Emergence of an oceanic CO_2 uptake hole under global warming

Domain: earth_science
Published: 2025-04-03
Editorial outcome: major_revision

Abstract
The ocean is a crucial sink for anthropogenic CO2 emissions, yet its future response remains uncertain. Here, using the Community Earth System Model (CESM2) under different CO2 emission rates, we find a pronounced weakening of ocean CO2 uptake in the Subpolar North Atlantic (SPNA), distinct from the global response. Initially, the SPNA is an effective CO2 sink, but due to the contraction of deep convection, the uptake decreases and the oceanic pCO2 exceeds the global average. Recognizing the importance of regional ocean circulation in CO2 uptake, we identify a nonlinear relationship between environmental conditions and uptake response, revealing consistent thresholds for the emergence of uptake weakening. These findings are also reproduced in the Coupled Model Intercomparison Project Phase 6, confirming their robustness. Overall, the distinct uptake response reflects the key role of regional dynamics in regulating the CO2 budget and geochemical environment, which is important for regional CO2 mitigation strategies.

Introduction
Human activities have led to a significant increase in carbon dioxide (CO2) emissions, with approximately 30% of this being absorbed by the ocean in the form of dissolved inorganic carbon (DIC)1. The ocean system is therefore an important buffer against anthropogenic CO2 emissions1,2. Naturally, a better understanding of the ocean carbon cycle is crucial for a reliable projection of atmospheric CO2 concentration under global warming. Earth System Models (ESMs) currently project that most of the ocean CO2 uptake would increase linearly in response to rising atmospheric CO2 levels, slowing towards the end of the simulation3–5. Interestingly, depending on the ocean basin, the CO2 uptake shows a different response. In both the Tropical and Southern Oceans, the uptake is particularly enhanced, with these regions expected to become more favorable for CO2 absorption due to changes in upwelling and sea ice6–8. In the North Atlantic Ocean (NA), previous studies suggest that the CO2 uptake may follow a distinct temporal evolution under global warming8–13. Therefore, as not all oceans respond in the same way, this highlight the need to focus on regions with unique characteristics.

The NA is currently known for its efficient CO2 uptake, covering only 15% of the global ocean area while holding more than 23% of the global oceanic anthropogenic CO2 14. This efficiency is closely linked to deep-water formation, facilitated by the cold and dense environment in the Subpolar North Atlantic (SPNA)14,15; excessive CO2 can be delivered into the deep ocean through the descending branch of the Atlantic Meridional Overturning Circulation (AMOC)14,16–18. Under greenhouse warming, changes in the AMOC naturally raise a potential loss of the effective CO2 uptake, which is indeed reported when the permanent collapse of the AMOC occurs8,9,11–13. For example, a recent modeling experiment suggests that the permanent collapse of the AMOC would diminish the NA CO2 uptake around the 22nd century8. Since such a permanent collapse is not projected to occur within 2100 in most of state-of-the-art ESMs19, one may reasonably expect that the SPNA will continue to serve as an effective sink for atmospheric CO2.

However, there remains the potential loss of the CO2 uptake efficiency in the SPNA under global warming scenarios. The deep convection around the SPNA, particularly Labrador- Irminger Seas (hereafter Lab-Irm), and the regional carbon cycle are also tightly connected to the subpolar gyre (SPG)15,20. Intense SPG maintains deep MLD through increased convection, while a weakened SPG suppresses the development of the active mixed layer21. While global warming weakens both the SPG and the AMOC22–24, the collapse of SPG cannot be regarded as identical to that of the collapse of the AMOC, particularly as their temporal evolutions may differ significantly21,25. Therefore, interpreting the SPNA CO2 uptake solely as a passive response to AMOC maybe a limited perspective. Consequently, there is the possibility of losing uptake efficiency triggered by abrupt changes in regional ocean dynamics, and this possibility remains largely unexplored.

Accordingly, this study aims to investigate changes in the SPNA CO2 uptake and local deep convection, with a particular focus on assessing potential nonlinear responses in CO2 uptake. Using a fully coupled Earth System Model (ESM) with varying CO2 emission rates, we reveal that the anomalous CO2 outgassing emerges with a distinct spatial pattern confined to the SPNA. Under greenhouse warming, the SPNA experiences unusual cooling—a phenomenon not observed in other regions—leading to the term “warming hole”26. In this context, we propose that the reversed CO2 flux exhibits similar unusual characteristics, with pronounced anomalous outgassing in the SPNA, while other regions continue to absorb more CO2. Drawing an analogy to the “North Atlantic warming hole”, we refer to this phenomenon as the “CO2 uptake hole”. Here, we show that the timing of the uptake hole occurrence coincides with the collapse of local deep convection, which could appear on relatively short timescales without substantial AMOC weakening.

Results
Ocean carbon cycle changes under the warming scenario
As the anthropogenic emission rate increases (Fig. 1a), the atmospheric CO2 concentration exponentially increases in each scenario, reaching approximately 1400 ppm, 1000 ppm, and 960 ppm at the end of the corresponding experiments (Fig. 1b). The global air-sea CO2 flux significantly uptakes the surplus atmospheric CO2. Thereby it absorbs a 3.5 PgC yr–1, 2.5 PgC yr–1, and 2 PgC yr–1 in ER1.5, ER1.0, and ER0.5, respectively, and reaches a quasi-steady state (Fig. 1d). This is mainly caused by decreasing the buffer capacity of the carbonate system—the efficiency of ocean CO2 absorption—due to change of multiple physical and chemical processes as anthropogenic CO2 dissolves into the ocean8,27,28. We further examine the spatial pattern of annual mean air-sea CO2 flux change in the last 20 years (years 2081–2100) relative to the initial 10 years (years 2001-2010) of ER1.5 (Fig. 1c). In line with previous studies, the global CO2 uptake is largely influenced by the Southern Ocean and tropics. In the Southern Ocean, turbulent exchange is expected to accelerate as sea ice cover decreases and westerly winds strengthen, while a weakening of the equatorial trade wind is followed by a weakening of the equatorial upwelling8. Both responses tend to provide fertile conditions to absorb the atmospheric CO2. Temporal evolutions of CO2 uptake changes in these regions well resemble that of the global ocean (comparing Fig. 1d and Supplementary Fig. 1c–d).

The SPNA, however, exhibits strong positive air-sea CO2 flux changes, indicating reduced CO2 uptake into the ocean relative to the initial 10 years (years 2001-2010, red box in Fig. 1c), which is consistently shown in different emission rate scenarios (i.e., ER0.5 and ER1.0; Supplementary Fig. 1a–b). The peak absorption is reached at 0.1 PgC yr–1, 0.05 PgC yr–1, and 0.03 PgC yr–1 by the years 2040, 2053, and 2075, denoted as the rebound time, which is the average of the maximum CO2 uptake peak times for each ensemble in the SPNA (vertical dashed line in Fig. 1e). From the rebound times, the SPNA begins to decrease the air-sea CO2 flux changes, thereby generating pronounced anomalous outgassing over the SPNA. It is already well known that the SPNA exhibits a distinct phenomenon compared to other regions, referred to as the “warming hole”26, which may also be manifested in the carbon cycle. Although the spatial patterns of the two features are not identical, we suggest referring to this significantly anomalous outgassing phenomenon as the “CO2 uptake hole”, drawing an analogy to the “warming hole”. Note that previous studies have already mentioned the possibility of the uptake hole phenomenon8–12. However, our results imply that its timescale could be much faster even without the AMOC collapse (Supplementary Fig. 2).

Furthermore, to assess the robustness of the uptake hole beyond the ER experiments, we extend our analysis using the Coupled Model Intercomparison Project phase 6 (CMIP6) (Methods). Regardless of the emission rates, the CMIP6 multi-model mean (MMM) shows similar features to the SPNA CO2 flux changes, although considerable uncertainty remains (Fig. 1e). The SPNA MLD, linked to the deep-water formation, exhibits significant inter model spread29 (Supplementary Fig. 3), contributing to considerable uncertainty, akin to the spread observed in the AMOC30,31. To underpin the robustness, we make a time-lag composite of SPNA air-sea CO2 flux based on the rebound time (Supplementary Fig. 4). The composite analysis is referenced to lag 0, corresponding to the timing of the local maximum CO2 uptake in each model. Notably, most models simulate this rebound feature, where the CO2 flux initially increases before subsequently decreasing. The result highlights that the CMIP6 MMM projects a reversed CO2 uptake trend, which is highly consistent with the ER experiments (comparing gray and colored thick lines in Supplementary Fig. 4). Furthermore, if we only select the models that closely simulate the observed MLD and CO2 flux (see “Methods”), the proposed rebound feature is much clearer (blue line in Supplementary Fig. 4). The spatial pattern of anomalous CO2 flux in the SPNA is also similar between CMIP6 MMM and ER1.5 (pattern correlation = 0.74) (Fig. 1c and Supplementary Fig. 5), which is also valid for the best models (pattern correlation = 0.81). It should be noted that other ER models and CMIP6 MMM also have high pattern correlation values in excess of 0.7. Thus, the results indicate that, in spite of the uncertainties, the CMIP6 generally shares a similar feature of the “CO2 uptake hole”.

Across different forcing scenarios and model frameworks, this points to the robustness of the CO2 uptake response to greenhouse warming. Therefore, we focus on understanding a different role of the SPNA region in CO2 uptake compared to the other oceans across all three global warming experiments.

Contribution of pCO2ocn change in the SPNA region
Air-sea CO2 flux is mostly proportional to the difference in partial pressure of CO2 between the ocean (pCO2ocn) and the atmosphere (pCO2atm), so-called the dpCO2 32–35. Here, we analyze the regional deviation from the global mean (denoted as ′) to examine distinct CO2 uptake changes over the SPNA region. Despite the interactive computation of atmospheric CO2, the pCO2atm0 is spatially quasi-homogeneous (not shown). Naturally, dpCO20 and pCO2ocn0 share similar spatial and temporal evolutions, implying that dpCO20 is largely attributed to changes in pCO2ocn0 (Fig. 2a). No matter what the emission rate, pCO20 exhibits a significant increasing trend, suggesting the SPNA pCO2ocn increases faster than the other oceans. Therefore, we suggest that the large reduction in dpCO20 is attributed to the rapid rise in pCO2ocn0 (Fig. 2a), consequently weakening the oceanic CO2 uptake in the SPNA (Fig. 1d).

The ocean pCO2 changes (ΔpCO2ocn) can be decomposed into the effects of several environmental factors by utilizing the first-order Taylor series expansion, which allows us to quantify how these environmental factors contribute to the ΔpCO2ocn (see “Methods”). Though there are some residuals near the end of the century, this method can decompose the contributions with negligible residuals (gray line in Fig. 2 and Supplementary Fig. 6). Thus, we use it to depict each contribution to the increase of the SPNA ΔpCO2ocn0 (Fig. 2b–d). Regardless of the experiments, sea surface temperature (SST), surface total alkalinity (ALK), and freshwater (FW) consistently contribute to a decrease in the SPNA ΔpCO2ocn0 (Fig. 2b–d). On the other hand, the surface DIC is the only major driver contributing to the increase of ΔpCO2ocn0. Furthermore, the DIC contributions are accelerated around the rebound time of the air-sea CO2 flux in each experiment (vertical dashed line in Fig. 2b–d), which also coincides with a notable increase of ΔpCO2ocn0 (black line in Fig. 2b–d). This indicates that the acceleration of DIC accumulation over the SPNA is the key reason for the emergence of the CO2 uptake hole. In fact, the SPNA is the only region, where the DIC contribution exceeds the total ΔpCO2ocn (rightmost column in Supplementary Fig. 6). It clearly shows that CO2 accumulates at the ocean surface much faster than in the atmosphere, pointing the need to understand the underlying physical dynamics.

Underlying mechanisms of the air-sea CO2 flux reversal
The DIC accumulation in the SPNA at a rate exceeding its sources implies the presence of unique processes, with the AMOC being an intuitive candidate. The AMOC is continuously weakening under greenhouse warming (Supplementary Fig. 2), contributing to the regional DIC accumulation as the DIC-poor water in the subtropics is less transported36–39. Current understanding suggests that SPNA CO2 uptake is likely to decline as the AMOC weakens and even collapses in future climate scenarios8–11. We further examine the CO2 flux response to the AMOC changes, which exhibits a nonlinear relationship in both the ER experiments and the CMIP6 MMM (Fig. 3a). The nonlinear relationship suggests that the CO2 uptake hole appears when the AMOC reaches a certain threshold. At the local minimum of the air-sea CO2 flux, the AMOC strength is about 15 Sv for ER experiments and 13 Sv for CMIP6 MMM. While there is intermodel spread in AMOC strength itself, the threshold is still highly distant from the permanent collapse8,40, reflecting an indirect role of AMOC in regulating the CO2 uptake hole and indicating the possibility of CO2 uptake hole prior to the permanent collapse. Accordingly, we need to invoke another process more responsible for the nonlinear CO2 uptake: the collapse of deep convection in the Lab-Irm.

The SPNA is one of the main regions where deep convection is observed in the Northern Hemisphere41–44 (Supplementary Fig. 3a), and the employed model also simulates the climatological MLD in the Lab-Irm extending beyond 1000m, active deep convection (black contour line in Supplementary Fig. 3b). MLD across the entire SPNA region exhibits a decrease under greenhouse warming. Interestingly, the Lab-Irm MLD—a footprint of deep convection—continues to decrease until it approaches about 200m and stays afterward, which can be considered as the shutdown of the Lab-Irm deep convection (black line in Fig. 4a and Supplementary Fig. 7a–b). The CMIP6 MMM also shows a marked reduction in deep convection, nearly vanishing at a depth of approximately 200m, a phenomenon even more distinctly captured by the best models (black line in Fig. 4b and Supplementary Fig. 7c). Recent studies have distinguished the shutdown of the Lab-Irm MLD from that of the AMOC, indicating that they are not the same phenomena. The regional contraction affects the AMOC but does not completely disrupt it21,25. Consistently, although the AMOC still takes a substantial role in the long-term adjustment of the climate system, the Lab-Irm MLD collapses on a much shorter time scale, initiating dynamic feedback to halt the deep CO2 transport and uptake there (Fig. 4a, b and Supplementary Fig. 7).

The local DIC accumulation upon the upper ocean is evident in the Hovmöller diagram of the vertical DIC deviations over the SPNA (Fig. 4a, b and Supplementary Fig. 7). For more detail, at the beginning of the experiments, the deep ocean exhibits significantly higher DIC concentration compared to the surface. This leads to effective surface-to-deep transport, despite the gradual decrease of the MLD. However, as the collapse gets closer, DIC accumulates explosively in the upper ocean (0–200m) because there is no longer active mixing between the upper and deeper ocean. Consistently, the CMIP6 MMM also shows increased DIC accumulation in the surface layer as the MLD approaches 200m, with this feature being clearer in the best models (Fig. 4b and Supplementary Fig. 7c). Hence, the Lab-Irm MLD collapse evidently acts as a positive feedback which amplifies a surface DIC accumulation over the SPNA. This mechanism ultimately has a direct impact on the CO2 uptake. It might not be surprising that the convection collapse is affected by the emission rates; however, it is quite striking that the collapse consistently aligns with the rebound of the air-sea CO2 flux, regardless of the emission rates (Fig. 1e). Therefore, in both the ER experiments and the CMIP6, the SPNA CO2 uptake is weakened as the Lab-Irm MLD begins to collapse, highlighting the importance of the proposed dynamical feedback (Fig. 3b).

Potential indicators of the uptake hole emergence
The surface DIC concentration that induces the CO2 uptake reversal also shows a nonlinear relationship with a local minimum around 2200–2250 mmol m–3 (Fig. 3c). After the minimum, the air-sea CO2 flux starts to decline despite continuous DIC increases. Therefore, the minimum serves as a threshold at which the uptake hole emerges. Overall, the chemical environment (i.e., DIC) is accompanied by regional ocean dynamics (i.e., MLD) and large-scale dynamics (i.e., AMOC), which together establish the nonlinear relationship with the CO2 uptake hole in the SPNA (Fig. 3). Although individual models exhibit varying threshold points over time due to differences in rebound timing of the CO2 flux (Supplementary Fig. 4), our finding is that all three indicators show a similar threshold leading to CO2 reversal within a reliable timeframe—particularly compared in the CMIP6 MMM and ER experiments—suggesting that the emergence of a CO2 uptake hole would not be unavoidable (comparing vertical lines and color transitions in Fig. 3).

We could utilize these relationships to assess the potential emergence of the uptake hole under climate change scenarios projected by current climate models. By comparing modeled thresholds with present-day observations, we estimate the remaining distance to the onset of the uptake hole. Examining the recent climatology obtained from the RAPID array and reanalysis data (SODA, ECCO, ORAS5 and the OceanSODA-ETHZ): about 17 Sv, 800m, 2230 mmol m–3 (vertical lines in Fig. 3) may provide valuable insights for predicting anomalous outgassing in the SPNA. While we refrain from overemphasizing the thresholds in determining exact timing, it is highly plausible that the uptake hole’s emergence will occur on a shorter timescale than the anticipated AMOC collapse.

So far, the analyses have only focused on the temporal evolution in terms of the multi-model mean, raising a query of its credibility in the context of inter-model spread. We hence compare the timing of the SPNA CO2 flux rebound to the local minimum and the MLD shutdown to approximately 200m (Fig. 4c). Despite the existence of models that do not simulate the Lab-Irm MLD (Supplementary Fig. 3), the inter-model relationship depicts a linear relationship (r=0.53, p value < 0.05). This suggests that, in difference to the previous finding that the loss of CO2 uptake capacity in the SPNA is associated with the collapse of the AMOC, regional deep convection acts as a direct modulator of CO2 capacity regulation in the SPNA.

Methods
CO2 emission rate (ER) experiments design
The Community Earth System Model, version 2 (CESM2) was used to conduct an idealized CO2 emission-driven simulation in this study. This model is composed of physical components, including the atmosphere, ocean, land, and cryosphere, which are fully integrated with the land and ocean carbon cycles59. The components of the model, including the Community Atmospheric Model, Version 6 (CAM6) for the atmosphere, and the Community Land Model Version 5 (CLM5) for the land60, are implemented with a spatial resolution of approximately ~1° and 32 vertical levels. The sea ice component is the Community Ice CodE version 5 (CICE5)61. The ocean component is the Parallel Ocean Program version 2 (POP2) with a nominal horizontal resolution of 1° (the meridional resolution was about ~0.27° near the equator), and 60 vertical levels are used59,62. The ocean carbon cycles in CESM2 are depicted by the Marine Biogeochemistry Library (MARBL)63.

We designed a series of idealized simulations with different CO2 emission rate (ER) to explore the pathway dependency (Fig. 1a). We first conducted an experiment in which anthropogenic CO2 emissions increase linearly to 30 PgC yr–1 from 2001 to 2100, based on the business-as-usual scenario (SSP5-8.5), referred to ER1.0. We further integrated ER1.5, in which anthropogenic CO2 emissions increase linearly to 45 PgC yr–1 from 2001 to 2100, which is 1.5 times the emission rate of ER1.0. Similarly, ER0.5 is conducted with half of the ER1.0 emission rate, so its anthropogenic emissions increase linearly to 21 PgC yr–1 from 2001 to 2150. Note that ER0.5 is integrated into 2150 (an additional 50 years) to simulate substantial global warming. Across all experiments, we conducted a total of 7 ensemble members, each initialized with slightly different initial conditions. These experiments are driven by the emission, which means that the atmospheric CO2 concentration is determined not only by prescribed anthropogenic CO2 emissions but also by oceanic and terrestrial CO2 fluxes interactively, thereby reflecting changes in CO2 concentration via the carbon cycle-climate feedback. Note that, all model datasets are re-gridded to a horizontal resolution of 1°×1°.

Coupled Model Intercomparison Project phase 6 (CMIP6)
Ensuring consistent representation of the CO2 uptake hole across different models and experimental configurations is crucial. To show its robustness, we analyzed 17 CMIP6 models using historical simulations (2001–2014) and SSP5-8.5 (2015–2100) simulations (Supplementary Table. 1)64. The models were selected based on the availability of the necessary variables, including air-sea CO2 flux, Atlantic Meridional Overturning Circulation (AMOC), dissolved inorganic carbon (DIC), and salinity. Each model dataset was converted to a horizontal resolution of 1°×1°. Furthermore, to choose more reliable model to simulate our key processes, the pattern correlation of the annual average from 2001 to 2010 between the reanalysis data and CMIP6 historical simulation was conducted (Supplementary Fig. 8b). The pattern correlations of the SPNA CO2 flux in each model show a strong linear relationship with that of the mixed layer depth (MLD) (r=0.84), highlighting that the MLD is a critical determinant of the SPNA CO2 flux, serving as an essential mechanism for the physical CO2 pump in the present day. It is also noted that the CESM2 used for the ER experiment is among the closest models to the observation. The linear relationship enables the selection of the 8 good models (CNRM-ESM2-1, MPI-ESM1-2-LR, NorESM2-MM, CESM2, CESM2-WACCM, UKESM1-0-LL, GFDL-CM4, and GFDL-ESM4) that closely simulate both MLD and CO2 flux in agreement with reanalysis data. These models exceed the mean of the SPNA CO2 flux and the MLD correlation coefficient, respectively (above the horizonal and vertical gray line in Supplementary Fig. 8b).

The linear relationship between the SPNA MLD and the CO2 flux, represented by the pattern correlation values between reanalysis data and the CMIP6 historical simulation, suggests that the performance of ocean biogeochemical cycles is influenced by the performance of ocean physical processes, contributing to multi-model diversity. Notably, CESM2 has been reported to represent the major ocean circulations relatively well among the current Earth System Models (ESMs). For example, the strength of the Antarctic Bottom Water, North Atlantic Deep Water, and AMOC in CESM2 are more comparable to observations than those in other models25,29,31,65. In particular, the overflow parameterization is implemented, which provides a better projection of deep ocean dynamics29,66. At this point, we quantified the spread in MLD climatology based on the area-averaged deep convection site (Supplementary Fig. 8a). The deep convection area is defined as the region where the MLD exceeds 1000m around the Labrador-Irminger Seas in maximum MLD in March (black contours in Supplementary Fig. 3b). Compared with the MLD reanalysis data, CESM2 is one of the best-performing models that simulate SPNA MLD strength and pattern with reliable consistency (Supplementary Fig. 3 and Supplementary Fig. 8).

Observational data and reanalysis products
The RAPID-MOCHA (Rapid Climate Change-Meridional Overturning Circulation and Heat flux Array) is an international project to observe real-time AMOC, which put array monitoring the AMOC at 26°N since April 200467. We downloaded the data from www.rapid.ac.uk/rapidmoc and calculated annual averages from the years 2004 to 2017. The AMOC strength in this study is defined as the maximum zonally integrated Atlantic overturning stream function (on the y-z plane) at 26°N and between 500 and 2000m.

We examined the Labrador-Irminger Seas MLD reanalysis data from the Simple Ocean Data Assimilation (SODA) version 3.15.268 ocean, Estimating the Circulation and Climate of the Ocean (ECCO)69, and Ocean Reanalysis System 5 (ORAS5)70 from 2001 to 2017 in March. The Max Planck Institute for Meteorology Self-Organizing Map Feed Forward Neural Network (MPI-SOMFFN)71, OceanSODA-ETHZ72, and CSIR-ML673 provides an estimate of the contemporary air-sea CO2 flux for the open ocean, used for the year 2001–2010. For the analysis, we computed the average of the three reanalysis datasets in the MLD and air-sea CO2 flux. The OceanSODA-ETHZ data for the annual averages of surface DIC and salinity spanning the period 2004 to 2017 are also employed. This data was generated by extrapolating observations of the surface ocean carbonate system in time and space using the newly developed Geospatial Random Cluster Ensemble Regression (GRaCER) method72.

