Spatial coherence of monsoon onset over Western and Central Sahel (1950-2000)

International audience The spatial coherence of boreal monsoon onset over Western and Central Sahel (Senegal, Mali, Burkina-Faso) is studied through the analysis of daily rainfall data for 103 stations from 1950 to 2000. Onset date is defined using a local agronomic definition, i.e. the first wet day (> 1 mm) of 1 or 2 consecutive days receiving at least 20 mm without a 7-day dry spell receiving less than 5 mm in the following 20 days. Changing either the length and/or the amplitude of the initial wet spell, or the length of the following dry spell modify the long-term mean local-scale onset date but has only a weak impact either on its interannual variability or its spatial coherence. Onset date exhibits a seasonal progression from southern Burkina-Faso (mid May) to northwestern Senegal and Saharian edges (early August). Interannual variability of the local-scale onset date does not seem to be strongly spatially coherent. The amount of common or covariant signal across the stations is far weaker than the inter-station noise at the interannual time scale. In particular, a systematic spatially-consistent, advance or delay of the onset is hardly observed across the whole Western and Central Sahel. In consequence, the seasonal predictability of local-scale onset over the Western and Central Sahel associated for example with large-scale sea surface temperatures, is, at best, weak.