Figure 1.

The main features of the rainfall climatology in West Africa. (a) Mean May–October rainfall (color shading); location of the African Easterly Jet (AEJ, magenta contour) as indicated by the 9 m/s contour of the easterly wind at 600 hPa; maximum African Easterly Waves (AEW, orange color; contour retraced from Figure 5a in Thorncroft & Hodges, 2001) as indicated by the a track density scaled to number density per unit area (10⁶ km²) per season (MJJASO) greater than 6; and near surface (925 hPa) wind (streamlines; ERA40, Uppala et al., 2006). All fields are for the 1979–1998 period, for consistency with Thorncroft and Hodges (2001). (b) The seasonal cycle of sector mean (20°W‐30°E) surface air temperature (warm shaded colors; CPC Monthly Global Surface Air Temperature, Fan & van den Dool, 2008), precipitation (contours, same colors as in (a); GPCP Huffman et al., 1997), and the intertropical discontinuity (ITD, indicated in blue by the zero contour of the 925 hPa meridional wind from ERA40 reanalysis). Note how the advance of the ITD is connected with the warming of the Sahara and how the rain band stays to the south of the ITD