Synoptic drivers of the August 2024 record-breaking rainfall in the Chadian Sahara: Dynamics, thermodynamics, and socio-economic consequences

Wamba Tchinda, Claudin; Saha, Fréderic; T. Tamoffo, Alain

doi:10.5194/egusphere-2026-725

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https://doi.org/10.5194/egusphere-2026-725

Preprints

16 Feb 2026

| 16 Feb 2026

Synoptic drivers of the August 2024 record-breaking rainfall in the Chadian Sahara: Dynamics, thermodynamics, and socio-economic consequences

Claudin Wamba Tchinda, Fréderic Saha, and Alain T. Tamoffo

Abstract. This study examines the atmospheric mechanisms behind the extreme rainfall event of August 2024 in the northern Chad, and their devastating socio-economic impacts. Analysis of the hydro-climatic regime over the region reveals a major structural transition marked by a statistical tipping point in 2003, shifting from historical aridity to a phase of intensified rainfall that culminated in the record high of August 2024. Our analysis of lower-tropospheric convergence, specific humidity, vertical velocity (ω), and moist static energy (MSE) reveals a major shift from the typical West African monsoon regime. In August 2024, the Intertropical Front (ITF) shifted abnormally northward, reaching 20–22° N, which allows moist moisture air to penetrate deep into the Saharan zone. This shift was driven by strengthened convergence at 850 hPa and a significant increase in low-level humidity. Furthermore, negative ω anomalies throughout the troposphere indicate a northward extension of the monsoon's upward branch. Strong positive MSE anomalies over desert regions further highlight a thermodynamic enrichment of the atmospheric column. Together, these signals point to a highly effective dynamic-thermodynamic coupling that fueled intense convective systems. Ultimately, the synchronization between these atmospheric condition and the synoptic forcing of African easterly waves generated local rainfall anomalies exceeding 100 %, redefining the hydrological balance of the Lake Chad basin between aquifer recharge and increased risks of flash flooding. This hydro-climatic shift had immediate and devastating socio-economic impacts: the resulting flooding affected nearly 20,000 people across four desert provinces in Chad. In Tibesti alone, sixty lives were lost due to drowning or building collapses, alongside significant losses of livestock and infrastructure.

Received: 06 Feb 2026 – Discussion started: 16 Feb 2026

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Claudin Wamba Tchinda, Fréderic Saha, and Alain T. Tamoffo

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-725', Christophe Lavaysse, 20 Apr 2026
Review of “Synoptic drivers of the August 2024 record-breaking rainfall in the Chadian Sahara: Dynamics, thermodynamics, and socio-economic consequences”
The manuscript investigates the atmospheric mechanisms responsible for the exceptional rainfall that occurred in northern Chad in August 2024 and its socio-economic impacts. The authors identify a climatological shift around 2003 toward increased rainfall and argue that the 2024 event resulted from an unusual northward displacement of the Intertropical Front, enhanced low-level convergence, increased moisture, and positive moist static energy anomalies. They conclude that the event reflects a strong dynamic–thermodynamic coupling within an intensified West African monsoon.
General assessment
Although the topic is very interesting and relevant, the analysis appears somewhat simplistic. The connection between the presented results and the conclusions is not always clearly demonstrated, and some interpretations seem speculative. In particular, additional and clearer analyses would be needed to substantiate the proposed climatological shift around 2003 and its relevance to the 2024 event.
Major comments
Abstract: The link between the proposed 2003 tipping point (climatological evolution) and the specific 2024 event is not clearly established.

Section 3.1 and Figure 2: The authors mention a sharp break in trends visible in both datasets of Figure 2, but there is no clear visual evidence supporting this claim. Instead, the series appear closer to a linear trend over the whole period with a few atypical positive or negative anomalies. This weakens the argument for a structural shift emphasized in the abstract and throughout the text. In addition, the hydrological consequences discussed at the end of the paragraph are not supported by sufficient evidence. Hydrological regimes depend on many factors beyond rainfall during a single month, including antecedent conditions, evaporation, and water use. This part therefore seems overly speculative.

Section 3.2: To compare the temporal evolution of precipitation with atmospheric drivers, it would be useful to include a figure showing the temporal evolution of an integrated indicator of wind convergence or humidity. This would help assess whether similar variability and trends are present as those shown in Figure 2.

Minor comments
l113: Does MSE decrease CIN or increase CAPE? Please clarify.

l120: Not sure about the reference Panthou et al. (2020); it is also missing from the reference list.

ll126: I do not agree with the term “low rainfall variability.” The previous sentences could be improved. There is confusion between extreme hazard values and impacts. This part should be clarified.

l129: Clarify the meaning of “true” in the text.

l149: “Extreme thermal amplitude and negligible annual rainfall” should be defined more scientifically.

