Figure 4: The same as Figure 2 but for zonal wind speeds at levels of (a) 200 mb, (b) 600 mb, and (c) 850 mb derived from (top-down) NCEP1, NCEP2, MERRA, and ERA40/I. The solid blue lines indicate the Sahel rainband migration. The dotted lines indicate linear trends of the designated value. Note the different latitudinal scales for each level. In ERA40/I, year 1989 is masked out to highlight the transition from ERA40 to ERA-Interim.