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the Creative Commons Attribution 4.0 License.
| 15 Aug 2024
Responses of polar energy budget to regional SST changes in extra-polar regions
Abstract. Surface temperature at polar regions is not only affected by local forcings and feedbacks, but also depends on teleconnections between polar regions and low latitude regions. In this study, the responses of energy budget in polar regions to remote SST changes are analysed using a set of idealized SST patch experiments. The results show that responses of polar energy budget to remote sea surface warmings are regulated by changes in atmospheric energy transport, and radiative feedbacks also contribute to the polar energy budget at both the top-of-atmosphere (TOA) and surface. An increase of poleward atmospheric energy transport to polar regions results in an increase of surface and air temperature, leading to a radiative warming at surface and radiative cooling at TOA. In response to sea surface warmings in most midlatitude regions, the poleward atmospheric energy transport to polar regions in the corresponding hemisphere increases. Sea surface warming over most tropical regions enhances the polar energy transport to both Arctic and Antarctic regions, except that an increase in the Indian Ocean's temperature results in a decrease in poleward atmospheric energy transport to the Arctics due to different responses of stationary waves. Sensitivity of Arctic energy budget to tropical SST changes is generally stronger than that of Antarctic energy budget, and poleward atmospheric heat transport is dominated by dry static energy, with a lesser contribution from latent heat transport. Polar energy budget is not sensitive to SST changes in most subtropical regions.
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RC1: 'Reviewer Comment on egusphere-2024-2379', Anonymous Referee #1, 13 Sep 2024
The authors use sea-surface temperature (SST) warming patch experiments to quantify how the Arctic energy budget is impacted by remote warming. They find a central role for changes in atmospheric heat transport, primarily due to stationary eddies, for connecting tropical SST changes to changes in Arctic surface and top-of-atmosphere radiation. They also highlight the opposite response of Arctic radiation to warming in the Indian Ocean versus Western Pacific, as a result of differences in the stationary eddy response.
The premise of this paper is novel and interesting: while past studies have explored the remote impacts of SST-patch changes on atmospheric circulation and global warming, the polar warming response has been less explored, and there are many open questions about the role of atmospheric heat transport for polar warming. This paper will be of interest to research communities studying polar climate change, the pattern effect and climate sensitivity, and the teleconnections between tropical SSTs and atmospheric circulation. However, I recommend some changes to the analysis to more clearly and mechanistically interpret the results and situate them within the context of previous literature.
Major Comments:
Rossby wave response to tropical SST forcing
- There are some previous relevant papers that would be helpful to add when discussing the opposite response to Indian Ocean versus tropical Pacific warming. Annamalai et al (2007; https://doi.org/10.1175/JCLI4156.1) have a review of some of these in their introduction paragraph 4, with a focus on how SST anomalies from different ocean basins affect the Pacific-North American (PNA) pattern. Barsugli and Sardeshmukh (2002) use SST patch experiments to show that warm SST anomalies in the tropical Pacific produce positive PNA index values, while warm SST anomalies in the Indian Ocean produce negative PNA index values, both by triggering a Rossby wave response. Others like Ding et al. (2014; https://doi.org/10.1038/nature13260) have connected this atmospheric circulation response to changes in Arctic warming. It seems like your experiments are consistent with these results: the Indian Ocean and tropical Pacific generate opposite temperature responses in the Arctic by producing different Rossby wave responses and changes in stationary eddy heat transport.
- As in the references above, to investigate this Rossby wave response, can the authors plot the 200-hPa geopotential height response to these two SST experiments? It would be helpful to more clearly illustrate this mechanism linking tropical SST perturbations to changes in Arctic temperature and radiation.
- I think Equation (7) is wrong: In Kaspi and Schneider (2013) Equation (3), the stationary eddy response is defined as Vbar*Sbar – Vbar,bracket*Sbar,bracket, but the authors here have written (V*S)bar – Vbar,bracket*Sbar,bracket, which is actually equal to the stationary plus transient eddy response. This will impact the results shown in Figure 8. Also, Figure 8 has 16 panels—consider whether all are necessary.
