The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity

Haywood, Alan M.; Tindall, Julia C.; Dowsett, Harry J.; Dolan, Aisling M.; Foley, Kevin M.; Hunter, Stephen J.; Hill, Daniel J.; Chan, Wing-Le; Abe-Ouchi, Ayako; Stepanek, Christian; Lohmann, Gerrit; Chandan, Deepak; Peltier, W. Richard; Tan, Ning; Contoux, Camille; Ramstein, Gilles; Li, Xiangyu; Zhang, Zhongshi; Guo, Chuncheng; Nisancioglu, Kerim H.; Zhang, Qiong; Li, Qiang; Kamae, Youichi; Chandler, Mark A.; Sohl, Linda E.; Otto-Bliesner, Bette L.; Feng, Ran; Brady, Esther C.; von der Heydt, Anna S.; Baatsen, Michiel L. J.; Lunt, Daniel J.

doi:https://doi.org/10.5194/cp-16-2095-2020

Articles | Volume 16, issue 6

https://doi.org/10.5194/cp-16-2095-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

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https://doi.org/10.5194/cp-16-2095-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 6

Research article

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04 Nov 2020

Research article | Highlight paper |

| 04 Nov 2020

The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity

Alan M. Haywood, Julia C. Tindall, Harry J. Dowsett, Aisling M. Dolan, Kevin M. Foley, Stephen J. Hunter, Daniel J. Hill, Wing-Le Chan, Ayako Abe-Ouchi, Christian Stepanek, Gerrit Lohmann, Deepak Chandan, W. Richard Peltier, Ning Tan, Camille Contoux, Gilles Ramstein, Xiangyu Li, Zhongshi Zhang, Chuncheng Guo, Kerim H. Nisancioglu, Qiong Zhang, Qiang Li, Youichi Kamae, Mark A. Chandler, Linda E. Sohl, Bette L. Otto-Bliesner, Ran Feng, Esther C. Brady, Anna S. von der Heydt, Michiel L. J. Baatsen, and Daniel J. Lunt

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Final revised paper (published on 04 Nov 2020)
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Preprint (discussion started on 02 Jan 2020)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC2: 'Review of Haywood et al.', Anonymous Referee #2, 03 Feb 2020
- AC1: 'Response to Reviewer 2', Julia Tindall, 29 Feb 2020
RC3: 'review of Haywood et al.', Tim Herbert, 30 Mar 2020
- AC2: 'Response to Tim Herbert', Julia Tindall, 16 Apr 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (20 Apr 2020) by Appy Sluijs

AR by Julia Tindall on behalf of the Authors (23 May 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Jun 2020) by Appy Sluijs

RR by Anonymous Referee #2 (17 Jul 2020)

Suggestions for revision or reasons for rejection

The revised version of the manuscript by Haywood et al. is improved over the original manuscript. I especially appreciate their inclusion of closer comparisons with PlioMIP1. As I said in my original review, as an analysis of a model intercomparison project, the manuscript is fine and some aspects of the study are especially interesting. In my original review, I suggested removing the climate and earth sensitivity analysis. The authors have decided not to do that. I’m okay with that decision. Personally, I find the manuscript to be too long and meandering, and that the climate sensitivity results would be more impactful if presented on their own.

The presentation of the manuscript is uneven. I still object to the introduction (lines 61-84), which remains a list of prior publications, and that the introduction doesn’t explain the point of the paper. I suggested a justification in my previous review, which the authors chose to ignore. I don’t think the fact that the MP has been studied for more than 25 years (lines 53-55) is a good justification for continuing to do so. The end of the Discussion (lines 670-675) provides a more compelling reason, that the Pliocene may have some lessons for near future climate change. I would encourage the authors to consider introducing these ideas in the Introduction.

Below I list some specific comments:

L. 136-137. “The standard version of the PRISM4 boundary conditions provides the best possible…” If these are the “best possible” set of boundary conditions, then why is there an “enhanced” set? (This is confusing.) It’s also worth stating in the manuscript that all but one model uses the “enhanced” boundary conditions.

L. 162. “minimum of 500 simulated years” It might be worth briefly noting that all but two models were run for 1000 yrs or more. Some of the models are quite far from TOA radiation balance. For this reason, the authors should add the SAT for the last 100 years of each model in Supplemental Fig. 1.

L. 186. This estimate of ESS assumes that the sensitivity to CO2 is linear. We know this is not necessarily the case (e.g. Caballero and Huber, PNAS, 2013; Zhu et al., Science Advances, 2019). The authors should discuss this in their list of errors in the estimate of ESS (line 189).

L. 212. “In general, PlioMIP1 models…” This sentence is confusing. I don’t think (though I’m not sure because it doesn’t make sense to me) that the sentence is correct as written.

L. 216-217. “The larger ensemble mean…rather than an increase in the temperature anomaly due to the change in boundary conditions.” This is an example of some of the clumsy writing that pops up throughout the manuscript. An ensemble mean temperature cannot increase by adding models that have the same temperature response as the mean. The authors intend to write that “The increase in the ensemble mean in PlioMIP2 is due to the addition of models with a large temperature sensitivity to the boundary conditions, rather than an increase in the temperature response of the former PlioMIP1 models to changes in boundary conditions.”

