Tropical belt expansion and its impacts on climate variability in the Mediterranean from 1980 to 2022

Darrag, Mohamed; Jin, Shuanggen; Calabia, Andres; Samy, Aalaa; Radevski, Ivan; M. Radwan, Ali

doi:10.5194/egusphere-2025-6521

Preprints

https://doi.org/10.5194/egusphere-2025-6521

Preprints

28 Jan 2026

| 28 Jan 2026

Tropical belt expansion and its impacts on climate variability in the Mediterranean from 1980 to 2022

Mohamed Darrag, Shuanggen Jin, Andres Calabia, Aalaa Samy, Ivan Radevski, and Ali M. Radwan

Abstract. The Mediterranean region's climatic parameters are undergoing substantial changes driven by climate change and tropical belt expansion. This paper explores the long-term variability of climatic parameters over the Mediterranean region and their relationship with tropical belt expansion by employing observational and reanalysis data from January 1980 to December 2022 through multiple approaches including correlation, linear regression, singular value decomposition (SVD), wavelet coherence (WTC), and principal component analysis (PCA). The lapse rate tropopause height (LRT-H) exhibits upward trends of approximately 80.58 m/dec. Concurrently, the tropical belt shows poleward expansion of approximately 0.14 °/dec and 0.27 °/dec from both applied methods. The tropical edge latitudes (TELs) of both applied approaches are in phase and have a strong positive correlation with LRT-H, surface temperature, and tropospheric temperature while being of strong negative correlation and out of phase with precipitation. However, there is no substantial correlation for the Standardized Precipitation Evapotranspiration Index (SPEI) with TELs or other climatic parameters. The results of SVD analysis depict that the surface temperature and lower tropospheric temperature (TRP-1) have evident coupling with the TELs in the eastern Mediterranean. Furthermore, surface temperature and TRP-1 have significant increasing trends, which are abundant in the eastern and northern Mediterranean. On the other hand, both precipitation and SPEI exhibit downward trends. Over the Mediterranean, an apparent spatial and temporal variation of climatic parameters is observed. The results confirm the linkage between climatic parameters' variability, coherence, and covariability with the TELs variability patterns over the study area, especially in the eastern Mediterranean region.

Received: 29 Dec 2025 – Discussion started: 28 Jan 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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06 Oct 2026

Tropical belt expansion and its impacts on climate variability in the Mediterranean from 1980 to 2022

Mohamed Darrag, Shuanggen Jin, Andrés Calabia, Aalaa Samy, Ivan Radevski, and Ali M. Radwan

Atmos. Chem. Phys., 26, 14031–14050, https://doi.org/10.5194/acp-26-14031-2026,https://doi.org/10.5194/acp-26-14031-2026, 2026

Short summary

Mohamed Darrag, Shuanggen Jin, Andres Calabia, Aalaa Samy, Ivan Radevski, and Ali M. Radwan

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-6521', Anonymous Referee #1, 20 Feb 2026
This study investigates the connection of the tropical edge latitude towards climatic parameters, such as temperature, precipitation, and SPEI. The authors hereby focus on the Mediterranean as an important region, which is undergoing significant changes with ongoing anthropogenic climate change. Utilizing ERA5 reanalysis data and some observational data, the authors explore multiple statistical methods. Overall, this is a comprehensive and detailed study. However, the writing and figures require some refinements, as outlined below, this would significantly improve the clarity and readability of the paper.
General comments
Throughout the manuscript, but especially in the figure captions the investigation period “over the period January 1980 to December 2022” is over-repeated. For example in the caption of Fig. 1 it is stated 3-times, and in the caption of Fig. 2 it is mentioned 4-times. Reduce most of the repetitions, particularly in figure captions. Moreover in the figures when time series are shown, it is not necessary to repeat the investigation period in the axis labelling – it is comprehensible from the time-axis and it is also referred to in the text. Rather state the unit e.g., “Time (years)”.

The mention of seasons is not consistent e.g., in Lines 196-201 seasons are stated e.g., as “summer”, “winter”, whereas in Lines 265-276 e.g., “Summer (JJA)”, “Winter (DJF)”, and in Line 278 as “Summer”, “Winter”. I suggest to introduce the seasons in the beginning what is meant with e.g., “summer” or use the short form e.g., “JJA” throughout the text. It does not need to be mentioned every time. Also, seasons are never capitalized.

The authors state for multiple plots that “statistically significant trends at p < 0.05 at most of the grid points”, could you add some hatching in the respective plots to indicate the grid points for which this statement valid?

The authors should be more consistent with the formatting of their figures e.g., where the labels of the sub panels are located – sometimes they are at the top left whereas in other figures they are at the top right. Also the font size of the axis labels needs to be increased in all figures. And it should be considered to add coastlines in map plots for better geographical orientation.

Use evenly distributed ticks for the colorbars and when possible round numbers in all the figures. This would greatly improve plot readability. Also, when using diverging colorbars, the authors should purposefully set the center, for example at zero, so that the colors are diverging from negative to positive values (e.g., Fig. 11d: Negative values are shown in teal, yellow and in brown, the same brown is then used for positive trend results. This makes the plot very difficult to interpret.)

