Examining the Impact of Climate Change on Stratosphere–Troposphere Exchange

While climate change is predicted to strengthen the stratospheric Brewer-Dobson circulation, the associated impact on stratosphere-troposphere exchange (STE) remains uncertain. Quantifying changes in the Brewer-Dobson driven STE contributes to better understanding the radiative and dynamical balance of the Upper Troposphere/Lower Stratosphere (UTLS) region and to identifying potential future increases in pollution and other hazards from chemical species such as ozone. This work quantifies these changes using the Community Earth System Model (CESM) with the Whole Atmosphere Community Climate Model (WACCM). This is a full Chemistry Climate Model with simulation of chemical and dynamical processes up to the lower thermosphere. We use monthly, zonal-mean output from fully-coupled runs spanning the time period 1955-2099 under an RCP 6.0 IPCC scenario. An inert tracer is utilized to quantify transport while excluding chemical effects. This study identifies increases in stratosphere-to-troposphere transport, especially in the subtropics. It additionally shows that the greatest concentration increases are primarily controlled by vertical transport across the subtropical tropopause, and that these transport-driven increases are also observed in modeled chemically active species such as ozone. By the end of the century, significant increases in transport are identified using the inert tracer concentrations – up to 30-40% in some seasons and regions. Results regarding observed change in STE will be presented including the rate and structure of change and the relation between dynamic forcing and these changes.