Multi-decadal pacemaker simulations with an intermediate-complexity climate model

Molteni, Franco; Kucharski, Fred; Farneti, Riccardo

doi:https://doi.org/10.5194/egusphere-2023-1103

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https://doi.org/10.5194/egusphere-2023-1103

Preprints

16 Jun 2023

| 16 Jun 2023

Multi-decadal pacemaker simulations with an intermediate-complexity climate model

Franco Molteni, Fred Kucharski, and Riccardo Farneti

Abstract. In this paper, we first describe the main features of a new version of the International Centre for Theoretical Physics global atmospheric model (SPEEDY) with improved simulation of surface fluxes, and the formulation of a 3-layer thermodynamic ocean model (TOM3) suitable to explore the coupled extratropical response to tropical ocean variability. Then, we present results on the atmospheric model climatology, highlighting the impact of the modifications introduced in the model code, and show how important features of interdecadal and interannual variability are simulated in a “pacemaker” coupled ensemble of 70-year runs, where portions of the tropical Indo-Pacific are constrained to follow the observed variability.

Despite the very basic representation of variations in greenhouse forcing and heat transport to the deep ocean (below the 300 m domain of the TOM3 model), the coupled ensemble reproduces the variations in surface temperature over land and sea with very good accuracy, confirming the role of the Indo-Pacific as a “pacemaker” for the natural fluctuations of global- mean surface temperatures found in earlier studies. Atmospheric zonal-mean temperature trends over 50 years are also realistically simulated in the extratropical lower troposphere and up to 100 hPa in the tropics.

On the interannual scale, SST variability in sub-tropical and tropical regions not affected by SST relaxation is underestimated (mostly because of the absence of dynamically induced variability), while extratropical SST variability during the cold seasons is comparable to observed statistics. Atmospheric teleconnections patterns and their connections with SST are reproduced with high fidelity, although with local differences in the amplitude of regional features (such as a larger-than-observed response of extratropical SST to North Atlantic Oscillation variability). The SPEEDY-TOM3 model also reproduces the observed connection between averages of surface heat fluxes over the oceans and land surface air temperature in the wintertime northern extratropics.

Overall, as in earlier versions of SPEEDY, the fidelity of the simulations (both in terms of climatological means and variability) is higher near the surface and in the lower troposphere, while the negative impacts of the coarse vertical resolution and simplified parametrizations are mostly felt in the stratosphere. However, the improved simulation of surface heat fluxes and their impact on extratropical SST variability in this model version (obtained at a very modest computational cost) make the SPEEDY-TOM3 model a suitable tool to investigate the coupled response of the extratropical circulation to interannual and inter-decadal changes of tropical SST in ensemble experiments.

Received: 24 May 2023 – Discussion started: 16 Jun 2023

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Journal article(s) based on this preprint

27 Feb 2024

Multi-decadal pacemaker simulations with an intermediate-complexity climate model

Franco Molteni, Fred Kucharski, and Riccardo Farneti

Weather Clim. Dynam., 5, 293–322, https://doi.org/10.5194/wcd-5-293-2024,https://doi.org/10.5194/wcd-5-293-2024, 2024

Short summary

Franco Molteni, Fred Kucharski, and Riccardo Farneti

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1103', Noel Keenlyside, 20 Aug 2023
The paper presents the new version of SPEEDY, its coupling to a three-layered thermodynamic ocean model, and some pace maker experiments. The paper is overall well written, but there a couple of points that should be addressed to make it suitable for publication.
Major comments
The idea of prognostic equations for sea ice thickness is interesting (mode 2), but I found the discussion of mode 1 (prescribed sea ice thickness) and mode 2 confusing. As I understand, mode 2 has not been tested. Given this and that you don't use mode 2 in the paper, it does not seem appropriate to introduce the prognostic computation of sea ice thickness in this paper.

In section 4 you begin with a simulation of SPEEDY-TOM3 in pacemaker mode. However, there was no discussion about the model's climate and how it compares to the prescribed SST runs described in section 3. Has coupling with TOM3 altered the model climatology? Also, to what extent does the model’s reproduction of global warming relate to the pacing of Indo-Pacific SST (and prescribed sea ice). The performance of the SPEEDY-TOM3 configuration without pacing the SST and prescribed sea ice should be described, as this would be a more natural control experiment (which prospective model users would need to perform).

