the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Mar 2024
Transport into the polar stratosphere from the Asian monsoon region
Abstract. The South-East Asian boundary layer has witnessed alarming pollution levels in recent years, which even affects the trace gas composition in the southern hemisphere by inter-hemispheric transport. We use SF6 observations and the Lagrangian chemistry transport model CLaMS, driven by the ERA5 reanalysis data for the period 2010–2014, to assess the impact of the Asian monsoon (AM) region [15° N, 45° N, 30° E, 120° E] as a significant source of pollutants for the stratosphere, in particular in polar regions. We examine the contribution of transport from the AM region to the Northern Hemisphere polar region (NP) [60° N, 90° N] and to the Southern Hemisphere polar region (SP) [60° S, 90° S]. Despite the smaller geographical size of the AM region when compared to the Southern Hemisphere subtropics [15° S, 45° S] and tropics [15° S, 15° N], our findings reveal that the air mass fractions from the AM to the polar regions are approximately 1.5 times larger than the corresponding contributions from the Southern Hemisphere subtropics and roughly two times smaller than those from the tropics. The transport of air masses from the AM boundary layer to the stratospheric polar vortex primarily occurs above an altitude of about 450 K and over timescales exceeding 2 years. In contrast, transport timescales to the polar regions situated below the vortex are shorter, typically less than about 2 years. Furthermore, the transport contribution from the AM region to the polar regions exhibits distinctive inter-annual variability, significantly influencing the distributions of pollutants. Our analysis of detrended SF6 from ACE-FTS over the polar regions reveals a strong correlation with the fraction of relatively young air (less than two years old) originating from the AM, Southern Hemisphere subtropics, and tropics. Importantly, our reconstructed SF6 data indicates that approximately 20 % of SF6 in both the northern and southern polar stratosphere originates from the AM boundary layer. The largest fraction of SF6 in the polar stratosphere still originates from the tropical boundary layer, contributing about 50 % of SF6.
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RC1: 'Comment on egusphere-2024-782', Anonymous Referee #1, 06 May 2024
The paper presents an analysis of the contribution of air masses originating in the Asian monsoon boundary layer region to the polar stratosphere using ClaMS model with ERA-5 trajectories. The results show a relevant role of the AM, which contributes to approximately 20% of the air mass in the polar stratosphere in both hemispheres. The major contribution is of air from the tropics, consistent with upwelling year-round in that region. The results are novel and I suggest publication after addressing the following minor comments requesting clarification on the interpretation of some aspects.
- L91-92: if the boundary condition is maintained during 30 years this would not be a delta function, but rather a step function?
- L122-123: does this 0.5% refer to the contribution to the polar regions? I would think that substantial transport from midlatitudes to poles occurs within the troposphere. It would be beneficial to discuss the limitations in the tropospheric transport representation.
- L143: ‘We use zonally averaged meridional wind’ → zonal wind
- L141-142: ‘In the tropics…’ Why are you referring to wintertime and summertime air here? The tropical ascent does not only happen in wintertime, and I do not understand the ‘surplus of summertime air’ in high latitudes,
- L149-150: ‘The strong Pacific westerly ducts during boreal autumn and winter enable large cross-hemispheric transport (see Yan et al., 2019).’ In which features do you see this? According to the patterns in figures 1 and 2, cross-hemispheric transport of AM air into the SH peaks in SON near the tropopause, but cross-hemispheric transport of subTR-SH into the NH peaks in MAM, also near the tropopause.
- L155-156: ‘Evidently, newly released AM air primarily undergoes transport from the troposphere to the stratosphere during boreal summer and autumn’. Maybe this could be rephrased to avoid confusion. The positive anomalies of AM air are limited to the troposphere in JJA, and seen in the stratosphere in SON and DJF. In general, the results suggest that the AM air remains confined during JJA and only reaches the extratropical stratosphere in SON.
- L165-169: In this aragraph it is unclear whether you are referring to the upper levels or the lower levels, please clarify.
- L171: ‘Notably, the seasonality of the total diabatic heating rate over the Antarctic region exhibits a six-month shift compared to that over the Arctic.’ Not really a 6-month shift, MAM downwelling is larger than JJA in the SH, while in the NH it peaks in DJF.
- L185-186: ‘Notably, the air from the three source regions exhibits its youngest mean age of air (AoA) in SON over the Arctic.’ For tropical air it is actually JJA, no? In general throughout Section 3.2, it is unclear if you are referring to levels above/below 450 K, or average of all levels?
- L194-195: ‘Transit times from the AM to the Arctic are longer (shorter) than those from the subTR-SH and tropics during JJA (DJF).’ How can this be? I understand having efficient inter-hemispheric transport on the upper branch of the Hadley cell towards the winter hemisphere, but how can subtropical air from the SH penetrate the NH efficiently?
- L210: ‘Consequently, air from these three source regions over the Antarctic region exhibits its youngest mean AoA in MAM.’ At which levels? (see comment on L185-186).
- Figure 5 a-f. There is a marked tilt in the maxima in the NH panels but not in the SH ones. Do you have an idea on what this means?
- L231-232: ‘The inter-annual variability of AMFs over the southern polar region exhibits a 6-month shift compared to that of the northern polar region.’ I cannot see this clearly. Could you point to specific features where this becomes clear?
- L254: ‘underrepresented vertical transport in ClaMS’. How important is this? Why and to what extent is it underrepresented? It would improve the paper if the limitations of the model in this regard were clearly stated. How much does this affect the transit timescales below 2 years?
