the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Apr 2025
Interannual variability of the Asian summer monsoon anticyclone
Abstract. The definition of the boundary of the Asian summer monsoon anticyclone (ASMA) in the upper troposphere and lower stratosphere (UTLS) (350–410 K) is a known challenge that highly impacts the information about the anticyclone's behavior and affects the results when studying of its interannual variability. We present a novel method based on the absolute vortex moments that defines the ASMA boundaries by solving an optimization problem. A 44-year ASMA climatology (1980–2023) will be shown using the ERA5 reanalysis provided by ECMWF. Here, we address the ASMA's climatology (1980–2023), interannual variability, the variability of the start and end dates and the duration of the anticyclone peak phase calculated with help of the defined novel method. In addition, three individual years – 2017, 2022 and 2023 are highlighted during which aircraft campaigns took place to measure air inside the ASMA or its outflow (StratoClim, ACCLIP, PHILEAS). The interannual analysis is based on the anticyclone's centroid latitude and longitude, excess kurtosis, angle, aspect ratio using 4 isentropic surfaces: 350, 370, 390 and 410 K. Our findings show that the ASMA area decreases over the period 1980–2023 in contrast to previous studies. Further, we provide evidence of possible bimodality of the ASMA by showing clustering of values (the Montgomery streamfunction values minus an optimized background value) around two centers on climatological data over 44 years as well as counting the number of days when two anticyclones (or two modes) where found simultaneously.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1670', Anonymous Referee #1, 20 May 2025
General comments
The authors present the ASMA interannual variability and properties based on an ERA5 climatology of the time period 1980-2023. They introduce a novel technique to derive the edge of the ASMA region, which is applicable on all time scale and was used in the past to described the polar vortex. The results agree well with former studies but further provide more detailed information describing the ASMA region such as bimodality events and shifts of the centroid during the season. Overall, the manuscript is well written and provides a rigorous analysis to describe AMSA region properties and adds information to the open question of the community on how to describe the ASMA region best.
In most figures, Fig.4-10, B1, C1, C2, E1 and E2, a “rainbow” colourmap or a derivative is used. Following the Copernicus guidelines for authors during the manuscript preparation (see https://publications.copernicus.org/for_authors/manuscript_preparation.html), it is important that the colour scheme used allows readers with colour vision deficiencies to correctly interpret the findings. Therefore, please refrain from using the “rainbow”, “hsv” or similar colourmaps in the figures of the manuscript.
Currently, I am wondering why this method should give a threshold that serves the purposes described in L108, i.e. outlining the strongest confinement of the anti-cyclone. How do we know, that this novel analysis is better than the analyses presented before? At various instance it is written that these previous methods produce “noise”. Connected to this how do we know, that the ASMA area actually decreases as stated in the abstract? Furthermore, does the optimization method really work to serve the purpose of finding a physically based boundary or does it simply (most of the time) lead to selecting the MSF threshold value that is the maximum value outside of the ASMA box. Then, to me it is not clear why this value should be connected to the strongest circulation.
I think it needs to be highlighted, what is new in this study. This is stated in the method section but should be made stated in a more prominent section of the text.
Is the detailed information on the campaigns needed? Currently, there is a lack of motivation for this and also for showing the 3
It is stressed, that the novel method works on any time scale. So, do the methods in Manney and Santee as well, correct?
As a personal preference, I would associate “peak phase“ with the strongest period of the ASMA; here it seems that you a referring to the period where the ASMA is fully developed.
This is a personal view: you write about background values (maybe was this the term used before?). I would suggest to call it a threshold value. But at least make clear why the term “background”
Specific comments
The title could be updated to directly mention that an new/updated method is introduced here. This seems to be one of the core information in this manuscript but this is not reflected in the title.
Next to the climatology over the time period 1980 to 2023, three specific years are picked for comparison. The three years are motivated with the aircraft measurement campaigns that took place in the years. However, no data and no conclusions from the campaigns are used for instance to validate the novel method. This raises the question, if the focus on these three years is necessary. It leads to unfulfilled expectations. If these years are given to be able to reference this paper in potential upcoming papers from these missions, then this should be stated.
