Review report on “QBO modulation of the Asian Monsoon water vapour” by Cristina Peña-Ortiz et al. 

This paper analyzes satellite water vapour and temperature data, radiosonde wind data taken at Singapore, and reanalysis ERA5 “fraction of cloud cover” and other data to investigate the possible response of water vapour in the Asian summer monsoon anticyclone to the quasi-biennial oscillation (QBO) in the tropical zonal winds. 

While the topic chosen is very interesting, I am afraid that I do not fully agree with authors’ discussion. In the following, I make several major questions. 

(1)    The use of ERA5 “fraction of cloud cover” data 

The authors use vertically resolved “fraction of cloud cover” data from ERA5 as a proxy of, in the end, tropical deep convection that provides source of large-scale diabatic heating to produce equatorial Kelvin and Rossby wave response (or the Matsuno-Gill response). First, reanalysis cloud data are product of forecast model, without observations assimilated, and thus in general less reliable compared to e.g. NOAA OLR data. Second, the authors focus on the region 150-100 hPa for these cloud data and discuss deep convection, but the clouds in this altitude region are primarily cirrus and anvil clouds, and may not be directly related to the core of deep convection that results in large-scale diabatic heating to produce the Matsuno-Gill type response. Because authors’ discussion very heavily relies on detailed structure/distribution in ERA5 fraction of cloud cover data at 150-100 hPa, and because (as discussed below) the features that the authors point out and emphasize are not very clear to me, I started to wonder whether the chosen cloud data set is appropriate or not. 

(2)    The Matsuno-Gill response 

In Section 4, in Figures 6-8, the authors show the ERA5 cloud data at 125-150 hPa and ERA5 temperature and wind anomalies, and mention that these are the Matsuno-Gill response. While it is well known that if we give diabatic heating at the equator or slightly off-equatorial region, we see the so-called Matsuno-Gill response in the wider regions of the tropical-to-subtropical atmosphere including the tropopause region, it is not clear to me in the current specific cases which group of deep convection, shown on the figures, is actually responsible for the specific 100 hPa temperature and wind anomalies over the Asian monsoon region. The authors need to clearly show the heating-response pair for each set of figures, and to show somehow (using e.g. a very simple model) the justification that they are actually the pairs. 

(3)    The choice of QBO indices 

The authors used monthly mean Singapore zonal wind data at 10 hPa, 20 hPa, 30 hPa, 50 hPa, and 70 hPa for (potential) QBO indices. While the authors’ approach is understandable as first trials, I think that the final choice should be made in terms of direct relevance to the current problem. What we need here is e.g. vertical displacement, or temperature anomaly, or temperature gradient (static stability) anomaly around e.g. the tropopause over the tropics and over the Asian monsoon region due to the QBO and/or its secondary circulation. In other words, please explain the relevance (or the phase relationship) to the tropopause-level variables of the 10 hPa Singapore winds for July and the 20 hPa Singapore winds for August. 

(4)    The QBO secondary circulation, and then the potential Matsuno-Gill response 

(This may be more like a comment, not a strong suggestion.) To me, it is more logical that the (zonal mean) QBO secondary circulation is first explained and analyzed, and then the anomalous Asian monsoon region is pointed out. Then, the tropical convection anomalies are analyzed in the Indian-Ocean and Indonesian sector. Then, the potential link of those convection anomalies to the Asian monsoon region through the Matsuno-Gill response is proposed. 

The exact latitude where the subtropical part of secondary circulation maximizes might not be very clear in the past works, but the following paper may be a good starting point: 

Hitchman et al., 2021, https://doi.org/10.2151/jmsj.2021-012

The one for specific months needs to be analyzed by the authors (and actually shown in the manuscript). 

Furthermore, it is not very clear to me what is the final process that mainly controls the water vapor in the Asian monsoon anticyclone. Is it local dehydration in association with the temperature anomalies or the wet/dry air transport changes in association with the wind anomalies, when the authors discuss the Rossby-wave part of the Matsuno-Gill response? 

(5)    Different processes are proposed for July and August 

Based on the analysis results, the authors suggest that different processes are operating in the month of July and August at the QBO time scales. I am not fully convinced whether this is possible/reasonable. If this were true, the seasonal progress of Asian summer monsoon should be quite robust in each year, and the seasonal features are clearly different between July and August. Or, do I misunderstand something? 

This leads me to the question (1). The features shown in the manuscript might be heavily dependent on the data set used, and ERA5 100-150 hPa fraction of cloud cover data might not be appropriate to be used as the observation of convective heating. 

I cannot make (minor) comments at this time. (I did not find any typos.) This is because my issue was to understand the logic by reading the manuscript. After reading through it, I tried to re-construct the logic by myself (e.g. above (4)) but I still do not find the solution myself for the issue (5). I hope that the authors provide sound explanation, which will also be reflected to the revised manuscript finally. 

Review for ‘QBO modulation of the Asian Monsoon water vapour’ by Peña-Ortiz et al.

