the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jan 2024
The impact of polar warming on global atmospheric circulation and mid-latitude baroclinic waves
Abstract. The results of experimental and numerical modeling of Arctic warming in a laboratory dishpan configuration are presented. The Arctic warming is reproduced by varying the size of a local cooler in the "atmospheric" regime, in which the flow structure is similar to the general atmospheric circulation. It is shown that a significant variation in cooling power and boundary (slip and non-slip) conditions leads to quantitative changes in the structure and intensity of baroclinic waves. The size of the cooler and boundary conditions applied to its surface play a crucial role in the structure and intensity of circulation at small radii. The laboratory Arctic warming leads to a weakening of a polar cell analog and mean zonal flows. The most important result of this study is a noticeable transformation of the mean temperature field. Namely, the central region and most of the lower layer become warmer, while most of the upper layer and the peripheral (equatorial) part of the lower layer become colder. The nature of this phenomenon is closely related to the changes in radial heat fluxes. Laboratory Arctic warming leads to a significant decrease in the negative heat flux near the bottom, which inevitably leads to an increase in temperature. Our results provide a plausible explanation for Arctic warming amplification.
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RC1: 'Comment on egusphere-2023-2797', Anonymous Referee #1, 19 Mar 2024
General comments:
The manuscript addresses a very interesting and timely problem, and investigates a laboratory model of polar amplification in a laboratory setting. The experimental apparatus, developed recently by the authors is a novel, and surprisingly realistic model of the atmospheric circulation of a hemisphere of Earth (in this case, the Southern hemisphere, as the tank is rotating in the clockwise direction). This setup with the applied thermal boundary condition is a significant improvement to the widely used baroclinis annulus settings, introduced by the groups of Fultz and Hide in the 1950s. To the best of my knowledge, this is the very first such differentially heated rotating setting that is able to capture qualitatively the three-cell convection of a hemisphere (baroclinic annulus models are typically restricted to one-cell sideways convective meridional flows). The two experiments, in which the Rossby waves and jets were traced using aluminum flakes are supported by a series of numerical simulations as well, which yielded a superb agreement with the observed surface patterns, and also revelaed the likely structure of the three- (plus one) cell dynamics and the strength of the zonal flow. The most important finding of the paper is surprising: the mean temperature field is reorganized in such a way in a polar warming scenario that the central domain and the larger part of the lower layer became warmer, whereas the upper layer and the "equatorial" part of the lower layer became colder. I would remark that this type of dynamics may be of relevance for the deeper understanding of the ongoing climatic processes: in the present-day global warming at the lower levels at the polar region a marked warming is observed in the lower layers, but at the tropics the warming indeed happens at higher levels of the troposphere/stratosphere, which is rather similar to what is observed here, and therefore, I believe that this finding is rather significant, as it clearly demonstrates that even in a fluid dynamic model like this one, where latent heat is not involved, this situation can develop. Therefore I believe that this is an important paper which should be published. The presentation is clear and concise, the paper is well written and the language is easy to follow.
Specific comments:
- The simulations applied a constant heat flux boundary condition at the free surface, whereas, in reality, I believe that a boundary condition with a heat flux given by dT(x,y)/dt = 1/tau * (T0-T(x,y)) would be more realistic, with T(x,y) being the surface temperature field, T0 the room temperature and tau is the timescale determined by the material properties (e.g. specific heat) of the fluid. Certainly not for the present paper, but in the future it may be interesting to see whether such a surface forcing would give different results in the numerical simulations.
- The nice images in Fig.4 suggest that using video recordings of these patterns and PIV software such as VidPIV it would be possible to get measurment data for the mean zonal and meridional surface velocities in the experiment. Was such analysis conducted, or can it be done relatively easily? If yes, I think it would be nice to compare those from the simulated mean zonal and meridional fields, to see, e.g. whether the size of the cells are indeed such as seen in the numerical results.
- See my general comment above: I think it is very important that this experiments provides similar patterns than the actual atmosphere subjest to polar warming. I believe that it would underline the significance of the paper if some discussion on this would be given in the Conclusions section.
Technical corrections/suggestions:
Figure 1b: In color it is nice, but in the printed (grayscale) version the blues and reds in the azimuthal flow field cannot be destinguished. I would suggest to replot this with similar light-to-dark scales that are used in pretty much all the other such diagrams of the paper.
Line 93: The wording here is a bit misleading, when it says "The experimental model is a rectangular tank..." instead, I would suggest to reformulate the sentence like this: "The experimental model is a tank of a rectangular cross-section....", as the tank itself is not rectangular but cylindrical.
