The ocean plays a crucial role in redistributing heat within the Earth's climate system, transporting excess heat from the tropics towards higher latitudes. This transport is primarily achieved through ocean currents like the Atlantic Meridional Overturning Circulation (AMOC), a significant northward flow of surface and thermocline waters in the Atlantic. The Indo-Pacific Ocean's OHT is more symmetric around the equator, largely driven by subtropical cells (STCs). These processes help regulate global climate by moderating equator-to-pole temperature gradients. Under the conditions of global warming, atmospheric heat transport (AHT) is anticipated to increase due to enhanced latent heat transport through water vapor in warmer air. However, changes in OHT are expected to counteract this effect, resulting in a weakening response not immediately evident through first principles. The concept of Bjerknes compensation, where changes in OHT and AHT largely cancel out when ocean heat uptake is neglected, does not hold true when considering the significant oceanic heat uptake (>90% of excess heat) in a global warming scenario. This disrupts the radiative heat imbalance at the top of the atmosphere, meaning the sum of OHT and AHT changes is not zero. Despite this complexity, climate change projections consistently show an opposing response between OHT and AHT to global warming. This suggests that changes in the sum of OHT and AHT are significantly smaller than changes in either OHT or AHT individually. This study directly addresses the question of how OHT decreases in future climate projections and whether this decrease is consistent across all ocean basins. While warming generally leads to increased stratification and enhanced OHT, a decrease signifies a weakening of ocean circulation. Models suggest that changes in ocean circulation, particularly in the North Atlantic, significantly impact the response to greenhouse gas warming. A warmer and wetter atmosphere reduces heat loss from the ocean and increases high-latitude freshwater input, reducing the density of convecting water masses and thus weakening the AMOC. This weakening, in turn, reduces OHT in midlatitudes, resulting in observable patterns of weaker ocean warming. The extent of AMOC reduction varies significantly among models, with differing behaviors observed between CMIP6 and CMIP5. Existing research primarily focuses on the Atlantic, Arctic, and Southern Oceans, leaving a gap in our understanding of the Indo-Pacific's response. This study fills this gap through a detailed analysis of OHT changes using both CMIP5 and CMIP6 archives.

Prior research has established the fundamental role of ocean circulation in global heat redistribution, particularly highlighting the AMOC's influence on northward heat transport in the Atlantic. Studies have also characterized the equatorward flow in the South Atlantic and the generally poleward OHT across most latitudes, emphasizing the climate-regulating effects of this redistribution. However, the specific response of OHT to global warming remains a subject of ongoing investigation. While the expected increase in AHT due to increased atmospheric moisture capacity is well-understood, the counteracting effects on OHT have not been fully elucidated. Previous works have explored the relationship between global warming and OHT changes in specific ocean regions (Atlantic, Arctic, Southern Ocean), predominantly using a process-based approach to understand circulation mechanisms. While the weakening of the AMOC under global warming is frequently observed in models, the Indo-Pacific OHT's response has received less attention. Several studies indicate that OHT and AHT changes, even in the context of significant ocean heat uptake, exhibit an opposite and compensating response. The precise mechanism behind this compensating response and the uniformity of the OHT decline across different ocean basins, however, require further detailed investigation.

This study utilized data from 52 ensemble members of 22 CMIP5 models and 52 ensemble members of 24 CMIP6 models. Data from historical simulations and future climate projections (RCP 2.6/SSP 1-2.6 and RCP 8.5/SSP 5-8.5) were analyzed. Ocean heat transport (OHT) was calculated using monthly mean temperature and meridional velocity fields on the respective model grids. The study accounted for variations in model grid types. To avoid biases introduced by differences in the methods used for calculating OHT in various estimates, the authors took care to use consistent methodologies throughout their analysis. The computation used a weighted average across all ensemble members for each model and employed a two-sample weighted Kolmogorov-Smirnov test to assess statistical significance. The global ocean was subdivided into Atlantic, Indo-Pacific (extending to 34°S), and Southern Ocean (south of 34°S) basins for analysis. Prior to OHT computation, the section average velocity was removed within each basin separately. The total OHT was then computed using the formula: OHT(y,t) = ∫∫Tdxdz x cp x ρo/10^15, where cp is the specific heat capacity (4000 J kg^-1 K^-1) and ρo is the mean density of seawater (1026 kg m^-3). The reference periods used were 1970-1999 for historical simulations and 2070-2099 for future climate scenarios. Changes in OHT were expressed as the difference between future and historical reference periods. The total OHT was further decomposed into overturning (zonal mean) and azonal (gyre) components. These components were analyzed to understand the contributions from velocity changes, temperature changes, and nonlinear effects using a decomposition technique where the temperature or velocity fields were set to the historical mean. The AMOC was computed at each latitude and for each basin using the meridional velocity with the section-averaged velocity removed to isolate the overturning component of the circulation. The impact of the higher climate sensitivity of CMIP6 compared to CMIP5 was investigated by scaling CMIP6 OHT changes using global mean temperature (GMT) changes, allowing for a comparison of OHT responses under similar radiative forcing conditions. The analysis included assessing the changes in the Indonesian throughflow in the Indo-Pacific region. For the Southern Ocean, the study addressed the challenge of limited data by comparing OHT and ocean temperature transport (OTT) estimates (including and excluding parameterized eddy effects) and compared those to observational data. The impacts of eddies were addressed by comparing those models with bolus velocity available for OTT computations.

