On the variations in the frequency of 25\u201370-day intraseasonal oscillations in Central Africa using wavelet-based indices

Abstract

Atmospheric variability at the intraseasonal timescale remains of great concern in tropical Africa because of the vulnerability of the population to variations in the distribution and amount of rainfall within a season. Then, the parameterization of the processes that induce the intraseasonal variability of the rainfall is still a challenge for the sub-seasonal-to-seasonal forecast in the tropics. In the study of intraseasonal oscillations (ISOs) in Central Africa, almost all of the authors focused only on the amplitude of the oscillations, even though the frequency is also very important because it also undergoes strong spatiotemporal variations. The novelty of this study is that we applied wavelet transform on the 2.5° × 2.5° daily Outgoing Long-wave Radiation (OLR) to extract the frequency (period) of intraseasonal oscillations (ISO) and then study its spatiotemporal variations over Central Africa (CA) within the period 1981–2015 (35 years). By the algorithm used, we obtained a dataset of daily ISO Period Indices (ISOPI) within the study period, with the same dimensions as the original OLR datasets. The analyses showed that the mean ISOPI globally fluctuates between 32 and 52 days, but undergoes strong day-to-day variations. The ISO frequency is highly seasonal, with high ISOPI (low frequency) during December–February and June–August, and short low ISOPI (high frequency) during March–May and September–November. The composites of OLR and 850 hpa zonal winds revealed that the low-frequency ISOs (LFISOs) are predominant in Eastern Central Africa and around the Cameroon Volcanic Line, while the long-frequency events (HFISOs) are mostly found in Western Central Africa, especially around the Congo basin. The plots of yearly mean ISOPI showed that the ISO period exhibits strong interannual variations with years of very high ISOPI such as 1983, 1985, 1987, 1989, 1999, 2002 and 2009, and years of lower ISOPI as 1988, 1994, 1995. Finally, it was proved in this study that there is an enhancement of rainfall during LFISOs, especially in northern hemisphere, while HFISOs are generally associated with normal or suppressed rainfall regime.