Detailed comments.
L21 "observed statistics" -> "that observed"
L76-80, it is not entirely clear why this information is included in the introduction. Is it an important new feature of the model?
L110-111, why is there an interpolation to a finer horizontal grid?
L230, I assume that you must then add a corresponding term to the third layer (equal and opposite to the fluxes in the upper two layers) to account for the convective mixing also.
L261, Have you used a value of 1.5m for ice thickness in this paper? Please make this clear.
L322, Fluxes from AGCM simulations are known to be inconsistent with SST in regions where the SST variations are driven by turbulent fluxes, as in such AGCM experiments the SST are not influenced by the fluxes. Thus, it is not clear to me why you prefer to use an AGCM experiment for estimating fluxes.
L333-338, This could be written a little more clearly, by indicating the difference for the global average solar fluxes, and then writing "global average of the net surface heat flux" (rather than "net global averages").
In figures 4 and 5, it would be useful to also show differences maps. I can see differences between the ERA and SPEEDY patterns, but the text glosses over them.
It would be useful to show biases for rainfall shown in figure 7.
It is confusing to refer to "ens. 653" in the caption and the figure titles.
L498, it is not possible to understand the initial conditions for the TOM3 from this sentence.
L531-533, it would be useful to see a simulation without such changes. How does the model behave without prescribing changes in tropical SST?
L583-585, It should be explained that SPEEDY-TOM3 as described here has an imposed tropical SST warming. This may explain the large agreement with observations.
L616-617, Is the increased signal of the PNA in the North Atlantic Ocean an indication that O-A is important for this teleconnection? Or does it reflect some differences in the model climatology between the SST and TOM3 experiments?
L686, I think you mean "coupled and prescribed SST ensemble simulations". I am wondering whether the analysis from the models is computed using the ensemble means, and how this might affect the comparison with the observations (which is one realisation).
L688-689, this information is missing from the caption.
L698, do you mean "both coupled and uncoupled ensemble simulations"?
L709, I don't think that you need "natural" here.
L711, "runs of" -> "runs"
L769-770, There is a typo here as Tau_1 is not indicated.
L801, I don't follow why you say the ice mass cannot grow beyond an ice fraction. I think you mean the maximum rate occurs when fi = 0.5^0.5; It is also confusing that you discuss thickness above, but actually you use ice fraction in A6.
Citation: https://doi.org/10.5194/egusphere-2023-1103-RC1
- AC1: 'Reply on RC1', Fred Kucharski, 24 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1103/egusphere-2023-1103-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1103-AC1
RC2:
'Comment on egusphere-2023-1103', Anonymous Referee #2, 04 Sep 2023

Review of the manuscript 'Multi-decadal pacemaker simulations with an intermediate-complexity climate model'

by Molteni et al.
The authors describe the major changes (new features) of a new version of the AGCM SPEEDY, and the formulation of a 3-layer thermodynamic ocean model (TOM3).

The most important change of the AGCM SPEEDY is in the interface between atmospheric and surface variables, and the

calculation of the surface fluxes. They present results on the climatology of the atmospheric model,

highlighting the influence of the modifications introduced in the AGCM SPEEDY, and shown how important features of interdecadal

and interannual variability are simulated in a pacemaker coupled ensemble of 70-year runs, where regions of the tropical

Indo-Pacific are constrained to follow the observed SST variability.
Major comments:
- the text is well written
- model seems to be a very useful tool
- Section 3.1: A comparison with the heat fluxes of the previous version of the model would be useful in the main text (similar to section 3.2). Have the heat fluxes improved in the newer version due to the modifications?
- Section 4.1: Figure 10 a and b: I think it would be useful to have similiar Figures for the NH extratropical SSWT and extratropical land TAS anomaly. Wondering if

the good agreement for the global means are dominated by the tropics - large parts of the tropical SSWT are prescribed.
- Section 4.3: Figure 14 and corresponding text: My impression is that there are substantial differences between the coupled model and ERA.

The coupled model patterns have a structure that reminds me of the NAO, while the ERA patterns remind me more of the East Atlantic pattern.

Therefore, the physical processes involved in the COWL-pattern might be different in the model and in the reanalysis data - in particular the importance of a dynamic ocean cannot be ruled out.

Minor comments:
- line 401 fig 7b => Fig. 7b
- line 686 Using data from both our coupled and ensembles => Using data from both our coupled and uncoupled ensembles?