- Figure 6 and L199-212: I didn’t find this figure helpful, other than illustrating that the Pacific patch warms the Arctic while the Indian Ocean patch cools the Arctic (although I would suggest a smaller scale for the color bar in 6a,e to be able to see the Arctic response). How do zonal-mean V and Q changes in Figure 6c and 6d help us understand this response (given that the authors later show stationary eddies are key, the covariance of V and MSE anomalies from the zonal mean would be more relevant)—please add mechanistic interpretation or remove this.
Mechanisms of Arctic warming response
- Introduction: Where is this statement that 50-85% of Arctic warming is induced by non-local drivers coming from (L48)? Some of the papers cited here (e.g. Stuecker et al., 2018) actually show the opposite—that very little polar amplification results from non-polar forcing. Papers like Dai et al. (2019) also show that local feedbacks due to sea-ice loss are needed to produce strong polar amplification. Pithan and Mauritsen (2014), Goosse et al. (2018), and Hahn et al. (2021) show that the local lapse-rate and albedo feedbacks contribute most to Arctic warming, followed by changes in poleward moisture transport. A more nuanced summary is needed: past studies have suggested a dominant role for local processes in driving polar amplification, but have also suggested that poleward moisture transport is another important contributor, and would support Arctic amplification even in the absence of local sea-ice feedbacks (e.g. Alexeev 2005). Moreover, local and remote processes interact, so remote heat transport may further contribute by amplifying local feedbacks.
- To understand the polar feedback and atmospheric heat transport response, I would recommend dividing the TOA radiation response (and heat transport convergence) (in W/m2) by the Arctic near-surface temperature response (in K), as in Kay et al. (2012; https://doi.org/10.1175/JCLI-D-11-00622.1). This would better show how remote warming impacts Arctic feedbacks and heat transport convergence. I would also consider expanding the current feedback decomposition to include the water vapor feedback and to split the temperature feedback into a Planck and lapse-rate response, consistent with previous studies. Similarly to Figure 9, can the authors also show the sensitivity of the Arctic-averaged near-surface temperature to the local SST changes? I also find Figure 9 with the Green’s function approach to be more informative than figures with the individual patch responses like Figure 1, so would suggest combining the patch experiments to create maps like Figure 9 for the feedback analysis, too.
Minor Comments
L18: Suggest adding a sentence to the abstract to indicate why the reader should care about these results—what’s the key takeaway, and what are the implications.
L23: “its lower albedo”—I don’t think this is true, and would delete. Also would add a reference for Southern Ocean heat uptake in L24 (like Armour et al. 2016) alongside the elevation/feedback references that are here already (Salzmann and Hahn).
L27: after “snow cover” add something like “and melts sea ice” (a huge contributor to the albedo feedback)
L30-33: Suggest editing this incomplete description of the temperature feedback’s contribution to AA. The main mechanism in the cited Pithan and Mauritsen reference is the lapse-rate feedback—in which surface warming is trapped by surface temperature inversions and contributes little to warming at higher altitudes (unlike in the tropics), which leads to less efficient radiative cooling in the Arctic than in the tropics. The Planck feedback also contributes to AA—in part because surface warming starting from colder temperatures in the Arctic produces less outgoing longwave radiation than when starting from warmer temperatures in the tropics, following the Stefan-Boltzmann equation.
L41: The phrasing of “efficiency” is vague—I would reword this. Also, the main point of the cited Stuecker et al. (2018) paper is the opposite of the point of this paragraph—they find that polar amplification is dominated by local, not remote forcing.
L52: Consider just writing out “polar energy budget”—I don’t think PEB is very common as an acronym, and it would be easier to read without the acronym.