Contribution decomposition of pCO2ocn
To understand the factors driving changes in ocean CO2 partial pressure (pCO2ocn), we calculated a first-order Taylor series expansion of pCO2ocn as a function of sea surface temperature (SST), surface dissolved inorganic carbon (DIC), surface total alkalinity (ALK), and sea surface salinity (SSS)34. In this study, to separate the influence of freshwater (FW) flux, we adopted normalized DIC (sDIC = Sref/S * DIC) and ALK (sALK = Sref/S * ALK) with reference salinity Sref (35 psu), and the equation is expressed as follow (e.g., Sarmiento and Grubuer 2006; Landschützer et al. 2018):

ΔpCO2ocn = (∂pCO2ocn / ∂SST) * ΔSST + (∂pCO2ocn / ∂DIC) * ΔsDIC + (∂pCO2ocn / ∂ALK) * ΔsALK + (∂pCO2ocn / ∂FW) * ΔFW (1)

where Δ indicates response to the changes relative to the initial value. Note that the PyCO2SYS is used to compute the sensitivities of pCO2ocn to DIC and ALK74. Furthermore, we repeat this decomposition with the deviations from the global mean to calculate the regional differences.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The study by Lee and co-authors investigates emission-driven future scenarios with the Earth System CESM2 and finds an extraordinary future outgassing of CO2 in the subpolar North Atlantic. The article identifies a shutdown of deep convection in the Labrador and Irminger Sea as a reason for this outgassing of CO2 in the subpolar North Atlantic.\nI really enjoyed reading this article and found the findings interesting and exciting. I regret to still recommend major revisions, but I believe that the article needs more work to be suitable for publication in Nature Communications.\nPlease find more details in the attachment.", "reviewer_2": "Summary:\nThis manuscript describes an abrupt weakening of the oceanic sink for atmospheric carbon dioxide in the sub-polar North Atlantic Ocean that may occur in the future, related to the slowdown of Labrador-Irminger Sea deep convection and related slowing of the Atlantic Meridional Overturning Current (AMOC). The authors first describe the emergence of this slowdown, especially anomalous in relation to other ocean basins; then identify the mechanism driving carbon system variability (DIC); then show that the buildup of surface DIC corresponds to the weakening of deep convection. In the discussion, the authors suggest that the abrupt onset of this feature suggests the existence of a threshold; that is to say, deep convection does not significantly impact the SPNA CO2 flux until the AMOC weakens below 15 Sv.\nMajor Comments:\nWhile this result sounds notable, it is in fact a well-known prediction that future climate simulations are likely to limit the global CO2 sink. This has been investigated thoroughly by the community, most notably including Chikamoto and DiNezio, 2021. This publication highlights the identical feature profiled by this submission: a slowdown of CO2 flux related to the weakening of AMOC and associated deep convection. The SPNA hotspot was directly highlighted there (see Figure 3). The authors themselves highlight this same contribution (their reference 12). If the authors are describing a unique simulation or significant divergence from this previous work, they do not highlight this significant advance in this work. Perhaps the most notable component of the work here is to relate the sudden slowdown of CO2 flux to a particular AMOC threshold, but the authors spend only one paragraph describing this potential threshold. Accordingly, this manuscript feels rushed and the analysis somewhat shallow.\nAs another example, the authors essentially provide the same information in three successive paragraphs: Lines 110-122, lines 123-134, and lines 137-165 all showcase the exact same conclusion: that DIC builds up in the surface ocean after deep convection stops. Again, this is a highly predictable result that already appears in the literature; given the high standard for novelty required by Nature journals, the attention to detail on this point seems out of place.\nLast, the authors themselves point out the weakness of using a limited ensemble to explore this phenomenon. When publishing extraordinary results, the evidence must be correspondingly extraordinary; if one does choose to consider this slowdown a novel result, this analysis does not seem sufficiently robust to prove such a claim is not a model artefact.\nOverall, the authors have not shown the necessary novelty of their results to merit publication in a journal of this caliber, although a more detailed accounting of this depth of analysis may be suitable for other journals. Multiple previous publications have pointed out similar processes and mechanisms before, even in this geographic region. The parts of the analysis that may seem novel—such as tying the weakening of the SPNA sink to a particular AMOC threshold— are given as an afterthought to the rest of an otherwise fairly routine analysis process. I strongly encourage the authors to spend more time in a deeper analysis of their data, and to showcase the truly novel parts of their work; this is a worthy subject and no doubt will be an important conversation in the decades to come. There is no need to sensationalize small steps when extraordinary results are likely to emerge with a little more effort.\nAs another example, the authors essentially provide the same information in three successive paragraphs: Lines 110-122, lines 123-134, and lines 137-165 all showcase the exact same conclusion: that DIC builds up in the surface ocean after deep convection stops. Again, this is a highly predictable result that already appears in the literature; given the high standard for novelty required by Nature journals, the attention to detail on this point seems out of place.\nLast, the authors themselves point out the weakness of using a limited ensemble to explore this phenomenon. When publishing extraordinary results, the evidence must be correspondingly extraordinary; if one does choose to consider this slowdown a novel result, this analysis does not seem sufficiently robust to prove such a claim is not a model artefact.\nOverall, the authors have not shown the necessary novelty of their results to merit publication in a journal of this caliber. Multiple previous publications have pointed out similar processes and mechanisms before, even in this geographic region. The parts of the analysis that may seem novel—such as tying the weakening of the SPNA sink to a particular AMOC threshold— are given as an afterthought to the rest of an otherwise fairly routine analysis process. I strongly encourage the authors to spend more time in a deeper analysis of their data, and to showcase the truly novel parts of their work; this is a worthy subject and no doubt will be an important conversation in the decades to come. There is no need to sensationalize small steps when extraordinary results are likely to emerge with a little more work.\nGiven the recommendation of rejection, minor line-by-line comments are not provided on this text."}, "author_response": "The response to Reviewer #1:\nThe study by Lee and co-authors investigates emission-driven future scenarios with the Earth System\nCESM2 and finds an extraordinary future outgassing of CO in the subpolar North Atlantic. The article\n2\nidentifies a shutdown of deep convection in the Labrador and Irminger Sea as a reason for this\noutgassing of CO in the subpolar North Atlantic.\nrecommend major revisions, but I believe that the article needs more work to be suitable for publication\nin Nature Communications.\nWe appreciate the reviewer for the thoughtful and constructive comments. Based on the reviewer’s\ncomments, we analyze CMIP6, which corroborates the robustness of our findings. We also carefully\nrevised the manuscript, clarifying language and comparing our findings with the previous literature.\nPlease see more specific responses below in blue.\nMajor points\n1. This article needs language editing. I started writing down the formulations that needed to be\ncorrected, but then I quickly realized that this concerned nearly every sentence of the manuscript.\nResponding to the reviewer’s comment, we have carefully checked every sentence, and rephrased them\naccordingly.\n2. The article considers idealized emission-driven simulations with the Earth System Model CESM2.\nHere, the considered simulation ER1.0 is based on the future scenario SSP5-8.5. It is not directly clear,\nwhy the CMIP6-scenario ‘esm-SSP5-8.5’ was not used for this study (the emission driven version of\nSSP5-8.5). ‘esm-SSP5-8.5’ was simulated by 14 Earth System Models (ESMs) and would allow to\nidentify if the results presented here are an outlier or if a similar outgassing of CO2 in the subpolar NA\nis also seen in other ESMs. Also, via comparison with results from SSP5-8.5, it would be possible to\nsee if these results are a product of the emission-driven version or if they are also seen in the ‘regular’\nSSP5-8.5. This information is important to identify the importance of the findings presented here.\nI would like to urge the authors to at least show the temporal evaluation of air-sea CO2-fluxes for an\nensemble of CMIP6 models (including CESM2) for both ‘esm-SSP5-8.5’ and ‘SSP5-8.5’ for the\nsubpolar North Atlantic. I believe that the statements in L255-256 and L274-275 simplify this issue\nway too much and that these statements should be rephrased substantially.\nThank you for the good suggestion. According to the reviewer’s suggestion, we analyzed the carbon\nflux responses in the CMIP6, and added the results in the revised manuscript.\n2-1) SSP5-8.5 scenario\nWe analyzed the CMIP6 historical and SSP5-8.5 scenario to examine the robustness of our findings,\nCO uptake hole, regardless of the model configuration. As the mechanism of the uptake hole is largely\n2\ndriven by the large-scale dynamics, the CMIP6 multi-model mean (MME) reproduces consistent results.\nWe first compared the CMIP6 MME CO flux response over the SPNA (Figure 1e and Supplementary\nL96-98: “Despite model uncertainties, the Coupled Model Intercomparison Project phase 6 (CMIP6) multi-\nmodel mean closely follows the air-sea CO flux changes with ER1.0 (grey line in Fig. 1e).”\n2\n2\nemission rate scenarios (i.e., ER0.5 and ER1.0; Supplementary Figs. 1a-b) and CMIP6 multi-model ensemble\n(Supplementary Figs. 2). The consistency across different forcing scenarios and model frameworks highlights\nthe robustness of the carbon uptake response to greenhouse warming.”\nFigure 1 | Carbonate system responses to the climate change. (a) Anthropogenic CO emission forcing and (b)\n2\ntemporal evolution of CO concentration in ER1 (green), ER0.5 (yellow), and ER1.5 (red). (c) Air-sea CO flux\n2 2\nchanges in ER1.5, which is the last 20 years (year 2080-2100) interval average in relative to initial 10 years (year\n2000-2010, hereafter initial year). Temporal evolution of integrated air-sea CO flux changes in relative to initial\n2\nyear of (d) global ocean and (e) Subpolar Northern Atlantic Ocean (SPNA, 45°N-65°N, 80°W-0°W). All line\ncolors are the same as noted above, and the grey line indicates the CMIP6 multi-model mean of the SSP5-8.5. In\nthe CMIP6 air-sea CO flux changes, the historical annual mean from 1980 to 2014 is used for the initial year.\n2\nabsorption peak trend\nincluded in all ensembles in the ER experiments. Negative (positive) air-sea CO flux indicates oceanic CO\n2 2\nuptake (outgassing). All values of colored lines represent the ensemble mean and shading shows the uncertainty\nof the projections calculated by the inter-ensemble spread (±1 𝑠.𝑑). These time series are calculated after taking\nthe 11-year running mean.\nSupplementary Figure 2 | Ocean carbonate system changes in the SSP5-8.5 scenario. Air-sea CO flux\n2\nchanges in the CMIP6 multi-model mean of the SSP5-8.5, i.e. the last 20 years (years 2080-2100), relative to the\n2\nhistorical annual mean from 1980 to 2014. The red box indicates the region of Subpolar Northern Atlantic Ocean\n(SPNA, 45°N-65°N, 80°W-0°W). Negative (positive) air-sea CO flux indicates oceanic CO uptake (outgassing).\n2 2\nMoreover, we described that the nonlinear relationships, CO flux with AMOC, MLD, and SPNA\n2\nsurface DIC concentration, also appear consistently in the CMIP6 MME (Figure 3). We added these\nresults in the revised manuscript as follows:\nL148-153: “Beyond the simple linear relation, it is evident the AMOC and the CO flux show a nonlinear\n2\nrelationship in both the ER experiments and the CMIP6 models, implying that a CO uptake hole emerges when\n2\nthe AMOC reaches a certain threshold (Fig. 3a). These results indicate that the nonlinear relationship exists\nrobustly, regardless of whether it is concentration-driven or emission-driven, the global warming pathway, or\nthe climate model used.”\nL184-187: “The relationship between the two variables shows that the CO2 absorption is attenuated in both the\nER experiments and the CMIP6 as the Lab-Irm MLD begins to collapse, also supporting the importance of the\nproposed dynamic feedback (Fig. 3b).”\nL190-198: “The surface DIC concentration, which reverses CO uptake response, naturally exhibits a nonlinear\n2\nrelationship. Consistently, there is a threshold that is universally adapted to different emission rates (~ 2250\nmmol m-3) (Fig. 3c). Before reaching the criteria, CO uptake strengthens with rising DIC. However,\n2\nsubsequent behavior in air-sea CO flux demonstrates a weakening of the uptake, despite continued DIC\n2\nincreases. These features generally appear in the CMIP6 multi-model with a comparable threshold (blue line in\nFig. 3c). Overall, we consistently examine the nonlinear relationship between SPNA carbon uptake and\nchemical environment (i.e., DIC), regional ocean dynamics (i.e., MLD), and large-scale dynamics (i.e.,\nAMOC).”\nFigure 3 | Nonlinear relationship with SPNA carbon uptake. Relationship between air sea CO flux change\n2\nand (a) AMOC, (b) Lab-Irm MLD, and (c) DIC in the SPNA. Scatter is shaped depending on their CO emission\n2\nrates: ER1.0 (triangle), ER0.5 (circle), and ER1.5 (rectangle) and shapes represent all ensembles in each\nexperiment. Blue line indicates the CMIP6 multi-model mean and blue scatter shows each 10 models. DIC is\naveraged surface layer (0-100m) and normalized by salinity (the standard salinity is 35 psu). AMOC strength is\ndefined as the average of the annual mean Atlantic meridional stream function from 26°N at a latitude and a\nmaximum depth below 500m. The grey vertical solid line indicates the current observation data in the decades\naverage and the grey vertical dashed lines represent the uncertainty in the decades (±1 𝑠.𝑑). The AMOC strength\nobservation (RAPID) from 26°N annually averaged during the year 2011-2021. The MLD (SODA) and DIC (The\nOceanSODA-ETHZ) reanalysis products annually averaged during the year 2004-2020.\nNote that, among the 12 models, 2 models are excluded in this analysis as it does not well reproduce\nthe observational March mixed layer over the Labrador-Irminger Sea, which is discussed in the\nMethod section (compared with the Supplementary Figure 5a and Figures 5c-n).\nER experiments. Note that among the 12 models, 2 models (CanESM5 and IPSL-CM6A-LR) are excluded from\nthe analysis because they do not reproduce the observed March mixed layer over the Labrador-Irminger Sea\nwell (Supplementary Fig.5b).”\nSupplementary Figure 5 | Mixed layer depth (MLD) climatology in March. Spatial pattern of (a) Ocean Data\nAssimilation (SODA) reanalysis data for the annual average from 2010 to 2020, (b) ER experiment, and (c)\nCMIP6 individual models. Black contour line in the (b) represents the deep convection index, where the MLD is\nover the 1000m. Note that CanESM5 and IPSL-CM6A-LR are excluded from the analysis due to the low\nreproducibility of the observed March mixed layer around the Labrador-Irminger Sea.\n2-2) esm-SSP5-8.5\nFollowing the reviewer’s suggestion, we also examined the CMIP6 emission-driven SSP5-8.5 scenario\n(esm-SSP5-8.5), which would give a valuable implication of the impact of configuration into the uptake\nhole. Hence, we gather all esm-SSP5-8.5 experiments that provide both air-sea CO flux and MLD (12\n2\nmodels). The esm-SSP5-8.5 also shows the reversal of the air-sea CO flux in terms of the multi-model\n2\naverage with a very similar time frame (gray dashed line in Figure R1-1). However, its intermodel\nspread is much larger than that of the SSP5-8.5 (comparing shading in Figure R1-1).\n2). The March MLD is generally inconsistent with the current observation (Supplementary Figure 5a)\nand, at the same time, the intermodel diversity is even larger than that of SSP5-8.5 (compared with\nSupplementary Figures 5c-n and Figure R1-2). Under these circumstances, we conclude that, although\nthe air-sea CO flux is rebounded in the SPNA region, the emission-driven multi-model comparison\n2\nmay be yet adequate to compare with our CESM2 experiments due to the unrealistic climatology.\nTherefore, in this manuscript, we present the mechanism analysis results for the SSP5-8.5 scenario\nalone, as it examines more reliable climatology.\nFigure R1-1 | Temporal Figure R1-2 | Mixed layer depth (MLD) climatology in March\nevolution of SPNA air-sea CO under the esm-SSP5-8.5 scenarios.\n2\nflux in ER1 (green), ER0.5\n(yellow), ER1.5 (red), CMIP6\nmulti-model mean of SSP5-8.5\n(black line) and esm-SSP5-8.5\n(grey dashed line)\n3. This study does not put their results into context of other studies that have used ESMs and\nhave studied the future carbon uptake of the North Atlantic (Goris, 2018, 2023; Park et al., 2023).\nSpecifically, Park et al. (2023) show that the CESM2 has rather a high salinity in the subpolar North\nAtlantic (in comparison to the CMIP6-model ensemble) and that this leads to an increase in dissolved\ninorganic carbon concentration, and thus to a reduction in the North Atlantic carbon uptake. This\nshould be noted in the discussion. In the introduction, the study here quotes observation-based studies\nin L43-45 but does not make it clear that these are observation-based studies about the past changes of\nthe North Atlantic carbon uptake. At the same time, studies with Earth System Models about the\nfuture carbon uptake of the North Atlantic are missing here.\nThank you for pointing this out and suggested references. We substantially revised both the introduction\nand discussion as following the reviewer’s suggestion:\n3-1) Introduction\nThanks for pointing this out. We substantially revised the introduction by reviewing more references\non the future NA carbon uptake studies using ESMs and pointing out their missing points, which are\nmotivation of our study.\nslow down, which will weaken dynamic sequestration and regional carbon storage7,8,10–12. For example, recent\nmodeling experiment suggests that the permanent collapse of the AMOC would diminish the NA carbon uptake\naround the 22nd century7. Despite the NA's smaller contribution to the global carbon budget, the projection\nuncertainty in ESMs is even greater for this region18–20, making it imperative to understand future carbon cycle\nimpacts and changes in physical processes there.”\nL49-55: “However, increasing evidence cautions against understanding the NA carbon cycle as merely a simple\nreflection of the AMOC response. The NA carbon uptake does not respond uniformly to global warming. The\nSubpolar North Atlantic region (SPNA) shows a weaker uptake response compared to the rest of the NA, due to\nthe strong coupling of the subpolar gyre (SPG) to the regional carbon cycle21. At the same time, it is proposed\nthat the collapse of AMOC should not be treated identically to the collapse of Labrador convection and\nSPG21,22. ”\n3-2) Discussion\nIn terms of discussion, we have revised the section comprehensively, including the suggested\nreference studies; Park et al. (2023) and Goris et al. (2018). Specifically, we outlined the implications\nof each paper's findings in relation to our study.\nL245-256: “Recently, efforts have been made to reduce the projection uncertainty of the NA climate by utilizing\nan emergent constraint. Park et al.37 found that the amount of future cumulative CO inventory depends on the\n2\nuptake hole existence with faster CO saturation in the future. Rather, Goris et al.38 directly examined tendency\n2\nchange of carbon uptake. They proposed that models with realistic NA simulation, stronger winter Lab-Irm\nmixing, and CO uptake in the NA, tend to commence the limited CO storage before the end of the 21st century\n2 2\ndue to occurs the vanished the mixing (i.e., CO flux tendency disappears). Reconciling with our findings, the\n2\nflattened CO storage could be understood as the onset of the uptake hole.”\n2\n4. The extraordinary outgassing of the subpolar North Atlantic is indicated via Figure 1 and\nSupplementary Figure 1. Both figures show changes in the air-sea CO2 flux, but it is not clear which\nperiods are compared (year 2080-21 compared to which period?). In order to verify the extraordinary\noutgassing, the temporal evolution of the subpolar North Atlantic carbon uptake is compared to that of\nthe Tropics (30°S-30°N) and the Southern extratropics (south of 40°S). This is a bit surprising, as the\nmap of changes in the air-sea CO flux illustrates that another region of declining CO2-uptake is\n2\nbetween approximately 30°S-45°S in Indian Ocean and Pacific Ocean. While the extraordinary\noutgassing in the subpolar North Atlantic is still interesting and worth a study about it, the selected\nregions hide that another region might also turn into a CO2-source.\n4-1) Compared periods\nTo clarify the period of air-sea CO flux changes in Figure 1 and Supplementary Figure 1, we rephrased\n2\nboth captions to “….in relative to initial 10 years (year 2000-2010).” Also, we reflected it in the\nmanuscript.\nL81-83: “We further examine the spatial pattern of annual mean air-sea CO flux change in the last 21 years\n2\n(year 2080-2100) in relative to the initial 10 years (year 2001-2010) of ER1.5 (Fig. 1c).”\n4-2) Other regions of declining CO uptake\n2\n2\nuptake as the reviewer pointed out (red and green box in Figure R1-3a). As shown in Figure R1-3,\nthese regions also turn into a CO uptake reverse, but the magnitude is much weaker than that in the SPNA.\nanomalous outgassing signal, but it will be worthwhile to analyze further in a future study.\nFigure R1-3 | (a) Air-sea CO flux changes in ER1.5, which is the last 21 years (year 2080-2100) interval average\n2\nin relative to initial 10 years (year 2001-2010). Temporal evolution of air-sea CO flux changes in the region of\n2\n(b) red box and (c) green box at (a). Each colored line indicates in ER1 (green), ER0.5 (yellow), and ER1.5 (red).\n5. The figure captions and legends of Figure 2 and supplementary Figure 2 suggest that these figures\nshow the same, yet the results look vastly different in magnitude.\nSorry for the confusion regarding the caption of Figure 2 and Supplementary Figure 2. For simplicity,\nFigure 2 displays the regional deviation (denoted as ') of SPNA relative to the global mean (SPNA-\nglobal mean) as derived from Supplementary Figure 2. Consequently, the legends and figure captions\nremain consistent because the same environmental variables are used in the decomposition.\nFurthermore, we substantially revised the descriptions of Figure 2 and Supplementary Figure 2 to\nreduce the potential confusion in the manuscript.\nL123-140: “The ocean 𝑝𝐶𝑂 change (∆𝑝𝐶𝑂𝑜𝑐𝑛) can be decomposed into the effects of several environmental\n2 2\nfactors by utilizing the first-order Taylor series expansion, which allows us to quantify how these environmental\nfactors contribute to the ∆𝑝𝐶𝑂𝑜𝑐𝑛 (see Methods). Though there are some residuals near the end of the century,\n2\nthis method decomposes the contributions with negligible residuals (gray line in Fig. 2 and Supplementary Fig.\n3). Thus, we could use it to depict each contribution to the increase of the SPNA ∆𝑝𝐶𝑂𝑜𝑐𝑛′ (Figs. 2b-d).\n(FW) consistently contribute to a decrease in the SPNA ∆𝑝𝐶𝑂𝑜𝑐𝑛′ (Figs. 2b-d). On the other hand, the surface\n2\naccelerated around the rebound time of the air-sea CO flux in each experiment (vertical dashed line in Figs.\n2\n2b-d), which also coincides with a notable increase of ∆𝑝𝐶𝑂𝑜𝑐𝑛′ (black line in Figs. 2b-d). This indicates that\n2\nacceleration of DIC accumulation over the SPNA is the key reason for the emergence of the CO uptake hole. In\n2\nfact, the SPNA is the only region, where the DIC contribution exceeds the total ∆𝑝𝐶𝑂𝑜𝑐𝑛 (rightmost column in\n2\natmosphere, underscoring the need to understand the underlying physical dynamics.”