Be consistent with the post comma digits (significant digits) e.g., Line 268 its 38.83°, Line 269 29.2° and Line 275 41°. Another example is Line 273 “-0.07 ± 0.001”.

Throughout the manuscript there are many instances where the term “Figure” is used in text, although it should be “Fig.” (e.g., Line 211 “Figure 1e” should be “Fig. 1e”).

Some references are missing the DOI, please check.

Specific comments
Line 182: consider adding the period investigated from the other authors
Figure 2:
“TEL TPD” and “TEL TPG” should be “TPD TEL” and “TPG TEL”, check for consistency

In sub panels (c) and (d) remove the redundant tick marks that make it look like a barcode, this is what the distribution function is showing. Also, use the same tick marks for these two sub panels (i.e., x-axis: 25, 30, 35, 40, 45, 50).

Line 261 & Line 262: The abbreviation for standard deviation “SD” has already been introduced in Line 203.
Figure 3 & Figure 4:
Be consistent with the labelling, replace TEL-D with TPD TEL and TEL-G with TPG TEL.

Line 403-405: The figure caption is unnecessary and the repetition can be misleading. Refer to the caption of Figure 5. Suggestion: “As in Fig. 5, but for TPG TEL.”
Figure 5 & Figure 6:
“TEL TPD” and “TEL TPG” should be “TPD TEL” and “TPG TEL”, check for consistency

Add x-axis labelling and the respective units

Figure 7:
I suggest to have a uniform color range, so one can see how TPD TEL and TPG TEL correlate with the individual climatic parameters. Like this they are very hard to compare.

Consider explaining in the text why you are now also looking at TRP-1 and TRP-2. This seems like an interesting approach, but why was it not included in the other tests and figures?

Figure 8 & Figure 9:
As for Fig. 7, consider using a uniform range for the colorbar of the individual sub panels.

Line 496-500: Remove the repetitive figure caption. Refer to the caption of Figure 8. Suggestion: “As in Fig. 8, but for TPG TEL.”
Lines 552-560: This caption is very repetitive – please improve it.
Line 616: Consider discussing the lack of a significant relationship of SPEI.
Technical corrections
Line 39: “greenhouse gases (GHGs) emissions” should be “greenhouse gas (GHG) emissions”
Line 59: it looks like a upper case zero not a “°”
Line 69: add a “the” before “Hadley circulation”
Line 270: remove hyphen between TPD-TEL
Line 541, Line 544: “45° N-50° N” should be “45° N to 50° N”
Line 546: “20° N-30° N” should be “20° N to 30° N”
Line 595: “GHGs” should be “GHG”
Citation: https://doi.org/10.5194/egusphere-2025-6521-RC1
- AC1: 'Reply on RC1', Mohamed Darrag, 20 Apr 2026
  
  We thank Referee #1 for the careful review of our manuscript and for the constructive and insightful comments. These suggestions have helped us improve the clarity and quality of the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6521-AC1
RC2:
'Comment on egusphere-2025-6521', Anonymous Referee #2, 22 Feb 2026
This paper investigates the seasonality and annual trend of the tropical belt edge over Mediterranean, giving insights on the local impact of climate change. However, the analyzed data are interpolated from pressure-level ERA5 to a 100m vertical resolution, which requires further justification. The writing also needs significant improvement, as there are many repetitions throughout the manuscript.
Major concerns:
In session 2.3, pressure-level ERA5 data are interpreted to 100m resolution to calculate the tropopause. I’m wondering how accurate the interpolated data are. The cited paper (Hindley et al., 2015) used COSMIC data, which has a much higher vertical resolution than pressure-level ERA5. Have you tried using 137 model level ERA5 data and see if there is any difference in calculating the tropopause?

“TPD” and “TPG” have very high correlation (0.9 shown in figure 3) and they showed very similar results. Session 3.2 have a lot of descriptions about what it’s shown in the figures from both methods. Figure 5 and Figure 6 are very similar. Left and right columns in Figure 7 are similar. Figure 8 and Figure 9 are similar. It looks redundant to me to include the discussion for both methods.

There are many abbreviations throughout the paper. Reducing them may improve readability.

