The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part II: The Boreal Autumn

This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October-December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east-west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Nino(La Nina)-like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features. The vital impact of the east-west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.