The impact of oceanic heat transport on the atmospheric circulation

Knietzsch, M.-A.; Schröder, A.; Lucarini, V.; Lunkeit, F.

doi:https://doi.org/10.5194/esd-6-591-2015

Articles | Volume 6, issue 2

https://doi.org/10.5194/esd-6-591-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/esd-6-591-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 6, issue 2

Research article

|

21 Sep 2015

Research article |

| 21 Sep 2015

The impact of oceanic heat transport on the atmospheric circulation

M.-A. Knietzsch, A. Schröder, V. Lucarini, and F. Lunkeit

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Final revised paper (published on 21 Sep 2015)
Preprint (discussion started on 04 Nov 2014)

Interactive discussion

Status: closed

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

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RC C583: 'Review', Anonymous Referee #1, 03 Dec 2014
- AC C733: 'Response to referee 1', Frank Lunkeit, 11 Feb 2015
- AC C734: 'New manuscript', Frank Lunkeit, 11 Feb 2015
RC C596: 'Comment on "Oceanic heat transport on the atmospheric circulation" (Knietzsch et al)', Anonymous Referee #2, 05 Dec 2014
- AC C735: 'Response to referee 2', Frank Lunkeit, 11 Feb 2015
- AC C736: 'Streamfunction forcing for all simulations', Frank Lunkeit, 11 Feb 2015
- AC C737: 'New Manuscript', Frank Lunkeit, 11 Feb 2015
RC C634: 'Comments on ESD-2014-61', Anonymous Referee #3, 12 Dec 2014
- AC C738: 'Response to referee 3', Frank Lunkeit, 11 Feb 2015
- AC C739: 'Streamfunction forcing for all simulations', Frank Lunkeit, 11 Feb 2015
- AC C740: 'New manuscript', Frank Lunkeit, 11 Feb 2015

Peer-review completion

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

AR by Frank Lunkeit on behalf of the Authors (17 Feb 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Feb 2015) by Daniel Kirk-Davidoff

RR by Frédéric Laliberté (08 Apr 2015)

Suggestions for revision or reasons for rejection

Review for “The impact of oceanic heat transport on the atmospheric circulation” by
M.-A. Knietzsch, A. Schröder, V. Lucarini, and F. Lunkeit

The authors use an intermediate complexity model with coarse resolution to quantify the impact of artificially changing poleward oceanic heat transport in a slab ocean. It follows a suite of similar studies that appeared in recent years and innovates by using advanced diagnostics to measure the impact of oceanic heat transport on the atmospheric circulation. Among other diagnostics, it describes the atmospheric circulation using the concept of the “climate machine”. It is my impression that the conclusions that follow from this climate machine perspective will help readers gain physical intuition about what makes the atmospheric circulation weaken when ocean heat transports are increased.

For these reasons I think this manuscript will be an important contribution to climate science. I have however serious concerns about parts of the Ferrel cell analysis. These will be discussed later in my first and second major comments. I have also the impression that the manuscript lacks a clear focus with none of their many results really standing out. Because this will likely affect the potential impact of this manuscript, I would suggest that the authors pick one of their results and then organize their development to clearly emphasize that result. In my third major comment the authors will find a more extensive explanation for this suggestion. For these reasons, I would recommend that the manuscript undergo a round of major revisions.

The authors should feel free to contact me if they have any questions about my review.

Frederic Laliberte

Major Comments:

1. On lines 170, 295, 550 and 660, the authors refer to Czaja and Marshall (2006, hereafter CM2006) but seem to mischaracterize their work. CM2006 did not show that “the atmospheric heat transport can be represented by the product of the strength of the TEM residual circulation and the vertical contrast in moist static energy if the eddy transport of the theta is replaced by the transport of theta_e” as claimed on line 660. Instead they have shown that this statement is true only if in addition the vertical gradient of theta in the equation on line 655 is replaced by the vertical gradient of theta_e. This can be easily verified. The problem is that in Earth-like conditions the vertical gradient of theta_e vanishes in the free troposphere from the tropics to well within the midlatitudes. Because of these two features, Figure 10b would look drastically different: It would be negative in the lower troposphere, be undefined in the lower free troposphere, and positive in the upper free troposphere. In fact, this problem was discussed in Pauluis et al. (2011, “A Statistical generalization…”) and to a lesser extent in Laliberte et al. (2012). The conclusion at the moment is that there is no simple way to represent a well-defined (all its values finite) moist isentropic circulation in the latitude-pressure plane.

If I understand well, the analysis to create Figure 10b involved only replacing the eddy transport of theta with the eddy transport of theta_e and not replacing the vertical gradient of theta with the vertical gradient of theta_e. In this case, Figure 10b gives the moist atmospheric heat transport only if multiplied by the vertical gradient of theta (or the dry static stability) and not if it is multiplied by the vertical gradient of theta_e (or the moist static stability), as claimed on line 325. As a consequence, the discussion of Figure 11 might not be right in this context. If the residual moist streamfunction was not computed using the vertical gradient of theta_e (or, more accurately, the STEM of Pauluis et. or an equivalent approximation) then its strength and the way it evolves under increased ocean heat transport might not be right. Although I am not expecting an increase as in Figure 3 (upper left) of Laliberte and Pauluis (2010), the moist circulation might not collapse as much as implied in Figure 11.

