the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2026
Methane in the Asian Monsoon Anticyclone and global UTLS behavior observed in ACE-FTS satellite data
Abstract. Methane in the upper troposphere and lower stratosphere (UTLS) plays a critical role in atmospheric composition and radiative forcing, yet the processes governing its vertical structure and variability remain insufficiently understood. Here, we use high-quality satellite observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), together with in situ measurements, to characterize the vertical structure and global seasonal behavior of methane in the UTLS, with particular emphasis on the role of Asian Summer Monsoon (ASM). Methane exhibits a relative maximum that peaks near the tropopause inside the ASM anticyclone. The largest relative enhancement – defined as the difference between air inside and outside the anticyclone–occurs above the tropopause and extends poleward to the north. This behavior contrasts sharply with carbon monoxide, which is confined within the anticyclone. The seasonal cycle of methane near the tropical tropopause reaches maximum in boreal autumn, following the summer peak in surface fluxes and deep convection. This phase lag reflects the combined effects of ASM transport and subsequent large-scale redistribution following the breakdown of the anticyclone. The enhanced methane signal propagates upward into the lower stratosphere, consistent with the upwelling branch of the Brewer–Dobson circulation (BDC). Beyond the seasonal cycle, ACE-FTS data show a pronounced hemispheric asymmetry in lower stratospheric methane trends, consistent with previously reported changes in other long-lived trace constituents. Together, these results provide new observational evidence that ASM transport imprints a distinct methane structure above the tropopause and plays a key role in shaping UTLS variability on a global scale.
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RC1: 'Comment on egusphere-2026-2644', Anonymous Referee #1, 24 Jun 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2644/egusphere-2026-2644-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-2644-RC1 -
RC2: 'Comment on egusphere-2026-2644', Anonymous Referee #2, 21 Aug 2026
Review of „Methane in the Asian Monsoon Anticyclone and global UTLS behavior observed in ACE-FTS satellite data“ by Yi et al.
General comments:
The paper presents an analysis of ACE-FTS CH4 and CO data related to the Asian monsoon region. The data are compared climatologically to AirCore and aircraft campaign data, and general agreement is found. The distributions are then analysed relative to their vertical and horizontal confinement by the Asian monsoon anticyclone. In a second part, the paper moves to analysis of global distributions of methane (also from ACE-FTS), and studies its seasonality in the tropics and its global trends. Main findings are that CO is more strongly confined due to its shorter chemical lifetime, while methane is transported out of the Asian monsoon anticyclone vertically, and to the North and into the tropics. A tropical stratospheric tape recorder of methane is claimed to be found, with its positive phase starting in October, and its negative phase starting in August. Methane trends are found to show hemispheric asymmetry in the lower stratosphere which is related to a weakened shallow branch of the Brewer-Dobson circulation in the Northern hemisphere relative to the Southern hemisphere.
In general, the paper is well written, the data analysis seems to be thorough (except for the restrictions I make below), and the figures are instructive. Nevertheless, I have two general comments.
First, the specific observational restrictions of ACE-FTS seem to be widely ignored in the data analysis. As an occultation instrument in a near-polar orbit, ACE-FTS is not able to cover all latitudes at the same time. In particular, the low latitudes (between 20S and 20N) are probed during four months of the year only: February, April, August and October. Other latitudinal regions are probed at other times of the year. The authors are well aware of this fact, nevertheless they worked with seasonal mean data when producing global maps or global distributions. This might produce some confusion and, in particular, some artificial structures. A warning in the text would be required, at least. While I can live with this procedure (mainly since it has been done the same way many times before and it wouldn‘t be fair to critizise the authors for an approach that has been more or less the common - however not exact - way to proceed), I have more severe problems with deriving a tape recorder signal from just four data points over the year. As I understand, the related figure (Fig. 9) is constructed from a climatology constructed from all years covered by ACE-FTS. Only four data points (Feb, Apr, Aug, Oct) are really probed, all other data in between seem to come from interpolation(?). Deriving a tape recorder signal and assessing its uplift velocity from these four data points seems to be quite far-fetched to me. I have made suggestions below how to improve on this analysis.
Second, I think this work needs to be put better into context of previous work wrt several aspects. The Asian monsoon anticyclone and its specific transport capabilities have be analysed in great detail in recent years. The authors should refer in more detail to this previous work, sharpen their statements by doing this, and demonstrate more clearly what their specific new findings are. I have listed a number of publications below that came immediately to my mind, but there are certainly far more.
Specific comments:
Abstract, line 15: What do you refer to when saying “relative maximum”? Vertically, horizontally?
line 35: Where do the methane enhancements (“rich in methane …”) come from? What are the sources and where are they?
line 38: Please cite also more recent publications, of e.g. Vogel et al. (see my suggestions below).
line 42: Where does this statement come from? Can you cite review papers or assessments where this deficiency is stated?
line 51: It needs to be stated here that ACE-FTS measures rarely in the tropics, only during four months per year.
line 66: Four months per year are hardly enough to derive a seasonal cycle (see more detailed comment below).
line 69: This validation refers certainly to an earlier data version. Which one? Same for comparisons to MIPAS.
line 77: For the tropics and the ASM, this means that DJF is actually February, MAM is actually May, JJA is actually August, and SON is actually October. The temporal coverage shifts for other latitude bands. Seasonal averaging masks this effect. How far is this accounted for in the global maps and zonal mean distributions?
line 91 to 95: How do you explain the enhancements in the tropopause region, but not below, in the troposphere? Does This mean that the air was not uplifted locally (and vertically), but tropopause air was transported horizontally before the observation, from areas with enhanced Co and CH4 amounts?
line 120: The selected profiles should not be called “collocated”, since they are within a wide spatial span, and they have a vague temporal coincidence only. The term “collocated” is mostly used within profile-to-profile validation, while your comparisons have the character of climatological comparisons, looking for typical profile shapes.