Figure 1: Improve the caption by explaining the different panels. The right panel is not clearly described.

Subsection 2.3: The detailed computation of climatological anomalies and convergence is quite obvious and may not be necessary.

l274: Is this region displayed in Figure 1, panel b?

l275: What is meant by “structural volatility”?

Figure 3: The anomaly seems to depict a cyclonic wind circulation pattern, but this is not associated with a southward shift of the ITCZ on the western side of the circulation. Could the authors comment on this?

Figure 5: The authors do not comment on the very strong descending wind anomaly below 700 hPa during summer 2024. How do they explain this feature?

l484: Same comment as previously regarding CIN.

l514: The title of subsection 3.2.5 sounds unclear; please rephrase.
Citation: https://doi.org/10.5194/egusphere-2026-725-RC1
RC2:
'Comment on egusphere-2026-725', Erwan Cornillault, 06 May 2026
Summary of the study :
The authors provide results on the atmospheric environment of Northern Chad during August 2024, an exceptional monsoon season. The region recorded the highest amount of monthly rainfall, associated with major socioeconomic consequences.
They use two satellite products for estimating rainfall to assess this rainfall record and compare this particular month with all Augusts between 1983 and 2023 (the climatology defined by the authors). They describe the mean monthly environment in Northern Chad and conclude that the large amount of rainfall is due to a stronger monsoon flow (moister and further north) during August 2024 compared with all other Augusts. This humidity excess, coupled with high temperatures, gives a high MSE positive anomaly, which fuels or supplies convective systems.
The study concludes with the socio-economic impacts of the rainfall, which spotlights the consequences of these events.
I have several comments and suggestions that should be considered before another publication. Those revisions fall within the major revisions category.

General comments :
This paper investigates a high-impact event, the exceptionally rainy month in August 2024 in Northern Chad, a region rarely covered by the literature, and where floods can be devastating. The socio-economic impacts of the rainfall presented are of interest since they are an example of an issue that Chadian public policies have to tackle.
The scientific questions are valuable and totally within the scope of the journal.
This study presents a description of the monthly environmental conditions over northern Chad during August 2024, with a comparison to the climatology 1983-2023. Unfortunately, the analysis of these conditions remains shallow. The only comparison to the climatology is not sufficient to characterize the exceptional nature of the large-scale context in Northern Chad. To what point was August 2024 an exceptional month? For example, what is the return period of the rainfall? What quantile does the specific humidity anomaly (for example) reach in August 2024, compared to its distribution between 1980 and 2024? And, as you mentioned, L282, other wet episodes like 1999 or 2018-2020 were recorded. What are the differences between August 2024 and these other wet seasons?
Moreover, the methods and presented material are inadequate regarding the title and the content of the abstract. You do not discuss at all the potential contribution of MCSs and AEWs to the rainfall amount in August 2024. You only mention them several times as potential drivers (L341, L396, L455, L526, L545) without explaining the direct link between these drivers and the total amount of the month. The first sentence of your conclusion does not correspond at all to your study (no multi-scale analysis is made). More generally, I am also quite annoyed by the fact that some results in your study are only inferred and not totally justified, mainly at the end of the subsections.
I strongly recommend that you either deepen this study to include a clear investigation of the role of these drivers and the exact justification of what you conclude; either you change your title, abstract, and conclusion to make them truly correspond to your material presented here. I also recommend that you proofread your paper carefully before another submission. No gross error of language is noted, but the plan announced at the end of the introduction does not correspond to the plan followed during the rest of the paper.