Line-by-line comments:
Lines 24-25: “both applied methods” is confusing because the methods are not stated earlier. I suggest revising this sentence to something like “the tropical belt shows poleward expansion ranging approximately between 0.14 °/dec and 0.27 °/dec.”
Line 25: “of both applied approaches” seems redundant.
Line 40: “climate change has been documented at a magnitude that exceeds global averages” is very broad. Does climate change here means temperature, precipitation, or something else?
I notice this sentence is rephrasing “As mentioned before, Mediterranean climate change has been observed at a magnitude exceeding global means, despite the fact that the emission of greenhouse gases (GHGs) in Mediterranean countries lie at relatively low levels” from the reference. In this paper, this sentence is missing context. I’m not sure if this sentence is a good one to put at the beginning.
Lines 47-48: This sentence repeated “the Mediterranean region has experienced significant shifts of climate in the past” in Line 45.
Lines 59-69: There is no subject in this sentence. A suggestion is “In astronomy and cartography, the tropical boundaries are at latitudes of ~23.5⁰ north and south”.
Lines 63-64: A better flow is needed. The previous sentence is about the seasonal change and “Tropical belt expansion” in this sentence is due to climate forcing. A suggestion is adding “Under climate change,” at the beginning of the sentence.
Lines 66-67: “sea-surface temperature (SST) variability over the tropics” and “ absorbing aerosols hemispheric asymmetry” don’t appear to be the changes driven by the climate change.
Lines 71-73: “even slight expansion of the tropical belt could cause substantial changes in the global climate system” repeats “The expansion of the tropics could cause substantial changes in the global climate system”.
Lines 76-77: I suggest revising “one of the most vulnerable regions to the effects of climate change” to “one of the most vulnerable regions affected by climate change”.
Line 79: “increasing” to “being”.
Line 82: Add “continue to” before “rise”.
Lines 98-99: “This accelerated…and biodiversity” repeats Lines 88-89.
Lines 115-131: Most of session 2.1 repeats the content already shown in the introduction. Please consider shortening or deleting session 2.1.
Lines 180-181: The captions in Figures are repeated in the test. Same in Lines 192-193.
Lines 181-183: Previous literature also shows an increasing trend, but the values presented in this paper are much larger.
Line 185: Add “in LRT-T also” after “our findings”.
Line 189: “consistent with earlier research” appears three times in this paragraph. Please consider revising to avoid repetition.
Line 207: “most of the grid points” is vague. Please consider stippling the regions where p<0.05. Same for line 213, line 510, line 530, line 570.
Figure 2: Repetition in the caption. For example, “Monthly time series of the TPD TEL method over the Mediterranean region for the period January 1980 to December 2022 (a) and monthly time series of the TPG TEL method over the Mediterranean region for the period January 1980 to December 2022 (b).” can be revised as “Monthly time series of the (a) TPD TEL method and (b) TPG TEL method over the Mediterranean region for the period January 1980 to December 2022”.
Line 244: “major tropical systems” is vague.
Line 250: Why stronger land-sea contrast leads to the following statement “resulting in higher TEL values in this region”?
Lines 283-284: “even minor expansion would have significant consequences” is repetition.
Figure 3: It will be more straightforward to put the correlation numbers on the top right panels.
Figure 10: Repetition in the description of panels.
Citation: https://doi.org/10.5194/egusphere-2025-6521-RC2
- AC2: 'Reply on RC2', Mohamed Darrag, 20 Apr 2026
  
  We would like to thank Referee #2 for the thorough review and valuable comments on our manuscript. We appreciate the reviewer’s constructive suggestions, which have helped us significantly improve the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6521-AC2

Peer review completion

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

AR by Mohamed Darrag on behalf of the Authors (20 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (26 Apr 2026) by Kevin Grise

RR by Anonymous Referee #2 (09 May 2026)

RR by Anonymous Referee #1 (13 May 2026)

ED: Reconsider after major revisions (15 May 2026) by Kevin Grise

The authors have satisfactorily addressed the reviewer comments. The reviewers have only minor technical comments on the revised manuscript.

However, the authors have not adequately addressed my initial editorial comments on the manuscript. Here is my original comment for reference:

“I would encourage the authors to clarify how the seasonal cycle is accounted for in the statistical analysis. To focus on climate variability and trends, most studies remove the seasonal cycle. The current figures (with the seasonal cycle retained in time series) make it difficult to see interannual variability and trends. The authors should justify that the strong relationship among variables documented here (such as in time series in Fig. 8 and 9) does not arise entirely from the inclusion of the seasonal cycle.”

In my interpretation, the seasonal cycle has still been retained in all of the statistical analyses in the manuscript, which greatly inflates the degree of co-variability among the respective variables. The following evidence shows the seasonal cycle has been retained in the results:

a) An annually periodic behavior in all time series shown in the paper (Figs. 1a-b, Figs. 2a-b, etc.), including in the right column of Figures 8 and 9 when the authors claim that they have removed the seasonal cycle (Line 164). Only the SPEI time series does not show a clear annual cycle.

b) Anomalies that have a clear seasonal cycle, with positive anomalies maximizing in one season and minimizing in another (Figs. 1c-d)

c) Wavelet analysis showing maximum coherence around 12 months

The authors need to be transparent to the reader about the effects of retaining the seasonal cycle in the results, particularly in the following instances:

1) Lines 184–196, Figures 1c-1d: The term anomalies must be clearly defined. Are these anomalies with respect to the long-term average (i.e., leaving the seasonal cycle in the data)? If so, then the anomalies predominantly highlight the seasonal cycle in these parameters, not natural variability and climate change (lines 184-185). The text should be adjusted to reflect that this is what is shown. If the authors are intending to remove the seasonal cycle from these figures (to highlight natural variability and climate change), I would strongly encourage them to double check their methods, as correctly calculated anomalies (about a seasonally varying climatology) do not have such a distinct seasonality noted on lines 187-193.