\n6. The authors suggest in in L99 that ‘Air-sea CO2 flux is mostly proportional to the difference in\npartial pressure of CO2’. It would be good to show evidence for this (in their specific model\nsimulation) and to disentangle this a bit more.\n2 2\nclearly shows that it is the same feature as the evolution of the air-sea CO flux (Figure R1-4). Therefore,\n2\nwe propose omitting the dpCO analysis from the manuscript, as it has been comprehensively covered\n2\nin the referenced studies (McKinley et al., 2004; Takahashi et al., 1997; 2002; 2009).\nL112-113: “Air-sea CO flux is mostly proportional to the difference in partial pressure of CO between the ocean\n2 2\n(𝑝𝐶𝑂𝑜𝑐𝑛) and the atmosphere (𝑝𝐶𝑂𝑎𝑡𝑚), so-called the 𝑑𝑝𝐶𝑂 25–28.”\n2 2 2\nFigure R1-4 | Temporal evolution of the difference in partial pressure of CO\n2\n(dpCO\n2\n)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Please see my attached review.", "reviewer_3": "The authors use a series of model simulations to illustrate a projected “hole” in North Atlantic carbon uptake, similar to the documented global warming hole in the North Atlantic. The modeled carbon uptake hole presented here, results from a reduction in deep vertical mixing, which generates a buildup of DIC in surface waters, thereby reducing ocean carbon uptake. Overall, I think this manuscript is interesting and worthy of consideration for the journal. I will also just note that I was not one of the original 2 reviewers of this manuscript, so some of my comments may be inconsistent with previous reviewer comments or changes made by the authors in response. That being said, my impression from reading the previous reviews is that the manuscript is greatly improved as a result of the authors incorporating the reviewer comments. I have some additional comments listed below which I think will help further benefit the manuscript.\nThe final sentence of the abstract makes a connection with regional strategies for carbon dioxide removal, but this is never actually mentioned in the manuscript. I think the authors could include some additional discussion here to further enhance the novelty of the results, which was an issue highlighted in the previous round of reviews. For example, the emerging research on ocean alkalinity enhancement tends to view the North Atlantic as one of the more unsuitable locations for deployment, because the deep vertical mixing removes the enhanced alkalinity water before most of the potential carbon uptake can occur. The work presented here though, suggests that may change in the not too distant future, and that maybe this could be a good location if convection slows down and the natural CO2 sink is reduced due to a buildup of anthropogenic DIC?\nIn the CMIP6 Models subsection of the methods, I’m confused why its stated that 10 CMIP6 models were used, and then in the next paragraph that there were 12 CMIP6 models but 2 were omitted from the analysis? If it was just 10 total models used from the CMIP6 archive, then why mention 12 later? There are far more than 12 models in the CMIP6 archive, so it’s not unusual to subset the archive based on the specific variables and temporal-spatial resolution required.\nAs noted by a previous reviewer, I think there is some language editing that this manuscript could still benefit from. However, I was mostly able to follow the text so it did not substantially impact my ability to understand the underlying science. Therefore, I think the additional editing could also occur with assistance by Nature after a manuscript decision is made.\nMinor comments:\nLines 49-57: The start of this paragraph seems to jump around between different mechanisms and effects for linking AMOC and NA carbon uptake, but doesn’t seem to build-up to the key topic that the authors are addressing, namely the shorter term response of NA carbon uptake to changes in climate/AMOC. Suggest rephrasing this a bit to ensure that the references are building towards this key final point.\nLines 51-53: This sentence is a bit vague to me, please describe what the “strong coupling of the subpolar gyre to the regional carbon cycle” is referring to?\nLine 62-63: Reference global warming hole here? It’s introduced much later in the results section, so I think a description earlier in the introduction would be beneficial.\nLine 227-228: “…which increase DIC uniformly”. This is not accurate, as differences in the Revelle Factor modify dDIC/dpCO2.\n(Remarks on code availability)\nI was able to download the netcdf files from the Zenodo link successfully. While I was eventually able to load the output, I would encourage the authors to take another look through their metadata attached to the .nc files to get everything standardized. The variable names are often missing/using a non-descriptive placeholder (e.g. __xarray_dataarray_variable__), or are sometimes incorrectly labeled (e.g. the ER05 and ER15 sDIC_surface_SPNA variables are labeled as \"SALT\", when they are sDIC). While it can deduced what the variables are without the metadata, I was having some issues loading the netcdf files that appear to result from metadata inconsistencies."}, "author_response": "Response to reviews\nThe response to Reviewer #1:\nThe study by Lee and co-authors investigates emission-driven future scenarios with the Earth System\nCESM2 and finds an extraordinary future outgassing of CO2 in the subpolar North Atlantic. The\narticle identifies a shutdown of deep convection in the Labrador and Irminger Sea as a reason for this\noutgassing of CO2 in the subpolar North Atlantic.\nI can see that the authors have made an extensive effort to address my comments from the previous\nreview and I am happy to see this. However, with the newly added CMIP6 results, it looks like the\nresults are not very robust and there is no agreement in the models that the CO2 sink will turn into a\nsource (despite claims of the authors that the finding is robust). The article is missing a justification\nwhy this specific result of CESM2 is plausible. Further major points are listed below. I regret to say\nthat I still believe that the article needs more work to be suitable for publication in Nature\nCommunications.\nWe truly appreciate the reviewer for the thoughtful and constructive comments. While we tried to\nsupport the robustness of our findings by adding CMIP6, there may be a lack of explanation on the\ndetails. We recognized the concern and carefully revised the manuscript based on the reviewer’s\ncomments to provide appropriate explanation and context. Our responses to the specific comments are\nas follows.\nMajor comments\n1. This article needs language editing. I started writing down the formulations that needed to be\ncorrected, but then I quickly realized that this concerned nearly every sentence of the manuscript.\nIn response to the reviewer’s comments, we have thoroughly reviewed and revised each sentence.\n2. The results added for the CMIP6 models must be put into context and be described more nuancedly\nand it must be justified that the results of CESM2 are plausible. In my opinion, the following is\nneeded.\nThank you for pointing this out. We acknowledge that the intermodel spread of CMIP6 were too\noverlooked, which was also not our intention. However, despite the considerable intermodel spread,\nwe believe that the CMIP6 models consistently simulate the rebounding of the CO flux evolution and\n2\nexcessive anomalous outgassing in SPNA compared to the global mean. We hence revised manuscript\nto appropriately explain that the timing of the CO flux rebound is closely related to the shutdown of\n2\nthe mixed layer depth (MLD) over Labrador-Irminger Sea (hereafter Lab-Irm) in CMIP6 models,\nwhich is consistent with our key argument.\nTo clarify the robustness of the air-sea CO flux reversal, we firstly investigate the CMIP6 17 models,\n2\nwhich are all available models that provide the variables needed for the analysis. With sufficient\nmodel configurations, we identified the timing of the CO flux reversal. Interestingly, all models\n2\nsimulate the reversed trend, where the CO flux first increases before subsequently decreasing,\n2\nalthough the timing of the rebound varies significantly across models. Therefore, we calculated the\nlag composite of the SPNA air-sea CO flux based on the timing (Figure R1-1). The lag 0 represents\n2\nthe time corresponding to the local minimum of the CO flux for each model. The composite shows\n2\nthat the CMIP6 multi-model mean (MMM) projects a reversed CO uptake trend, which is highly\n2\ncomparable with the projection of our ER experiments (compared to the grey and green lines in\n2\nof the local minimum in the CO flux, the CO flux reversal is consistently represented across all\n2 2\nmodels.\nFurthermore, we chose 7 models (CNRM-ESM2-1, NorESM2-LM, NorESM2-MM, CESM2,\nCESM2-WACCM, GFDL-CM4, and GFDL-ESM4) that are most similar to the observational MLD\nand CO flux (see the detail in the comment 2-1). The CO flux evolution from these models is more\n2 2\nsimilar to our ER experiments, further supporting the robustness of CO flux reversal (red line in\nincrease, the SPNA CO₂ flux exhibits the “CO₂ uptake hole” of “CO flux reversal” phenomenon\n2\nacross various forcing scenarios and model frameworks, albeit with different rebound timings in each\nmodel. Therefore, we have carefully revised the manuscript to ensure the robustness of the mean\nresponse and to avoid exaggerating our interpretations. Please see our detailed responses to each\ncomment.\nL114-132: It should be noted that the uptake hole is highly dependent on the model configuration,\nwhich calls for assessment of its robustness in the Coupled Model Intercomparison Project phase 6\n(CMIP6) (Methods). Although the CMIP6 multi-model mean (MMM) closely follows the SPNA CO\n2\nflux changes with ER1.0 (comparing grey and green lines in Fig. 1e), there is considerable\nuncertainty. In a manner similar to the AMOC31,32, the SPNA MLD, the descending branch, also\nsuffers from substantial intermodel spread33 (Supplementary Fig. 3), which would be a source of\nuncertainty. To underpin the robustness, we make a time-lag composite of SPNA air-sea CO flux\n2\nbased on the rebound time (Supplementary Fig. 4). Notably, most models simulate this rebound\nfeature, where the CO flux initially increases before subsequently decreasing. The composite shows\n2\nthat the CMIP6 MMM projects a reversed CO uptake trend, which is highly consistent with the ER\n2\n1.0 projection (comparing grey and green lines in Supplementary Fig. 4). Furthermore, if we only\nselect the models that closely simulate the observed MLD and CO flux (Methods), the proposed\n2\nrebound is much more clear (red line in Supplementary Fig. 4).\nFigure R1-1 | Time-lag composite of the SPNA air-sea CO flux. Colored lines indicate each CMIP6\n2\nmodels (light blue), CMIP6 MMM (grey), best CMIP6 best 7 models (CNRM-ESM2-1, NorESM2-LM,\n(green), respectively. Lag 0 shows times corresponding to the local minimum in the CO flux.\n2\n2-1. The chosen CMIP6 models and their performance should be introduced. How well do they\nrepresent the AMOC and deep convection (see, e.g., Heuze, 2021, Swingedouw et al., 2022) in\ncomparison to CESM2? It seems that most of the chosen CMIP6 models have a larger AMOC and\nmore deep convection than observed. Moreover, the two models with less deep convection\n(CanESM5, IPSL -CM6A-LR ) are even left out of the analysis because “they do not reproduce the\nobserved March mixed layer over the Labrador-Irminger Sea well” (L317-321), though it can be\nclaimed that the remaining models have a biased high MLD and hence also do not represent this well.\nIn summary, it appears as if the chosen CMIP6 model ensemble is biased in one direction. To clarify\nthe robustness of the results, it would be best to choose a CMIP6 ensemble where half of the members\nshow too little deep convection and a too weak AMOC. Moreover, a justification should be provided\nas to why CESM2 is better performing, and its results have high value.\nThank you for the reviewer’s comments. To address the reviewer’s concern, we first repeated our\nanalysis by gathering all available CMIP6 models that provide the variables for our analysis. Firstly,\n23 models are currently available for the air-sea CO flux (FCO2) variable. Among them, only 17\n2\nmodels provide the necessary variables (i.e., AMOC, DIC, MLD, and salinity). Therefore, we used\nthem all in this revised version. We then checked how well the models simulate the MLD climatology\nover the SPNA. As shown in the MLD climatology in March, both the CMIP6 MMM and our ER\nexperiment well simulate the March MLD climatology over the Labrador-Irminger Sea comparing to\nthe SODA reanalysis data (comparing Figure R1-2a with Figure R1-2b, c). There are models with\nabsence of MLD in the Labrador-Irminger Sea (e.g., Figure R1-2d,i), which was excluded in previous\nanalysis. However, as the reviewer’s suggestion, we included them in current revision\nWe further quantified the spread in MLD climatology among CMIP6 in terms of both magnitude and\nspatial pattern. The former is measured by the area-averaged MLD climatology over the deep\nconvection site, defined as regions where the MLD exceeds 1000 m (black line in Figure R1-2b). The\nlatter is quantified by the pattern correlation between over the SPNA between reanalysis and\nindividual model. As already hinted in Figure R1-2, CMIP6 models exhibit a substantial spread in\nboth magnitude and pattern compared to the reanalysis (Figure R1-3). However, in spite of the biases\nof the individual members, the CMIP6 MMM simulates a compatible MLD strength and pattern,\nwhich is also the case of our ER experiment. Therefore, our ER experiment (CESM2) is one of the\nmodels that simulates SPNA MLD appropriately. Furthermore, we calculated the pattern correlation between the reanalysis dataCO₂ flux in MPI-\nSOMFFN climatology (year 2021) and MLD in SODA (average over 2010 - 2020)and CMIP6\n(averaged over 1980–2014). The pattern correlation of the CO flux shows a strong linear relationship\n2\nwith that of the MLD (r = 0.88), highlighting that the MLD is a critical determinant of the SPNA CO₂\nfluxes, serving as an essential mechanism for the physical CO pump in the present day. It is also\n2\nnoted that our ER experiments and the CESM2 models are among the closest models to the\nobservation. The linear relationship allows us to select 7 best models (CNRM-ESM2-1, NorESM2-\nLM, NorESM2-MM, CESM2, CESM2-WACCM, GFDL-CM4, and GFDL-ESM4). These are the\nmodels that simulate both MLD and CO flux significantly similar to the reanalysis (r > 0.5) (Fig. R1-\n2\n3b). Throughout the response, we consistently showed the best model response is highly similar to our\nER experiments (Figure R1-1, R1-4, R1-5). Therefore, in response to the reviewer’s comments, these\nresults suggest that CESM2 closely aligns with current observations, reinforcing the significant value\nof the findings presented in the manuscript.\nperformance of the employed CMIP6 models, are discussed in the manuscript.\nL326-336: Note that, it has been reported that CESM2 represents the major ocean circulations\nrelatively well among the current Earth System Models (ESMs). For example, the strength of the\nAntarctic Bottom Water, North Atlantic Deep Water, and AMOC in our model are more comparable\nto observations than those in other models27,31,33,65. In particular, the overflow parameterization is\nimplemented, which provides a better projection of deep ocean dynamics33,66. At this point, the spread\nin MLD climatology magnitude is quantified based on the area-averaged MLD climatology over the\ndeep convection site, defined as regions where the MLD exceeds 1000 m (black contours in\nSupplementary Fig. 3b). Compared with the MLD reanalysis data, CESM2 is one of the best-\nperforming models that simulate SPNA MLD strength and pattern with reliable consistency\n(Supplementary Fig. 3 and Supplementary Fig. 8a).\nL356-370: The presence of the CO uptake hole is examined using 17 models from the Coupled Model\n1)68. The models are selected based on the availability of the necessary variables, including air-sea\nCO flux, AMOC, DIC, and salinity. Each model dataset was converted to a horizontal resolution of\n2\n1° × 1°. Furthermore, the pattern correlation is conducted between the reanalysis data (CO flux in\npattern correlation of the CO flux shows a strong linear relationship with that of the MLD (r = 0.88),\n2\nhighlighting that the MLD is a critical determinant of the SPNA CO flux, serving as an essential\n2\nmechanism for the physical CO pump in the present day. It is also noted that the CESM2 used for the\nselection of the 7 best models (CNRM-ESM2-1, NorESM2-LM, NorESM2-MM, CESM2, CESM2-\nWACCM, GFDL-CM4, and GFDL-ESM4), which simulate both MLD and CO flux significantly\n2\nsimilar to the reanalysis (r > 0.5) (Supplementary Fig. 8b).\nAssimilation (SODA) reanalysis data for the annual average from 2010 to 2020, (b) our ER experiment for the\ninitial 10 years (year 2001 to 2010) annual average, (c) historical CMIP6 multi-model mean (MMM) and (d-t)\nits individual models for the annual average from 1980 to 2014. The black line represents the deep convection\nsite, where the MLD is over 1000 m in the (b) our ER experiment.\nIrminger Sea in March. Deep convection is defined as an area-averaged March MLD over the Labrador-\nIrminger Sea (black contour line in Figure R1-2b). Grey horizontal dashed lines represent the uncertainty in the\nMLD reanalysis data (±1 𝑠.𝑑). (b) Pattern correlation between reanalysis and individual models. Reanalysis\ndata are the Simple Ocean Data Assimilation (SODA) reanalysis for the annual average from 2010 to 2020 in\nthe MLD and MPI-SOMFFN climatology reanalysis data for year 2021 in the SPNA air-sea CO flux.\nmodel mean. Our ER experiments are used as the initial 10 averaged years (year 2001-2010). Grey horizontal\nand vertical dashed lines show the correlation coefficient is 0.5.\n2-2. The chosen CMIP6-ensemble shows that the pattern of CO -outgassing does not locally coincide\n2\nwith that of the warming hole, but it is located more to the west. I consider the term “CO uptake\n2\nhole” hence less fitting and L101-102 should be reformulated.\nThank you for bringing this to our attention. First, we removed the sentence in lines 101-102 to avoid\nany confusion regarding the new term, “CO uptake hole”. It is important to note that the term “CO\n2 2\nuptake hole” encompasses more than just spatial pattern resemblance. A phenomenon that is distinctly\nanomalous in the SPNA and not observed in the surrounding regions is referred to as the \"warming\nhole\" under greenhouse warming. In this context, we propose that the current CO flux anomalies\n2\nshow a similar structural pattern that is opposite to the global mean in the SPNA region, although the\nspecific location of the \"CO uptake hole\" and the \"warming hole\" are not exactly same. Accordingly,\n2\nwe have chosen to retain the term \"hole\" and have clearly defined and explained this terminology in\nthe Introduction section.\nL71-77: Under greenhouse warming, the SPNA experiences unusual cooling—a phenomenon not\nobserved in other regions—leading to the term \"warming hole\"28. In this context, we propose that the\nreversed CO flux exhibits similar structural characteristics, with significant anomalous outgassing\n2\noccurring in the SPNA, while other areas continue to absorb more carbon. Drawing an analogy to the\n\"North Atlantic warming hole\", we refer to this phenomenon as the “CO uptake hole”.\nL107-111: It is already well known that the SPNA will show a distinct phenomenon compared to\nother regions, which is called “warming hole”28, and it is repeated in the carbon cycle. Although the\nspatial pattern is not exactly the same, we hence propose that this phenomenon is the “CO uptake\n2\nhole” by analogy to the \" warming hole”.\n6\n2-3. The CMIP6-ensemble shows that the CO -outgassing at the end of the simulation is NOT a\n2\nrobust feature, neither is the DIC- threshold and there is also no linear relationship between the\nAMOC strength when the air-sea CO flux is reversed and when the SPNA MLD is shallower than\n2\n200 m. Throughout the text, the authors claim the contrary. Please check Fig. 3 and 4d more carefully\nand rephrase L104-105, L105-107, L151-153, L190-196 and L213-214, L238 to make clear that the\nresults are NOT robust. As the results are NOT robust, it must be justified why CESM2 deserves extra\nattention (see 2a).\nThank you for the comments. Following the reviewer’s comments, we have clarified the manuscript\nwith respect to the robustness of CMIP6.\n1) Reliable air-sea CO flux reversal in CMIP6\n2\na huge intermodal spread. However, all models simulate the reversed trend of the CO flux without\n2\nany exception and the time-lag composite of CMIP6 (Figure R1-1) show consistent results, suggesting\nthat the reversal of CO flux are robust response in climate models. We revised the manuscript to\n2\nprovide clarified explanation on the context.\nL114-132: It should be noted that the uptake hole is highly dependent on the model configuration,\nwhich calls for assessment of its robustness in the Coupled Model Intercomparison Project phase 6\n(CMIP6) (Methods). Although the CMIP6 multi-model mean (MMM) closely follows the SPNA CO\n2\nflux changes with ER1.0 (comparing grey and green lines in Fig. 1e), there is considerable\nuncertainty. In a manner similar to the AMOC31,32, the SPNA MLD, the descending branch, also\nsuffers from substantial intermodel spread33 (Supplementary Fig. 3), which would be a source of\nuncertainty. To underpin the robustness, we make a time-lag composite of SPNA air-sea CO flux\n2\nbased on the rebound time (Supplementary Fig. 4). Notably, most models simulate this rebound\nfeature, where the CO flux initially increases before subsequently decreasing. The composite shows\n2\nthat the CMIP6 MMM projects a reversed CO uptake trend, which is highly consistent with the ER\n2\n1.0 projection (comparing grey and green lines in Supplementary Fig. 4). Furthermore, if we only\nselect the models that closely simulate the observed MLD and CO flux (Methods), the proposed\n2\nrebound is much more clear (red line in Supplementary Fig. 4).\nHere, we further support the robustness of the “CO uptake hole” by comparing spatial resemblance\n2\nbetween CMIP6 and our ER experiments. First of all, in the manuscript, we showed that the reversed\ntrend of CO flux highly coincides with MLD contraction and near-surface DIC accumulation (Figure\nare minor near-surface DIC accumulation at the beginning, it indicates that similar processes also\nhappen in the CMIP6, hence supporting the reliability of our findings. We further tested whether this\nsimilarity still holds for the best models: the models that simulate well both CO flux and MLD\n2\nclimatology (see details in response 2-1; Figure R1-3b). The best models, which closely simulate\nobservations, show a more similar hovmöller diagram with our ER experiment: the initial near surface\nDIC disappeared (Figure R1-4c). Then we looked at the air-sea CO flux to check the horizontal\n2\npattern (Figure R1-5). The horizontal pattern of anomalous air-sea CO flux is also highly similar\n2\nbetween CMIP6 MME and ER1.0 (pattern correlation = 0.72; Figure R1-5a, b), which is also valid for\nthe best models (pattern correlation = 0.79; Figure R1-5a, c).\nexperiments. The former indicates that primary driver of the “CO uptake hole”, MLD contraction and\n2\nnear-surface DIC accumulation, is consistently shown in the CMIP6. Consequently, the latter\nindicates that the CMIP6 and our ER experiment share an anomalous CO flux pattern. The models\n2\nthat closely resemble the reanalysis further support the findings. Overall, these results underpin the\nreliability of the “CO uptake hole”. Hence, we carefully reflected this context in the revised\n2\nmanuscript, including the paragraphs that the reviewer requested to be revised.\nL128-132: The spatial pattern of anomalous CO flux in the SPNA is also similar between CMIP6\nwhich is also valid for the best models (pattern correlation = 0.79). The results indicate that, in spite\nof the uncertainties, the CMIP6 generally shares a similar feature of the “CO uptake hole”.\n2\nnonlinear relationship in both the ER experiments and the CMIP6 MMM (Fig. 3a). The nonlinear\n10\nrelationship suggests that the CO uptake hole appears when the AMOC reaches a certain threshold.\n2\nfor CMIP6 MMM. While there is intermodel spread in AMOC strength itself, the threshold is still\nhighly distant from the permanent collapse8,42, reflecting an indirect role of AMOC in regulating the\nCO uptake hole and indicating the possibility of CO uptake hole prior to the permanent collapse.