L59: Suggest rewording this sentence, as the literature supports a large role of synoptic-scale waves for poleward heat transport—synoptic-scale transient eddies contribute significantly to both mean-state poleward heat transport and its changes under increased CO2 (e.g., Donohoe et al., 2020: https://doi.org/10.1175/JCLI-D-19-0797.1). I think the authors are saying that planetary waves are more important for the response to tropical warming, but should make this clearer.
L90: What magnitude of SST anomaly, A, is imposed?
L111: Should say “western and central tropical Pacific,” not eastern?
L110-120: This comes across as a descriptive list rather than telling a cohesive and interesting story. The authors might instead consider first discussing the advective, TOA, and surface responses to the tropical Pacific warming, and then the advective, TOA, and surface responses to the Indian Ocean warming. It would be helpful to add some mechanistic interpretation here, too, like these results suggest that in response to tropical Pacific warming, there is increased poleward atmospheric heat transport, which warms the Arctic atmosphere and therefore increases TOA radiative cooling and surface radiative heating. Also considering the rest of the paper, it would be generally helpful to include more mechanistic interpretation.
L252: Should be Kaspi and Schneider (2013). Many of the other citations in the text are also missing “et al.”—suggest checking the citation formatting throughout the paper.
Citation: https://doi.org/10.5194/egusphere-2024-2379-RC1 -
RC2: 'Comment on egusphere-2024-2379', Anonymous Referee #2, 19 Oct 2024
The authors investigated the influence of the regional SST change on polar amplification through a set of idealized SST patch experiments. Their findings indicate that sea surface warming in most tropical regions enhances poleward energy transport, with the exception of the Indian Ocean, which is due to different responses of stationary waves. The innovative method employed is commendable, and the results are reasonable. I would recommend a minor revision for this paper.
Arctic is experiencing a faster warming rate than the global average during the recent decade, and the underlying reasons for this amplification remain somewhat unclear. Previous energy budget analyses, such as Pithan & Mauritsen (2014), showed the significance of local feedbacks. Stucker et al. (2018) demonstrated through model simulations that Arctic amplification is primarily driven by local forcing and feedbacks. However, Ding et al. (2017) highlighted the role of circulation in influencing September sea-ice extent. Some observations also indicate short-period warming events in the Arctic often follow a period of anomalous energy transport. This highlights the necessity for a deeper understanding of how changes in poleward energy transport interact with local feedbacks in the Arctic region. This paper makes a valuable contribution to addressing this important question.
Major Comments:
- The motivation of this paper could be further clarified in the Introduction section. The authors provide a substantial summary of the ongoing debate regarding the drivers of Arctic amplification (AA), specifically whether it is driven by local processes or remote factors. While it seems the authors will discuss the importance of atmospheric heat transport (AHT) later on, this point is not revisited in detail. The authors summarized that 50%-85% of Arctic warming is induced by non-local drivers in the Introduction, which also seems overstated. Given that this paper specifically focuses on how SST warming patterns influence the Arctic rather than quantifying the relative contributions from local and nonlocal drivers, I recommend that the authors either include a discussion or quantification of how their results support the importance of AHT in AA, or step back to enhance the literature review in the Introduction regarding the influence of SST warming patterns on AA. This would help readers understand what has been explored and what this paper aims to contribute.
- The slower warming rate of the Antarctic is another interesting question. Since the authors have quantified how SST patches influence the energy budget in both the Arctic and Antarctic, I wonder if it could be possible to further discuss how the SST warming patterns might influence the asymmetry of AHT in polar regions. This may provide additional insights into the contrasting warming rates observed in the two areas.
Minor Comments:
L8: “The results show…”. This sentence is quite general; it would be beneficial to provide more specific details.
L23: “which is also applicable to the Antarctic”. The mechanisms of Antarctic warming are different from the Arctic. The mechanism studies regarding the two regions are always separate.
L48: “50%-85% of Arctic warming”. This number is followed by several cited papers. This number is much higher than expected. I recommend the authors clarify the scenarios and methods used to obtain the number to avoid confusion.
