The study presented here makes use of about 300 daily rain gauges covering a 1 700 000 km2 area in order to characterize the rainfall regimes of West Africa at hydrological scales. The rainfall regime is analyzed as a combination of two variables, the average number of events over a given period of time (nT) and the average cumulative rainfall per event (h). These two parameters are a measure of the occurrence rate and magnitude of the convective storms that generate most of the rainfall in this region. They define the average water input to the hydrological systems and the average time available for this water to be redistributed into the continental hydrological cycle before a new input occurs. By analyzing for a period of 40 yr (1951–90), the space and time variations of these two parameters, it is possible to better understand how the intraseasonal to decadal rainfall variability may impact on the hydrological cycle. The analysis is carried out in two steps. First, the annual cycle and migrat... Abstract The study presented here makes use of about 300 daily rain gauges covering a 1 700 000 km2 area in order to characterize the rainfall regimes of West Africa at hydrological scales. The rainfall regime is analyzed as a combination of two variables, the average number of events over a given period of time (nT) and the average cumulative rainfall per event (h). These two parameters are a measure of the occurrence rate and magnitude of the convective storms that generate most of the rainfall in this region. They define the average water input to the hydrological systems and the average time available for this water to be redistributed into the continental hydrological cycle before a new input occurs. By analyzing for a period of 40 yr (1951–90), the space and time variations of these two parameters, it is possible to better understand how the intraseasonal to decadal rainfall variability may impact on the hydrological cycle. The analysis is carried out in two steps. First, the annual cycle and migrat...

Ongoing global warming raises the hypothesis of an intensification of the hydrological cycle, extreme rainfall events becoming more frequent. However, the strong time–space variability of extreme rainfall makes it difficult to detect meaningful trends in the regime of their occurrence for recent years. Using an integrated regional approach, it is shown that over the last 10 years, the Sahelian rainfall regime is characterized by a lasting deficit of the number of rainy days, while at the same time the extreme rainfall occurrence is on the rise. As a consequence, the proportion of annual rainfall associated with extreme rainfall has increased from 17% in 1970–1990 to 19% in 1991–2000 and to 21% in 2001–2010. This tends to support the idea that a more extreme climate has been observed over 2001–2010: this climate is drier in the sense of a persisting deficit of rainfall occurrence compared to 1950–1969, while at the same time there is an increased probability of extreme daily rainfall.

The Standardized Precipitation Index (SPI) is usually defined as the arithmetic mean of the normalized precipitation recorded at several stations over a region of interest where the standard deviation computed at each station over a period of reference is used as the normalizing factor. It is common to use this index in order to diagnose whether the Sahelian region can be considered as wet or dry for a given year. There are several key factors interfering with the relevance of the SPI as a measure of how rainy is a season over the Sahel. The strong spatial variability of the Sahelian rainfall at the annual scale, the uneven distribution of the raingauge network and the mean interannual climatological gradients across the region are the most important of these factors, and their influence is studied in detail here. Using an optimal interpolation algorithm to compute the SPI, the effects of various sampling schemes are first studied showing that the SPI computed as a single mean value over the whole Sahelian region is relatively robust with respect to these effects. However, the central key question remains that computing a single mean SPI over the Sahelian region hides the strong underlying spatial variability of this index. For instance, 2006 was a significantly dry season over the Sahel as a whole, but working at the 0.5° × 0.5° resolution shows that in fact only 28% of the area was significantly dry, while 15% of the Sahel was significantly wet. From conditional empirical distributions a distribution function is proposed to determine the spatial distributions of the 0.5° × 0.5° SPI values for a given mean regional SPI value. Studying in detail the space–time pattern of the SPI over the 1950–2006 period, also shows that recent years are characterized by a greater interannual variability than the previous 40 years, and by a contrast between the western Sahel remaining dry and the eastern Sahel returning to wetter conditions. Copyright © 2008 Royal Meteorological Society

AMMA—the African Monsoon Multidisciplinary Analysis—is the biggest programme of research into environment and climate ever attempted in Africa. AMMA has involved a comprehensive field experiment bringing together ocean, land and atmospheric measurements, on time‐scales ranging from hourly and daily variability up to the changes in seasonal activity over a number of years. Many of the publications in this special issue make use of subsets of the AMMA measurements, collected from a diverse set of sensors.As a general introduction to the special issue, this paper provides a comprehensive overview of the AMMA observational programme, and summarises the scientific strategy which has defined the field deployment. The relationship between the existing observational monitoring networks of the region and the new sensors deployed for AMMA, and for the future, is described. Making use of regional and sub‐regional maps, the main groups of sensors are described in terms of their deployment periods and their spatial co‐ordination. The key linkages between different groups of measurements are also outlined, in terms of the strategy for their combined use and in terms of their interdependence. Some brief summaries of conditions sampled during the three years of the AMMA Extended Observing Period are also given. Copyright © 2009 Royal Meteorological Society and Crown Copyright