Subseasonal Predictability and Rossby Wave Dynamics of Blocking High during Transitional Seasons: Insights from Three Successive Events in May&ndash;June 2023

Li, Zhixiang; Lu, Jianhua; Liu, Yimin

doi:10.5194/egusphere-2026-1156

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https://doi.org/10.5194/egusphere-2026-1156

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23 Mar 2026

| 23 Mar 2026

Subseasonal Predictability and Rossby Wave Dynamics of Blocking High during Transitional Seasons: Insights from Three Successive Events in May–June 2023

Zhixiang Li, Jianhua Lu, and Yimin Liu

Abstract. Extended-range prediction during the transitional seasons remains particularly challenging due to the volatile large-scale circulation background. This study investigates the dynamical linkages, Rossby wave characteristics, and subseasonal predictability of three successive atmospheric blocking events in May–June 2023, which contributed to severe Canadian wildfires, anomalous East Asian precipitation, and European heatwaves. The results indicate that the three blocking episodes are not independent but interconnected through downstream propagation of quasi-stationary Rossby wave energy. Spatiotemporal local diagnostics of phase speed, amplitude, and zonal wavenumber further reveal that all episodes are dominated by slowly propagating, large-amplitude planetary-scale Rossby waves in the troposphere, with zonal wavenumbers typically below the climatological zonal wavenumber. Day-to-day evolution shows abrupt transitions during blocking onset from an eastward-propagating synoptic-scale small-amplitude regime to a quasi-stationary or westward-propagating planetary-scale large-amplitude regime, with the reverse during blocking’s decaying stage. ECMWF ensemble forecasts exhibit high predictability of 500 hPa geopotential height at 15–19-day lead times, with the best performance for the Ural blocking, where skillful members capture both the amplification of wave amplitude and spatial scale. In contrast, forecasts for the Canadian and European blocking show limited growth in wave parameters and associated geopotential height. For all episodes, skillful subseasonal predictions depend on capturing upstream quasi-stationary troughs over the North Pacific or North Atlantic, potentially influenced by exceptionally high sea surface temperatures in these basins during May–June 2023. These findings underscore that the growth in both amplitude and scale contributes to forecast errors in blocking circulation, while upstream wave precursors and external boundary forcing provide key sources of subseasonal predictability for persistent blocking circulation.

Received: 28 Feb 2026 – Discussion started: 23 Mar 2026

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Zhixiang Li, Jianhua Lu, and Yimin Liu

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-1156', Anonymous Referee #1, 16 Apr 2026
A review of EGUsphere-2026-1156 (Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: Insights from three successive events in May–June 2023) by Zhixiang Li et al. (2026)
Recommendation: Major revisions
Li et al. (2026) takes a case study approach to examining three consecutive blocking events across the Northern Hemisphere in May and June 2023. Through the calculation of certain blocking diagnostics, the paper helps to establish a dynamic connection between the three successive events. In the second half of the paper, the predictability of these events is examined using the ECMWF subseasonal-range ensemble prediction system.
The paper is interesting and offers a nice foundation for a climatological assessment of similar dynamically linked blocking events. The scope and analysis are of high quality, but I find that there is insufficient context that motivates many of the analysis choices and how the motivation of extreme weather is incorporated into the main paper. I think with additional context, this will be a highly successful paper.
General comments
While I am familiar with the metrics and calculations used in this paper (wave activity flux, stationary wavenumber, Tibaldi and Molteni blocking index), there is little context for what these metrics show and what the benefits/limitations of these metrics are. For example, what should the reader take away from the observed zonal wavenumber (k) falling below the stationary wavenumber (K_s)? I think a better description of how these metrics help us to understand the dynamics of these events could improve the paper greatly.

The figures in this paper, especially figures 3 and 4, need to be enlarged for ease of viewing. Much of the results of section 3.1 rely on interpretation of these figures, and the WAF vectors are very difficult to see.

Outside of the introduction, there is very little mention of the connection between the three blocking events and the sensible weather extremes of Canadian wildfires, extreme precipitation in Italy and East Asia, and a heatwave over Northwest Europe. There is also very little literature cited that looks at the dynamic connection between blocking and these weather extremes outside of these specific events. Adding this context would help to connect this work to the broader literature on blocking better.

In section 3.3, the predictability and potential sources of said predictability in the ECMWF ensemble forecasts are discussed. Given that this paper emphasizes the dynamical connection between the three events, could the enhanced predictability of the Ural block in late May be linked to the blocking over Canada in early May? For example, do the best ensemble members for the Ural block successfully capture the block over Canada? This is not a necessary edit, but I wanted to suggest it as a possibility.

Line-by-line comments
Lines 32–33: The three blocking episodes are not defined before being mentioned here

Line 48: “Northern Hemisphere” should be capitalized

Lines 169–172: The timing and occurrence of the blocking events have only been alluded to at this point through Figure 1. It might make sense to more explicitly state when and where the blocks occurred, and how they are temporally related to the weather extremes mentioned in lines 53–63.

Line 223: What is meant by “scale effects” here?

Line 384: You mentioned that k<K_s, but there is not much context for the significance or interpretation of this fact. Tying into my first general comment, more text discussing the implications of your calculations would be helpful.

Line 401: How is “meaningful” benchmarked in terms of predictability? Probabilities of blocking also fell in subsequent forecasts, so is the enhanced predictability just random change?

Line 402: Change “ensembles” to “ensemble members”

Lines 425–435: If SSTs are considered a possible driver of planetary-scale wave structures, why were they not examined in the ECMWF ensemble?

