A large annual cycle in ozone above the tropical tropopause

Near-equatorial ozonesonde observations from the SHADOZ network reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ~16-19 km, with approximately a factor of two change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with, the well-known annual variation in tropical temperature. We show that the large annual variation in ozone occurs primarily due to variations in vertical transport associated with mean upwelling in the lower stratosphere; the maximum relative amplitude peak in the lower stratosphere is co- located with the strongest background vertical gradients in ozone. The observed ozone variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.