In figure 10, the presence of bimodality of the ASMA is shown per season. The bimodality is something shown in observations and therefore should be found in the analysis of the ASMA? There is no statement about whether this is more reasonable than former methods.
Absolute Vortex momentum method by Matthewman et al. 2009: Is it applicable for the ASMA in the first place? What are the differences in the polar vortex and the ASMA?
Is it planned to provide the data used for this work along with the paper?
L5-6: Remove “A 44-year … by ECMWF.” And add info on ERA5 from ECMWF to the next sentence?
L14: In my opinion traditionally “two modes” are not the same as “two centres” or “two anticyclones”; the method by Zhang et al. does not allow for multiple centres;
Eq. 1: Could be introduced in the methodology instead of the introduction? This part feels a little jumpy
L20-21: Add references to this sentence.
L25-28: Please link to the previous sentence to make clear why it is “important”
L40: tautology: defining the edge influences the edge of the anticyclone…; I think the question is rather that you would like to find the boundary of maximum confinement and that there is a debate on how to do that in the best way
L50: I am not 100% sure but IMO Zhang et al do not mention splits of the ASMA; their bimodality is based on their graph 2; and the mode is Iranian if the center is over the Iranian Plateau and Tibetan if it is above the Tibetan Plateau.
L53: “background” value sounds strange “threshold value”
L56: do they use different values for different monsoon seasons, please clarify
L57: to not enclose the boundaries: do you mean that there is no closed contour?
L60: Turn around sentence for better bridging of paragraphs.
L104: what is daily data at noon? Could the results be influenced by this choice instead of taking full daily averages or performing the analysis on more timesteps per day?
L106: First two sentences sound very much like in the introduction. Can be removed?
Eq. 2: Is there any area weighting included? Following the description this does not seem to be since delta x is in degree instead of km. Not performing any area weighting while calculating half hemispheric or ¾ global means seems sounds wrong and potentially affects the results! There is some motivation necessary why the original method of Matthewman is applicable for the ASMA in the first place.
L117: integers k and l are not introduced appropriately. Integers “depending on what we want to calculate” – please elaborate further
L125 What is meant with “bipolarity”.
L126: What are eddy shedding events? They are not mentioned in the results, but in the section title
L132-137: I do not understand what these lines are supposed to tell the reader.
L142: Necessary to define the ASMA region over the quarter of the globe? 60/65° N would be enough? Following Fig. 6, the region should reach eastward of 0° E.
L145: Don’t understand what k and l mean. It is M_in and M_out
L146: So, the idea means, that the other regions of the globe with larger streamfunctions are weighted down by the large area (3/4 of the globe) with low Montgomery streamfunction?
Caption Fig.1: The statement “There MSF values inside of the box are larger than outside.” is wrong as this statement is too simplified.
L152: Introduce/cite Dual Annealing algorithm
L154: time dependence of Eq. 1 not mentioned before. Add it or mention it
L164f: I can hardly imagine, that (almost) all MSF outside of the ASMA box are zero, and none inside is. The sum is maxed to differ the most. Not necessary for such an absolute statement
L174: “small noise”: Is this really small noise? Isn’t this how the optimization is supposed to work. Otherwise simply the maximum value outside the ASMA box could be used as threshold value. (see general comment)
L180 onwards strange?
L196 (and following): “unwanted noise” in Santee et al. 2017 How can you be sure, that this is true? Why can you be sure that your method is really better.
Fig. 3: Why only Santee and not (also) Manney? This is the only time Santee is used as comparison. Why dashed line? To account for colour-blindness?
L203f: Show results of Plöger et al. 2015 in Fig. 3 as well?
L213: What does robust mean in this case?
L213: What index? The anticyclone area? Clarify. Can you explain why robust start end days could not be identified via the area? Also, the PCA is not described in detail and it would be good to know why 14 days are set.