2) Figure 3: The authors need to be clear that the seasonal cycle is retained in these relationships, so two variables that both show an increase in summer may be highly correlated, even if they have no physical relationship with one another. For example, surface temperature shows a strong negative correlation with precipitation, as temperatures are warmer in summer and the Mediterranean is drier in summer. But, the reason the Mediterranean is drier in summer is not necessarily because it is warmer, but because the prevailing weather systems that drive rainfall predominantly occur in winter. A more meaningful correlation would be to assess the relationship between temperature and precipitation within an individual season (i.e., are warmer winters associated with less precipitation in winter?, are warmer summers associated with less precipitation in summer?).

3) Figures 5–6: Again, the authors need to be transparent that the seasonal cycle is retained in these relationships. Because the seasonal cycle is retained in this data, it is to be expected that there should be coherence among many of these variables at periods of around 12 months. More interesting results to discuss would be coherence at periods not associated with the seasonal cycle.

4) Figures 7-9: Again, the authors need to be clear that the seasonal cycle is retained in these relationships, as they are dominated by seasonality. The regular annual cycles in the time series in the right column of Figures 8 and 9 illustrate that the seasonal cycle is still retained in this analysis. I think the SVD analysis is simply just reflecting the annual cycle. If the authors simply plotted the magnitude of the annual cycle (DJF average – JJA average), I think they would find similar patterns.

My recommendation would be for the authors to make a version of Figure 3 and Figure 7 with the seasonal cycle removed to assess this impact on the results, as well as correct the results in Figures 8-9 to actually remove the seasonal cycle (as they have stated in the text). However, at a minimum, the authors need to acknowledge that the seasonal cycle dominates the results in their present form and discuss how the results would be different if the seasonal cycle were removed. If I am mistaken in my interpretation, the authors are welcome to provide definitive proof in a response that seasonality is not dominating their results and can disregard the comments above.

Other minor recommended editorial fixes:

Lines 60–61: Good to clarify here that the seasons noted correspond to the Northern Hemisphere tropical edge. The Southern Hemisphere tropical edge would expand poleward in DJF.

Line 69: This value is too large. The value quoted in the IPCC sixth assessment report is 0.1–0.5 degrees per decade. I have quoted the relevant passage below:

“Since AR5 several studies based upon a range of metrics and different reanalyses products have suggested that the annual mean HC extent has shifted poleward at an approximate rate of 0.1°–0.5° latitude per decade over the last about 40 years”

Figure 7: The term spatial correlation is misleading, as it typically refers to the similarity between two spatial maps. In this case, I think the authors are referring to temporal correlations on a grid point by grid point basis. If so, the authors should clarify their terminology.

Make sure the captions clearly explain all the symbols used in the figures:
• Figure 1: Light shading behind time series in panels a and b needs to be explained.
• Figure 2: Shading along trend line (panels a and b), PDFs in panels c and d, and all symbols in panels g and h need to be defined.
• Figure 3: Axes need to be labeled with proper units for all variables.
• Figures 5-6: Color bar should be labeled with appropriate quantity and units.
• Figures 8-9: Color bar should be labeled with appropriate quantity and units.
• Figures 10, 11: Light shading behind time series in left column needs to be explained.
• Figure 11: Color schemes are incorrect in right column. Browns/oranges should indicate drier conditions.

Proofread the following sentence for English grammar:
• Lines 256-258

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AR by Mohamed Darrag on behalf of the Authors (20 Jul 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Jul 2026) by Kevin Grise

RR by Anonymous Referee #2 (03 Aug 2026)

Suggestions for revision or reasons for rejection

The revised manuscript focuses more on the analysis of climatological patterns by removing some comparisons between different tropical belt methods. This paper provides insights on the widening tropical belt is changing Mediterranean studies under a warming climate.

Major Comments:
There appear to be unphysical vertical stripes in the left column of figures 4–7, suggesting that the values are uniform across latitude at each longitude. If these panels represent the spatial patterns of the first SVD mode, I would expect them to be two-dimensional latitude–longitude maps rather than one-dimensional patterns. Please verify the SVD calculation and plotting.

Minor Comments:
Line 155: Does Trp mean the “average” of temperature from the surface to the LRT level? Same for lines 267&268, are Trp-1&Trp-2 calculated as layer-averaged temperatures?
Lines 193-195: This paragraph starts by discussing the seasonal variability (max and min) in LRT-H and LRT-T over the decades. It’s not clear that this is then described as being consistent with the standard deviation of the monthly anomalies.
Line 203: The argument that “The LRT-T regression map reveals a spatial pattern opposite to that of LRT-H” doesn’t seem to hold between 40-50N, which accounts for nearly one-third of the study area. In figure 1f, temperature change between 40-50N is positive and insignificant, while LRT-H in figure 1e shows a significantly increase in the same area.
Line 231: Please clarify “TEL variability in the TPG method is higher than in the TPD method”. It’s hard to see the difference from the figures.
Lines 271-288: Could you expand the discussion of the correlation results? For example, why is the correlation between Ts and TPD TEL higher than that between Ts and TPG TEL?
Figure 8: The significance stippling appears to be identical in 8d and 8f. Is this expected, or could it be a plotting issue?

