West African Monsoon Intraseasonal Variability: A Precipitable Water Perspective

Abstract

est African monsoon intraseasonal variability has important implications for food security and drought early warnings. In the present study, intraseasonal variability over the Sahel is assessed from the perspective of precipitable water, as provided by model reanalyses and GPS measurements. In the eastern Sahel, precipitable water variability is dominated by time scales longer than 10 days, whereas synoptic scales dominate in the western Sahel, especially because of African easterly waves (AEWs).The present work then focuses on the moisture footprint of AEWs along the northern side of the African easterly jet, as detected and analyzed directly from the main synoptic disturbances associated with precipitable water. Composite wet and dry precipitable water anomalies within AEWs propagate westward with a 5?6-day period. Their robustness, consistency, and spatial footprint, as well as their significant modulation of the convective activity, imply potential skill for short- to medium-range forecasts of wet and dry events over the Sahel. A composite moisture budget points out the key processes involved in the evolution of moisture anomalies. Advection processes are shown to be dominant during their life cycle. A linear adiabatic analysis of the propagation and growth of AEW precipitable water anomalies captures the main observed properties well, even though a key role of diabatic processes such as rain evaporation is needed to fully understand the life cycle of such precipitable water anomalies, especially their growth over the continent.