Downstream development of extratropical transition of tropical cyclones over the Southern Hemisphere

Jin, Chenhui; Ritchie, Elizabeth A.; Barnes, Michael A.; Holbrook, Neil J.

doi:10.5194/egusphere-2026-5018

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https://doi.org/10.5194/egusphere-2026-5018

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31 Aug 2026

| 31 Aug 2026

Downstream development of extratropical transition of tropical cyclones over the Southern Hemisphere

Chenhui Jin, Elizabeth A. Ritchie, Michael A. Barnes, and Neil J. Holbrook

Abstract. Tropical cyclones (TCs) occasionally move into the midlatitudes and transition into extratropical cyclones. The process is known as extratropical transition (ET), which can affect the weather further downstream. The present study conducts a climatological analysis of the dynamics of the downstream development associated with ET over the Southern Hemisphere. These ET events are objectively identified and further grouped into four distinct ET clusters using K-means clustering. ET events in clusters 2 and 3 cause pronounced downstream development. The dynamics of the TC-midlatitude flow interaction are diagnosed from both potential vorticity (PV) and eddy kinetic energy (K_e) perspectives. From the PV viewpoint, the anticyclonic PV tendency due to the PV advection by diabatically-driven divergent outflow contributes substantially to downstream ridge amplification, supplemented by the nonlinearities arising from PV anomalies advected by flow induced by PV anomalies. From the K_e viewpoint, the transitioning TC injects additional K_e through baroclinic conversion into the midlatitude flow. The energy is then distributed by ageostrophic geopotential fluxes downstream, thereby amplifying the downstream wave packet. We identified two different pathways of downstream development. One pathway (cluster 2) exhibits a classic picture of downstream development, with Rossby wave packets amplifying and propagating farther downstream along a strong and straight jet, whereas the other pathway (cluster 3) is more localised and characterised by a pronounced ridge immediately downstream. These distinct pathways appear to be predominantly determined by the characteristics of the jet, in which the more pronounced downstream ridge is associated with a weak, short, and poleward-orientated jet. Furthermore, our results show that the strong ET events in the southwest Indian Ocean downstream can cause significantly enhanced high-impact weather frequency over Australia.

Received: 20 Aug 2026 – Discussion started: 31 Aug 2026

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Chenhui Jin, Elizabeth A. Ritchie, Michael A. Barnes, and Neil J. Holbrook

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-5018', Anonymous Referee #1, 10 Oct 2026
General comments
Jin et al. investigate the dynamics of the downstream development associated with extratropical transition (ET) of tropical cyclones over the Southern Hemisphere. Based on objective clustering, they classified ET events into four categories, two of which are characterized by pronounced downstream development. Using potential vorticity and eddy kinetic energy perspectives, the authors argue that the time evolution of the two clusters is distinct. One cluster is accompanied by a classic picture of downstream development, with Rossby wave packets along a strong jet. By contrast, the other cluster is more localized and is accompanied by a pronounced ridge immediately downstream. They also discuss their influence on high-impact weather in downstream regions.
Overall, this study is comprehensive and provides results with implications for advancing our understanding of ET in the Southern Hemisphere. Thus, I believe that this work will be suitable for publication in WCD. However, there are several points that need to be addressed, as specifically commented below, before the possibility of publication can be further considered.

Major comments (not necessarily in order of significance)
Logical structure and guidance

I find that the motivation of this work is very clear and the structure is nicely presented in the introduction. However, while reading Section 3, I found myself lost in the interpretation of the results and the accompanying discussion. In my view, this is at least in part because Section 3 is somewhat a list of results without sufficiently clarifying what should be inferred from each result. For example, the relationship between the PV budget and the Ke framework is not sufficiently clear (see also Other Comment 2). Another issue is that maps for C2 and C3 are generally similar, and it is thus not straightforward to identify which signals should be focused on. Therefore, the authors should revise the manuscript to guide readers more clearly on what should be interpreted from which analysis. In my understanding, one of the most important arguments of this study is:
For C3 ET, the stronger upper-level divergent wind around TCs than in C2 acts to modify the shape of western flank of the ridge just downstream of ET to be more meridional, particularly through the nonlinear term (and an additional contribution of the interaction term). The more prominent upper-level divergent wind is related to the stronger mid-level updraft that is (somehow) associated primarily with upper-level forcing, with a secondary contribution of low-level forcing. The more meridional shape of the western flank of the ridge leads to the poleward propagation of the system, suppressing its downstream development compared to C2, together with the weaker westerly wind speed. Is this correct? Please correct me if I misunderstand. Anyway, clarifying the most important components and their connections would largely improve the readability of the study.

