The 6–9 day wave and rainfall modulation in northern Africa during summer 1981

Abstract

Zonal and meridional wind components and geopotential height from European Centre for Medium‐Range Weather Forecasts model analyses and daily rainfall data from the Institut Français de Recherche Scientifique pour le Développement en Coopération (ORSTOM) are used to study westward propagating 6–9 day waves and rainfall modulation in northern Africa during summer 1981. The 6–9 day wave structure is determined using a composite method. In this structure, there are two vortices of opposite circulation on either side of the latitude 12.5°N. The rainfall maxima are associated with cyclonic vortices and the rainfall minima with anticyclonic vortices, coinciding with the minima and the maxima of geopotential height anomalies, respectively. The composite variability shows that the 6–9 day wave is associated with positive rainfall anomalies in West Africa in the band of latitude 7.5°–17.5°N, in the western part of the area around Senegal and Guinea and in the center toward Lake Chad. The rainfall anomalies are linked to the zonal wind anomalies, and the increase in rainfall is associated with large modulation of the African Easterly Jet zonal wind component, mainly in the cyclonic circulation. The main zones of decreasing rainfall appear north of 17.5°–20°N, toward Sudan, and south of 8°N, near Ivory Coast.