Trans-basin Atlantic-Pacific connections further weakened by common model Pacific mean SST biases

A robust eastern Pacific surface temperature cooling trend was evident between ~1990–2013 that was considered as a pronounced contributor to the global surface warming slowdown. The majority of current climate models failed to reproduce this Pacific cooling trend, which is at least partly due to the underrepresentation of trans-basin teleconnections. Here, we investigate whether common Pacific mean sea surface temperature biases may further diminish the Atlantic-Pacific trans-basin induced Pacific cooling. Our results suggest that background Pacific SST biases act to weaken the trans-basin teleconnection by strengthening the Atlantic atmospheric stability and reducing Atlantic convection. These Pacific SST biases also act to substantially undermine the positive zonal wind-SST feedback. Furthermore, when combined, the Pacific and Atlantic SST biases led to Pacific cooling response that is almost non-existent (underestimated by 89%). Future efforts aim at reducing the model mean state biases may significantly help to improve the simulation skills of trans-basin teleconnections.

Other comments 1) Lines 21-30: "Tropical central-to-eastern Pacific sea surface temperatures (SSTs) have experienced notable cooling during ~1990-2013, together with the strengthening of Pacific trade winds ……." => The recent surface global warming hiatus has been largely attributed to IPO (PDO), which is intrinsic mode of variability. So, it is not surprising that CMIP5 historical simulation ensemble mean (or multimodel mean) cannot reproduce the hiatus (but, individual ensemble member does have IPO). As such, it is questionable if it is appropriate to use the recent surface global warming hiatus to motivate this study.
The last sentence "The observed Pacific La Niña-like change during this period has been related to both external forcing (e.g., changes in aerosols9 and solar radiation10), and internal variability, such as Interdecadal Pacific Oscillation (IPO)" is also out of place. It should appear earlier in this paragraph. The two references (9 and 10) discussed the potential effects of aerosol and solar activity, which are not directly related to anthropogenic global warming.
2) Lines 62-64: "Previous studies suggested that the Atlantic warming alone induces upward motion over the tropical Atlantic with descending air in the central-to-eastern Pacific Ocean, which is consistent with the Matsuno-Gill pattern" => Matsuno-Gill model produces descending motion all over the global topics away from the heating source. So, it cannot explain the descending air in the central-to-eastern Pacific Ocean, in particular.
3) Lines 80-82: "Thus, a robust weakened Atlantic warming-Pacific cooling teleconnection is evident when the model simulation encloses CMIP5-like SST background bias in the Pacific." => It is hard to digest "weakened Atlantic warming-Pacific cooling teleconnection". Perhaps, it is better to break this sentence into two. 4) Lins 86-88: "In fact, resulting in a simulated central-to-eastern Pacific cooling that is weakened by ~89% (Supplementary Table 2)" => This sentence is not complete. Figure 2: => Did you convert the unit for the vertical velocity from Pa/sec to mm/sec? So, this is not omega? 6) Lines 108-110: "The introduction of the Pacific region background SST bias leads to a significant reduction in upward motion over the tropical Atlantic region (Fig. 2e) in the Atmosphere-only simulations" => I don't see much change in the Atlantic vertical velocity between the two AGCM experiments. Perhaps, plotting the difference (EXP3 -EXP1) may help. 7) Figure S3a-c => Please add divergent velocity vector. 8) Line 119: "… the warm effect is stronger and more expanded in the higher layers…" => It is better to use "upper atmosphere" to replace "higher layers" 9) Line 123: "This reduced Atlantic heating response in the experiment…." => This is very confusing. Please be more specific. In what experiment? 10) Lines 136-139: "The net surface heat flux response distribution is highly consistent with the latent heat flux response ( Fig. 4d-f), which indicates the central Pacific wind-evaporation SST is the dominant mechanism underpinning the underestimated Pacific cooling trend in the simulation that includes CMIP5 biases background SST in the Pacific region." => Pleased break it down into two sentences. 11) Line 146: "raising" => rising 12) Line 170: "leaded to" => led to 13) Lines 178-179: "However, the important role of Indian Ocean warming in modulating the Pacific climate has also been noticed" => I think the following two papers should be referenced in here:

