In July 2022, Senegal experienced an exceptional wet spell characterized by widespread flooding and anomalously high rainfall intensity. This study investigates the multi-scale hydroclimatic mechanisms underlying this event, focusing on the combined roles of sea surface temperature (SST) anomalies, African Easterly Waves (AEWs), and intraseasonal variability associated with the Madden-Julian Oscillation (MJO). We combine satellite-based precipitation products (IMERG, CHIRPS, and TAMSAT), ERA5 reanalysis fields, and daily SST data (OISST v2) to diagnose the large-scale and mesoscale processes involved. AEWs and mesoscale convective systems (MCSs) are objectively tracked to quantify their contribution to rainfall variability. The results show that the event coincided with a pronounced tropical Atlantic SST dipole, characterized by anomalous warming in the North Tropical Atlantic and cooling along the equatorial Atlantic. This configuration enhanced the meridional SST gradient, strengthened the low-level monsoon flow, and increased moisture availability over West Africa. During the same period, total column water vapor exceeded 59 mm over Senegal, while a sequence of long-lived and coherently propagating AEWs was observed. Moreover, this event occurred during an active MJO phase (phases 8 - 1), which is known to favor enhanced convection over West Africa. The MJO likely acted as an intraseasonal modulator, reinforcing large-scale ascent and moisture convergence, thereby amplifying convection triggered by AEWs. The extreme rainfall arose from a compound mechanism involving persistent AEW activity, a moisture-rich background state, and favorable intraseasonal conditions linked to the MJO. These results underscore the role of multi-scale interactions in shaping extreme rainfall over west Africa and offer valuable insights for improving subseasonal forecasting and early warning systems.
References
[1]
Field, C.B., Barros, V.R. and Intergovernmental Panel on Climate Change (2014) Climate Change 2014: Impacts, Adaptation, and Vulnerability: Working Group II Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.
[2]
Badji, A., Mohino, E., Diakhaté, M., Mignot, J. and Gaye, A.T. (2022) Decadal Variability of Rainfall in Senegal: Beyond the Total Seasonal Amount. Journal of Climate, 35, 5339-5358.
[3]
Lavender, S.L. and Matthews, A.J. (2009) Response of the West African Monsoon to the Madden-Julian Oscillation. Journal of Climate, 22, 4097-4116. https://doi.org/10.1175/2009jcli2773.1
[4]
Niang, C., Mohino, E., Gaye, A.T. and Omotosho, J.B. (2017) Impact of the Madden Julian Oscillation on the Summer West African Monsoon in AMIP Simulations. Climate Dynamics, 48, 2297-2314. https://doi.org/10.1007/s00382-016-3206-4
[5]
Diakhaté, M. and Dieng, A.L. (2026) Key Mechanisms Leading to the East-West Asymmetric Impacts of Convectively Coupled Kelvin Waves on the West African Monsoon. Open Journal of Modern Hydrology, 16, 84-98. https://doi.org/10.4236/ojmh.2026.161006
[6]
Lafore, J., Beucher, F., Peyrillé, P., Diongue-Niang, A., Chapelon, N., Bouniol, D., et al. (2017) A Multi-Scale Analysis of the Extreme Rain Event of Ouagadougou in 2009. Quarterly Journal of the Royal Meteorological Society, 143, 3094-3109. https://doi.org/10.1002/qj.3165
[7]
Lavaysse, C., Flamant, C., Janicot, S., Parker, D.J., Lafore, J.-., Sultan, B., et al. (2009) Seasonal Evolution of the West African Heat Low: A Climatological Perspective. Climate Dynamics, 33, 313-330. https://doi.org/10.1007/s00382-009-0553-4
[8]
Lavaysse, C., Flamant, C. and Janicot, S. (2010) Regional-Scale Convection Patterns during Strong and Weak Phases of the Saharan Heat Low. Atmospheric Science Letters, 11, 255-264. https://doi.org/10.1002/asl.284
[9]
Atiah, W.A., Amekudzi, L.K. and Danuor, S.K. (2023) Mesoscale Convective Systems and Contributions to Flood Cases in Southern West Africa (SWA): A Systematic Review. Weather and Climate Extremes, 39, Article ID: 100551. https://doi.org/10.1016/j.wace.2023.100551
[10]
Schmetz, J., Pili, P., Tjemkes, S., Just, D., Kerkmann, J., Rota, S., et al. (2002) Supplement to an Introduction to Meteosat Second Generation (MSG). Bulletin of the American Meteorological Society, 83, 992-992. https://doi.org/10.1175/1520-0477(2002)083<0992:staitm>2.3.co;2
[11]
Ocasio, K.M.N., Evans, J.L. and Young, G.S. (2020) Tracking Mesoscale Convective Systems That Are Potential Candidates for Tropical Cyclogenesis. Monthly Weather Review, 148, 655-669.
