259529.fig.006a
(a)
259529.fig.006b
(b)
259529.fig.006c
(c)
Figure 6: The same as Figure 5 but for (a) meridional gradient of Ertel potential vorticity at 600 mb, (b) vorticity source due to vortex stretching at 600 mb, and (c) vorticity source at 925 mb averaged between 30°–15°W. Black dotted lines are linear trends of the designated value. The eastern Sahel conditions are shown in the right with respect to (a) and (b).