the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jul 2025
Cross-Hemispheric Transport of the Hunga Aerosol Plume: In Situ Evidence and Radiative Effects from the Northern Hemisphere
Abstract. The Hunga Tonga–Hunga Ha’apai eruption (20° S) in January 2022 injected a substantial amount of water vapor and aerosols into the stratosphere, primarily impacting the Southern Hemisphere and tropics. Using a combination of satellite observations and in situ measurements with optical particle counters, we show that a significant portion of the aerosol plume was transported into the Northern Hemisphere (NH) mid-latitudes. This cross-hemispheric transport occurred within the tropically controlled transition zone, within the shallow branch of the Brewer–Dobson circulation. By October 2022, enhanced aerosol concentrations were observed up to 50° N, at altitudes between 17–23 km with some dense plumes at around 21–22 km. In situ observations reveal an effective radius of around 330 nm, comparable to what was observed in the Southern Hemisphere (SH). Aerosol extinction coefficients in the mid-latitudes (30–50° N) increased by ∼50 % over background levels, corresponding to an aerosol optical depth (AOD) increase of (1–2) × 10−3. These enhancements led to a modest, but not negligible, shortwave top-of-atmosphere (TOA) radiative forcing of −0.05 ± 0.01 W m−2 between October 2022 and April 2023. Our results show that the moderate aerosol impact of the Hunga eruption in the SH produced non-negligible radiative impacts in the NH, emphasizing the importance of considering both hemispheres when analysing the total impact.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-2091', Yun He, 28 Jul 2025
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RC1: 'Comment on egusphere-2025-2091', Anonymous Referee #1, 12 Aug 2025
This manuscript of 'Cross-Hemispheric Transport of the Hunga Aerosol Plume: In Situ Evidence and Radiative Effects from the Northern Hemisphere' by Kloss et al. investigated the cross-hemispheric transport and the impact of the southern hemispheric Hunga eruption on the northern hemisphere mid-latitudes, which is a meaningful supplement to previous studies primarily concerning the southern hemisphere and tropics. In this study, both satellite observations and in situ measurements with OPCs were used to analyze the significant aerosol plumes transported into the NH mid-latitudes and their transport pathway. Balloon borne OPCs observations revealed the total bimodal distribution and an effective radius of ~ 330 nm within the aerosol plume layer. Based on analysis of aerosol extinction coefficients from SAGE III/ISS, the Hunga eruption was estimated to have a shortwave top-of-atmosphere (TOA) radiative forcing of −0.05±0.01W m−2 during October 2022 to April 2023. These results underscored that even a volcanic eruption with moderate aerosol output can have measurable effects in the opposite hemisphere.
Some minor issues:
Line 76: 2.5 mm --> 3.0 mm
Line 140: It should be noted at which levels no background aerosol conditions are observed.
Figure 1: I maybe miss the time interval for this figure, daily or twice daily?
Figure 1: It speaks twice of vertical dashed line.
Figure 1: It's seen that the most enhanced signals are located at the levels of mean tropopause height, but obviously these signals are not from volcano eruption, probably from cirrus in the upper tropopause at the lower latitude. Therefore, my suggestion is to use the highest tropopause instead of the mean tropopause at the latitude range, in this way, all the enhanced signals will be below the tropopause.
Line 144-150: Could you draw the outline of the plumes from Hunga eruption in Figure 1? It's not easy to get what you said.
Line 152: Could you show us the exact values of aerosol extinction from these two eruptions? It's not easy to see a factor of around 1.5.
Figure 2: It's not easy to tell the difference among different profiles. Could you plot the figures above 15 km, below which is not used?
Figure 3: This figure shows the trajectories originated from 20.6 km. How about the lower levels, such as 16-18 km also with volcanic eruption plumes, where the flow should be different?
Figure 4: Could you check the tropopause height in the figures, particularly in Fig. 4a, which changes too rapid?
Figure 5:What's the time interval for this figure?
Figure 6: Could you draw the outline of the plumes from Hunga eruption in Figure 6? It's not easy to get what you said.
Line 276: 676 nm --> 756 nm.
Line 281: Why use the size distributions obtained with satellite observations, not use those from POPS measurements?
Citation: https://doi.org/10.5194/egusphere-2025-2091-RC1 -
RC2: 'Comment on egusphere-2025-2091', Anonymous Referee #4, 17 Oct 2025
This paper describes the impact of the Hunga-Tonga eruption on the norther mid latitudes. Overall this point is established by the paper, but the presentation is poor and needs a lot of work to bring it up to a publishable standard. Presently it is difficult and cumbersome to read and some figures need a lot of work. The writing includes lots of numbers and details on locations and times of measurements, yet these are of no real use to the reader, who would like to know, not the location of each individual measurement, but the separation in space and time between the measurements. The dates are presented in three or four different formats challenging the reader to figure out what format is used for each figure. Yet important details like the extinction levels used to trace the Hunga plume, or the wavelengths used for the AOD and Angstrom exponent calculations, are left out. The writing is a bit convoluted, using lots of parenthetical expressions to explain what is meant, when a little care in the writing, and shorter sentences, would eliminate this necessity.