2. I found the analysis of the Kuo-Eliassen equation slightly confusing and I would have greatly benefitted from a clearer connection between the Eulerian-mean decomposition (Fig. 8) and the TEM decomposition (Fig. 10a,c,e). Here’s a suggestion: Rewrite equation on line 620 by adding and removing the last term on the RHS of the equation on line 650. This will split the eulerian-mean circulation into 5 terms instead of 4: heating, friction, eddy heating minus vertical EP flux, eddy momentum minus horizontal EP flux, and the EP flux. This way the TEM reconstruction then becomes simply equal to the sum of the first, second and last term. In the TEM framework, the third and fourth term are therefore representing the effective mass transport by eddy heat fluxes and eddy momentum fluxes, respectively. This would likely put much more physical intuition into Fig. 9 because the different terms will have a clearer physical meaning. At the moment, it seems that heat and momentum transport do not change (Fig. 9b) with increased ocean heat transport and yet the residual streamfunction (which is basically driven by the heat and momentum transports) changes appreciably (Fig. 11, here I’m assuming that the picture for the dry residual streamfunction looks more or less similar).

3. As indicated above, I find that there are many interesting results but that they are not tied together. My understanding is that most of the results are in support of Figs. 14-18 but at the moment it feels more like three sets of disjoint results. One set is about the circulation, the second set is about the Lorenz cycle and the last is about the climate machine. One suggestion would be to begin with Figs 1-4 and then discuss Figs. 12-14 and Fig. 15a. These fit nicely with Fig. 2. Then the authors could present Fig. 17 and Fig. 18, their main results. Fig. 18 should be followed by Fig. 5 because the responses are consistent. Then Figs. 6-11 explain what is happening to the circulation and Fig. 16 confirms its role in the Lorenz energy cycle. Then Fig. 15b shows that what is happening in the cycle is consistent with the efficiency, thus confirming the relevance of the climate machine perspective. I feel that this would make it easier for the authors to clearly emphasize their innovative point of view on the atmospheric circulation.

Minor comments:

1. I know that publication was concurrent but it would seem relevant to relate some of these results to the recently published Laliberte et al. (2015).
2. See my minor comments / questions / typos in the annotated pdf attached to this review. Better to view with Adobe Reader.

Referee Report: PDF

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RR by Anonymous Referee #5 (17 Apr 2015)

ED: Reconsider after major revisions (03 May 2015) by Daniel Kirk-Davidoff

AR by Frank Lunkeit on behalf of the Authors (09 Jul 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Jul 2015) by Daniel Kirk-Davidoff

RR by Frédéric Laliberté (30 Jul 2015)

Suggestions for revision or reasons for rejection

Review of “The impact of oceanic heat transport on the atmospheric circulation"
by M.-A. Knietzsch, A. Schröder, V. Lucarini, and F. Lunkeit

This is my second review of this manuscript and after thoroughly re-reading the revised manuscript, it is my opinion that it is ready for publication except for minor technical corrections. I found several typos or phrasing decisions that could be improved, described in an annotated pdf file. Most of these corrections are either minor or, when they are concerning phrasing, should be taken merely as suggestions.

I have only one more important comment, concerning a citation of my own work. Around line 185, the authors cite Laliberte & Pauluis (2010) to explain why the moist isentropic circulation cannot be easily cast onto the latitude-pressure plane but this publication does not really mention the problem. In fact, it is Laliberte et al. (2012) that addresses this problem and I think it would be a preferable citation.

Finally, I have a comment about the interpretation of Figure 10. In this figure, I would be tempted to say that the apparent "negative contribution” from surface fluxes is simply a consequence of the choice of moist entropy variable. If I'm not mistaken, the way the material entropy production is computed corresponds to a moist entropy where moisture is converted to heat only when it precipitates (i.e. the liquid-water entropy, see Emanuel 1994). This is why the precipitation leads to large entropy production in Figure 10. For some people (and maybe many) in the atmospheric community this could be a little bit confusing. Usually, precipitation is associated with pseudo-adiabatic motions and therefore intuitively with no material entropy production ("adiabatic"). It is my intuition that if you were instead computing latent heat fluxes corresponding to a moist entropy that is mostly conserved in pseudo-adiabatic motions (i.e. theta_e) then Figure 10 would show only small contributions from precipitations and most contributions would come from surface fluxes and rain-snow conversions. This would then tie in nicely with the results of Laliberte et al. (2015) and several of the papers by Pauluis et al, allowing the authors (if they wished to!) to explain some of the changes they are observing in terms of moistening inefficiencies. At this point in the publication process, I do not recommend that the authors change their analysis. This comment could however allow them to reformulate paragraphs 365 and 645 slightly to achieve more impact.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by Editor) (10 Aug 2015) by Daniel Kirk-Davidoff

AR by Frank Lunkeit on behalf of the Authors (28 Aug 2015) Author's response Manuscript

ED: Publish subject to technical corrections (03 Sep 2015) by Daniel Kirk-Davidoff

AR by Frank Lunkeit on behalf of the Authors (03 Sep 2015) Manuscript

Short summary

A general circulation model with an aquaplanet setup is used to study the impact of changes in the oceanic heat transport (OHT) on the atmospheric circulation. The atmosphere counterbalances the imposed changes in OHT. A stronger OHT leads to a decline in the intensity and a poleward shift of the maxima of both the Hadley and Ferrel cells. The efficiency of the climate machine, the intensity of the Lorenz energy cycle and the material entropy production of the system decline with increased OHT.