Hide

RR by Anonymous Referee #1 (07 Aug 2026)

Suggestions for revision or reasons for rejection

The authors have substantially improved the manuscript during their last revision. I recommend publication after addressing the minor (technical) revisions listed below.

• As there are many different acronyms in the manuscript I suggest considering adding a list of acronyms. This could tremendously help readers to better understand the text.
• Abstract: are all these acronyms really necessary in the Abstract? For example, “LRT-H”, “TPD”, “TPG”, “SVD1, and “SVD2” are introduced but never reused in the Abstract.
• Line 75: either write “in the ocean circulation” or “in ocean circulations”
• Line 125-127: Please check if this sentence is correct. Do you really employ multiple datasets of monthly temperature data on pressure levels from ERA5 to determine the lapse rate tropopause (LRT) parameters (height and temperature)? Which “multiple temperature datasets from ERA5” do you refer to? In case, additional information would be helpful.
• Line 149: Is the additional term “poleward” necessary? I suggest: “maximum LRT-H meridional gradient”. This is also more similar to how the cited papers phrase it.
• Line 193-195: This sentence only refers to LRT-H, so moving it right after your LRT-H result in Line 190 would improve the flow for the reader. If this sentence stays in its current place, I as a reader, miss a similar reference for LRT-T.
• Line 254: I suggest: “due to warming at the surface and in the troposphere”.
• Line 259-261: This sentence is really complicated to understand; please rephrase it.
• Line 445, 451, 452: remove the comma in “TPD, TEL” and “TPG, TEL”; I assume you mean “TPD TEL” and “TPG TEL”.
• Line 511: The sentence is grammatically not complete, my suggestion is: “reported that the Eastern Mediterranean and the Middle East are”.
• Line 513: The last part of the sentence (“and particular locations far more drastic changes.”) sounds vague, please improve or remove it.
• Line 514: It should be “shows” instead of “showing”.
• Line 529-531: This sentence sounds slightly misleading. Consider removing “over height ranges” to improve the clarity of the sentence.
• Line 553-557: This sentence is grammatically incorrect and very long, please improve.
• Line 572: Please remove “While” or rephrase the sentence so that it is grammatically correct.
• Line 586: Add “ patterns” after “LRT-H anomaly”.
• Line 587-588: The text says: “Moreover, LRT-H shows dominant upward trends over the study area, […]”. This appears repetitive with your newly added sentence in lines 585-587 where you state that LRT-H anomalies are predominantly positive which indicates a progressive rise.
• Line 617: remove “nearly”
• Line 641: remove the comma after variability

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ED: Publish subject to minor revisions (review by editor) (10 Aug 2026) by Kevin Grise

AR by Mohamed Darrag on behalf of the Authors (19 Aug 2026) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (19 Aug 2026) by Kevin Grise

I thank the authors for responding to my editorial comments on the revised manuscript, but they did not respond to the comments from the 2 reviewers on the revised manuscript. I have pasted them below for reference. Please address the reviewer comments below before the paper can be accepted for publication.

Reviewer 1

The revised manuscript focuses more on the analysis of climatological patterns by removing some comparisons between different tropical belt methods. This paper provides insights on the widening tropical belt is changing Mediterranean studies under a warming climate.

Major Comments:
There appear to be unphysical vertical stripes in the left column of figures 4–7, suggesting that the values are uniform across latitude at each longitude. If these panels represent the spatial patterns of the first SVD mode, I would expect them to be two-dimensional latitude–longitude maps rather than one-dimensional patterns. Please verify the SVD calculation and plotting.

Minor Comments:
Line 155: Does Trp mean the “average” of temperature from the surface to the LRT level? Same for lines 267&268, are Trp-1&Trp-2 calculated as layer-averaged temperatures?
Lines 193-195: This paragraph starts by discussing the seasonal variability (max and min) in LRT-H and LRT-T over the decades. It’s not clear that this is then described as being consistent with the standard deviation of the monthly anomalies.
Line 203: The argument that “The LRT-T regression map reveals a spatial pattern opposite to that of LRT-H” doesn’t seem to hold between 40-50N, which accounts for nearly one-third of the study area. In figure 1f, temperature change between 40-50N is positive and insignificant, while LRT-H in figure 1e shows a significantly increase in the same area.
Line 231: Please clarify “TEL variability in the TPG method is higher than in the TPD method”. It’s hard to see the difference from the figures.
Lines 271-288: Could you expand the discussion of the correlation results? For example, why is the correlation between Ts and TPD TEL higher than that between Ts and TPG TEL?
Figure 8: The significance stippling appears to be identical in 8d and 8f. Is this expected, or could it be a plotting issue?

Reviewer 2

The authors have substantially improved the manuscript during their last revision. I recommend publication after addressing the minor (technical) revisions listed below.