L. 378. “Some models show a different seasonal cycle to the annual mean..” I can’t make sense of why there is a comparison of the seasonal cycle with the annual mean. Perhaps the authors simply mean that the seasonal cycle varies between models.

L. 407. “is representing a state in which the associated feedbacks are in equilibrium.” How do the authors know this, and what is the point? Is there evidence that the climate is in equilibrium at any point in time?

L. 537. “A particularly robust feature…over the modern Sahara Desert…” Indicate what the feature is, i.e. “A particularly robust feature across the ensemble is an increase in precipitation over the Saharan Desert and Asian monsoon regions…”

L. 549-551. I’m not sure what the point is here. Given that the present atmospheric CO2 level now exceeds that of the mid-Pliocene, I think this is a questionable statement. Anyway, it’s the differences between the mid-Pliocene and the modern that are interesting and scientifically important.

L. 556. The use of “release date” as a model characteristic is misguided and (forgive me) lazy. It implies that newer is better, and it is a shortcut that avoids a discussion of the changes in model parameterizations that are responsible for the change in sensitivity. As in my first review, I encourage the authors not to use release date in the manuscript and instead to briefly summarize some of the reasons for changes in sensitivity that have been recently published in the literature.

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RR by Tim Herbert (19 Jul 2020)

Suggestions for revision or reasons for rejection

I am not sure if the response to my earlier query is satisfied:
[TDH6]: I would have thought that reduction in winter sea ice and/or lower land surface albedo would have generated a larger winter warming relative to the mean anomaly. [lines 230-240].
The amount of winter sea ice has very little effect on temperature. This is because there is no sunlight over the winter pole, and so the value of the surface albedo is irrelevant.
The reduction in winter snow cover away from the pole will affect only be a small proportion of the northern hemisphere surface. Therefore, it can only have a limited effect on hemisphere averaged temperatures
My understanding is that sea ice has an important effect not through albedo, but by capping off the surface ocean which has a large capacity to buffer atmospheric cooling in the winter. My query comes from studies that suggest that extensive sea ice during the younger Dryas, for example, led to very pronounced winter temperature anomalies. Likewise, I based the land albedo comment on reading that winter anomalies are reduced when tundra is replaced by denser vegetation. Can the authors comment?
The revised manuscript has this sentence in the introduction:
“Modelled sea-ice responses were studied by Howell et al. (2016), who demonstrated a significant decline in Artic sea-ice extent, with some models simulating a seasonally sea-ice free Arctic Ocean driving polar amplification of the warming. “
I would note that the revised manuscript has more discussion of seasonality and the timing of seasonal temperature maxima (p 19-20) that are very interesting and useful to a proxy person.
Likewise, I’m not sure the response to this comment hits the nail on the head
[TDH9]: To me, the pattern of data anomalies exceeding model anomalies near the gyre boundaries is quite robust (see MyClymont or example) and is a general feature of “warm climate” reconstructions- see Brierley for an earlier Pliocene time slice, and many others for the Miocene and Eocene. I think there is a fundamental model deficiency here. [line 401-406]
We agree that there could be a fundamental model deficiency in these regions and have stated near line 520 “The simulation of upwelling systems is particularly challenging for global numerical climate models due to the spatial scale of the physical processes involved, and the capability of models to represent changes in the structure of the water column (thermocline depth) as well as cloud/surface temperature feedbacks”. However, the interpretation of data in upwelling regions is also not trivial and we also discuss this in our paper.

The movement of SST gradients well poleward from the present gyre boundaries is a robust feature of warm climate SST reconstructions. Whether this actually represents extending the gyres is another question implied by the SST data but not proven. But I do not consider gyre boundaries to be “upwelling zones”- perhaps the authors misunderstood my comment. The gyre boundaries today represent major areas of thermocline ventilation and generally where mode waters form- very distinct processes from my understanding of “upwelling”. I think the problem of the models is much larger than a failure to simulate upwelling , which of course depends a lot on model resolution, coastline resolution etc.

I would also like the authors to extend their evaluation of upwelling biases in alkenone SST: Benguela is not the only instance where there is no detectable upwelling bias. We wrote a paper on the California margin (Herbert et al., ) and one can also look at the Arabian Sea and Peru-Chile margin in vain for large upwelling-related anomalies.
Herbert, T.D., J.D. Schuffert, D. Thomas, K. Lange, A. Weinheimer, and J.-C. Herguera, 1998, Depth and seasonality of alkenone production along the California margin inferred from a core-top transect, Paleoceanography, 13: 263-271.

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ED: Publish subject to minor revisions (review by editor) (20 Jul 2020) by Appy Sluijs

AR by Julia Tindall on behalf of the Authors (24 Jul 2020) Author's response Manuscript

ED: Publish as is (29 Jul 2020) by Appy Sluijs

AR by Julia Tindall on behalf of the Authors (04 Aug 2020)

Short summary

The large-scale features of middle Pliocene climate from the 16 models of PlioMIP Phase 2 are presented. The PlioMIP2 ensemble average was ~ 3.2 °C warmer and experienced ~ 7 % more precipitation than the pre-industrial era, although there are large regional variations. PlioMIP2 broadly agrees with a new proxy dataset of Pliocene sea surface temperatures. Combining PlioMIP2 and proxy data suggests that a doubling of atmospheric CO₂ would increase globally averaged temperature by 2.6–4.8 °C.