the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2026
Projected changes in African Easterly Waves and rainfall events in West Africa with kilometre-scale global coupled simulations
Abstract. African easterly waves (AEW) are a key modulator of West African rainfall variability with impacts on tropical cyclon activity. However, climate change projections of AEW show contrasting results, underpinned by the coarseness of some simulations and the lack of consistency with surface conditions in others. Here we analyse the performance and projected changes in 3-5 day AEWs using two 30-yr long simulations consistent with historical (1990-2019) and near-future climate conditions (2020-2049) by IFS-FESOM model at kilometer scale horizontal resolution.
We show that the model captures the spatial structure and energetics of AEW with reasonable accuracy, although it overestimates associated precipitation variability. The model projects an intensification of AEW activity over land associated with enhanced barotropic and baroclinic conversion and stronger diabatic generation of eddy available potential energy. Over the central and eastern Sahel, the model projects a shift towards more extreme rainfall events and fewer moderate ones, with an overall enhancement of rainfall. This response is not exclusively related to AEW activity, as a similar response is observed when AEW activity is low. Conversely, over the westernmost Sahel, the projected drying is related to a decrease in extreme events during AEW days and in moderate events during non-AEW days.
Over eastern Africa, where AEWs originate, the simulated increase in the reversal of the meridional potential vorticity - caused by the increase in static stability as a response to tropical climate warming - and the enhancement of upstream convective variability set the scene for more frequent and stronger seeding disturbances for AEWs. These results underline the importance of improving AEW representation in climate models to enhance seasonal forecasting, early warning systems, and adaptation strategies in the region.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2879', Anonymous Referee #1, 07 Aug 2026
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RC2: 'Comment on egusphere-2026-2879', Anonymous Referee #2, 04 Sep 2026
Two 30-year coupled model experiments with ECWMF IFS-FESOM at 9 km resolution for a historic (1990-2019) and future (2020-2049, SSP3-7.0) period were investigated for West Africa and the adjacent Atlantic Ocean. A weakly active convection scheme has been used in the experiments. AEW activity is assessed based on 2-5 day bandpass-filtering of 700-hPa meridional wind. From this, Eddy Kinetic Energy (EKE) and energy conversion terms were derived. To understand the changes in AEW activity, the meridional PV gradient, static stability and relative vorticity was considered. To assess projected changes in African Easterly Wave (AEW) activity and related extreme and moderate rainfall events, both “AEW days” and “non AEW” days were defined. MSWEP and CHIRPS were utilized for observed rainfall and other “observed” meteorological parameters were taken from ERA5. A plausibility check of ECWMF IFS-FESOM is first carried out using these observational data sets. Overall, this is an interesting and well-written study, yet some issues need clarification.
Major:
- What is a “weakly active convection scheme”? The authors say that it has a reduced cloud base mass flux. The approach will likely not be familiar to many readers. What is the purpose of this? What implication does it have for West Africa where highly organized convective systems occur? In the literature, explicit convection experiments have been successfully used at about 9 km resolution for this region. Can the authors state what they expect in terms of organized convective system when using IFS-FESOM with explicit, parameterized and weakly active convection, respectively?
- The authors write „To isolate AEW days, the JJAS 700 hPa filtered meridional wind at the same base point (i.e. 11.5°N, 1°W) was used. AEW days were identified as those corresponding to wind anomalies exceeding the 90thpercentile or falling below the 10th percentile of the filtered distribution “. The rationale is not clear to me. Does this mean that „AEW“ and „non-AEW“ days were defined sub-continent wide relying on the base point and not locally for a grid point? And selecting the upper/lower deciles of the 2-5 day bandpass-filtered meridional wind at the base point: Doesn’t this mean that you look at days with large AEW-related northerlies and southerlies respectively, i.e. to a region ahead or behind an AEW trough? Would the passage of the AEW trough axes at the base point not be associated with new zero meridional filtered winds but still be a potential AEW day? And why did the authors only use one base point and not others up- and downstream? Clearly, since these definitions are central to the following, more explanations and justifications are needed.
- Related to the previous comment: As also mentioned by the authors in the Introduction, there is a northerly and southerly AEW track. In previous literature, the southerly track was investigated using bandpass-filtered 700 hPa meridional wind for example, but the mostly dry northerly waves which grow due to baroclinic conversions are better captured by the 850 or even 925 hPa level. Why did the authors only stick to the 700 hPa level, so essentially to the southerly AEW track? While they are more important to rainfall, the northerly AEWs might grow in intensity or shift due to changes in the meridional temperature gradient.
- Again, related to comment 2): While I acknowledge that the bandpass-filtered meridional wind at 700 hPa has been used to create AEW composites, the AEW is an object, and numerous object-tracking algorithms are freely available now. Defining the AEW trough and tracking it would yield a much more obvious AEW-rainfall relationship than used in the present study. Can the authors reflect on the implication of using their statistical approach vs. an object-following approach in the conclusion.
Minor:
- 375: It would be helpful to refer to previous literature and add a sentence or so why the amplified Sahara warming occurs. The reasoning given is not clear.
- When IFS is compared to ERA5 like in Figs. 1 and 2, it would really help to add a third column showing the difference. It would really make it easier to comprehend the many statements made by the authors on the differences.
- Caption of Fig. 3: “11.5°N”, the “N” is missing
Citation: https://doi.org/10.5194/egusphere-2026-2879-RC2 -
EC1: 'Comment on egusphere-2026-2879', Peter Knippertz, 12 Sep 2026
Dear authors,
first of all, I would like to apologize for the time it took to get the two reviews. Vacation time and other issues unfortunately led to various delays.
Looking at them, you can see that both reviewers value your work and only ask for some further explanations and clarifications. I suggest that you should send a short reply stating in general terms how you want to address their points. After this we can move to the actual revision.
Best regards,
Peter
Citation: https://doi.org/10.5194/egusphere-2026-2879-EC1
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General:
Two 30-year simulations representing 1990-2019 with observed greenhouse gases and 2020-2049 using the SSP3-7.0 greenhouse gas forcing are analyzed. The model is a coupled GCM with 9-km resolution in the atmosphere. At this resolution, some explicit convection occurs but parameterized convection is still needed – the authors described the convective parameterization as “weakly active”. The reference to “kilometre-scale” modeling in the title is misleading, since this term is used to refer to convective-permitting modeling at resolutions under 4 km.
The precipitation analysis is especially interesting, with the decomposition into “AEW days” and “non-AEW days”. The PV analysis is also really nice – perhaps you would compare with Hsieh and Cook (2008) who discuss how PV gradient reversals interact with the jet and the waves.
The paper is well-written and informative, and the analysis is substantial. A very nice contribution.
Major:
Minor:
Abstract: Cyclone not cyclon