Convective Mixing near the Tropical Tropopause: Insights from Seasonal Variations

It has been suggested that convection remains important in the budget of water vapor up to the tropical tropopause or even higher. But observed seasonal variations of CO 2 and water vapor in the lower stratosphere, and their timing, call the required convective transport into question. Here, these seasonal variations are modeled using several idealized representations of convection. First, a CO 2 -like tracer is added to a previously published climate model in which convection explicitly transports air to a variety of heights ranging from 14 to 19 km, in a manner sufficient to dehydrate the stratosphere. It is found that these motions are consistent with the observed lags and amplitudes of seasonal variations near and above the tropopause, including a 2-month phase lag in CO 2 at 390 K relative to surface values and a similar lag in H 2 O relative to tropopause temperatures. This result is explained in terms of the model's mixing physics.