In Fig. 2 the authors present extinction coefficient calculations from in situ measurements and from SAGE III measurements, yet do not offer any quantitative extinction comparison between the different OPCs used and the SAGE III measurements for either the Hunga plume or the surrounding background. Considering that all three measurements, two OPCs, and SAGE III, occur in relative coincidence it seems this would be a natural thing to include.
Oddly the authors sneak in an OPC comparison in the appendix with an OPC not described in the text. Why is this done and what does it have to do with this paper? It should be eliminated and presented somewhere else where that OPC is presented.
Here follows, by line number, figure, and table specific comments to improve the manuscript.
36 should be “Antarctic”
61 What is meant by “partial aerosol concentrations”? Shouldn’t the wavelength of the laser be listed above rather than qualifying “aerosol size distribution”? Presumably the aerosol size distribution does not depend on the wavelength of the laser.
65 This description is poorly worded. If the sampling frequency of LOAC is 0.1 Hz why does that lead to a final profile with a sampling frequency of 1/60 to 1/600 Hz? Presumably the 0.1 Hz samples have to be averaged from 6 to 60 samples to achieve a final resolution? This is confusing. But in any case isn’t the important thing the vertical resolution, which can be calculated once the sampling frequency used is known, assuming a nominal balloon ascent rate. This is the quantity of most interest for the reader.
74 “aerosol properties” very generic. Be specific. What does it measure?
76 again, “partial aerosol concentrations”?
80 Good. Such a similar statement should be included with the LOAC description.
90-91. Do the authors mean that for this study they use the extinction measurements at 675 nm? OMPS measures and reports extinction at many wavelengths.
97 Does anyone use the limb scatter data from SAGE III? Is an extinction product even produced from this data? If not it should not really be mentioned.
106 The current SAGE III/ISS version is 6.0. It seems the authors should use a later data version as these have been available for some time. It may make no difference, but the version used should be more up to date.
124 Why “i.e.” and the parenthetical expression. Just state … with output of potential temperature, ….
Fig. 1 caption. Why the repetition that Tonga is represented by the dashed vertical line(s). Only one line is shown.
Fig. 1 If this figure were started in Jan 2021 then the whole La Soufriere cycle could be seen and the background prior to La Soufriere? Then the La Soufriere plume would be clear and contrast with the Hunga plume. Such inclusion would not significantly expand the figure.
142-143 “From Fig. 1, the increase is evident up to around 22 km altitude until early 2022.” This statement is hardly supported by the figure. Considering the region from 20-22 km, there is little difference between late 2021 and near Sept. 2022. Again it would be worthwhile to show the data prior to La Soufriere to let the reader get a sense of the changes and the timing of the changes. With the current figure only the period late 2022 to fall 2023 seems to be really different than the prior and subsequent aerosol.
144 “Hunga eruption (at 20◦S, 175◦W) in January 2022 produced a slight influence on the northern-hemispheric stratosphere as early as March 2022” Perhaps this can be stated from the OMPS data but it is not apparent in Fig. 1.
147 “… the actual beginning of the general influence of the Hunga eruption on the NH in terms of increased aerosol values cannot clearly be identified …” Have the authors considered looking at the water vapor signal to get a better handling on the timing? There the timing of the Hunga plume may be more clearly identifiable.
152-153 This statement would be much clearer if the authors include earlier data in Fig. 1.
153 change to … extinction, is a factor of around 1.5 smaller than the Hunga eruption ...
160 19/10?
158-162, 80-82, …, and figures. Date format: day/month/year or month/day/year or year/month/day!? It doesn’t matter which is used, but it should be consistent throughout the text and figures. Here the authors switch between all three and even a fourth leaving the reader floundering. Please be consistent.
Figure 2 needs a lot of work. The interest is above the tropopause so start the figure there. This then allows the extinction scale, covering 4 orders of magnitude, to be reduced to 2 or maybe even 1 so the reader can compare the estimates from the different instruments. Something the authors should also consider doing. Fix the date format and organize the dates chronologically ascending or descending but consistent throughout. In a,d) dates ascend. In b,c) they descend. (c) “LOAC observations of aerosol plumes of volcanic origin from MeteoModem, France 48.3◦N, 2.6◦E” Why is this panel included? What volcanic plumes are sampled? How is it known they are volcanic plumes? Why isn’t the 14/11/22 plume included in b)? In fact all the c) LOAC profiles could appear in b). While it is good the authors know the locations of the SAGE III plumes in d,e), providing the latitude/longitude to the reader doesn’t add any useful information. Much more helpful would be the distance and time difference of the satellite measurements from the in situ measurements. And to the extent possible, such information should be in the figure legend. This would not be hard, just two numbers following each date and adjustments to the legend location. Do the latitude longitude and time of each in situ measurement need to be included in the figure caption? What is the reader to do with that information. Just a general location statement would be enough. There is no description of panel (d) in the figure caption.