Lines 429–431: I am not sure how your results “highlight the importance of midlatitude SST anomalies for the evolution of circulation patterns” when the analysis is only mentioned at the very end of the summary and discussion section.
Citation: https://doi.org/10.5194/egusphere-2026-1156-RC1
- AC1: 'Reply on RC1 and RC2', Jianhua Lu, 06 Jun 2026
  
  The replies to both RC1 and RC2 are includes in the attached file, which indicts what and how we shall revise the manuscript after the editor makes a decision on the invitation for submitting the revision.
  
  Citation: https://doi.org/10.5194/egusphere-2026-1156-AC1
RC2:
'Review of Subseasonal Predictability and Rossby Wave Dynamics of Blocking High during Transitional Seasons: Insights from Three Successive Events in May–June 2023', Anonymous Referee #2, 11 May 2026
Recommendation: Major Revisions
General Comments:
This study analyzes three consecutive blocking events that occurred in May-June 2023. Li et al. (2026) present a novel analysis of each event through the diagnosis of Rossby wave parameters, as calculated using a Hilbert transformation. Through this, the authors successfully demonstrate the interconnected nature of these three events. Additional work is shown to examine the predictability of the three blocking events using the ECMWF S2S model. They show maxima in predictability for each event at 15-19 day lead times and shortly before the initiation of each blocking event.
The analysis presented, especially those of Rossby wave parameters, are excellent and do well to establish a useful perspective on the temporal and spatial evolution of blocking events. My primary concern for improving this manuscript is the need for additional contextualization of the methodology utilized. Additionally, there is a real need to reevaluate several figures, both in their visual presentation and the utilization of the analysis within the text. Further, the motivation for this study could use bolstering. Upon revision, I believe this study would prove an excellent addition to literature of atmospheric blocking.
Specific Comments:
Many figures are difficult to decipher due to their size within the manuscript, specifically Figs. 1, 3, and 4. Figure 3 is far too small, especially given how critical this figure proves to be for the analysis presented in sections 3.1 and 3.2. It is very difficult to distinguish the WAF vectors from the stippling in the left column. One other potential improvement to Figure 3 could be the contouring of WAF convergence, given that this quantity proves important to the interpretation of the figure. If this makes the figure too busy and less legible, it would be understandable to leave out.

1 is seemingly the motivation for the study, as it shows the extreme impacts of the three blocking events; however, these results are utilized throughout the study. There is little reference to either Fig. 1 or to literature to specifically tie the blocking events to the weather extremes. How is the blocking event in Canada explicitly connected to the wildfires? What connects the blocking event in the Ural Mountains to the anomalous precipitation in East Asia? What dynamics force the European heatwave? I do not mean to suggest more analysis needed; rather, the text needs to explicitly address these questions through references to Fig. 1 and literature.

In the current form of the manuscript, references to SST add minimal to the analysis produced. If the authors intend to use SST as a source of predictability, the current analysis should be expanded. This should likely be included in the results section, following the discussion of predictability in the S2S model, rather than at the end of the summary and discussion.

The mentions of WAF and K_s could use additional physical contextualization. While these methods are often encountered in other studies, the present work would be improved with a brief discussion in the methods section or results of how these two measures are used to bolster the compelling evidence presented from the Rossby wave dynamics analysis.

2b-c are only referenced once within the text, despite showing an interesting result. Based on this figure, there is a key difference in episode 2, relative to the other two periods, as the primary blocking signal seems to be dominated by higher wavenumber variations, whereas episodes 1 and 3 are more attributable to planetary scale waves.

Why was 2.5-degree resolution used for ERA5 data? It’s my understanding that native resolution is .25-degree. This is a minor concern, but I am just curious if this was a data choice or if the data were regridded and, if so, why.

The paragraph at the end of section 3.3 is structured in a confusing manner to me. I think it would make more sense to discuss the predictability of the 3 episodes in the order in which they occurred, rather than beginning with episode 2.

Technical Corrections:
The grey lines in Figs. 5 and 6 are very difficult to make out for me. Perhaps these could be darkened slightly.

The previous studies mentioned in L57-58 should be cited.

It would be worthwhile to cite literature on the interactions between planetary- and synoptic-scale waves at L228.

The vertical levels of ERA5 data utilized should be mentioned at L107.

A number of minor word choice suggestions or grammatical corrections are listed below:
Insert ‘the’ at the end of L48

Use ‘is’ instead of ‘are’ in L61

Insert ‘the’ at beginning of L78

The phrase ‘scale itself’ at the end of L82 is slightly awkward

Use ‘has’ instead of ‘have’ in L94

Use ‘shifts’ instead of ‘shifted’ at L237

Use ‘approaches’ instead of approaching at L241

Use ‘occurring’ instead of occurred at L245

Use ‘predict’ instead of predicting at L310

Use ‘research’ or ‘literature’ or similar instead of ‘researches’ at line 416

Insert a comma after ‘amplitude’ on L417
Citation: https://doi.org/10.5194/egusphere-2026-1156-RC2

Zhixiang Li, Jianhua Lu, and Yimin Liu

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Short summary

This work studies the dynamical linkages, wave parameters, and subseasonal predictability of three successive atmospheric blocking events that are associated with continual extreme events during May–June 2023. We suggest that these interconnected blocking events are dominated by the regime of slowly propagating, large-amplitude planetary-scale waves. The skillful subseasonal predictions depend on capturing upstream quasi-stationary troughs, highlighting sources of subseasonal predictability.


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