On the factors modulating the intensity of the tropical rainbelt over West Africa

摘要

This article presents a case study of a wet and a dry year over West Africa. These 2 years, 1955 and 1983, are characterized by rainfall anomalies of one sign throughout West Africa, including the Sahel and Guinea Coast (the no-dipole cases defined in previous studies). The contrast in rainfall is related to a general weakening and intensification of the tropical rainbelt in the dry year (1983) and the wet year (1955), respectively. The Study, limited to the month of August, examines the factors that modulate the intensity of the rainbelt. The more intense rainbelt of 1955 is associated with an anomalously strong tropical easterly jet (TEJ) and a slight northward displacement of the African easterly jet of the Northern Hemisphere (AEJ-N). The link between the TEJ and rainfall is a causal one, with the strong TEJ enhancing rainfall by enhancing upper-level divergence and sustaining a strong Hadley-type overturning over West Africa, with strong vertical motion. In 1983, a weak TEJ and vertical alignment of the AEJ-N and TEJ axes promote a weak rainbelt and ubiquitous drought. The factors that produce a rainfall dipole (i.e. an opposition in the sign of anomalies between the Sahel and the Guinea Coast) are notably different. The dipole results from meridional displacements of the circulation and rainbelt, rather than changes in rainbelt intensity. Although the strong TEJ is a prime factor, the development of the dipole requires the development of strong and vertically extensive equatorial westerlies. These are produced by strong surface pressure gradients over the Atlantic and West Africa.