Title: Untangling the Annual Cycle of the Tropical Tropopause Layer with an Idealized Moist Model

Presenter:  Edwin Gerber

Abstract: 
The boundary between the troposphere and stratosphere in the tropics is less distinct than its high latitude counterpart, the tropopause, such that we often refer to the region between 12 and 17 kilometers (from the cloud tops to the cold point) as the tropical tropopause layer, or TTL.  The TTL plays a critical role in regulating the chemical composition of the stratosphere, and its impact on middle atmosphere water vapor has implications for the surface energy balance.  The processes regulating the climatology and annual cycle of the tropical tropopause layer and cold point, however, are not fully understood.  Three main drivers have been identified: planetary-scale equatorial waves excited by tropical convection, planetary-scale extratropical waves associated with the deep Brewer–Dobson circulation, and synoptic-scale waves associated with the midlatitude storm tracks. In both observations and comprehensive atmospheric models, all three coexist, making it difficult to separate their contributions.

I'll present a new, intermediate-complexity atmospheric model that we developed to untangle these process in the TTL.  It reveals a remarkably generic response of TTL temperatures over a range of configurations, suggesting that the details of the wave forcing are unimportant, provided there is sufficient variation in the upward extent of westerly winds over the annual cycle.  This explains in part why previous authors have come to differing conclusions, and more generally, provides a nice case study of why one must be very careful in developing a mechanistic explanation of a nonlinear phenomenon.