Line 200: I'm not sure about the terminology here. The text says: "leads to an additional circulation known as the Ekman pumping". I think Ekman pumping specifically refers to the extra downwelling flow created here, as a consequence of Ekman transport, but the circulation itself, I believe is not to be called Ekman pumping.
Citation: https://doi.org/10.5194/egusphere-2023-2797-RC1 -
RC2: 'Comment on egusphere-2023-2797', Tim Woollings, 28 Mar 2024
This paper describes a set of lab experiments and accompanying numerical solutions as potential analogues to the problem of amplified Arctic warming. The experiments are novel and the results interesting. I believe the paper ultimately has promise, even if only as a proof-of-concept, but I do have some major concerns over the current framing of the work.
Major comments:
1. The main conclusion that a transformation of polar cell structure is a plausible explanation for Arctic amplification is not justified. This is based on the temperature structure with warming over the pole near the surface, but the forcing seems very different in the experiments (changes in upper level friction, cooling and strong descent) to the real context (changes in surface heat fluxes associated with sea ice loss, among several other mechanisms). If the authors want to make this argument, perhaps as one of several contributing mechanisms, it should be supported with a discussion or comparison to recent observed trends or future projections, to see if there are any similar changes in the polar cell in that context.
Presumably the numerical model is quite flexible, so one way to test this would be to compare with numerical experiments adding an additional heating at the polar surface, instead of changing the upper level cooling.
2. I would question the significance of the changes in mode energies which are highlighted as another key result (9b). There appear to be some differences here but the contribution of internal variability should be quantified to test this. This could involve adding error bars to fig 9c, and/or similar figures, to show whether differences are statistically significant.
Despite the focus on the meridional cells in the text, the eddy results are potentially very interesting in that the changes are relatively small. An emerging consensus from climate modelling is that the storm track response to Arctic warming is weak (eg https://www.nature.com/articles/s41612-023-00562-5) and an alternative interpretation of the current results is that these very different experiments also support this conclusion.
Minor comments:
1. The title should be adjusted to include the nature of the work, ie concerning laboratory analogues
2. The introduction should mention some of the literature on storm track responses to Arctic warming.
3. It should also note the central role of anthropogenic climate change in observed and modelled Arctic warming, as reported in many studies and IPCC reports etc.
4. Note also the work of Blackport and Screen casting doubt on claims of increased zonal flow meandering - eg DOI: 10.1126/sciadv.aay2880.
5. The introduction motivates the study as attempting to resolve uncertainty around the transition between regular and irregular baroclinic waves in lab experiments. This doesn’t seem particularly relevant and the paper doesn’t really answer this anyway. I would have thought a better motivation is simply to test Arctic warming-like changes in a broader range of physical situations.
6. Fig 1 is confusing, partly due to the caption. Presumably the greens in panel a are speeds in the meridional plane, while the shading in b is the zonal wind? What are the lines in panel a, just indicative directions? It doesn’t appear to be streamfunction.
7. Relatedly, ‘azimuthal’ is used repeatedly as well as meridional and zonal. It would be good to simplify and stick with one set of words - I suggest meridional and zonal.
8. Fig 1 also shows relatively large vertical velocities, which is at odds with the real atmosphere in which there is a clear scale separation between horizontal and vertical velocities associated with the aspect ratio. This contributes to doubts over how realistic the polar cell changes are.
9. How long are the simulations compared to a typical baroclinic wave timescale in the system?
10. What is actually plotted in fig 5?
11. Around fig 6 - when introducing the different cases, it would be nice to discuss which of these are more or less realistic and in what ways.
12. Lines 213-14: the largest differences actually seem to be in the polar cell, not the central one.
13. Fig 8 is of limited use in telling the difference between simulations.
14. The cooling seen at upper levels and larger radius (eg fig 13) is in striking contrast to observed and modelled Arctic warming. This is presumably a consequence of the fixed mean temperature and heating/cooling applied, so some discussion of this should be given. The real system is of course not in equlibrium due to the warming effect of greenhouse gases.
15. It’s nice to see the temperature measurements from the physical experiment in fig 13c. These do show some discrepancy from the numerical model though, is there a hypothesis to explain this?
Citation: https://doi.org/10.5194/egusphere-2023-2797-RC2 -
AC1: 'Comment on egusphere-2023-2797', Andrei Sukhanovskii, 19 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2023-2797/egusphere-2023-2797-AC1-supplement.pdf
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