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 CO2 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 CO2-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 CO2 at 390 K relative to surface values and a similar lag in H2O relative to tropopause temperatures. This result is explained in terms of the model's mixing physics. Next the ability of other models is considered, where convective outflows are confined below some ceiling at or below the cold-point tropopause, to account for the observed seasonal cycles. Behavior of such models is governed by the placement of the ceiling relative to a known stagnation surface in the radiatively balanced vertical velocity. It is found that convection must reach to within 1 km of the cold point in order for realistic seasonal cycles to exist above the tropopause in these simulations. Importantly, the properties of air entering the stratosphere must be determined by those of the planetary boundary layer rather than the upper troposphere. This work reinforces the view that convective mixing must evanesce gradually in importance through a tropical tropopause layer of substantial thickness, rather than stopping at any particular height.