The Sahelian standardized rainfall index revisited

The Standardized Precipitation index (SPI) IS usually defined as the arithmetic mean of the normalized precipitation recorded at several stations over it re-ion of interest where (lie standard deviation computed at each Station over it period of reference is Used as the normalizing factor It is common to use this index in order to diagnose whether the Sahehan region can be considered as wet or dry for a given year There ire several key factors interfering with [lie relevance of the SPI as I measure of how ramy is it season over the Sahel The strong spatial variability of the Sahehan rainfall at the annual scale. the uneven distribution of the ramguage 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 10 compute the SPI, [lie effects Of various sampling schemes are first Studied showing that the SPI computed its a single mean value over the whole Sahehan region is relatively robust Willi respect to these effects. However, the central key question remains that computing I single mean SPI over the Sahehan region hides the strong underlying][12 Spatial variability of this index For instance, 2006 was I significantly dry season over the Sahel as it whole, but working at the 0 5 degrees x 0.5 degrees 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 I distribution function is proposed to determine the spatial distributions of the 0 5 degrees x 0 5 degrees 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,11 variability than the previous 40 years. and by,I contrast between the western Sahel remaining dry and the eastern Sahel returning to wetter conditions. Copyright (C) 2008 Royal Meteorological Society