Extreme Indian Ocean dipole and rainfall variability over Central Africa

Using both observational and reanalysis datasets, this study examines the influence of extreme Indian Ocean Dipole (IOD) events on Central Africa (CA) rainfall from September to December over the period 1980–2016. It is shown that during extreme positive IOD events (hereafter pIOD), CA experiences enhanced moisture supply from the Indian Ocean that results in increased rainfall while the opposite process is recorded during the negative phase (hereafter nIOD), with the magnitude of pIOD events anomalies stronger than those of nIOD events. Furthermore, the inflow is strongest in the lower troposphere over the ocean while it is strongest in the mid-troposphere over CA because much of the lower-level inflow is blocked by the topography to the east of CA. The intensification of mid-tropospheric moisture transport over CA is associated with the African Easterly Jet which is present during these months. From 1980 to 2016 the IOD and the El Niño-Southern Oscillation (ENSO) showed a strong positive relationship so that extreme pIOD (nIOD) are mostly coincident with El Niño (La Niña). To assess the possible role of non-ENSO influences on the IOD influence over CA, the linear relationship between the Niño-3.4 and dipole mode index (DMI) was removed from the DMI. It appears that the influence of the pIOD on CA rainfall is significant with the non-El Niño effect and this is characterized by an increase in moisture advection toward CA that contributes to an enhancement of rainfall intensity. However, the IOD seems to contribute strongly to the rainfall variability in the CA regions south of 5°N, especially over the eastern Democratic Republic of Congo, and over the area from Gabon to the Republic of Congo where about 20–32% of the total rainfall variance is associated with the IOD independently of ENSO from November to December.