How Will Climate Change Impact Large-Scale Transport in the Upper Troposphere and Lower Stratosphere (UTLS)?

Long-term (1955-2099) simulations of the Whole Atmosphere Community Climate Model (WACCM) carried out for the Chemistry-Climate Model Initiative (CCMI) are used to investigate future changes in large-scale transport. In order to isolate transport changes from future changes in emissions and chemical processes, we focus on the artificial passive tracer e90, specifically designed to highlight the transition region between troposphere and stratosphere. The e90 tracer has constant and homogeneous emissions at the surface and decay time scale throughout the atmosphere, such that trends in e90 are exclusively due to changes in transport. The results show increased e90 concentrations in the lower stratosphere and reduced concentrations in the troposphere. These trends emphasize a key role of the rising tropopause height with climate change, which despite being a small altitude change (~30 m/decade), has a strong impact on UTLS dynamics and thus tracer transport. The tracer trends are then traced back to changes in the different transport terms, including resolved processes (meridional residual circulation and eddy transport) and sub-grid parameterized processes.