Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa: Statistical Analysis and Seasonal Variations

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In several case studies, tropical–extratropical interactions (TEIs) have been shown to contribute to the transition-season precipitation in subtropical northwest Africa. Such TEI situations are characterized by a moisture source in the Tropics, a midlevel moisture transport into the subtropics to the east of an upper-level trough, and precipitation generation over northwest Africa through upper-level divergence and orographic effects in the Atlas Mountains. In this paper, an automatic algorithm to identify TEI episodes on the basis of a 20-yr (December 1978–November 1998) climatology of 4-day backward trajectories starting at 400 hPa over northwest Africa, calculated from ECMWF (re-) analysis, is introduced. Twelve-hourly precipitation reports from 36 synoptic stations in northwest Africa are used to investigate the climatological relevance of TEI situations for different seasons. Results show that the region with the highest relative importance of TEIs is the semiarid southern foothills of the High Atlas (up to 40% of the annual precipitation amount). Relevance clearly decreases toward the much wetter Atlantic coast. TEI contributions are largest in the transition seasons, when TEI situations are most distinct, and in summer, when TEI situations are most frequent. It is suggested to consider TEIs in future studies on the observed and modeled precipitation variability of the region around the Atlas chain.