\n2 2\nL189-196: MLD across the entire SPNA region exhibits a decrease under greenhouse warming.\nInterestingly, the Lab-Irm MLD—a footprint of deep convection—continues to decrease until it\napproaches about 200 m and stays afterward, which can be considered as the shutdown of the Lab-\nIrm deep convection (black line in Fig. 4a and Supplementary Figs. 7a-b). The CMIP6 MMM also\nshows a marked reduction in deep convection, nearly vanishing at a depth of approximately 200 m, a\nphenomenon even more distinctly captured by the best models (black line in Fig. 4b and\nSupplementary Fig. 7c).\nL207-214: However, as the collapse gets closer, DIC accumulates explosively in the upper ocean (0-\n200 m) because there is no longer active mixing between the upper and deeper ocean. Consistently,\nthe CMIP6 MMM also exhibits increased DIC accumulation in the surface layer as the MLD\napproaches 200 m, with this feature being clearer in the best models (Fig. 4b and Supplementary Fig.\n7c). Hence, the Lab-Irm MLD collapse evidently acts as positive feedback which amplifies a surface\nDIC accumulation over the SPNA. This mechanism ultimately has a direct impact on CO uptake.\n2\nnonlinear relationship with a local minimum around 2200 to 2250 mmol m-3 (Fig. 3c). After the\nminimum, the air-sea CO flux starts to decline despite continuous DIC increases. Therefore, the\n2\nminimum serves as a threshold at which the uptake hole emerges. Overall, the chemical environment\n(i.e., DIC) is accompanied by regional ocean dynamics (i.e., MLD) and large-scale dynamics (i.e.,\nAMOC), which together establish the nonlinear relationship with the CO₂ uptake hole in the SPNA\n(Fig. 3). While individual models exhibit varying threshold points over time due to differences in\nrebound timing of the CO flux (Supplementary Fig. 4), our finding is that all three indicators show a\n2\nsimilar threshold leading to CO reversal within a reliable timeframe—particularly compared in the\n2\nnegligible (comparing vertical lines and color transitions in Fig. 3).\nHovmöller diagram of the SPNA DIC concentration MMM removing total column average in (a) ER1.5\nensemble mean, (b) CMIP6 MMM, and (c) CMIP6 best 7 models (CNRM-ESM2-1, NorESM2-LM, NorESM2-\nMM, CESM2, CESM2-WACCM, GFDL-CM4, and GFDL-ESM4). The temporal evolution of MLD extracted\nover 1000m depth regions in March, where deep convection forms around the Labrador Sea and Irminger Sea\n(black solid line). Shading is the uncertainty of the projections represented by inter-ensemble spread (±1 𝑠.𝑑).\nFigure R1-5 | Air-sea CO flux changes of CMIP6 and ER experiment. (a) Air-sea CO flux changes over\n2 2\nthe SPNA for the CMIP6 multi-model mean (MMM) and (b) for the ER1.5 experiment. For CMIP6,\nthe anomaly is calculated by comparing the last 10 years (2091–2100) of the SSP5-8.5 scenario to the\nhistorical scenario (1980–2014). For ER1.5, the anomaly is calculated by comparing the last 10 years\n(2091–2100) to the initial 10 years (2001–2010).\n2) Regarding Figure 4.\nThe inter-model relationship in Figure 4, updated with 17 CMIP6 models, demonstrates an\nexplanatory linearity (r = 0.53) between the timing of the SPNA CO flux rebound to the local\n2\nminimum and the MLD shutdown to approximately 200 m (Figure R1-6). As shown in Figure R1-2,\nwhile some models do not accurately simulate MLD around the Labrador-Irminger Sea, they\nreasonably capture the general linearity that indicates regional deep convection would act as a direct\nmodulator of CO capacity regulation in the SPNA. We therefore revised the manuscript with updated\n2\nanalyses.\nL243-251: So far, the analyses have only focused on the temporal evolution in terms of the multi-\nmodel mean, raising a query of its credibility in the context of inter-model spread. We hence compare\nthe timing of the SPNA CO₂ flux rebound to the local minimum and the MLD shutdown to\napproximately 150 m (Fig. 4c). Despite the existence of models that do not simulate the MLD around\nthe Labrador-Irminger Sea (Supplementary Fig. 3), the inter-model relationship depicts a linear\nrelationship (r = 0.53*). This suggests that in difference to the previous finding that the loss of CO₂\n9\nuptake capacity in the SPNA is associated with the collapse of the AMOC, regional deep convection\nwould act as a direct modulator of CO₂ capacity regulation in the SPNA.\nFigure R1-6 | Scatter plot between the timing in the Larbador-Irminger sea MLD shutdown versus the SPNA\nair-sea CO flux rebound time. The MLD shutdown indicates when it becomes shallower than 200 m. The air-sea\n2 2\nindicates statistically significant correlation at the 95% confidence level.\n3. The analysis leading to thresholds seems a bit rushed and shallow as pointed out by reviewer\n2. Based on Figure 3, I only see a threshold for MLD, but not for AMOC or DIC. Please extend your\njustification with further analysis and rephrase all related text. Also consider that there is a lot of\nroom between an MLD of 800m and an MLD collapse, so please rephrase L204-208\nWe apologize for the confusion. First, we updated Figure 3 with all CMIP6 17 available models.\nAlthough some models do not accurately reproduce the MLD (Figure R1-3), the nonlinear trends in\nthe CMIP6 MMM are well captured and closely match our ER experiments (compared to black lines\nand green lines in Figure R1-7). We indicated the threshold that is the point at which a local minimum\nis reached as shown in Figure R1-1 and Figure R1-7. As discussed in Major Comment 2 with the\ntime-lag composite of air-sea CO flux, we acknowledge that the timing of the rebound in SPNA CO\n2 2\nflux varies across models (Figure R1-1), resulting in diverse thresholds for the local minimum over\ntime. However, what we want to focus on in this analysis is that the CMIP6 MMM closely follows the\nnonlinear shape observed in our ER experiment and can be used as a prior signal for the emergence of\nthe CO uptake hole to some extent by comparing it with the current state. Therefore, recognizing\n2\nthese points, we rephrased the manuscript following the reviewer's comments.\nL176-183: We further examine the CO flux response to the AMOC changes, which exhibits a\n2\nnonlinear relationship in both the ER experiments and the CMIP6 MMM (Fig. 3a). The nonlinear\n10\nrelationship suggests that the CO uptake hole appears when the AMOC reaches a certain threshold.\n2\nfor CMIP6 MMM. While there is intermodel spread in AMOC strength itself, the threshold is still\nhighly distant from the permanent collapse8,42, reflecting an indirect role of AMOC in regulating the\nCO uptake hole and indicating the possibility of CO uptake hole prior to the permanent collapse.\n2 2\nL222-233: The surface DIC concentration, which reverses CO uptake response, also displays a\n2\nnonlinear relationship with a local minimum around 2200 to 2250 mmol m-3 (Fig. 3c). After the\nminimum, the air-sea CO flux starts to decline despite continuous DIC increases. Therefore, the\n2\nminimum serves as a threshold at which the uptake hole emerges. Overall, the chemical environment\n(i.e., DIC) is accompanied by regional ocean dynamics (i.e., MLD) and large-scale dynamics (i.e.,\nAMOC), which together establish the nonlinear relationship with the CO₂ uptake hole in the SPNA\n(Fig. 3). While individual models exhibit varying threshold points over time due to differences in\nrebound timing of the CO flux (Supplementary Fig. 4), our finding is that all three indicators show a\n2\nsimilar threshold leading to CO reversal within a reliable timeframe—particularly compared in the\n2\nnegligible (comparing vertical lines and color transitions in Fig. 3).\nL234-242: We could utilize these relationships to assess the potential emergence of the uptake hole\nunder climate change scenarios projected by current climate models. By comparing modeled\nthresholds with present-day observations, we estimate the remaining distance to the onset of the\nuptake hole. Examining the recent climatology obtained from the RAPID array and reanalysis data\n(SODA and the OceanSODA-ETHZ): about 17 Sv, 800 m, 2230 mmol m-3 (vertical lines in Fig. 3)\nmay provide valuable insights for predicting anomalous outgassing in the SPNA. While we refrain\nfrom overemphasizing the thresholds in determining exact timing, it is highly plausible that the uptake\nhole's emergence will occur on a shorter timescale than the anticipated AMOC collapse.\nFigure R1-7 | Nonlinear relationship with SPNA carbon uptake. Same as Figure 3. Relationship between air-sea CO flux change\n2\nand (a) AMOC, (b) Lab-Irm MLD, and (c) DIC in the SPNA. Scatter is shaped depending on their CO emission\n2\nrates: ER1.0 (triangle), ER0.5 (circle), and ER1.5 (rectangle), and shapes represent all ensembles in each\nexperiment. The green line indicates the CMIP6 multi-model mean and the green scatter shows each 17 models.\nDIC is averaged surface layer (0-100m) and normalized by salinity (the standard salinity is 35 psu). AMOC\nstrength is defined as the average of the annual mean Atlantic meridional stream function from 26°N at a\nlatitude and a maximum depth from 500 m to 2000 m. The grey vertical solid line indicates the current\nobservation and reanalysis data averaged during the year 2004-2017 and the grey vertical dashed lines\nrepresent the uncertainty in the average period (±1 𝑠.𝑑).\nMinor comments\n11\n1. Title, abstract etc: I am not familiar with the term “ocean CO2 uptake hole”. It is somewhat\nintuitive as there is also the “warming hole” in the subpolar North Atlantic, but these two “holes” do\nnot seem to regionally coincide. Hence, I would like the term to be introduced and it not to be used as\nif it was a commonly used term. While the authors have now introduced the term, I still think that the\ntitle “Emergence of the ocean CO2 uptake hole under global warming” seems to imply that this is a\ncommonly used term. I would like the title to at least be changed to “Emergence of an oceanic ‘CO2\nuptake hole’ under global warming” or something similar.\nWe appreciated the reviewer’s suggestion. We revised the title accordingly.\nTitle: Emergence of an oceanic “CO uptake hole” under global warming\ndetail in the comments 2-2) and then referred to them in the rest of the context.\nL71-77: Under greenhouse warming, the SPNA experiences unusual cooling—a phenomenon not\nobserved in other regions—leading to the term \"warming hole\"28. In this context, we propose that the\nreversed CO flux exhibits similar structural characteristics, with significant anomalous outgassing\n2\noccurring in the SPNA, while other areas continue to absorb more carbon. Drawing an analogy to the\n\"North Atlantic warming hole\", we refer to this phenomenon as the “CO uptake hole”.\nregions, which is called “warming hole”28, and it is repeated in the carbon cycle. Although the\nspatial pattern is not exactly the same, we hence propose that this phenomenon is the “CO2 uptake\nhole” by analogy to the \" warming hole”.\nAbstract\n2. L10: The oceanic carbon system is an important reservoir for anthropogenic CO2 emissions, yet\nuncertainties persist regarding its future changes.” --> please change to “The ocean an important\nsink for anthropogenic CO2 emissions, yet uncertainties persist regarding its future changes.”\nWe have followed the reviewer’s suggestion.\nL12: The ocean serves as an important sink for anthropogenic CO emissions, yet uncertainties\n2\npersist regarding its future changes.\n3. L13: please specify the model's name and add that this article also includes the results of an\nensemble of CMIP6 models.\nTo reflect the reviewer's comment, we specify the model’s name and CMIP6 results in the abstract.\nL13-15: Here, we employ the Community Earth System Model (CESM2) with different CO emission\n2\nrates to systematically investigate the response of ocean CO uptake.\nIntercomparison Project Phase 6, providing the reliability of the phenomena.\n4. L15: “weakening of the uptake–CO2 uptake hole”– please change to “weaking of the CO2\nuptake” and do not refer to the CO2 uptake hole without introducing the term.\nand we do not mention it prior to the definition.\nL15-19: We find the possibility of a pronounced weakening of CO uptake in the Subpolar North\nserves as an effective carbon sink, but this uptake weakens as deep convection contracts around the\nLabrador-Irminger Seas and oceanic pCO rises beyond the global average.\n2\n5. L18-22: these sentences are unclear. Please revise such that their meaning is clear.\nWe substantially revised those sentences to clarify the meaning.\nL15-21: We find the possibility of a pronounced weakening of CO uptake in the Subpolar North Atlantic\n2\n(SPNA), which is distinct from the global ocean response. In the beginning, the SPNA serves as an effective\ncarbon sink, but this uptake weakens as deep convection contracts around the Labrador-Irminger Seas and\noceanic pCO rises beyond the global average. Given its connection to ocean circulation, we assess uptake\n2\nresponse with regional climate change, establishing consistent thresholds for the emergence of the uptake\nweakening.\nIntroduction\n6. L29-34: Many studies with ESMs do not show a linear response of the oceanic CO2 uptake to\nrising atmospheric CO2 levels, neither globally nor regionally (e.g. Kessler & Tjiputra, 2016).\nThank you for the reviewer’s comment. We intended to highlight the distinct variation in carbon\nuptake in the North Atlantic Ocean compared to the others. To clarify this point, we revised the\nsentence as follows:\nL33-40: Earth System Models (ESMs) currently project that most of the ocean carbon uptake would\nincrease linearly and saturate at the end of the simulation in response to rising atmospheric CO\n2\nlevels3–5. The carbon uptake is especially enhanced in both the Tropical and Southern Ocean,\nexpected to be a more favorable environment for CO uptake due to changes in upwelling and sea\n2\nice6–8. Not all the oceans can be explained by a simple relation. In the North Atlantic Ocean (NA),\nprevious studies suggest that the CO uptake may follow a distinct temporal evolution under global\n2\nwarming8–13.\n7. L49-57: It is unclear to me what the authors want to say here.\nHere, we tried to explain that changes in regional dynamics, such as deep convection and the\nsubpolar gyre (SPG), can occur more rapidly than the long-term collapse of the AMOC, and these\nfaster changes can significantly impact carbon uptake in the SPNA region on shorter timescales. We\nrevised the sentences to reflect this perspective.\nL53-66: However, there are limitations to interpreting the NA carbon cycle solely as a passive\nresponse to the AMOC. The NA carbon uptake does not respond uniformly to global warming. The\nSubpolar North Atlantic region (SPNA) especially shows a weaker uptake response compared to the\nrest of the NA, due to the strong coupling of the subpolar gyre (SPG) to the regional carbon\ncycle15,22. While several studies have suggested that global warming weakens both the SPG and the\nAMOC23–25, the connection between the processes remains a topic of ongoing debate. Nevertheless,\nit is worth noting that their temporal evolution can be distinct from one another 26,27. In particular,\n13\nthe mixed layer depth (MLD) is strongly influenced by SPG circulation, with an intense SPG\nenhancing MLD through increased convection, while a weakened SPG suppresses the development\nof the active mixed layer26. Consequently, although AMOC changes are known to significantly\nreduce NA carbon uptake, there remains the possibility of losing absorption capacity on much\nshorter timescales with respect to the SPG, and this possibility remains largely unexplored.\n8. L60-61: please change to “an emerging CO2 sink in the SPNA region, coined “CO2 uptake hole”\ndue to its overlap in location with the so-called warming hole”\nThank you for your suggested sentence. However, we thought it to be somewhat ambiguous and\npotentially confusing, so we have decided to remove it. Additionally, in the introduction section, we\nintroduced and clearly defined the new term \"CO uptake hole\" to enhance clarity and\n2\nunderstanding.\nL71-77: Under greenhouse warming, the SPNA experiences unusual cooling—a phenomenon not\nobserved in other regions—leading to the term \"warming hole\"28. In this context, we propose that\nthe reversed CO flux exhibits similar structural characteristics, with significant anomalous\n2\noutgassing occurring in the SPNA, while other areas continue to absorb more carbon. Drawing an\nanalogy to the \"North Atlantic warming hole\", we refer to this phenomenon as the “CO uptake\n2\nhole”.\n9. L62: The connection between deep convection and AMOC is in debate (Li et al., 2019,2022;\nMenary et al., 2020), so please rephrase.\nThank you for the reviewer's important point and reference. We also agree with the reviewer's\ncomment, which has been reflected as follows:\nL57-60: While several studies have suggested that global warming weakens both the SPG and the\nAMOC23–25, the connection between the processes remains a topic of ongoing debate. Nevertheless,\nit is worth noting that their temporal evolution can be distinct from one another 26,27.\n10. L 66-69: meaning of sentence is unclear. Please either add missing details such that an\nunderstanding is possible or rephrase.\nWe apologize for the lack of clarity in the original sentence and have removed it.\nResults\n11. L83: There is no linear increase (see Figure 1d), please rephrase\nTo reflect the reviewer’s suggestion, we modified the sentence.\nL92-93: In line with previous studies, the global carbon uptake is largely influenced by the Southern\nOcean and tropics.\n12. L90: please specify at what point in time\nThank you for the detailed suggestion. The specific time is added.\n2\ncarbon uptake into the ocean relative to the initial 10 years (year 2001-2010, red box in Fig. 1c),\nwhich is consistently shown in different emission rate scenarios (i.e., ER0.5 and ER1.0;\nSupplementary Figs. 1a-b).\n13. L112-122: what about changes in the wind-forcing?\nLines 112–122 note a homogeneous atmospheric 𝑝𝐶𝑂𝑎𝑡𝑚 distribution; hence, the 𝑑𝑝𝐶𝑂\n2 2\ndistribution primarily depends on oceanic 𝑝𝐶𝑂𝑜𝑐𝑛. Considering this point, we are unsure whether\n2\nan explanation of wind-forcing is necessary to account for the heterogeneous oceanic 𝑝𝐶𝑂𝑜𝑐𝑛.\n2\n14. L142: please rephrase the title of the section – its meaning is not clear\nWe have updated the title to enhance the clarity of the section.\nL169: Underlying mechanisms of the air-sea CO₂ flux reversal\n15. L143: no “additional” process is needed, just a change in a process\nThank you. We have followed the reviewer’s suggestion.\nL170-171: The DIC accumulation in the SPNA at a rate exceeding its sources implies the presence\nof a unique process, with the AMOC being an intuitive candidate.\n16. L157: “Therefore” is not the right word here, the sentence is not a direct consequence (as\ndifferent options are possible)\nTo reflect the reviewer's comment, we rephrased the word.\nL183-185: Accordingly, we need to invoke another process more responsible for the nonlinear CO\n2\nuptake: the collapse of deep convection in the Labrador and Irminger Seas (hereafter Lab-Irm).\n17. L161: the SPNA is not an ocean and deep convection also is present in the Southern Ocean\nWe agree with the reviewer’s opinion and modified the sentence to reflect the nuances suggested by\nthe reviewer.\nL186-189: The SPAN is one of the main regions where deep convection is observed in the Northern\nHemisphere43–46 (Supplementary Fig. 3a), and the employed model also simulates the climatological\nMLD in the Lab-Irm extending beyond 1000 m, active deep convection (black contour line in\nSupplementary Fig. 3b).\n18. L263-265: I think that this sentence is factually wrong.\nIn response to the reviewer's comment, we removed this sentence due to its vagueness and\nambiguity.\nThe authors have addressed all of my previous comments and I think the manuscript is ready for\nacceptance. I have just 2 remaining additional, very minor comments:\n1. Line 17: In the beginning of what?\nWe clarified the sentence.\nL17: In the beginning of the scenarios, the SPNA is an effective CO sink, but this uptake decreases\n2\ndue to the weakening of deep convection around the Labrador–Irminger Seas region and the rising of\nits oceanic pCO beyond the global average.\n2\n2. Line 186: Should be SPNA\nCorrected.\nP\nA\nG\nE\nThe response to Reviewer #4:\nThe study by Lee et al., investigates the ocean CO2 uptake response under different atmospheric CO2\nemission scenarios using the CESM2 model. The authors found an unusual behavior with respect to\nthe global ocean in the SPNA, a crucial region for ocean CO2 uptake. Regardless of the emission\nscenario, if atmospheric CO2 concentration continues to rise, the authors show that the SPNA will\nreverse its CO2 uptake. This phenomenon, referred to as the “CO2 uptake hole” by the authors, is\nattributed to surface DIC saturation caused by strong DIC accumulation in shallow layers due to a\ncontraction of the Irminger-Labrador Sea convection, as a consequence of the MLD becoming\nshallower. Moreover, they find that this phenomenon could occur without significant AMOC\nweakening.\nThe article has strong points, such as the “CO2 uptake hole” concept, an important finding, and the\nresearch conducted to determine the reason why it happens and when. The relevance of the finding is\nwell framed in the last paragraph of the discussion. Among other aspects, I also find the results\npresented in panel (e) of Figure 1 and the decoupling of the phenomenon with an AMOC weakening\nparticularly relevant (although the figure shows a huge dispersion within the CMIP6 models, and the\nAMOC analysis still has some issues). The overall idea is good, but there are still major aspects to\naddress that require further work before publication. I encourage the authors because their work is\nvaluable and, with the necessary corrections and work, I believe that it would be a good contribution\nto the scientific community.\nI have noticed that this is not the original submitted manuscript, so my comments may conflict or be\ninconsistent with previous comments by the other reviewers.\nWe truly appreciated the reviewer for the thoughtful and constructive comments. Based on the\nreviewer’s comments, we carefully revised the manuscript. We hope that the revised manuscript\nwould be plausible and suitable to resolve the concerns. Please see more specific responses below in\nblue.\nMajor comments\n1.⁠ ⁠The writing style of the article makes it tedious to read. I recommend a careful review of the\nlanguage, as several phrases are unclear. Moreover, the flow of the text is disrupted in several\nsections, creating an unstructured discourse. I recommend reviewing these aspects thoroughly. I have\nhighlighted the most important issues in some minor comments, but these issues can be extrapolated\nto the entire manuscript. Overall, the introduction seems to me to be the most poorly structured\nsection.\nResponding to the reviewer’s comment, we have carefully checked the sentence in the general\nmanuscript and rephrased them accordingly. In particular, we have substantially revised the\nIntroduction to improve its structure and organization.\n2. If the possibility of a CO2 uptake hole was already suggested by other studies (line 111) you\nshould mention it in the introduction and explain clearly how your research differs or it is\npioneering. From the way it is written, it seems that this is the first time the CO2 uptake hole\nP\nA\nG\nE\nhas been identified. Moreover, the analogy with the “warming hole” needs to be used carefully,\nas the ‘CO2 uptake hole’ does not spatially coincide with the ‘warming hole’.\n2-1. Novelty\nAs we briefly noted in the manuscript (L111), previous studies hint the possibility of the “CO uptake\n2\nhole,” primarily indicating a reduction in CO uptake. The main difference is that Chikamoto and\naveraged CO flux and the AMOC time series (Figures 4 and S1 in CD21), suggesting a simplistic\n2\ninterpretation of the phenomenon as solely AMOC-related. However, our study demonstrates that the\nCO flux response in the SPNA is reversed. We also suggested that it is more coupled to the deep\n2\nconvection over the Labrador-Irminger Sea, the Subpolar Gyre (SPG) than to the weakening or\ncollapse of the AMOC. Current climate models tend to support that the permanent collapse of the\nAMOC would not be happen before 2100. However, our findings indicates that the distinct uptake\nhole could emerge much earlier without the collapse of the AMOC. Although the previous study\nhinted the possibility, our study is the first study to investigate a distinct evolution of SPNA CO\n2\nuptake. We revised the introduction to clarify this novelty.