L68: “(Lee, 2011; 2012; 1204”. Typo, the closing parenthesis is missing.
L111: “The response of …”. This is an important conclusion, but this sentence is difficult to understand. Suggest rephrasing.
L252: “Y ohai”. Typo, there’s an extra space.
L357: “This knowledge…”. This paper emphasizes the role of AHT, rather than the observed radiation in the Arctic.
Figures 1-5: The colors are a bit faint, making it difficult to clearly distinguish the points.
Figure 7: The black is not bolded, while the blue line is bolded.
Citation: https://doi.org/10.5194/egusphere-2024-2379-RC2 -
EC1: 'Comment on egusphere-2024-2379', Yuan Wang, 04 Nov 2024
Comments below are from an anonymous reviewer:
This study, through extensive simulation experiments and complex diagnostic analyses, explores the response of polar energy budget to sea surface temperature anomalies. The conclusion of this paper is clear. However, this paper still requires revision and further clarification.
Major comments:
- The model description is unclear. I suppose the model used in this paper is CAM (only uncoupled atmosphere component), not CESM (coupled). The patch experiments are also incomplete. Does each warm or cold patch experiment consist of 80 sub-experiments, and every sub-experiments utilize different SST anomalies? What is the integration time of the sub-experiment?
- In the method section, authors should provide a detailed introduction to the radiative kernels technology.
- Lines 48-49, “50%-85% of Arctic warming is induced by non-local drivers”, this conclusion is a great shock and is certainly not a mainstream view. The references provided by the authors is also not compelling.
- Lines 173-175, The responses of Arctic RLH and RSH are negative. This is very interesting, and I encourage the authors further discuss it in detail. I suppose that the warm Arctic caused by energy transport from low latitude suppresses the Arctic surface turbulent heat flux (increase the downward turbulent heat flux). Since the directions of LH and SH are positive upward (see minor comments #3), the responses of LH and SH are negative. It is worth noting that if the Arctic warming is driven by the local drivers, such sea ice reduction, it will lead to upward turbulent heat flux anomalies on the Arctic surface. The surface heat flux in the Arctic has shown an upward anomaly in recent years, and the warming of the Arctic should be dominated by local factors (see major comments #3).
- Line 222, Should a barotropic mass-flux correction be applied before the computation of the energy transport? See the paper for more details.
Graversen RG. 2006. Do changes in midlatitude circulation have any impact on the Arctic surface air temperature trend?. J. Clim. 19: 5422–5438.
Minor comments:
- Lines 31-33, I cannot agree that the temperature feedback proposed by the authors creates a feedback loop.
- Line 80, remove the brackets.
- Line 98. The directions of SH and LH are not defined, I assume they are positive upward.
- Line 111, western tropical Pacific?
- Line 112, western tropical Pacific?
- Lines 115-117, the descriptions of the Rsfc responses are not consistent with Figure 1b.
- Line 119-120, the descriptions of the Radv responses are not consistent with Figure 1c.
- Lines 128-129, I can’t understand how the Radv calculated at 60° What is the physical meaning? I suppose this should be the mean Radv north of 60°N.
- Line 124, The negative response of Rsfc around 60°S may attributed to the Antarctic mean calculations of Rsfc (60°S-90°S). The imposed warm SST south of 60°S will increase the surface upward heat flux, thus the negative Rsfc.
- Line 152, the tropical western Pacific.
- Lines 154-155, the contribution of cloud is important because it is statistically significant, despite the value is relatively small.
- Line 158, western tropical pacific.
- Line 159, eastern tropical pacific.
- Lines 191-192, Do the authors consider Radv to partly represent AHT?
- There is no significance test in Figure 6.
- Line 240, Figure 7a and Figure 7b.
- Lines 276-277, I didn’t noticed the poleward propagation of SE in the tropical warm pool.
Citation: https://doi.org/10.5194/egusphere-2024-2379-EC1
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