Sorting sudden stratospheric warmings with the downward tropospheric influence using ERA5 and CESM2-WACCM

Lu, Rongzhao; Rao, Jian

doi:10.5194/egusphere-2025-1178

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https://doi.org/10.5194/egusphere-2025-1178

Preprints

11 Jul 2025

| 11 Jul 2025

Sorting sudden stratospheric warmings with the downward tropospheric influence using ERA5 and CESM2-WACCM

Rongzhao Lu and Jian Rao

Abstract. Sudden Stratospheric Warming events (SSWs) can have a downward impact on the troposphere, but the mechanism remains uncertain. To improve the understanding of this uncertainty, this study explores the characteristics of SSWs that have different impacts on the troposphere. Using the ERA5 data and CESM2-WACCM outputs, 52 SSWs are identified in ERA5 and 273 in CESM2- WACCM, with 33 and 119 downward-propagating SSWs (DWs), respectively. The DWs are classified into three types based on cold surges over Eurasia (EA), North America (NA), and both (BOTH), respectively. Both DWs and non-downward-propagating SSWs (NDWs) correspond to weakened and/or deformed the polar vortex, but DWs induce stronger negative Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) responses. For DWs, the anomalous high develops in the polar region, which deflects to lower latitudes, consistent with the frequent appearance of the polar high and the midlatitude blockings. The shape of the anomalous polar high varies with the DW type, and the extension and shift of the anomalous high lead to different surface responses. The DWs are also accompanied by a southward shift of the precipitation belt, especially over the oceanic and coastal regions. NDWs show relatively weaker impact on the troposphere, which is primarily related to the weaker stratospheric disturbance amplitude. The three types of DWs differ in spatiotemporal evolutions of the NAM and NAO pattern, different forcing by planetary waves, and varying ratios between displacement and split. This study reveals the diversity of the DWs and distinguishes their potential impacts on both continents in the Northern Hemisphere.

Received: 13 Mar 2025 – Discussion started: 11 Jul 2025

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Journal article(s) based on this preprint

16 Mar 2026

Sorting sudden stratospheric warmings with the downward tropospheric influence using ERA5 and CESM2-WACCM

Rongzhao Lu and Jian Rao

Atmos. Chem. Phys., 26, 3723–3742, https://doi.org/10.5194/acp-26-3723-2026,https://doi.org/10.5194/acp-26-3723-2026, 2026

Short summary

Rongzhao Lu and Jian Rao

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1178', Anonymous Referee #1, 06 Aug 2025
This study investigates how different types of downward-propagating sudden stratospheric warming (SSW) events are associated with distinct regional surface cold extremes and classifies them based on their surface impacts, using reanalysis data and CESM-WACCM simulations. According to the abstract, introduction and conclusion, the paper aims not only to classify cases of surface cold extremes associated with SSWs, but also to explain the mechanisms behind how different types of surface cold extremes are caused by difference in SSWs. To this end, the study presents a comprehensive analysis using both observations and CESM-WACCM. One of the main contributions of the paper is the classification of downward-propagating SSW events based on the regional characteristics of the associated surface cold extremes, followed by a global analysis of the related dynamical fields. However, the main analyses are mostly descriptive, such as “this type tends to have these features,” without sufficient efforts to clarify the causal connections between processes. This creates a mismatch between the intended positioning of the paper and the actual description of the results, which may confuse readers about the central message of the paper. Moreover, even if the main purpose was the classification of SSW types, there are places where the descriptions overinterpret differences between the types that may naturally result from the way they are classified.
Nevertheless, I believe the paper has potential, since the analyses are extensive and well-organized. It would be publishable as a descriptive classification study if the authors revise the manuscript to focus on the meaningful dynamical differences revealed by the classification, while excluding the differences that naturally arise from the way the types are defined.

Major Comments
Unclear objective: classification or mechanism?
If the goal is to understand the cause of different SSW surface impacts, the paper needs to move beyond descriptive comparisons and provide a clearer explanation of how variables interact across steps. If the goal is to classify the types, the paper should clarify up front that it is intended as a classification study based on observed differences.

Insufficient interpretation and unclear physical connections
The authors should avoid wording that may be misinterpreted as implying a causal relationship between features that simply co-occur. It would be helpful to clarify which parts of the discussion are supported by physical reasoning and which are more descriptive.
Some differences reported in the results seem to directly stem from how the cases were defined, but this point is not acknowledged. For example, the EA-type cold extremes are defined to cover a broader longitudinal range. Since the NAM index more reflects zonal-mean changes, it is not surprising that EA cases show a stronger and persistent NAM signal than NA cases. This might be a natural result of the classification.