\nL43-66: The NA is currently known for its efficient CO uptake, covering only 15% of the global\n2\nocean area while holding more than 23% of the global oceanic anthropogenic CO 14. This efficiency\n2\nis closely linked to deep-water formation, facilitated by the cold and dense environment in the\nSubpolar North Atlantic (SPNA)14,15; excessive CO can be delivered into the deep ocean through the\n2\ndescending branch of the Atlantic Meridional Overturning Circulation (AMOC)14,16–18. Under\ngreenhouse warming, changes in the AMOC naturally raise a potential loss of the effective CO\n2\nuptake, which is indeed reported when the permanent collapse of the AMOC occurs8,9,11–13. For\nexample, a recent modeling experiment suggests that the permanent collapse of the AMOC would\ndiminish the NA CO uptake around the 22nd century. Since such a permanent collapse is not\n2\nprojected to occur within 2100 in most of state-of -the-art ESMs19, one may reasonably expect that the\nSPNA will continue to serve as an effective sink for atmospheric CO .\n2\nwarming scenarios. The deep convection around the SPNA, particularly Labrador Sea and Irminger\nSea, and the regional carbon cycle are also tightly connected to the subpolar gyre (SPG)15,22. Intense\nSPG maintains deep MLD through increased convection, while a weakened SPG suppresses the\ndevelopment of the active mixed layer26. While global warming weakens both the SPG and the\nAMOC23–25, the collapse of SPG can not be regarded as identical to that of the collapse of the AMOC,\nparticularly as their temporal evolutions may differ significantly26,27. Therefore, interpreting the\nSPNA CO uptake solely to a passive response to AMOC may be a limited perspective. Consequently,\n2\nthere remains the possibility of losing uptake efficiency which is triggered by abrupt changes in\nregional ocean dynamics, and this possibility remains largely unexplored.\n2-2. Regarding the CO uptake hole terminology\n2\npattern. However, in broader context, the distinct evolution over the SPNA is consistent between two\nphenomena, which we adapt the term with expression “Although the spatial pattern is not exactly the\nsame, we hence…”.\nWe clarified the manuscript in order to address the reviewer's concern.\nP\nA\nG\nE\nL104-111: From the rebound times, the SPNA begins to decrease the air-sea CO flux changes,\n2\nthereby generating pronounced anomalous outgassing over the SPNA. It is already well known that\nthe SPNA exhibits a distinct phenomenon compared to other regions, referred to as the “warming\nhole”28, which may also be manifested in the carbon cycle. Although the spatial patterns of the two\nfeatures are not identical, we suggest referring to this significantly anomalous outgassing\nphenomenon as the “CO uptake hole”, drawing an analogy to the “warming hole”.\n2\n3. The authors use the CMIP6-models results to validate the CESM2 model performance in the\nexperiments, as the uptake hole seems to be highly variable depending on the model configuration\n(line 114). Does this mean that the seven simulation results for each scenario are very different from\none another? I believe it is very important for your results that their uncertainty is well explained and\ndiscussed, to ensure they are sufficiently robust.\nIn addition, the CMIP6-related information is not well-integrated, nor the reason why you use it.\nI would add the necessary discussion about why you use the CMIP6 models, perhaps at the beginning\nof the CMIP6 Methods section. In addition, lines 331-336 should be moved to the CMIP6 methods\nsection, as this part describes the MLD comparison with CMIP6 models before introducing them.\nSimilarly, lines 352-35 should be restructured, as you introduce the use of ESMs without sufficient\ncontext.\nThank you for the reviewer’s constructive comments.\nThe ER experiments exhibit relatively small spread across ensembles, as indicated by the colored\nshading in Figure 1e, supporting the CO uptake hole is not driven by internal variability but forced\n2\nresponse. Rather, we use CMIP6 to show that our main conclusions are robust, not depending on\nspecific model configuration of CESM2. We acknowledged that L114 leads to confusion and revised\nit to make it clearly express the reason.\nL114-116: Furthermore, to assess the robustness of the uptake hole beyond the ER experiments, we\nextend our analysis using the Coupled Model Intercomparison Project phase 6 (CMIP6) (Methods).\nWe also carefully revised and improved the CMIP6 Method section to align with the reviewer’s\nsuggestions, including discussion on purpose of CMIP6 analysis, relocation of MLD comparison,\nrestructuring of paragraph.\nL346-378: Ensuring consistent representation of the CO₂ uptake hole across different models and\nexperimental configurations is crucial. To show its robustness, we analyzed 17 CMIP6 models using\nhistorical simulations (2001–2014) and SSP5-8.5 (2015-2100) simulations (Supplementary Table.\n1)62. The models were selected based on the availability of the necessary variables, including air-sea\nCO flux, AMOC, DIC, and salinity. Each model dataset was converted to a horizontal resolution of\n2\n1° × 1°. Furthermore, to choose more reliable model to simulate our key processes, the pattern\ncorrelation of the annual average from 2001 to 2010 between the reanalysis data and CMIP6\nhistorical simulation was conducted (Supplementary Fig. 8b). The pattern correlations of the SPNA\nCO flux in each model show a strong linear relationship with that of the MLD (r = 0.84),\n2\nhighlighting that the MLD is a critical determinant of the SPNA CO flux, serving as an essential\n2\nmechanism for the physical CO pump in the present day. It is also noted that the CESM2 used for the\nselection of the 8 good models (CNRM-ESM2-1, MPI-ESM1-2-LR, NorESM2-MM, CESM2, CESM2-\nWACCM, UKESM1-0-LL, GFDL-CM4, and GFDL-ESM4) that closely simulate both MLD and CO\n2\nflux in agreement with reanalysis data. These models exceed the mean of the SPNA CO₂ flux and the\nMLD correlation coefficient, respectively (above the horizonal and vertical gray line in\nSupplementary Fig. 8b).\nP\nA\nG\nE\nThe linear relationship between the SPNA MLD and the CO flux, represented by the pattern\n2\ncorrelation values between reanalysis data and the CMIP6 historical simulation, suggests that the\nperformance of ocean biogeochemical cycles is influenced by the performance of ocean physical\nprocesses, contributing to multi-model diversity. Notably, CESM2 has been reported to represents\nmajor ocean circulations relatively well among the current Earth System Models (ESMs). For\nexample, the strength of the Antarctic Bottom Water, North Atlantic Deep Water, and AMOC in\nCESM2 are more comparable to observations than those in other models27,63–65. In particular, the\noverflow parameterization is implemented, which provides a better projection of deep ocean\ndynamics63,66. At this point, we quantified the spread in MLD climatology based on the area-averaged\ndeep convection site. The deep convection area is defined as the region where the MLD exceeds 1000\nm around the Labrador-Irminger sea in maximum MLD in March (black contours in Supplementary\nFig. 3b). Compared with the MLD reanalysis data, CESM2 is one of the best-performing models that\nsimulate SPNA MLD strength and pattern with reliable consistency (Supplementary Fig. 3 and\nSupplementary Fig. 8).\n4. There are several inconsistencies in the analysis that should be refined. For example, in Figure 1\npanel (c) you show ER 1.5 with relative years (2001-2010) vs (2081-2100). However, in Supp Fig 5\nyou show CMIP6 SSP5-8.5 (only comparable to your ER 1.0, if I understood correctly) and with\nrelative years (1980-2014) vs (2081-2100). The same issue appears in Figure 4, where you show\nER1.5 and MMM of CMIP6.\n4-1) Caption of Supplementary Figure 5\nWe apologize for the mistake. The relative year was calculated based on the years 2001 to 2010\nreferenced in Figure 1. Therefore, we corrected the Supplementary Fig. 5 caption.\nL55-60: Supplementary Figure 5 | Ocean carbonate system changes in the CMIP6 SSP5-8.5\nscenario. Air-sea CO flux changes in the CMIP6 multi-model mean of the SSP5-8.5, i.e. the last 20\n2\nyears (years 2081-2100), relative to the historical simulation annual mean from 2001 to 2010. The\nred box indicates the region of Subpolar Northern Atlantic Ocean (SPNA, 45°N-65°N, 80°W-0°W).\nNegative (positive) air-sea CO flux indicates oceanic CO uptake (outgassing).\n2 2\n4-2) ER1.5 as a representative experiment\nAs shown in Supplementary Figure 1a and 1b, other ER experiments also exhibit the strong\nanomalous outgassing behavior of the SPNA. This pattern is also evident in Figure 4 and\nSupplementary Figure 7. Thus, sensitivity to the emission rate has a minimal impact on our\nconclusion. We compared ER1.5 and the multi-model mean (MMM) of CMIP6 because the strongest\nemission rate experiment (ER1.5) clearly reveals the CO uptake hole phenomenon. We revised the\n2\nmanuscript to emphasize the independency.\nL116-118: Regardless of the emission rates, the CMIP6 multi-model mean (MMM) shows similar\nfeatures to the SPNA CO flux changes, although considerable uncertainty remains (Fig. 1e).\n2\nhighly consistent with the ER experiments (comparing grey and colored thick lines in Supplementary\nFig. 4).\nIn Figure 4, using the DIC minus total column average instead of DIC solely can be misleading. Why\ndo you need to subtract the total column average? (and how is this average computed? by year, by the\nmean over the period…?). Also, in Figure 4, as in others, there is a lack of specification regarding the\nP\nA\nG\nE\nregions you use for some computations. For example, ‘the temporal evolution of MLD is extracted\nover 1000m depth regions in March’, which regions?\n4-3) The DIC vertical deviation\nThe vertical deviation of the SPNA DIC was computed by total DIC value minus entire depth mean of\nthe total DIC by year. DIC increases monotonically from the surface to the deep ocean as atmospheric\nCO₂ levels rise (Figure R4-1). Thus, the vertical deviation is an effective way to highlight that DIC\nincreases more rapidly at the surface than in the deep ocean, which reflects a role of the regional\ndynamics. To enhance clarity in the computational explanation, we rephrased the sentence in the\nfigure caption.\nL671-674: Hovmöller diagram of the SPNA DIC concentration, calculated as total DIC minus total\ncolumn average of total DIC by year in (a) ER1.5 ensemble mean, (b) CMIP6 multi-model mean\n(MMM) of the historical (years 2001-2014) and the SSP5-8.5 (years 2015-2100) simulations.\nFigure R4-1 | Hovmöller diagram of the SPNA DIC changes. Hovmöller diagram of the SPNA DIC\nconcentration changes in relative to the initial year (year 2001-2010) in (a) ER1.0 and (b) CMIP6 multi-\nmodel mean (MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulation.\n4-4) Definition of SPNA MLD\nWe apologize for the lack of specification about SPNA deep-ocean convection. We defined the deep-\nocean convection region as the area where the MLD exceeds 1000 m over the Labrador Sea and\nIrminger Sea (black contours in Supplementary Fig. 3b) and computed its area-averaged MLD. We\nincluded the detailed computation in the Methods section and revised the Figure 4 caption to enhance\nclarity in the explanation.\nL373-376: At this point, we quantified the spread in MLD climatology based on the area-averaged\ndeep convection site (Supplementary Fig. 8a). The deep convection area is defined as the region\nwhere the MLD exceeds 1000 m around the Labrador-Irminger sea in maximum MLD in March\n(black contours in Supplementary Fig. 3b).\nL674-677: The temporal evolution of the MLD represents an area-averaged value extracted from\nregions with exceeding 1000 m depth in March, where deep convection forms around the Labrador\nSea and Irminger Sea (black contours in Supplementary Fig. 3b) (black solid line).\nP\nA\nG\nE\nOn the other hand, when analyzing the MLD you state that, from Supp. Figures 3 and 8, the CESM2\nis among the best performing models. This conclusion is somewhat questionable based on the Supp\nFig3, because you are comparing different periods for the products (annual average 2010-2020 for\nSODA, 2001-2010 for ER experiment, I assume 1.0, and 1980-2014 for the CMIP6 models). In\naddition, the vertical grey line in panel (b) of Figure 3 (SODA reanalysis from 2004-2020) is far from\ncoinciding with your results for the same period (in light yellow). I have written more examples as\nminor comments, but I suggest reviewing all these issues and discussing the results accordingly.\n4-5) Regarding analyzing periods\nThank you for pointing out the important issue. We agree that the analyzing periods are not exactly\nmatched with each other. Therefore, we adjusted all data to the initial year from 2001 to 2010 in the\nrevised manuscript. In addition, we updated the reanalysis data, MLD and CO flux, from one to\n2\nthree. In the case of the MLD, we used the Simple Ocean Data Assimilation (SODA) version 3.15.2,\nEstimating the Circulation and Climate of the Ocean (ECCO), and Ocean Reanalysis System 5\n(ORAS5). For the CO flux, the Max Planck Institute for Meteorology Self-Organizing Map Feed\nin the analysis. Note that for Figure 3 (Figure R4-5), the period 2004-2017 was selected to align with\nthe time range of the available AMOC observation data and MLD reanalysis datasets.\nIn Supplementary Figure 3, we compared the spatial pattern of the SPNA MLD climatology.\nExamining the reanalysis data for MLD over the annual mean from 2001 to 2010, we overall observed\nthe formation of a deep MLD around the Labrador-Irminger Sea, although its magnitude varies\ndepending on the data set (Figure R4-2). In addition, for MLD climatology with three reanalysis data\naverages compared to simulation data, we found that the MLD above 1000 m depth—a footprint of\ndeep convection—is well simulated (Figure R4-3a to c). Therefore, in the case of Supplementary\nFigure 8, examining the deep convection index (Figure R4-4a), which represents the area-averaged\nMLD within the black contour of Figure R4-3b, along with the pattern correlation between reanalysis\nand simulation data (Figure R4-4b), we believe that it is reasonable to select the CMIP6 8 good\nmodels than the previous version.\nRegarding to the lack of coincidence for the period in Figure 3b, when we updated to the three MLD\nreanalysis datasets and adjusted the period, the vertical grey line in Figure R4-5b more closely aligns\nwith our results for the same period (light yellow in Figure R4-5b).\nFigure R4-2 | Mixed layer depth (MLD) climatology in March. Spatial pattern of the reanalysis datasets (a)\nSimple Ocean Data Assimilation (SODA), (b) Estimating the Circulation and Climate of the Ocean (ECCO), and\n(c) Ocean Reanalysis System 5 (ORAS5). All values represent the initial annual averages for the period 2001–\n2010.\nP\nA\nG\nE\nFigure R4-3 | Mixed layer depth (MLD) climatology in March. Spatial pattern of (a) mean of the reanalysis\ndatasetsSimple Ocean Data Assimilation (SODA), Estimating the Circulation and Climate of the Ocean\n(ECCO), and Ocean Reanalysis System 5 (ORAS5), (b) ER experiment, (c) CMIP6 multi-model mean (MMM)\nin the historical simulation, and (d-t) its individual models. All values represent the initial annual averages for the\nperiod 2001–2010.\nP\nA\nG\nE\nFigure R4-4 | Performance of employed CMIP6 models. (a) Deep convection climatology values, which are\ndefined as an area-averaged near the Labrador-Irminger Sea in March (black contour line in Supplementary Figure\n3b). All values represent the annual mean over the period 2001–2010. Black horizontal dashed line indicates the\nmean of the reanalysis datasetsSODA, ECCO, and ORAS5. Grey horizontal dashed lines represent the\nuncertainty in the average period (±1 𝑠.𝑑). (b) Pattern correlation in the SPNA between reanalysis data and\nindividual models. In the reanalysis datasets, the average of SODA, ECCO, and ORAS5 is used for the MLD and\nMPI-SOMFFN, OceanSODA-ETHZ, and CSIR-ML6 for the air-sea CO₂ flux. All values are used as the initial\n10 averaged years (year 2001-2010). Grey horizontal and vertical dashed lines show the mean of the SPNA CO₂\nflux and the MLD correlation coefficient, respectively.\nFigure R4-5 | Nonlinear relationship with SPNA CO uptake. Relationship between air-sea CO flux change\n2 2\nand (a) AMOC, (b) Lab-Irm MLD, and (c) DIC in the SPNA. Scatter is shaped depending on their CO emission\n2\nrates: ER1.0 (triangle), ER0.5 (circle), and ER1.5 (rectangle), and shapes represent all ensembles in each\nexperiment. The green line indicates the CMIP6 multi-model mean and the green scatter shows each 17 models.\nDIC is averaged surface layer (0-100m) and normalized by salinity (the standard salinity is 35 psu). AMOC\nstrength is defined as the average of the annual mean Atlantic meridional stream function from 26°N at a\nlatitude and a maximum depth from 500 m to 2000 m. The grey vertical solid line indicates the current\nobservation and reanalysis data averaged during the years 2004-2017 and the grey vertical dashed lines\nrepresent the uncertainty in the average period (±1 𝑠.𝑑).\nP\nA\nG\nE\nMinor comments\nAbstract:\n1. Line 14: consider eliminating “systematically” and adding ‘scenarios’ after “emission rates”.\nCorrected.\n2. Line 15: rephrase “possibility”.\nCorrected.\n3. Line 18: consider replacing “weakens” by ‘decreases’ and “contracts” by ‘weakens’.\nCorrected.\n4. Line 19: add ‘its’ before oceanic pCO2?\nCorrected.\n5. Line 19-20: I do not get the link between the two phrases between “Given its connection… with\nregional climate change’.\nCorrected.\n6. Line 23: consider replacing “providing” by ‘supporting’.\nCorrected.\n7. Line 24: add ‘regional’ before dynamics?\nCorrected.\nIntroduction:\n8. when referring to carbon uptake, I suggest using a unique term, such as CO2. For example, in line\nP\nA\nG\nE\nThank you for the suggestion. As the reviewer suggested, we changed the “carbon uptake” to “CO\n2\nuptake” in the whole manuscript.\n9. Line 34: I suggest rephrasing, as uptake does not saturate but DIC does; saturation is a consequence\nof the uptake.\nTo reflect the reviewer's comment, we revised the sentence.\nL34-36: Earth System Models (ESMs) currently project that most of the ocean CO uptake would\n2\nincrease linearly in response to rising atmospheric CO levels, slowing towards the end of the\n2\nsimulation3–5.\n10. Lines 35-40: You introduce projections about how the ocean will behave in the future. Then you\ndescribe two specific regions. After that, you generalize that it cannot be expected that all oceans have\nthe same response, and then you describe again a specific region (NA). May consider rephrasing ‘Not\nall the oceans can be explained by a simple relation’ to something as (for example) ‘Not all oceans are\nexpected to respond in the same way’; then describe the Tropical and SO future behaviour, and after\nthis, start describing the NA.\nThank you for the constructive reviewer’s comment. We rephrased the sentence structures following\nthe suggestion.\nL36-42: Interestingly, depending on the ocean basin, the CO uptake shows a different response. In\n2\nboth the Tropical and Southern Oceans, the uptake is particularly enhanced, with these regions\nexpected to become more favorable for CO absorption due to changes in upwelling and sea ice6–8. In\n2\nthe North Atlantic Ocean (NA), previous studies suggested that the CO uptake may follow a distinct\n2\ntemporal evolution under global warming8–13. Therefore, as not all oceans respond in the same way,\nthis highlights the need to focus on regions with unique characteristics.\n11. Lines 42-43: Consider rephrasing as follows: “… is tightly coupled with deep-water formation,\nfacilitated by the cold and dense environment, at the Labrador…”.\nThank you for the comments. Following the reviewer’s comments, we rephrased the sentence.\nL45-48: This efficiency is closely linked to deep-water formation, facilitated by the cold and dense\nenvironment in the Subpolar North Atlantic (SPNA)14,15; excessive CO can be delivered into the deep\n2\nocean through the descending branch of the Atlantic Meridional Overturning Circulation\n(AMOC)14,16–18.\n12. Lines 49-52. Consider rephrasing or deleting ‘despite de NA’s smaller contribution to the global\ncarbon budget’.\nThank you for the suggestion. We deleted the sentence and rephrased it.\nP\nA\nG\nE\nL52-54 : Since such a permanent collapse is not projected to occur within 2100 in most of state-of -\nthe-art ESMs19, one may reasonably expect that the SPNA will continue to serve as an effective sink\nfor atmospheric CO .\n2\n13. Lines 53-66: I suggest better organization and structure.\nWe apologized for the lack of clarity in the paragraph and fully rephrased it to better convey our\nintended message. This comment is closely related to Major Comment 2; please refer to it for further\ndetails.\n14.Lines 63-64: if you keep “AMOC changes” I suggest using ‘modulate/influence’ or similar instead\nof “reduce NA carbon uptake”; if keeping “reduce” then replace “changes” by ‘weakening is’.\nThis comment is closely related to Major Comment 2; please refer to it for further details.\nResults.\n15. Even though you explain it in Methods, I was a bit confused the first time I read the manuscript\nabout what was the ensemble mean of the ERs and what ER was. It could be beneficial to quickly\nexplain this in the figures’ legends, for example, as ‘7 simulations for each scenario with slightly\ndifferent initial conditions’.\nTo reflect the reviewer's suggestion, we modified the Figure1 legend to add ER experimental\nensemble details.\nL637-641: 7 ensembles are conducted for each scenario with slightly different initial conditions (see\nthe Method for more details on ER experiments). All values of colored lines represent the ensemble\nmean and shading shows the uncertainty of the projections calculated by the inter-ensemble spread\n(±1 𝑠.𝑑). These time series are smoothed with an 11-year running mean.\n16. Line 87: ‘by a decrease’ or ‘by decreasing the’.\nCorrected.\n17. Lines 93-96: “as the open ocean increases”, consider rephrasing.\nAccording to the reviewer’s comment, we rephrased the sentence below:\nL92-95: In the Southern Ocean, turbulent exchange is expected to accelerate as sea ice cover\ndecreases and westerly wind strengthens, while a weakening of the equatorial trade wind is followed\nby a weakening of the equatorial upwelling8.\nP\nA\nG\nE\n18. Lines 106-107: consider rephrasing for “ thus formulating..”\nWe revised the sentence to clarify the meaning.\nL104-106: From the rebound times, the SPNA begins to decrease the air-sea CO flux changes,\n2\nthereby generating pronounced anomalous outgassing over the SPNA.\n19. Line 113: how do you quantify the ‘collapse’?\nWe define “collapse” as the point at which the MLD continues to decrease and then stops.\nSpecifically, when the MLD reaches approximately 200 meters and the gradient no longer changes,\nwe identify this point as the MLD collapse.\n20. Lines 114-115: what do you mean by “model configuration”?\nThe term “model configuration” refers to the specific setup and settings of a numerical model used in\nsimulations.\n21. Line 119: rephrase.\nWe rephrased the sentence to make it clearer.\nL118-120: The SPNA MLD, linked to the deep-water formation, exhibits significant intermodel\nspread33 (Supplementary Fig. 3), contributing to considerable uncertainty, similar to the spread\nobserved in the AMOC31,32.\n22. Line 186: replace SPAN by SPNA.\nCorrected.\n23. Line 248: the “*” after “0.53” is explained in Figure 4 legend but not here. Can be replaced by ‘p-\nvalue < 0.05’.\nSorry for the confusion. We included the information both in the text and caption.\nL250-252: Despite the existence of models that do not simulate the MLD around the Labrador-\nIrminger Sea (Supplementary Fig. 3), the inter-model relationship depicts a linear relationship (r =\n0.53, p-value < 0.05).\nL683: …* mark indicates statistically significant correlation at the 95% confidence level.\nP\nA\nG\nE\n24. Lines 278-279: I do not understand what you want to say.\nWe apologized for the unclear sentence and thoughtfully revised it.\nL281-282: Recent studies have aimed to reduce the uncertainty in the NA future projections by\napplying an emergent constraint approach.\n25. Line 291: climate change impact because of? I suggest rephrasing.\nAs suggested by the reviewer, we rephrased the sentence more clearly.