 

This work studies the mechanisms of how Quasi-Biennial Oscillation (QBO) will influence the tropical tropopause layer water vapor by regulating temperature and convection. This work focuses on the Asian Monsoon (AM) region, a region where it is important while there remains a lot of uncertainty in the water vapor budget. By analyzing observational data, the authors explain how QBO will influence the water vapor from a dynamics perspective, filling current knowledge gaps. Most of the analytical work in this paper is clear and logical, and the paper is well-written. I recommend the paper be accepted after considering the following suggestions.

 

Major comments:

 

1.    It would be helpful for readers to better understand the processes involved if the authors include figures showing the climatology, QBO-w mean, and QBO-e means of water vapor, temperature, outgoing longwave radiation (OLR), wind fields. This additional visual representation could enhance comprehension.

 

2.    Table 1 indicates the number of QBOe and QBOw cases but doesn't provide information on their strength. With a sample size of 5-9 cases, there remains substantial uncertainty in the results. For instance, the differences observed between July and August may be influenced by variations in QBO strength due to the limited record. To address this, consider:

a.    When discussing the relationship between water vapor and temperature, use MLS water vapor data, and temperature over 2005-2020.

b.    When discussing how QBO modulates the deep convection and thus temperature, use a longer record (ERAi, NOAA OLR, and Singapore QBO data all have a longer record).

c.      Instead of counting the number of QBO events, consider presenting variables like wind (U) or the correlation between U and H2O to strengthen your analysis.

 

3.    The paper currently discusses the choice of different QBO levels extensively, which may distract from the main topic. The conclusion that 10 hPa/20 hPa is the best proxy may be sample-size dependent and not universally applicable so can be removed from the result section. Consider consolidating this discussion with Table 1 and relocating it to Section 2, allowing Sections 3 and 4 to focus on scientific results.

 

4.    Need more discussion on using OLR as a proxy of the deep convection. Although Randel et al. (2015) conclude that deep convective cooling plays a main role in the water vapor budget over AM and North American regions, there are also many recent studies using radar with a much finer resolution to observe deep convection and overshooting over the North American regions, and conclude that the deep convection impact is moistening (Chang et al., 2023; Smith et al., 2017; Tinney & Homeyer, 2021; Yu et al., 2020). Asian monsoon region is different from the NA region and there is no very good coverage radar product over the AM region to test convective moistening/drying, so convective impact still could be cooling over this region. However, a discussion on the shortcoming of using OLR as a proxy is necessary.

 

Specific comment:

1.     After first introducing the region, consider adding latitude information (e.g., Indian, tropical Indian Ocean).

2.     Line 16-18: The sentence is long and slightly challenging to comprehend. Consider breaking it into two or more sentences for clarity.

3.     Figure 3: This figure could benefit from subtitles and adding a significant level to figures b and d. Additionally, consider adding a second x-axis with lag dates to improve readability.

4.     Figure 5: The dots in this figure are unclear when overlapping with the wind field.

5.     Line 205: It's unclear how this paragraph differs from the one starting at line 140, as they both discuss Figure 3. Please clarify the distinction.

6.     Line 289: The sentence is unclear. Isn’t the secondary circulation over the mid-latitudes, and this paragraph is discussing the equatorial region? Could you please explain more?

7.     Line 359: In the sentence, "and a secondary meridional circulation of the QBO...," Is the secondary circulation transport the equatorial temperature anomaly?

 

Reference

 

Chang, K.-W., Bowman, K. P., & Rapp, A. D. (2023). Transport and Confinement of Plumes From Tropopause-Overshooting Convection Over the Contiguous United States During the Warm Season. Journal of Geophysical Research: Atmospheres, 128(2), e2022JD037020. https://doi.org/10.1029/2022JD037020

Smith, J. B., Wilmouth, D. M., Bedka, K. M., Bowman, K. P., Homeyer, C. R., Dykema, J. A., Sargent, M. R., Clapp, C. E., Leroy, S. S., Sayres, D. S., Dean-Day, J. M., Paul Bui, T., & Anderson, J. G. (2017). A case study of convectively sourced water vapor observed in the overworld stratosphere over the United States. Journal of Geophysical Research: Atmospheres, 122(17), 9529–9554. https://doi.org/10.1002/2017JD026831

Tinney, E. N., & Homeyer, C. R. (2021). A 13‐year Trajectory‐Based Analysis of Convection‐Driven Changes in Upper Troposphere Lower Stratosphere Composition Over the United States. Journal of Geophysical Research: Atmospheres, 126(3), e2020JD033657. https://doi.org/10.1029/2020jd033657

Yu, W., Dessler, A. E., Park, M., & Jensen, E. J. (2020). Influence of convection on stratospheric water vapor in the North American monsoon region. Atmospheric Chemistry and Physics, 20(20), 12153–12161. https://doi.org/10.5194/acp-20-12153-2020