While climate change is predicted to strengthen the stratospheric Brewer-Dobson circulation, the impacts on stratosphere-troposphere exchange (STE) remain uncertain. The evaluation of changes in the Brewer-Dobson circulation driven STE contributes to better understanding the radiative and dynamic... Show moreWhile climate change is predicted to strengthen the stratospheric Brewer-Dobson circulation, the impacts on stratosphere-troposphere exchange (STE) remain uncertain. The evaluation of changes in the Brewer-Dobson circulation driven STE contributes to better understanding the radiative and dynamical balance of the stratosphere and to quantifying future pollution from chemical species such as ozone. In this work, STE changes are quantified in the Community Earth System Model (CESM) with the Whole Atmosphere Community Climate Model (WACCM), a full Chemistry Climate Model with resolution of chemical and dynamical processes up to the lower thermosphere. Data is used in monthly output from fully-coupled runs spanning the time period from 1955-2099 under a RCP 6.0 IPCC scenario. An inert tracer, with fixed concentration above 80 hPa and with an e-folding timescale of 25 days, enables quantification of transport with exclusion of chemical effects. This study focuses on the Upper Troposphere/Lower Stratosphere (UTLS) region and identifies increases in transport across the tropopause, especially in the subtropics. Relative to a 1955-1984 baseline, upper tropospheric annual-mean tracer concentrations increase by as much as 25% by the end of the century, and lower troposphere concentrations by 5-10% in the mid-latitudes and 15-20% in the north-polar and equatorial latitudes. The maximum seasonal increases (primarily in boreal autumn/winter) approach 40% in the upper troposphere, and in the lower troposphere 15-20% in the mid-latitudes and 20-30% in the north-polar and equatorial latitudes. The tropospheric tracer burden increases by 1.7 ± 0.3% per decade, following a significant linear trend. The greatest concentration increases are largely controlled by vertical transport through the subtropical tropopause layer, and these transport-driven increases are also observed in chemically-active species such as ozone. Show less
