African Easterly Jet: Structure and Maintenance

Abstract

Abstract This article investigates the African easterly jet (AEJ), its structure, and the forcings contributing to its maintenance, critically revisiting previous work that attributed the maintenance of the jet to soil moisture gradients over tropical Africa. A state-of-the-art global model in a high-end computer framework is used to produce a three-member 73-yr ensemble run forced by observed SST to represent the control run. The AEJ as produced by the control is compared with the representation of the AEJ in the 40-yr ECMWF Re-Analysis (ERA-40) and other observational datasets and found to be very realistic. Five experiments are then performed, each represented by sets of three-member 22-yr-long (1980–2001) ensemble runs. The goal of the experiments is to investigate the role of meridional soil moisture gradients, different land surface properties, and orography. Unlike previous studies, which have suppressed soil moisture gradients within a highly idealized framework (i.e., the so-called bucket model), terrestrial evaporation control is here achieved with a highly sophisticated land surface treatment and with an extensively tested and complex methodology. The results show that the AEJ is suppressed by a combination of absence of meridional evaporation gradients over Africa and constant vegetation, even if the individual forcings taken separately do not lead to the AEJ disappearance, but only its modification. Moreover, the suppression of orography also leads to a different circulation in which there is no AEJ. This work suggests that it is not just soil moisture gradients but a unique combination of geographical features present only in northern tropical Africa that causes and maintains the jet.