Fig. 2: It is interesting to see that the bulge in the ACCLIP mean CO profile (red line, top right panel) comes from a bimodal distribution of CO, hinting towards some inter-annual variability. Have you looked into this fact in more detail?
Fig. 2, figure caption: what does “near the location of AirCore campaign” exactly mean? As described in the text lines 117-121? Please specify.
Line 148 and Fig.3: The description “near global climatological distributions of CH4 and CO in JJA” is not wrong, but not fully correct either. ACE-FTS samples different latitudes at different times, which means that for JJA e.g. tropics are observed in August, while Northern midlatitudes are observed in June, and Northern (sub-)polar regions in July. The averaging over a season provides the wrong impression that the distributions in Northern high latitudes are observed at the same time as those in the tropics. I am fully aware that this is a property of ACE-FTS observations that cannot be avoided. Nevertheless, at least a sentence or two should explaining these specific observational details should be provided.
Fig. 4: Same comment as for Fig. 3
Line 153: What do you refer to with “more pronounced”? Relatively? In percent per latitude? Or do you mean that CO spreads farther longitudinally outside the AMA? Please clarify!
Line 174-175 and Fig.5: I assume these enhancements refer also to JJA? This should be mentioned in the text and the figure caption.
Line 180: Eddy-shedding transport towards the North-East is discussed in detail by Vogel et al., 2014 (ACP, 14, 12745-12762, 2014; doi: 10.5194/acp-14-12745-2014) and other colleagues of this team more recently (e.g. Clemens et al., 2022, doi:10.5194/acp-22-3841-2022). Respective references should be added.
Fig.6: It should be emphasised more strongly that here, we look at a zonal mean distribution covering all longitudes, while before, e.g. in Fig. 4, the data were from the region of the AMA only. Further, the same comment regarding seasonal coverage applies here as for Figs.3 and 4.
Lines 201-203: The CH4 maximum near the tropical tropopause is interesting, as it cannot come from strictly vertical transport of air masses at the same latitudes, but must involve meridional transport. Vogel et al., 2019 (ACP, 19, 6007–6034, 2019, https://doi.org/10.5194/acp-19-6007-2019) have looked into this in great detail (compare their Fig. 14). Reference to this publication should be made and your findings should be discussed in the light of this publication.
Line 209/210: Reference to Vogel et al., 2019 should be made.
Line 216 - 218: It should be made clear at this point that the CH4 surface fluxes are not derived form ACE-FTS data in this work, but stem from another publication and are based on other data.
Line 221-223: As demonstrated by Vogel et al., 2019, the transport from the Asian monsoon anticyclone (AMA) does already happen when the AMA is still intact, and not only after its break-up. Please discuss your findings versus this publication.
Fig. 9 and related text (lines 231-239): This figure and the related discussion is a bit tricky due to the limited sampling of ACE-FTS at low latitudes (as the authors indicate in line 234). Within the latitude band 20S-20N, there are indeed only ACE-FTS data available for the month February, April, August, and October). Indeed we find the positive and negative maxima of the anomalies for these months. How have the data for the other months been generated? By interpolation? If so, it is very bold to derive a tape recorder and assess its uplift velocity from interpolated data. I recommend strongly to reconsider inclusion of this figure. At least, these statements need to be checked with CH4 data that are available from satellite observation all year round (e.g. HALOE, MIPAS). Or, as an alternative approach, apply a regression fit to time series (as shown in Fig. 10, left panel) at various stratospheric altitudes and derive the tape recorder and its uplift velocity from the phase shift between the altitude levels.
Line 252: How do these trends fit to tropopheric methane trends from other studies?
Lines 255 -260: The hemispheric asymmetry of trends of the Brewer-Dobson circulation has been analysed in a number of papers. It was also observed by e.g. Mathieu et al., 2014 (doi:10.1038/nature13857) and Haenel et al., 2015 (doi:10.5194/acp-15-13161-2015), analysed in detail by Stiller et al., 2017 (https://doi.org/10.5194/acp-17-11177-2017), and finally confirmed to appear in CCMI model simulations as well by Ivaniha et al., 2026 (10.1029/2025JD045069), tracing it back to QBO-BDC interactions. A more thorough referencing of previous work is desirable here.
Summary and discussion: I recommend to put the findings of this work in closer context to findings of previous publications, some of which I have listed above. In particular, there is rich literature on transport out of the anticyclone to the North-East (lines 275-280) and into the tropics (lines 281-286) and on the hemispheric asymmetry of BDC trends (lines 290-294) that should be considered in the discussion. By doing so, the authors should sharpen their statements and demonstrate more clearly what their specific new findings are.
Citation: https://doi.org/10.5194/egusphere-2026-2644-RC2
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