- Figure 7: Why are the relative contributions for subTR-NH and subTR-SH shifted by -10% and +10%, respectively? It does not seem there is a need for this scale shift (I understand the need in the case of the tropical air mass contribution), and it makes the quantitative comparison between panels quite difficult.
- L321-324: Is the difference with previous works relative to inter-hemispheric transport due to more transport in the lower stratosphere (shallow branch of the Brewer-Dobson circulation), or in the upper troposphere Hadley cell? The fact that the tracers are now injected in the boundary layer and not in the upper troposphere may suggest that transport in the troposphere plays an important role, right?
- L335-336: ‘The easterly phase of the Quasi-Biennial Oscillation (QBO) in 2010 and 2013 (e.g. Anstey et al., 2022) corresponded with positive anomalies in 2010 and 2013 in the northern polar stratosphere and in 2011 and 2014 in the southern polar stratosphere’. Positive anomalies of what?
- L355-357: ‘Additionally, strong downwelling and jet streams in the polar stratosphere play a vital role in isolating stratospheric air masses within the polar vortex during local autumn and winter. Consequently, this phenomenon leads to an increase in AM tracers in the polar stratosphere during these seasons.’ This reasoning is confusing: if the polar stratosphere is well isolated why is there ‘an increase in AM tracers’? Rather, in view of the results, I would say that in JJA there is more tropical air in the Arctic stratosphere because of weaker jets and enhaced mixing in the lower stratosphere, while AM air remains mostly confined within the anticyclone. In SON the AM air is liberated from its confinement and can reach polar latitudes. In winter the pole is more isolated so the AM air that was already there remains there. Is that a correct interpretation?
- L370: ‘Impressively’ seems an objective term, change to importantly?
Citation: https://doi.org/10.5194/egusphere-2024-782-RC1 -
RC2: 'Comment on egusphere-2024-782', Anonymous Referee #2, 30 May 2024
Review for “transport into the polar stratosphere from the Asian monsoon region” by Yan et al.,
This paper discusses the transport of surface air from the Asian monsoon region to the stratosphere, particularly the polar regions, based on the ClaMS trajectory model driven by ERA5 data. Overall, the analysis in this paper is of very high scientific quality, and the writing and figures are very clear. The emphasis on dynamics is strong and well-presented. However, in my opinion, the lack of necessary discussion on the chemistry weakens the motivation and overall context of the paper. The lack of adequate motivation necessitates a major revision before acceptance. But I believe this issue is not difficult to address and this manuscript has a potential to be a good one. Please refer to general comment #1.
General Comments:
1. In the introduction, lines 49-57, in addition to stating "there is less study on pollutant transport to the polar region" and "CFC-11, HCN, and air can be transported to the polar region," please consider adding a sentence or two to emphasize "how harmful these pollutants are to the polar region and humans." The argument that "this is not studied" is insufficient motivation for a study; "why it is important" should be the key focus.
Also, this paper discusses the transport of air, thus using SF6, an extremely long-lived gas. This experimental design is good. However, when discussing "how harmful" these pollutants are, more discussion is needed on pollutants with shorter lifetimes. While a detailed analysis of whether these short-lived pollutants can be transported to the polar region is not necessary for this work, a relevant discussion is important, especially since the first sentence of this work states, "Over the past few decades, rapid economic development in South-East Asia has been associated with a notable increase in the emissions of various pollutants."
2. Lines 187-188: "when the tracers are released during boreal summer." I'm confused here.
Isn't Figure 3 showing the age of air observed in each season? How can we judge whether the tracers were released during JJA or DJF, etc.? Tracers released in DJF can also be counted when calculating the age of air in JJA, right?
Even if you can determine this from Figure 3, is the difference significant? Visually, I can see the difference, but I'm not sure if it is statistically significant. Significance tests are also necessary for other conclusions that compare seasonality.Specific Comments:
Line 26-27: “the elevated emissions..”: please be more specific. In some of your citations, e.g., Rosenlof et al., (1997) and Solomon et al., (2010), they seem to talk about water vapor instead of pollutants. Is water vapor a pollutant?
Line 90: “pulsing 40 different species”: what are these species? Please list the example of the most important ones for the polar stratospheric chemistry
Figure 1: why remove 40% mass fraction? Then the conclusion in line 134-135 “2-3 times smaller” is not obvious at all.
Figure 3& 4: how to explain the stripe pattern of the age of air?
Line 201-202: “the pollutants from the source regions released during summer”: summer of which hemisphere? The hemisphere of release point, or Antarctica?
Line 230-234 & Figure 5: the interannual variability is very interesting: over polarLS_NH (Figure 5-i), the interannual variability above and below 450 K shows a near opposite pattern, so the interannual variability may from deep branch of the BDC above 450 K and from the shallow branch below 450 K. Over polarLS_SH, the anomaly is consistent throughout all altitudes (Figure 5j-l), and the tilted pattern indicate that the transport of the interannual variability in Figure5j-l is mostly from deep branch of the BDC. Please consider adding corresponding more detailed analysis.
Line 299: “another interesting fact is that the positive SF6 anomaly in the reconstruction over the southern polar region in 2012..”why? is it related to the strength of the polar vortex, BDC, or the AM anticyclone?
Citation: https://doi.org/10.5194/egusphere-2024-782-RC2 - AC1: 'Comment on egusphere-2024-782', Xiaolu Yan, 25 Oct 2024
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