On the variations in the frequency of 25–70‑day intraseasonal oscillations in Central Africa using wavelet‑based indices

摘要

Atmospheric variability at the intraseasonal timescale remains of great concern in tropical Africa because of the vulnerabilityof the population to variations in the distribution and amount of rainfall within a season. Then, the parameterizationof the processes that induce the intraseasonal variability of the rainfall is still a challenge for the sub-seasonal-to-seasonalforecast in the tropics. In the study of intraseasonal oscillations (ISOs) in Central Africa, almost all of the authors focusedonly on the amplitude of the oscillations, even though the frequency is also very important because it also undergoesstrong spatiotemporal variations. The novelty of this study is that we applied wavelet transform on the 2.5° × 2.5° dailyOutgoing Long-wave Radiation (OLR) to extract the frequency (period) of intraseasonal oscillations (ISO) and then studyits spatiotemporal variations over Central Africa (CA) within the period 1981–2015 (35 years). By the algorithm used, weobtained a dataset of daily ISO Period Indices (ISOPI) within the study period, with the same dimensions as the originalOLR datasets. The analyses showed that the mean ISOPI globally fluctuates between 32 and 52 days, but undergoes strongday-to-day variations. The ISO frequency is highly seasonal, with high ISOPI (low frequency) during December–Februaryand June–August, and short low ISOPI (high frequency) during March–May and September–November. The compositesof OLR and 850 hpa zonal winds revealed that the low-frequency ISOs (LFISOs) are predominant in Eastern Central Africaand around the Cameroon Volcanic Line, while the long-frequency events (HFISOs) are mostly found in Western CentralAfrica, especially around the Congo basin. The plots of yearly mean ISOPI showed that the ISO period exhibits stronginterannual variations with years of very high ISOPI such as 1983, 1985, 1987, 1989, 1999, 2002 and 2009, and years oflower 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.