L215: Is it reasonable to average pressure and PV over ¼ of the globe?
L217: larger than zero?
L217: Why 14 days? Did you test other time periods? How sensitive is your results on this threshold?
L238f: Shift is present in Northward es well as Eastward direction. Why only northward shift mentioned?
L258: “does not capture the western tail of the ASMA”. This is present in this work, but not certain, that is should be there. Rephrase it, so it is more objective. You find a difference.
Fig. 9: Caption. Reddish colour is probably not the centroid longitudinal position, but the cluster of the maximum residual MSF. Otherwise this does not make sense for bimodal case. Would agree to L285
Fig. 10: To optimized background values are shown, but not/ helpful to understand the left plot. Nothing necessary to see directly. Put into appendix?
L284: The threshold is set arbitrarily?
L327: “despite their higher variability”. Is this not always true?
L343ff: I do not understand the message of the sentence
L354-L359: This part can be shortened. Good agreement with Manney et al 2021.
L362ff: Generally true, but the focus on the years does not suit the overall results
L369: What is the index? “the/a” PCA technique. Not well enough introduced in Sec. 2
L379: Did you test for statistical significance?
Fig. 16: Neutral ENSO case is for exactly for value 0.5? So many cases are neutral?
L437: shedding events are not mentioned in this work other than an epithet
L451f: Influence of ENSO is not analysed! Compare to statement in L382f. This finding is not supported in that strength
Fig. E1: caption. Last sentence probably not correct
Technical corrections
L5: potentially boundaries -> boundary
L10: “, aspect ratio” -> “and aspect ratio”
L10: “4” -> “four”
L24: Check formatting “.” before references
L48: edges -> edge
L58: special cases”,” instead of “.”
L93: our instead of own
L93: comparison to Manney et al. 2021 instead of Santee et al. 2017?
L100: 1° x 1°
L100ff: 1° x 1° looks formatted. Adapt it to suit text font
L129: remove (e.g. ) and just write , myu_b
L130: change phrasing. “mean poleward 45° N value of PV” to “mean PV value poleward of 45° N”
L130: Space between ° and N
L134: “leads to changes” or “leads to a change”
L138f: Sentence is hard to follow. Suggestion: splitting the globe into the ASMA box and the rest, defining the ratio of the individual absolute vortex moments as objective function
L141: optimized background value. Write the tilde only over myu? More consistent with \hat{\myu}
Fig.1: Caption. Last sentence. Add comma after “There”. Not all MSF values are larger inside
Eq.4: Not sure about the syntax
L150: Following CF standard units and copernicus guidelines (https://publications.copernicus.org/for_authors/manuscript_preparation.html), the unit should be m2 s-2. For consistency, this would have to be changed in the axis labels of Fig. 1,2 and 3 as well.
L151: argmax has different font. Does not seem appropriate
L165: only non-zero values -> only in the box non-zero….
L181: Different method than Manney, so better other formulation. With our approach we capture more ASMA boundaries than Manney et al. 2021.
L182f: The range itself does not affect the two cases. Remove “During the chosen time range (1980-2023)”
L183: ⪅ is not smaller equal that is typically used
L189: Feels redundant
L216: by retrieving standard scores/ the standard score
Fig.4: Caption. 370 K and 390 K (both cases the unit, following the guidelines)
L230: Add unit K in both missing cases
L264: obsolete.. better outdated?
Fig. 5-8: The colorbar range is chosen to large. Only half the range is visible in the plots. Add space for isentrope surface level between value and unit
Fig. 6-8: Would it be better to expand/shift the ASMA region westward, to fully capture it.
Fig.8: For clarification, PV colormap is also based on data from Plöger et al. 2015?
