| 23 Jan 2026
Synoptic climatology of extratropical transition of tropical cyclones over the Southern Hemisphere
Abstract. Tropical cyclones that move into the midlatitudes undergo changes in their structure and transition into extratropical cyclones. The process is known as extratropical transition (ET). ET can result in severe weather locally and also affect the weather downstream. Although the importance of ET has been recognised primarily in the Northern Hemisphere, there are only a handful of studies focusing on the Southern Hemisphere. The current study conducts a comprehensive synoptic-climatological analysis of ET over the Southern Hemisphere. We use a state-of-the-art low-pressure system detection and classification scheme to objectively track tropical cyclones and detect those that undergo ET based on ERA5 data. Our results show that ET preferentially occurs in the southwest Indian Ocean, off the northwest coast of Australia, and in the southwest Pacific. The ET fraction is higher in March–May and lower in January and February, and the latitude of ET also changes strongly with season. The observed seasonality is associated with meridional shifts in the large-scale circulation and sea surface temperature pattern. The changes in structural characteristics and background environment during ET are investigated via cyclone-centred composites. In general, the transitioning cyclone lies on the equatorward side of the jet entrance, with an upper-level trough approaching from the west and a ridge developing downstream. Highly asymmetric fields of vertical velocity and equivalent potential temperature advection are indicative of warm, moist, ascending (cold, dry, descending) air to the east (west), responsible for an increasingly asymmetric precipitation pattern. Case-to-case variability of the ET structure is further examined by clustering ET events into four clusters. In particular, Clusters 2 and 3 feature the transitioning cyclone with a relatively strong intensity and high precipitation, accompanied by enhanced latent heat release in its southeastern sector. In the upper troposphere, the cyclone-associated divergent outflow impinges on the waveguide and enhances the potential vorticity gradient, leading to downstream jet streak formation and contributing to ridge development.
General Comments
The manuscript provides a valuable and thorough synthesis of extratropical transition (ET) in the Southern Hemisphere (SH). While the methodology is robust and the literature review is extensive, the manuscript would benefit from structural reorganizations in the Introduction, Methodology, and Results sections to improve the narrative flow and logical progression. Below are detailed suggestions to address these structural issues, along with specific comments regarding figures and data interpretation.
Major Comments & Structural Suggestions
1. Introduction Structure and Narrative Flow
The Introduction provides an excellent and thorough synthesis of existing ET literature. However, the narrative arc currently struggles with structural flow. Specifically, the establishment of the central knowledge gap—that SH ET cases are understudied compared to the Northern Hemisphere (NH)—is introduced and reiterated multiple times, causing the text to loop back on itself and disrupt the logical progression.
For instance, the lack of SH studies is explicitly stated in Line 57 ("However, most studies focus on basins in the Northern Hemisphere..."), repeated in Line 75 ("There are substantially fewer studies of ET in the Southern Hemisphere..."), and stated a third time in Line 95 ("To our knowledge, only a handful of studies have investigated the climatology..."). Furthermore, the text jumps from SH/NH case studies (Lines 35–55), to the SH knowledge gap (Lines 56–59), back to NH basin climatologies (Lines 60–74), and then back to the SH gap (Lines 75–89).
Recommendation: To strengthen the narrative arc and improve readability, reorganize the Introduction into the following sequence: 1) Broad Concept & Physics, 2) Hazards & Downstream Impacts, 3) Established NH Climatologies, 4) The Knowledge Gap & Existing SH Literature, and 5) Methodological Gap & Paper Objectives.
2. Methodology Organization
ET Detection Flow: The methodology used for ET detection is robust, but the flow of Section 2.3 is slightly disjointed. The text goes through the conceptual definitions of the new parameters, the quantitative thresholds, a specific case study (TC Debbie), the total event count, and then the traditional Cyclone Phase Space (CPS) framework. Since CPS parameters are the foundation of past ET work and are analyzed later in this study, the introduction to this section could start with CPS to help ground readers first. Then, the RH-DeepShear method could be introduced and discussed within that established context.
Clustering Methodology Placement: The paragraphs detailing the clustering methodology (Lines 289–302) currently reside in the Results section. These should probably be moved to the Methods section. In Lines 293–296, the authors mentioned that the optimal cluster number (K=4) was determined via the silhouette score and Davies-Bouldin index, but state "(figures not shown)." To ensure transparency and traceability, this reviewer recommends adding these validation plots to a Supplementary Material document.
3. Results Section Structure
The current placement of Section 3.2 ("Precipitation and wind") is out of place and disrupts the logical flow of the results. It separates related concepts and would be better consolidated with other lat-lon composites (currently in Section 3.3.2).
Recommendation: Consider adopting the following structure: 3.1 Climatology and seasonality; 3.2 Mean Characteristics (3.2.1 Evolution of cyclone structure and background environment and 3.2.2 Composite analyses of ET structure (consolidating current 3.2 and 3.3.2 here); 3.3 ET Morphologies & Clusters.
Specific and Minor Comments
1. Line 191 & Figure 4: The SST contours are a bit dense, making it hard to see the 26°C contour lines. They could be thickened or highlighted in other ways.
2. Lines 238–239: The mean values of B and low-level shear are consistent with the ET definition using the CPS. However, their spreads around the critical thresholds suggest the ETs detected via RH-DeepShear are not perfectly aligned with the ETs identified with the CPS (Line 239). A note explaining why would add physical depth here. Additionally, this would be a good place to discuss why the CPS method (N=600; Line 164) yields ~20% more ETs than the RH-DeepShear method (N=506; Line 153).
3. Section 3.4.3 Title: The section title ("Mechanisms") is vague. Please consider specifying whether it describes the mechanisms of extratropical transitions or downstream impacts.
4. Lines 395–396: "The reduced divergent flow…" The text refers to subplots of C3 instead of C1. Please check whether the right plots were referred to.
5. Cluster Summary: It might be highly beneficial to provide a summary table that consolidates the key findings and characteristics of the four distinct clusters.
6. Lat-lon Composite Analyses (General): Could the statistically significant anomalies be highlighted in these lat-lon composite plots?
7. Figure 2: The track density appears quite smooth for an analysis using 1.25-degree grid boxes. Was any smoothing applied to this data? Please clarify in the text or caption.
8. Figure 5: ERA5 data are known for their biases (e.g., 10-m wind speeds being too low for TCs) and limitations (e.g., precipitation not being directly constrained by observations). It would be helpful to briefly acknowledge these issues in the text.
9. Figure 6i and 10: Consider rotating this plot by 90 degrees so that its x-axis is aligned with the other subplots in the figure.
10. Figure 8: Could the lead-lag composites be extended to include adjacent time steps, similar to the other composite plots? Also, please specify in the caption whether the thick magenta contours indicate -2 PVU.
11. Figure 10: Are there any reasons for excluding SST and DeepShear from this figure, unlike Figure 6?
12. Figure 13: Please specify the vertical level of the divergent flow in the caption or text.