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Intraseasonal atmospheric variability and its interannual modulation in Central Africa

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Abstract

The spatial and temporal structures of the intraseasonal atmospheric variability over central Africa is investigated using 2.5° × 2.5° daily outgoing longwave radiation (OLR) and National Centers for Environmental Prediction (NCEP) Reanalysis zonal winds for the period 1980–2010. The study begins with an overview of the Central African rainfall regime, noting in particular the contrast amongst Western and Eastern parts, with different topography and surface conditions features. The annual mean rainfall and OLR over the region revealed a zone of intense convective activity centered on the equator near 30°E, which extends southward and covers almost all the Congo forest. The annual cycle of rainfall reflects the classical bi-annual shift of Inter-Tropical Convergence Zone across the equatorial belt, between 10°S and 10°N. The result of the empirical orthogonal functions (EOFs) analysis has shown that the three leading EOF modes explain about 45 % of total intraseasonal variability. The power spectra of all the three corresponding principal components (PCs) peak around 45–50 days, indicating a Madden–Julian Oscillation (MJO) signal. The first mode exhibits high positive loadings over Northern Congo, the second over Southern Ethiopia and the third over Southwestern Tanzania. The PCs time series revealed less interannual modulation of intraseasonal oscillations for the Congo mode, while Ethiopian and Tanzanian modes exhibit strong interannual variations. Hövmoller plots of OLR, 200 and 850 hPa NCEP zonal winds found the eastward propagating features to be the dominant pattern in all the three times series, but this propagation is less pronounced in the OLR than in the 850 and 200 hpa zonal wind anomalies. An index of MJO strength was built by averaging the 30–50 day power for each day. A plot of MJO indices and El Niño Southern Oscillation (ENSO) cycle confirm a strong interannual modulation of MJO over Eastern central Africa partially linked with the ENSO events (El Niño and La Niña). Strong MJO activity is observed during La Niña years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Niño episodes.

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Acknowledgments

Two anonymous reviewers provided insightful comments on the manuscript. Most of the scripts used in this study were produced from the NCL website http://www.ncl.ucar.edu. The NCEP and OLR data were obtained from the NOAA websites, http://www.cdc.noaa.gov for the NCEP zonal winds and http://www.esrl.noaa.gov for the OLR. All the administrator members of these websites are gratefully acknowledged for maintaining the updated data. We also thank the senior researchers of LEMAP for helpful comments.

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  1. Laboratory for Environmental Modeling and Atmospheric Physics, Department of Physics, Faculty of Science, University of Yaoundé 1, P.O. Box 812, Yaoundé, Cameroon

    Alain Tchakoutio Sandjon

  2. Institut Universitaire de Technologie Fotso Victor de Bandjoun, Université de Dschang, Dschang, Cameroun

    Armand Nzeukou

  3. Laboratory of Mechanics, Department of Physics, Faculty of Science, University of Yaoundé 1, Yaoundé, Cameroon

    Clément Tchawoua

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  1. Alain Tchakoutio Sandjon
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Correspondence to Alain Tchakoutio Sandjon.

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Sandjon, A.T., Nzeukou, A. & Tchawoua, C. Intraseasonal atmospheric variability and its interannual modulation in Central Africa. Meteorol Atmos Phys 117, 167–179 (2012). https://doi.org/10.1007/s00703-012-0196-6

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