The Intraseasonal Variability of Precipitable Water Vapor during West African Monsoon

The aim of this study is to improve the understanding of the mechanisms involved in the intraseasonal variability of the West African Monsoon. We focus here on the precipitable water (PW), which is a key variable for convective processes. To characterize the PW variability, four datasets are used and compared: the two reanalyses ECMWF/ERA-Interim and NCEP/DOE AMIP-II, the ECMWF operational analysis, and the PW retrieved from GPS observations during the AMMA field campaign. These datasets give very consistent results in terms of climatology and intraseasonal variability: PW has a meridional structure marked by a strong North-South gradient, maximum in the Sahelian zone. Several cores of variance are located along this gradient. The consistency between the datasets allows us to focus subsequently on the ERA-Interim reanalysis, which presents a good compromise between spatial and temporal resolutions (0.75°, 6h) and a large covered period (1989-2007). The intraseasonal variance represents 65% of the total PW variability in the Sahelian belt with a predominant variability at scales lower than 10 days in the West and a comparable weight of all scales in the East. We then focus on the 1-10-day timescale variability. A composite analysis of “wet” and “dry” events reveals a strong and robust westward propagating pattern of humidity (wet/dry) similar to the African Easterly Waves structure : Periodicity ~ 5-6 days, wavelength ~ 3000 km and propagating speed ~ 8-9 m/s. Nevertheless, there are some differences with AEWs traditionally identified using the vorticity or the meridional wind. Indeed, this mode has a longer period and a larger spatial and temporal footprint than AEWs and presents different phase-lag relationships between precipitation, PW and wind fields. The composite analysis of this PW mode reveals a strong link with convective activity (OLR and rainfall). Humidity and temperature budgets then emphasize the role of advections for the amplification and the propagation of the PW anomalies. Although linear parts of advection terms are predominant, the contributions of non-linear advective and diabatic terms are necessary to completely explain this mode. The structure of diabatic terms revealed by the composite budget of this PW mode is consistent with our understanding of the impact of convection, surface fluxes and radiation in the Sahel band as documented by the apparent heat source and moisture sink.