\nL293-294: Current climate models project that the NA will undergo substantial changes in the future,\ndriven by the slowdown of the AMOC and the contraction of convection.\n26. Lines 298: aerosols, meltwater fluxes in plural?\nIn response to the reviewer’s comment, we changed the words to plural.\nMethods:\n27. AMOC calculation is not explained, only briefly in supp. fig 2. It is computed for all products at\nRAPID latitude? Could the difference between your study area and RAPID latitude influence your\nresults?\nWe updated the explanation of the AMOC calculation in the method. In addition, we used 26°N as the\nlatitude for calculating the AMOC index in both the RAPID observation and our ER experiments.\nTherefore, we used the same latitude for AMOC computation, it does not affect the results.\nL385-386: The AMOC strength in this study is defined as the maximum zonally integrated Atlantic\noverturning stream function (on the y-z plane) at 26°N and between 500 m and 2000 m.\n28. Line 334-336. This is true for Supp Information figure 8b but questionable from Supp Information\nFigure 3 as there are no metrics.\nSorry for the confusion. We revised the figure reference as follow.\nL376-378: Compared with the MLD reanalysis data, CESM2 is one of the best-performing models\nthat simulate SPNA MLD strength and pattern with reliable consistency (Supplementary Fig. 3 and\nSupplementary Fig. 8).\n29. Line 348: Do you mean the CO2 concentration-driven simulation as an example of a typical\nsimulation, or do you mean that you use it?\nP\nA\nG\nE\nTo avoid any confusion, we removed the sentence about the CO concentration-driven simulation and\n2\nonly focused on the emission-driven simulation, which is designed for our study.\nL339-342: These experiments are driven by the emission, which means that the atmospheric CO\n2\nconcentration is determined not only by prescribed anthropogenic CO emissions but also by oceanic\n2\nand terrestrial CO fluxes interactively, thereby reflecting changes in CO concentration via the\n2 2\ncarbon cycle-climate feedback.\n30. Lines 360-370.\n30-1. Line 361: the pattern correlation of what?\nSorry for the unclear sentence. we revised it for clarity.\nL351-354: Furthermore, to choose more reliable model to simulate our key processes, the pattern\ncorrelation of the annual average from 2001 to 2010 between the reanalysis data and CMIP6\nhistorical simulation was conducted (Supplementary Fig. 8b).\n30-2. Line 363: CO2 flux of what, the ensemble, for each model?\nWe apologized for the confusion and updated the sentence.\nL354-357: The pattern correlations of the SPNA CO flux in each model show a strong linear\n2\nrelationship with that of the MLD (r = 0.84), highlighting that the MLD is a critical determinant of\nthe SPNA CO flux, serving as an essential mechanism for the physical CO pump in the present day.\n2 2\n30-3. From line 360, I suggest starting a new paragraph describing the MLD, why you analyse it,\nand to bring the CESM2 MLD analysis here and the comparison with the CMIP6 results.\nThank you for your constructive suggestion. We substantially revised the Method section. Also, this\ncomment is closely related to Major Comment 3; please refer to it for further details.\n31. Lines 397-398. I do not understand what you mean here.\nSorry for the confusion. To reflect the reviewer's comment, we revised the sentence.\nL403-406: In this study, to separate the influence of freshwater (FW) flux, we adopted normalized\nDIC (𝑠𝐷𝐼𝐶 = 𝑆 ⁄𝑆∙𝐷𝐼𝐶) and ALK (𝑠𝐴𝐿𝐾 = 𝑆 ⁄𝑆∙𝐴𝐿𝐾) with reference salinity 𝑆 (35\n𝑟𝑒𝑓 𝑟𝑒𝑓 𝑟𝑒𝑓\npsu), and the equation is expressed as follow (e.g., Sarmiento and Grubuer 2006; Landschützer et al.\n2018):\n32. Figure 1 legend.\n32-1) Red box in panel (c) is not described in the legend. Add “atmospheric” in (b) temporal\nevolution of “atmospheric” CO2 concentration. You could refer the reader to Methods after the word\nSSP5-8.5.\nP\nA\nG\nE\nCorrected.\n32-2) After (e), when you explain what rebound timing is, you say it is defined by the maximum CO2\nabsorption peak trend. What do you mean by “peak trend”? What do you mean by ‘after taking the\n11-year running mean’?\nAdditionally, we are sorry for any confusion regarding the meaning of “peak trend.” Clearly, this is\nthe “peak timing”, that is, a maximum uptake in the SPNA sea-air CO flux and defined by the\n2\nhighest trend of the sea-air CO flux with smoothing 11 years since 2001. The meaning of ‘after\n2\ntaking the 11-year running mean’ is the meaning of “time series are smoothed with an 11-year\nrunning mean”. Therefore, we generally revised the legend to make it clear.\nL625-642: Figure 1 | Carbonate system responses to the climate change. (a) Anthropogenic CO\n2\nemission forcing and (b) temporal evolution of atmospheric CO concentration in ER1 (green), ER0.5\n2\n(yellow), and ER1.5 (red). (c) Air-sea CO flux changes in ER1.5, which is the last 20 years (year\n2\n2081-2100) interval average relative to initial 10 years (years 2001-2010, hereafter initial year). The\nred box indicates the region of Subpolar Northern Atlantic Ocean (SPNA, 45°N-65°N, 80°W-0°W).\nTemporal evolution of integrated air-sea CO flux changes relative to initial year of (d) global ocean\n2\nand (e) SPNA. All line colors are the same as noted above, and the grey line indicates the CMIP6\nmulti-model mean (MMM) of the historical simulation (years 2001-2014) and the SSP5-8.5 (years\n2015-2100) (see the Method for more details on CMIP6.). The dashed lines in panel (e) show the\nrebound timing defined by the maximum CO absorption peak times, which is the highest trend of the\n2\nsea-air CO flux for smoothing 11 years since 2001, included in all ensembles in the ER experiments.\n2 2\nAll values of colored\nlines represent the ensemble mean and shading shows the uncertainty of the projections calculated by\nthe inter-ensemble spread (±1 𝑠.𝑑). These time series are smoothed with an 11-year running mean.\n33. Figure 2. First time I looked at this figure I did not realize the y-axis scale changes between panel\nb,c and d; you could mention it, or even use the same scale. Regarding DIC and ALK average, is the\n100m threshold defined by something other than a generic criteria? Does this threshold have an\nimpact on your results?\nFirst, we mention it in the Figure 2 legend. Also, we borrowed from previous studies (Chikamoto and\nDiNezio, 2021) to define and calculate upper ocean DIC and ALK average. This upper ocean\nthreshold has no impact on our results when we changed, for example 200m.\nL643-646: Temporal evolution of (a) global-mean removed 𝑝𝐶𝑂𝑜𝑐𝑛(𝑝𝐶𝑂𝑜𝑐𝑛′) and 𝑑𝑝𝐶𝑂\n2 2 2\n(𝑑𝑝𝐶𝑂𝑜𝑐𝑛′) and (b-d) contributions in 𝑝𝐶𝑂𝑜𝑐𝑛′changes in the SPNA region. Note that the y-axis\n2 2\nscales differently depending on the experiments.\n34. Figure 4. Add “is” in line 652 between “MLD” and “extracted”.\nCorrected.\n35. Supp. Figure 2 legend: what do you mean by “Historical and SSP58.5”? Vertical dashed lines are\nnot described.\nP\nA\nG\nE\nSorry for the confusion. We revised the legend of Figure 2 to clarify the meaning and removed the\nvertical dashed line in the figure (Figure R4-X).\nL30-32: Color lines indicate ER1 (green), ER0.5 (yellow), ER1.5 (red), and CMIP6 multi-model mean\n(MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulations (grey).\nFigure R4-6 | Time evolution of the AMOC strength. Time evolution of the Atlantic meridional overturning\ncirculation (AMOC) in each simulation. Color lines indicate ER1 (green), ER0.5 (yellow), ER1.5 (red), and\nCMIP6 multi-model mean (MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulations (grey).\nAMOC strength is calculated from the maximum stream function at depths from 500m to 2000m at the 26oN. All\nvalues of colored lines represent the ensemble mean and shading is the inter-ensemble spread (±1 𝑠.𝑑). It is\nsmoothed with an 11-year running mean.\n36. Supp. Figure 3 legend: again, what do you mean by historical? If the deep convection index is not\nexplained, maybe it would be better to delete it from here.\nWe apologize for any ambiguity in terminology that may have made it difficult for reviewers to\nunderstand. We used the term to refer to the historical run simulation of CMIP6. To clarify the\nmeaning, we rephrased the sentence. Also, we deleted the explanation about the deep convection\nindex as follow the reviewer’s suggestion.\nL40-45: Spatial pattern of (a) mean of the reanalysis datasetsSimple Ocean Data Assimilation\n(SODA), Estimating the Circulation and Climate of the Ocean (ECCO), and Ocean Reanalysis System\n5 (ORAS5), (b) ER experiment, (c) CMIP6 multi-model mean (MMM) in the historical simulation, and\n(d-t) its individual models. All values represent the initial annual averages for the period 2001–2010.\n37. Supp. Figure 4 legend: how do you compute the time lags?\nWe apologize for the lack of detail. First, we identified the timing of the CO flux reversal in all\n2\navailable models, which is the maximum CO uptake peak. Then, we made the time-lag composite of\n2\nthe SPNA air-sea CO flux. The composite analysis is aligned based on the local maximum CO\n2 2\nP\nA\nG\nE\nuptake time for each model, corresponding to lag 0. For better understanding, we added more detail\nabout the time-lag composite calculation in the legend and the manuscript.\nL120-123: To underpin the robustness, we make a time-lag composite of SPNA air-sea CO flux\n2\nbased on the rebound time (Supplementary Fig. 4). The composite analysis is referenced to lag 0,\ncorresponding to the timing of the local maximum CO uptake in each model.\n2\nfor each model with smoothing 11 years since 2001, serving as the reference point for aligning the\ncomposite analysis.\n38. Supp. Figure 8 legend: I think you meant “Supp. Figure 3b” instead of “Supp. 5b”.\nCorrected.\nP\nA\nG\nE\nReviewer 1 attachment:\nEmergence of the ocean CO2 uptake hole under global warming\nThe study by Lee and co-authors investigates emission-driven future scenarios with the Earth System\nCESM2 and finds an extraordinary future outgassing of CO in the subpolarNorth Atlantic. The article\n2\nidentifies a shutdown of deep convection in the Labrador and Irminger Sea as a reason for this\noutgassing of CO in the subpolar North Atlantic.\nreview and I am happy to see this. However, with the newly added CMIP6 results, it looks like the\nresults are not very robust and there is no agreement in the models that the CO sink will turn into a\n2\nsource (despite claims of the authors that the finding is robust). The article is missing a justification\nwhy this specific result of CESM2 is plausible. Further major points are listed below. I regret to say that I still\nbelieve that the article needs more work to be suitable for publication in Nature Communications.\nM1) ajoThr ipso ainrtitsc le needs language editing. I started writing down the formulations that needed to be\ncorrected, but then I quickly realized that this concerned nearly every sentence of the manuscript.\n2) The results added for the CMIP6 models must be put into context and be described more nuancedly\nand it must be justified that the results of CESM2 are plausible. In my opinion, the following is\nneeded:\na. The chosen CMIP6 models and their performance should be introduced. How well do they\nrepresent the AMOC and deep convection (see, e.g., Heuze, 2021, Swingedouw et al.,\n2022) in comparison to CESM2? It seems that most of the chosen CMIP6 models have a\nlarger AMOC and more deep convection than observed. Moreover, the two models with\nless deep convection (CanESM5, IPSL -CM6A-LR ) are even left out of the analysis because\n“they do not reproduce the observed March mixed layer over the Labrador-Irminger Sea\nwell” (L317-321), though it can be claimed that the remaining models have a biased high\nMLD and hence also do not represent this well. In summary, it appears as if the chosen\nCMIP6 model ensemble is biased in one direction. To clarify the robustness of the results,\nit would be best to choose a CMIP6 ensemble where half of the members show too little\ndeep convection and a too weak AMOC. Moreover, a justification should be provided\nas to why CESM2 is better performing, and its results have high value.\nb. The chosen CMIP6-ensemble shows that the pattern of CO -outgassing does not locally\n2\ncoincide with that of the warming hole, but it is located more to the west. I consider the\nterm “CO uptake hole” hence less fitting and L101-102 should be reformulated.\n2\nc. The CMIP6-ensemble shows that the CO -outgassing at the end of the simulation is NOT\n2\na robust feature, neither is the DIC- threshold and there is also no linear relationship\nbetween the AMOC strength when the air-sea CO flux is reversed and when the SPNA MLD\n2\nis shallower than 150m. Throughout the text, the authors claim the contrary. Please check\nFig. 3 and 4d more carefully and rephrase L104-105, L105-107, L151-153, L190-196 and\nL213-214, L238 to make clear that the results are NOT robust. As the results are NOT\nrobust, it must be justified why CESM2 deserves extra attention (see 2a).\n3) The analysis leading to thresholds seems a bit rushed and shallow as pointed out by reviewer 2.\nBased on Figure 3, I only see a threshold for MLD, but not for AMOC or DIC. Please extend your\njustification with further analysis and rephrase all related text. Also consider that there is a lot of\nroom between an MLD of 800m and an MLD collapse, so please rephrase L204-208\nMinor comments:\n• Title, abstract etc: I am not familiar with the term “ocean CO uptake hole”. It is somewhat\n2\nintuitive as there is also the “warming hole” in the subpolar North Atlantic, but these two\n“holes” do not seem to regionally coincide. Hence, I would like the term to be introduced and\nit not to be used as if it was a commonly used term. While the authors have now introduced\nthe term, I still think that the title “Emergence of the ocean CO uptake hole under global\n2\nwarming” seems to imply that this is a commonly used term. I would like the title to at least be\nchanged to “Emergence of an oceanic ‘CO uptake hole’ under global warming” or something\n2\nsimilar.\n• L10: The oceanic carbon system is an important reservoir for anthropogenic CO emissions, yet\n2\nuncertainties persist regarding its future changes.” --> please change to “The ocean an\nimportant sink for anthropogenic CO emissions, yet uncertainties persist regarding its future\n2\nchanges.”\n• L13: please specify the model's name and add that this article also includes the results of an\nensemble of CMIP6 models.\n• L15: “weakening of the uptake–CO uptake hole”– please change to “weaking of the CO\n2 2\nuptake” and do not refer to the CO uptake hole without introducing the term.\n2\n• L18-22: these sentences are unclear. Please revise such that their meaning is clear.\n• L29-34: Many studies with ESMs do not show a linear response of the oceanic CO uptake to\n2\nrising atmospheric CO levels, neither globally nor regionally (e.g. Kessler & Tjiputra, 2016).\n2\n• L49-57: It is unclear to me what the authors want to say here.\n• L60-61: please change to “an emerging CO sink in the SPNA region, coined “CO uptake hole”\n2 2\ndue to its overlap in location with the so-called warming hole”\n• L62: The connection between deep convection and AMOC is in debate (Li et al., 2019,2022;\nMenary et al., 2020), so please rephrase.\n• L 66-69: meaning of sentence is unclear. Please either add missing details such that an\nunderstanding is possible or rephrase.\n• L83: There is no linear increase (see Figure 1d), please rephrase\n• L90: please specify at what point in time\n• L112-122: what about changes in the wind-forcing?\n• L142: please rephrase the title of the section – its meaning is not clear\n• L143: no “additional” process is needed, just a change in a process\n• L157: “Therefore” is not the right word here, the sentence is not a direct consequence (as\ndifferent options are possible)\n• L161: the SPNA is not an ocean and deep convection also is present in the Southern Ocean\n• L263-265: I think that this sentence is factually wrong.\nReferences:\n• Swingedouw et al., 2022: AMOC recent and future trends: a crucial role for Oceanic\nresolution and Greenland melting? Front. Clim. 4, 32.\n• Heuzé, 2021: Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models,\nOcean Sci., 17, 59–90, https://doi.org/10.5194/os-17-59-2021, 2021.\n• Li FL, Lozier MS, Danabasoglu G, Holliday NP, Kwon YO, Romanou A, Yeager SG, Zhang R. 2019\nLocal and Downstream Relationships between Labrador Sea Water Volume and North Atlantic\nMeridional Overturning Circulation Variability. J. Climate 32, 3883-3898. (doi:10.1175/jcli-d-\n18-0735.1)\n• Menary MB, Jackson LC, Lozier MS. 2020 Reconciling the Relationship Between the AMOC\nand Labrador Sea in OSNAP Observations and Climate Models. Geophys. Res. Lett. 47,\ne2020GL089793. (doi:10.1029/2020gl089793)\n• Li F et al. 2021 Subpolar North Atlantic western boundary density anomalies and the\nMeridional Overturning Circulation. Nature Comm. 12, 3002."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have addressed all of my previous comments and I think the manuscript is ready for acceptance. I have just 2 remaining additional, very minor comments:\nLine 17: In the beginning of what?\nLine 186: Should be SPNA\n(Remarks on code availability)\nWith the revisions made by the authors I was able to access the code and will note that it has more sufficient metadata now included.", "reviewer_4": "The study by Lee et al., investigates the ocean CO2 uptake response under different atmospheric CO2 emission scenarios using the CESM2 model. The authors found an unusual behavior with respect to the global ocean in the SPNA, a crucial region for ocean CO2 uptake. Regardless of the emission scenario, if atmospheric CO2 concentration continues to rise, the authors show that the SPNA will reverse its CO2 uptake. This phenomenon, referred to as the “CO2 uptake hole” by the authors, is attributed to surface DIC saturation caused by strong DIC accumulation in shallow layers due to a contraction of the Irminger-Labrador Sea convection, as a consequence of the MLD becoming shallower. Moreover, they find that this phenomenon could occur without significant AMOC weakening.\nThe article has strong points, such as the “CO2 uptake hole” concept, an important finding, and the research conducted to determine the reason why it happens and when. The relevance of the finding is well framed in the last paragraph of the discussion. Among other aspects, I also find the results presented in panel (e) of Figure 1 and the decoupling of the phenomenon with an AMOC weakening particularly relevant (although the figure shows a huge dispersion within the CMIP6 models, and the AMOC analysis still has some issues). The overall idea is good, but there are still major aspects to address that require further work before publication. I encourage the authors because their work is valuable and, with the necessary corrections and work, I believe that it would be a good contribution to the scientific community.\nI have noticed that this is not the original submitted manuscript, so my comments may conflict or be inconsistent with previous comments by the other reviewers.\nMajor comments\n1.⁠ ⁠The writing style of the article makes it tedious to read. I recommend a careful review of the language, as several phrases are unclear. Moreover, the flow of the text is disrupted in several sections, creating an unstructured discourse. I recommend reviewing these aspects thoroughly. I have highlighted the most important issues in some minor comments, but these issues can be extrapolated to the entire manuscript. Overall, the introduction seems to me to be the most poorly structured section.\n2. If the possibility of a CO2 uptake hole was already suggested by other studies (line 111) you should mention it in the introduction and explain clearly how your research differs or it is pioneering. From the way it is written, it seems that this is the first time the CO2 uptake hole has been identified. Moreover, the analogy with the “warming hole” needs to be used carefully, as the ‘CO2 uptake hole’ does not spatially coincide with the ‘warming hole’.\n3. The authors use the CMIP6-models results to validate the CESM2 model performance in the experiments, as the uptake hole seems to be highly variable depending on the model configuration (line 114). Does this mean that the seven simulation results for each scenario are very different from one another? I believe it is very important for your results that their uncertainty is well explained and discussed, to ensure they are sufficiently robust. In addition, the CMIP6-related information is not well-integrated, nor the reason why you use it. I would add the necessary discussion about why you use the CMIP6 models, perhaps at the beginning of the CMIP6 Methods section. In addition, lines 331-336 should be moved to the CMIP6 methods section, as this part describes the MLD comparison with CMIP6 models before introducing them. Similarly, lines 352-35 should be restructured, as you introduce the use of ESMs without sufficient context.\n4. There are several inconsistencies in the analysis that should be refined. For example, in Figure 1 panel (c) you show ER 1.5 with relative years (2001-2010) vs (2081-2100). However, in Supp Fig 5 you show CMIP6 SSP5-8.5 (only comparable to your ER 1.0, if I understood correctly) and with relative years (1980-2014) vs (2081-2100). The same issue appears in Figure 4, where you show ER1.5 and MMM of CMIP6.\nIn Figure 4, using the DIC minus total column average instead of DIC solely can be misleading. Why do you need to subtract the total column average? (and how is this average computed? by year, by the mean over the period…?). Also, in Figure 4, as in others, there is a lack of specification regarding the regions you use for some computations. For example, ‘the temporal evolution of MLD is extracted over 1000m depth regions in March’, which regions?\nOn the other hand, when analyzing the MLD you state that, from Supp. Figures 3 and 8, the CESM2 is among the best performing models. This conclusion is somewhat questionable based on the Supp Fig3, because you are comparing different periods for the products (annual average 2010-2020 for SODA, 2001-2010 for ER experiment, I assume 1.0, and 1980-2014 for the CMIP6 models). In addition, the vertical grey line in panel (b) of Figure 3 (SODA reanalysis from 2004-2020) is far from coinciding with your results for the same period (in light yellow). I have written more examples as minor comments, but I suggest reviewing all these issues and discussing the results accordingly.\nMinor comments\nAbstract. Line 14: consider eliminating “systematically” and adding ‘scenarios’ after “emission rates”. Line 15: rephrase “possibility”. Line 18: consider replacing “weakens” by ‘decreases’ and “contracts” by ‘weakens’. Line 19: add ‘its’ before oceanic pCO2? Line 19-20: I do not get the link between the two phrases between “Given its connection… with regional climate change’. Line 23: consider replacing “providing” by ‘supporting’. Line 24: add ‘regional’ before dynamics?\nIntroduction: when referring to carbon uptake, I suggest using a unique term, such as CO2. For example, in line 44 I would replace “carbon” by ‘CO2’.\nLine 34: I suggest rephrasing, as uptake does not saturate but DIC does; saturation is a consequence of the uptake.\nLines 35-40: You introduce projections about how the ocean will behave in the future. Then you describe two specific regions. After that, you generalize that it cannot be expected that all oceans have the same response, and then you describe again a specific region (NA). May consider rephrasing ‘Not all the oceans can be explained by a simple relation’ to something as (for example) ‘Not all oceans are expected to respond in the same way’; then describe the Tropical and SO future behaviour, and after this, start describing the NA.\nLines 42-43: Consider rephrasing as follows: “… is tightly coupled with deep-water formation, facilitated by the cold and dense environment, at the Labrador…”.\nLines 49-52. Consider rephrasing or deleting ‘despite de NA’s smaller contribution to the global carbon budget’.\nLines 53-66: I suggest better organization and structure.\nLines 63-64: if you keep “AMOC changes” I suggest using ‘modulate/influence’ or similar instead of “reduce NA carbon uptake”; if keeping “reduce” then replace “changes” by ‘weakening is’.\nResults. Even though you explain it in Methods, I was a bit confused the first time I read the manuscript about what was the ensemble mean of the ERs and what ER was. It could be beneficial to quickly explain this in the figures’ legends, for example, as ‘7 simulations for each scenario with slightly different initial conditions’.