Recent rainfall trends across tropical West Africa: Observations and potential causes

Based on the results of Nicholson and Palao (1993) and Moron (1994), tropical West Africa can be divided into three quasi-homogeneous climatic regions with respect to interannual to decadal rainfall variability: The West Sahel, the Central Sahel and the Guinea Coast regions. Until the end of the 1990s, corresponding rainfall index time series based on standardized anomalies of June-September rainfall for 7, 14, and 10 stations located within the above-mentioned rainfall climates were maintained at the Colorado State University (CSU) by Prof. Gray and Dr. Landsea. The CSU time series started in 1950 and was calculated with respect to the 1950–1990 mean.

The CSU time series has been extended both backward (since 1921) and forward (until 2004) in time by the authors. Due to a collaboration between the University of Cologne and the West African AGRHYMET and ACMAD (African Centre of Meteorological Application for Development) pan-national institutions under the umbrella of the international AMMA (African Monsoon – Multidisciplinary Analysis) research initiative, monthly data for almost all 31 stations could be obtained for the recent five years. Results show that despite two anomalously wet years (1999 and 2003, and – based on preliminary data - 2005) in the large Central Sahel region, the last decade has seen about average rainfall in this area in terms of the 1950-1990 mean. In the West Sahel, the decadal rainfall is still below average. As a result, the usefulness of the West Sahel rainfall as a predictand of the upcoming Atlantic hurricane activity has declined since 1995. Along the climatologically wet Guinea Coast, the June-September rainfall was below average since 1990 in all years except 1991 and 1996. It will be shown that that the June-September rainfall at the Guinea Coast is a good proxy for the goodness of the monsoon rains that extend over the period March–October. Potential reasons for the regional rainfall trends, as well as some salient regional features (e.g., trends across Benin from a dense rain gauge network, recent wetness in Kano, central Nigeria) will be discussed.