authors acknowledge this and\nprovide explanation for why this may be the case in the western pacific, and I think the\nqualitative results still bare out their thesis.\nI do still request one change to be made prior to this being accepted for publication. New line\n261 refers to the acceleration of the ACC and references ref 58. I think it should be tweaked\nslightly to say ‘...acceleration of some regions in the north of the ACC58 (although not through\nDrake Passage) and warming of the transported waters…’.\nOnce this minor change is made I would consider it appropriate for publication. Thank you to\nthe authors for making the changes requested and significantly improving the paper.\nResponse:\nWe sincerely thank the reviewer’s thoughtful and constructive comments, as well as for\nacknowledging the improvements made in our revision. We are pleased that the reviewer now\nfinds our analysis and presentation significantly strengthened, particularly with the addition of\nthe CMIP6 results and expanded discussion of previous work.\nRegarding the remaining point, we fully agree with the reviewer’s suggestion about the wording\nin Line 261. We have revised the sentence accordingly in the manuscript:\nLines 226-228: “In CESM1-LE, the ZOHT has increased at both 150°E and 70°W, reflecting\nthe acceleration of some regions in the north of the ACC58 (although not through Drake Passage)\nand warming of the transported waters carried by the ACC.”\nWe appreciate the reviewer’s careful attention to detail, and we believe this change further\nclarifies our description.\nReviewer #2\nWe are grateful to the reviewer for the thoughtful and valuable feedback, and we have\nrevised the manuscript accordingly.\nThe point-to-point responses to Reviewer #2’s comments are given below:\nThe authors have carried out extensive further analysis in response to my previous comments\nand I am now mostly satisfied that their results are robust. The only remaining cause for concern\nis that the additional 1990-2020 0-700m Atl-IO and Pac trends they have calculated are not\nsignificantly different at 1 sd level. They note that this could reflect decadal variability but the\nlack of a significant difference does weaken their argument somewhat. Nevertheless, the 1960-\n2020 and 2004-2020 trends do differ between basins. These point should be made more strongly\nin their next revision and the 1990-2020 trends should be included in Extended Data Fig.3. so\nthat the reader can form their own judgement of the significance of the results.\nResponse:\nWe sincerely thank the reviewer for this helpful comment. We agree that the 1990-2020 trends\nshould be included in Extended Data Fig. 3 and the reader can form their own judgement of the\nsignificance. We modified Extended Data Fig. 3 and corresponding statement in the manuscript.\nLines 121-122: “The weaker contrast over 1990-2020 implies the influence of decadal\nvariability on shorter-period change (Extended Data Fig. 3).”\nExtended Data Fig. 3 | Linear trends and uncertainty of 0-700 m temperature in\nthe Pacific and Atlantic-Indian sectors. Averaged linear trends and one standard\ndeviation ranges (colored bars) of averaged 0-700 m temperature in the Pacific and\nAtlantic-Indian sectors over 1960-2020, 1990-2020, 2004-2020 (the Argo era), and\n1960-2003 (pre-Argo era) based on observational datasets. Square, triangle, and circle\ndenote linear trends based on IAP, EN4.2.1, and Ishii, respectively. The range\nindicates one standard deviation."}]

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