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Luo, J. J., Sasaki, W., & Masumoto, Y. (2012). Indian ocean warming modulates pacific climate change. Proceedings of the National Academy of Sciences, 109, 18,701-18,706. https://doi.org/10.1073/pnas.1210239109 Zhang, L., Han, W., Karnauskas, K. B., Meehl, G. A., Hu, A., Rosenbloom, N., & Shinoda, T. (2019). Indian Ocean warming trend reduces Pacific warming response to anthropogenic greenhouse gases: An interbasin thermostat mechanism. Geophysical Research Letters, 46, Dear Editor and Reviewers, Thank you very much for your time and constructive comments of our manuscript. We have revised the original manuscript on the basis of the reviewers' suggestions. The specific original comments of the reviewers follow, in italics, with our responses in blue. The corrected text lines and figures in the revised manuscript are shown in yellow.

Reviewer #1 (Remarks to the Author):
Review of " This is a very nice study. The AGCM experiments showed very clearly that the warm SST bias in the tropical Pacific and Atlantic weakens the Atlantic-to-Pacific interbasin teleconnection as well summarized in Figure 1. This result has an important implication for the future projection of the tropical Pacific under the increasing anthropogenic greenhouse gases (i.e., El Nino-like or La Nina-like). So, I recommend a publication in Nature communications after a revision.
We thank the review very much for your helpful comments and positive recommendation. Responses to each comment are as follows. Figures S3-S5 "Results from a previous study suggest that adding the CMIP5 Atlantic SST bias, with a warm SST bias in the southeastern tropical Atlantic along with a cold bias prevailing in the northwestern Atlantic, acts to alter the regions that are above or below the threshold for deep convection 20 . This results in a weakening of the Atlantic atmospheric heating response to the prescribed tropical Atlantic warming trend along with reducing the ascending motion between 90°W to 30°W and a shift of the maximum ascending motion eastward. This weakening and eastward shift of the Atlantic heating response leads to a reduction in the descending motion of the central Pacific (180°E to 135°W), further reducing the anomalous easterly wind response in the central equatorial Pacific (Fig. 2 of Ref 21 ). Thus, an underestimated strengthening of Pacific Walker circulation and a weakened Pacific cooling response is expected when Atlantic warming is imposed on top of common model background SSTs in the tropical Atlantic region. Results from our Atlantic bias experiment generally confirm those of the previous study, finding a similar eastward shift of the atmospheric circulation response (Fig. 2a,c,d,f). In the remainder of this manuscript, we will focus on exploring the impact of CMIP5 ensemble mean background SST biases over the Pacific region." The above discussion can be found in the revised manuscript Lines 106-121.

2) Lines 48-49: "a slab mixed-layer ocean in the Pacific basin" => This is an important limitation of this study. Please discuss if and how the absence of dynamic ocean model component may affect the results.
[Response 2]: Thanks for pointing this out. We understand that both the surface process and ocean dynamics are important for tropical Pacific variability. The choice of slab ocean model for the current study is based on two main reasons: (1) the simplified slab ocean allows us to adjust the Pacific mean SST to observed or CMIP5-like mean state, while it's hard to control the SST mean state in the fully coupled ocean model; (2) two previous modelling studies, which use same atmospheric GCM but coupled with slab ocean (McGregor et al. 2014 Ref13 ) and active dynamical ocean (Li et al. 2016 Ref20 ), respectively, get a similar Pacific SST response to the Atlantic warming forcing. Li et al. 2016 Ref20 further suggested the initial SST response is first set by changes to the surface fluxes and amplified by the wind-evaporation-SST feedback. The ocean dynamical Bejerknes feedback provides a secondary amplification of the SST response. The similarity results from slab ocean and dynamical ocean model lead us to believe that the results based on the slab ocean (which can represent the surface heat flux changes and WES feedback) can provide us a good 'first look' to the atmosphere teleconnection, albeit the Pacific SST response magnitude may change if ocean dynamics are included.
We have noted the limitation of slab ocean in the revised manuscript as: "It is important to note that although the use of slab ocean model allows us to adjust the Pacific SST mean state and focus on the atmospheric dynamics, the absence of oceanic dynamics (which can provide an amplification effect 20 ) may lead to an underestimated Pacific SST response to the Atlantic forcing. Further studies are needed to address the role of SST mean state while including active ocean dynamics." Please refer to Lines 187-191.
3) Lines 53-55: "To achieve these differing background SSTs, flux adjustment schemes are used to mimic the observed or the biased CMIP5-like mean state in the respective Pacific Ocean basin ( Supplementary Fig. 2)" => Different strategies are used to for the background SSTs in the Atlantic (prescribed) and Pacific (flux adjustment). Please discuss if and how the result may differ if the Atlantic background SSTs are also flux adjusted.
[Response 3]: We have followed two different approaches for the Atlantic and the Pacific, as we are interested in different aspects for the Atlantic and the Pacific. For this study we treat the Atlantic as a pacemaker, and we are interested in the Pacific response to the Atlantic forcing. While it may be possible to conduct such a pacemaker experiment with heat flux corrections (i.e., the flux correction approach is able to largely represent the SST background (as shown in Supplementary Fig. 2 a, b) and simulate the warm trend pattern in the Atlantic), it would be more complicated compared with the traditional pacemaker experiments (with fixed SST). To our knowledge this has not been done in most (or any) previous pacemaker experiments. The different approaches may lead to slightly differences of the SST background in the pacemaker region (Atlantic). However, the present study focuses on the Pacific response, thus, we believe that using flux adjustment instead of prescribing the Atlantic background SST would have limited impact to our main results.

