Spatial Coherence of Monsoon Onset over Western and Central Sahel (1950–2000)

Abstract

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.

Seasonal forecasts for regional onset of the West African monsoon

“…They are often formulated for a particular region. Marteau et al (2009) show that local onset dates that are defined in this way tend to be characterised by weak spatial coherence of interannual variability across stations reporting rainfall. If at the individual station level small-scale processes are important in controlling local onset then that may turn out to be difficult to predict at seasonal timescales.…”

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

“…Some have defined it in terms of simple rainfall characteristics. Marteau et al [40], for example, use threshold values of daily rainfall at individual stations to quantify the magnitude and length of the initial wet spell and any subsequent breaks.…”

“…The onset proceeds slowly and is characterized by a succession of active and inactive phases (breaks in the monsoon) [43]. There is little spatial coherence in the onset dates and, consequently, little predictability [40]. The lack of coherence is a result of the fragmented nature of the active and break phases [44].…”

Rainfall Intra-Seasonal Variability and Vegetation Growth in the Ferlo Basin (Senegal)

“…Most previous studies have focused only on average or cumulative rainfall during the rainy season, and the start [4] and end date of the season [5]-the variability within the monsoon has generally been neglected. However, the length and frequency of dry spells [6] and the frequency, amount and duration of rainfall events [7] are essential variables controlling the relationships between rainfall and vegetation growth.…”