the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Feb 2025
Significant Response of Methane in the Upper Troposphere to Subseasonal Variability of the Asian Monsoon Anticyclone
Abstract. Substantial methane (CH4) emissions in Asia are efficiently transported to the upper troposphere through the monsoon dynamical system, which forms a remarkable seasonal CH4 enhancement in the upper troposphere. Using a chemical transport model GEOS-Chem driven by surface optimized CH4 flux, the CH4 enhancement over the Asian monsoon region is explored as a combined effect of the monsoon dynamical system and regionally increased emissions during late monsoon season. The spatial distributions of CH4 at the upper troposphere show strong subseasonal variability, which is closely tied to the east-west oscillation of Asian monsoon anticyclone (AMA). Besides, the AMA patterns influence the vertical structure of methane. The AMA center around 80° E favors the upward transport from north India and Bangladesh while the AMA center around 105° E favors the source from southwest China transported to the upper troposphere. The AMA center over the Iranian Plateau suppresses the vertical transport and favors the horizontal redistribution. According to our model sensitivity study, the differences in the upper tropospheric CH4 anomalies caused by large-scale circulation is 1–2 times of that caused by regional surface emissions. Our research highlights the complex interaction between monsoon dynamics and surface emissions to determine the upper tropospheric methane.
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Status: open (until 31 Mar 2025)
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RC1: 'Comment on egusphere-2024-4188', Anonymous Referee #2, 07 Mar 2025
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The authors simulated upper tropospheric methane distributions using GEOS-Chem driven with “optimized” surface methane flux data and showed strong subseasonal variation linked to the east-west oscillation of the Asian monsoon anticyclone (AMA). What is new about this study is twofold, the use of a surface methane flux dataset optimized with observational data and the finding that upper tropospheric methane concentrations peaked in September due to rice paddy emissions from Southeast Asia. Extensive work has been conducted to examine the AMA’s role in redistributing CO, water vapor, HCN, hydrocarbons and aerosols in the upper troposphere, as summarized in the Introduction, and this paper added CH4 into that list. The study clearly shows how subseasonal variations in upper tropospheric CH₄ over the Iranian Plateau to the Tibetan Plateau align with AMA variability and surface emissions. Below are my comments.
First, my technical review suggested that the authors provide a clear definition of “anomaly” at the outset, as the term requires a reference point, particularly in climatology. While this may be self-evident to the authors, an explicit definition would benefit ACP’s broader readership. However, such a definition is still missing. Moreover, their “anomaly” appears synonymous with “enhancement.” For consistency, the authors should consider using one term throughout. In cases where they quantify upper tropospheric “enhancement” over the Asian monsoon region (lines 64–65), they must clarify the reference used to determine these values, preventing ambiguity for readers.
Second, the case study seems unnecessary. The authors should move directly to the composites of the AMA modes and their corresponding methane distributions. It is also unclear whether they identified AMA modes independently and what methodology they used.
Specific comments:
Lines 116–127: When describing the probability distribution of AMA center positions and classifying AMA modes, including one or two figures would improve clarity. Additionally, the statement in lines 121–122 requires a concise summary of the different effects of AMA modes. This would not only make the paper more self-contained but also help explain the differing distributions of upper tropospheric methane concentrations associated with each AMA mode.
Study period: The study period should be stated upfront rather than buried near the end of Section 2.
Lines 132: Figure 1 – Explain what ΔCH4 in Figure 1b represents.
Lines 133-134: The statement about thermal heating in the Tibetan Plateau (TP) and subsequent westward migration due to instability needs supporting references or evidence.
Line 134: “the AMA center over predominantly hovered east of 75°E” – It doesn’t read right.
Line 135: Provide a brief explanation of how the AMA position influences the ΔCH₄ distribution.
Lines 146-148: Clarify what is meant by “stirring interaction.” Additionally, explain the mechanism by which boundary layer air is lifted, as this is key to transporting surface methane emissions into the upper troposphere. The authors should support these statements with references or their own evidence.