• As there are many different acronyms in the manuscript I suggest considering adding a list of acronyms. This could tremendously help readers to better understand the text.
• Abstract: are all these acronyms really necessary in the Abstract? For example, “LRT-H”, “TPD”, “TPG”, “SVD1, and “SVD2” are introduced but never reused in the Abstract.
• Line 75: either write “in the ocean circulation” or “in ocean circulations”
• Line 125-127: Please check if this sentence is correct. Do you really employ multiple datasets of monthly temperature data on pressure levels from ERA5 to determine the lapse rate tropopause (LRT) parameters (height and temperature)? Which “multiple temperature datasets from ERA5” do you refer to? In case, additional information would be helpful.
• Line 149: Is the additional term “poleward” necessary? I suggest: “maximum LRT-H meridional gradient”. This is also more similar to how the cited papers phrase it.
• Line 193-195: This sentence only refers to LRT-H, so moving it right after your LRT-H result in Line 190 would improve the flow for the reader. If this sentence stays in its current place, I as a reader, miss a similar reference for LRT-T.
• Line 254: I suggest: “due to warming at the surface and in the troposphere”.
• Line 259-261: This sentence is really complicated to understand; please rephrase it.
• Line 445, 451, 452: remove the comma in “TPD, TEL” and “TPG, TEL”; I assume you mean “TPD TEL” and “TPG TEL”.
• Line 511: The sentence is grammatically not complete, my suggestion is: “reported that the Eastern Mediterranean and the Middle East are”.
• Line 513: The last part of the sentence (“and particular locations far more drastic changes.”) sounds vague, please improve or remove it.
• Line 514: It should be “shows” instead of “showing”.
• Line 529-531: This sentence sounds slightly misleading. Consider removing “over height ranges” to improve the clarity of the sentence.
• Line 553-557: This sentence is grammatically incorrect and very long, please improve.
• Line 572: Please remove “While” or rephrase the sentence so that it is grammatically correct.
• Line 586: Add “ patterns” after “LRT-H anomaly”.
• Line 587-588: The text says: “Moreover, LRT-H shows dominant upward trends over the study area, […]”. This appears repetitive with your newly added sentence in lines 585-587 where you state that LRT-H anomalies are predominantly positive which indicates a progressive rise.
• Line 617: remove “nearly”
• Line 641: remove the comma after variability

Hide

AR by Mohamed Darrag on behalf of the Authors (21 Aug 2026) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (27 Aug 2026) by Kevin Grise

In my assessment, the SVD section of this manuscript still does not meet the standards for an ACP publication. The authors have made an effort to respond to Reviewer 1's major comment, but the response (on lines 401-404) does not make sense. The method and explanation of the results in this section remain difficult to understand. The SVD methodology needs to be accessible and understandable for the reader to interpret. I recommend that the authors:

1. More carefully describe the methodology: In my understanding, there should be one TEL value at each longitude, but the authors state that they are repeating TEL values along all longitudes for each timestep. I think they mean to say that they are using the same TEL value at each longitude (i.e., repeating the same TEL value at all latitudes for a given longitude).

2. Explain to readers how to interpret the coupled SVD maps in Figs. 4–7: The text in this section is entirely descriptive and does not attempt to explain to the reader how to interpret the patterns shown in Figs. 4-7. In other words, if the map in the left column of these figures shows a west-to-east gradient in TEL (Fig. 4a) and the map in the right column shows a spatially varying map (Fig. 4b), what does this mean? For example, stating in words that Fig. 4a shows southward TELs in the western Mediterranean and northward TELs in the eastern Mediterranean and then explaining the corresponding pattern in Fig. 4b and why it is linked to the west-to-east gradient in TEL. This certainly doesn't need to be done for every panel in Figs. 4–7, but providing the reader with a few examples on how to interpret these patterns would greatly improve the readability of this section.

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AR by Mohamed Darrag on behalf of the Authors (16 Sep 2026) Author's response Author's tracked changes Manuscript

EF by Polina Shvedko (17 Sep 2026) Manuscript Author's tracked changes

EF by Polina Shvedko (18 Sep 2026) Manuscript Author's tracked changes

ED: Publish as is (25 Sep 2026) by Kevin Grise

AR by Mohamed Darrag on behalf of the Authors (26 Sep 2026) Author's response Manuscript

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-6521', Anonymous Referee #1, 20 Feb 2026
This study investigates the connection of the tropical edge latitude towards climatic parameters, such as temperature, precipitation, and SPEI. The authors hereby focus on the Mediterranean as an important region, which is undergoing significant changes with ongoing anthropogenic climate change. Utilizing ERA5 reanalysis data and some observational data, the authors explore multiple statistical methods. Overall, this is a comprehensive and detailed study. However, the writing and figures require some refinements, as outlined below, this would significantly improve the clarity and readability of the paper.
General comments
Throughout the manuscript, but especially in the figure captions the investigation period “over the period January 1980 to December 2022” is over-repeated. For example in the caption of Fig. 1 it is stated 3-times, and in the caption of Fig. 2 it is mentioned 4-times. Reduce most of the repetitions, particularly in figure captions. Moreover in the figures when time series are shown, it is not necessary to repeat the investigation period in the axis labelling – it is comprehensible from the time-axis and it is also referred to in the text. Rather state the unit e.g., “Time (years)”.

