Interannual variations in the amplitude of 25–70-day intraseasonal oscillations in Central Africa and relationship with ENSO

Abstract

The interannual variations in the intraseasonal oscillations (ISO) activity have long been investigated in different geographical areas around the world. However, Central Africa (CA) remains a serious gap in climate variability studies, especially at intraseasonal timescales because very few of these studies focused on this region. In this paper, we investigated the interannual variations in the amplitude of intraseasonal oscillations (ISO) in CA, using an ISO index (ISOI), extracted from the wavelet transform on the 25–70-day filtered daily Outgoing Longwave Radiation (OLR) data. Results showed that the ISOI in CA undergoes large interannual variations with some years of high as 1985 and 1990, and years of very low ISO as 1981 and 2001. The composite analysis revealed that the warm El Nino Southern Oscillation (ENSO) years are characterized by negative ISOI in most part of CA, while cold ENSO years are dominated by positive ISOIs. The lagged correlations computed between ISOIs and Nino3.4 index showed that the relationship ISOI-ENSO varies much spatially, but the correlation coefficients are globally low, with maxima around ± 0.5. The empirical orthogonal function (EOF) analysis applied on monthly ISOI revealed that two main spatial structures characterize ISOI variability, which explain 15.7% and 8.4% of the total variance, respectively. The first is marked by mostly positive loadings, crossing CA from northwest to southeast, and the second emerges as a quasi-meridional, with positive loadings located around the latitude equator. The principal component (PC) time series confirm the strong interannual variations of the ISOI amplitude. The extreme ENSO years are characterized by an anomalous easterly winds coming from the Indian Ocean, which reverses around Eastern Central Africa borders and blow back into the ocean. These easterly winds bring warm (or cold) air fluxes from the Ocean, which may be responsible for the modulation of the atmospheric anomalous convection at intraseasonal timescales.