Citation: https://doi.org/10.5194/egusphere-2023-1103-RC2
- AC2: 'Reply on RC2', Fred Kucharski, 24 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1103/egusphere-2023-1103-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1103-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1103', Noel Keenlyside, 20 Aug 2023
The paper presents the new version of SPEEDY, its coupling to a three-layered thermodynamic ocean model, and some pace maker experiments. The paper is overall well written, but there a couple of points that should be addressed to make it suitable for publication.
Major comments
The idea of prognostic equations for sea ice thickness is interesting (mode 2), but I found the discussion of mode 1 (prescribed sea ice thickness) and mode 2 confusing. As I understand, mode 2 has not been tested. Given this and that you don't use mode 2 in the paper, it does not seem appropriate to introduce the prognostic computation of sea ice thickness in this paper.

In section 4 you begin with a simulation of SPEEDY-TOM3 in pacemaker mode. However, there was no discussion about the model's climate and how it compares to the prescribed SST runs described in section 3. Has coupling with TOM3 altered the model climatology? Also, to what extent does the model’s reproduction of global warming relate to the pacing of Indo-Pacific SST (and prescribed sea ice). The performance of the SPEEDY-TOM3 configuration without pacing the SST and prescribed sea ice should be described, as this would be a more natural control experiment (which prospective model users would need to perform).

Detailed comments.
L21 "observed statistics" -> "that observed"
L76-80, it is not entirely clear why this information is included in the introduction. Is it an important new feature of the model?
L110-111, why is there an interpolation to a finer horizontal grid?
L230, I assume that you must then add a corresponding term to the third layer (equal and opposite to the fluxes in the upper two layers) to account for the convective mixing also.
L261, Have you used a value of 1.5m for ice thickness in this paper? Please make this clear.
L322, Fluxes from AGCM simulations are known to be inconsistent with SST in regions where the SST variations are driven by turbulent fluxes, as in such AGCM experiments the SST are not influenced by the fluxes. Thus, it is not clear to me why you prefer to use an AGCM experiment for estimating fluxes.
L333-338, This could be written a little more clearly, by indicating the difference for the global average solar fluxes, and then writing "global average of the net surface heat flux" (rather than "net global averages").
In figures 4 and 5, it would be useful to also show differences maps. I can see differences between the ERA and SPEEDY patterns, but the text glosses over them.
It would be useful to show biases for rainfall shown in figure 7.
It is confusing to refer to "ens. 653" in the caption and the figure titles.
L498, it is not possible to understand the initial conditions for the TOM3 from this sentence.
L531-533, it would be useful to see a simulation without such changes. How does the model behave without prescribing changes in tropical SST?
L583-585, It should be explained that SPEEDY-TOM3 as described here has an imposed tropical SST warming. This may explain the large agreement with observations.
L616-617, Is the increased signal of the PNA in the North Atlantic Ocean an indication that O-A is important for this teleconnection? Or does it reflect some differences in the model climatology between the SST and TOM3 experiments?
L686, I think you mean "coupled and prescribed SST ensemble simulations". I am wondering whether the analysis from the models is computed using the ensemble means, and how this might affect the comparison with the observations (which is one realisation).
L688-689, this information is missing from the caption.
L698, do you mean "both coupled and uncoupled ensemble simulations"?
L709, I don't think that you need "natural" here.
L711, "runs of" -> "runs"
L769-770, There is a typo here as Tau_1 is not indicated.
L801, I don't follow why you say the ice mass cannot grow beyond an ice fraction. I think you mean the maximum rate occurs when fi = 0.5^0.5; It is also confusing that you discuss thickness above, but actually you use ice fraction in A6.
Citation: https://doi.org/10.5194/egusphere-2023-1103-RC1
- AC1: 'Reply on RC1', Fred Kucharski, 24 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1103/egusphere-2023-1103-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1103-AC1
RC2:
'Comment on egusphere-2023-1103', Anonymous Referee #2, 04 Sep 2023

Review of the manuscript 'Multi-decadal pacemaker simulations with an intermediate-complexity climate model'

by Molteni et al.
The authors describe the major changes (new features) of a new version of the AGCM SPEEDY, and the formulation of a 3-layer thermodynamic ocean model (TOM3).

