Isotopes, seasonal signals, and transport near the tropical tropopause

A recent model study (Sherwood and Dessler 2001) found that dehydration of air within energetic overshooting convective cells, followed by slow lofting into the stratosphere, could account for moisture levels in the lower stratosphere and upper troposphere given realistic horizontal transports and diabatic circulations. Here, we present augmented model simulations of transport across the tropical tropopause that include simulated stable isotopes of water vapor and a conserved CO -like tracer, including steady and transient seasonal behavior. The augmented model predicts isotopic ratio profiles that depart from the simple Rayleigh model consistently with observed departures. The model also predicts reasonable seasonal cycles of a passive, CO -like tracer and water vapor. Simulations in models that do not include energetic, overshooting convection predict a TTL and stratosphere that is more depleted than observed. Further, because of effective vertical diffusion caused by horizontal variations in vertical velocity near the tropopause, these models significantly under-predict the seasonal cycle of passive tracer entering the stratosphere. This problem disappears if one allows an energetic role for overshooting convection, rather than radiation, in explaining the horizontal vertical velocity variations. We conclude that the isotopic abundance and seasonal cycle simulations lend further support to the importance of penetrative convective elements near the tropical tropopause.