Related to the above, Sections 4 and 5 could substantially benefit from shortening by focusing more on the important arguments and logical flow. Specifically, in the current form, Section 4 (Discussion) is too lengthy and contains much of the summary of the results, not just discussions. In my view, for example, most of the part in L391-404 could be removed without loss of important information.
Dynamical interpretations

This manuscript presents multiple lines of analyses based on different frameworks. However, the dynamical interpretations of these analyses are relatively thin, in particular the connection between the results based on different frameworks, leaving questions of the reason behind them. Specific comments on this issue are shown below as “other comments”.
Schematic figure

Related to Comments A and B, it is not straightforward for readers to follow the overall argument and identify the core findings related to the dynamics and differences between the two clusters, as the two clusters are seemingly similar in many ways. Therefore, I would strongly recommend adding a schematic figure to summarize the findings obtained by this study. It would largely help clarify the most important components and their connections (as commented in Comment A).
Regarding ET clustering

The present study utilizes a particular clustering of ET events into four clusters. As this clustering is an important basis for this work, it needs to be explained in more detail, so that readers do not necessarily need to refer to previous papers. The most crucial question is what criteria were used for the clustering (e.g., track shape, intensity, duration, translation speed, life cycle, geographical location, etc.). For example, there seems to be a large difference in the typical intensity of TCs in each of the ET clusters (as mentioned in the later part of the manuscript). Is this related to the configuration (and motivation) of the clustering? I also wonder if there is a discernible difference in the typical longitude of each cluster. Such clarification would help readers figure out why this study focuses on C2 and C3 only and separately.
Influence on the Australian region

It is nice to show the downstream influence of ET on the Australian region. However, I wonder why the authors combine C2 and C3 to investigate their downstream influence, given that the characteristics of their downstream development are distinct (as argued in the prior parts of the manuscript). To focus on the difference between C2 and C3 would fit the flow of this work much better.
Regionality

In the present study, the discussion on regionality of ET event dynamics is limited. It may be true that the Southern Hemisphere is more zonally symmetric than the Northern Hemisphere, but the effect of landmass and zonal asymmetry in mean states (such as jet streams) can have an impact on ET event dynamics differently in different basins. If the four clusters reflect the difference in the typical longitudinal sector of ET events, such a zonal asymmetry in basic states is likely to play an important role. In addition, it would also be beneficial to add a discussion on the influence of oceanic fronts (especially in the Southern Indian Ocean) on ET event dynamics. I suspect that considering the regionality of the clusters would help elucidate the importance of the pre-existing midlatitude flow for the ET characteristics.

Other comments
Introduction: It would be helpful to clarify the focus of this study on the downstream influence of ET on the Australian region (L95) and its importance before Section 2.

L36-68: I found that this part can benefit from focusing more on literature relevant for the present study. It would also be beneficial to add an explanation of the difference between the two frameworks (i.e., pros and cons, and why we need both in this study).

L149: available potential energy -> eddy available potential energy?

Figure 2: The caption says the contour interval is 1hPa. Is this correct? If this is correct, I wonder why anomalies particularly in C1 and C4 are so weak.

The titles of subsections 3.2 and 3.3 are very similar. In my view, it would be beneficial to change the title of Section 3.3 to represent its content more effectively.

Figure 5: It seems that the more prominent negative QGomega_top just downstream of the ridge in C3 (Fig. 5b) compared to C2 is a very important factor characterizing their different nature in the downstream development. Can the authors explain what makes this signal (e.g., upper-level vorticity advection by something)?

L265-266: Can the authors explain why “this tendency pattern” is in phase with the propagation terms? I wonder how this is related to the weakening TC cyclonic PV tower (L266). This applies to L355-356. Please elaborate a little more on the mechanisms.

Figures 6 and 7: These ET events exhibit a clear signal of anticyclonic Rossby wave breaking (AWB) downstream (as touched on in the later part of the manuscript). Can the authors relate such AWB signature and the downstream influence around the Australian region, for example, by the climatological-mean AWB frequency distribution?

L271: “60 and 90E” seems misleading, as this is relative longitude in reference to the ET. Please consider revising it.

Figure 8: I guess that the shading indicates convergence, not divergence. Please check it. In addition, I wonder why the authors show selected contours of geopotential height at 500-hPa. Perhaps other variables would work better (e.g., wind speed to show jet streams).

L388: Figs -> Fig

L419-420: There is an error around “Jin et al., 2026”.