The paper tends to overinterpret co-occurring patterns as physical links. The paper lists many local differences across types but does not sufficiently explain whether these patterns are meaningfully connected.

the paper lists a number of detailed local differences between SSW types and presents them as if they are physically meaningful, and this seems to depend on the statistical significance of the composite. However, it is not always evident whether these differences are statistically significant. There are figures which show that composite for NDW show widespread dotted areas of statistically significant values (e.g. Fig. 6d), and the statistical testing method is not clearly explained. This raises the concern that some of these apparent differences may not be meaningful physical distinctions, especially in relation to the difference in SSW downward impact.

The anomalies suggested as precursors are not clearly distinguished as being part of the SSW-related signal or independent tropospheric variability.

Methodology
Even if the main objective of the study was the type classification, more careful methodological design would be necessary. For example, EA DW cases already show strong cold anomalies before the SSW occurred, which raises questions about whether the cold extremes are truly caused by SSW. If the anomalies are computed based on a fixed climatology, long-term signals may be included. However, there is also no clear method for determining whether post-SSW cold events are actually caused by the SSW. This makes it difficult to rule out the possibility that some tropospheric anomalies (or anomalies of longer timescale) developed independently of the SSW.

How CESM-WACCM contributes to the overall interpretation is not clear. Is it simply to provide more cases, or to test mechanisms? For example, when CESM-WACCM and observational results differ, the physical meaning of the differences is not discussed. Although the study analyzes both ERA5 and CESM-WACCM, the interpretation is mostly based on ERA5.

Lack of structural and editorial refinement
Although the analysis is limited to the Northern Hemisphere, this is not explicitly stated in the methodology.

The structure of the methodology section also lacks consistency. For example, the CESM-WACCM model description is under the subsection “2.1 Reanalysis Data.”

While the study defines four geographical domains (Europe, Asia, the United States, and Canada) the classification later becomes just two types (NA and EA), without explanation of the criteria used to group these regions. Furthermore, there is no explanation for why those particular domains were chosen for each region.

A few grammatical errors are present in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-1178-RC1
- AC1: 'Reply on RC1', Rongzhao Lu, 10 Sep 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1178-AC1
RC2:
'Comment on egusphere-2025-1178', Anonymous Referee #2, 08 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1178-RC2
- AC2: 'Reply on RC2', Rongzhao Lu, 10 Sep 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1178-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1178', Anonymous Referee #1, 06 Aug 2025
This study investigates how different types of downward-propagating sudden stratospheric warming (SSW) events are associated with distinct regional surface cold extremes and classifies them based on their surface impacts, using reanalysis data and CESM-WACCM simulations. According to the abstract, introduction and conclusion, the paper aims not only to classify cases of surface cold extremes associated with SSWs, but also to explain the mechanisms behind how different types of surface cold extremes are caused by difference in SSWs. To this end, the study presents a comprehensive analysis using both observations and CESM-WACCM. One of the main contributions of the paper is the classification of downward-propagating SSW events based on the regional characteristics of the associated surface cold extremes, followed by a global analysis of the related dynamical fields. However, the main analyses are mostly descriptive, such as “this type tends to have these features,” without sufficient efforts to clarify the causal connections between processes. This creates a mismatch between the intended positioning of the paper and the actual description of the results, which may confuse readers about the central message of the paper. Moreover, even if the main purpose was the classification of SSW types, there are places where the descriptions overinterpret differences between the types that may naturally result from the way they are classified.
Nevertheless, I believe the paper has potential, since the analyses are extensive and well-organized. It would be publishable as a descriptive classification study if the authors revise the manuscript to focus on the meaningful dynamical differences revealed by the classification, while excluding the differences that naturally arise from the way the types are defined.

Major Comments
Unclear objective: classification or mechanism?
If the goal is to understand the cause of different SSW surface impacts, the paper needs to move beyond descriptive comparisons and provide a clearer explanation of how variables interact across steps. If the goal is to classify the types, the paper should clarify up front that it is intended as a classification study based on observed differences.

