Within the ongoing German IMPETUS ("An Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa") project, the relation between the rainfall variability in subtropical northwest Africa/tropical West Africa and the large-scale atmospheric circulation on the one hand and the associated synoptic activity on the other hand is investigated. In contrast to the winter-rain dominated region along the Atlantic and Mediterranean coasts in northwest Africa, the semi-arid to arid southern foothills of the Atlas Mountains receive significant contributions to their annual rainfall amounts from rainy episodes in late summer/early autumn and in spring. The description of the synoptic evolution of two significant rainfall events, one in September 1990, the other in April 2002, in the latter region is the first focus of the present contribution. Secondly, emphasis is put on a disastrous rainfall event in January 2002 in the Sahelian climate zone of Senegal and Mauritania and the Cape Verde Islands. This tropical region receives the bulk of the annual rainfall during the July-September wet season, while in the climatological mean, only traces of rain are observed during the core of the dry season in December-February.

Common to these cases, all of which represent the same type of an important tropical- extratropical interaction mechanism, is the presence of (a) an upper-level trough in the subtropics that extends into the tropical belt, of (b) a subtropical jet streak at the eastern flank of the trough that is collocated with a "tropical moisture outburst", of (c) a low-level moisture source in the deep West African/East Atlantic Tropics from which the water vapor is advected into the rainfall region, and (d) a forcing of rainfall due to dynamical lifting under the inflection point of the subtropical jet. Several differences in synoptic evolution and rainfall-generating factors between the presented cases will be highlighted. For the Saharan foothills of the Atlas Mountains, this type of tropical-extratropical interaction is responsible for up to 40% of the climatological rainfall. In a companion paper, the dynamical aspects that lead to the formation of the subtropical trough will be dealt with.