\nLine 87: ‘by a decrease’ or ‘by decreasing the’.\nLines 93-96: “as the open ocean increases”, consider rephrasing.\nLine 106-107: consider rephrasing for “ thus formulating..”\nLine 113: how do you quantify the ‘collapse’?\nLine 114-115: what do you mean by “model configuration”?\nLine 119: rephrase.\nLine 186: replace SPAN by SPNA.\nLine 248: the “*” after “0.53” is explained in Figure 4 legend but not here. Can be replaced by ‘p-value < 0.05’.\nLines 278-279: I do not understand what you want to say.\nLine 291: climate change impact because of? I suggest rephrasing.\nLines 298: aerosols, meltwater fluxes in plural?\nMethods. AMOC calculation is not explained, only briefly in supp. fig 2. It is computed for all products at RAPID latitude? Could the difference between your study area and RAPID latitude influence your results?\nLine 334-336. This is true for Supp Information figure 8b but questionable from Supp Information Figure 3 as there are no metrics.\nLine 348: Do you mean the CO2 concentration-driven simulation as an example of a typical simulation, or do you mean that you use it? This confused me a bit.\nLines 360-370. Line 361: the pattern correlation of what? Line 363: CO2 flux of what, the ensemble, for each model? From line 360, I suggest starting a new paragraph describing the MLD, why you analyse it, and to bring the CESM2 MLD analysis here and the comparison with the CMIP6 results.\nLines 397-398. I do not understand what you mean here.\nFigure 1 legend. Red box in panel (c) is not described in the legend. Add “atmospheric” in (b) temporal evolution of “atmospheric” CO2 concentration. You could refer the reader to Methods after the word SSP5-8.5. After (e), when you explain what rebound timing is, you say it is defined by the maximum CO2 absorption peak trend. What do you mean by “peak trend”? What do you mean by ‘after taking the 11-year running mean’?\nFigure 2. First time I looked at this figure I did not realize the y-axis scale changes between panel b,c and d; you could mention it, or even use the same scale. Regarding DIC and ALK average, is the 100m threshold defined by something other than a generic criteria? Does this threshold have an impact on your results?\nFigure 4. Add “is” in line 652 between “MLD” and “extracted”.\nSupp. Figure 2 legend: what do you mean by “Historical and SSP58.5”? Vertical dashed lines are not described.\nSupp. Figure 3 legend: again, what do you mean by historical? If the deep convection index is not explained, maybe it would be better to delete it from here.\nSupp. Figure 4 legend: how do you compute the time lags? .\nSupp. Figure 8 legend: I think you meant “Supp. Figure 3b” instead of “Supp. 5b”.\n(Remarks on code availability)"}, "author_response": "The response to Reviewer #3:\nThe authors have addressed all of my previous comments and I think the manuscript is ready for\nacceptance. I have just 2 remaining additional, very minor comments:\n1. Line 17: In the beginning of what?\nWe clarified the sentence.\nL17: In the beginning of the scenarios, the SPNA is an effective CO sink, but this uptake decreases\n2\ndue to the weakening of deep convection around the Labrador–Irminger Seas region and the rising of\nits oceanic pCO beyond the global average.\n2\n2. Line 186: Should be SPNA\nCorrected.\nP\nA\nG\nE\nThe response to Reviewer #4:\nThe study by Lee et al., investigates the ocean CO2 uptake response under different atmospheric CO2\nemission scenarios using the CESM2 model. The authors found an unusual behavior with respect to\nthe global ocean in the SPNA, a crucial region for ocean CO2 uptake. Regardless of the emission\nscenario, if atmospheric CO2 concentration continues to rise, the authors show that the SPNA will\nreverse its CO2 uptake. This phenomenon, referred to as the “CO2 uptake hole” by the authors, is\nattributed to surface DIC saturation caused by strong DIC accumulation in shallow layers due to a\ncontraction of the Irminger-Labrador Sea convection, as a consequence of the MLD becoming\nshallower. Moreover, they find that this phenomenon could occur without significant AMOC\nweakening.\nThe article has strong points, such as the “CO2 uptake hole” concept, an important finding, and the\nresearch conducted to determine the reason why it happens and when. The relevance of the finding is\nwell framed in the last paragraph of the discussion. Among other aspects, I also find the results\npresented in panel (e) of Figure 1 and the decoupling of the phenomenon with an AMOC weakening\nparticularly relevant (although the figure shows a huge dispersion within the CMIP6 models, and the\nAMOC analysis still has some issues). The overall idea is good, but there are still major aspects to\naddress that require further work before publication. I encourage the authors because their work is\nvaluable and, with the necessary corrections and work, I believe that it would be a good contribution\nto the scientific community.\nI have noticed that this is not the original submitted manuscript, so my comments may conflict or be\ninconsistent with previous comments by the other reviewers.\nWe truly appreciated the reviewer for the thoughtful and constructive comments. Based on the\nreviewer’s comments, we carefully revised the manuscript. We hope that the revised manuscript\nwould be plausible and suitable to resolve the concerns. Please see more specific responses below in\nblue.\nMajor comments\n1.⁠ ⁠The writing style of the article makes it tedious to read. I recommend a careful review of the\nlanguage, as several phrases are unclear. Moreover, the flow of the text is disrupted in several\nsections, creating an unstructured discourse. I recommend reviewing these aspects thoroughly. I have\nhighlighted the most important issues in some minor comments, but these issues can be extrapolated\nto the entire manuscript. Overall, the introduction seems to me to be the most poorly structured\nsection.\nResponding to the reviewer’s comment, we have carefully checked the sentence in the general\nmanuscript and rephrased them accordingly. In particular, we have substantially revised the\nIntroduction to improve its structure and organization.\n2. If the possibility of a CO2 uptake hole was already suggested by other studies (line 111) you\nshould mention it in the introduction and explain clearly how your research differs or it is\npioneering. From the way it is written, it seems that this is the first time the CO2 uptake hole\nP\nA\nG\nE\nhas been identified. Moreover, the analogy with the “warming hole” needs to be used carefully,\nas the ‘CO2 uptake hole’ does not spatially coincide with the ‘warming hole’.\n2-1. Novelty\nAs we briefly noted in the manuscript (L111), previous studies hint the possibility of the “CO uptake\n2\nhole,” primarily indicating a reduction in CO uptake. The main difference is that Chikamoto and\naveraged CO flux and the AMOC time series (Figures 4 and S1 in CD21), suggesting a simplistic\n2\ninterpretation of the phenomenon as solely AMOC-related. However, our study demonstrates that the\nCO flux response in the SPNA is reversed. We also suggested that it is more coupled to the deep\n2\nconvection over the Labrador-Irminger Sea, the Subpolar Gyre (SPG) than to the weakening or\ncollapse of the AMOC. Current climate models tend to support that the permanent collapse of the\nAMOC would not be happen before 2100. However, our findings indicates that the distinct uptake\nhole could emerge much earlier without the collapse of the AMOC. Although the previous study\nhinted the possibility, our study is the first study to investigate a distinct evolution of SPNA CO\n2\nuptake. We revised the introduction to clarify this novelty.\nL43-66: The NA is currently known for its efficient CO uptake, covering only 15% of the global\n2\nocean area while holding more than 23% of the global oceanic anthropogenic CO 14. This efficiency\n2\nis closely linked to deep-water formation, facilitated by the cold and dense environment in the\nSubpolar North Atlantic (SPNA)14,15; excessive CO can be delivered into the deep ocean through the\n2\ndescending branch of the Atlantic Meridional Overturning Circulation (AMOC)14,16–18. Under\ngreenhouse warming, changes in the AMOC naturally raise a potential loss of the effective CO\n2\nuptake, which is indeed reported when the permanent collapse of the AMOC occurs8,9,11–13. For\nexample, a recent modeling experiment suggests that the permanent collapse of the AMOC would\ndiminish the NA CO uptake around the 22nd century. Since such a permanent collapse is not\n2\nprojected to occur within 2100 in most of state-of -the-art ESMs19, one may reasonably expect that the\nSPNA will continue to serve as an effective sink for atmospheric CO .\n2\nwarming scenarios. The deep convection around the SPNA, particularly Labrador Sea and Irminger\nSea, and the regional carbon cycle are also tightly connected to the subpolar gyre (SPG)15,22. Intense\nSPG maintains deep MLD through increased convection, while a weakened SPG suppresses the\ndevelopment of the active mixed layer26. While global warming weakens both the SPG and the\nAMOC23–25, the collapse of SPG can not be regarded as identical to that of the collapse of the AMOC,\nparticularly as their temporal evolutions may differ significantly26,27. Therefore, interpreting the\nSPNA CO uptake solely to a passive response to AMOC may be a limited perspective. Consequently,\n2\nthere remains the possibility of losing uptake efficiency which is triggered by abrupt changes in\nregional ocean dynamics, and this possibility remains largely unexplored.\n2-2. Regarding the CO uptake hole terminology\n2\npattern. However, in broader context, the distinct evolution over the SPNA is consistent between two\nphenomena, which we adapt the term with expression “Although the spatial pattern is not exactly the\nsame, we hence…”.\nWe clarified the manuscript in order to address the reviewer's concern.\nP\nA\nG\nE\nL104-111: From the rebound times, the SPNA begins to decrease the air-sea CO flux changes,\n2\nthereby generating pronounced anomalous outgassing over the SPNA. It is already well known that\nthe SPNA exhibits a distinct phenomenon compared to other regions, referred to as the “warming\nhole”28, which may also be manifested in the carbon cycle. Although the spatial patterns of the two\nfeatures are not identical, we suggest referring to this significantly anomalous outgassing\nphenomenon as the “CO uptake hole”, drawing an analogy to the “warming hole”.\n2\n3. The authors use the CMIP6-models results to validate the CESM2 model performance in the\nexperiments, as the uptake hole seems to be highly variable depending on the model configuration\n(line 114). Does this mean that the seven simulation results for each scenario are very different from\none another? I believe it is very important for your results that their uncertainty is well explained and\ndiscussed, to ensure they are sufficiently robust.\nIn addition, the CMIP6-related information is not well-integrated, nor the reason why you use it.\nI would add the necessary discussion about why you use the CMIP6 models, perhaps at the beginning\nof the CMIP6 Methods section. In addition, lines 331-336 should be moved to the CMIP6 methods\nsection, as this part describes the MLD comparison with CMIP6 models before introducing them.\nSimilarly, lines 352-35 should be restructured, as you introduce the use of ESMs without sufficient\ncontext.\nThank you for the reviewer’s constructive comments.\nThe ER experiments exhibit relatively small spread across ensembles, as indicated by the colored\nshading in Figure 1e, supporting the CO uptake hole is not driven by internal variability but forced\n2\nresponse. Rather, we use CMIP6 to show that our main conclusions are robust, not depending on\nspecific model configuration of CESM2. We acknowledged that L114 leads to confusion and revised\nit to make it clearly express the reason.\nL114-116: Furthermore, to assess the robustness of the uptake hole beyond the ER experiments, we\nextend our analysis using the Coupled Model Intercomparison Project phase 6 (CMIP6) (Methods).\nWe also carefully revised and improved the CMIP6 Method section to align with the reviewer’s\nsuggestions, including discussion on purpose of CMIP6 analysis, relocation of MLD comparison,\nrestructuring of paragraph.\nL346-378: Ensuring consistent representation of the CO₂ uptake hole across different models and\nexperimental configurations is crucial. To show its robustness, we analyzed 17 CMIP6 models using\nhistorical simulations (2001–2014) and SSP5-8.5 (2015-2100) simulations (Supplementary Table.\n1)62. The models were selected based on the availability of the necessary variables, including air-sea\nCO flux, AMOC, DIC, and salinity. Each model dataset was converted to a horizontal resolution of\n2\n1° × 1°. Furthermore, to choose more reliable model to simulate our key processes, the pattern\ncorrelation of the annual average from 2001 to 2010 between the reanalysis data and CMIP6\nhistorical simulation was conducted (Supplementary Fig. 8b). The pattern correlations of the SPNA\nCO flux in each model show a strong linear relationship with that of the MLD (r = 0.84),\n2\nhighlighting that the MLD is a critical determinant of the SPNA CO flux, serving as an essential\n2\nmechanism for the physical CO pump in the present day. It is also noted that the CESM2 used for the\nselection of the 8 good models (CNRM-ESM2-1, MPI-ESM1-2-LR, NorESM2-MM, CESM2, CESM2-\nWACCM, UKESM1-0-LL, GFDL-CM4, and GFDL-ESM4) that closely simulate both MLD and CO\n2\nflux in agreement with reanalysis data. These models exceed the mean of the SPNA CO₂ flux and the\nMLD correlation coefficient, respectively (above the horizonal and vertical gray line in\nSupplementary Fig. 8b).\nP\nA\nG\nE\nThe linear relationship between the SPNA MLD and the CO flux, represented by the pattern\n2\ncorrelation values between reanalysis data and the CMIP6 historical simulation, suggests that the\nperformance of ocean biogeochemical cycles is influenced by the performance of ocean physical\nprocesses, contributing to multi-model diversity. Notably, CESM2 has been reported to represents\nmajor ocean circulations relatively well among the current Earth System Models (ESMs). For\nexample, the strength of the Antarctic Bottom Water, North Atlantic Deep Water, and AMOC in\nCESM2 are more comparable to observations than those in other models27,63–65. In particular, the\noverflow parameterization is implemented, which provides a better projection of deep ocean\ndynamics63,66. At this point, we quantified the spread in MLD climatology based on the area-averaged\ndeep convection site. The deep convection area is defined as the region where the MLD exceeds 1000\nm around the Labrador-Irminger sea in maximum MLD in March (black contours in Supplementary\nFig. 3b). Compared with the MLD reanalysis data, CESM2 is one of the best-performing models that\nsimulate SPNA MLD strength and pattern with reliable consistency (Supplementary Fig. 3 and\nSupplementary Fig. 8).\n4. There are several inconsistencies in the analysis that should be refined. For example, in Figure 1\npanel (c) you show ER 1.5 with relative years (2001-2010) vs (2081-2100). However, in Supp Fig 5\nyou show CMIP6 SSP5-8.5 (only comparable to your ER 1.0, if I understood correctly) and with\nrelative years (1980-2014) vs (2081-2100). The same issue appears in Figure 4, where you show\nER1.5 and MMM of CMIP6.\n4-1) Caption of Supplementary Figure 5\nWe apologize for the mistake. The relative year was calculated based on the years 2001 to 2010\nreferenced in Figure 1. Therefore, we corrected the Supplementary Fig. 5 caption.\nL55-60: Supplementary Figure 5 | Ocean carbonate system changes in the CMIP6 SSP5-8.5\nscenario. Air-sea CO flux changes in the CMIP6 multi-model mean of the SSP5-8.5, i.e. the last 20\n2\nyears (years 2081-2100), relative to the historical simulation annual mean from 2001 to 2010. The\nred box indicates the region of Subpolar Northern Atlantic Ocean (SPNA, 45°N-65°N, 80°W-0°W).\nNegative (positive) air-sea CO flux indicates oceanic CO uptake (outgassing).\n2 2\n4-2) ER1.5 as a representative experiment\nAs shown in Supplementary Figure 1a and 1b, other ER experiments also exhibit the strong\nanomalous outgassing behavior of the SPNA. This pattern is also evident in Figure 4 and\nSupplementary Figure 7. Thus, sensitivity to the emission rate has a minimal impact on our\nconclusion. We compared ER1.5 and the multi-model mean (MMM) of CMIP6 because the strongest\nemission rate experiment (ER1.5) clearly reveals the CO uptake hole phenomenon. We revised the\n2\nmanuscript to emphasize the independency.\nL116-118: Regardless of the emission rates, the CMIP6 multi-model mean (MMM) shows similar\nfeatures to the SPNA CO flux changes, although considerable uncertainty remains (Fig. 1e).\n2\nhighly consistent with the ER experiments (comparing grey and colored thick lines in Supplementary\nFig. 4).\nIn Figure 4, using the DIC minus total column average instead of DIC solely can be misleading. Why\ndo you need to subtract the total column average? (and how is this average computed? by year, by the\nmean over the period…?). Also, in Figure 4, as in others, there is a lack of specification regarding the\nP\nA\nG\nE\nregions you use for some computations. For example, ‘the temporal evolution of MLD is extracted\nover 1000m depth regions in March’, which regions?\n4-3) The DIC vertical deviation\nThe vertical deviation of the SPNA DIC was computed by total DIC value minus entire depth mean of\nthe total DIC by year. DIC increases monotonically from the surface to the deep ocean as atmospheric\nCO₂ levels rise (Figure R4-1). Thus, the vertical deviation is an effective way to highlight that DIC\nincreases more rapidly at the surface than in the deep ocean, which reflects a role of the regional\ndynamics. To enhance clarity in the computational explanation, we rephrased the sentence in the\nfigure caption.\nL671-674: Hovmöller diagram of the SPNA DIC concentration, calculated as total DIC minus total\ncolumn average of total DIC by year in (a) ER1.5 ensemble mean, (b) CMIP6 multi-model mean\n(MMM) of the historical (years 2001-2014) and the SSP5-8.5 (years 2015-2100) simulations.\nFigure R4-1 | Hovmöller diagram of the SPNA DIC changes. Hovmöller diagram of the SPNA DIC\nconcentration changes in relative to the initial year (year 2001-2010) in (a) ER1.0 and (b) CMIP6 multi-\nmodel mean (MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulation.\n4-4) Definition of SPNA MLD\nWe apologize for the lack of specification about SPNA deep-ocean convection. We defined the deep-\nocean convection region as the area where the MLD exceeds 1000 m over the Labrador Sea and\nIrminger Sea (black contours in Supplementary Fig. 3b) and computed its area-averaged MLD. We\nincluded the detailed computation in the Methods section and revised the Figure 4 caption to enhance\nclarity in the explanation.\nL373-376: At this point, we quantified the spread in MLD climatology based on the area-averaged\ndeep convection site (Supplementary Fig. 8a). The deep convection area is defined as the region\nwhere the MLD exceeds 1000 m around the Labrador-Irminger sea in maximum MLD in March\n(black contours in Supplementary Fig. 3b).\nL674-677: The temporal evolution of the MLD represents an area-averaged value extracted from\nregions with exceeding 1000 m depth in March, where deep convection forms around the Labrador\nSea and Irminger Sea (black contours in Supplementary Fig. 3b) (black solid line).\nP\nA\nG\nE\nOn the other hand, when analyzing the MLD you state that, from Supp. Figures 3 and 8, the CESM2\nis among the best performing models. This conclusion is somewhat questionable based on the Supp\nFig3, because you are comparing different periods for the products (annual average 2010-2020 for\nSODA, 2001-2010 for ER experiment, I assume 1.0, and 1980-2014 for the CMIP6 models). In\naddition, the vertical grey line in panel (b) of Figure 3 (SODA reanalysis from 2004-2020) is far from\ncoinciding with your results for the same period (in light yellow). I have written more examples as\nminor comments, but I suggest reviewing all these issues and discussing the results accordingly.\n4-5) Regarding analyzing periods\nThank you for pointing out the important issue. We agree that the analyzing periods are not exactly\nmatched with each other. Therefore, we adjusted all data to the initial year from 2001 to 2010 in the\nrevised manuscript. In addition, we updated the reanalysis data, MLD and CO flux, from one to\n2\nthree. In the case of the MLD, we used the Simple Ocean Data Assimilation (SODA) version 3.15.2,\nEstimating the Circulation and Climate of the Ocean (ECCO), and Ocean Reanalysis System 5\n(ORAS5). For the CO flux, the Max Planck Institute for Meteorology Self-Organizing Map Feed\nin the analysis. Note that for Figure 3 (Figure R4-5), the period 2004-2017 was selected to align with\nthe time range of the available AMOC observation data and MLD reanalysis datasets.\nIn Supplementary Figure 3, we compared the spatial pattern of the SPNA MLD climatology.\nExamining the reanalysis data for MLD over the annual mean from 2001 to 2010, we overall observed\nthe formation of a deep MLD around the Labrador-Irminger Sea, although its magnitude varies\ndepending on the data set (Figure R4-2). In addition, for MLD climatology with three reanalysis data\naverages compared to simulation data, we found that the MLD above 1000 m depth—a footprint of\ndeep convection—is well simulated (Figure R4-3a to c). Therefore, in the case of Supplementary\nFigure 8, examining the deep convection index (Figure R4-4a), which represents the area-averaged\nMLD within the black contour of Figure R4-3b, along with the pattern correlation between reanalysis\nand simulation data (Figure R4-4b), we believe that it is reasonable to select the CMIP6 8 good\nmodels than the previous version.\nRegarding to the lack of coincidence for the period in Figure 3b, when we updated to the three MLD\nreanalysis datasets and adjusted the period, the vertical grey line in Figure R4-5b more closely aligns\nwith our results for the same period (light yellow in Figure R4-5b).\nFigure R4-2 | Mixed layer depth (MLD) climatology in March. Spatial pattern of the reanalysis datasets (a)\nSimple Ocean Data Assimilation (SODA), (b) Estimating the Circulation and Climate of the Ocean (ECCO), and\n(c) Ocean Reanalysis System 5 (ORAS5). All values represent the initial annual averages for the period 2001–\n2010.\nP\nA\nG\nE\nFigure R4-3 | Mixed layer depth (MLD) climatology in March. Spatial pattern of (a) mean of the reanalysis\ndatasetsSimple Ocean Data Assimilation (SODA), Estimating the Circulation and Climate of the Ocean\n(ECCO), and Ocean Reanalysis System 5 (ORAS5), (b) ER experiment, (c) CMIP6 multi-model mean (MMM)\nin the historical simulation, and (d-t) its individual models. All values represent the initial annual averages for the\nperiod 2001–2010.\nP\nA\nG\nE\nFigure R4-4 | Performance of employed CMIP6 models. (a) Deep convection climatology values, which are\ndefined as an area-averaged near the Labrador-Irminger Sea in March (black contour line in Supplementary Figure\n3b). All values represent the annual mean over the period 2001–2010. Black horizontal dashed line indicates the\nmean of the reanalysis datasetsSODA, ECCO, and ORAS5. Grey horizontal dashed lines represent the\nuncertainty in the average period (±1 𝑠.𝑑). (b) Pattern correlation in the SPNA between reanalysis data and\nindividual models. In the reanalysis datasets, the average of SODA, ECCO, and ORAS5 is used for the MLD and\nMPI-SOMFFN, OceanSODA-ETHZ, and CSIR-ML6 for the air-sea CO₂ flux. All values are used as the initial\n10 averaged years (year 2001-2010). Grey horizontal and vertical dashed lines show the mean of the SPNA CO₂\nflux and the MLD correlation coefficient, respectively.\nFigure R4-5 | Nonlinear relationship with SPNA CO uptake. Relationship between air-sea CO flux change\n2 2\nand (a) AMOC, (b) Lab-Irm MLD, and (c) DIC in the SPNA. Scatter is shaped depending on their CO emission\n2\nrates: ER1.0 (triangle), ER0.5 (circle), and ER1.5 (rectangle), and shapes represent all ensembles in each\nexperiment. The green line indicates the CMIP6 multi-model mean and the green scatter shows each 17 models.