Romain Marteau

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

The spatial coherence of boreal monsoon onset over the 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, that is, 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 or the amplitude of the initial wet spell, or both, or the length of the following dry spell modifies the long-term mean of 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 interstation 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.

Implication of the Madden―Julian Oscillation in the 40-Day Variability of the West African Monsoon

Abstract Madden–Julian oscillations (MJOs) are extracted over the Indo-Pacific basin using a local mode analysis. The convective perturbations are then projected over a larger domain to evaluate their remote consequences over the West African monsoon (WAM) intraseasonal variability. Rather weak (4–6 W m−2) convective fluctuations occurring in phase with those over the southern Indian basin are found over Africa, confirming the results of Matthews. In reverse, 40-day fluctuations in the WAM, similarly detected and projected over a widened area, demonstrate that a large majority of these events are embedded in the larger-scale patterns of the MJO. The regional amplitude of intraseasonal perturbations of the West African convection is not statistically associated with the amplitude of the MJO over the Indian basin but is instead closely related to background vertical velocity anomalies over Africa, possibly embedded in changes in the regional Walker-type circulation. Subsiding motion over Africa is recorded ...

La question de la vulnérabilité et de l'adaptation de l'agriculture sahélienne au climat au sein du programme AMMA

Rural populations in Soudano-Sahelian Africa are known to be particularly vulnerable to climate change due to a combination of naturally high levels of climate variability, high reliance on climate sensitive activities such as rain-fed agriculture and limited economic and institutional capacity to cope with and adapt to climate variability and change. Urgent actions are required to tackle the issues raised by climate change in Soudano-Sahelian Africa and these actions need to be supported by the best knowledge available. The AMMA program is attempting to revitalize research in Africa in this field through an integrated interdisciplinary framework that aims to increase our understanding of the problem and support decision making for the future.The first phase of AMMA tackled three major issues: (i) increase our understanding of the relationships between climate and agriculture (ii) quantify the vulnerability of agriculture productivity to climate change and (iii) explore pathways to cope with climate variability and change. This synthesis will show results from the first phase of the AMMA program to illustrate progress in each of these three issues.