The appendix figure A1 displays the data as it should be in Figure 1, but then A1d) makes no sense and is off topic of this paper. It appears the authors are trying to sneak in a figure showing results from another OPC, which hasn’t been introduced except in the appendix, and while the measurement comparison is encouraging it has nothing to do with the subject of this paper. Such information has to appear in another paper.
Why is figure A2 not included as part of Fig. 1, replacing the current panel c)? Note in this figure the dates are now in a third format.
166 14/11? This statement would be clearer if the 14/11 plume were in panel b) particularly if the scale as in A1 was used for the abscissa.
- This statement cannot be confirmed by the figure. Where are the optical depth calculations that might confirm it?
169-171 What are the authors saying. If there is a “general aerosol enhancement … up to 20 km” why do the 2022 data agree with the background gray lines in 2018?
174 and Fig. 3 Why don’t the air parcels end near 21.5 km? 20.6/4 km is at the base of the perturbed layer.
176-178. Why all the parenthetical clauses? There is no reason for them. They are very cumbersome to read. If the information is important include as part of the sentence. If not leave it out.
Fig. 4 caption is brief and not very helpful. Try: Altitude, latitude cross sections of aerosol extinction from OMPS at 675 nm for the time period 12-20 October 2022. Each panel is a two day average(?) over the longitude ranges listed in each panel. The arrow identifies an aerosol plume believed to arrive over France at the end of the time period…
180-182 The longitude ranges are confusing. They begin encompassing only two of the CLAMS backtrajectory plumes, then move further west while the plume moves east. Is there a labelling error?
183-193 The paragraph indications are confusing. It seems that lines 183-187 form a paragraph, but there is no indentation of the first line. While directly below there is an indentation indicating a new paragraph. How is the reviewer supposed to read this? Similar problems exist throughout.
192-195. While the CALIOP data confirm an aerosol layer at 22 km at the time of the balloon flights, the figure is also suggestive that the source of this aerosol is from a plume originating at 18 km 4 days earlier on 15 October. How can this be separated from the Hunga plume? The authors should mention this coincidence and explain why the lower and upper plume are not connected.
Fig. 6 Why are 5 days chosen for each time period, why not a monthly average? If that were the case then the panel labelling would be much easier and the reader could follow the progression of the aerosol since the panels would be labelled e.g. as January 2022, March 2022, May 2022, … With the current date labelling the reader has to struggle to figure out the time sequence and correlating the figure to the discussion. At the very least each panel should be labelled clearly with the year and month in easy to read format.
198-206 What aerosol extinction are the authors using to identify the Hunga plume. This should be stated so the reader can follow the discussion. Likewise what is meant by “light aerosol plumes”? For example in the March 2022 panel aerosol extinction > 0.008-0.01 km-1 is observed to barely 30 N, while the authors state, “…but rather light aerosol plumes (in terms of aerosol extinction enhancements) are observed being transported to higher latitudes in the NH to around 35◦N.” Where is this seen? And again why the parenthetical clause? Clear writing does not require such clauses.
Fig. 7 Do the authors means as presented in Fig. 2? Is the fit to an average of the data from 20-22.5 km, or to a single measurement within that altitude range? If the former it would be nice to show the standard deviation of the concentration data at each size.
Fig. 8 Since these profiles are averages it would be appropriate to shade in, or otherwise present, the standard deviation along with the means.
274-276. All these numbers are lost on the reader. They are already in Table 2, or should be, so just refer to Table 2 and point out the important point of the Hunga contribution to AOD. In addition these numbers don’t agree with Table 2, where the Hunga contribution is estimated at 0.0011, while the text here states the Hunga impact at 0.009 – 0.003. The difference is because the latter are at different wavelengths, but the wavelengths aren’t specified. If this information is important include it, and the wavelengths in the table. Otherwise leave it out. There is plenty of space in the table to add columns at wavelengths in addition to 756 nm. In fact that would be useful and easily done. Table 2 could be made much more useful with a little bit of work, it has plenty of space. So add a column to the left indicating: Background, Background+Hunga, Hunga only. Then add AOD at the wavelengths the authors wish to quote in the text. An Angstrom Exponent is always calculated between two wavelengths. Here those wavelengths are not specified. Finally the SW TOA RF, what is the time period, does it include both background & Hunga, or just Hunga?
277-279. An Angstrom Exponent without specifying the wavelengths is useless. Be specific or eliminate this text. What is the difference between an experimental value and a theoretical value of an Angstrom exponent. These exponents are all calculated the same way. What does the theoretical value use for extinction coefficients?
282 Presumably the TOA RF is calculated using the UVSPEC radiative transfer model. If so it should be so stated. The current text rather suggests the number is from Sellitto et al.
Citation: https://doi.org/10.5194/egusphere-2025-2091-RC2
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Thank you very much for authors' work regarding the cross-hemispheric transport of the Hunga aerosol plume. Please find some supplement informations in the uploaded file.