\nDIC is averaged surface layer (0-100m) and normalized by salinity (the standard salinity is 35 psu). AMOC\nstrength is defined as the average of the annual mean Atlantic meridional stream function from 26°N at a\nlatitude and a maximum depth from 500 m to 2000 m. The grey vertical solid line indicates the current\nobservation and reanalysis data averaged during the years 2004-2017 and the grey vertical dashed lines\nrepresent the uncertainty in the average period (±1 𝑠.𝑑).\nP\nA\nG\nE\nMinor comments\nAbstract:\n1. Line 14: consider eliminating “systematically” and adding ‘scenarios’ after “emission rates”.\nCorrected.\n2. Line 15: rephrase “possibility”.\nCorrected.\n3. Line 18: consider replacing “weakens” by ‘decreases’ and “contracts” by ‘weakens’.\nCorrected.\n4. Line 19: add ‘its’ before oceanic pCO2?\nCorrected.\n5. Line 19-20: I do not get the link between the two phrases between “Given its connection… with\nregional climate change’.\nCorrected.\n6. Line 23: consider replacing “providing” by ‘supporting’.\nCorrected.\n7. Line 24: add ‘regional’ before dynamics?\nCorrected.\nIntroduction:\n8. when referring to carbon uptake, I suggest using a unique term, such as CO2. For example, in line\nP\nA\nG\nE\nThank you for the suggestion. As the reviewer suggested, we changed the “carbon uptake” to “CO\n2\nuptake” in the whole manuscript.\n9. Line 34: I suggest rephrasing, as uptake does not saturate but DIC does; saturation is a consequence\nof the uptake.\nTo reflect the reviewer's comment, we revised the sentence.\nL34-36: Earth System Models (ESMs) currently project that most of the ocean CO uptake would\n2\nincrease linearly in response to rising atmospheric CO levels, slowing towards the end of the\n2\nsimulation3–5.\n10. Lines 35-40: You introduce projections about how the ocean will behave in the future. Then you\ndescribe two specific regions. After that, you generalize that it cannot be expected that all oceans have\nthe same response, and then you describe again a specific region (NA). May consider rephrasing ‘Not\nall the oceans can be explained by a simple relation’ to something as (for example) ‘Not all oceans are\nexpected to respond in the same way’; then describe the Tropical and SO future behaviour, and after\nthis, start describing the NA.\nThank you for the constructive reviewer’s comment. We rephrased the sentence structures following\nthe suggestion.\nL36-42: Interestingly, depending on the ocean basin, the CO uptake shows a different response. In\n2\nboth the Tropical and Southern Oceans, the uptake is particularly enhanced, with these regions\nexpected to become more favorable for CO absorption due to changes in upwelling and sea ice6–8. In\n2\nthe North Atlantic Ocean (NA), previous studies suggested that the CO uptake may follow a distinct\n2\ntemporal evolution under global warming8–13. Therefore, as not all oceans respond in the same way,\nthis highlights the need to focus on regions with unique characteristics.\n11. Lines 42-43: Consider rephrasing as follows: “… is tightly coupled with deep-water formation,\nfacilitated by the cold and dense environment, at the Labrador…”.\nThank you for the comments. Following the reviewer’s comments, we rephrased the sentence.\nL45-48: This efficiency is closely linked to deep-water formation, facilitated by the cold and dense\nenvironment in the Subpolar North Atlantic (SPNA)14,15; excessive CO can be delivered into the deep\n2\nocean through the descending branch of the Atlantic Meridional Overturning Circulation\n(AMOC)14,16–18.\n12. Lines 49-52. Consider rephrasing or deleting ‘despite de NA’s smaller contribution to the global\ncarbon budget’.\nThank you for the suggestion. We deleted the sentence and rephrased it.\nP\nA\nG\nE\nL52-54 : Since such a permanent collapse is not projected to occur within 2100 in most of state-of -\nthe-art ESMs19, one may reasonably expect that the SPNA will continue to serve as an effective sink\nfor atmospheric CO .\n2\n13. Lines 53-66: I suggest better organization and structure.\nWe apologized for the lack of clarity in the paragraph and fully rephrased it to better convey our\nintended message. This comment is closely related to Major Comment 2; please refer to it for further\ndetails.\n14.Lines 63-64: if you keep “AMOC changes” I suggest using ‘modulate/influence’ or similar instead\nof “reduce NA carbon uptake”; if keeping “reduce” then replace “changes” by ‘weakening is’.\nThis comment is closely related to Major Comment 2; please refer to it for further details.\nResults.\n15. Even though you explain it in Methods, I was a bit confused the first time I read the manuscript\nabout what was the ensemble mean of the ERs and what ER was. It could be beneficial to quickly\nexplain this in the figures’ legends, for example, as ‘7 simulations for each scenario with slightly\ndifferent initial conditions’.\nTo reflect the reviewer's suggestion, we modified the Figure1 legend to add ER experimental\nensemble details.\nL637-641: 7 ensembles are conducted for each scenario with slightly different initial conditions (see\nthe Method for more details on ER experiments). All values of colored lines represent the ensemble\nmean and shading shows the uncertainty of the projections calculated by the inter-ensemble spread\n(±1 𝑠.𝑑). These time series are smoothed with an 11-year running mean.\n16. Line 87: ‘by a decrease’ or ‘by decreasing the’.\nCorrected.\n17. Lines 93-96: “as the open ocean increases”, consider rephrasing.\nAccording to the reviewer’s comment, we rephrased the sentence below:\nL92-95: In the Southern Ocean, turbulent exchange is expected to accelerate as sea ice cover\ndecreases and westerly wind strengthens, while a weakening of the equatorial trade wind is followed\nby a weakening of the equatorial upwelling8.\nP\nA\nG\nE\n18. Lines 106-107: consider rephrasing for “ thus formulating..”\nWe revised the sentence to clarify the meaning.\nL104-106: From the rebound times, the SPNA begins to decrease the air-sea CO flux changes,\n2\nthereby generating pronounced anomalous outgassing over the SPNA.\n19. Line 113: how do you quantify the ‘collapse’?\nWe define “collapse” as the point at which the MLD continues to decrease and then stops.\nSpecifically, when the MLD reaches approximately 200 meters and the gradient no longer changes,\nwe identify this point as the MLD collapse.\n20. Lines 114-115: what do you mean by “model configuration”?\nThe term “model configuration” refers to the specific setup and settings of a numerical model used in\nsimulations.\n21. Line 119: rephrase.\nWe rephrased the sentence to make it clearer.\nL118-120: The SPNA MLD, linked to the deep-water formation, exhibits significant intermodel\nspread33 (Supplementary Fig. 3), contributing to considerable uncertainty, similar to the spread\nobserved in the AMOC31,32.\n22. Line 186: replace SPAN by SPNA.\nCorrected.\n23. Line 248: the “*” after “0.53” is explained in Figure 4 legend but not here. Can be replaced by ‘p-\nvalue < 0.05’.\nSorry for the confusion. We included the information both in the text and caption.\nL250-252: Despite the existence of models that do not simulate the MLD around the Labrador-\nIrminger Sea (Supplementary Fig. 3), the inter-model relationship depicts a linear relationship (r =\n0.53, p-value < 0.05).\nL683: …* mark indicates statistically significant correlation at the 95% confidence level.\nP\nA\nG\nE\n24. Lines 278-279: I do not understand what you want to say.\nWe apologized for the unclear sentence and thoughtfully revised it.\nL281-282: Recent studies have aimed to reduce the uncertainty in the NA future projections by\napplying an emergent constraint approach.\n25. Line 291: climate change impact because of? I suggest rephrasing.\nAs suggested by the reviewer, we rephrased the sentence more clearly.\nL293-294: Current climate models project that the NA will undergo substantial changes in the future,\ndriven by the slowdown of the AMOC and the contraction of convection.\n26. Lines 298: aerosols, meltwater fluxes in plural?\nIn response to the reviewer’s comment, we changed the words to plural.\nMethods:\n27. AMOC calculation is not explained, only briefly in supp. fig 2. It is computed for all products at\nRAPID latitude? Could the difference between your study area and RAPID latitude influence your\nresults?\nWe updated the explanation of the AMOC calculation in the method. In addition, we used 26°N as the\nlatitude for calculating the AMOC index in both the RAPID observation and our ER experiments.\nTherefore, we used the same latitude for AMOC computation, it does not affect the results.\nL385-386: The AMOC strength in this study is defined as the maximum zonally integrated Atlantic\noverturning stream function (on the y-z plane) at 26°N and between 500 m and 2000 m.\n28. Line 334-336. This is true for Supp Information figure 8b but questionable from Supp Information\nFigure 3 as there are no metrics.\nSorry for the confusion. We revised the figure reference as follow.\nL376-378: Compared with the MLD reanalysis data, CESM2 is one of the best-performing models\nthat simulate SPNA MLD strength and pattern with reliable consistency (Supplementary Fig. 3 and\nSupplementary Fig. 8).\n29. Line 348: Do you mean the CO2 concentration-driven simulation as an example of a typical\nsimulation, or do you mean that you use it?\nP\nA\nG\nE\nTo avoid any confusion, we removed the sentence about the CO concentration-driven simulation and\n2\nonly focused on the emission-driven simulation, which is designed for our study.\nL339-342: These experiments are driven by the emission, which means that the atmospheric CO\n2\nconcentration is determined not only by prescribed anthropogenic CO emissions but also by oceanic\n2\nand terrestrial CO fluxes interactively, thereby reflecting changes in CO concentration via the\n2 2\ncarbon cycle-climate feedback.\n30. Lines 360-370.\n30-1. Line 361: the pattern correlation of what?\nSorry for the unclear sentence. we revised it for clarity.\nL351-354: Furthermore, to choose more reliable model to simulate our key processes, the pattern\ncorrelation of the annual average from 2001 to 2010 between the reanalysis data and CMIP6\nhistorical simulation was conducted (Supplementary Fig. 8b).\n30-2. Line 363: CO2 flux of what, the ensemble, for each model?\nWe apologized for the confusion and updated the sentence.\nL354-357: The pattern correlations of the SPNA CO flux in each model show a strong linear\n2\nrelationship with that of the MLD (r = 0.84), highlighting that the MLD is a critical determinant of\nthe SPNA CO flux, serving as an essential mechanism for the physical CO pump in the present day.\n2 2\n30-3. From line 360, I suggest starting a new paragraph describing the MLD, why you analyse it,\nand to bring the CESM2 MLD analysis here and the comparison with the CMIP6 results.\nThank you for your constructive suggestion. We substantially revised the Method section. Also, this\ncomment is closely related to Major Comment 3; please refer to it for further details.\n31. Lines 397-398. I do not understand what you mean here.\nSorry for the confusion. To reflect the reviewer's comment, we revised the sentence.\nL403-406: In this study, to separate the influence of freshwater (FW) flux, we adopted normalized\nDIC (𝑠𝐷𝐼𝐶 = 𝑆 ⁄𝑆∙𝐷𝐼𝐶) and ALK (𝑠𝐴𝐿𝐾 = 𝑆 ⁄𝑆∙𝐴𝐿𝐾) with reference salinity 𝑆 (35\n𝑟𝑒𝑓 𝑟𝑒𝑓 𝑟𝑒𝑓\npsu), and the equation is expressed as follow (e.g., Sarmiento and Grubuer 2006; Landschützer et al.\n2018):\n32. Figure 1 legend.\n32-1) Red box in panel (c) is not described in the legend. Add “atmospheric” in (b) temporal\nevolution of “atmospheric” CO2 concentration. You could refer the reader to Methods after the word\nSSP5-8.5.\nP\nA\nG\nE\nCorrected.\n32-2) After (e), when you explain what rebound timing is, you say it is defined by the maximum CO2\nabsorption peak trend. What do you mean by “peak trend”? What do you mean by ‘after taking the\n11-year running mean’?\nAdditionally, we are sorry for any confusion regarding the meaning of “peak trend.” Clearly, this is\nthe “peak timing”, that is, a maximum uptake in the SPNA sea-air CO flux and defined by the\n2\nhighest trend of the sea-air CO flux with smoothing 11 years since 2001. The meaning of ‘after\n2\ntaking the 11-year running mean’ is the meaning of “time series are smoothed with an 11-year\nrunning mean”. Therefore, we generally revised the legend to make it clear.\nL625-642: Figure 1 | Carbonate system responses to the climate change. (a) Anthropogenic CO\n2\nemission forcing and (b) temporal evolution of atmospheric CO concentration in ER1 (green), ER0.5\n2\n(yellow), and ER1.5 (red). (c) Air-sea CO flux changes in ER1.5, which is the last 20 years (year\n2\n2081-2100) interval average relative to initial 10 years (years 2001-2010, hereafter initial year). The\nred box indicates the region of Subpolar Northern Atlantic Ocean (SPNA, 45°N-65°N, 80°W-0°W).\nTemporal evolution of integrated air-sea CO flux changes relative to initial year of (d) global ocean\n2\nand (e) SPNA. All line colors are the same as noted above, and the grey line indicates the CMIP6\nmulti-model mean (MMM) of the historical simulation (years 2001-2014) and the SSP5-8.5 (years\n2015-2100) (see the Method for more details on CMIP6.). The dashed lines in panel (e) show the\nrebound timing defined by the maximum CO absorption peak times, which is the highest trend of the\n2\nsea-air CO flux for smoothing 11 years since 2001, included in all ensembles in the ER experiments.\n2 2\nAll values of colored\nlines represent the ensemble mean and shading shows the uncertainty of the projections calculated by\nthe inter-ensemble spread (±1 𝑠.𝑑). These time series are smoothed with an 11-year running mean.\n33. Figure 2. First time I looked at this figure I did not realize the y-axis scale changes between panel\nb,c and d; you could mention it, or even use the same scale. Regarding DIC and ALK average, is the\n100m threshold defined by something other than a generic criteria? Does this threshold have an\nimpact on your results?\nFirst, we mention it in the Figure 2 legend. Also, we borrowed from previous studies (Chikamoto and\nDiNezio, 2021) to define and calculate upper ocean DIC and ALK average. This upper ocean\nthreshold has no impact on our results when we changed, for example 200m.\nL643-646: Temporal evolution of (a) global-mean removed 𝑝𝐶𝑂𝑜𝑐𝑛(𝑝𝐶𝑂𝑜𝑐𝑛′) and 𝑑𝑝𝐶𝑂\n2 2 2\n(𝑑𝑝𝐶𝑂𝑜𝑐𝑛′) and (b-d) contributions in 𝑝𝐶𝑂𝑜𝑐𝑛′changes in the SPNA region. Note that the y-axis\n2 2\nscales differently depending on the experiments.\n34. Figure 4. Add “is” in line 652 between “MLD” and “extracted”.\nCorrected.\n35. Supp. Figure 2 legend: what do you mean by “Historical and SSP58.5”? Vertical dashed lines are\nnot described.\nP\nA\nG\nE\nSorry for the confusion. We revised the legend of Figure 2 to clarify the meaning and removed the\nvertical dashed line in the figure (Figure R4-X).\nL30-32: Color lines indicate ER1 (green), ER0.5 (yellow), ER1.5 (red), and CMIP6 multi-model mean\n(MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulations (grey).\nFigure R4-6 | Time evolution of the AMOC strength. Time evolution of the Atlantic meridional overturning\ncirculation (AMOC) in each simulation. Color lines indicate ER1 (green), ER0.5 (yellow), ER1.5 (red), and\nCMIP6 multi-model mean (MMM) of the historical (2001-2014) and SSP5-8.5 (2015-2100) simulations (grey).\nAMOC strength is calculated from the maximum stream function at depths from 500m to 2000m at the 26oN. All\nvalues of colored lines represent the ensemble mean and shading is the inter-ensemble spread (±1 𝑠.𝑑). It is\nsmoothed with an 11-year running mean.\n36. Supp. Figure 3 legend: again, what do you mean by historical? If the deep convection index is not\nexplained, maybe it would be better to delete it from here.\nWe apologize for any ambiguity in terminology that may have made it difficult for reviewers to\nunderstand. We used the term to refer to the historical run simulation of CMIP6. To clarify the\nmeaning, we rephrased the sentence. Also, we deleted the explanation about the deep convection\nindex as follow the reviewer’s suggestion.\nL40-45: Spatial pattern of (a) mean of the reanalysis datasetsSimple Ocean Data Assimilation\n(SODA), Estimating the Circulation and Climate of the Ocean (ECCO), and Ocean Reanalysis System\n5 (ORAS5), (b) ER experiment, (c) CMIP6 multi-model mean (MMM) in the historical simulation, and\n(d-t) its individual models. All values represent the initial annual averages for the period 2001–2010.\n37. Supp. Figure 4 legend: how do you compute the time lags?\nWe apologize for the lack of detail. First, we identified the timing of the CO flux reversal in all\n2\navailable models, which is the maximum CO uptake peak. Then, we made the time-lag composite of\n2\nthe SPNA air-sea CO flux. The composite analysis is aligned based on the local maximum CO\n2 2\nP\nA\nG\nE\nuptake time for each model, corresponding to lag 0. For better understanding, we added more detail\nabout the time-lag composite calculation in the legend and the manuscript.\nL120-123: To underpin the robustness, we make a time-lag composite of SPNA air-sea CO flux\n2\nbased on the rebound time (Supplementary Fig. 4). The composite analysis is referenced to lag 0,\ncorresponding to the timing of the local maximum CO uptake in each model.\n2\nfor each model with smoothing 11 years since 2001, serving as the reference point for aligning the\ncomposite analysis.\n38. Supp. Figure 8 legend: I think you meant “Supp. Figure 3b” instead of “Supp. 5b”.\nCorrected.\nP\nA\nG\nE\nReviewer 1 attachment:\nEmergence of the ocean CO2 uptake hole under global warming\nThe study by Lee and co-authors investigates emission-driven future scenarios with the Earth System\nCESM2 and finds an extraordinary future outgassing of CO in the subpolarNorth Atlantic. The article\n2\nidentifies a shutdown of deep convection in the Labrador and Irminger Sea as a reason for this\noutgassing of CO in the subpolar North Atlantic.\nreview and I am happy to see this. However, with the newly added CMIP6 results, it looks like the\nresults are not very robust and there is no agreement in the models that the CO sink will turn into a\n2\nsource (despite claims of the authors that the finding is robust). The article is missing a justification\nwhy this specific result of CESM2 is plausible. Further major points are listed below. I regret to say that I still\nbelieve that the article needs more work to be suitable for publication in Nature Communications.\nM1) ajoThr ipso ainrtitsc le needs language editing. I started writing down the formulations that needed to be\ncorrected, but then I quickly realized that this concerned nearly every sentence of the manuscript.\n2) The results added for the CMIP6 models must be put into context and be described more nuancedly\nand it must be justified that the results of CESM2 are plausible. In my opinion, the following is\nneeded:\na. The chosen CMIP6 models and their performance should be introduced. How well do they\nrepresent the AMOC and deep convection (see, e.g., Heuze, 2021, Swingedouw et al.,\n2022) in comparison to CESM2? It seems that most of the chosen CMIP6 models have a\nlarger AMOC and more deep convection than observed. Moreover, the two models with\nless deep convection (CanESM5, IPSL -CM6A-LR ) are even left out of the analysis because\n“they do not reproduce the observed March mixed layer over the Labrador-Irminger Sea\nwell” (L317-321), though it can be claimed that the remaining models have a biased high\nMLD and hence also do not represent this well. In summary, it appears as if the chosen\nCMIP6 model ensemble is biased in one direction. To clarify the robustness of the results,\nit would be best to choose a CMIP6 ensemble where half of the members show too little\ndeep convection and a too weak AMOC. Moreover, a justification should be provided\nas to why CESM2 is better performing, and its results have high value.\nb. The chosen CMIP6-ensemble shows that the pattern of CO -outgassing does not locally\n2\ncoincide with that of the warming hole, but it is located more to the west. I consider the\nterm “CO uptake hole” hence less fitting and L101-102 should be reformulated.\n2\nc. The CMIP6-ensemble shows that the CO -outgassing at the end of the simulation is NOT\n2\na robust feature, neither is the DIC- threshold and there is also no linear relationship\nbetween the AMOC strength when the air-sea CO flux is reversed and when the SPNA MLD\n2\nis shallower than 150m. Throughout the text, the authors claim the contrary. Please check\nFig. 3 and 4d more carefully and rephrase L104-105, L105-107, L151-153, L190-196 and\nL213-214, L238 to make clear that the results are NOT robust. As the results are NOT\nrobust, it must be justified why CESM2 deserves extra attention (see 2a).\n3) The analysis leading to thresholds seems a bit rushed and shallow as pointed out by reviewer 2.\nBased on Figure 3, I only see a threshold for MLD, but not for AMOC or DIC. Please extend your\njustification with further analysis and rephrase all related text. Also consider that there is a lot of\nroom between an MLD of 800m and an MLD collapse, so please rephrase L204-208\nMinor comments:\n• Title, abstract etc: I am not familiar with the term “ocean CO uptake hole”. It is somewhat\n2\nintuitive as there is also the “warming hole” in the subpolar North Atlantic, but these two\n“holes” do not seem to regionally coincide. Hence, I would like the term to be introduced and\nit not to be used as if it was a commonly used term. While the authors have now introduced\nthe term, I still think that the title “Emergence of the ocean CO uptake hole under global\n2\nwarming” seems to imply that this is a commonly used term. I would like the title to at least be\nchanged to “Emergence of an oceanic ‘CO uptake hole’ under global warming” or something\n2\nsimilar.\n• L10: The oceanic carbon system is an important reservoir for anthropogenic CO emissions, yet\n2\nuncertainties persist regarding its future changes.” --> please change to “The ocean an\nimportant sink for anthropogenic CO emissions, yet uncertainties persist regarding its future\n2\nchanges.”\n• L13: please specify the model's name and add that this article also includes the results of an\nensemble of CMIP6 models.\n• L15: “weakening of the uptake–CO uptake hole”– please change to “weaking of the CO\n2 2\nuptake” and do not refer to the CO uptake hole without introducing the term.\n2\n• L18-22: these sentences are unclear. Please revise such that their meaning is clear.\n• L29-34: Many studies with ESMs do not show a linear response of the oceanic CO uptake to\n2\nrising atmospheric CO levels, neither globally nor regionally (e.g. Kessler & Tjiputra, 2016).\n2\n• L49-57: It is unclear to me what the authors want to say here.\n• L60-61: please change to “an emerging CO sink in the SPNA region, coined “CO uptake hole”\n2 2\ndue to its overlap in location with the so-called warming hole”\n• L62: The connection between deep convection and AMOC is in debate (Li et al., 2019,2022;\nMenary et al., 2020), so please rephrase.\n• L 66-69: meaning of sentence is unclear. Please either add missing details such that an\nunderstanding is possible or rephrase.\n• L83: There is no linear increase (see Figure 1d), please rephrase\n• L90: please specify at what point in time\n• L112-122: what about changes in the wind-forcing?\n• L142: please rephrase the title of the section – its meaning is not clear\n• L143: no “additional” process is needed, just a change in a process\n• L157: “Therefore” is not the right word here, the sentence is not a direct consequence (as\ndifferent options are possible)\n• L161: the SPNA is not an ocean and deep convection also is present in the Southern Ocean\n• L263-265: I think that this sentence is factually wrong.\nReferences:\n• Swingedouw et al., 2022: AMOC recent and future trends: a crucial role for Oceanic\nresolution and Greenland melting? Front. Clim. 4, 32.\n• Heuzé, 2021: Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models,\nOcean Sci., 17, 59–90, https://doi.org/10.5194/os-17-59-2021, 2021.\n• Li FL, Lozier MS, Danabasoglu G, Holliday NP, Kwon YO, Romanou A, Yeager SG, Zhang R. 2019\nLocal and Downstream Relationships between Labrador Sea Water Volume and North Atlantic\nMeridional Overturning Circulation Variability. J. Climate 32, 3883-3898. (doi:10.1175/jcli-d-\n18-0735.1)\n• Menary MB, Jackson LC, Lozier MS. 2020 Reconciling the Relationship Between the AMOC\nand Labrador Sea in OSNAP Observations and Climate Models. Geophys. Res. Lett. 47,\ne2020GL089793. (doi:10.1029/2020gl089793)\n• Li F et al. 2021 Subpolar North Atlantic western boundary density anomalies and the\nMeridional Overturning Circulation. Nature Comm. 12, 3002."}]

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