Specific comments :
1. Data
Between L559 and L565, you mentioned rainfall amounts registered by the national meteorological service of Chad. Where was the record of 126,5 mm over 8 days registered? Do you have access to this data and different time series to sample your region of interest spatially?
A subsidiary question is about the quality of the datasets that you used for this study. TAMSAT and CHIRPS registered respectively 70 and 22 mm for August 2024 (on average in northern Chad). So even if August 2024 is the month with the highest amount of rainfall, the two datasets are not consistent. In fact, even if satellite products are fair to sample the climatological rainfall in the tropics, most studies (e.g., Alexander et al. 2020, Masunaga et al. 2019, Sanogo et al. 2022) show that they still have some discrepancies about extreme rainfall. If you have access to in situ data, a discussion of the skill of these two datasets compared with rain gauges (focused on August 2024) will be a great added value for this study.
Finally, is the climatological period 1980-2023 (indicated by Figures 4, 5, and 7) or 1983-2023?
2. Rainfall of August 2024 and trend analysis
First, I would suggest that you move Figure 7 and the associated analysis to Section 3.1? I think it is more appropriate to first describe the rainfall before the atmospheric environment. It would also be more consistent to present it with the time series in Figure 2. Moreover, in Figure 7, is it CHIRPS or TAMSAT?
Second, I am not convinced at all by the trend analysis as it is presented. The time series over such a long period is very useful, and I could not agree more with you that August 2024 is the wettest month of the period. But the year 2003 should not be considered as a tipping point and a transition between a dry period and a wetter one. 2003 is just the middle of the period 1983-2024, and by construction of the linear regression, the middle point of a linear regression always corresponds to the average.
3. Organization of section 3.2
Currently, the results in section 3.2 are organized for each figure as follows: a description of the climatology, a description of the situation of August 2024, and finally, the difference between the two. This organization is quite burdensome and can be improved by first describing the climatological context for all variables and then the context for August 2024 and the difference with the climatology. This second organization can clarify the presentation of the results.
4. Methods
Figure 3: You should consider downloading the divergence field directly from ERA5 instead of computing it from u and v. Because of that, you have large bands of strong divergence or convergence on the northern and southern edges of your domain. After this correction, you should adapt your colorbar to better indicate the stronger convergence in Northern Chad. Then, section 2.3.c is not necessary.
Figures 3 to 5: except for Figure 4, the significance of values is not mentioned. Can you add it for those Figures and mention its computation in the “Methods” section?
Moreover, “Moisture Flux Convergence” is mentioned in L251 but not discussed in the study. Please remove it or add the corresponding analysis.
5. Analysis of MSE anomaly
No mention of the equivalent potential temperature is made before Figure 6. Please add it to the “Methods” section. Moreover, in section 3.2.4, what is the link between MSE and the potential temperature? Please, be more precise. Maybe I can suggest some computations or an MSE budget to reveal the link.
You also indicate a link between MSE and CIN. I think there is a misinterpretation of the literature. Romps (2015) indicates, on the contrary, a link between MSE and CAPE.
In section 3.2.4, the different references used are sometimes irrelevant. Please clarify this section and fit your references accordingly to your statements.
6. Further suggestions
To deepen further your work and make it correspond to the true objectives of your paper, I made you the following suggestions that can add interesting elements to your study. But, I understand if you do not have the time to do it. I will not take it into account in the next review if the title and the abstract are consistent with the scientific content.
First, I suggest that you investigate the role of other synoptic drivers like convectively-coupled equatorial waves (CCEWs). In fact, CCEWs can help trigger extreme precipitation events in Africa (Peyrillé et al. 2023) and are the main mode of variability of rainfall in the Tropics (behind the AEWs in Africa, Schlueter et al. 2019, Kiladis et al. 2009). The recent literature shows that CCEW activity is key to understanding large-scale mechanisms behind rainfall events during the monsoon season in Africa.
To help you with this study, the website Misva (https://misva.aeris-data.fr/) and the North Carolina Institute for Climate Studies (https://ncics.org/pub/mjo/archive/2024/2024-08-19/v2/) propose real-time monitoring of equatorial waves in the whole tropics. Archives for August 2024 are available on both websites and may guide you in the investigation. A quick exploration of maps and Hovmoller diagrams suggests, in fact, a strong CCEW activity during this period.
Also, a case study may add great value to the paper. As you mention in section 3.3, the period between 9 and 14^th August seems very interesting to focus on. I suggest you focus on this period or some days of this period with the highest amount of daily precipitation.
To give you one more reference which could be helpful, the study of Lafore et al. (2016) presents a complete multi-scale study of an extreme precipitation event that occurred in Ouagadougou, Burkina Faso, in September 2009, which you can get inspired by. This paper presents many mechanisms involved in the event, from the background environment to the particular behaviour of the convective system that triggers the extreme rainfall.

