Between January and March 1999, a Citation jet aircraft equipped with a variety of chemical instrumentation took part in the Indian Ocean Experiment (INDOEX). During the winter monsoon (December--March), the Intertropical Convergence Zone (ITCZ) over the Indian Ocean lies between 10¿S and the equator, effectively dividing the polluted Northern Hemisphere from the relatively pristine Southern Hemisphere. Analyzed here are four vertical profiles (0.5--12.5 km) of CO, O3 mixing ratios, and relative humidity, north and south of the ITCZ. From these profiles and accompanying back-trajectory analyses, we identify two possible mechanisms for the influx of dry, ozone-rich stratospherically influenced air observed in the middle tropical troposphere. The first involves rapid advection and subsidence of air associated with the subtropical jet stream in the Northern Hemisphere. The second is associated with subsidence ahead of a tropical cyclone in the Southern Hemisphere. Moreover, high mixing ratios of CO in Northern Hemisphere boundary layer air and the Southern Hemisphere middle atmosphere are interpreted as a case of interhemispheric exchange, leading to elevated levels of pollutants in the 8- to 10-km altitude range of the Southern Hemisphere.