Please find my review in the attached document.

Review of “Utilizing a Multi-Proxy to Model Comparison to Constrain the Season and Regionally Heterogeneous Impacts of the Mt. Samalas 1257 Eruption” by Laura Wainman et al.

The authors present a multi-proxy to model comparison study of the Mt. Samalas eruption, the largest explosive sulfur-rich eruptions of the last millennium, which eruption season/year is still not known. As potential eruption dates NH summer 1257 and early 1258 are discussed. To achieve a more-precise constraint of the year and season of the Mt. Samalas eruption, the authors run ensemble simulations with the UK Earth System Model (UKSEM1) for a range of eruption scenarios and initial conditions for a NH summer and winter eruption and compare them with spatially resolved multi-proxy data. This allows them to robustly distinguish between both eruption dates. The authors suggest July 1257 as the most likely initial date due to its better agreement with spatially averaged and regionally resolved proxy surface temperature reconstructions. Overall, it is solid piece of work and important for further applications, but needs some clarifications and improvements. I therefore recommend publication after revisions, see below.

General comments

 In my opinion, the discussion part needs some revision as some important points are not mention at all or only briefly touched.

1. I miss a dedicated paragraph about volcanic forcing uncertainties. The authors mention in one sentence that there might be uncertainties in the VSSI estimate. There was recently a study published in Climate of the Past by Lücke et al. (2023) who addressed the effect of uncertainties in natural forcing records on simulated temperature during the last millennium with a volcanic forcing ensemble. In Lücke et al. (2023) also the large uncertainties around the Samals eruption were addressed, thus it would be good to discuss your results with respect to their work. Timmreck et al. (2021) also discussed forcing uncertainties in comparison with multiple-proxy data for the 1809 eruption showing that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure.  As the spatial structure of the forcing pattern is quite important, I wonder, if the spatial volcanic forcing distribution is similar for the different realizations of each starting date and how does it differ between them. Observations show that some tropical eruptions had a hemispherical asymmetric aerosol load e.g. Agung 1963 or El Chichon 1982. The spatial structure might also be a potential source of uncertainty and should be addressed in the discussion section.
2. I also miss in the discussion a dedicated paragraph about the strength and the weaknesses of the applied global aerosol model. The recent global aerosol model intercomparison studies (Marshall et al. 2018, Clyne et al. 2021, Quaglia et al. 2023) reveal several difficulties, which the current generation of global aerosol model has to face too. Marshall et al. (2018) demonstrate for example that the ratio of the hemispheric atmospheric sulfate aerosol burden after the eruption to the average ice sheet deposited sulfate varies between models by up to a factor of 15. The study by Qualia et al. (2023) where the different model results are compared to satellite observations after the Pinatubo episode show a stronger transport towards the NH extratropics, suggesting a much weaker subtropical barrier in all the models. Hence, I wonder how model specific are your results? How much are the results presented here influenced by biases or specific features of the UKESM model. Would not a multi-model multi-proxy intercomparison the best suitable way to move forward?
3. I wonder why you run only a July and a January scenario and not an experiment for the autumn season. Toohey at al. (2011) demonstrate that the modulation by the annual cycle for many variables is not linear. An experiment with the initial date at the 1^st of October could have been a very valuable set up.

Specific comments

• Lines 18 ff.: The description of the initialization of the volcanic cloud misses some important details. For me it is not clear, how you initialize your volcanic cloud on the horizontal grid. Do you inject your sulfur emission in one grid box around the location of the volcano or over several grid boxes or even in a zonal band at 8 S. As shown by Quaglia et al. (2023), the results could be very different for the UKESM depend on the initialization of the eruption cloud. Please, give some more details here and also modify Table 1 accordingly as “8 S” is a bit unspecific in the respect.
• Lines 45-46 Please add references
• Line 200 : As you discuss also in 3.1.2 only the NH data, it might be appropriate to change the subsection title to “NH hemispheric mean” or something along this line.
• Line 200 ff.: I wonder a bit why you calculate your own uncertainties for the tree ring reconstruction and do not use the ensemble spread of tree ring ensemble reconstruction from Büntgen et al. (2021), see for example Figure 6 in van Dijk et al (2022).
• Line 201 and elsewhere: I suggest that you give the two experiments dedicated names e.g JUN1257 or JAN1258 to avoid confusion by just saying the date
• Line 418 : How many individual realizations have a positive ENSO phase in summer 1258 and 1259? You can also look to relative SSTs instead of raw SSTs here.
• Line 450: Does a best estimate for the emission height really exist?
• Lines 459-60: Reference is missing
• Lines 462: Not clear to me. According to their analysis of speleothem data from Mesoamerica, Ridley et al (2015) showed that SH volcanic eruptions, including those at low southerly latitudes (e.g. Tambora 1815) force the ITCZ to the north and lead to wetter conditions. Your figure S11 shows for Mexico a similar response for Tambora. Tejedor et al. (2021) showed on the other hand results for a super epoch analysis.
• Line 491ff:: You should not forget to discuss the model deficits in this paragraph; nine  realizations might not be a sufficient number for each model experiment to obtain statistically significant pattern of tropical hydroclimate changes, large scale meridional transport and sulfate deposition are  also strongly model dependent, see Marshall et al. (2018), Quaglia et al. (2023)
• Line 495: Another exemplary study in this respect is the paper by van Dijk et al. (2023) which you could cite here as well
• Lines 780 ff.: References of Wade is listed twice, also indicated as 2020a and 2020b

