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African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon

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Abstract

Janicot and Sultan (Geophys Res Lett 28(3):523–526, 2001) and Sultan et al. (J Clim 16(21):3389–3406, 2003) showed evidence of an intraseasonal signal of Sahelian rainfall corresponding to wet and dry sequences of the West African Monsoon. Using NCEP/NCAR reanalysis, NOAA outgoing longwave radiation (OLR) and observed daily rainfall over West Africa from 1968 to 1990, this paper investigates the variability of 3 to 5-day African Easterly Waves (AEWs), convection and their relationship with rainfall in these wet and dry sequences. The mean daily value rainfall during wet sequences is twice the mean value during dry sequences but the number of dry or wet sequences per year is not correlated with the annual rainfall. Wet sequences account for 39% of the annual accumulated rainfall while dry sequences account for 22%. The number of 3 to 5-day AEWs increases during wet years in wet sequences and the activity tends to be larger during wet years in both wet and dry sequences. These AEWs explain 40% of the accumulated rainfall during wet sequences whereas they contribute to 26% of the accumulated rainfall observed during dry sequences. Generally, they contribute to the increase of rainfall during these sequences. Mean convection is stronger and there are twice as many low OLR days (<225W/m2) during wet than dry sequences. The mean rainfall for days with high convective activity (convective days) is also twice as great during wet sequences. Rainfall that occurs during days without low OLR (weak convection with warm cloud tops or isolated deep convection) contributes to 69% of the total rainfall during dry sequences and 45% during wet sequences. A composite study was performed from day D 0−10 to day D 0+10 in each sequence. Wet (dry) sequences of the African monsoon start with a decrease (slight increase) of the negative meridional Ertel Potential Vorticity (PV) gradient at 700 hPa, associated with an increase (decrease) of the spectral density of AEWs. During the wet sequence, the African Easterly Jet (AEJ), detected by 700 hPa zonal wind, decreases and moves northward, whereas the Tropical Easterly Jet (TEJ), detected at 200 hPa, increases and shifts southward. Convective activity increases from D 0−6 to D 0−3 and remains high for 4 days in wet sequences. The daily rainfall increases (decreases) between D 0−6 and D 0 and returns to the mean value at D 0+4 for wet (dry) sequences.

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Acknowledgments

The authors are very thankful to Climate Diagnostics Center (NOAA, Boulder, CO) for providing the NCEP/NCAR Reanalysis dataset. Authors also thank ASECNA (Agence pour la Securite de la Navigation Aerienne en Afrique et a Madagascar) and African National Meteorological and Hydrological Services for collecting the rainfall data and IRD for the ORE AMMA-CATCH database.

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Authors and Affiliations

  1. LTHE, BP 53, 38051, Grenoble Cedex 9, France

    Christophe Lavaysse, Henri Laurent & Thierry Lebel

  2. IRD-Niger, BP 11416, Niamey, Niger

    Arona Diedhiou

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  1. Christophe Lavaysse
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  2. Arona Diedhiou
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  3. Henri Laurent
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  4. Thierry Lebel
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Correspondence to Henri Laurent.

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Lavaysse, C., Diedhiou, A., Laurent, H. et al. African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon. Clim Dyn 27, 319–332 (2006). https://doi.org/10.1007/s00382-006-0137-5

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  • DOI: https://doi.org/10.1007/s00382-006-0137-5

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