Technical corrections :
L45: “moist moisture air” → more moisture air?
L93: “arainfall” → space?
L165: “thrend” → trend?
L189-193, L305-315 : check the font
L259 : “g = 9,81 m.s-1” → m.s-2, it’s an acceleration
L260: Can you add the unit of the latent heat of evaporation Lv?
L264: section “2,5”: 2.3.e or 2.4?
L302: What is a Sen Slope? Figure 2 indicates Sen’s slope. Moreover, this feature is not explained anywhere. Similarly, what is the relative magnitude?
L482: Please, add the Figure corresponding to this sentence (Figure 5, I assume).
Figure 1: The legends and text are quite small to read. Can you be more precise with the caption? For all the next Figures with maps, can you add the borders of each region?
Captions of Figures 3 to 5: I suggest you can shorten captions for Figures 4 and 5 by writing “same as Figure 3” or something similar.
Figure 5: As you depict a latitude-pressure cross-section, the circulation is “meridional-vertical” and not “zonal-vertical” and the vectors are the combined meridional and vertical winds. Please check the caption.
Figure 6: Is it the anomaly of August 2024 regarding the climatology? What pressure level do you consider for the MSE and equivalent potential temperature?
Figure 7: Except for the eastern part, the positive anomalies are not very visible. Can you adapt the colorbar?
L662-665 and 797-804: You cite twice the studies of Biasutti (2019) and Taylor et al. (2017) in your references

References :
Alexander, Lisa V., Margot Bador, Rémy Roca, Steefan Contractor, Markus G. Donat, et Phuong Loan Nguyen. 2020. « Intercomparison of Annual Precipitation Indices and Extremes over Global Land Areas from in Situ, Space-Based and Reanalysis Products ». Environmental Research Letters 15 (5): 055002. https://doi.org/10.1088/1748-9326/ab79e2.

Kiladis, George N., Matthew C. Wheeler, Patrick T. Haertel, Katherine H. Straub, et Paul E. Roundy. 2009. « Convectively Coupled Equatorial Waves ». Reviews of Geophysics 47 (2). https://doi.org/10.1029/2008RG000266.

Lafore, Jean-Philippe, Florent Beucher, Philippe Peyrillé, et al. 2017. « A Multi-Scale Analysis of the Extreme Rain Event of Ouagadougou in 2009 ». Quarterly Journal of the Royal Meteorological Society 143 (709): 3094‑109. https://doi.org/10.1002/qj.3165.

Masunaga, Hirohiko, Marc Schröder, Fumie A. Furuzawa, Christian Kummerow, Elke Rustemeier, et Udo Schneider. 2019. « Inter-Product Biases in Global Precipitation Extremes ». Environmental Research Letters 14 (12): 125016. https://doi.org/10.1088/1748-9326/ab5da9.

Peyrillé, Philippe, Romain Roehrig, et Sidiki Sanogo. 2023. « Tropical Waves Are Key Drivers of Extreme Precipitation Events in the Central Sahel ». Geophysical Research Letters 50 (20): e2023GL103715. https://doi.org/10.1029/2023GL103715.

Sanogo, Sidiki, Philippe Peyrillé, Romain Roehrig, Françoise Guichard, et Ousmane Ouedraogo. 2022. « Extreme Precipitating Events in Satellite and Rain Gauge Products over the Sahel ». Journal of Climate. Journal of Climate 35 (6): 1915‑38. https://doi.org/10.1175/JCLI-D-21-0390.1.

Schlueter, Andreas, Andreas H. Fink, Peter Knippertz, et Peter Vogel. 2019. « A Systematic Comparison of Tropical Waves over Northern Africa. Part I: Influence on Rainfall ». Journal of Climate. Journal of Climate 32 (5): 1501‑23. https://doi.org/10.1175/JCLI-D-18-0173.1.
Citation: https://doi.org/10.5194/egusphere-2026-725-RC2
EC1: 'Comment on egusphere-2026-725', Peter Knippertz, 06 May 2026

Dear authors,
now that we have two high-quality assessments of your work, we should discuss potential ways forward for a revision before you go into any detail.
The issues both reviewers see are serious and if you cannot cure them, may jeopardize the publication of this paper. In particular, I am referring to statements that the analysis is simplistic or shallow and that inferences are not substantiated by your results.
Please answer to this comment with a high-level strategy of how you plan to improve the paper. Once we have agreed on a plan, you can go ahead and implement the concrete changes.
Best regards,
Peter

Citation: https://doi.org/10.5194/egusphere-2026-725-EC1

Claudin Wamba Tchinda, Fréderic Saha, and Alain T. Tamoffo

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Short summary

Northern Chad's hydro-climatic regime has shifted since 2003 from aridity to heavy rain, peaking in August 2024. This resulted from Hadley cell reorganization, pushing the convective band northward. The West African monsoon reached deeper, replacing Saharan subsidence with moisture from the Gulf of Guinea. This thermodynamic change boosted moist static energy, enabling vertical ascent. Coupled with easterly waves, this created immense rainfall, altering Lake Chad’s water balance.


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