An analysis of tropospheric O3 over the Indian Ocean during spring 1995 is presented based on O3 soundings and results from the European Centre Hamburg (ECHAM) chemistry-general circulation model. The ECHAM model is nudged toward actual meteorology using European Centre for Medium-Range Weather Forecasts analyses, to enable a direct comparison between model results and in situ observations. The model reproduces observed CO levels in different air mass categories. The model also reproduces the general tendencies and the diurnal variation in the observed surface pressure, although the amplitude of the diurnal variation in the amplitude is underestimated. The model simulates the general O3 tendencies as seen in the sonde observations. Tropospheric O3 profiles were characterized by low surface concentrations (<10 ppbv), midtropospheric maxima (60--100 ppbv, at 700--250 hPa) and upper tropospheric minima (<20 ppbv, at 250--100 hPa). Large-scale upper tropospheric O3 minima were caused by convective transport of O3-depleted boundary layer air in the intertropical convergence zone (ITCZ). Similarly, an upper tropospheric O3 minimum was caused by Cyclone Marlene south of the ITCZ. The midtropospheric O3 maxima were caused by transport of polluted African air. The ECHAM model appears to overestimate surface O3 levels and does not reproduce the diurnal variations very well. This could be related to unaccounted multiphase O3 destruction mechanisms involving low level clouds and aerosols, and missing halogen chemistry. ¿ 1999 American Geophysical Union