the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2026
QBO-induced anomalous transport in the Northern Hemisphere stratosphere: the exceptional 2018/2019 late boreal winter
Abstract. Large positive anomalies of nitrous oxide (N2O) were observed in the northern hemisphere lower stratosphere in the late 2018/2019 boreal winter. Thanks to its long lifetime in the lower stratosphere, N2O is a robust tracer for stratospheric transport. This study investigates the magnitude, vertical structure, and dynamical origin of the late 2018/2019 boreal winter N2O anomaly using multiple chemical transport model simulations, chemical reanalyses, a specified-dynamics chemistry-climate model, and merged satellite observations. All datasets consistently show pronounced N2O positive anomalies in February 2019 in the northern mid-latitudes at 50 hPa. The N2O Transformed Eulerian Mean budget indicates that the N2O anomalies are primarily driven by enhanced meridional residual advection, which is in turn determined by enhanced planetary wave forcing. The extratropical effects of the quasi-biennial oscillation (QBO) determine these transport anomalies: the poleward QBO secondary circulation was unusually strong in the late 2018/2019 boreal winter, and a marked northward displacement of the zero wind line induced the enhanced planetary wave forcing. The combined strengthening of both the northward QBO secondary circulation and the planetary wave forcing led to unusually strong poleward advection in the northern lower stratosphere, which ultimately built up the N2O positive anomalies. These results indicate the value of long-lived stratospheric tracers such as N2O for diagnosing dynamically driven extreme events and for disentangling the contributions of different transport processes. We highlight the importance of the QBO teleconnections, particularly as a warming climate may change the frequency and intensity of extreme events thereby impacting these teleconnections.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3449', Anonymous Referee #1, 02 Aug 2026
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RC2: 'Comment on egusphere-2026-3449', Anonymous Referee #2, 15 Aug 2026
Review of Minganti et al, 2026
The study by Minganti et al investigates the strong stratospheric tracer (in particular N2O) anomaly in northern mid-latitudes in early 2019 based on satellite data and a large range of observation-constrained model data. They show that the anomaly was among the strongest on record, that it is well captured by the large range of datasets and models, and that it was associated with enhanced meridional advection. They further analyse the dynamical causes of the enhanced meridional advection and suggest that the QBO phase in this period was decisive through a) its secondary circulation that enhanced lower stratospheric poleward transport, and b) leading to a enhanced wave forcing of the residual circulation because of the northward displaced zero wind line in the tropics. Overall, the study is a valuable contribution in investigating the observed tracer anomaly and showing that it is well captured in many observationally-constrained models, and thus will be worth publishing in ACP. However, I have some concerns in particular 1) on the presentation style, which needs to be condensed and more focused, 2) on the qualitative nature of inter-model comparisons, and 3) on the discussion of the dynamical mechanism, which I believe misses some important points. Therefore, I suggest to return the paper to the authors for major revisions.
Major comments:
1) Focus of paper, presentation style.
The paper expands in many places on a lot of details, and is very long. I believe the paper would benefit from condensing the writing by keeping a clearer focus. Examples are given in the specific comments below and in the annotated pdf. A particular concern is, in my opinion, the merged discussion of, on the one hand, inter-model comparisons, and on the other hand the investigation of the processes that led to the early 2019 anomalies. In particular in section 3 this makes the text hard to follow. Therefore, I suggest that the authors either put less focus on the discussion of inter-model differences (given the overall conclusion appears to be that models agree well), or they
re-structure their paper into two sections, one with a clear focus on the tracer and dynamical anomalies (possibly with a reduced number of data sets), and another section with a focus on inter-model comparisons (i.e., do all models capture the tracer anomalies and the suggested mechanism). In this case, the latter needs to be more quantitative, see next comment.
2) Qualitative nature of inter-model comparison.
If the authors decide to keep one focus on inter-model comparison, it needs, in my opinion, to be more quantitative (see e.g. below suggestion in comment on line 363ff). From the current Figures, the inter-model differences are hard to see, let alone quantify. For Example, I find it hard to understand what the concluding statement that the ERA5-driven CTM is superior to other reanalysis is based on (line 639).
3) Discussion of dynamical mechanism
As the authors noted, a SSW occurred in January 2019, which is the focus month of the dynamical analysis in the paper. This is also visible in the quite weak mean zonal winds in the polar vortex region in Figs. 6/7. However, the authors do not consider the contribution of the extratropical circulation anomalies to the anomalous meridional advection of N2O presented in their study. After the SSW, the negative winds will block vertical planetary wave propagation, so that they might rather dispose their momentum in the lower instead of the middle stratosphere. This is, in my opinion, a possible explanation of the vertical dipole seen in the EP flux divergence in Figure 8, with enhanced negative EPFD in the lower stratosphere in mid-latitudes, and reduced above. Indeed, this is to me more plausible than arguing with the poleward shift of the subtropical zero wind line to 15°N, given enhanced wave forcing occurs rather poleward of 30°N. I strongly suggest the authors include discussion of the effects of the SSW on the circulation anomalies.
