Characteristics of daily rainfall totals in tropical West Africa over the last 100 years

Tropical West Africa is known for its large decadal rainfall variability. Recent decades have not only seen a recovery of rains from the drought in the 1970s and 1980s, but also devastating fluvial floods in various major river basins of the sub-continent. In addition, a significant warming trend has been observed in most months, thus suggesting that enhanced recent rainfall intensities are related to larger water vapor holding capacity of the atmosphere. However, West Africa is also known for its sparsity of continuously recording rainfall stations, thus rendering the investigations of any long term trend in daily rainfall characteristics difficult.

From the “Karlsruhe African Station Surface- Data Base (KASS-D)” more than 100 rainfall stations, most of them recording since the 1920s and having more than 80% of data availability, were extracted and each of the station was subjected to a thorough quality check. The latter is still ongoing, especially for some dubiously high rainfall values for some stations in francophone countries in the 1920s-1940s that are in the original records and have thus widely been just in past studies.

For each station, as well as countries and Sahel/Guinea Coast regions, the “Expert Team on Climate Change Detection and Indices (ETCCDI)” were calculated and linear and higher order trends were determined. Results show that variability and trends were dominated by well-known decadal-scale rainfall variations in the region. Particularly, it is not clear to what quantitative extent the recent increase in rainfall intensity in the Sahel is related to the warming trend of recent years vs. potentially more favorable rainfall conditions by the decadal-scale drivers. ETCCDI indices at the Guinea Coast partly indicate no recent strong increase in intensity of the daily rainfall totals with the 1960s having higher values. Equally high or higher values in the 1920s to 1940s are currently investigated in terms of their plausibility.