L271: Add units K for clarity
L272: Missing space between ° and N
L273: right and left have to be swapped
Fig. 9, 10: Caption. Space between ° and N. No fraction in figure axis label for units. following guidelines by copernicus
L278, L285: right = east
L284: left = west
L284f: mean_longitude is the mean centroid? Use same font
Fig. 11: Space between value and unit in figure title
L298f: Sentence is redundant, has nothing to do with work
L303: Space between ° and N, missing unit K after 370
L305: Space between ° and N
L306: Is the interseasonal variability not also/already indicated by the standard deviation?
Fig. 12: Space between value and unit in figure title
L312: Space between ° and E
L313: Space between ° and E, missing unit K after 370
L314: Space between ° and E
L305f & L316: Copy paste of sentence The difference of/between the latitude/longitude centroid position for the years 2017, 2022 and 2023 to the climatological mean indicate the strong interseasonal variability of the ASMA.
Fig. 13: Space between value and unit in figure title. Missing unit K for 370. Maybe larger dots? %hem an unusual unit, maybe better “Area of hemisphere [%]”?
L323 % L325: missing space between value and unit
L325f, L333: Missing unit K for clarity (SI nomenclature)
L352: Add unit K for clarity
L357: Missing space between value and unit
L267: Missing space between value and unit
L372: “end” and “start” not recursively
L375f: Add units for clarity
Fig. 15 & 16: Space in isentropic level between value and unit, missing unit in caption. The longer figure explanation follows in Fig. 16. Move it to Fig. 15? Rename y axis name?
L382: Add units for clarity
L387: Add units for clarity
Fig. 17: Space in isentropic level between value and unit. Add also ENSO phase? Y axis name.
L402: Add units for clarity
L411: Add units for clarity
L416, L422, L423: same sentence structure “in contrast to previous/other studies/work”
L417, L419, L427: Add units for clarity
L432f: Add space between ° and E
L445: increased over the analysed time period. Recent year is too undefined
L454: what does “robust” mean?
Eq. A1: k and l are not understandable
E1. A5: what is this? Kappa_4 is not further mentioned or explained
L475f: Missing white spaces between ° and N/E
L478: “that area safe to add” something is missing here
Fig B1: unit connotation in colorbar label, unit missing for background value range in crossed region
L484: Add unit for clarity
Fig. C1: caption. Add unit for clarity
L485ff: Add unit for clarity
Fig. D1, D2, D3: caption. Add unit for clarity
Fig. E1: caption. Space missing ° and N
Citation: https://doi.org/10.5194/egusphere-2025-1670-RC1 - AC1: 'Reply on RC1', Oleh Kachula, 24 Jul 2025
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RC2: 'Comment on egusphere-2025-1670', Anonymous Referee #2, 21 May 2025
Please see the attached document for my comments.
- AC2: 'Reply on RC2', Oleh Kachula, 24 Jul 2025
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RC3: 'Comment on egusphere-2025-1670', Sanjay Kumar Mehta, 30 May 2025
Comment on “Interannual variability of the Asian summer monsoon anticyclone”. This paper presents a more robust definition for identifying the ASMA boundaries, shedding light on various important features of the ASMA, such as its temporal and spatial structures. It has provided a concise understanding of the ASMA variability and important further development in ASMA research. However, the Authors heavily compare their results with Ploeger et al. (2017) and Manney et al. (2021), which appears redundant at several places. Authors also completely ignored in some recent studies by Musaid et al. 2024a,b needs to be discussed it is very much aligned with the present work. The paper has been written well, but there are places which need improvement in English writing. Thus, I recommend major revision, and my detailed comments are listed below.
What is the purpose of defining the ASMA on a day-to-day basis? The main emphasis of the recent work is the interannual variability and long-term trend in the ASMA boundaries, which can be simply obtained by the monthly representation of the ASMA boundaries using the existing methods.
The title of the paper can be modified as “Temporal variability of the Asian summer monsoon anticyclone” as it covers variation from day to day to long-term trends.