The mention of seasons is not consistent e.g., in Lines 196-201 seasons are stated e.g., as “summer”, “winter”, whereas in Lines 265-276 e.g., “Summer (JJA)”, “Winter (DJF)”, and in Line 278 as “Summer”, “Winter”. I suggest to introduce the seasons in the beginning what is meant with e.g., “summer” or use the short form e.g., “JJA” throughout the text. It does not need to be mentioned every time. Also, seasons are never capitalized.

The authors state for multiple plots that “statistically significant trends at p < 0.05 at most of the grid points”, could you add some hatching in the respective plots to indicate the grid points for which this statement valid?

The authors should be more consistent with the formatting of their figures e.g., where the labels of the sub panels are located – sometimes they are at the top left whereas in other figures they are at the top right. Also the font size of the axis labels needs to be increased in all figures. And it should be considered to add coastlines in map plots for better geographical orientation.

Use evenly distributed ticks for the colorbars and when possible round numbers in all the figures. This would greatly improve plot readability. Also, when using diverging colorbars, the authors should purposefully set the center, for example at zero, so that the colors are diverging from negative to positive values (e.g., Fig. 11d: Negative values are shown in teal, yellow and in brown, the same brown is then used for positive trend results. This makes the plot very difficult to interpret.)

Be consistent with the post comma digits (significant digits) e.g., Line 268 its 38.83°, Line 269 29.2° and Line 275 41°. Another example is Line 273 “-0.07 ± 0.001”.

Throughout the manuscript there are many instances where the term “Figure” is used in text, although it should be “Fig.” (e.g., Line 211 “Figure 1e” should be “Fig. 1e”).

Some references are missing the DOI, please check.

Specific comments
Line 182: consider adding the period investigated from the other authors
Figure 2:
“TEL TPD” and “TEL TPG” should be “TPD TEL” and “TPG TEL”, check for consistency

In sub panels (c) and (d) remove the redundant tick marks that make it look like a barcode, this is what the distribution function is showing. Also, use the same tick marks for these two sub panels (i.e., x-axis: 25, 30, 35, 40, 45, 50).

Line 261 & Line 262: The abbreviation for standard deviation “SD” has already been introduced in Line 203.
Figure 3 & Figure 4:
Be consistent with the labelling, replace TEL-D with TPD TEL and TEL-G with TPG TEL.

Line 403-405: The figure caption is unnecessary and the repetition can be misleading. Refer to the caption of Figure 5. Suggestion: “As in Fig. 5, but for TPG TEL.”
Figure 5 & Figure 6:
“TEL TPD” and “TEL TPG” should be “TPD TEL” and “TPG TEL”, check for consistency

Add x-axis labelling and the respective units

Figure 7:
I suggest to have a uniform color range, so one can see how TPD TEL and TPG TEL correlate with the individual climatic parameters. Like this they are very hard to compare.

Consider explaining in the text why you are now also looking at TRP-1 and TRP-2. This seems like an interesting approach, but why was it not included in the other tests and figures?

Figure 8 & Figure 9:
As for Fig. 7, consider using a uniform range for the colorbar of the individual sub panels.

Line 496-500: Remove the repetitive figure caption. Refer to the caption of Figure 8. Suggestion: “As in Fig. 8, but for TPG TEL.”
Lines 552-560: This caption is very repetitive – please improve it.
Line 616: Consider discussing the lack of a significant relationship of SPEI.
Technical corrections
Line 39: “greenhouse gases (GHGs) emissions” should be “greenhouse gas (GHG) emissions”
Line 59: it looks like a upper case zero not a “°”
Line 69: add a “the” before “Hadley circulation”
Line 270: remove hyphen between TPD-TEL
Line 541, Line 544: “45° N-50° N” should be “45° N to 50° N”
Line 546: “20° N-30° N” should be “20° N to 30° N”
Line 595: “GHGs” should be “GHG”
Citation: https://doi.org/10.5194/egusphere-2025-6521-RC1
- AC1: 'Reply on RC1', Mohamed Darrag, 20 Apr 2026
  
  We thank Referee #1 for the careful review of our manuscript and for the constructive and insightful comments. These suggestions have helped us improve the clarity and quality of the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6521-AC1
RC2:
'Comment on egusphere-2025-6521', Anonymous Referee #2, 22 Feb 2026
This paper investigates the seasonality and annual trend of the tropical belt edge over Mediterranean, giving insights on the local impact of climate change. However, the analyzed data are interpolated from pressure-level ERA5 to a 100m vertical resolution, which requires further justification. The writing also needs significant improvement, as there are many repetitions throughout the manuscript.
Major concerns:
In session 2.3, pressure-level ERA5 data are interpreted to 100m resolution to calculate the tropopause. I’m wondering how accurate the interpolated data are. The cited paper (Hindley et al., 2015) used COSMIC data, which has a much higher vertical resolution than pressure-level ERA5. Have you tried using 137 model level ERA5 data and see if there is any difference in calculating the tropopause?

“TPD” and “TPG” have very high correlation (0.9 shown in figure 3) and they showed very similar results. Session 3.2 have a lot of descriptions about what it’s shown in the figures from both methods. Figure 5 and Figure 6 are very similar. Left and right columns in Figure 7 are similar. Figure 8 and Figure 9 are similar. It looks redundant to me to include the discussion for both methods.