4) Broader implication
The main result of this study has an important implication for the future projection of the tropical Pacific under the increasing anthropogenic greenhouse gases (i.e., El Nino-like or La Nina-like). Please discuss this border impact (or implication), toward the end of the paper, in more detail. The following references may be helpful: Li, G., Xie, S. P., Du, Y., & Luo, Y. (2016) Thank you for providing these references, they are very helpful. We added a paragraph to discuss the implication of our results to the future projection of tropical Pacific as: "Given the importance of the future Pacific mean state to regional climate changes in precipitation (

Other comments 1) Lines 21-30: "Tropical central-to-eastern Pacific sea surface temperatures (SSTs) have experienced notable cooling during ~1990-2013, together with the strengthening of Pacific trade winds ……." => The recent surface global warming hiatus has been largely attributed to IPO (PDO), which is intrinsic mode of variability. So, it is not surprising that CMIP5 historical simulation ensemble mean (or multi-model mean) cannot reproduce the hiatus (but, individual ensemble member does have IPO). As such, it is questionable if it is appropriate to use the recent surface global warming hiatus to motivate this study.
[Response 1a]: Thank you for your helpful advice. Yes, we agree with the reviewer that we cannot expect the CMIP5 multi-model ensemble mean to reproduce the Pacific cooling trend ~1990-2013, as it is largely related to the natural variability (i.e., IPO/PDO). But we may expect this observed cooling trend to be presented inside of the model range. However, none of the CMIP5 historical simulations can reproduce the recent eastern Pacific cooling trend ( The last sentence "The observed Pacific La Niña-like change during this period has been related to both external forcing (e.g., changes in aerosols9 and solar radiation10), and internal variability, such as Interdecadal Pacific Oscillation (IPO)" is also out of place. It should appear earlier in this paragraph. The two references (9 and 10) discussed the potential effects of aerosol and solar activity, which are not directly related to anthropogenic global warming. . However, the majority of Coupled Model Intercomparison Project phase 5 8 (CMIP5) historical simulations, which are generated by perturbed initial conditions and historical anthropogenic forcing, produced a consistent Pacific warming trend in the past decades 9-11 . Further to this, the observed Pacific cooling (Fig. 1 Fig. 2 of Ref 13 ) are both entirely outside of the model ranges. This indicates that some model common biases may lead to underestimate the eastern Pacific cooling contribution." Please refer to Lines 21-31.

