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Weather regimes over Senegal during the summer monsoon season using self-organizing maps and hierarchical ascendant classification. Part I: synoptic time scale

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Abstract

The aim of this work is to define over the period 1979–2002 the main synoptic weather regimes relevant for understanding the daily variability of rainfall during the summer monsoon season over Senegal. “Pure” synoptic weather regimes are defined by removing the influence of seasonal and interannual time scales, in order to highlight the day by day variability of the atmospheric circulation. The Self-Organizing Maps (SOM) approach, a clustering methodology based on non-linear artificial neural network, is combined with a Hierarchical Ascendant Classification to compute these regimes. Nine weather regimes are identified using the mean sea level pressure and 850 hPa wind field as variables, and gathered into three classes. Two of these weather regimes represent the classical 3–5-day African easterly waves with a mean wavelength of about 3,000 km. Three others are characterized by a modulation of the semi-permanent trough located along the western coast of West Africa and might be interpreted in terms of the 6–9-day easterly waves. The last four weather regimes are characterized by a more or less strong north–south dipole of circulation. They can be interpreted as a northward/southward displacement of the Saharan Heat Low for two of them, and a filling/deepening of this depression for the other two. The circulation patterns of all these nine weather regimes are very consistent with the associated anomaly patterns of precipitable water, mid-troposphere vertical velocity, outgoing longwave radiation, and finally rainfall. Rainfall distribution is also highlighted over the southwestern area of Senegal.

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Acknowledgments

We are thankful to NOAA-CIRES Climate Diagnostics Center (Boulder, CO, USA) for providing the NCEP/NCAR Reanalysis dataset and the Interpolated OLR dataset from their Web site at http://www.cdc.noaa.gov/. We thank the forecasters team of the Direction de la Météorologie Nationale (ANAMS, Dakar, Senegal) for very fruitful discussions on the African meteorology. We thank also Dr. Crépon and Dr. Brajard for helpful comments, and the two reviewers who contributed significantly to clarify this paper. A. K. Gueye has been supported by IRD. Based on French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially from France, UK, US and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available on the AMMA International website http://www.amma-international.org.

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Authors and Affiliations

  1. ESP, UCAD, Dakar, Senegal

    A. K. Guèye

  2. LOCEAN/IPSL, IRD, Université Pierre et Marie Curie, Boite 100, 4 Place Jussieu, 75252, Paris Cedex 05, France

    Serge Janicot, Benjamin Sultan & S. Thiria

  3. LTI, ESP/UCAD, Dakar, Senegal

    A. Niang

  4. LTI, EPT, Thiès, Senegal

    S. Sawadogo

  5. ANAMS, Dakar, Senegal

    A. Diongue-Niang

Authors
  1. A. K. Guèye
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  2. Serge Janicot
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  3. A. Niang
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  4. S. Sawadogo
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  5. Benjamin Sultan
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  6. A. Diongue-Niang
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  7. S. Thiria
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Correspondence to Serge Janicot.

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Guèye, A.K., Janicot, S., Niang, A. et al. Weather regimes over Senegal during the summer monsoon season using self-organizing maps and hierarchical ascendant classification. Part I: synoptic time scale. Clim Dyn 36, 1–18 (2011). https://doi.org/10.1007/s00382-010-0782-6

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  • DOI: https://doi.org/10.1007/s00382-010-0782-6

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