The present study states that the importance of defining the ASMA boundaries; however, they did not discuss the recent work by Musaid et al. (2024 providing the definition for ASMA boundaries based on the Jet stream cores. Provide the citations sentence in the L42-43: “Different ways to define the boundary of the ASMA exist”
L 60-64: The Authors proposed a method to identify MSF background values for an individual point in time per isentropic surface, which is an optimised MSF value to describe the ASMA boundaries, which can capture its day-to-day variability. This is one important aspect of Musaid et al. (2024) to study to day variation of the ASMA to understand its variation during active and break phases of the monsoon. They also proposed a new GPH method to define the ASMA for the active and break phases of the monsoon based on the GPH values at the tropical easterly jet (TEJ) and subtropical westerly jet (STJ) locations. Their main conclusion is that other methods are not suitable to capture the ASMA boundaries on a shorter scale, especially on the day-to-day scale. This aspect needs to be discussed here.
Musaid P. P., Sanjay Kumar Mehta, Susann Tegtmeier, Masatomo Fujiwara, Siddarth Shankar Das, and Someshwar Das (2024), Characteristics of Convection and Advection associated with the Asian Summer Monsoon Anticyclone, Clim Dyn 62, 8929–8945. https://doi.org/10.1007/s00382-024-07371-3
Musaid, P. P., Sanjay Kumar Mehta, Susann Tegtmeier, Masatomo Fujiwara, Pooja Purushotham, Sachin Philip, Betsy, K. B., Seetha, C. J. (2024), Identification of the Asian summer monsoon anticyclone based on tropical easterly and subtropical westerly jets during active and break phases, International Journal of Climatology, 44 (12), 4260-4285, https://doi.org/10.1002/joc.8580
L83-90: The title of this paper emphasises on Interannual variability of the ASMA; however, the authors fail to provide novelty, rather they end up highlighting the meridional transport of the tracers and pollutants to the global stratosphere. Authors need to focus and provide the main objectives carried out in this paper.
L104: Why specific time 12:00 UTC, considered for the analysis instead of the daily average?
L230: Figure 4 describes climatological mean residual MSF; however, the description provided is very brief.
L245: Traditionally, researchers use the withdrawal phase instead of the break-up phase of the monsoon
So, the sentence can be revised as “During the withdrawal phase in September, the shape of the mean residual MSF is elongated and shrunk along the latitude axis,
indicating the break-up of the anticyclonic circulation. It is suggested to use the withdrawal phase instead of the breakup phase throughout the manuscript.
L248: The authors mentioned that residual MSF (L232) is not used to obtain the ASMA boundaries. How are ASMA boundaries obtained? Descriptions about Figures 5-7 are very short, they can be kept as subplots in one figure. Figure 8 can also be included. Why not comparisons with existing methods discussed here?
L250: change 15.07.2017 as 15 July 2017 and elsewhere in the manuscript.
L255: Replace “our boundary” with “ASMA boundary obtained using proposed method”
L261: Replace “our method” with “proposed method”
L255-262: Rewrite these sentences
L261: What do you mean by aligns with wind velocity?
L271 & Figures 9- 10: The Authors show the seasonal mean representations of the residual MSF based on the proposed method. How are these representations different from the one obtained from the existing method? It provides a concise way to interannual variations in the ASMA boundaries as well as its horizontal structure, demonstrating the bimodality during different years.
Figure 11: “Number of days per year during monsoon season (JJA)” would be more meaningful. Do you observe bimodality during May and September when ASMA is very weak?
L319-321: Rewrite the sentences
L354: Overall, a good agreement. Delete study.
L354-355: “Our method…variability” is redundant
L355-60: The Authors claim the comparison with Manney et al (2021) needs to be rewritten.
Subsection 3.4: (at 370 K; Fig. 15) What is the significance of the trends in the start date and end dates? Why are start dates becoming early (In May) and end dates late (in September -October) over the year?
Subsection: 3.5 Interannual variability and trends of the ASMA area and location
Section 4: Discussion and Conclusions
Provide implications towards the conclusions obtained in the present study
Citation: https://doi.org/10.5194/egusphere-2025-1670-RC3 - AC3: 'Reply on RC3', Oleh Kachula, 24 Jul 2025
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