Multi-scale analyses of dry spells across Niger and Senegal [Analyses multi-échelles des pauses pluviométriques au Niger et au Sénégal]

International audienceThe intra-seasonal episodes of "rainless days" or dry spells are hazardous events for traditional rainfed agriculture in the West African Sahel. The objective of this study is to identify the seasonal and multi-site distribution of dry spells (DS), their seasonality and their implications in local rainfall variability. The results define a regional dry spell and show the prevalence of extreme dry spells after onset and before cessation of the rainy season. These types of dry spells mark the false start of the rainy season. Dry spells 15 days (in May-June-July) and 8-14 days (in August-September) are of low frequency of occurrence and correlated to rainfall deficits. Periods of these types of dry spells in Niger are similar to those found in the North Guinean regions of Senegal. Approximately a one-month lag is found between the Sahelian regions of the two countries. The years in which extreme dry spells were found to be coherent over at least one third of the observation network at onset and cessation of the rainy season are identified for Niger and the Sahel/Northern Guinean regions of Senegal. These results lead to a better understanding of the intraseasonal distribution of dry spells over the West African Sahel