The study's key findings reveal a projected decrease in poleward OHT in response to global warming. This decrease is evident across all Northern Hemisphere latitudes and south of 10°S. The multi-model mean reduction in poleward OHT for the Atlantic at 26.5°N and Indo-Pacific at 20°S is substantial, ranging from 0.093-0.304 PW and 0.097-0.194 PW respectively, depending on the scenario and CMIP phase. Similarly, the reduction in poleward OHT at 55°S in the Southern Ocean ranges from 0.071-0.268 PW. The projected changes are significantly stronger in CMIP6 compared to CMIP5, even when corrected for its larger climate sensitivity. This difference is particularly noticeable in the Atlantic Ocean under weaker forcing scenarios (SSP 1-2.6/RCP 2.6), where the decrease in OHT at 26.5°N is 2.5 times larger in CMIP6. In the Atlantic, the reduction in northward OHT is primarily driven by changes in the overturning circulation, particularly a velocity-driven decline mirroring the weakening AMOC. Temperature-driven changes partially offset this reduction. In the Indo-Pacific, changes in OHT are more pronounced in the Southern Hemisphere and are primarily caused by overturning circulation changes, mainly in the velocity-driven component. In the Southern Hemisphere Indo-Pacific, the dominant factor is the weakening of the deep overturning cell associated with reduced Antarctic Bottom Water (AABW) formation, resulting from warming and freshening around Antarctica. The Southern Ocean shows a decrease in southward OHT in the future projections, with some scenarios even resulting in a reversal of OHT direction at 55°S due to an increase in northward overturning-driven OHT. The change is largely temperature-driven, influenced by the warming of the upper layers of the Southern Ocean.

The projected reduction in poleward OHT has significant implications for global climate, leading to reduced polar amplification and impacting sea surface temperatures. This is evident in the formation of warming holes in the Atlantic, where reduced temperatures in the warming hole lead to less heat loss to the atmosphere. The observed discrepancies between CMIP5 and CMIP6 projections suggest that CMIP6 models may be projecting a larger response to future climate change. The strongest difference is in the Atlantic, driven by the stronger response of the AMOC in CMIP6. Whether or not this stronger response in CMIP6 is more realistic is still debated, however, proxy-based evidence suggests that both CMIP5 and CMIP6 may underestimate the sensitivity of the AMOC to climate change. Until resolved, the findings from CMIP6 models serve as a strong warning that the impact of changes in Atlantic circulation due to global warming may be greater than previously thought. The observed differences between CMIP5 and CMIP6 models may partly be attributed to the differing radiative forcing increase rates between RCP and SSP scenarios. While analysis of a single model simulating both RCP and SSP scenarios shows larger responses in SSP scenarios, the differences are smaller than in the multi-model mean ensembles, suggesting that other factors beyond scenario design contribute to the CMIP5/CMIP6 differences.

This study provides compelling evidence of a projected decrease in poleward ocean heat transport under future climate scenarios, with CMIP6 models indicating a stronger response than CMIP5, even when accounting for differences in climate sensitivity. The Atlantic Ocean is particularly vulnerable, experiencing a substantial decline in OHT due to AMOC weakening. The Indo-Pacific and Southern Ocean also exhibit significant changes. These findings highlight the critical need for further research to refine our understanding of ocean circulation changes and their impact on global climate, emphasizing the potential for more severe consequences than initially anticipated.

The study acknowledges limitations inherent in using CMIP model outputs, including model-dependent variations in AMOC response and potential biases related to resolution and parameterization of eddies, particularly in the Southern Ocean. The impact of aerosol forcing on AMOC decline and climate sensitivity variations between CMIP5 and CMIP6 remain subjects of ongoing research and might influence the interpretation of the findings. Additionally, the reliance on monthly mean data might overlook some smaller timescale changes in OHT. The lack of a perfect agreement with observation-based estimates could also be a limitation, though the paper attempts to account for this by comparing multiple types of OHT estimates.