This study presents regular observations from the MOZAIC data base over Africa (1994--2004) to highlight the role of Hadley cells in tropical ozone production. We show the presence in the African upper troposphere (9--12 km) of a minimum of ozone and of a maximum of relative humidity that both follow the meridional migration of the Inter-Tropical Convergence Zone (ITCZ). We suggest that mechanisms contributing to meridional ozone gradients (0.3 to 1.5 ppbv deg-1) on both sides of the ozone minimum in the 200S--200N latitude band are (1) the vertical transport of relatively poor ozone air masses by tropical convection, and (2) the photochemical production of ozone in upper-level branches of the Hadley circulation. Depending on the season and on the meridional side of the ITCZ, ozone precursors injected in the ascending branch of the ITCZ include biomass burning, biogenic, lightning and anthropogenic emissions.