Influence of large-scale atmospheric moisture fluxes on the interannual to multidecadal rainfall variability of the West African Monsoon

The West African monsoonal rainfall exhibits a large variability on interannual to decadal time scales. Besides the long term rainfall variations, food security in the region depends, among non-climatic factors, on the onset of the rainy season, as well as the intensity of dry spells within the growing period. In order to better understand the intraseasonal to multidecadal rainfall variations, various components of the continental-scale atmospheric water budgets have been assessed using data from ERA-40 (1957–2002) and ECMWF as well as NCEP operational analyses.

The atmospheric water vapour budgets and fluxes are discussed by means of the aerial runoff vector “Q” and the columnar moisture divergence “div Q”. Besides annual and monthly means, we considered the pre-monsoon (April-May), full monsoon (June-September) and post-monsoon (October-November) seasons. The atmosphere was split up into three distinct vertical layers, the entire troposphere (surface to 300 hPa), the monsoon layer (surface to 850 hPa) and the African Easterly Jet (AEJ) layer (700 to 500 hPa). Finally, the moisture enrichment of the West African continent up to the Sahel zone is determined via trans-boundary fluxes into a box bounded by 5° N to 15° N and 17° W to 5° E. The AMMA EOP years of 2005–2007 will be compared to the particularly interesting earlier years, as well as to the long-term means of 1979–2001.

Emphasis will be put on the role of low-level moisture influx anomalies from the Mediterranean, and meridional vs. zonal, near-surface moisture flux anomalies from the Atlantic Ocean. It will be discussed in how far and at which time scales variations in the moisture export to the Atlantic Ocean by the mid-level AEJ supports low-level moisture convergence anomalies. In a next step, the daily moisture sources and fluxes, as well as flux convergence fields will be included into the analyses of individual African Easterly Waves (AEW) in the AMMA SOP year of 2006. The diagnostics focuses on the AEW's ability to initiate and maintain meso-scale convective systems (MCS) that provide the bulk of the annual rainfall in the Sahel. Thus, the investigation shall further our understanding of how more abundant moisture over West Africa is converted into rainfall.