of Ref 12 ) and Pacific trade wind intensification (Supplementary
2) Lines 62-64: "Previous studies suggested that the Atlantic warming alone induces upward motion over the tropical Atlantic with descending air in the central-to-eastern Pacific Ocean, which is consistent with the Matsuno-Gill pattern" => Matsuno-Gill model produces descending motion all over the global topics away from the heating source. So, it cannot explain the descending air in the central-to-eastern Pacific Ocean, in particular.
[Response 2]: Thanks for the suggestion. Yes, the Matsuno-Gill model produces descending motion all over the global tropical away from the heating source, but not with the same rate of decent everywhere. We agreed with the reviewer that it's improper to directly connect the descending air in central-to-eastern Pacific with the Atlantic warming Gill-pattern. In the revised manuscript, we explained how the Atlantic warming causes eastern Pacific cooling as following: "Previous studies (McGregor et al. 2014 Ref13 ;Li et al. 2016 Ref20 ;Polo et al. 2014 Ref23 ) suggested that the Atlantic warming induced atmospheric deep convection will trigger easterly anomaly in the Indo-western Pacific along with westerly anomaly along the eastern equatorial Pacific, reminiscent of the classic Gill-type 24 atmospheric response. The easterly wind anomaly supresses the local convection and tends to warm the Indo-western Pacific. In the eastern Pacific, on the other hand, the Rossby-wave induced off-equatorial easterly anomaly acts to intensify the trade wind and cool the equatorial-off eastern Pacific. These SST-atmosphere interactions enhance the Pacific walker circulation with anomalous descending air in the central-to-eastern Pacific, and eventually cools the central-to-eastern Pacific through the wind-evaporation-SST effect and Bjerknes feedback 20 ." Please refer to Lines 63-71.

3) Lines 80-82: "Thus, a robust weakened Atlantic warming-Pacific cooling teleconnection is evident when the model simulation encloses CMIP5-like SST background bias in the Pacific." => It is hard to digest "weakened Atlantic warming-Pacific cooling teleconnection". Perhaps, it is better to break this sentence into two.
[Response 3]: We revised this sentence as: "Thus, simulations that utilize CMIP5-like SST background bias in the Pacific produce a robust weakening of the Pacific trade wind intensification and SST cooling in response to the prescribed Atlantic warming." Please refer to Lines 85-87. Table 2)" => This sentence is not complete.

4) Lins 86-88: "In fact, resulting in a simulated central-to-eastern Pacific cooling that is weakened by ~89% (Supplementary
[Response 4]: Thanks for pointing this out. We have completed this sentence as: "The combined impacts of these background state biases result in a simulated central-toeastern Pacific cooling that is weakened by ~89% relative to the observed climatology simulation." Please refer to Lines 91-94. Figure 2: => Did you convert the unit for the vertical velocity from Pa/sec to mm/sec? So, this is not omega? [Response 5]: Yes, it is not omega. The model output we used here is called "upward air velocity" on pressure levels with unit mm/sec. Thus, the positive value represents upward motion and vice versa. We have added a detailed description in the Fig.2 caption. Please refer to Lines 394-395.

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6) Lines 108-110: "The introduction of the Pacific region background SST bias leads to a significant reduction in upward motion over the tropical Atlantic region (Fig. 2e) in the Atmosphere-only simulations" => I don't see much change in the Atlantic vertical velocity between the two AGCM experiments. Perhaps, plotting the difference (EXP3 -EXP1) may help.
[Response 6]: Thank you for your suggestion. In the Fig.S3 a-c, we show the low-layers (950-500hPa) averaged velocity potential response, which indicates the vertical velocity change. We can see a significant reduced upward motion over the Atlantic region in the difference plot Fig.S3 (c) (EXP3-EXP1; Pacific bias -observed mean state). Please refer to the supplementary Figure 3c.

8) Line 119: "… the warm effect is stronger and more expanded in the higher layers…"
=> It is better to use "upper atmosphere" to replace "higher layers".
[Response 8]: We have changed to "upper atmosphere". Please refer to Line 132. 9) Line 123: "This reduced Atlantic heating response in the experiment…." => This is very confusing. Please be more specific. In what experiment? [Response 9]: We have changed this sentence into: "In corresponding to the weakened Atlantic heating response in the Pacific bias experiment, a reduced descending motion is shown in the central Pacific (Fig. 2b, e and Supplementary  Fig.3c, f). Please refer to Lines 136-138.
10) Lines 136-139: "The net surface heat flux response distribution is highly consistent with the latent heat flux response (Fig. 4d-f), which indicates the central Pacific wind-evaporation SST is the dominant mechanism underpinning the underestimated Pacific cooling trend in the simulation that includes CMIP5 biases background SST in the Pacific region." => Pleased break it down into two sentences.
[Response 10]: We have changed this sentence into: "The net surface heat flux response distribution is highly consistent with the latent heat flux response (Fig. 4d-f). This suggests that the underestimated Pacific cooling trend in the Pacific bias simulation is largely contributed by the central Pacific wind-evaporation SST feedback." Please refer to Lines 151-154. 11) Line 146: "raising" => rising 12) Line 170: "leaded to" => led to [Response 11,12]: Thank you for your careful reading. We have corrected them. Please refer to Line 161 and Line 185.