Figures:

• Figure 2: Maybe you include here in one of the panels the specific position of the tree-rings
• Figure 3: Difficult to interpret the proxies in the two lower rows. The colors in the upper row probably not refer to the colormap at the bottom, so please use different colors instead of red and blue here. Which meaning has the cyan color here? I also wonder if it would make sense to show here only the NH as you do not discuss the two proxies from the SH here. They could be shown in the supplements.
• Figure S2, S3: As the number of individual realizations are not import in the context, I suggest to combine both figures into a new one with two panels one for each starting date and thin lines for the individual realizations and a thick one for the ensemble mean
• Figure S11: Please list the reference of the reconstruction

Data availability: Please make sure that the data are available before the submission of the revised version.

Literature

Büntgen, U., Allen, K., Anchukaitis, K. J., et al.: The influence of decision-making in tree ring-based climate reconstructions, Nat. Commun., 12, 1–10, 2021.

Clyne, M., Lamarque, J.-F., Mills, M. J., Khodri, M., Ball, W., Bekki, S., Dhomse, S. S., Lebas, N., Mann, G., Marshall, L., Niemeier, U., Poulain, V., Robock, A., Rozanov, E., Schmidt, A., Stenke, A., Sukhodolov, T., Timmreck, C., Toohey, M., Tummon, F., Zanchettin, D., Zhu, Y., and Toon, O. B.: Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble, Atmos. Chem. Phys., 21, 3317–3343, https://doi.org/10.5194/acp-21-3317-2021, 2021.  -

Marshall, L., Schmidt, A., Toohey, M., Carslaw, K. S., Mann, G. W., Sigl, M., Khodri, M., Timmreck, C., Zanchettin, D., Ball, W. T., Bekki, S., Brooke, J. S. A., Dhomse, S., Johnson, C., Lamarque, J.-F., LeGrande, A. N., Mills, M. J., Niemeier, U., Pope, J. O., Poulain, V., Robock, A., Rozanov, E., Stenke, A., Sukhodolov, T., Tilmes, S., Tsigaridis, K., and Tummon, F.: Multi-model comparison of the volcanic sulfate deposition from the 1815 eruption of Mt. Tambora, Atmos. Chem. Phys., 18, 2307–2328, https://doi.org/10.5194/acp-18-2307-2018, 2018. 

Lücke, L. J., Schurer, A. P., Toohey, M., Marshall, L. R., and Hegerl, G. C.: The effect of uncertainties in natural forcing records on simulated temperature during the last millennium, Clim. Past, 19, 959–978, https://doi.org/10.5194/cp-19-959-2023, 2023.

Quaglia, I., Timmreck, C., Niemeier, U., Visioni, D., Pitari, G., Brodowsky, C., Brühl, C., Dhomse, S. S., Franke, H., Laakso, A., Mann, G. W., Rozanov, E., and Sukhodolov, T.: Interactive stratospheric aerosol models' response to different amounts and altitudes of SO2 injection during the 1991 Pinatubo eruption, Atmos. Chem. Phys., 23, 921–948, https://doi.org/10.5194/acp-23-921-2023, 2023.

Ridley H, Asmerom Y, Baldini JUL, Breitenbach SFM,Aquino VV,Prufer KM, Culleton BJ, Polyak V, Lechleitner FA, Kennett DJ, Zhang M,Marwan N, Macpherson CG, Baldini LM, Xiao T, PeterkinJL, Awe J, Haug GH. Aerosol forcing of the position of the intertropical convergence zone since AD 1550. Nat Geosci 2015,8: 195–200, doi: 10.1038/ngeo235.

Tejedor, E., Steiger, N.J., Smerdon, J.E., Serrano-Notivoli, R. and Vuille, M. (2021). Global hydroclimatic response to tropical volcanic eruptions over the last millennium. Proceedings of the National Academy of Sciences, 118(12), p.e2019145118. doi:10.1073/pnas.2019145118.

Timmreck, C., Toohey, M., Zanchettin, D., Brönnimann, S., Lundstad, E., & Wilson, R. (2021). The unidentified eruption of 1809: a climatic cold case. Climate of the Past, 17(4), 1455-963 1482. https://doi.org/10.5194/cp-17-1455-2021.

Toohey, M., Krüger, K., Niemeier, U. and Timmreck, C. (2011). The influence of eruption season on the global aerosol evolution and radiative impact of tropical volcanic eruptions. Atmospheric Chemistry and Physics, 11(23), pp.12351–12367. doi:10.5194/acp-11-12351-2011.

van Dijk, E., Jungclaus, J., Lorenz, S., Timmreck, C., and Krüger, K.: Was there a volcanic-induced long-lasting cooling over the Northern Hemisphere in the mid-6th–7th century?, Clim. Past, 18, 1601–1623, https://doi.org/10.5194/cp-18-1601-2022, 2022.

van Dijk, E., Mørkestøl Gundersen, I., de Bode, A., Høeg, H., Loftsgarden, K., Iversen, F., Timmreck, C., Jungclaus, J., and Krüger, K.: Climatic and societal impacts in Scandinavia following the 536 and 540 CE volcanic double event, Clim. Past, 19, 357–398, https://doi.org/10.5194/cp-19-357-2023, 2023.