The revised manuscript presented by Visioni et al provides an interesting new perspective on sulfate geoengineering which was not previously available from the literature. Lack of novelty in the manuscript was a concern of both myself and reviewer #1, but for my part I believe this concern to now have been addressed. The authors now confront directly the question of interactions between the QBO, sulfate geoengineering, and impacts (using the proxy of sulfate deposition), which adds a valuable contribution relative to existing work by Kravitz et al. and highlights the novelty of these findings. The authors have also made an effort to further caveat their findings and to highlight differences between modeled geoengineering, observed volcanic impacts, and the true effects of any future geoengineering, which was my second major concern and one highlighted by another commenter. As such I believe that the manuscript presents results which are both novel and interesting enough to warrant publication in ACP.
The authors have addressed all of my comments from the first review directly and to my satisfaction, in each case either instituting significant rewrites or providing compelling explanations for their decision not to do so. However, I believe that there are still some areas where the manuscript would benefit from changes. As such, I recommend that the manuscript undergo some minor technical corrections. I have one concern regarding scope which is presented as a major comment, followed by a list of purely typographical or stylistic corrections and suggestions presented as minor comments.
The study addresses the question of the effects that injection of sulfur dioxide might have on sulfur deposition, but this is not the only way in which sulfate geoengineering has been proposed. Alternative methods such as direct emissions of sulfur acid have been discussed (e.g. Pierce et al 2010, Keith and Macmartin 2015) and may produce different outcomes with respect to aerosol size distribution. For moderate-to-large injection rate scenarios such as that presented in this study, the rate of cross-tropopause flux as a function of latitude may be affected by this assumption. The authors should caveat that some (although certainly not all) of the results may not hold or may change if direct injection of sulfur trioxide or sulfuric acid is used, due to the different microphysical circumstances.
The new section dedicated to QBO impacts on sulfate deposition (section 4.2) is a welcome addition, in particular the new Figure 14. However, it is clear that the opening text of the section was written separately from the rest of the section. An opening sentence for section 4.2 to introduce the purpose of the section would help to make the manuscript flow better. It would also help the reader to have this section referenced more explicitly throughout the text.
Throughout the introduction (e.g. page 2, line 15) the authors discuss prior investigations of sulfate geoengineering but do not state the size of the injections in each case. I recommend that, where possible, the injection sizes are quantified to provide context to the reader.
There are a number of typographical and grammatical errors in the manuscript. I have done my best to find them (listed below) but recommend the authors take another pass to clear any remaining errors from the manuscript. For each entry below, page X, line Y is listed as PX LY:
P1 L22: “and the sequent” should be “and the subsequent”
P1 L25: “is one of techniques” should be “is one of the techniques”
P2 L20: “such those” should be “such as those”
P3 L25: “pointing out to the” should be “pointing out the”
P16 L9: “seeTrepte” is missing a space
P16 L15: “so that they result anti-correlated” should be “such that they are anti-correlated”
P16 L20 (and throughout): the “mu” for “micro” should not be italicized
P22 L12: this should be “1-2 days”, rather than using a division sign
P22 L33: “results to be” should be “is”
P24 L22: two more instances of an incorrect division sign
P29 L19: “The QBO important role” should be “The QBO’s important role”
P29 L29: “consistently” should be “consistent”
P34 L3: “point out to an” should be “point to an”
P34 L16: Need an “and” before “circulation changes”
Keith D W and MacMartin D G 2015 A temporary, moderate and responsive scenario for solar geoengineering Nat. Clim. Chang. 5 nclimate2493
Pierce J R, Weisenstein D K, Heckendorn P, Peter T and Keith D W 2010 Efficient formation of stratospheric aerosol for climate engineering by emission of condensible vapor from aircraft Geophys. Res. Lett. 37 Online: http://doi.wiley.com/10.1029/2010GL043975