Insufficient interpretation and unclear physical connections
The authors should avoid wording that may be misinterpreted as implying a causal relationship between features that simply co-occur. It would be helpful to clarify which parts of the discussion are supported by physical reasoning and which are more descriptive.
Some differences reported in the results seem to directly stem from how the cases were defined, but this point is not acknowledged. For example, the EA-type cold extremes are defined to cover a broader longitudinal range. Since the NAM index more reflects zonal-mean changes, it is not surprising that EA cases show a stronger and persistent NAM signal than NA cases. This might be a natural result of the classification.

The paper tends to overinterpret co-occurring patterns as physical links. The paper lists many local differences across types but does not sufficiently explain whether these patterns are meaningfully connected.

the paper lists a number of detailed local differences between SSW types and presents them as if they are physically meaningful, and this seems to depend on the statistical significance of the composite. However, it is not always evident whether these differences are statistically significant. There are figures which show that composite for NDW show widespread dotted areas of statistically significant values (e.g. Fig. 6d), and the statistical testing method is not clearly explained. This raises the concern that some of these apparent differences may not be meaningful physical distinctions, especially in relation to the difference in SSW downward impact.

The anomalies suggested as precursors are not clearly distinguished as being part of the SSW-related signal or independent tropospheric variability.

Methodology
Even if the main objective of the study was the type classification, more careful methodological design would be necessary. For example, EA DW cases already show strong cold anomalies before the SSW occurred, which raises questions about whether the cold extremes are truly caused by SSW. If the anomalies are computed based on a fixed climatology, long-term signals may be included. However, there is also no clear method for determining whether post-SSW cold events are actually caused by the SSW. This makes it difficult to rule out the possibility that some tropospheric anomalies (or anomalies of longer timescale) developed independently of the SSW.

How CESM-WACCM contributes to the overall interpretation is not clear. Is it simply to provide more cases, or to test mechanisms? For example, when CESM-WACCM and observational results differ, the physical meaning of the differences is not discussed. Although the study analyzes both ERA5 and CESM-WACCM, the interpretation is mostly based on ERA5.

Lack of structural and editorial refinement
Although the analysis is limited to the Northern Hemisphere, this is not explicitly stated in the methodology.

The structure of the methodology section also lacks consistency. For example, the CESM-WACCM model description is under the subsection “2.1 Reanalysis Data.”

While the study defines four geographical domains (Europe, Asia, the United States, and Canada) the classification later becomes just two types (NA and EA), without explanation of the criteria used to group these regions. Furthermore, there is no explanation for why those particular domains were chosen for each region.

A few grammatical errors are present in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-1178-RC1
- AC1: 'Reply on RC1', Rongzhao Lu, 10 Sep 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1178-AC1
RC2:
'Comment on egusphere-2025-1178', Anonymous Referee #2, 08 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1178-RC2
- AC2: 'Reply on RC2', Rongzhao Lu, 10 Sep 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1178/egusphere-2025-1178-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1178-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Rongzhao Lu on behalf of the Authors (10 Sep 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Oct 2025) by Peter Haynes

RR by Anonymous Referee #2 (30 Oct 2025)

Suggestions for revision or reasons for rejection

I appreciate the authors’ efforts to respond to my comments and improve their manuscript, making it more accurate and clearer.
I have two comments.
1. I don’t think the authors actually answer some of my comments. For example, the comments below:
L212-L217: How do these different behaviors before SSWs determine, influence, or play a role in different types of downward impacts after SSWs?
Figure 7 and the relevant description
My question is about the physical mechanism of the coupling. However, the authors’ response is primarily descriptive and repeated as in their own text without any new insight provided. For example, “Consistent with” does not explain the HOW or the mechanism by which the pre-SSW conditions (the "different behaviors") determine, influence, or play a role in the downward impact. "low-level circulation is continuous" or "has not changed much." This is not a satisfying physical explanation.
It is fine if the authors didn’t have an answer to these questions. However, I think the authors should be cautious not to overstate their conclusions by implying a causal link where no physical mechanism explanation.
2. Figure 11 and its discussion describe the dynamical features in a chain. I am a bit confused whether the authors implied a causal chain with the black arrows that connect these features. I don’t think the causal link can be concluded based on the analyses in the paper. For example, how does the “joint action of wave-1 and wave-2” lead to “three centers in stratospheric height anomaly”? Figures 8-10 show cross-sections, which can only show the different features in the three types of DWs, but there is no analysis linking the wave patterns (spatial map) with the three centers in the height anomaly as shown in Figure 7. Again, I am not asking authors to perform more analyses, but I suggest the authors should be cautious not to overstate their conclusions by implying a causal link where no physical mechanism explanation.
I would recommend acceptance or a slight modification of Figure 11.

