Atmospheric circulation patterns that trigger heavy rainfall in West Africa

Classification of atmospheric circulation patterns (CP) is a common tool for downscaling rainfall, but it is rarely used for West Africa. In this study, a two-step classification procedure is proposed for this region, which is applied from 1989 to 2010 for the Sudan-Sahel zone (Central Burkina Faso) with a focus on heavy rainfall. The approach is based on a classification of large-scale atmospheric CPs (e.g., Saharan Heat Low) of the West African Monsoon using a fuzzy rule-based method to describe the seasonal rainfall variability. The wettest CPs are further classified using meso-scale monsoon patterns to better describe the daily rainfall variability during the monsoon period. A comprehensive predictor screening for the seasonal classification indicates that the best performing predictor variables (e.g., surface pressure, meridional moisture fluxes) are closely related to the main processes of the West African Monsoon. In the second classification step, the stream function at 700 hPa for identifying troughs and ridges of tropical waves shows the highest performance, providing an added value to the overall performance of the classification. ... mehrThus, the new approach can better distinguish between dry and wet CPs during the rainy season. Moreover, CPs are identified that are of high relevance for daily heavy rainfall in the study area. The two wettest CPs caused roughly half of the extremes on about 6.5% of days. Both wettest patterns are characterized by an intensified Saharan Heat Low and a cyclonic rotation near the study area, indicating a tropical wave trough. Since the classification can be used to condition other statistical approaches used in climate sciences and other disciplines, the presented classification approach opens many different applications for the West African Monsoon region.