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Accueil du site → Doctorat → France → 2007 → L’Oscillation de Madden-Julian et la variabilité pluviométrique régionale en Afrique Subsaharienne

Université de Bourgogne (2007)

L’Oscillation de Madden-Julian et la variabilité pluviométrique régionale en Afrique Subsaharienne

Pohl Benjamin

Titre : L’Oscillation de Madden-Julian et la variabilité pluviométrique régionale en Afrique Subsaharienne

The Madden-Julian Oscillation and regional rainfall variability over Sub-Saharan Africa

Auteur : Benjamin Pohl

Université de soutenance : Université de Bourgogne

Grade : Doctorat Discipline : Géographie Option climatologie 2007

The role of the Madden-Julian Oscillation ("MJO") on rainfall variability in Sub-Saharan Africa is examined, based on daily rain-gauge records and the NCEP-DOE AMIP-II reanalyses. The convective and dynamical signal associated with the MJO is extracted using two differing methodologies, the BMRC daily indices (Wheeler & Hendon 2004) on the one hand, and a Local Mode Analysis ("LMA", Goulet & Duvel 2000) on the other hand. The temporal variability of the MJO (in terms of period, amplitude, seasonality and location of the convective anomalies) is first analysed. Though the overall amplitude of the signal is not related to El Niño, the oscillations occurring under El Niño (La Niña) conditions tend to be shorter (longer). Surface conditions such as sea surface temperature and the seasonal cycle in the Tropics are two features that play a predominant role on the location of the associated convective clusters. The response of the African rainfall to the MJO is then examined for 3 distinct regional indices. The rainy season(s) is (are) in each case significantly triggered. In Equatorial East Africa Kenya and northern Tanzania) the Highlands show the succession of a dry and a humid phase over the MJO cycle. The associated rain-causing mechanisms mainly involve deep convection. In the eastern plains and on the coast, the peak of rainfall is out-of-phase and results from a strengthening of the trade winds over the Indian basin. A significant influence of the MJO is also detected over Southern Africa (south of 15°S) and depends on an intraseasonal modulation of the SW Indian Ocean heights. Anticyclonic circulations over Tropical Africa favour northerly anomalies that advect moisture over the region from the tropical Indian Ocean. Over West Africa, the MJO is responsible of recurrent dry conditions, alternating with a weak wet phase. The latter does not seem to result from deep convection only. The case of the East African Long Rains (March through May) is finally considered to investigate scale interactions depending on the MJO. At the "shorter" timescales, the diurnal cycle of convection is modified, from one phase of the MJO to another. The onset of the rains and the extremely wet events are also strongly locked on the intraseasonal cycle. At the "longer" timescales, a significant fraction of the interannual variability of the Long Rains is statistically related to the MJO seasonal amplitude during the corresponding trimester. The MJO therefore "adds" rainfall over East Africa, rather than simply enhancing the differences between the humid and the wet phases of the Long Rains.

Mots clés : Oscillation de Madden-Julian - variabilité intrasaisonnière - pluviométrie - Afrique Subsaharienne - dynamique atmosphérique - interactions d’échelle - diagnostic climatique

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Page publiée le 13 octobre 2008, mise à jour le 24 novembre 2017