Impact of Different Processes on Tropical Lower-Stratospheric Water Vapor as Simulated by Chemistry–Climate Models

We use a linear regression model to investigate processes driving the trends and variability in 21st century tropical lower-stratospheric water vapor, as simulated by 12 state-of-the-art chemistry-climate models (CCMs). We find the long-term trend in stratospheric humidity to be primarily driven by a warming troposphere within simulations. A strengthening Brewer-Dobson circulation cools the tropical tropopause layer (TTL), thereby offsetting some of the long-term increase. To compare results to observations, we compare these mode-based results to an analysis of MLS data. We find many, but not all, CCMs compare well with observations, lending credibility to their predictions. One clear deficiency in the models is that they tend to underestimate the impact of the Quasi-Biennial oscillation on lower-stratospheric water vapor. These results are published in Smalley et al. (2017) and will be presented at the conference.