There are many abbreviations throughout the paper. Reducing them may improve readability.

Line-by-line comments:
Lines 24-25: “both applied methods” is confusing because the methods are not stated earlier. I suggest revising this sentence to something like “the tropical belt shows poleward expansion ranging approximately between 0.14 °/dec and 0.27 °/dec.”
Line 25: “of both applied approaches” seems redundant.
Line 40: “climate change has been documented at a magnitude that exceeds global averages” is very broad. Does climate change here means temperature, precipitation, or something else?
I notice this sentence is rephrasing “As mentioned before, Mediterranean climate change has been observed at a magnitude exceeding global means, despite the fact that the emission of greenhouse gases (GHGs) in Mediterranean countries lie at relatively low levels” from the reference. In this paper, this sentence is missing context. I’m not sure if this sentence is a good one to put at the beginning.
Lines 47-48: This sentence repeated “the Mediterranean region has experienced significant shifts of climate in the past” in Line 45.
Lines 59-69: There is no subject in this sentence. A suggestion is “In astronomy and cartography, the tropical boundaries are at latitudes of ~23.5⁰ north and south”.
Lines 63-64: A better flow is needed. The previous sentence is about the seasonal change and “Tropical belt expansion” in this sentence is due to climate forcing. A suggestion is adding “Under climate change,” at the beginning of the sentence.
Lines 66-67: “sea-surface temperature (SST) variability over the tropics” and “ absorbing aerosols hemispheric asymmetry” don’t appear to be the changes driven by the climate change.
Lines 71-73: “even slight expansion of the tropical belt could cause substantial changes in the global climate system” repeats “The expansion of the tropics could cause substantial changes in the global climate system”.
Lines 76-77: I suggest revising “one of the most vulnerable regions to the effects of climate change” to “one of the most vulnerable regions affected by climate change”.
Line 79: “increasing” to “being”.
Line 82: Add “continue to” before “rise”.
Lines 98-99: “This accelerated…and biodiversity” repeats Lines 88-89.
Lines 115-131: Most of session 2.1 repeats the content already shown in the introduction. Please consider shortening or deleting session 2.1.
Lines 180-181: The captions in Figures are repeated in the test. Same in Lines 192-193.
Lines 181-183: Previous literature also shows an increasing trend, but the values presented in this paper are much larger.
Line 185: Add “in LRT-T also” after “our findings”.
Line 189: “consistent with earlier research” appears three times in this paragraph. Please consider revising to avoid repetition.
Line 207: “most of the grid points” is vague. Please consider stippling the regions where p<0.05. Same for line 213, line 510, line 530, line 570.
Figure 2: Repetition in the caption. For example, “Monthly time series of the TPD TEL method over the Mediterranean region for the period January 1980 to December 2022 (a) and monthly time series of the TPG TEL method over the Mediterranean region for the period January 1980 to December 2022 (b).” can be revised as “Monthly time series of the (a) TPD TEL method and (b) TPG TEL method over the Mediterranean region for the period January 1980 to December 2022”.
Line 244: “major tropical systems” is vague.
Line 250: Why stronger land-sea contrast leads to the following statement “resulting in higher TEL values in this region”?
Lines 283-284: “even minor expansion would have significant consequences” is repetition.
Figure 3: It will be more straightforward to put the correlation numbers on the top right panels.
Figure 10: Repetition in the description of panels.
Citation: https://doi.org/10.5194/egusphere-2025-6521-RC2
- AC2: 'Reply on RC2', Mohamed Darrag, 20 Apr 2026
  
  We would like to thank Referee #2 for the thorough review and valuable comments on our manuscript. We appreciate the reviewer’s constructive suggestions, which have helped us significantly improve the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6521-AC2

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AR by Mohamed Darrag on behalf of the Authors (20 Apr 2026) Author's response Author's tracked changes Manuscript

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AR by Mohamed Darrag on behalf of the Authors (16 Sep 2026) Author's response Author's tracked changes Manuscript

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ED: Publish as is (25 Sep 2026) by Kevin Grise

AR by Mohamed Darrag on behalf of the Authors (26 Sep 2026) Author's response Manuscript

Journal article(s) based on this preprint

06 Oct 2026

Tropical belt expansion and its impacts on climate variability in the Mediterranean from 1980 to 2022

Mohamed Darrag, Shuanggen Jin, Andrés Calabia, Aalaa Samy, Ivan Radevski, and Ali M. Radwan

Atmos. Chem. Phys., 26, 14031–14050, https://doi.org/10.5194/acp-26-14031-2026,https://doi.org/10.5194/acp-26-14031-2026, 2026

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Mohamed Darrag, Shuanggen Jin, Andres Calabia, Aalaa Samy, Ivan Radevski, and Ali M. Radwan

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This study examines the variability of climatic parameters over the Mediterranean and their teleconnection with tropical belt expansion. The results exhibit a dominant upward trend of tropopause height, with maximum trends in the eastern Mediterranean region, which shows more poleward occurrences of tropical edge latitudes. The analysis confirms the link between climatic parameter variability and tropical edge latitude occurrences.


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