On the study of seasonal patterns of 25–70-day intraseasonal oscillations in Central Africa

Abstract

With its strong influence on weather and climate, the Madden–Julian Oscillation (MJO) plays a crucial role in the tropical climate system. However, one of the greatest challenges of climate modeling remains definition of suitable MJO-related metrics for the efficient use in climate models. This requires a good documentation of the MJO variation patterns throughout the year and from tear to another. As contribution to the definition of these MJO-related metrics in General Circulation Models (GCMs), this paper addresses the seasonal patterns of 25–70-day Intraseasonal Oscillations (ISO) in CA using the 2.5° × 2.5° daily Outgoing Longwave Radiation (OLR), for the period 1981–2015 (35 years). One novelty in this study is the use of a Discrete Wavelet Transform (DWT) model to compute ISO amplitude and frequency indices and then investigate their variations from one season to another. The analyses showed that the ISO characteristics (amplitude, frequency and propagation speed) strongly vary from season to another. The ISO intensity (ISOI) is extremely high during December–February (DJF) and March–May (MAM) and lower during June–August (JJA) and September–November (SON) seasons. As for the ISO period (ISOP), the low-frequency ISO (high ISOP) are observed during DJF and JJA and the high frequency (low ISOP) during MAM and SON seasons. However, for the four seasons, the period is predominantly contained between 40 and 50 days, suggesting the dominance of MJO. The investigation of propagation characteristics for the selected Active Intraseasonal Events (AIEs) found the eastward propagation to be the dominant feature in all seasons but the propagation speed is lower for DJF and MAM, when compared with JJA and SON. At global scale, we found that the incursions of the moisture fluxes from Atlantic and Indian oceans are responsible for the convection and subsequently the rainfall enhancement over CA at intraseasonal timescales. We also accessed the impact of ISO on rainfall using and index called the Impact Rate (IR). The plots of spatial distribution of IR during the four seasons showed that the influence of ISO on rainfall varies from one season to another, with some zones of very high IR and other zones of lower IR. Moreover, the zone of high IR propagates northwards from one season to another.