The MJO’s impact on rainfall trends over the Congo rainforest

Abstract

A significant declining trend in rainfall over the Congo basin has been observed over the past three decades. Since the Madden–Julian oscillation (MJO) is a major forcing mechanism for tropical convection and rainfall, the interannual variability and trend in rainfall over the Congo may be partly attributable to variability or changes in the MJO. This study explores the long-term (1979–2018) relationship between the active MJO diagnosed by the real-time multivariate (RMM) MJO phase index data and observed rainfall and cloud data over the Congo during October–March. Since the MJO may significantly enhance rainfall during the wet phases or suppress rainfall during the dry phases, the crux of this paper includes how trends in MJO activity may impact the overall observed precipitation trend over the Congo. The relationship between MJO activity and rainfall over the Congo was documented using statistical techniques and composite analysis. A new, yet simple approach was developed to partition seasonal rainfall depending on the MJO phase (i.e., wet, dry, inactive, and other). Results show a significant correlation between the number of wet and dry MJO days, and rainfall enhancement and suppression over the Congo. While there exists considerable interannual variability in MJO activity and rainfall over the Congo, there is a significant increase in the number of dry MJO days (3.47 days decade−1) which tends to intensify the large-scale drying trend over the Congo during October–March. The increasing trend in the number of dry MJO days is likely enhancing the net drying trend by 13.6% over the Congo.