|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.
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.
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.