[12]
Huffman, G.J., Bolvin, D.T., Braithwaite, D., Hsu, K., Joyce, R.J., Kidd, C., et al. (2020) Integrated Multi-Satellite Retrievals for the Global Precipitation Measurement (GPM) Mission (IMERG). In: Levizzani, V., et al., Eds., Satellite Precipitation Measurement, Springer International Publishing, 343-353. https://doi.org/10.1007/978-3-030-24568-9_19
[13]
Houngnibo, M.C.M., Minoungou, B., Traore, S.B., Maidment, R.I., Alhassane, A. and Ali, A. (2023) Validation of High-Resolution Satellite Precipitation Products over West Africa for Rainfall Monitoring and Early Warning. Frontiers in Climate, 5, Article ID: 1185754. https://doi.org/10.3389/fclim.2023.1185754
[14]
Dezfuli, A.K., Ichoku, C.M., Mohr, K.I. and Huffman, G.J. (2017) Precipitation Characteristics in West and East Africa from Satellite and in Situ Observations. Journal of Hydrometeorology, 18, 1799-1805. https://doi.org/10.1175/jhm-d-17-0068.1
[15]
Maranan, M., Fink, A.H., Knippertz, P., Amekudzi, L.K., Atiah, W.A. and Stengel, M. (2020) A Process-Based Validation of GPM IMERG and Its Sources Using a Mesoscale Rain Gauge Network in the West African Forest Zone. Journal of Hydrometeorology, 21, 729-749.
[16]
Boluwade, A. (2020) Remote Sensed-Based Rainfall Estimations over the East and West Africa Regions for Disaster Risk Management. ISPRS Journal of Photogrammetry and Remote Sensing, 167, 305-320. https://doi.org/10.1016/j.isprsjprs.2020.07.015
[17]
Funk, C., Peterson, P., Landsfeld, M., Pedreros, D., Verdin, J., Shukla, S., et al. (2015) The Climate Hazards Infrared Precipitation with Stations—A New Environmental Record for Monitoring Extremes. Scientific Data, 2, Article ID: 150066. https://doi.org/10.1038/sdata.2015.66
[18]
Sanogo, S., Peyrillé, P., Roehrig, R., Guichard, F. and Ouedraogo, O. (2022) Extreme Precipitating Events in Satellite and Rain Gauge Products over the Sahel. Journal of Climate, 35, 1915-1938. https://doi.org/10.1175/jcli-d-21-0390.1
[19]
Diakhaté, M., Rodríguez-Fonseca, B., Gómara, I., Mohino, E., Dieng, A.L. and Gaye, A.T. (2019) Oceanic Forcing on Interannual Variability of Sahel Heavy and Moderate Daily Rainfall. Journal of Hydrometeorology, 20, 397-410. https://doi.org/10.1175/jhm-d-18-0035.1
[20]
Faye, D., Kaly, F., Dieng, A.L., Wane, D., Fall, C.M.N., Mignot, J., et al. (2024) Regionalization of the Onset and Offset of the Rainy Season in Senegal Using Kohonen Self-Organizing Maps. Atmosphere, 15, Article No. 378. https://doi.org/10.3390/atmos15030378
[21]
Maidment, R.I., Grimes, D., Black, E., Tarnavsky, E., Young, M., Greatrex, H., et al. (2017) A New, Long-Term Daily Satellite-Based Rainfall Dataset for Operational Monitoring in Africa. Scientific Data, 4, Article ID: 170063. https://doi.org/10.1038/sdata.2017.63
[22]
Tarnavsky, E., Grimes, D., Maidment, R., Black, E., Allan, R.P., Stringer, M., et al. (2014) Extension of the TAMSAT Satellite-Based Rainfall Monitoring over Africa and from 1983 to Present. Journal of Applied Meteorology and Climatology, 53, 2805-2822. https://doi.org/10.1175/jamc-d-14-0016.1
