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Abstract:The spatial coherence of boreal monsoon onset over Western and Central Sahel (Senegal, Mali, BurkinaFaso) 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 7day 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 longterm mean localscale 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 BurkinaFaso (mid May) to northwestern Senegal and Saharian edges (early August). Interannual variability of the localscale 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 interstation noise at the interannual time scale. In particular, a systematic spatiallyconsistent, advance or delay of the onset is hardly observed across the whole Western and Central Sahel. In consequence, the seasonal predictability of localscale onset over the Western and Central Sahel associated for example with largescale sea surface temperatures, is, at best, weak.

The intraseasonal time scale is critical in West Africa where resources are highly rainfall dependent. Three main modes of variability have been identified, two with a mean periodicity of 15 days and one with a mean periodicity around 40 days. These modes have a regional scale and can strongly influence precipitation and convective activity. They are mainly controlled by atmospheric dynamics and land-surface interactions. They can also modulate the very specific phase of the African summer monsoon onset. A better knowledge of the mechanisms controlling this scale is necessary to improve its predictability.