Review’s comments for the manuscript egusphere-2025-3389, entitled "European sulphate aerosols were a key driver of the early twentieth-century intensification of the Asian summer monsoon"

General comments

By using ensemble experiments based on CESM, this study investigates impacts of increased European sulphate aerosol emissions in shaping Asian summer monsoon multidecadal changes during the early 20th century. Results suggest that that European aerosols played an important role in modulating the Asian summer monsoon through aerosol induced changes in large scale atmospheric circulation and surface climate. Results are interesting and they are well presented in the study. However, the paper needs some clarifications by addressing the following specific comments. Therefore, the paper is acceptable for publication after minor revisions.

Specific comments

1. Line 60. “NH”, please spell it out when it appears first time in the paper.
2. Line 77. “driving monsoon” to “driving Asian summer monsoon”.
3. Lines 162-170. It is better to add a comment to compare trends between observations and model simulations.
4. Line 163. “BoB” has not defined yet.
5. Lines 172-179. What are grey bars in two observations in Supplementary Fig.S4? From text, it seems that they are trends based on PI control. If they are, why are the two grey bars in GPCC and CRU are not the same as those shown in ALL and fixEU?
6. Lines 172-179. There is no description about Supplementary Fig. S5.
7. Figure 1 caption. It is better to emphasize that the color scale of precipitation trends in GPCC and model simulations are different.
8. Figure 2b. Please use same color scale for vertically integrated moisture transport divergence trend and precipitation trend shown in Figure 1d for easy comparison.
9. Line 214. “AOD”. Please spell it out when it appears first time.
10. Lines 216-218. Please use the same color scale for Fig. 3b and Supplementary Fig. S6e to these descriptions.
11. Lines 237-238. The magnitude of SLP trend is only about 10% of those based on observations or CESM ALL simulation. This implies that SLP trends over Europe in CESM ALL might not be resulted from European sulphate aerosol emissions. Please comment.
12. Lines 256-262. The wave train shown in Fig.4 is in mid-high latitude while anomalous ascents and precipitation are over 10-20N across India into southern China and northeastern Indochina. It is therefore not clear how does the wave train trigger these anomalous ascents and precipitation. Fig.2 shows circulation changes over the western tropical Pacific induced by European sulphate aerosol. The reviewer wonder whether there is any local feedback between ocean and atmosphere.

Review of European sulphate aerosols were a key driver of the early twentieth-century intensification of the Asian summer monsoon:

This paper presents a very interesting modeling study on the remote effects of European aerosol emission changes on the Asian summer monsoon. The authors show evidence that there is a connection between European sulphate aerosol emissions and the Asian monsoon. This paper presents very interesting findings that will be a meaningful addition to the field. Overall, this paper is well written and presents the findings in a logical manner, with only a few places that need clarifying. Thus, I recommend this paper be published following minor revisions.

Comments:

1. The authors state in lines 152-153 that “the ALL ensemble mean captures the main spatio-temporal characteristics of the observed precipitation trends”. I interpret this to mean that Figure 1a and 1b share some similarity. Visually I am not convinced that I see this. Firstly, the two figures are not plotted on the same color bar, so all of the changes in the ALL ensemble are much weaker than those from GPCC. Additionally, it does not appear to me that the locations with the strongest positive and negative anomalies are collocated. Is there some spatial correlation between the maps in Figures a and b? That would help to solidify this argument.

1. There are numerous times throughout this manuscript that references “not shown” results (for example, line 214, 221, 325). Any results that are important enough to mention in the manuscript should be at least shown in a supplement.

1. There are also a couple times where later figures are referenced by the statement “see below” (for example line 219, 228). It would benefit the reader to instead reference a specific figure or section.

1. The figure referencing is at times inconsistent. Sometimes it is Figure X, and others Fig. X. One should be used consistently throughout the manuscript.