Conditions for ET: While the authors mention that investigating conditions that determine ET characteristics (i.e., clusters) is future studies in Section 5, I would recommend adding some discussions on it. Obviously, one potentially important factor is a jet stream (as mentioned in L371), but other factors, such as preexisting wave-like anomalies (as seen in Fig. 3b), can also play a role.
Citation: https://doi.org/10.5194/egusphere-2026-5018-RC1
RC2:
'Comment on egusphere-2026-5018', Anonymous Referee #2, 17 Oct 2026
Review of “Downstream development of extratropical transition of tropical cyclones over the Southern Hemisphere” by C. Jin et al.
This manuscript extends previous work by Jin et al. (2026) about the extratropical transition (ET) of tropical cyclones (TCs) occurring in the Southern Hemisphere (SH), this time focusing on their downstream impact in terms of upper-level flow evolution and effects on Australian surface weather. Results are based on the same subsets of TCs as Jin et al. (2026), obtained through cluster analysis. The climatological analysis is conducted and explained carefully, and brings a few interesting insights into the downstream impact of SH ETs. The most important is arguably the presence of two different “flavors” of flow amplification downstream of ET, likely dependent on the state of the waveguide at the time of TC-midlatitude flow interaction. The enhanced likelihood of warm days and heavy precipitation events over Australia in the days following the ET event is also an interesting result, consistent with the expectations.
My comments mostly concern the way results are presented, which is at times too "descriptive" and does not highlight the aspects of novelty of this piece of work with respect to previous studies. These issues make the manuscript appear less sharp, potentially limiting its contribution to the field, so I encourage the authors to address them.
Nevertheless, this manuscript could be a valuable contribution to the literature about the downstream impact of ET, and would fit the readership of Weather and Climate Dynamics.
General comments
While it is true that SH ET has not been targeted as much in the past, the simple focus on an understudied region does not necessarily constitute sufficient motivation. In this regard, it would be useful to understand whether there are some SH-specific features that make studying ET in this basin particularly interesting with respect to other basins. Or do the authors expect to find the same behavior as in the North Pacific and the North Atlantic? I encourage the authors to emphasize these aspects in the Introduction and in the discussion of the results, highlighting specific SH aspects more explicitly (e.g., the first coming to mind is the fact that the SH jet is generally more zonal than over the Northern Hemisphere).

Related to the previous point, I have the impression that the results of this study are not properly contextualized with respect to the previous literature that specifically focused on the downstream impact of ET. For instance, the presence of different “pathways” or “flavours” of flow amplification downstream of ET had been discussed by Riboldi et al. (2019) for the western North Pacific basin and by Brannan and Chagnon (2020) for the North Atlantic, which are not cited here: do the C2/C3 subsets discussed in this paper correspond to those previous categorizations, or do they highlight something different and/or peculiar to the SH? I am thinking in particular to the results of Brannan and Chagnon (2020), who noticed that in some cases North Atlantic ET can correspond to flow de-amplification – are there potentially similarities with cluster C3, with the wave propagation interrupted by wave breaking? This contextualization would deserve, in my opinion, a dedicated paragraph of discussion and maybe more space in the Introduction.

A Rossby wave breaking diagnostic could be applied to better understand why Rossby wave propagation is halted in C3 with respect to C2, and to put into context the results obtained from the PV and Ke budgets.

In principle, one should be able to read and understand the analysis without having to go back and read Jin et al. (2026), too. For instance, in the manuscript there is no explanation of the adopted definition to define ET events (i.e., based on Hart’s cyclone phase space) or of the rationale driving the cluster analysis. I would suggest discussing those aspects specifically in Sect. 3.1 or, even better, in a separate methodology paragraph. I also do not understand why seasonality of different clusters is the discussed there in Sect 3.1, as it is not further mentioned or addressed in the remaining parts of the paper.

The paper has several descriptive sections in which the novel results get quite diluted (I am thinking in particular to Sect. 3.3.2, 3.3.3 and 3.3.4): the authors should try to emphasize more what is learnt from each analysis, rather than merely describing the features that are visible in them and their difference. A way to do so could be to conclude each paragraph in those sections with a short statement about the relevance/what has been learnt in the previous lines.

