Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa

Abstract

In contrast to the winter rain-dominated region along the Atlantic and Mediterranean coasts in northwest Africa, the semiarid to arid southern foothills of the Atlas Mountains receive significant contributions to their annual rainfall amounts from rainy episodes in late summer/early autumn. Three such cases (September 1988, September 1990, August–September 1999) are studied with respect to the sources and the vertical and horizontal transports of moisture, as well as local factors for precipitation generation. Besides station reports of precipitation, the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses and Meteosat water vapor images are considered. All three cases presented reveal similar tropical–extratropical interactions. Convective cloud clusters or squall lines over tropical West Africa and the adjacent tropical Atlantic Ocean, several of them associated with low-level African easterly waves, could be identified as midlevel moisture source regions by the use of trajectory analysis. The moisture is transported northward to the east of an mid- to upper-level subtropical trough, which extends anomalously deep into the Tropics. Most of the transport occurs above the dry Saharan planetary boundary layer. The moisture converges at midlevels (700–400 hPa) over northwestern Africa underneath a strong upper-level divergence center at the inflection point of the trough. The dynamically forced ascent in connection with orographic lifting at the Atlas Mountains in the southerly flow and surface heating over the elevated terrain triggers convective rainfalls, which occur preferably close to and downwind of the mountain chain. The three cases differ with respect to the synoptic evolution of the upper-level subtropical trough and the paths of the moisture export from the Tropics. At the end of the episode in September 1988, the tropical air over northwest Africa is displaced by polar air connected with some heavy rainfall events. The presented cases are compared to studies of tropical plumes and Soudano–Saharan depressions.