[23]
Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Mu？oz-Sabater, J., et al. (2020) The ERA5 Global Reanalysis. Quarterly Journal of the Royal Meteorological Society, 146, 1999-2049. https://doi.org/10.1002/qj.3803
[24]
Soci, C., Hersbach, H., Simmons, A., Poli, P., Bell, B., Berrisford, P., et al. (2024) The ERA5 Global Reanalysis from 1940 to 2022. Quarterly Journal of the Royal Meteorological Society, 150, 4014-4048. https://doi.org/10.1002/qj.4803
[25]
Reynolds, R.W., Smith, T.M., Liu, C., Chelton, D.B., Casey, K.S. and Schlax, M.G. (2007) Daily High-Resolution-Blended Analyses for Sea Surface Temperature. Journal of Climate, 20, 5473-5496. https://doi.org/10.1175/2007jcli1824.1
[26]
Banzon, V., Smith, T.M., Chin, T.M., Liu, C. and Hankins, W. (2016) A Long-Term Record of Blended Satellite and in Situ Sea Surface Temperature for Climate Monitoring. Geophysical Research Letters, 43, 6353-6361.
[27]
Huang, B., Liu, C., Banzon, V., Freeman, E., Graham, G., Hankins, B., et al. (2021) Improvements of the Daily Optimum Interpolation Sea Surface Temperature (DOISST) Version 2.1. Journal of Climate, 34, 2923-2939. https://doi.org/10.1175/jcli-d-20-0166.1
[28]
Thorncroft, C. and Hodges, K. (2001) African Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity. Journal of Climate, 14, 1166-1179. https://doi.org/10.1175/1520-0442(2001)014<1166:aewvai>2.0.co;2
[29]
Berry, G., Thorncroft, C. and Hewson, T. (2007) African Easterly Waves during 2004—Analysis Using Objective Techniques. Monthly Weather Review, 135, 1251-1267. https://doi.org/10.1175/mwr3343.1
[30]
Bain, C.L., Williams, K.D., Milton, S.F. and Heming, J.T. (2013) Objective Tracking of African Easterly Waves in Met Office Models. Quarterly Journal of the Royal Meteorological Society, 140, 47-57. https://doi.org/10.1002/qj.2110
[31]
Ullrich, P.A. and Zarzycki, C.M. (2017) Tempestextremes: A Framework for Scale-Insensitive Pointwise Feature Tracking on Unstructured Grids. Geoscientific Model Development, 10, 1069-1090. https://doi.org/10.5194/gmd-10-1069-2017
[32]
Enyew, B.D. and Mekonnen, A. (2021) The Interaction between African Easterly Waves and Different Types of Deep Convection and Its Influence on Atlantic Tropical Cyclones. Atmosphere, 13, Article No. 5. https://doi.org/10.3390/atmos13010005
[33]
Jonville, T., Cornillault, E., Lavaysse, C., Peyrillé, P. and Flamant, C. (2025) Distinguishing North and South African Easterly Waves with a Spectral Method: Implication for Tropical Cyclogenesis from Mergers in the North Atlantic. Quarterly Journal of the Royal Meteorological Society, 151, e4909. https://doi.org/10.1002/qj.4909
[34]
Dieng, A.L., Sall, S.M., Eymard, L., Leduc-Leballeur, M. and Lazar, A. (2017) Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic. Monthly Weather Review, 145, 599-616. https://doi.org/10.1175/mwr-d-15-0277.1
[35]
Ocasio, K.M.N., Evans, J.L. and Young, G.S. (2020) A Wave-Relative Framework Analysis of AEW-MCS Interactions Leading to Tropical Cyclogenesis. Monthly Weather Review, 148, 4657-4671.
