Measurements from the SHADOZ ozonesonde network and Aura MLS are used to show that there are seasonal cycles in ozone (O3) and carbon monoxide (CO) at the tropical tropopause (~17 km). A simple model, driven by cloud free radiative mass fluxes, is used to investigate the origin of these seasonal cycles. The seasonal cycle in O3 is due to seasonal changes in upwelling and high altitude convective outflow. The seasonal cycle in CO at 100 hPa is influenced by both seasonal changes in dynamics and seasonal changes in CO at lower altitudes. The age of air at 17 km varies from 40 days during NH winter to 70 days during NH summer. The seasonal cycle in clear sky radiative heating at the tropical tropopause is mainly due to seasonal cycles in O3 and temperature. The seasonal cycle in O3 can be expected to amplify the seasonal cycle in cold point temperature.