Intraseasonal Land-Atmosphere Coupling in the West African Monsoon

Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.