Impact de la variabilité intra-saisonnière sur les variations interannuelles de la saison des pluies en Afrique Centrale

Abstract

Central Africa is a region particularly sensitive to rainfall variations, which significantly affect agriculture, food security, and economic development. This thesis examines the influence of 25-70 day intraseasonal variability (ISV) on the interannual variability of the seasonal mean rainfall in Central Africa (CA). Our study, based on observational data and reanalyses (1980-2019), aims to understand how different modes of climate variability influence seasonal rainfall (March-April-May) in the region. We first analyzed the impact of ISV amplitudes on 137 intraseasonal events (71 strong and 66 weak). Then, we studied the influence of the 25-70 day intraseasonal variability and the Madden-Julian Oscillation (MJO) phases on rainfall. Finally, we explored the dynamic and thermodynamic mechanisms underlying these influences and their impact on the interannual variability of rainfall regimes. The results indicate that strong intraseasonal events (SIE) are associated with increased rainfall over almost all of Central Africa, while weak intraseasonal events (WIE) induce a marked east-west contrast: an increase in rainfall in the west of the region and a decrease in the east. Phases 4 and 5 of the ISV induce an increase in rainfall in the equatorial belt (10°S - 10°N) between 10°E and 45°E, a similar effect to that observed during phases 1 and 2 of the MJO. Conversely, phases 1 and 8 of the ISV, as well as phases 5 and 6 of the MJO, are associated with a decrease in rainfall in the region. The impact of the ISV phases varies greatly from one phase to another and from one region to another in CA. The areas experiencing a significant increase in rainfall show a high sensitivity to the ISV, with an impact rate that can reach 80%. Similarly, the regions where rainfall decreases are strongly correlated with the ISV, with impact rates that can drop to -80%. Linear correlation analyses confirm this strong association, with coefficients reaching 0.6 and -0.6 in certain regions. The disturbances in atmospheric circulation and the thermodynamic anomalies associated with the different phases of the ISV largely explain the spatial distribution of convection and precipitation anomalies. A non-centered model correlation analysis reveals a close relationship between the ISV amplitude and the MJO phases, and their combined impact on the rainfall distribution during the MAM season. The analysis of the interannual variations of the 25-70 day wave reveals three types of dominant spatial structures: a positive phase characterized by a positive anomaly of amplitude over the entire region, a negative phase with a negative anomaly of amplitude, and a mixed phase with an east-west dipolar structure. These different phases are associated with distinct rainfall regimes: positive years (1987, 1989, 1991, 1994, 1996, 1999, 2005 and 2008) favor an increase in rainfall on the Atlantic coasts and west of Ethiopia, while the south of the Congo Basin and Kenya experience a decrease. Conversely, negative years (1988, 1990, 1992, 1997, 2003, 2004, 2010, 2015 and 2017) show an opposite pattern. Mixed years (1983, 1985, 2000, 2018 and 2019) are marked by a decrease in rainfall in the west and an increase in the east of Central Africa. These interannual modulations of rainfall are linked to corresponding variations in atmospheric circulation at low and high altitudes. The 25-70 day waves thus play a key role in the interannual variability of rainfall in Central Africa, with potentially significant implications for agriculture and livelihoods.