Specific points
Line 8: what are these “nonlinearities”? Could it be specified here?
Lines 28-31: a relevant citation here is Pohorsky et al. (2019), that specifically focused on heavy precipitation events downstream of North Atlantic ET events related to upper-level flow amplification.
Line 36-48: Would it be possible to highlight in which aspects the eddy kinetic energy perspective of ET differs with respect to the eddy kinetic energy analysis of classic baroclinic development?
Line 64: what is meant here with “cyclonic PV advection”? Shouldn’t it be “anticyclonic” (as at line 78)?
Sec. 2.3: please specify already here (rather than only at the end of the manuscript) that the diabatic term is not included, why it is so and in which regard this is supposed to affect the results.
Sect. 2.4: is the Ke budget closed? How large are the residuals?
Fig. 6f-j: would it be possible to show here, rather than repeating the divergent wind in the second column? It would match better the description of the PV advection pattern by the eddy-related nondivergent wind shown in those sub-plots and discussed at lines 257-268. Would it be also possible to show and highlight the location of the PV tower, which is discussed several times in Sect 3.3.3?
Section 3.3.4: This whole section might sound rather descriptive. I believe the manuscript would profit here from a more explicit acknowledgment of the novel scientific insights gained from such a detailed analysis of the Ke composites, also with respect to Quinting and Jones (2016) who used the same approach.
Line 316: maybe I missed it, but how is this “strong ET” subset defined?
Line 317: it seems like C2 dominates the subset of “strong ET”. What is the reason behind merging it together with the fewer C3 cases? How would the composites of Fig. 10 look like if C2/C3 cases were to be considered separately?
Lines 356-359: this statement is quite general and does not sound so novel. Would it be possible to continue instead on the line of the previous sentence, and keep discussing the differences between C2 and C3 downstream impacts from the Ke perspective?
Lines 364-366: I am not fully following the reasoning behind the comparison here. Latent heat release is a different phenomenon than baroclinic conversion and is not part of the eddy kinetic energy framework discussed in the methodology. In terms of conversion of available potential energy into kinetic energy, latent heat release can even act in an opposite direction than baroclinic development (e.g., Federer et al. 2025). Could the authors provide more explanations and rephrase if needed?
Section 4 (Discussion): this section is rather long and could be re-organized thematically to give a more logical flow, because the topics follow each other without apparent order. My suggestion would be to “de-structure” this section, putting some of the discussed points in the corresponding Results sub-sections (alleviating the problem of the excessively descriptive style), and the rest in a novel Conclusion section (see next point).
Section 5 (Conclusions): The part dedicated to the Conclusions is rather small in this section, as it consists only of the first and the last paragraph and the rest is a discussion concerning limitations and of outlook. I would suggest merging the Discussion and the Conclusion in a single section, possibly with sub-sections (e.g., “Main findings”, “Discussion (including limitations)”, “Outlook”, …).
Technical/Grammar/Typos
Line 24: “a” ridge-trough couplet?
Lines 112-114: do the authors mean “features”?

Bibliography
Brannan, A. L., and J. M. Chagnon, 2020: A Climatology of the Extratropical Flow Response to Recurving Atlantic Tropical Cyclones. Mon. Wea. Rev., 148, 541–558, https://doi.org/10.1175/MWR-D-19-0216.1.
Federer, M., et al.: Synoptic perspective on the conversion and maintenance of local available potential energy in extratropical cyclones, Weather Clim. Dynam., 6, 211–230, https://doi.org/10.5194/wcd-6-211-2025 , 2025.
Pohorsky, R., et al., 2019: The Climatological Impact of Recurving North Atlantic Tropical Cyclones on Downstream Extreme Precipitation Events. Mon. Wea. Rev., 147, 1513–1532, https://doi.org/10.1175/MWR-D-18-0195.1.
Riboldi, J., et al., 2019: A Phase Locking Perspective on Rossby Wave Amplification and Atmospheric Blocking Downstream of Recurving Western North Pacific Tropical Cyclones. Mon. Wea. Rev., 147, 567–589, https://doi.org/10.1175/MWR-D-18-0271.1.
Citation: https://doi.org/10.5194/egusphere-2026-5018-RC2

Chenhui Jin, Elizabeth A. Ritchie, Michael A. Barnes, and Neil J. Holbrook

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Chenhui Jin, Elizabeth A. Ritchie, Michael A. Barnes, and Neil J. Holbrook

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Short summary

This study investigates how the extratropical transition (ET) of tropical cyclones influences downstream weather in the Southern Hemisphere. Two main ET downstream developments are identified: a propagating Rossby wave train along a strong, straight jet, and a more localised downstream ridge associated with a weaker, poleward-oriented jet. The strong ET events in the southwest Indian Ocean can increase the occurrence of high-impact weather over Australia.


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