Line 176: The term “pathway” should be reconsidered unless streamlines or variables indicating a dynamic process are presented.
Line 198: The phrase “(unit: %, referring to zonal mean)” is unclear. The authors should explicitly define how anomalies were calculated (per the first comment) and clarify what is being presented. It appears they are referring to zonal means of anomalies within their domain.
Citation: https://doi.org/10.5194/egusphere-2024-4188-RC1 -
RC2: 'Comment on egusphere-2024-4188', Anonymous Referee #1, 13 Mar 2025
reply
Sihong Zhu et al.,
https://doi.org/10.5194/egusphere-2024-4188
This is a interesting and important study, which merits its publication in ACP. The scientific content, the quality of the study and its presentation is good, however I suggest some revisions before publication by ACP.
General comment:My principle concern is that the role of convection within the study by Zhu et al. (2025) is not discussed sufficiently. 'The complex interaction between monsoon dynamics and surface emissions to determine the upper tropospheric methane' is highlighted as main result of the paper. However, the definition of 'monsoon dynamics' remains unclear. My impression is that 'monsoon dynamics' stands here for the spacial-temporal variability of the Asian summer monsoon anticyclone in the upper troposphere and lower stratosphere (east-west and south-north shift, Iranian and Tibetan mode). However, convection that uplift methane to altitudes of the upper troposphere plays a major role within the monsoon dynamics. Maybe there is a misunderstanding, therefore, I recommend to improve the study by clarify the role of convection. More specific comments to this issue will follow below.
Major comments:p1 L26: 'The AMA center over the Iranian Plateau suppresses the vertical transport'.
This formulation is confusing. Over the Iranian Plateau shallower and less intense convection occurs during summer compared to regions further east (e.g Indian subcontinent, Bay of Bengal, China). Therefore a lower amount (or rather no) methane can be transported from surface levels to the upper troposphere over the Iranian Plateau (see Fig. 3 in Zhu et al., 2025). Thus if the anticyclone is over IP far off the strong convective sources (e.g Indian subcontinent, Bay of Bengal, China), methane can not be uplifted locally over the Iranian Plateau into altitudes of the anticyclone in contrast to the TP mode.
What is meant with 'suppresses the vertical transport'? That the location of the AMA over the Iranian Plateau suppress convection over the Iranian Plateau? Please clarify.
p2 L40: '(Park et al., 2009; Pan et al., 2016; Randel et al., 2010; Rosenlof et al., 1997; Yu et al., 2017)'Please add here some more recent publications e.g. from aircraft campaigns that measured air within the AMA during StratoClim 2017 and ACCLIP 2022 showing enhanced tropospheric tracers within the anticyclone.
p2 L2: 'with a vertical velocity of about 1–1.5 K/day'
This value depends on the used reanalysis. 1–1.5 K/day is related to ERA-Interim. Please clarify.
p3 L3: 'The debate persists over whether the seasonal increase of UT methane in the Asian monsoon region is due to enhanced summer emissions from regional rice paddies (Zhang et al., 2020) or the upward transport by the monsoonal circulation'.
Could it also be the combination of both?
p3 Sect. 2.1:
Please provide some information about the treatment of convection in the GEOS-Chem model. Is there an additional convection scheme included or is just used the convection included in MERRA-2?
p4 Sect. 2.2:
A map showing the regions of the different modes (IP, WTP, ETP) would be very helpful.
p6 Sect. 3.1:Please provide a map showing the regional/spacial distribution (fluxes) of CH4 emission in Asia in the model at surface.
p5 L131: 'shows the anomalies of GPH as well as methane on 150 hPa'
Please explain how 'anomalies' are defined / calculated here.
p6 L157: 'The timing of this CH4 surge aligns closely with the seasonal emissions peak from rice paddy
cultivation.'Please provide more information about subseasonal variability of CH4 at model boundary layer
p7 L176: 'Under IP mode, the horizontal redistribution is remarkable with a weak vertical pathway.'I think this statement should be made a bit more clear such as:
'Over the Iranian Plateau shallower and less intense convection occurs during summer compared to regions in WTP and ETP, therefore CH4 enhancements over the IP are caused by horizontal westward transport in the UT caused by the AMA and not by local vertical transport.'