[36]
Nú？ez Ocasio, K.M. and Dougherty, E.M. (2024) The Effect of Pseudo-Global Warming on the Weather-Climate System of Africa in a Convection-Permitting Model. Geophysical Research Letters, 51, e2024GL112341. https://doi.org/10.1029/2024gl112341
[37]
Liu, P., Zhang, Q., Zhang, C., Zhu, Y., Khairoutdinov, M., Kim, H.-M., et al. (2016) A Revised Real-Time Multivariate MJO Index. Monthly Weather Review, 144, 627-642. https://doi.org/10.1175/mwr-d-15-0237.1
Wheeler, M.C. and Hendon, H.H. (2004) An All-Season Real-Time Multivariate MJO Index: Development of an Index for Monitoring and Prediction. Monthly Weather Review, 132, 1917-1932. https://doi.org/10.1175/1520-0493(2004)132<1917:aarmmi>2.0.co;2
[40]
Poan, D.E., Roehrig, R., Couvreux, F. and Lafore, J. (2013) West African Monsoon Intraseasonal Variability: A Precipitable Water Perspective. Journal of the Atmospheric Sciences, 70, 1035-1052. https://doi.org/10.1175/jas-d-12-087.1
[41]
Diedhiou, S., Rauch, M., Lahat Dieng, A., Bliefernicht, J., Sy, S., Sall, S.M., et al. (2024) Extreme Rainfall in Dakar (Senegal): A Case Study for September 5, 2020. Frontiers in Water, 6, Article ID: 1439404. https://doi.org/10.3389/frwa.2024.1439404
[42]
Engel, T., Fink, A.H., Knippertz, P., Pante, G. and Bliefernicht, J. (2017) Extreme Precipitation in the West African Cities of Dakar and Ouagadougou: Atmospheric Dynamics and Implications for Flood Risk Assessments. Journal of Hydrometeorology, 18, 2937-2957. https://doi.org/10.1175/jhm-d-16-0218.1
[43]
Laing, A.G. and Michael Fritsch, J. (1997) The Global Population of Mesoscale Convective Complexes. Quarterly Journal of the Royal Meteorological Society, 123, 389-405. https://doi.org/10.1002/qj.49712353807
[44]
Janiga, M.A. and Thorncroft, C.D. (2016) The Influence of African Easterly Waves on Convection over Tropical Africa and the East Atlantic. Monthly Weather Review, 144, 171-192. https://doi.org/10.1175/mwr-d-14-00419.1
[45]
Panthou, G., Vischel, T. and Lebel, T. (2014) Recent Trends in the Regime of Extreme Rainfall in the Central Sahel. International Journal of Climatology, 34, 3998-4006. https://doi.org/10.1002/joc.3984
[46]
Wane, D., Dieng, A.L., Niang, C. and Gaye, A.T. (2023) Northeastern Tropical Atlantic SST and Sahel Rainfall Variability. Atmospheric and Climate Sciences, 13, 431-454. https://doi.org/10.4236/acs.2023.134024
[47]
Fall, M., Dieng, A.L., Sall, S.M., Sane, Y. and Diakhaté, M. (2020) Synoptic Analysis of Extreme Rainfall Event in West Africa: The Case of Linguère. American Journal of Environmental Protection, 8, 1-9.