Furthermore, the authors convincingly show that the meridional advection of N2O was anomalous in the period of interest, resulting in the strong N2O anomalies. They argue an important ingredient was the anomalous meridional circulation component v*, associated with enhanced lower stratospheric wave driving and the secondary circulation from the QBO. However, anomalies in v* are not shown, and in particular the latter point on the secondary circulation is therefore rather inferred than directly shown. Therefore, I suggest the authors include analysis of v*, and/or anomalies in the residual streamfunction; in particular the latter is well suited to reveal the secondary circulation component (showing as closed cells of circulation anomalies extending from the equator to the subtropics).
Specific comments:
(Note that additional minor comments, in particular suggestions to re-phrase/ condense and typos can be found in the annotated pdf).
Line 101 ff: Is this the first study that uses the N2O SWOOSH data? If yes, it would be good to provide a more comprehensive description of the merging procedure, or point to a report or similar where this information can be found.
If no, please refer to previous studies on the description.
Line 168: Not clear what is meant here by FWmaSD - is this a specific set-up of the nudging? Please specify, and/or refer to the relevant paper.
Line 195ff:
The definition of the EP fluxes as in Equ. 4a and 4b is inconsistent with the EPFD in Equ 6a and 6b. Please only provide the Equations for the version of the actual calculation you have performed; the calculated EP fluxes and the EP flux divergence have to follow the same assumptions.
Line 240 to 264: I find the discussion of differences between the datasets quite difficult to follow. Also, I wonder whether the details are important given the focus of the paper on explaining the 2018/19 N2O anomaly. Consider condensing these paragraphs, or at least clarify and re-phrase, focusing on the most important messages.
Line 275 to 290: Again, I find the discussion of the differences in the data products difficult to follow, and I think there is too much detail here that might not be as important. For me, the main message is that the products are overall all able to reproduce the strong anomaly, with some differences in absolute value. Please condense and simplify.
A good example would be, in my opinion, your description of inter-model differences in the ozone anomaly timeseries (line 304ff)
line 312: If AO/NAO was the circulation pattern associated with the ozone anomaly, there still has to be a stratospheric transport anomaly that causes the ozone anomaly. And given you don't see an N2O anomaly, was this of different nature compared to the other transport anomalies? Please comment on this.
line 356 to 362: As stated in line 357, biases in the mean BDC do not necessarily reflect in the strength of anomalies in a particular year. So I'm not sure this discussion on the difference in the anomaly strength in different products is very meaningful.
Line 363ff: For making the point on the relation of anomalous transport contributions versus the actually anomaly in a more quantitative way, it would be useful to produce e.g. scatter plots of A_y (and/or A_y+A_z) in Jan 2019 versus the N2O anomaly in Feb 2019 across the individual models. Such a Figure would give a much more quantitative view of the relation of transport anomalies and tracer anomalies across model. As is now, this discussion appears a bit selective and qualitative.
Line 387: I'd strongly suggest to show the sum of EPFD_z and EPFD_y. If the latter has no strong anomalies, the former will dominate also in the sum, but that way you make sure you capture the full forcing.
Line 391ff: I find those differences in WACCM difficult to see from the Figure (mostly lines are on top of each other, and differences between data sets are difficult to see). Also, I think those differences appear to be minor, so not necessarily worth discussing. Thus I suggest to drop this paragraph, also because the fact that the free-running WACCM versus the SD-version has different planetary wave forcing is rather obvious.
Line 504ff: Did the authors attempt to quantitatively determine the contributions of individual TEM budget terms by time-integrating the budget equation to obtain the tracer time-series? I know this can be difficult in practice due to large cancellation and residual terms, but the authors might want to acknowledge that this is in principle possible.
Figure 7: The label on the colorbar seems to be wrong (says w* instead of A_y)
Line 519: Given the important role of the secondary circulation in the discussion, it would be great to show the mean residual circulation anomaly of the month of interest (e.g. overlaying contours of the residual streamfunction anomaly in Fig. 7 could be very helpful).
Line 608/Figure 10: Why don't you show v* ? This would be much more consistent.
Line 670-697: This reads like a nice motivation and outlook for stratospheric science in general, but I think goes far beyond "Summary and Conclusions" related to the current manuscript. Please condense significantly.
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Please find attached the review comments.