VARIABILITY OF SUMMER RAINFALL OVER TROPICAL NORTH-AFRICA (1906-92) -OBSERVATIONS AND MODELING

The nature and causes of seasonal (July to September (JAS)) rainfall v ariability over tropical north Africa are investigated using a combina tion of empirical and modelling approaches. Concentration is focused o n three regions: the Sahel, Soudan and Guinea Coast. The variability o f seasonal rainfall through the twentieth century is analysed for each region. The well known dipole of anomalies between the Sahel and Guin ea Coast is evident, and new analysis reveals that this fluctuates pri marily on time-scales of five years or less. Attention is then focusse d on the causes of rainfall variability in each region, by examining t he relationships with global sea-surface-temperature (SST) patterns; a partitioning of the data into low- and high-frequency components is f ound to be particularly useful. So as to attribute more convincingly t he primary cause of seasonal rainfall variability to global SST forcin g, a suite of general circulation model experiments are performed, aim ed at simulating JAS rainfall anomalies for ten past years between 194 9 and 1990. Each is forced by the observed SST patterns for the approp riate year. In almost every case, the model quite skilfully simulates the magnitude and pattern of JAS rainfall anomalies across tropical no rth Africa, thus strengthening the idea that global SST variations are indeed responsible for most of the variability of seasonal rainfall. The relative impact of two additional sources of variability is also i nvestigated. First the role of internal atmospheric variability is qua ntified (using an 'analysis of variance' technique), and found to be s mall in all three regions. Second, and perhaps more controversially, t he possible effects of land-surface-moisture feedback are explored. Th is is done by replacing the normal interactive soil-moisture scheme wi th a model-derived climatology; results suggest that in some years moi sture evaporated from the land surface may play a key role, but that i n general SST forcing still dominates. Finally, an assessment of the m odel's skill at sub-seasonal time scales reveals that fluctuations of monthly rainfall about each year's seasonal mean (intraseasonal variat ions) are insensitive to SST forcing, in part due to a larger influenc e of internal atmospheric variations.