Hide

RR by Anonymous Referee #1 (09 Dec 2025)

ED: Reconsider after major revisions (10 Dec 2025) by Peter Haynes

AR by Rongzhao Lu on behalf of the Authors (07 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (01 Feb 2026) by Peter Haynes

My impression is that this version of the paper is now closer to resolving the general concerns of the referees -- specifically that the stated conclusions have been too strong and cannot be justified either by logical argument or by the evidence available. (One aspect was the identification of physical mechanisms, the other was interpreting as important aspects of the different classes identified that essentially resulted directly from the definition.)

In looking through the paper myself I have identified below some points that need to be addressed -- ranging from typographical errors to lack of clarity or overstatement. Please can you address these specific points and check the whole paper again carefully -- making changes where necessary -- bearing in mind the general concerns of the referees. Your paper will be more valuable if it is free of errors and if what has been achieved is clearly stated. Please provide a clear list of the changes that you have made, with justification if needed.

I then expect to accept the paper for publication.

Detailed comments:

49: should be Haynes et al 1991. It was easy for me to spot that but please check all other citations/references carefully -- there are others where similar error have been made.

69: why 'in observations' -- omit?

70: 'As a consequence, the tropospheric response signals vary in extent, area and scope.' -- this is an example of invoking causality in a vague manner. 'As a consequence of what' -- all we know is that the tropospheric response varies and -- from observations along we cannot be sure what the 'response' is.

82: 'dripping' -- many readers will not understand what this means -- later you refer to 'dripping paint' features in Baldwin and Dunkerton (2001) -- only then does it become clear what you mean.

87: 'This study is mainly concerned with two questions: (1) What causes the inter-case difference in the DW influence on the troposphere although by definition the NAM during DWs shows downward propagation to the lower troposphere? (2) What can we learn from the flavor identification for DWs?' -- as the referees have commented -- you cannot directly address (1) -- you cannot find a 'cause'. Then you need to explain what you mean by 'the flavor identification' -- this is the first time you have mentioned the term. Then you need to include a sentence that makes it clear while you cannot answer (1) you believe that the 'classification' -- i.e. the 'flavor identification' -- why use 2 different terms for the same thing? -- provides useful information that may in due course help to answer (1).

156: 'we further divide DWs based on the inland temperature anomalies' -- these are surface temperature anomalies? I don't believe that you have stated that explicitly.

270: 'random sample of 2000 winters' -- in WACCM presumably.

566: 'The mean intensity of NDW events in terms of the NAM and the circulation anomalies in the stratosphere and troposphere are nearly half weaker than DW events.' -- this comes back to the criticisms of the referees -- to what extent does this difference results from the difference in definition of NDW and DW events? Make it clear what does and what does not.

570: 'dipping' should be 'dripping'

573: 'An isentropic vorticity analysis reveals that' -- that should be 'potential vorticity analysis'.

635: 'Further, the distribution of various types of events (see Table 1) shows strong interdecadal variability in past decades. Whether this change is an internal climate variability or forced by global warming due to anthropogenic emission is still unknown, worth exploring in the future.' -- to be honest, the way in which the information is presented in Table 1 makes it very difficult to determine anything about interdecadal variability apart from the fact there is some.

Hide

AR by Rongzhao Lu on behalf of the Authors (03 Feb 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 Feb 2026) by Peter Haynes

AR by Rongzhao Lu on behalf of the Authors (06 Mar 2026)

Journal article(s) based on this preprint

16 Mar 2026

Sorting sudden stratospheric warmings with the downward tropospheric influence using ERA5 and CESM2-WACCM

Rongzhao Lu and Jian Rao

Atmos. Chem. Phys., 26, 3723–3742, https://doi.org/10.5194/acp-26-3723-2026,https://doi.org/10.5194/acp-26-3723-2026, 2026

Short summary

Rongzhao Lu and Jian Rao

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The downward impact of sudden stratospheric warming events (SSWs) on the troposphere is still controversial. We further classify downward-propagating SSWs (DWs) into three types that are followed by cold surges over Eurasia (EA), over North America (NA), and over both (BOTH), respectively. This study reveals the diversity of the DWs and distinguishes their potential impact on both continents in the Northern Hemisphere.


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