The relationship between the Sahelian and previous 2nd Guinean rainy seasons: A monsoon regulation by soil wetness?

Abstract

The correlation of 0.61 found between observed July-September Sahelian rainfall in year 0 and September-November Guinean rainfall in year - 1, led us to explore the statistical relationships between precipitation, soil moisture and near surface Moist Static Energy (MSE) gradients in West Africa. These analyses were performed over successive 30-year periods and specifically, the most recent period between 1968-1998. It is shown from observations, National Centers for Environmental Predictions and National Center for Atmospheric Research (NCEP/NCAR) reanalyses and from the Global Soil Wetness Project (GSWP) database, that wet Sahelian rainy seasons are preceded by abnormally wet soils over the Sudan-Guinean belt during northern winter. Such moisture anomalies tend to hold during the dry season, then generate increasing MSE gradients just above the continent by March-April. These gradients have been shown to be of prime importance for monsoon dynamics and associated rainfall.