The most important change of the AGCM SPEEDY is in the interface between atmospheric and surface variables, and the

calculation of the surface fluxes. They present results on the climatology of the atmospheric model,

highlighting the influence of the modifications introduced in the AGCM SPEEDY, and shown how important features of interdecadal

and interannual variability are simulated in a pacemaker coupled ensemble of 70-year runs, where regions of the tropical

Indo-Pacific are constrained to follow the observed SST variability.
Major comments:
- the text is well written
- model seems to be a very useful tool
- Section 3.1: A comparison with the heat fluxes of the previous version of the model would be useful in the main text (similar to section 3.2). Have the heat fluxes improved in the newer version due to the modifications?
- Section 4.1: Figure 10 a and b: I think it would be useful to have similiar Figures for the NH extratropical SSWT and extratropical land TAS anomaly. Wondering if

the good agreement for the global means are dominated by the tropics - large parts of the tropical SSWT are prescribed.
- Section 4.3: Figure 14 and corresponding text: My impression is that there are substantial differences between the coupled model and ERA.

The coupled model patterns have a structure that reminds me of the NAO, while the ERA patterns remind me more of the East Atlantic pattern.

Therefore, the physical processes involved in the COWL-pattern might be different in the model and in the reanalysis data - in particular the importance of a dynamic ocean cannot be ruled out.

Minor comments:
- line 401 fig 7b => Fig. 7b
- line 686 Using data from both our coupled and ensembles => Using data from both our coupled and uncoupled ensembles?

Citation: https://doi.org/10.5194/egusphere-2023-1103-RC2
- AC2: 'Reply on RC2', Fred Kucharski, 24 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1103/egusphere-2023-1103-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1103-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Fred Kucharski on behalf of the Authors (02 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Nov 2023) by Gwendal Rivière

RR by Noel Keenlyside (13 Dec 2023)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

rejected

Were a revised manuscript to be sent for another round of reviews:

I would be willing to review the revised manuscript.

I would not be willing to review the revised manuscript.

Suggestions for revision or reasons for rejection

The authors have addressed all but one of my concerns. I still find the description of how sea ice is treated in mode 1 and mode 2 confusing. I think the authors can be easily address this issue with a little rewriting. Below is a summary of my understanding and points of confusion.

After reading through the relevant sections several times, I have understood the following for mode 1: The fraction of ice mass is computed using eq 2a, 2b, A7a from the prescribed sea ice concentration. However, at line 264, it also states that thickness is also prescribed. Thus, the exact setting for mode 1 are ambiguous.

The sea ice temperature is computed using equations 7a, 7b, and A6, using the fraction of ice mass. The introduction of an adjustment for sea ice temperature that accounts for the formation/melting of ice is interesting. However, the values in equations A6 are A7a could be a little better justified.

I am sorry but it is still not clear to me why including equations A7b – A10 is necessary, and how it helps to explain the formulation in equation A6. Furthermore, it is unclear why you use Si<1 and Si=1 rather than Si<=1. Doesn’t the formulation introduce a discontinuity? And as far as I could tell the values of d1 and d2 are not given. The description of the prognostic formulation for the fraction of sea ice mass, then the Appendix “Relationship between sea-ice concentration and thickness” could benefit from reordering to first discuss mode 1, and then discuss mode 2.

Minor comments

L34, several is an understatement. Haven’t there been “numerous” studies with SPEEDY.
L64, fullstop missing.

Hide

RR by Anonymous Referee #2 (20 Dec 2023)

ED: Publish subject to minor revisions (review by editor) (20 Dec 2023) by Gwendal Rivière

AR by Fred Kucharski on behalf of the Authors (09 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (11 Jan 2024) by Gwendal Rivière

AR by Fred Kucharski on behalf of the Authors (15 Jan 2024)

Journal article(s) based on this preprint

27 Feb 2024

Multi-decadal pacemaker simulations with an intermediate-complexity climate model

Franco Molteni, Fred Kucharski, and Riccardo Farneti

Weather Clim. Dynam., 5, 293–322, https://doi.org/10.5194/wcd-5-293-2024,https://doi.org/10.5194/wcd-5-293-2024, 2024

Short summary

Franco Molteni, Fred Kucharski, and Riccardo Farneti

Supplement

https://doi.org/10.5194/egusphere-2023-1103-supplement

Franco Molteni, Fred Kucharski, and Riccardo Farneti

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Short summary

We describe here some new features of an intermediate-complexity coupled model, including a 3-layer thermodynamic ocean model suitable to explore the extratropical response to tropical ocean variability. We present results on the model climatology and show that important features of interdecadal and interannual variability are realistically simulated in a “pacemaker” coupled ensemble of 70-year runs, where portions of the tropical Indo-Pacific are constrained to follow the observed variability.


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