p9 L207: 'we demonstrate that variability in UTLS methane over the Asian region is influenced by two main factors: firstly, the dynamical east-west oscillation of the ASM, which substantially modulates methane distribution and
influence the upward transport pathways; secondly, the increase in methane emissions from rice paddy cultivation in late August and early September'I am missing in this discussion the role of convection. The upward transport depends on the location of strong convection. If the AMA anticyclone is over convective regions enhanced CH4 can be injected into the AMA (WTP, ETP), subsequently by horizontal displacement enhanced CH4 can be transported horizontally westward (IP) here no local injection of CH4 into the AMA by convection occurs.
p10 L211: 'ASM dynamics' = convection + horizontal shift of AMA ?
p10 L220-225: 'What about differences in strength and location of convection in your model between the different years from 2015 to 2020 such as modulations by ENSO?Alladi et al (2024) shows, that during Asian monsoon 2015, a substantial reduction of the tropospheric species at 100 hPa and 146 hPa in contrast to 2022 where observed mainly attributed to the weaker updrafts in 2015 owing to strong El Nino conditions.
Hemanth Kumar Alladi, P.R. Satheesh Chandran, Venkat Ratnam M,
Impact of ENSO on the UTLS chemical composition in the Asian Summer Monsoon Anticyclone,
Atmospheric Research, Volume 309, 2024, 107551, ISSN 0169-8095,
https://doi.org/10.1016/j.atmosres.2024.107551.
(https://www.sciencedirect.com/science/article/pii/S0169809524003338)The lower CH4 mixing rations in 2015 shown in Fig. 5 (Zhou et al., 2025) could be caused by weaker
updrafts in 2015.p13 L284-L287: 'the subseasonal oscillations of the Asian Monsoon Anticyclone (AMA)
significantly influence the methane transport pathway and its efficiency from the lower boundary to the UTLS (upper troposphere and lower stratosphere'This sentence is somewhat misleading. Please clarify 'transport pathways'.
p14 L295: 'Based on AMA mode composites, we confirm that the dynamic nature of the AMA, in terms of its subseasonal modes, modulates the vertical structure of CH4 over the monsoon region.'--> modulates the horizontal structure. Please clarify.
p14 L296: 'In particular, AMA centering around 80°E (WTP mode) favors the vertical
transport of air from north India and Bangladesh to the upper troposphere, which contributes most significantly to the total CH4 monsoon plume at the UT.'-->
'The local coincidence of CH4 emissions, strong convection and the location of the anticyclone around 80°E (WTP mode) favors the vertical transport of air from north India and Bangladesh to the upper troposphere, which contributes most significantly to the total CH4 monsoon plume at the UT.'
Minor/technical comments:p2 L34: 'upper atmosphere' -> 'upper troposphere' or 'UTLS'
p4 L114: 'tracer transport' -> 'transport of trace gases'
p4 L117: 'Our statistics show that the frequency of the Tibetan mode (the eastern
phase of the distribution) is nearly twice that of the Iranian mode.'
Please specify the time period for the statistics. 2015-2020?p5 L130: 'on the vertical distribution of CH4 in 2020 summer' ->
'on the horizontal distribution of ... at 150hPa'p6 L157: 'in Figure.1' -> 'in Figure 1'
p7 L176: 'horizonal' -> 'horizontal'
p7 L179: 'in Figure.S1' -> 'in Figure S1'
p10 L216: 'late-summer emissions' ->
'higher emission during the JAS than in other years tend to ....'
Fig. 5: The red and blue lines are very thin. Please make them a bit thicker for better visibility.
Add in the figure caption the meaning of the dashed red and black boxes that are related to table 1.Tab. 1: Please add CH4 [ppbv] as subtitle in the table.
Citation: https://doi.org/10.5194/egusphere-2024-4188-RC2
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