Reviewer #2 (Remarks to the Author):
Review of the manuscript NCOMMS-20-22435-Tentitled "Trans-basin Atlantic-Pacific connections further weakened by common model Pacific mean SST biases", by Li, Dommegnet and McGregor. This work uses AGCM and AGCM-SOM simulations to study the impact of Pacific and Atlantic biases in the simulation of the Atlantic-Pacific tropical teleconnections. The results show that the warming in the southeastern Pacific reduces de teleconnection between the Atlantic and the Pacific oceans via the increase of the atmospheric stability in the tropics.
The work is well designed and structured, and the results are relevant. I thus recommend publication of the paper after a few minor issues are taken care off: Thank you for the very positive recommend. In the revised manuscript, we have included a discussion of the potential impacts of ocean dynamics and added more explanations for the PARCP experiments. For details, please see the following point-to-point response.

1) From line 140 onwards, the authors talk about the modification Bjerknes feedback in the
Pacific, but, in order to have proof of such modification, ocean dynamics have to be considered. Nevertheless, the model used in this work is a slab ocean model that, by definition only has thermodynamics included. The authors are referring to the atmospheric part of the Bjerknes feedback, but they can't really know if the ocean thermocline is going to respond to the anomalies (they can hypothesize it, but not really assert it). Moreover, adding ocean dynamics would reshape the SST-wind response in an unknown way. The authors should elaborate more on this subject in the paper.
[Response 1]: Thank you for the suggestion. We have double checked the use of word atmospheric Bjerknes feedback, and make sure it clearly refers to the zonal wind -SST feedback only. Besides, we have added a discussion of the potential impact of the ocean dynamics as: "It is important to note that although the use of slab ocean model allows us to adjust the Pacific SST mean state and focus on the atmospheric dynamics, the absent of oceanic dynamics (which can provide an amplification effect 20 ) may lead to an underestimated Pacific SST response to the Atlantic forcing. Further studies are needed to address the role of SST mean state while including active ocean dynamics." Please refers to Lines 187-191.
2) The methodology used in PARCP simulations needs to be better explained. How do you restore to a certain climatology in the Pacific but keep the model's Pacific response to the Atlantic warm pattern? [Response 2]: Thanks for your suggestions. We have added more detailed information of PARCP run in the Method part, as following: "Four sets of partially coupled experiments are performed in this study. To obtain the various Pacific (i.e., the partially coupled domain) SST background states, we estimate a flux correction that forces the model SSTs to closely follow the observed or CMIP5-like monthly mean state in the Pacific Ocean. The applied flux correction was generated using a series of iterative simulations. While in the Atlantic region (i.e., the pacemaker region), SSTs are prescribed to include the observed or CMIP5-like mean state, as detailed in the different experiments. To obtain the Pacific's response to Atlantic warming, we did one pair of simulations (i.e., the control run and the warm run) for each experiment, with the same heat flux forcing in the Pacific, but an additional prescribed Atlantic warm pattern added into the warm run. Thus, their difference can be considered as model's response to Atlantic warming under different SST backgrounds." Please refer to Lines 228-238.
Minor points: 1) Are the differences in the vertical velocities between control simulations (Figure 5b) significant? [Response 1]: Yes, the weakened Walker circulation rising branch is significant when including the Pacific mean state bias. We have added the significant test for Fig. 5b, as following: [Response 4]: Thank you for your careful reading. We have corrected this sentence, please refer to Line 79.

Lines 123-125: This response is not so clear in the AGCM experiment.
[Response 5]: Thanks for pointing this out. Yes, the reduced central Pacific descending motion in the Pacific bias AGCM simulation is much weaker but still significant for a small region near the dateline, in comparation with the coupled simulation. We think this is reasonable since the wind-evaporation-SST feedback, which is a dominant mechanism for the enhancement of Pacific Walker circulation, is absent in the AGCM simulation. In the revised manuscript, we have pointed out this difference between the PARCP and AGCM simulations as: "Note that the reduced central Pacific descending motion only becomes robust while including the surface coupling processes (c.f., Fig. 2b and Fig. 2e), indicates the vital role of surface heat flux changes in simulating the Atlantic-Pacific atmospheric teleconnection." Please refer to Lines 138-140.